Refine Search

New Search

Results in Journal Journal of Heat Transfer: 10,757

(searched for: journal_id:(1900996))
Page of 216
Articles per Page
by
Show export options
  Select all
Kyle Hassan, Robert F. Kunz, David Hanson, Michael Manahan
Published: 18 October 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052437

Abstract:
In this work, we study the heat transfer performance and particle dynamics of a high mass-loaded, compressible, particle-laden flow in a horizontally oriented pipe using an Eulerian–Eulerian (two-fluid) computational model. Previous experimental work by our group provides the basis for the study. Specifically, a 17 bar coflow of nitrogen gas and copper powder are modeled with inlet Reynolds numbers of 3 × 104, 4.5 × 104, and 6 × 104 and mass loadings of 0, 0.5, and 1.0. Eight binned particle sizes were modeled to represent the known powder properties. Significant settling of all particle groups is observed leading to asymmetric temperature distributions. Wall and core flow temperature distributions are observed to agree well with measurements. In high Reynolds number cases, the predictions of the multiphase computational model were satisfactorily aligned with the experimental results. Low Reynolds number model predictions were not as consistent with the experimental measurements.
Published: 13 October 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052435

Abstract:
Exploring parametric effects in pool boiling is challenging because the dependence of the resulting surface heat flux is often nonlinear, and the mechanisms can interact in complex ways. Historically, parametric effects in nucleate boiling processes have been deduced by fitting relations obtained from physical models to experimental data and from correlated trends in nondimensionalized data. Using such approaches, observed trends are often influenced by the framing of the analysis that results from the modeling or the collection of dimensionless variables used. Machine learning strategies can be attractive alternatives because they can be constructed either to minimize biases or to emphasize specific biases that reflect knowledge of the system physics. The investigation summarized here explores the use of machine learning methods as a tool for determining parametric trends in boiling heat transfer data and as a means for developing methods to predict boiling heat transfer. Results are presented that demonstrate how a genetic algorithm and deep learning can be used to extract heat flux dependencies of a binary mixture on wall superheat, gravity, Marangoni effects, and pressure. The results provide new insight into how gravity and Marangoni effects interact in boiling processes of this type. The results also demonstrate how machine learning tools can clarify how different mechanisms interact in the boiling process, as well as directly providing the ability to predict heat transfer performance for nucleate boiling. Each technique demonstrated clear advantages depending on whether speed, accuracy, or an explicit mathematical model was prioritized.
Published: 12 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052688

Abstract:
The extended meniscus and the intermolecular and capillary forces that govern its behavior and connection to change of phase heat transfer have been the subject of an increasing body of research over the past 50 years. We have been fortunate to be at the forefront of this effort starting from the development of a capillary feeder, in Earth's gravity, to stabilize film boiling to running a series of transparent heat pipe experiments aboard the International Space Station hoping to better understand the role of intermolecular forces in microgravity. The use of ellipsometry and interferometry to highlight the location and state of the vapor-liquid interface have been key to these studies and have helped to uncover many new, interesting, and sometimes unexpected, phenomena associated with fluid flow and change-of-phase heat transfer.
Richard Blythman, Sajad Alimohammadi, Nicholas Jeffers, ,
Published: 12 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052686

Abstract:
While numerous applied studies have successfully demonstrated the feasibility of unsteady cooling solutions, a consensus has yet to be reached on the local instantaneous conditions that result in heat transfer enhancement. The current work aims to experimentally validate a recent analytical solution (on a local time-dependent basis) for the common flow condition of a fully-developed incompressible pulsating flow in a uniformly-heated vessel. The experimental setup is found to approximate the ideal constant heat flux boundary condition well, especially for the decoupled unsteady scenario where the amplitude of the most significant secondary contributions (capacitance and lateral conduction) amounts to 1.2% and 0.2% of the generated heat flux, respectively. Overall, the experimental measurements for temperature and heat flux oscillations are found to coincide well with a recent analytical solution to the energy equation by the authors. Furthermore, local time-dependent heat flux enhancements and degradations are observed to be qualitatively similar to those of wall shear stress from a previous study, suggesting that the thermal performance is indeed influenced by hydrodynamic behaviour.
Published: 12 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052687

