ISSN: 0022-1481 eISSN: 1528-8943 |
Published by ASME International
No Issue Number
- A Hybrid Phonon Gas Model for Transient Ballistic-Diffusive Heat Transport
Yanbao Ma<br/> We present a continuum hybrid phonon gas model to describe transient ballistic-diffusive heat transport. In this model, heat energy is carried by a mixture of longitudinal and transverse phonon gases so that the distinction between longitudinal and transverse phonon excitations is taken into account ... [J. Heat Transfer 135, 044501 (2013)] published Wed Feb 20, 2013.
- Structure Controlled Synthesis of Vertically Aligned Carbon Nanotubes Using Thermal Chemical Vapor Deposition Process
Myung Gwan Hahm, Young-Kyun Kwon, Ahmed Busnaina, and Yung Joon Jung<br/> Due to their unique one-dimensional nanostructure along with excellent mechanical, electrical, and optical properties, carbon nanotubes (CNTs) become a promising material for diverse nanotechnology applications. However, large-scale and structure controlled synthesis of CNTs still have many difficul ... [J. Heat Transfer 133, 031001 (2010)] published Mon Nov 15, 2010.
- Special Issue on Advanced Thermal Processing
Wilson K. S. Chiu, Costas P. Grigoropoulos, and Ben Q. Li<br/> Abstract not available. [J. Heat Transfer 133, 030301 (2010)] published Mon Nov 15, 2010.
- Thermal Science of Weld Bead Defects: A Review
P. S. Wei<br/> Mechanisms for the formation of bead defects, such as humping, gouging, rippling, and other unexpected surface patterns, encountered in welding or drilling are interpreted and reviewed from thermal-fluid science viewpoint. These defects usually accompanying with porosity, undercut, segregation, stre ... [J. Heat Transfer 133, 031005 (2010)] published Mon Nov 15, 2010.
- Characteristics of Pool Boiling Bubble Dynamics in Bead Packed Porous Structures
Calvin H. Li, Ting Li, Paul Hodgins, and G. P. Peterson<br/> Spherical glass and copper beads have been used to create bead packed porous structures for an investigation of two-phase heat transfer bubble dynamics under geometric constraints. The results demonstrated a variety of bubble dynamics characteristics under a range of heating conditions. The bubble g ... [J. Heat Transfer 133, 031004 (2010)] published Mon Nov 15, 2010.
- Modeling of Ultrafast Phase Change Processes in a Thin Metal Film Irradiated by Femtosecond Laser Pulse Trains
Jing Huang, Yuwen Zhang, J. K. Chen, and Mo Yang<br/> Ultrashort laser pulses can be generated in the form of a pulse train. In this paper, the ultrafast phase change processes of a 1 [mu]m free-standing gold film irradiated by femtosecond laser pulse trains are simulated numerically. A two-temperature model coupled with interface tracking method is d ... [J. Heat Transfer 133, 031003 (2010)] published Mon Nov 15, 2010.
- Modeling of Ceramic Particle Heating and Melting in a Microwave Plasma
Kaushik Saha, Swetaprovo Chaudhuri, and Baki M. Cetegen<br/> A comprehensive model based on finite volume method was developed to analyze the heat-up and the melting of ceramic particles injected into a microwave excited laminar air plasma flow field. Plasma flow field was simulated as a hot gas flow generated by volumetric heat addition in the microwave coup ... [J. Heat Transfer 133, 031002 (2010)] published Mon Nov 15, 2010.
- Single-Phase Thermal Transport of Nanofluids in a Minichannel
Dong Liu and Leyuan Yu<br/> Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage and electronics cooling to thermal processing of materials. In spite of the extensive studies in the literature, a consensus is lack ... [J. Heat Transfer 133, 031009 (2010)] published Tue Nov 16, 2010.
- Theoretical Analysis of Microwave Heating of Dielectric Materials Filled in a Rectangular Waveguide With Various Resonator Distances
Phadungsak Rattanadecho and Waraporn Klinbun<br/> This paper proposes mathematical models of the microwave heating process of dielectric materials filled in a rectangular waveguide with a resonator. A microwave system supplies a monochromatic wave in a fundamental mode (TE mode). A convection exchange at the upper surface of the sample is considere ... [J. Heat Transfer 133, 031008 (2010)] published Tue Nov 16, 2010.
- Modeling of the Off-Axis High Power Diode Laser Cladding Process
Shaoyi Wen and Yung C. Shin<br/> Off-axis high power diode laser (HPDL) cladding is commonly used for surface quality enhancement such as coating, part repairing, etc. Although some laser cladding models are available in literature, little has been reported on the modeling of powder flow and molten pool for a rectangular beam with ... [J. Heat Transfer 133, 031007 (2010)] published Tue Nov 16, 2010.
- The Experimental and Theoretical Evaluation of an Indirect Cooling System for Machining
Jay C. Rozzi, John K. Sanders, and Weibo Chen<br/> Cutting fluids have been used in machining processes for many years to decrease the temperature during machining by spraying the coolant into the machining zone directly on the cutting tool and the part. This has the effect of decreasing the tool temperature, which increases tool life and improves t ... [J. Heat Transfer 133, 031006 (2010)] published Mon Nov 15, 2010.
- Titania Nanotubes: Novel Nanostructures for Improved Osseointegration
Nathan Swami, Zhanwu Cui, and Lakshmi S. Nair<br/> Nanostructured one dimensional titanium oxides such as nanotubes and nanowires have raised interest lately due to their unique electronic and optical properties. These materials also have shown significant potential as biomaterials because of their ability to modulate protein and cellular interactio ... [J. Heat Transfer 133, 034002 (2010)] published Tue Nov 16, 2010.
- Thermal Processing of Tissue Engineering Scaffolds
Alisa Morss Clyne<br/> Tissue engineering requires complex three-dimensional scaffolds that mimic natural extracellular matrix function. A wide variety of techniques have been developed to create both fibrous and porous scaffolds out of polymers, ceramics, metals, and composite materials. Existing techniques include fiber ... [J. Heat Transfer 133, 034001 (2010)] published Tue Nov 16, 2010.
- Modeling Transport in Porous Media With Phase Change: Applications to Food Processing
Amit Halder, Ashish Dhall, and Ashim K. Datta<br/> Fundamental, physics-based modeling of complex food processes is still in the developmental stages. This lack of development can be attributed to complexities in both the material and transport processes. Society has a critical need for automating food processes (both in industry and at home) while ... [J. Heat Transfer 133, 031010 (2010)] published Tue Nov 16, 2010.
- Review of Heat Conduction in Nanofluids
Jing Fan and Liqiu Wang<br/> Nanofluidsfluid suspensions of nanometer-sized particlesare a very important area of emerging technology and are playing an increasingly important role in the continuing advances of nanotechnology and biotechnology worldwide. They have enormously exciting potential applications and may revolutionize ... [J. Heat Transfer 133, 040801 (2011)] published Mon Jan 10, 2011.
- Thermodynamic Basis of Dual-Phase-Lagging Heat Conduction
Mingtian Xu<br/> The thermal vibration phenomenon occurring in the dual-phase-lagging heat conduction violates the second law of thermodynamics under the local equilibrium assumption. In order to resolve this paradox, two types of the extended irreversible thermodynamics are developed in the present work, which make ... [J. Heat Transfer 133, 041401 (2011)] published Tue Jan 11, 2011.
- Numerical Simulation of Dynamics and Heat Transfer Associated With a Single Bubble in Subcooled Boiling and in the Presence of Noncondensables
Jinfeng Wu and Vijay K. Dhir<br/> During phase change at the bubble-liquid interface, under subcooled boiling conditions, noncondensable gases dissolved in the liquid will be injected into the bubble along with vapor. Due to heat transfer into subcooled liquid, vapor will condense in the upper regions of the bubble while noncondensa ... [J. Heat Transfer 133, 041502 (2011)] published Mon Jan 10, 2011.
- Scaling of Natural Convection of an Inclined Flat Plate: Sudden Cooling Condition
Suvash C. Saha, John C. Patterson, and Chengwang Lei<br/> The natural convection boundary layer adjacent to an inclined plate subject to sudden cooling boundary condition has been studied. It is found that the cold boundary layer adjacent to the plate is potentially unstable to RayleighBenard instability if the Rayleigh number exceeds a certain critical va ... [J. Heat Transfer 133, 041503 (2011)] published Tue Jan 11, 2011.
- Correlation of Subatmospheric Pressure, Saturated, Pool Boiling of Water on a Structured-Porous Surface
Sean J. Penley and R. A. Wirtz<br/> Saturated pool-boiling experiments at 1 atm and subatmospheric pressure assess the utility of fine-filament screen-laminate enhanced surfaces as effective bubble nucleation sites. Experiments were conducted on vertically oriented, multilayer laminates in saturated distilled water at pressures of 0.2 ... [J. Heat Transfer 133, 041501 (2011)] published Thu Jan 6, 2011.
- Intraparticle Mass Transfer in Adsorption Heat Pumps: Limitations of the Linear Driving Force Approximation
Alexander Raymond and Srinivas Garimella<br/> Adsorption heat pumps and chillers (ADHPCs) can utilize solar or waste heat to provide space conditioning, process heating or cooling, or energy storage. In these devices, intraparticle diffusion is shown to present a significant mass transfer resistance compared with interparticle permeation. There ... [J. Heat Transfer 133, 042001 (2011)] published Thu Jan 6, 2011.
- Heat Transfer From Freely Suspended Bimaterial Microcantilevers
Arvind Narayanaswamy and Ning Gu<br/> Bimaterial atomic force microscope cantilevers have been used extensively over the last 15 years as physical, chemical, and biological sensors. As a thermal sensor, the static deflection of bimaterial cantilevers, due to the mismatch of the coefficient of thermal expansion between the two materials, ... [J. Heat Transfer 133, 042401 (2011)] published Thu Jan 6, 2011.
- Resonance of Natural Convection Inside a Bidisperse Porous Medium Enclosure
Arunn Narasimhan and B. V. K. Reddy<br/> An enclosure filled with blocks made of a microscale porous medium, separated by macrosize pores, can be treated as a bidisperse porous medium (BDPM) enclosure. This study investigates natural convection resonance inside such a BDPM enclosure subjected to time-periodic heat flux at a side wall, with ... [J. Heat Transfer 133, 042601 (2011)] published Thu Jan 6, 2011.
