Accepted Manuscripts

Danchen Luo, Congliang Huang and Zun Huang
J. Heat Transfer   doi: 10.1115/1.4038003
In this paper, we have studied the effect of short branches (side chains) on the thermal conductivity of a polyethylene (PE) chain. With a reverse non-equilibrium molecular dynamic method, thermal conductivities of the pristine PE chain and the PE-ethyl chain are simulated and compared. It shows that the branch has a positive effect to decrease the thermal conductivity of a PE chain. The thermal conductivity of the PE-ethyl chain decreases with the number density increase of branches, until the density becomes larger than about 8 ethyl per 200 segments, where the thermal conductivity saturates to be only about 40% that of a pristine PE chain. Because of different weights, different branches will cause a different decrease of thermal conductivities, and a heavy branch will leads to a lower thermal conductivity than a light one. This study is expected to provide some fundamental guidance to obtain a polymer with a low thermal conductivity.
Yue Zhou, Qiang Wang and Haiyang Hu
J. Heat Transfer   doi: 10.1115/1.4038004
In the present work, strategies for the grouping of the spectral absorption coefficients used in multi-scale multi-group full-spectrum k-distribution models were improved by considering the effects of variations in both temperature and species molar ratio on the correlated-k characteristics of the spectra of H2O-CO2-soot mixtures. The improvements in the accuracy of predictions of radiation heat transfer characteristics resulting from these new grouping strategies were evaluated using a series of semi-1D cases with significant temperature, participating species molar ratio and pressure inhomogeneities. Finally, evaluations of grouping strategies were presented on calculation of the full-spectrum thermal images of an actual aeroengine combustor.
Yuanpeng Yao, Huiying Wu and Zhenyu Liu
J. Heat Transfer   doi: 10.1115/1.4038006
The interstitial heat transfer coefficient is a key parameter in the two-energy equation model, which is usually used to investigate the thermal performance of high porosity open-cell metal foam/paraffin composite. Due to the existence of weak convection of liquid paraffin through metal foam during phase change process, the interstitial heat transfer coefficient (IHTC) should be carefully determined for a low Reynolds number range (Re = 0 - 1), which however has been rarely addressed in the literature. In this paper, a direct simulation at foam pore scale is carried out to determine the IHTC between paraffin and metal foam at Re = 0 - 1. For this purpose, the cell structures reflecting realistic metal foams are first constructed based on the 3D Weaire-Phelan foam cell to serve as the representative elementary volume (REV) of metal foam for direct simulation. Then, by solving the Navier-Stokes equations and energy equation for the REV, the influences of Reynolds number, Prandtl number, foam porosity and pore density (PPI) on IHTC are investigated. According to the numerical results, a correlation of IHTC at Re = 0 - 1 is proposed for metal foam/paraffin composite material, which covers both conduction dominated interstitial heat transfer region (Re < 0.1) and convection dominated interstitial heat transfer region (0.1 < Re < 1). Finally, the applicability of this correlation in the two-energy equation model for solid-liquid phase change of paraffin in metal foam is validated by comparing the model predicted melting front with that of experimental observations.
Technical Brief  
C.Y. Wang
J. Heat Transfer   doi: 10.1115/1.4038001
The constant flux forced convection in a rectangular duct with two highly conductive (H1) walls and two poorly conductive (H2) walls is studied for the first time. This mixed problem is solved analytically using a modified single series method. The Nusselt number is determined for various duct aspect ratios. Depending on the aspect ratio, hot spots may occur either on the H1 walls or on the H2 walls.
Zhonghao Rao, Yutao Huo and Yinmin Li
J. Heat Transfer   doi: 10.1115/1.4037908
The solid-liquid phase change process is of significant importance in the widely usage of phase change material (PCM), including thermal energy storage. In this paper, a phase change lattice Boltzmann (LB) model has been used to investigate the effects of inclination angle on the melting process in a cavity filled with PCM, with three kinds of heat flux distribution applied to the left wall: uniform distribution, linear distribution and parabolic symmetry distribution. The results show that for all the heat flux distributions, the slight clockwise rotation of cavity is able to accelerate the melting process. Furthermore, when more heat is transferred into the cavity through the middle part (parabolic symmetry distribution) or bottom part (linear distribution), the clockwise rotation of cavity leads to higher temperature of PCM than anticlockwise rotation.