Abstract:
Modelling of turbulence heat transfer for supercritical fluids using Computational Fluid Dynamics (CFD) software is always challenging due to the drastic property variations near critical point. Use of Artificial Neural Networks (ANN) along with numerical methods have shown promising results in predicting heat transfer coefficients of heat exchangers. In this study, accuracy of four different turbulent models available in the commercial CFD software - Ansys Fluent is investigated against the available experimental results. The k-e Re Normalization Group (RNG) model with enhanced wall treatment is found to be the best-suited turbulence model. Further, K-e RNG Turbulence Model is used in CFD for parametric analysis to generate the data for ANN studies. A total of 1,34,698 data samples were generated and fed into the ANN program to develop an equation that can predict the heat transfer coefficient. It was found that, for the considered range of values the absolute average relative deviation is 3.49%.
Eberhard Bänsch, Sara Faghih-Naini
Published: 11 October 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051601

Abstract:
A nonhomogeneous model is used to simulate convective transport in nanofluids. The model is a thermodynamically consistent version of the celebrated Buongiorno model. We study two situations in detail: flow through a pipe that is heated periodically in time at one lateral wall and a lid-driven cavity with a triangular heat source placed within. Both studies reveal the mechanisms of enhanced heat transfer by nanofluids through thermophoresis: the temperature gradient at the wall leads to a reduced concentration of nanoparticles. This reduces the concentration-dependent viscosity of the suspension close to the boundary, which in turn leads to a stronger convective transport.
Published: 11 October 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052154

Abstract:
A parametric analysis has been conducted for the phase change material (PCM)-air cooled battery pack. The system is composed of 26650 lithium-ion LiFePO4 batteries enclosed by PCM. A one-dimensional thermal model for the PCM domain is developed using the enthalpy method. The finite volume method is employed to solve the energy equation for both cell and PCM domain. The developed computational algorithm has been validated as a result of the simulations for the same conditions with the literature. The discharge process of the batteries has been investigated for 2C, 3C, and 5C rates. Thermal analyses have been performed for passive (natural convection) and active cooling (forced convection). It is aimed to keep the temperature of the battery cell under critical levels. A parametric investigation for crucial parameters like PCM layer thickness, the thermal conductivity of the PCM, arrangement of the batteries has been performed. Simulations have been conducted for the constant air velocity and the pumping power. Thanks to the constant pumping power analysis, thermally best-performing configuration have been sought by eliminating the hydrodynamic effect.
Published: 11 October 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052006

Abstract:
During the quenching process, the liquid bath is usually agitated to homogenize the temperature and to enhance convective heat transfer. The purpose of this paper is to characterize on the one hand the agitation of a water bath due to the movement of a three-blade turbine and on the other, the cooling of an Inconel 718 part being quenched in a stirred water bath. Velocity measurements were taken by particle image velocimetry (PIV) with and without the metallic part. We found that the velocity field became purely axial when we were far enough away from the turbine. Moreover, a high turbulent mixing level was shown for this type of jet. Velocity measurements were carried out for two agitation intensities. The axial velocity amplitude, as well as the turbulent kinetic energy, decreased dramatically as the rotational speed of the propeller decreased from 410 to 100 rpm. This caused the thermal behavior of the part to differ during quenching. Indeed, we found that the part cooled faster under stronger agitation. During the film boiling and transition phases, no appreciable effect of agitation could be observed. However, from the middle of the nucleate boiling phase, the part-bath heat transfer coefficient was found to decrease much less rapidly with the surface temperature if agitation was strong than if it was weak or if the bath was completely calm. In such a case of strong agitation, both nucleate boiling and convection concomitantly ensure part cooling.
Yuhao Lin, , Jia Sun, , Yanlong Cao
Published: 11 October 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052434