- Efficiency Comparison Between Circular and Semicircular Fins Circumscribing Circular Pipes
R. Chakraborty and A. Sirkar<br/> A new design of fin has been conceived, which is in the shape of a semicircular strip. A step-by-step straightforward detailed derivation of the governing equations for two-dimensional temperature distribution in a semicircular fin has been presented, assuming a third degree polynomial. Numerical re ... [J. Heat Transfer 133, 044501 (2011)] published Thu Jan 6, 2011.
- Erratum: Therporaoustic Convection: Modeling and Analysis of Flow, Thermal, and Energy Fields [Journal of Heat Transfer, 2009, 131(10), p. 101011]
Shohel Mahmud and Roydon Andrew Fraser<br/> Abstract not available. [J. Heat Transfer 133, 047001 (2011)] published Thu Jan 6, 2011.
- A New Technique to Determine Convection Coefficients With Flow Through Particle Beds
Xiaodong Nie, Richard Evitts, Robert Besant, and John Bolster<br/> A new method for determining the heat transfer coefficient for air flowing steadily through beds of particles is presented. In this technique, a step change in the inlet air temperature is applied to a small test bed and temperature distributions in the bed and at the air outlet are sampled over a s ... [J. Heat Transfer 133, 041601 (2011)] published Tue Jan 11, 2011.
- An Improved Volume of Fluid Method for Two-Phase Flow Computations on Collocated Grid System
Dong-Liang Sun, Yong-Ping Yang, Jin-Liang Xu, and Wen-Quan Tao<br/> An improved volume of fluid method called the accurate density and viscosity volume of fluid (ADV-VOF) method is proposed to solve two-phase flow problems. The method has the following features: (1) All operations are performed on a collocated grid system. (2) The piecewise linear interface calculat ... [J. Heat Transfer 133, 041901 (2011)] published Tue Jan 11, 2011.
- Heat Transfer Enhancement Using Laminar Gas-Liquid Segmented Plug Flows
Y. S. Muzychka, E. J. Walsh, and P. Walsh<br/> Heat transfer enhancement using segmented nonboiling gas-liquid flow is examined. Segmentation results in a two phase flow of liquid/gas having a constant liquid fraction; i.e., no phase change occurs. In this flow configuration, enhanced heat transfer occurs as a result of a shorter effective therm ... [J. Heat Transfer 133, 041902 (2011)] published Tue Jan 11, 2011.
- Phonon Transport Across Mesoscopic Constrictions
Dhruv Singh, Jayathi Y. Murthy, and Timothy S. Fisher<br/> Phonon transport across constrictions formed by a nanowire or a nanoparticle on a substrate is studied by a numerical solution of the gray Boltzmann transport equation (BTE) resolving the effects of two length scales that govern problems of practical importance. Predictions of total thermal resistan ... [J. Heat Transfer 133, 042402 (2011)] published Tue Jan 11, 2011.
- Global Spectral Methods in Gas Radiation: The Exact Limit of the SLW Model and Its Relationship to the ADF and FSK Methods
V. P. Solovjov and B. W. Webb<br/> The exact form of the spectral line weighted-sum-of-gray-gases (SLW) model is developed as a limiting case of the classical SLW method when the number of gray gases in the classical model approaches infinity. Solutions of the exact SLW radiative transfer equation are obtained in the case of isotherm ... [J. Heat Transfer 133, 042701 (2011)] published Mon Jan 10, 2011.
- Improved Discrete Ordinates Method for Ray Effects Mitigation
Zhi-Feng Huang, Huai-Chun Zhou, and Pei-feng Hsu<br/> A new and improved method based on the concept of discrete ordinates scheme with infinitely small weights (DOS+ISW) is developed for modeling radiative heat transfer in three-dimensional participating media. To demonstrate the effectiveness of the method in mitigating ray effects, the ray effects ca ... [J. Heat Transfer 133, 044502 (2011)] published Mon Jan 10, 2011.
- Heat Transfer Properties and Energy-Exergy Efficiency in a Finned Cross-Flow Heat Recovery Unit
Isak Kotcioglu and Ahmet Cansiz<br/> In this study, a cross-flow heat recovery-exchanger system operating with unmixed fluids was manufactured and tested. The thermodynamic analysis of the system was presented via determining the variations of exergy loss with Reynolds number. The analysis also included the effects of convergent and di ... [J. Heat Transfer 133, 044503 (2011)] published Thu Jan 13, 2011.
- Numerical Inversion of Laplace Transform for Time Resolved Thermal Characterization Experiment
J. Toutain, J.-L. Battaglia, C. Pradere, J. Pailhes, A. Kusiak, and W. Aregba<br/> The aim of this technical brief is to test numerical inverse Laplace transform methods with application in the framework of the thermal characterization experiment. The objective is to find the most reliable technique in the case of a time resolved experiment based on a thermal disturbance in the fo ... [J. Heat Transfer 133, 044504 (2011)] published Thu Jan 13, 2011.
- Enhancement and Prediction of Heat Transfer Rate in Turbulent Flow Through Tube With Perforated Twisted Tape Inserts: A New Correlation
J. U. Ahamed, M. A. Wazed, S. Ahmed, Y. Nukman, T. M. Y. S. Tuan Ya, and M. A. R. Sarkar<br/> An experimental investigation has been carried out for turbulent flow in a tube with perforated twisted tape inserts. The mild steel twisted tape inserts with circular holes of different diameters (i.e., perforation) are used in the flow field. An intensive laboratory study is conducted for heat tra ... [J. Heat Transfer 133, 041903 (2011)] published Fri Jan 14, 2011.
- Re-examining Electron-Fermi Relaxation in Gold Films With a Nonlinear Thermoreflectance Model
Patrick E. Hopkins, Leslie M. Phinney, and Justin R. Serrano<br/> In this work, we examine Fermi relaxation in 20 nm Au films with pump-probe themoreflectance using a thin film, intraband thermoreflectance model. Our results indicate that the Fermi relaxation of a perturbed electron system occurs approximately 1.100.05 ps after absorption of a 785 nm, 185 fs lase ... [J. Heat Transfer 133, 044505 (2011)] published Fri Jan 14, 2011.
- Measurement of Time-Averaged Turbulent Free Convection in a Tall Enclosure Using Interferometry
M. E. Poulad, D. Naylor, and P. H. Oosthuizen<br/> Laser interferometry is combined with high-speed digital cinematography to measure time-averaged transient and turbulent convective heat transfer rates. The method is applied to study free convection in a tall vertical air-filled enclosure. Measurements are made at three wall spacings in the turbule ... [J. Heat Transfer 133, 042501 (2011)] published Wed Jan 19, 2011.
- Effects of Film Evaporation and Condensation on Oscillatory Flow and Heat Transfer in an Oscillating Heat Pipe
Wei Shao and Yuwen Zhang<br/> An advanced theoretical model of a U-shaped minichannel, a building block of a closed-end oscillating heat pipe, has been developed. Thin film evaporation in the evaporator and thin film condensation in the condenser, axial variation of surface temperature, and pressure loss at the bend are incorpor ... [J. Heat Transfer 133, 042901 (2011)] published Wed Jan 19, 2011.
- Forced Convection in a Polygonal Duct With a Circular Core
C. Y. Wang<br/> The H1 forced convection through a regular polygonal duct with a centered core is studied using eigenfunction expansions and point match. Accurate Nusselt numbers for the full range of core radii are presented. ... [J. Heat Transfer 133, 044506 (2011)] published Wed Jan 19, 2011.
- Forced Convection Boundary Layer Flow Past Nonisothermal Thin Needles in Nanofluids
T. Grosan and I. Pop<br/> The classical problem of forced convection boundary layer flow and heat transfer past a needle with variable wall temperature using nanofluids is theoretically studied. The similarity equations are solved numerically for two types of metallic or nonmetallic, such as copper (Cu) and alumina (AlO) nan ... [J. Heat Transfer 133, 054503 (2011)] published Wed Feb 2, 2011.
- Mineral Fouling Control by Underwater Plasma Discharge in a Heat Exchanger
Yong Yang, Hyoungsup Kim, Alexander Fridman, and Young I. Cho<br/> The excessive mineral contents in water circulation systems could cause severe fouling in heat transfer equipment. The present study investigated the effect of underwater pulsed spark discharges on the mitigation of mineral fouling in a concentric counterflow heat exchanger. Artificial hard water wi ... [J. Heat Transfer 133, 054502 (2011)] published Tue Feb 1, 2011.
- On the Generalized Brinkman Number Definition and Its Importance for Bingham Fluids
P. M. Coelho and J. C. Faria<br/> In this technical note we discuss the importance of using a generalized Brinkman number definition for laminar pipe flow of a Bingham fluid, when viscous dissipation effects are relevant. We show that adapting the Brinkman number definition commonly used for Newtonian fluids directly to the more gen ... [J. Heat Transfer 133, 054505 (2011)] published Thu Feb 3, 2011.
- Evaluation of Compact and Effective Air-Cooled Carbon Foam Heat Sink
W. Wu, J. H. Du, Y. R. Lin, L. C. Chow, H. Bostanci, B. A. Saarloos, and D. P. Rini<br/> This study investigates a V-shaped corrugated carbon foam heat sink for thermal management of electronics with forced air convection. Experiments were conducted to determine the heat sink performance in terms of heat transfer coefficient and pressure drop. The test section, with overall dimensions o ... [J. Heat Transfer 133, 054504 (2011)] published Wed Feb 2, 2011.
- On the Cooling of Electronics With Nanofluids
W. Escher, T. Brunschwiler, N. Shalkevich, A. Shalkevich, T. Burgi, and B. Michel<br/> Nanofluids have been proposed to improve the performance of microchannel heat sinks. In this paper, we present a systematic characterization of aqueous silica nanoparticle suspensions with concentrations up to 31 vol %. We determined the particle morphology by transmission electron microscope imagi ... [J. Heat Transfer 133, 051401 (2011)] published Fri Feb 4, 2011.