TOPICS: Melting, Phase change materials, Cavities, Lattice Boltzmann methods, Rotation, Heat flux, Heat, Temperature, Thermal energy storage
Swastik Acharya and Sukanta K. Dash
J. Heat Transfer   doi: 10.1115/1.4037875
The continuity, momentum and the energy conservation equation for air around a hollow cylinder with inline or staggered holes have been solved in three dimension to assess the buoyancy driven flow and temperature field around the cylinder. From the thermal field the average surface Nu could be computed for hollow cylinders with inline or staggered holes and the heat loss from the cylinder could be compared with that of a hollow cylinder without holes. Interesting flow and thermal plume around the hollow cylinder with holes could be seen which could help to explain why there is more heat loss from a cylinder with staggered holes compared to a cylinder with inline holes at lower Ra of 10^5, whereas for higher Ra of 10^6 or more there exists an optimum d/D where the heat loss from the perforated cylinder has a maximum value and thereafter it reduces. There are interesting comparisons on Nu for the hollow cylinder with inline or staggered holes and new correlations for Nu versus many different pertinent input parameters have been developed for many cases which can be used practically in industry for designing perforated cylinder with heat loss.
TOPICS: Heat transfer, Natural convection, Cylinders, Heat losses, Flow (Dynamics), Buoyancy, Temperature, Dimensions, Plumes (Fluid dynamics), Design, Energy conservation, Momentum
Technical Brief  
Pranay Biswas and Suneet Singh
J. Heat Transfer   doi: 10.1115/1.4037874
The separation of variables (SOV) can be used for all Fourier, single phase lag (SPL) and dual phase lag (DPL) heat conduction problems with time independent source and/or boundary conditions (BCs). The Laplace transform (LT) can be used for problems with time dependent BCs and sources but requires large computational time for inverse LT. In the present work, the orthogonal eigenfunction expansion (OEEM) has been proposed as an alternate method for non-Fourier (SPL and DPL) heat conduction problem. However, the OEEM is applicable only for cases where BCs are homogeneous. Therefore, BCs of the original problem are homogenized by subtracting an auxiliary function from the temperature to get a modified problem in terms of a modified temperature. It is shown that the auxiliary function has to satisfy a set of conditions. However, these conditions do not lead to a unique auxiliary function. Therefore, an additional condition, which simplifies the modified problem, is proposed to evaluate the auxiliary function. The methodology is verified with SOV for time independent BCs. The implementation of the methodology is demonstrated with illustrative example which shows that this approach leads to an accurate solution with reasonable number of terms in the expansion.
TOPICS: Heat conduction, Eigenfunctions, Boundary-value problems, Temperature, Separation (Technology), Laplace transforms
Sheng-Yen Hsu
J. Heat Transfer   doi: 10.1115/1.4037823
In this study, a new model for intumescent coatings is developed including the radiation transfer equation. So, one of the important features of this model is to give the insight of the radiative heat transfer in the intumescent coating during expansion. In addition, the model equations are derived into a new coordinate system by introducing the expansion effect into the corresponding parameters. Consequently, the numerical results can be carried out by using a fixed grid system. The numerical results show that the radiative heat transfer near the exposed coating surface cannot be well simulated by the model of thermal radiation conductivity, which is widely used in the previous studies. So, it is suggested that the radiative heat transfer in the expanded char region should be formulated by a more considerate model. In addition, several thermal property parameters of coating (thermal conductivity, extinction coefficient and albedo) are tested and investigated under a radiant heat source. In addition to the transient response, the effects of these coating properties on the quasi-steady results are also discussed. It is found that the thermal conductivity and the extinction coefficient in the expanded char region both dominate the coating performance. For the thermal properties of virgin coating, the thermal conductivity may have significant effect when the coating has large incomplete pyrolysis region, while the extinction coefficient has little influence. Besides, the thermal conductivity and the albedo of virgin coating both alter the heating time to initial expansion but in different mechanisms.