- Parametric Numerical Study of Flow and Heat Transfer in Microchannels With Wavy Walls
Liang Gong, Krishna Kota, Wenquan Tao, and Yogendra Joshi<br/> Wavy channels were investigated in this paper as a passive scheme to improve the heat transfer performance of laminar fluid flow as applied to microchannel heat sinks. Parametric study of three-dimensional laminar fluid flow and heat transfer characteristics in microsized wavy channels was performed ... [J. Heat Transfer 133, 051702 (2011)] published Fri Feb 4, 2011.
- Diffusion-Thermo Effects on Free Convective Heat and Mass Transfer Flow in a Vertical Channel With Symmetric Boundary Conditions
Basant K. Jha and Abiodun O. Ajibade<br/> This article investigates the influence of diffusion-thermo effect on transient free convective heat and mass transfer flow in a channel bounded by two infinite vertical parallel plates. Fully developed laminar flow is considered when the boundaries are subjected to symmetric concentration and therm ... [J. Heat Transfer 133, 052502 (2011)] published Fri Feb 4, 2011.
- Transient Aspects of Heat Flux Bifurcation in Porous Media: An Exact Solution
Kun Yang and Kambiz Vafai<br/> The transient thermal response of a packed bed is investigated analytically. A local thermal nonequilibrium model is used to represent the energy transport within the porous medium. The heat flux bifurcation phenomenon in porous media is investigated for temporal conditions and two primary types of ... [J. Heat Transfer 133, 052602 (2011)] published Fri Feb 4, 2011.
- Flow and Heat Transfer Over a Continuously Moving Flat Plate in a Porous Medium
W. A. Khan and I. Pop<br/> The effects of the presence of an isotropic solid matrix and radiation on the forced convection boundary layer past a continuously moving flat plate is studied theoretically. The transformed nondimensional partial differential equations are solved numerically for some values of the governing paramet ... [J. Heat Transfer 133, 054501 (2011)] published Tue Feb 1, 2011.
- Pool Boiling Heat Transfer and Bubble Dynamics Over Plain and Enhanced Microchannels
Dwight Cooke and Satish G. Kandlikar<br/> Pool boiling is of interest in high heat flux applications because of its potential for removing large amount of heat resulting from the latent heat of evaporation and little pressure drop penalty for circulating coolant through the system. However, the heat transfer performance of pool boiling syst ... [J. Heat Transfer 133, 052902 (2011)] published Thu Feb 3, 2011.
- Visualization of Two-Phase Flows in Nanofluid Oscillating Heat Pipes
Qi-Ming Li, Jiang Zou, Zhen Yang, Yuan-Yuan Duan, and Bu-Xuan Wang<br/> Two-phase flows in an oscillating heat pipe (OHP) charged with deionized (DI) water and a nanofluid (0.268% v/v) were experimentally investigated. The OHP was made of quartz glass tube (with an inner diameter of 3.53 mm and an outer diameter of 5.38 mm) and coated with a transparent heating film in ... [J. Heat Transfer 133, 052901 (2011)] published Wed Feb 2, 2011.
- Second Law Analysis for Free Convection in Non-Newtonian Fluids Over a Horizontal Plate Embedded in a Porous Medium: Prescribed Surface Temperature
W. A. Khan and Rama Subba Reddy Gorla<br/> Second law characteristics of heat transfer and fluid flow due to free convection of non-Newtonian fluids over a horizontal plate with prescribed surface temperature in a porous medium are analyzed. Velocity and temperature fields are obtained numerically using an implicit finite difference method u ... [J. Heat Transfer 133, 052601 (2011)] published Wed Feb 2, 2011.
- Natural Convection in a Quadrantal Cavity Heated and Cooled on Adjacent Walls
Orhan Aydin and Gurkan Yesiloz<br/> In this study, experimental and numerical analyses of natural convection in a quadrantal cavity heated and cooled on adjacent walls have been made to examine heat and fluid flow. Experimental studies involve the use of the particle tracing method that enables us to visualize the flow pattern in the ... [J. Heat Transfer 133, 052501 (2011)] published Tue Feb 1, 2011.
- Prediction of Thermal Conductivity and Convective Heat Transfer Coefficient of Nanofluids by Local Composition Theory
M. S. Hosseini, A. Mohebbi, and S. Ghader<br/> In this study, a new method based on the local composition theory has been developed to predict thermal conductivity, convective heat transfer coefficient, and viscosity of nanofluids. The nonrandom two liquid (NRTL) model is used for this purpose. The effects of temperature and particle volume conc ... [J. Heat Transfer 133, 052401 (2011)] published Tue Feb 1, 2011.
No Issue Number
- Instantaneous Heat Flux Simulation of a Motored Reciprocating Engine: Unsteady Thermal Boundary Layer With Variable Turbulent Thermal Conductivity
<span class="paragraphSection">Due to the inherently unsteady environment of reciprocating engines, unsteady thermal boundary layer modeling may improve the reliability of simulations of internal combustion engine heat transfer. Simulation of the unsteady thermal boundary layer was achieved in the present work based on an effective variable thermal conductivity from different turbulent Prandtl number and turbulent viscosity models. Experiments were also performed on a motored, single-cylinder spark-ignition engine. The unsteady energy equation approach furnishes a significant improvement in the simulation of the heat flux data relative to results from a representative instantaneous heat transfer correlation. The heat flux simulated using the unsteady model with one particular turbulent Prandtl number model agreed with measured heat flux in the wide open and fully closed throttle cases, with an error in peak values of about 6% and 35%, respectively.</span>
- Measurement of Interface Thermal Resistance With Neutron Diffraction
<span class="paragraphSection">A noncontact, nondestructive neutron diffraction technique for measuring thermal resistance of buried material interfaces in bulk samples, inaccessible to thermocouple measurements, is described. The technique uses spatially resolved neutron diffraction measurements to measure temperature, and analytical or numerical methods to calculate the corresponding thermal resistance. It was tested at the VULCAN instrument of the Spallation Neutron Source, Oak Ridge National Laboratories on a stack of three 6061 alloy aluminum plates (heat-source, middle-plate, and heat-sink), held in dry thermal contact, at low pressure, in ambient air. The results agreed with thermocouple-based measurements. This technique is applicable to all crystalline materials and most interface configurations, and it can be used for the characterization of thermal resistance across interfaces in actual engineering parts under nonambient conditions and/or in moving/rotating systems.</span>
- Effect of Gravity Modulation on Electrothermal Convection in a Dielectric Fluid Saturated Anisotropic Porous Layer
<span class="paragraphSection">This paper deals with linear and nonlinear stability analyses of thermal convection in a dielectric fluid saturated anisotropic Brinkman porous layer subject to the combined effect of AC electric field and time-periodic gravity modulation (GM). In the realm of linear theory, the critical stability parameters are computed by regular perturbation method. The local nonlinear theory based on truncated Fourier series method gives the information of convection amplitudes and heat transfer. Principle of exchange of stabilities is found to be valid and subcritical instability is ruled out. Based on the governing linear autonomous system several qualitative results on stability are discussed. The sensitive dependence of the solution of Lorenz system of electrothermal convection to the choice of initial conditions points to the possibility of chaos. Low frequency g-jitter is found to have significant stabilizing influence which is in turn diminished by an imposed AC electric field. The role of other governing parameters on the stability threshold and on transient heat transfer is determined.</span>
- Predicting Thermal System Performance and Estimating Parameters for Systems Burdened With Uncertainties and Noise Using Hierarchical Bayesian Inference
<span class="paragraphSection">The precision of estimates of system performance and of parameters that affect the performance is often based upon the standard deviation obtained from the usual equation for the propagation of variances derived from a Taylor series expansion. With ever increasing computing power it is now possible to utilize the Bayesian hierarchical approach to yield improved estimates of the precision. Although quite popular in the statistical community, the Bayesian approach has not been widely used in the heat transfer and fluid mechanics communities because of its complexity and subjectivity. The paper develops the necessary equations and applies them to two typical heat transfer problems, measurement of conductivity with heat losses and heat transfer from a fin. Because of the heat loss the probability distribution of the conductivity is far from Gaussian. Using this conductivity distribution for the fin gives a very long tailed distribution for the heat transfer from the fin.</span>
- Algebraic Anisotropic Turbulence Modeling of Compound Angled Film Cooling Validated by Particle Image Velocimetry and Pressure Sensitive Paint Measurements
The complex structures in the film cooling flow field of gas turbines lead to the anisotropic property of the turbulent eddy viscosity and scalar diffusivity. An algebraic anisotropic turbulence model is developed aiming at a more accurate modeling of the Reynolds stress and turbulent scalar flux. In this study, the algebraic anisotropic model is validated by a series of in-house experiments for cylindrical film cooling with compound angle injection of 0, 45, and 90 deg. Adiabatic film cooling effectiveness and flow field are measured using pressure sensitive paint and particle image velocimetry techniques on film cooling test rig in Tsinghua University. Detailed analyses of computational simulations are performed. The algebraic anisotropic model gives a good prediction of the secondary vortices associated with the jet and the trajectory of the jet, therefore improves the prediction of the scalar field. On one hand, the anisotropic eddy viscosity improves the modeling of Reynolds stress and the predictive flow field. On the other hand, the anisotropic turbulent scalar-flux model includes the role of anisotropic eddy viscosity in modeling of scalar flux and directly improves the turbulent scalar flux prediction.
- Ultra Fast Cooling and Its Effect on the Mechanical Properties of Steel
The objective of this work is to study about the ultrafast cooling of a hot static 6 mm thick steel plate (AISI-1020) by air assisted spray cooling. The study covers the effect of air flow rate and the water impingement density on the cooling rate. The initial temperature of the plate, before the cooling starts, is kept at 900 °C. The spray was produced from a full cone high mass flux and low turn down ratio air atomizer at a fixed nozzle to plate distance. The cooling rate shows that low turn down ratio air atomized spray can generate ultra fast cooling (UFC) rate for a 6 mm thick steel plate. After cooling, the tensile strength and hardness of the cooled steel plate were examined. The surface heat flux and surface temperature calculations have been performed by using INTEMP software. The result of this study could be applied in designing of fast cooling system especially for the run-out table cooling.
- An Experimental Study of Passive and Active Heat Transfer Enhancement in Microchannels
An experimental study on single-phase heat transfer and fluid flow downstream a single microscale pillar in a microchannel was conducted. A secondary jet flow was issued from slits formed along the pillar. A comparison of the thermal performances of a plain microchannel, a microchannel with a pillar, and a microchannel with a jet issued from a pillar was performed to elucidate the merits of this heat transfer enhancement technique. It was found that the presence of a pillar upstream the heater enhanced the heat transfer; the addition of jet flow issued from a pillar further enhanced the heat transfer. At a Reynolds number of 730, an improvement of spatially averaged Nusselt number of 80% was achieved due to the combined effect of the pillar and the jet compared with the corresponding plain channel. Micro particle image velocimetry ( μ PIV) measurements provided planar velocity fields at two planes along the channel height, and allowed flow structure visualization. Turbulent kinetic energy (TKE) was used to measure flow mixing and to quantify the hydrodynamic effect of the jet. It was shown that the TKE is closely related to the Nusselt number.
- Thermal Conductivity Enhancement of Ethylene Glycol-Based Suspensions in the Presence of Silver Nanoparticles of Various Shapes
In this technical brief, the effect of adding silver (Ag) nanoparticles of various shapes on the thermal conductivity enhancement of ethylene glycol (EG)-based suspensions was investigated experimentally. These included Ag nanospheres (Ag NSs), Ag nanowires (Ag NWs), and Ag nanoflakes (Ag NFs). Measurements of the thermal conductivity of the suspensions were performed from 10 to 30 °C at an increment of 5 °C. It was shown that the thermal conductivity of the EG-based suspensions increases with raising the temperature. The Ag NWs of a high aspect ratio (∼500) caused greatest relative enhancement up to 15.6% at the highest loading of nearly 0.1 vol. %, whereas the other two shapes of nanoparticles, Ag NSs and Ag NFs with much smaller aspect ratios, only led to enhancements up to 5%. The formation of a network of Ag NWs that facilitates heat conduction was likely responsible for their better performance. The relative enhancement was also predicted by the Hamilton-Crosser model that takes the particle shape effect into consideration. It was shown that the predictions far underestimate the thermal conductivity enhancements but are qualitatively consistent with their shape dependence. As a penalty, however, the presence of Ag NWs was shown to give rise to significant increase in the viscosity of the EG-based suspensions.
- Determination of the Thermal Conductivity in Adobe With Several Models
The thermal conductivity of the earth materials conditions their ability as thermal isolator and its heating capacity, which has a direct impact on the energy consumption of the buildings built with these materials. Two original mathematical models have been developed (models MA-1 and MA-2) to calculate the effective thermal conductivity ( λE ) of adobes and their results have been compared with other models already known for other materials and with experimental measures done on adobes. The model MA-1 starts from the electric analogy of the transmission of heat in series and in parallel. The model MA-2 is obtained with a regression curve from experimental and literature values of λE in adobes. The λE in adobes has been measured by the thermal needle probe (TNP) procedure using 10 min as the measuring time. For dry adobes, with average environmental conditions of 19 °C and 41% of relative moisture, the values of λE measured were 0.80 W/(m·K) ± 10%. For natural hygroscopic moisture of 1.67% in the same environmental conditions, a λE of 0.90 W/(m·K) ± 10% was measured. Only five of the 18 models analyzed adjust to the values experimentally measured, and their precision depends on the values of λ of the components, which are obtained from the literature. Of the proposed models, the MA-1 fits for the values of the dry and wet material and with some determined values of the literature. The model MA-2 fits in all cases since it does not depend on the values of the literature but on the density of the material and its moisture content.
- Effect of Uncertainty in Blowing Ratio on Film Cooling Effectiveness
In this study, the effect of randomness of blowing ratio on film cooling performance is investigated by combining direct numerical simulations with a stochastic collocation approach. The geometry includes a 35-deg inclined jet with a plenum attached to it. The blowing ratio variations are assumed to have a truncated Gaussian distribution with mean of 0.3 and the standard variation of approximately 0.1. The parametric space is discretized using multi-element general polynomial chaos (ME-gPC) with five elements where general polynomial chaos of order 3 is used in each element. Direct numerical simulations were carried out using spectral element method to sample the governing equations in space and time. The probability density function of the film cooling effectiveness was obtained and the standard deviation of the adiabatic film cooling effectiveness on the blade surface was calculated. A maximum of 20% of variation in film cooling effectiveness was observed at 2.2 jet-diameter distance downstream of the exit hole. The spatially-averaged adiabatic film cooling effectiveness was 0.23 ± 0.02. The calculation of all the statistical properties were carried out as off-line post processing. A fast convergence of the polynomial expansion in the random space is observed which shows that the computational strategy is very cost-effective.
- A Reexamination of Phonon Transport Through a Nanoscale Point Contact in Vacuum
Using a silicon nitride cantilever with an integral silicon tip and a microfabricated platinum–carbon resistance thermometer located close to the tip, a method is developed to concurrently measure both the heat transfer through and adhesion energy of a nanoscale point contact formed between the sharp silicon tip and a silicon substrate in an ultrahigh vacuum atomic force microscope at near room temperature. Several models are used to evaluate the contact area critical for interpreting the interfacial resistance. Near field-thermal radiation conductance was found to be negligible compared to the measured interface thermal conductance determined based on the possible contact area range. If the largest possible contact area is assumed, the obtained thermal interface contact resistance can be explained by a nanoconstriction model that allows the transmission of phonons from the whole Brillouin zone of bulk Si with an average finite transmissivity larger than 0.125. In addition, an examination of the quantum thermal conductance expression suggests the inaccuracy of such a model for explaining measurement results obtained at above room temperature.
- Temperature Rise in Electroosmotic Flow of Typical Non-Newtonian Biofluids Through Rectangular Microchannels
<span class="paragraphSection">Electroosmosis is the main mechanism for flow generation in lab-on-a-chip (LOC) devices. The temperature rise due to the Joule heating phenomenon, associated with the electroosmosis, may be detrimental for samples being considered in LOCs. Hence, a complete understanding of the heat transfer physics associated with the electroosmotic flow is of high importance in design and active control of LOCs. The objective of the present study is to estimate the temperature rise and the thermal entry length in electroosmotic flow through rectangular microchannels, having potential applications in LOC devices. Along this line, the power-law rheological model is used to account for non-Newtonian behavior of the common biofluids encountered in these devices. A mixed type of thermal boundary condition is employed at the channel surface, instead of routinely presumed constant wall heat flux or constant wall temperature conditions. A finite difference-based numerical method is employed for solving the governing equations in dimensionless form. An approximate solution, based on the premise of a uniform temperature field throughout the channel cross section, is also obtained for the bulk mean temperature, which is found to be of high accuracy. This, accompanied by the assessments of the temperature profile, reveals that the temperature variations in the channel cross section are negligible, and as a result, the bulk mean temperature can be used as a very precise estimate of the maximum temperature in an LOC device. Moreover, the evaluation of the entry length shows that a thermally fully developed flow is hardly achieved in practical applications because of small length scales involved. Accordingly, the maximum temperature rise may significantly be smaller than what is calculated based on a thermally fully developed flow assumption.</span>
- An Experimental Investigation of Pressure Drop in Expanding Microchannel Arrays
The pressure effects of expanding the cross section of microchannels along the direction of flow are investigated across four rates of channel expansion in the flow boiling of R-134a. Prior investigation by the authors detailed the fabrication of four copper microchannel arrays and the pumped-loop apparatus developed to facilitate interchange of the microchannel specimens, allowing consistency across experiments. Significant beneficial pressure effects are observed to result from the expansion, including reduction by half of the pumping cost per flow rate at critical heat flux. The improvements are seen with small expansions, and greater expansion yields diminishing returns. The high pressure drops associated with microchannel evaporators are effectively reduced by expanding channel geometry, and the low-frequency system spectral response indicates that expanding channel arrays typically carry less energy in oscillations up to 2.5 Hz, suggesting amelioration of oscillatory instabilities. Results are discussed in light of a comparative force analysis, with the balance of these forces linked to the observed behavior of the pressure drop and heat flux relationship.
- Physics of the Interaction of Ultrasonic Excitation With Nucleate Boiling
Physics of ultrasound-assisted augmentation of saturated nucleate boiling through the interaction of multiphase fluid flow is revealed in the present work. Different regimes of influence of ultrasound, ranging from augmentation to deterioration and even no effect, as reported in literature in a contradictory fashion, have been observed. However unlike the previous studies, here it has been clearly demonstrated that this apparent anomaly lies in the different natures of interactions between the influencing parameters like heat flux, ultrasonic frequency, and pressure amplitude. The present results clearly bring out an interactive effect of these operating parameters with surface parameter like surface roughness. A mechanistic model unifying all these parameters has been presented to explain quantitatively the physics of the interaction. The model-based predictions match experimental results quite well suggesting the validity of the hypothesis on liquid–vapor-surface interaction through the process of nucleation and its site density, on which the model is built, and thus revealing the underlying physics.
- Convective Instability of the Darcy Flow in a Horizontal Layer With Symmetric Wall Heat Fluxes and Local Thermal Nonequilibrium
The linear stability of the parallel Darcy throughflow in a horizontal plane porous layer with impermeable boundaries subject to a symmetric net heating or cooling is investigated. The onset conditions for the secondary thermoconvective flow are expressed through a neutral stability bound for the Darcy–Rayleigh number associated with the uniform heat flux supplied or removed from the walls. The study is performed by taking into account a condition of local thermal nonequilibrium between the solid phase and the fluid phase. The linear stability analysis is carried out according to the normal modes' decomposition of the perturbations to the basic state. The governing equations for the disturbances are solved numerically as an eigenvalue problem leading to the neutral stability condition. If compared with the asymptotic condition of local thermal equilibrium, the regime of local nonequilibrium manifests an enhanced instability. This behavior is displayed by lower critical values of the Darcy–Rayleigh number, eventually tending to zero when the thermal conductivity of the solid phase is much larger than the conductivity of the fluid phase. In this special limit, which can be invoked as an approximate model of a gas-saturated metallic foam, the basic throughflow is always unstable to external disturbances of arbitrarily small amplitude.