TOPICS: Heat transfer, Coatings, Flame retardants, Modeling, Radiant heat, Thermal conductivity, Radiative heat transfer, Albedo, Thermal properties, Transients (Dynamics), Thermal radiation, Radiation (Physics), Heating, Pyrolysis
Andrew Sexton, Jeff Punch, Jason Stafford and Nicholas Jeffers
J. Heat Transfer   doi: 10.1115/1.4037786
Liquid micro-jets are emerging as candidate primary or secondary heat exchangers for the thermal management of next generation photonic integrated circuits (PICs). However, the thermal and hydrodynamic behavior of confined, low Reynolds number liquid slot jets is not yet comprehensively understood. This investigation experimentally examined jet outlet modifications - in the form of tabs and chevrons - as techniques for passive control and enhancement of single-phase convective heat transfer. The investigation was carried out for slot jets in the laminar flow regime, with a Reynolds number range, based on the slot jet hydraulic diameter, of 100 to 500. A slot jet with an aspect ratio of 4, and a fixed confinement height to hydraulic diameter ratio (H=Dh) of 1 was considered. Local surface heat transfer and velocity field characteristics were measured using infrared thermography and particle image velocimetry techniques. It was found that increases in area-averaged Nusselt number of up to 31% compared to the baseline case, could be achieved without incurring additional hydrodynamic losses. It was also determined that the location and magnitude of Nusselt number and velocity peaks within the slot jet stagnation region, could be passively controlled and enhanced through the application of outlet tabs of varying geometry and location.
TOPICS: Reynolds number, Jets, Passive control, Convection, Geometry, Integrated circuits, Thermal management, Waste heat, Thermography, Heat transfer, Particulate matter, Laminar flow
Julius Simon, Leizeronok Boris and Cukurel Beni
J. Heat Transfer   doi: 10.1115/1.4037775
Finite integral transform techniques are applied to solve the one-dimensional dual-phase hyperbolic heat conduction problem, and a comprehensive analysis is provided for general time-dependent heat generation and arbitrary combinations of various boundary conditions (Dirichlet, Neumann & Robin). Through the dependence on the relative differences in heat flux and temperature relaxation times, this analytical solution effectively models both parabolic and hyperbolic heat conduction. In order to demonstrate several exemplary physical phenomena, four distinct cases that illustrate the hyperbolic heat conduction behavior are presented. In the first model, following an initial temperature spike in a slab, the thermal evolution portrays immediate dissipation in parabolic systems, whereas the hyperbolic solution depicts wavelike temperature propagation - the intensity of which depends on the relaxation times. Next, the analysis of periodic surface heat flux at the slab boundaries provides evidence of interference patterns formed by temperature waves. In following, the study of Joule-heating driven periodic generation inside the slab demonstrates that the steady-periodic parabolic temperature response depends on the ratio of pulsatile electrical excitation and the electrical resistivity of the slab. As for the hyperbolic model, thermal resonance conditions are observed at distinct excitation frequencies. Building on findings of the other models, the case of moving constant-amplitude heat generation is considered, and the occurrences of thermal shock and thermal expansion waves are demonstrated at particular conditions.