- Free Convection in Antisymmetrically Heated Vertical Channels
<span class="paragraphSection">Free convection in a vertical channel with antisymmetrical heating is a special case that has not received a great deal of attention in the literature. Antisymmetrical heating is where the hot wall is heated above the ambient temperature by the same amount that the cold wall is cooled below the ambient, giving equal but opposing buoyancy forces inside the channel. An experimental model was constructed to study antisymmetrical heating inside an isothermally heated vertical channel. Flow visualization was used to obtain the flow field and laser interferometry was used to obtain the temperature field. Based on the measured temperature field, the local and average Nusselt numbers were determined, which were compared with numerical predictions obtained using <span style="text-transform:lowercase;font-variant:small-caps;">ansys fluent</span>. A range of Rayleigh numbers were studied for air with a Prandtl number of 0.71. The results show that an open-ended channel with antisymmetrical heating has some similarities to a tall enclosure. The average convective heat transfer can be approximated using an existing correlation for a tall enclosure from the literature.</span>
- Completely Spectral Collocation Solution of Radiative Heat Transfer in an Anisotropic Scattering Slab With a Graded Index Medium
<span class="paragraphSection">A completely spectral collocation method (CSCM) is developed to solve radiative transfer equation in anisotropic scattering medium with graded index. Different from the Chebyshev collocation spectral method based on the discrete ordinates method (SP-DOM), the CSCM is used to discretize both the angular domain and the spatial domain of radiative transfer equation. In this approach, the angular derivative term and the integral term are approximated by the high order spectral collocation scheme instead of the low order finite difference approximations. Compared with those available data in literature, the CSCM has a good accuracy for a wide range of the extinction coefficient, the scattering albedo, the scattering phase function, the gradient of refractive index and the boundary emissivity. The CSCM can provide exponential convergence for the present problem. Meanwhile, the CSCM is much more economical than the SP-DOM. Moreover, for nonlinear anisotropic scattering and graded index medium with space-dependent albedo, the CSCM can provide smoother results and mitigate the ray effect.</span>
- Natural Convection Heat Transfer Performance of Non-Newtonian Power-Law Fluids Enclosed in Cavity With Complex-Wavy Surfaces
<span class="paragraphSection">Numerical simulations are performed to investigate the natural convection heat transfer performance of non-Newtonian power-law fluids in a cavity bounded by wavy vertical walls with different temperatures and flat horizontal walls under adiabatic conditions. The results show that for Rayleigh numbers greater than 10<sup>3</sup>, the mean Nusselt number has a significantly increase as the flow behavior index is decreased. Moreover, it is shown that in the convection-dominated regime, the mean Nusselt number increases with an increasing Rayleigh number, while in the conduction-dominated regime, the mean Nusselt number remains approximately constant. Finally, it is shown that for a given fluid, the heat transfer performance can be optimized via an appropriate tuning of the wavelength and amplitude of the wavy surface depending on the Rayleigh number.</span>
- Heat Transfer Evaluation on Curved Boundaries in Thermal Lattice Boltzmann Equation Method
An efficient and accurate approach for heat transfer evaluation on curved boundaries is proposed in the thermal lattice Boltzmann equation (TLBE) method. The boundary heat fluxes in the discrete velocity directions of the TLBE model are obtained using the given thermal boundary condition and the temperature distribution functions at the lattice nodes close to the boundary. Integration of the discrete boundary heat fluxes with effective surface areas gives the heat flow rate across the boundary. For lattice models with square or cubic structures and uniform lattice spacing the effective surface area is constant for each discrete heat flux, thus the heat flux integration becomes a summation of all the discrete heat fluxes with constant effective surface area. The proposed heat transfer evaluation scheme does not require a determination of the normal heat flux component or a surface area approximation on the boundary; thus, it is very efficient in curved-boundary simulations. Several numerical tests are conducted to validate the applicability and accuracy of the proposed heat transfer evaluation scheme, including: (i) two-dimensional (2D) steady-state thermal flow in a channel, (ii) one-dimensional (1D) transient heat conduction in an inclined semi-infinite solid, (iii) 2D transient heat conduction inside a circle, (iv) three-dimensional (3D) steady-state thermal flow in a circular pipe, and (v) 2D steady-state natural convection in a square enclosure with a circular cylinder at the center. Comparison between numerical results and analytical solutions in tests (i)–(iv) shows that the heat transfer is second-order accurate for straight boundaries perpendicular to one of the discrete lattice velocity vectors, and first-order accurate for curved boundaries due to the irregularly distributed lattice fractions intersected by the curved boundary. For test (v), the computed surface-averaged Nusselt numbers agree well with published results.
- Numerical Prediction of Turbulent Flow and Heat Transfer Enhancement in a Square Passage With Various Truncated Ribs on One Wall
<span class="paragraphSection">Repeated ribs are often employed in the midsection of internal cooling passages of turbine blades to augment the heat transfer by air flowing through the internal ribbed passages. Though the research of flow structure and augmented heat transfer inside various ribbed passages has been well conducted, previous works mostly paid much attention to the influence of rib topology (height-to-pitch, blockage ratio, skew angle, rib shape). The possible problem involved in the usage of ribs (especially with larger blockage ratios) is pressure loss penalty. Thus, in this case, the design of truncated ribs whose length is less than the passage width might fit the specific cooling requirements when pressure loss is critically considered. A numerical study of truncated ribs on turbulent flow and heat transfer inside a passage of a gas turbine blade is performed when the inlet Reynolds number ranges from 8000 to 24,000. Different truncation ratio (truncated-length to passage-width) rib geometries are designed and then the effect of truncation ratio on the pressure drop and heat transfer enhancement is observed under the condition of constant total length. The overall performance characteristics of various truncated rib passages are also compared. It is found that the heated face with a rib that is truncated 12% in length in the center (case A) has the highest heat transfer coefficient, while the heated face with a rib that is truncated 4% at three locations over its length, in the center and two sides (case D), has a reduced pressure loss compared with passages of other designs and provides the lowest friction factors. Although case A shows larger heat transfer augmentation, case D can be promisingly used to augment side-wall heat transfer when the pressure loss is considered and the Reynolds number is relatively large.</span>
- Effect of Rotation on Detailed Heat Transfer Distribution for Various Rib Geometries in Developing Channel Flow
The effects of Coriolis force and centrifugal buoyancy have a significant impact on heat transfer behavior inside rotating internal serpentine coolant channels for turbine blades. Due to the complexity of added rotation inside such channels, detailed knowledge of the heat transfer will greatly enhance the blade designer's ability to predict hot spots so coolant may be distributed more effectively. The effects of high rotation numbers are investigated on the heat transfer distributions for different rib types in near entrance and entrance region of the channels. It is important to determine the actual enhancement derived from turbulating channel entrances where heat transfer is already high due to entrance effects and boundary layer growth. A transient liquid crystal technique is used to measure detailed heat transfer coefficients (htc) for a rotating, short length, radially outward coolant channel with rib turbulators. Different rib types such as 90 deg, W, and M-shaped ribs are used to roughen the walls to enhance heat transfer. The channel Reynolds number is held constant at 12,000 while the rotation number is increased up to 0.5. Results show that in the near entrance region, the high performance W and M-shaped ribs are just as effective as the simple 90 deg ribs in enhancing heat transfer. The entrance effect in the developing region causes significantly high baseline heat transfer coefficients thus reducing the effective of the ribs to further enhance heat transfer. Rotation causes increase in heat transfer on the trailing side, while the leading side remains relatively constant limiting the decrement in leading side heat transfer. For all rotational cases, the W and M-shaped ribs show significant effect of rotation with large differences between leading and trailing side heat transfer.
- Modeling and Experimental Characterization of Metal Microtextured Thermal Interface Materials
<span class="paragraphSection">A metal microtextured thermal interface material (MMT-TIM) has been proposed to address some of the shortcomings of conventional TIMs. These materials consist of arrays of small-scale metal features that plastically deform when compressed between mating surfaces, conforming to the surface asperities of the contacting bodies and resulting in a low-thermal resistance assembly. The present work details the development of an accurate thermal model to predict the thermal resistance and effective thermal conductivity of the assembly (including contact and bulk thermal properties) as the MMT-TIMs undergo large plastic deformations. The main challenge of characterizing the thermal contact resistance of these structures was addressed by employing a numerical model to characterize the bulk thermal resistance and estimate the contribution of thermal contact resistance. Furthermore, a correlation that relates electrical and thermal contact resistance for these MMT-TIMs was developed that adequately predicted MMT-TIM properties for several different geometries. A comparison to a commercially available graphite TIM is made as well as suggestions for optimizing future MMT-TIM designs.</span>
- Isoflux Nusselt Number and Slip Length Formulae for Superhydrophobic Microchannels
We analytically and numerically consider the hydrodynamic and thermal transport behavior of fully developed laminar flow through a superhydrophobic (SH) parallel-plate channel. Hydrodynamic slip length, thermal slip length and heat flux are prescribed at each surface. We first develop a general expression for the Nusselt number valid for asymmetric velocity profiles. Next, we demonstrate that, in the limit of Stokes flow near the surface and an adiabatic and shear-free liquid–gas interface, both thermal and hydrodynamic slip lengths can be found by redefining existing solutions for conduction spreading resistances. Expressions for the thermal slip length for pillar and ridge surface topographies are determined. Comparison of fundamental half-space solutions for the Laplace and Stokes equations facilitate the development of expressions for hydrodynamic slip length over pillar-structured surfaces based on existing solutions for the conduction spreading resistance from an isothermal source. Numerical validation is performed and an analysis of the idealized thermal transport behavior suggests conditions under which superhydrophobic microchannels may enhance heat transfer.
- Apparent Temperature Jump and Thermal Transport in Channels With Streamwise Rib and Cavity Featured Superhydrophobic Walls at Constant Heat Flux
This paper presents an analytical investigation of constant property, steady, fully developed, laminar thermal transport in a parallel-plate channel comprised of metal superhydrophobic (SH) walls. The superhydrophobic walls considered here exhibit microribs and cavities aligned in the streamwise direction. The cavities are assumed to be nonwetting and contain air, such that the Cassie–Baxter state is the interfacial state considered. The scenario considered is that of constant heat flux through the rib surfaces with negligible thermal transport through the air cavity interface. Closed form solutions for the local Nusselt number and local wall temperature are presented and are in the form of infinite series expansions. The analysis show the relative size of the cavity regions compared to the total rib and cavity width (cavity fraction) exercises significant influence on the aggregate thermal transport behavior. Further, the relative size of the rib and cavity module width compared to the channel hydraulic diameter (relative module width) also influences the Nusselt number. The spatially varying Nusselt number and wall temperature are presented as a function of the cavity fraction and the relative module width over the ranges 0–0.99 and 0.01–1.0, respectively. From these results, the rib/cavity module averaged Nusselt number was determined as a function of the governing parameters. The results reveal that increases in either the cavity fraction or relative module width lead to decreases in the average Nusselt number and results are presented over a wide range of conditions from which the average Nusselt number can be determined for heat transfer analysis. Further, analogous to the hydrodynamic slip length, a temperature jump length describing the apparent temperature jump at the wall is determined in terms of the cavity fraction. Remarkably, it is nearly identical to the hydrodynamic slip length for the scenario considered here and allows straightforward determination of the average Nusselt number for any cavity fraction and relative rib/cavity module width.
- Combined Kinetic Monte Carlo—Molecular Dynamics Approach for Modeling Phonon Transport in Quantum Dot Superlattices
A new kinetic Monte Carlo method for modeling phonon transport in quantum dot superlattices is presented. The method uses phonon scattering phase functions and cross sections to describe collisions between phonons and quantum dots. The phase functions and cross sections are generated using molecular dynamics simulation, which is capable of including atomistic effects otherwise unavailable in Monte Carlo approaches. The method is demonstrated for a test case featuring a Si-Ge quantum dot superlattice, and the model is compared against published experiments. It is found that molecular dynamics-derived cross sections must be weighted by diffuse mismatch model-type weighting factors in order to satisfy detailed balance considerations. Additionally, it is found that thin alloy “base layer” films strongly reduce thermal conductivity in these systems and must be included in the modeling to obtain agreement with published experimental data.
- Discussion on: Assis, E., Ziskind, G., and Letan, R., 2009, “Numerical and Experimental Study of Solidification in a Spherical Shell,” ASME J. Heat Transfer, 131, p. 024502
Assis et al. [ 1 ] gave a numerical model for solidification in spheres considering curvilinear geometry, shrinkage of phase change material (pcm) and air gap variation for the material RT27. Their numerical results of melt fraction (MF) and the product of Fourier and Stefan numbers (Fig. 5( b ) ) were correlated by Eq. (3)(3) [ 1 ]. This correlation is valid for the pcm RT27 which has a melting range (28–30 °C) and for Stefan number (Ste) ≤ 0.4.
- The Onset of Convection in an Internally Heated Nanofluid Layer
<span class="paragraphSection">We analytically studied the onset of convection, induced by internal heating, such as that produced by microwave heating or chemical reaction, in a horizontal layer of a nanofluid subject to Brownian motion and thermophoresis. This is a fundamentally different situation from traditionally studied heating from below. Convection, when it occurs, is now concentrated in the portion of the layer where the upward vertical gradient is negative, which is the upper portion of the layer. The situation of internal heating also allows employing more realistic boundary conditions than those hitherto used.</span>
- Heat in Computers: Applied Heat Transfer in Information Technology
<span class="paragraphSection">Since the advent of modern electronics technology, heat transfer science and engineering has served in the development of computer technology. The computer as an object of heat transfer research has a unique aspect; it undergoes morphological transitions and diversifications in step with the progress of microelectronics technology. Evolution of computer's hardware manifests itself in increasing packing density of electronic circuits, modularization of circuit assemblies, and increasing hierarchical levels of system internal structures. These features are produced by the confluence of various factors; the primary factors are the pursuit of ever higher processing performance, less spatial occupancy, and higher energy utilization efficiency. The cost constraint on manufacturing also plays a crucial role in the evolution of computer's hardware. Besides, the drive to make computers ubiquitous parts of our society generates diverse computational devices. Concomitant developments in heat generation density and heat transfer paths pose fresh challenges to thermal management. In an introductory part of the paper, I recollect our experiences in the mainframe computers of the 1980s, where the system's morphological transition allowed the adoption of water cooling. Then, generic interpretations of the hardware evolution are attempted, which include recapturing the past experience. Projection of the evolutionary trend points to shrinking space for coolant flow, the process commonly in progress in all classes of computers today. The demand for compact packaging will rise to an extreme level in supercomputers, and present the need to refocus our research on microchannel cooling. Increasing complexity of coolant flow paths in small equipment poses a challenge to a user of computational fluid dynamics (CFD) simulation code. In highly integrated circuits the paths of electric current and heat become coupled, and coupled paths make the electrical/thermal codesign an extremely challenging task. These issues are illustrated using the examples of a consumer product, a printed circuit board (PCB), and a many-core processor chip.</span>
- Mixed Convection Stagnation-Point Flow Past a Vertical Flat Plate With a Second Order Slip
An exact similarity solution of the steady mixed convection flow of a viscous and incompressible fluid in the vicinity of two-dimensional stagnation-point with a second-order slip condition has been investigated. Using appropriate similarity variable, the Navier–Stokes equations coupled with the energy equation governing the flow and heat transfer are reduced to a system of nonlinear ordinary (similarity) equations, which are well-posed. These equations are solved numerically in the buoyancy assisting and opposing flow regions. It is found that a reverse flow region develops in the buoyancy opposing flow case, and dual (upper and lower branch) solutions are found to exist in the case of opposing flow region for a certain range of the negative values of the mixed convection parameter. A stability analysis has been performed, which shows that the upper branch solutions are stable and physically realizable in practice, while the lower branch solutions are not stable and, therefore, not physically realizable in practice. The numerical results have been compared with those reported in the literature, the agreement being excellent.
- Nonlocal Modeling and Swarm-Based Design of Heat Sinks
<span class="paragraphSection">Cooling electronic chips to satisfy the ever-increasing heat transfer demands of the electronics industry is a perpetual challenge. One approach to addressing this is through improving the heat rejection ability of air-cooled heat sinks, and nonlocal thermal-fluid-solid modeling based on volume averaging theory (VAT) has allowed for significant strides in this effort. A number of optimization methods for heat sink designers who model heat sinks with VAT can be envisioned due to VAT's singular ability to rapidly provide solutions, when compared to computational fluid dynamics (CFD) approaches. The particle swarm optimization (PSO) method appears to be an attractive multiparameter heat transfer device optimization tool; however, it has received very little attention in this field compared to its older population-based optimizer cousin, the genetic algorithm (GA). The PSO method is employed here to optimize smooth and scale-roughened straight-fin heat sinks modeled with VAT by minimizing heat sink thermal resistance for a specified pumping power. A new numerical design tool incorporates the PSO method with a VAT-based heat sink solver. Optimal designs are obtained with this new tool for both types of heat sinks, the performances of the heat sink types are compared, the performance of the PSO method is discussed with reference to the GA method, and it is observed that this new method yields optimal designs much quicker than traditional approaches. This study demonstrates, for the first time, the effectiveness of combining a VAT-based nonlocal thermal-fluid-solid model with population-based optimization methods, such as PSO, to design heat sinks for electronics cooling applications. The VAT-based nonlocal modeling method provides heat sink design capabilities, in terms of solution speed and model rigor, that existing modeling methods do not match.</span>
- Thermal Insulators' Performances in Simulated Mars Environment
This paper describes the experimental characterization of the thermal insulation properties of a multilayer insulator (MLI) and of an aerogel. Materials characterization was performed to optimize the thermal control design of a small interferometer devoted to planetary observation. In order to simulate the Martian environment, tests were performed in a carbon dioxide atmosphere, with pressures between 10 and 10 4 Pa and temperatures from 193 to 353 K. MLI was tested at different levels of layers compression to investigate thermal insulation changes deriving from the constraining of the mechanical structure. The thermal conductivity was measured with a purposely designed guarded hot plate apparatus. Results showed that the aerogel exhibits a lower thermal conductivity for gas pressures larger than 100 Pa and that the layer compression of the MLIs does not affect the heat conduction for gas pressures above 10 3 Pa.
- The Influence of Carbon Nanotube Aspect Ratio on Thermal Conductivity Enhancement in Nanotube–Polymer Composites
We report and model a linear increase in the thermal conductivity ( κ ) of polymer composites incorporated with relatively low length/diameter aspect ratio multiwalled carbon nanotubes (CNTs). There was no evidence of percolation-like behavior in the κ , at/close to the theoretically predicted threshold, which was attributed due to the interfacial resistance between the CNT and the polymer matrix. Concomitantly, the widely postulated high thermal conductivity of CNTs does not contribute to the net thermal conductivity of the composites. Through estimating the interfacial resistance and the thermal conductivity of the constituent CNTs, we conclude that our experimental and modeling approaches can be used to study thermal transport behavior in nanotube–polymer composites. Keywords: carbon nanotubes, filled polymers, nanocomposites, percolation, thermal conductivity, effective medium theory, thermal interface resistance, and kaptiza resistance.