TOPICS: Heat conduction, Temperature, Slabs, Waves, Relaxation (Physics), Heat flux, Excitation, Heat, Energy dissipation, Boundary-value problems, Electrical resistivity, Thermal shock, Heating, Joules, Resonance, Thermal expansion
Ziqin Zhu, Minjie Liu, Nan Hu, Yuan-Kai Huang, Liwu Fan, Zitao Yu and Jian Ge
J. Heat Transfer   doi: 10.1115/1.4037776
The classical problem of inward solidification heat transfer inside a spherical capsule, with an application to thermal energy storage, was revisited in the presence of nano-enhanced phase change materials (NePCM). The model NePCM samples were prepared by dispersing graphite nanoplatelets (GNPs) into 1-tetradecanol (C14H30O) at loadings up to 3.0 wt.%. The transient phase change, energy retrieval, and heat transfer rates during solidification of the various NePCM samples were measured quantitatively using a volume-shrinkage-based indirect method. The data reduction and analysis were carried out under single-component, homogeneous assumption of the NePCM samples without considering the microscale transport phenomena of GNPs. It was shown that the total solidification time becomes monotonously shorter with increasing the loading of GNPs, in accordance with the increased effective thermal conductivity. The maximum relative acceleration of solidification was found to be more than 50% for the most concentrated sample, which seems to be appreciable for practical applications. In addition to enhanced heat conduction, the possible effects due to elimination of supercooling and viscosity growth were elucidated. The heat retrieval rate was also shown to be increased monotonously with raising the loading of GNPs, although the heat storage capacity is sacrificed. Despite the remarkable acceleration of the solidification time, the use of a high loading (e.g., 3.0 wt.%) was demonstrated to be possibly uneconomical because of the marginal gain in heat retrieval rate. Finally, correlations for the transient variations of the melt fraction and surface-averaged Nusselt number were proposed.
TOPICS: Heat transfer, Phase change materials, Solidification, Heat, Transients (Dynamics), Thermal conductivity, Shrinkage (Materials), Microscale devices, Heat storage, Viscosity, Heat conduction, Supercooling, Transport phenomena, Graphite, Thermal energy storage
Masao Furukawa
J. Heat Transfer   doi: 10.1115/1.4037748
The state of the art of thermally-excited oscillatory heat pipe technology is briefly mentioned to emphasize that there exists no oscillating/pulsating heat pipe suited to long-distance heat transport. Responding to such conditions, this study actively proposes a newly-devised conceptually novel type of oscillating/pulsating heat pipe. In that heat pipe, the adiabatic section works as it were the dream pipe invented by Kurzweg. This striking quality of the proposed new-style oscillating/pulsating heat pipe produces high possibilities of long- distance heat transport. To support such optimistic views, an originally planned mathematical model is introduced for feasibility studies. Hydraulic considerations have first been done to understand what conditions are required for sustaining bubble-train flows in a capillary tube of interest. Theoretical analysis has then been made to solve the momentum and energy equations governing the flow velocity and temperature fields in the adiabatic section. The obtained analytical solutions are arranged to give algebraic expressions of the effective thermal diffusivity, the performance index combined with the tidal displacement, and the require electric power. Computed results of those three are displayed in the figures to demonstrate the realizability of that novel oscillatory heat pipe.
TOPICS: Heat pipes, Pipes, Flow (Dynamics), Heat, Temperature, Electricity (Physics), Thermal diffusivity, Bubbles, Displacement, Theoretical analysis, Trains, Tides, Algebra, Momentum
Reda Ragab and Ting Wang
J. Heat Transfer   doi: 10.1115/1.4037641
Motivated by the need to further improve film cooling in terms of both cooling effectiveness and coolant coverage area, the mist/air film cooling scheme is investigated through experiments using fan-shaped holes over an extended downstream length in this study. Both an existing wind tunnel and test facility, used in previous work, have been retrofitted. The first modification was extending the length of the flat plate test section to cover longer distances downstream of the injection holes, up to X/D=100, in order to investigate whether mist cooling can be harnessed farther downstream where single-phase film cooling is not effective. The second modification was to incorporate a fan-shaped diffusion hole geometry in order to investigate whether mist can further enhance the film cooling performance of the already highly effective fan-shaped holes. A Phase Doppler Particle Analyzer (PDPA) system is employed to measure droplet size, velocity, and turbulence information. An infrared camera and thermocouples are both used for temperature measurements. Part 1 is focused on the heat transfer result on the wall, and Part 2 is focused on the two-phase droplet multiphase flow behavior. Three blowing ratios are investigated. The results show that, at low blowing ratios when the film is attached to the surface, the enhancement of the mist film cooling effectiveness, compared to the air-only case, on the centerline of the hole ranges from 40% in the near hole region to over 170% at X/D = 100.
TOPICS: Heat transfer, Flat plates, Film cooling, Drops, Cooling, Temperature measurement, Particulate matter, Turbulence, Coolants, Geometry, Test facilities, Thermocouples, Wind tunnels, Multiphase flow, Diffusion (Physics)
Reda Ragab and Ting Wang
J. Heat Transfer   doi: 10.1115/1.4037642
A Phase Doppler Particle Analyzer (PDPA) system is employed to measure the two-phase mist flow behavior including flow velocity field, droplet size distribution, droplet dynamics, and turbulence characteristics. Based on the droplet measurements made through PDPA, a projected profile describing how the air-mist coolant jet flow spreads and eventually blends into the hot main flow is prescribed for both cylindrical and fan-shaped holes. The mist film layer consists of two layers; a typical coolant film layer (cooling air containing the majority of the droplets) and a wider droplet layer containing droplets outside the film layer. Thanks to the higher inertia possessed by larger droplets (>20 ?m in diameter) at the injection hole, the larger droplets tend to shoot across over the coolant film layer, resulting in a wider droplet layer than the coolant film layer. The wider droplet layer boundaries are detected by measuring the droplet data rate (droplet number per second) distribution, and it is identified by a wedge-shaped enclosure prescribed by the data rate distribution curve. The coolant film layer is prescribed by its core and its upper boundary. The apex of the data rate curve, depicted by the maximum data rate, roughly indicates the core region of the coolant film layer. The upper boundary of the coolant film layer, characterized by active mixing with the main flow, is found to be close to relatively high values of local Reynolds shear stresses.
TOPICS: Dynamics (Mechanics), Drops, Two-phase flow, Flat plates, Film cooling, Coolants, Flow (Dynamics), Cooling, Particulate matter, Turbulence, Inertia (Mechanics), Jets, Shear stress, Wedges
Ya Ge, Feng Shan, Zhichun Liu and Wei Liu
J. Heat Transfer   doi: 10.1115/1.4037643
This paper proposes a general method combining evolutionary algorithm and decision-making technique to optimize the structure of a minichannel heat sink (MCHS). Two conflicting objectives, thermal resistance ? and pumping power P, are simultaneously considered to assess the performance of the MCHS. In order to achieve the ultimate optimal design, multi-objective genetic algorithm is employed to obtain the non-dominated solutions (Pareto solutions), while TOPSIS (technique for order preference by similarity to an ideal solution) is employed to determine which is the best compromise solution. Meanwhile, both the material cost and volumetric flow rate are fixed where this nonlinear problem is solved by applying the penalty function. The results show that ? of the Pareto solutions varies from 0.03707 K W-1 to 0.10742 K W-1, while P varies from 0.00307 W to 0.05388 W, respectively. After the TOPSIS selection, it is found that P is significantly reduced without increasing too much ?. As a result, ? and P of the optimal MCHS determined by TOPSIS are 35.82% and 52.55% lower than initial one, respectively.
TOPICS: Flow (Dynamics), Structural optimization, Computational fluid dynamics, Genetic algorithms, Heat sinks, Thermal resistance, Preferences, Design, Decision making, Evolutionary algorithms
Alyssa Recinella and Satish G. Kandlikar
J. Heat Transfer   doi: 10.1115/1.4037644
Over the past decade, research in two-phase liquid cooling has risen in popularity. Because of latent heat effects, boiling has the ability to dissipate vast amounts of heat. Flow boiling in particular has multiple advantages including higher heat transfer coefficients, bulk liquid and surface temperature uniformity, low coolant inventory and higher critical heat flux values. This study focuses on a radial flow configuration, comparing a radial configuration of microchannels and offset strip fins. The prospective results from a radial flow boiling geometry are promising due to an increase in flow cross-sectional area and a reduced pressure drop. Further modifications are made to improve results even further including the addition of a gapped manifold creating open microchannels. Various flow rates were tested for all geometries and configurations ranging from 120 mL/min - 320 mL/min. The offset strip fin geometry with an open manifold yielded the best performance for all tested flow rates. At a flow rate of 240 mL/min, a maximum heat flux of 618 W/cm2 at a ?T of 20.1°C and a pressure drop of 13.8 kPa. Increasing the flow rate to 320 mL/min resulted in a higher heat flux of 897 W/cm2 at a ?T of 63.7°C. The heat transfer performance can be further improved by increasing the flow rate and adjusting the gap to provide a pressure drop within acceptable limits. High speed videos were taken to understand the underlying mechanism.