- Effect of Temperature Dependent Fluid Properties on Heat Transfer in Turbulent Mixed Convection
The effect of the uniform fluid properties approximation (Oberbeck-Boussinesq (OB)) in turbulent mixed convection is investigated via direct numerical simulation (DNS) of water flows with viscosity ( μ ) and thermal expansion coefficient ( β ) both independently and simultaneously varying with temperature (non-Oberbeck-Boussinesq conditions (NOB)). Mixed convection is analyzed for the prototypical case of Poiseuille-Rayleigh-Bénard (PRB) turbulent channel flow. In PRB flows, the combination of buoyancy driven (Rayleigh-Bénard) with pressure driven (Poiseuille) effects produce a complex flow structure, which depends on the relative intensity of the flow parameters (i.e., the Grashof number, Gr, and the shear Reynolds number, Re τ ). In liquids, however, temperature variations induce local changes of fluid properties which influence the macroscopic flow field. We present results for different absolute values of the shear Richardson numbers (Riτ= Gr/Reτ2 ) under constant temperature boundary conditions. As Ri τ is increased buoyant thermal plumes are generated, which induce large scale thermal convection that increases momentum and heat transport efficiency. Analysis of friction factor ( Cf ) and Nusselt number (Nu) for NOB conditions shows that the effect of viscosity is negligible, whereas the effect of thermal expansion coefficient is significant. Statistics of mixing show that (i) mixing increases for increasing Ri τ (and decreases for increasing Re τ ) and (ii) the effect of thermal expansion coefficient on mixing increases for increasing Ri τ (and decreases for increasing Re τ ). A simplified phenomenological model to predict heat transfer rates in PRB flows has also been developed.
- Flow Boiling Heat Transfer of Liquid Nitrogen in Heated U-Tubes
<span class="paragraphSection">The flow boiling heat transfer characteristics of liquid nitrogen in three U-tubes with different curvature ratios were investigated experimentally. The effects of inlet pressure, heat flux, and curvature ratio on heat transfer characteristic are analyzed. The results indicate that the local heat transfer characteristics change obviously as fluid flows through the return bend, especially in the case of high heat flux. The drying out occurs near the inner wall of the return bend under high heat flux. A parameter <span style="font-style:italic;">Rh</span> (down/up), which is defined as the ratio of heat transfer coefficient between the downstream and upstream section of U-bend, is proposed to evaluate the contributions of the curvature ratio to the heat transfer. It is found that the Rh (down/up) increases with the decrease of the curvature ratios. Furthermore, the experiments results of the average heat transfer coefficient are compared with the calculated results of the empirical correlations.</span>
- Prediction of the Onset of Flow Instability in a Single Horizontal Microtube With an Inlet Orifice
A methodology to predict the onset of flow instability (OFI) in a single horizontal microtube with an inlet orifice is developed based on the predication of pressure drop. The predictive methodology states, for the same flow rate, the flow instability occurs as the single-phase liquid pressure drop under no heating condition equals the two-phase pressure drop under heating condition in a single microtube. The addition of inlet orifice increases the heat flux at the onset of flow instability by increasing the upstream pressure. The present methodology is validated by comparing the predicted heat flux at the onset of flow instability with our previous experimental data in the microtubes with three sizes of inlet orifices. The results show that the present method can predict the heat flux at the onset of flow instability with a deviation of 30% and mean absolute error of 13% at mass fluxes from 700 to 3000 kg/m 2 s. The effects of inlet orifice size and saturation pressure on the onset of flow instability are also studied based on the present methodology. It is found that, at mass fluxes from 100 to 2000 kg/m 2 s, the area ratio less than 15% eliminates the flow instability completely before the critical heat flux occurs.
- Impact of Flow Dynamics on the Heat Transfer of Bubbly Flow in a Microchannel
During nucleate flow boiling, the bubble dynamics affect the liquid flow field and the corresponding heat transfer process through several distinct mechanisms. At the microscale, this effect is different than at the macro scale partly because the bubble dimensions are comparable to the characteristic length scale of the channel. Since the process involves several mechanisms, an attempt to isolate and study them independently from one another is desired in order to extend knowledge. To remove the evaporation effect from the heat transfer process, noncondensable gas bubbles were introduced upstream of a 1 mm × 1 mm heater into a 220 μ m deep and a 1.5 mm wide microchannel and the heat transfer coefficient was measured and compared to single-phase liquid flow. High speed imaging and micro particle image velocimetry (μ-PIV) measurements were used to elucidate the bubble dynamics and the liquid velocity field. This, in turn, revealed mechanisms controlling the heat transfer process. Acceleration and deceleration of the liquid flow due to the presence of bubbles were found to be the main parameters controlling the heat transfer process.
- Performance Augmentation and Optimization of Aluminum Oxide-Water Nanofluid Flow in a Two-Fluid Microchannel Heat Exchanger
In this paper, laminar forced convection and entropy generation in a counter flow microchannel heat exchanger (CFMCHE) with two different working fluids in hot and cold channels, i.e., pure water and Al2O3–water nanofluid are investigated numerically using a three-dimensional conjugate heat transfer model. The temperature distribution, effectiveness, pumping power and performance index for various volume fractions between 0.01–0.04, three nanoparticles diameters, i.e., 29, 38.4, and 47 nm and a range of Reynolds number from 120 to 480 are given and discussed. According to second law of thermodynamics and entropy generation rate in the CFMCHE, the analysis of optimal volume fraction, particles size, Reynolds number as well as optimal placement of using nanoparticles in hot/cold channels is carried out. It is found that decreasing particles size and increasing nanoparticles concentration lead to higher effectiveness and pumping power as well as lower temperature in the solid phase of CFMCHE. Furthermore, the frictional contribution of entropy increases with decreasing particles size and increasing volume fractions while the trends for heat transfer contribution of entropy are reverse. Total entropy decreases as particles size decreases and volume fraction increases hence the maximum performance occurred at lower particles sizes and higher volume fractions. The Reynolds number has significant effect on performance of system and with decreasing it the effectiveness increases and heat transfer contribution of entropy decreases while the pumping power and frictional contribution of entropy decrease. Finally, it is seen that the capability of heat transfer of Al 2 O 3 –water nanofluids is higher when they are under heating conditions because the effectiveness of CFMCHE is higher when nanoparticles are used in cold channels.
- Nonlinear Heat Transfer in a Two-Layer Flow With Nanofluids by OHAM
<span class="paragraphSection">The problem of fully developed steady, laminar, incompressible flow in a vertical channel is studied analytically, one region is filled with water based copper nanofluid and the other region is filled with clear viscous fluid. The resulting coupled nonlinear ordinary differential equations (ODEs) are solved by optimal homotopy analysis method (OHAM). The convergence of our results is discussed by the so-called total average squared residual error. Analytical results are presented for different values of the physical parameters, such as the mixed convection parameters, the Brownian motion parameter, and thermophoresis parameter. Reversed flow is observed for sufficiently high buoyancy (mixed convection parameter). Further we investigate the effects of the Brownian motion parameter and thermophoresis parameter on the fluid flow and heat transfer at the interface of the two regions.</span>
- Experimental Investigation of Thermal and Hydraulic Performance of V-Shape Corrugated Carbon Foam
In air-cooled heat exchangers, air-side thermal resistance is usually the largest compared to conduction and liquid-side thermal resistances. Thus, reducing the air-side thermal resistance can greatly improve overall cooling performance. The performance of an air-cooled heat exchanger is usually characterized by the rate of heat which can be transferred and the pumping power required to convect the heat away. This paper presents a method of utilizing V-shape corrugated carbon foam to improve thermal performance. The air-side heat transfer coefficient and the pressure drop across the foam have been investigated using different V-shape foam geometrical configurations obtained by varying its length and height. Based on design considerations and availability, the foam length has been chosen to be 25.4, 38.1, and 52.1 mm, while its height is 4.4, 6.8, and 11.7 mm, resulting in nine different test pieces of foam with different heights and lengths. A total number of 81 experiments were carried out with different air face velocities (0.7-9m/s) and heat fluxes at the heater surface (0.5-2W/cm2). The pressure drop across the V-shape corrugated carbon foam as well as inlet air, exit air, foam, and ambient temperatures were measured. Of the nine V-shape configurations, the foam with the shortest length and tallest height gives the best performance. The present results are also compared with the results of prior work using different carbon foam geometries. It is shown that V-shape corrugated carbon foam provides better heat transfer coefficient and the overall performance.
- Effects of Brownian Diffusion and Thermophoresis on the Laminar Forced Convection of a Nanofluid in a Channel
A steady laminar forced convection in a parallel–plane channel using nanofluids is studied. The flow is assumed to be fully developed, and described through the Hagen–Poiseuille profile. A boundary temperature varying with the longitudinal coordinate in the thermal entrance region is prescribed. Two sample cases are investigated in detail: a linearly changing wall temperature, and a sinusoidally changing wall temperature. A study of the thermal behavior of the nanofluid is performed by solving numerically the fully–elliptic coupled equations. The numerical solution is obtained by a Galerkin finite element method implemented through the software package Comsol Multiphysics (© Comsol, Inc.). With reference to both the wall temperature distributions prescribed along the thermal entrance region, the governing equations have been solved separately both for the fully developed region and for the thermal entrance region. The analysis shows that if a linearly varying boundary temperature is assumed, for physically interesting values of the Péclet number the concentration field depends very weakly on the temperature distribution. On the other hand, in case of a longitudinally periodic boundary temperature, nonhomogeneities in the nanoparticle concentration distribution arise, which are wrongly neglected whenever the homogeneous model is employed.
- A Study on Gas–Liquid Film Thicknesses and Heat Transfer Characteristics of Vapor–Gas Condensation Outside a Horizontal Tube
Noncondensable gases deteriorate heat transfer in the condensation process. It is therefore necessary to study vapor–gas condensation heat transfer process and analyze main factors influencing the process. Based on the double-film theory and the Prandtl boundary layer theory, this investigation developed a mathematical model of gas–liquid film thicknesses and local heat transfer coefficient for studying laminar film condensation in the presence of noncondensable gas over a horizontal tube. Induced velocity in the gas film, gas–liquid interfacial shear stress, and pressure gradient were considered in the study. Importantly, gas–liquid film separations were analyzed in depth in this paper. It obtained the distributions of gas–liquid film thicknesses, local heat transfer coefficient, condensate mass flux, and gas–liquid interfacial temperature along the tube surface, and analyzed the influences of bulk velocity, total pressure, bulk mass concentration of noncondensable gas and wall temperature on them, providing a theoretical guidance for understanding and enhancing vapor–gas condensation heat transfer. Gas film thickness and gas–liquid film separations have certain effects on vapor–gas condensation heat transfer. The average dimensionless heat transfer coefficients are in agreement with the data from related literatures.