TOPICS: Flow (Dynamics), Boiling, Fins, Strips, Microchannels, Manifolds, Pressure drop, Radial flow, Heat flux, Geometry, Latent heat, Heat, Heat transfer, Cooling, Coolants, Temperature uniformity, Heat transfer coefficients, Critical heat flux
Yong Ren, Kai Seng Koh, Jit Kai Chin, Jing Wang, Conghua Wen and Yuying Yan
J. Heat Transfer   doi: 10.1115/1.4037338
With a novel platform of bilayer polydimethylsiloxane (PDMS) microchannel formed by bifurcated junction, we aim to investigate droplet formation and fission in a multiphase system with complex 3D structure; and understand the variations in mechanism associated with droplet formation and fission in the microstructure between shear-thinning/Newtonian system versus Newtonian/Newtonian system. The investigation concentrates on shear-thinning fluid because it is one of the most ubiquitous rheological properties of non-Newtonian fluids. Sodium carboxymethyl cellulose (CMC) solution and silicon oil have been used as model fluids and numerical model has been established to characterize the shear-thinning effect to formation of CMC-in-oil emulsions, as well as breakup dynamics when droplets flow through 3D bifurcated junction. The droplet volume and generation rate have been compared between two systems at the same Weber number and Capillary number. Variation in droplet fission has been found between two systems, demonstrating that the shear-thinning property and confining geometric boundaries significantly affect the deformation and breakup of each mother droplet into two daughter droplets at bifurcated junction. The understanding of the droplet fission in the novel microstructure will enable more versatile control over the emulsion formation and fission when non-Newtonian fluids are involved. The model systems in the study can be further developed to investigate the mechanical property of emulsion templated particles such as drug encapsulated microcapsules when they flow through complex media structures, such as blood capillaries or the porous tissue structure, which feature with bifurcated junction.
TOPICS: Nuclear fission, Drops, Shear (Mechanics), Microchannels, Junctions, Emulsions, Non-Newtonian fluids, Fluids, Flow (Dynamics), Ceramic matrix composites, Rheology, Deformation, Particulate matter, Computer simulation, Biological tissues, Blood, Drugs, Mechanical properties, Plasma desorption mass spectrometry, Dynamics (Mechanics), Silicon, Sodium
Lakehal Abdelhak, Nait Bouda Nora, Pellé Julien and Harmand Souad
J. Heat Transfer   doi: 10.1115/1.4037208
Both experimental and numerical studies of a turbulent flow in a bifurcating channel are performed to characterize the dynamical behavior of the flow and its impact on the convective heat transfer on the sides of the branch. This configuration corresponds to the radial vents placed in the stator vertically to the rotor-stator air gap in theelectrical machines. Indeed, our analysis focuses on the local convective heat transfer on the vents internal surface under a turbulent mass flow rate. The flow field measurements were carried with two components particle image velocimetry (PIV) system and the local heat transfer on the sides of the bifurcation branch was measured using an infrared thermography device. The convective heat transfer and the flow dynamics through the geometry are investigated numerically considering a three-dimensional flow. The closure system of the Navier- Stokes equations for steady and incompressible flow is based on the low-Reynolds numbers RSM-model (RSM-Stress-?). The comparison of the three-dimensional computed results with the measurements in the xy symmetry plane is satisfactory in the vertical and horizontal channels. The numerical prediction of the secondary flow in the vertical branch was analyzed and complement the experimental results. It particularly noticed that accelerated flow observed at the right side of the branch's inlet allows more pronounced heat transfer comparatively to the left side.
TOPICS: Flow (Dynamics), Convection, Heat transfer, Turbulence, Stators, Vents, Thermography, Stress, Machinery, Particulate matter, Rotors, Bifurcation, Geometry

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