- Characteristics of Fully Developed Flow and Heat Transfer in Channels With Varying Wall Geometry
Present study focuses on numerical investigation of fully developed flow and heat transfer through three channels having sine-shaped, triangle-shaped, and arc-shaped wall profiles. All computations are performed at Reynolds number of 600. Finite volume method on collocated grid is used to solve the time-dependent Navier–Stokes and energy equations in primitive variable form. For all the geometries considered in the study, the ratios H min /H max and L/a are kept fixed to 0.4 and 8.0, respectively. The thermal performances of all the three wall configurations are assessed using integral parameters as well as instantaneous, time-averaged and fluctuating flow fields. The geometry with the sinusoidal-shaped wall profile is found to produce the best thermal properties as compared to the triangle-shaped and the arc-shaped profiles though the obtained heat transfer is the highest for the arc-shaped geometry.
- The Wall Heat Transfer Phenomenon of Premixed CH 4 /Air Catalytic Combustion in a Pt Coated Microtube
In this paper, a 2D model with detailed heterogeneous chemical mechanism has been employed to investigate the heat transfer phenomenon of premixed CH 4 /air catalytic combustion in a Pt coated microtube. Especially, the thermal processes such as coupled heat transfer between the internal surface of the microtube and the gas phase, thermal conduction along the solid structure, convection and radiation between the external surface and the environment are comprised in the simulation. The results show that the thermal conductivity of different solid wall materials dramatically affects the uniformity of temperature distribution of the catalytic surface. To maintain stable combustion in the microtube, the thermal conductivity should exceed 0.49 W/m/K at least and conductive walls (FeCr alloy and corundum ceramic) are more appropriate to manufacture microcombustors. The extremely small Biot number at the external surface indicates that convective heat transfer coefficient and emissivity to the environment are the key factors determining the heat loss of the microtube. The amount of heat loss influences the reaction rate and residence time of the mixtures in the microtube, which would affect the conversion of CH 4 . An increase of the wall thickness improves the heat transfer along the solid structure, also increases the total heat loss.
- Cavitation Bubble Collapse Near a Heated Wall and Its Effect on the Heat Transfer
In the present work, a numerical investigation on the mechanism of heat transfer enhancement by a cavitation bubble collapsing near a heated wall has been presented. The Navier–Stokes equations and volume of fluid (VOF) model are employed to predict the flow state and capture the liquid-gas interface. The model was validated by comparing with the experimental data. The results show that the microjet violently impinges on the heated wall after the bubble collapses completely. In the meantime, the thickness of the thermal boundary layer and the wall temperature decrease significantly within the active scope of the microjet. The fresh low-temperature liquid and the impingement brought by the microjet should be responsible for the heat transfer reinforcement between the heated wall and the liquid. In addition, it is found that the impingement width of the microjet on the heated wall always keeps 20% of the bubble diameter. And, the enhancement degree of heat transfer significantly depends on such factors as stand-off distance, saturated vapor pressure, and initial bubble radius.
- Impact of Delta-Winglet Pair of Vortex Generators on the Thermal and Hydraulic Performance of a Triangular Channel Using Al 2 O 3 –Water Nanofluid
This paper presents the laminar forced convection of Al 2 O 3 –water nanofluid in a triangular channel, subjected to a constant and uniform heat flux at the slant walls, using delta-winglet pair (DWP) of vortex generator which is numerically investigated in three dimensions. The governing equations of mass, momentum, and energy are solved using the finite volume method (FVM). The nanofluid properties are estimated as constant and temperature-dependent properties. The nanoparticle concentrations and diameters are in ranges of 1–4% and 25–85 nm, respectively. Different attack angles of vortex generators are examined which are 7 deg, 15 deg, 30 deg, and 45 deg with range of Reynolds number from 100 to 2000. The results show that the heat transfer coefficient is remarkable dependent on the attack angle of vortex generators and the volume fraction of nanoparticles. The heat transfer coefficient increases as the attack angle increases from 7 deg to 30 deg and then diminishes at 45 deg. The heat transfer rate remarkably depends on the nanoparticle concentration and diameter, attack angle of vortex generator and Reynolds number. An increase in the shear stress is found when attack angle, volume fraction, and Reynolds number increase.
- Experimental Investigation of Flow Condensation in Microgravity
<span class="paragraphSection">Future manned space missions are expected to greatly increase the space vehicle's size, weight, and heat dissipation requirements. An effective means to reducing both size and weight is to replace single-phase thermal management systems with two-phase counterparts that capitalize upon both latent and sensible heat of the coolant rather than sensible heat alone. This shift is expected to yield orders of magnitude enhancements in flow boiling and condensation heat transfer coefficients. A major challenge to this shift is a lack of reliable tools for accurate prediction of two-phase pressure drop and heat transfer coefficient in reduced gravity. Developing such tools will require a sophisticated experimental facility to enable investigators to perform both flow boiling and condensation experiments in microgravity in pursuit of reliable databases. This study will discuss the development of the Flow Boiling and Condensation Experiment (FBCE) for the International Space Station (ISS), which was initiated in 2012 in collaboration between Purdue University and NASA Glenn Research Center. This facility was recently tested in parabolic flight to acquire condensation data for FC-72 in microgravity, aided by high-speed video analysis of interfacial structure of the condensation film. The condensation is achieved by rejecting heat to a counter flow of water, and experiments were performed at different mass velocities of FC-72 and water and different FC-72 inlet qualities. It is shown that the film flow varies from smooth-laminar to wavy-laminar and ultimately turbulent with increasing FC-72 mass velocity. The heat transfer coefficient is highest near the inlet of the condensation tube, where the film is thinnest, and decreases monotonically along the tube, except for high FC-72 mass velocities, where the heat transfer coefficient is enhanced downstream. This enhancement is attributed to both turbulence and increased interfacial waviness. One-<span style="font-style:italic;">ge</span> correlations are shown to predict the average condensation heat transfer coefficient with varying degrees of success, and a recent correlation is identified for its superior predictive capability, evidenced by a mean absolute error of 21.7%.</span>
- Experimental and Numerical Investigation on Natural Convection Heat Transfer of TiO 2 –Water Nanofluids in a Square Enclosure
An experimental and numerical investigation on natural convection heat transfer of TiO 2 –water nanofluids in a square enclosure was carried out for the present work. TiO 2 –water nanofluids with different nanoparticle mass fractions were prepared for the experiment and physical properties of the nanofluids including thermal conductivity and viscosity were measured. Results show that both thermal conductivity and viscosity increase when increasing the mass fraction of TiO 2 nanoparticles. In addition, the thermal conductivity of nanofluids increases, while the viscosity of nanofluids decreases with increasing the temperature. Nusselt numbers under different Rayleigh numbers were obtained from experimental data. Experimental results show that natural convection heat transfer of nanofluids is no better than water and even worse when the Rayleigh number is low. Numerical studies are carried out by a Lattice Boltzmann model (LBM) coupling the density and the temperature distribution functions to simulate the convection heat transfer in the enclosure. The experimental and numerical results are compared with each other finding a good match in this investigation, and the results indicate that natural convection heat transfer of TiO 2 –water nanofluids is more sensitive to viscosity than to thermal conductivity.
- Non-Darcy Natural Convection From a Vertical Cylinder Embedded in a Thermally Stratified and Nanofluid-Saturated Porous Media
<span class="paragraphSection">In recent years, nanofluids have attracted attention as a new generation of heat transfer fluids in building heating, heat exchangers, plants, and automotive cooling applications because of their excellent thermal performance. Various benefits of the application of nanofluids include improved heat transfer, heat transfer system size reduction, minimal clogging, microchannel cooling, and miniaturization of systems. In this paper, a study of steady, laminar, natural convection boundary-layer flow adjacent to a vertical cylinder embedded in a thermally stratified nanofluid-saturated non-Darcy porous medium is investigated. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis, and a generalized porous media model, which includes inertia and boundary effects, is employed. The cylinder surface is maintained at a constant nanoparticles volume fraction, and the wall temperature is assumed to vary with the vertical distance according to the power law form. The resulting governing equations are nondimensionalized and transformed into a nonsimilar form and then solved by Keller box method. A comparison is made with the available results in the literature, and our results are in very good agreement with the known results. A parametric study of the physical parameters is made, and a representative set of numerical results for the velocity, temperature, and volume fraction, as well as local shear stress and local Nusselt and Sherwood numbers, are presented graphically. The salient features of the results are analyzed and discussed. The results indicate that, when the buoyancy ratio or modified Grashof number increases, all of the local shear stress, local Nusselt number, and the local Sherwood number enhance while the opposite behaviors are predicted when the thermophoresis parameter increases. Moreover, increasing the value of the surface curvature parameter leads to increases in all of the local shear stress and the local Nusselt and Sherwood numbers while the opposite behaviors are obtained when either of the thermal stratification parameter or the boundary effect parameter increases.</span>
- Stable Nanofluids for Convective Heat Transfer Applications
Nanofluids are stable dispersions of ultrafine or nanoscale metallic, metal oxide, ceramic particles in a given base fluid. It is reported that nanofluids register an extraordinarily high level of thermal conductivity, and thus possess immense potential in improvement of heat transfer and energy efficiency of several industrial applications including vehicular cooling in transportation, nuclear reactors, and microelectronics. The key issues with nanofluids are: (i) a robust, cost-effective and scalable method to produce nanofluids to industrial scale has not yet been developed, (ii) stability in industrial applications is not yet established, and (iii) meaningful data in flow based heat transfer process do not exist. The present work attempts to address all these three issues. We have developed an in-situ technique for preparation of stable nanofluids by wet-milling of the metal oxide powder in the base fluid, and in the presence of a suitable dispersant. The nanofluids thus produced are tested for heat transfer efficiency under flow conditions in double pipe heat exchangers. Alumina nanofluids have been found to show enhancements of around 10–60% for various base fluids flown under different flow conditions. Thermal enhancements have been found to depend on the flow-rate, particle concentration, type of base fluid, and material of the thermal contact surface of the heat exchanger. The nanofluids thus obtained exhibit sustained stability (>30 months) and their stability remains unaltered for several heating-cooling cycles.