Abstract:
The combination of microstructured surface and microchannel flow boiling is expected to solve the thermal management problems of high-heat-flux devices. In this study, the experimental investigation of subcooled flow boiling in a high aspect ratio, one-sided heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods were synthesized on the titanium surface to be used as the heated surface compared with the bare titanium surface. A facile image tool is utilized to process the flow patterns photographed by a high-speed camera, which is analyzed with the heat transfer characteristics. The flow pattern of isolated bubbly flow reveals the large number of nucleation sites formed on the microrod surface but the heat transfer performance deteriorates with increasing mass flux because of the smaller bubble area and weaker nucleation. With increasing heat flux, the flow pattern changes from isolated bubbly flow to alternating bubbly/slug flow and alternating slug/annular flow. The latter flow pattern is confirmed to bring a higher heat transfer coefficient due to the larger area of thin-film evaporation. Compared with the bare surface, a higher heat transfer coefficient is achieved on the ZnO microrod surface for up to 37% due to the more nucleate sites and strengthened convective evaporation. Therefore, this surface might be suitable for heat dissipation in the watercraft or aerospace industry considering the low density, strong intensity, and corrosion resistance of titanium.
Published: 7 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052655

Abstract:
The Fourier and the hyperbolic heat conduction equations were solved numerically to simulate a frequency-domain thermoreflectance (FDTR) experiment. Numerical solutions enable isolation of pump and probe laser spot size effects, and use of realistic boundary conditions. The equations were solved in time domain and the phase lag between the temperature of the transducer (averaged over the probe laser spot) and the modulated pump laser signal, were computed for a modulation frequency range of 200 kHz to 200 MHz. Numerical calculations showed that extracted values of the thermal conductivity are sensitive to both the pump and probe laser spot sizes, while analytical solutions (based on Hankel transform) cannot isolate the two effects, although for the same effective (combined) spot size, the two solutions are found to be in excellent agreement. If the substrate (computational domain) is sufficiently large, the far-field boundary conditions were found to have no effect on the computed phase lag. The interface conductance between the transducer and the substrate was found to have some effect on the extracted thermal conductivity. The hyperbolic heat conduction equation yielded almost the same results as the Fourier heat conduction equation for the particular case studied. The numerically extracted thermal conductivity value (best fit) for the silicon substrate considered in this study was found to be about 82-108 W/m/K, depending on the pump and probe laser spot sizes used.
Published: 7 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052657

Abstract:
This article provides the author's perspectives on the current molecular-level understanding of thermophysical properties and transport processes in liquids. After illustrating peculiarities of the thermophysical properties of some common liquids using experimental data on their specific heat, thermal conductivity and viscosity, the article moves on with a summary of existing molecular pictures and theoretical approaches on liquids, followed with elaborations on different models developed for the specific heat, thermal conductivity, and viscosity. The review shows that current understanding of thermophysical properties of liquids is still poor and theoretical tools to study them are not well developed. The article provides personal views of the author on what are missing in current theories and underlying mechanisms for some experimental observations, and includes some suggestions for potential directions of future research.
Published: 7 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052656

Abstract:
2-D numerical experiments are performed to investigate the flow instabilities and mixing of different non-isothermal counterflowing jets in a Passive-Mixer. The fluid is modelled as a binary mixture with thermal and solutal buoyancy effects considered through the Boussinesq approximation. The streams are arranged in a thermal and solutal buoyancy aiding configuration. Computations are carried out for three different ratios of the upper jet bulk velocity to the lower jet bulk velocity (VR), namely, VR = 0.5, 1.0 and 2. Within the parametric domain of RiT and RiC defined by region (RiT + RiC) = 3, the instability causing transition from steady to unsteady flow regime is observed for VR = 1 and 2 while no transition is found to occur at VR = 0.5. Using Landau theory, it is established that the transition from steady to unsteady flow regime is a supercritical Hopf bifurcation. A complete regime map identifying the steady and unsteady flow regimes, within the parametric space of the present study, is obtained by plotting the neutral curves of RiC and RiT (obtained using Landau theory) for different values of VR. POD analysis of the unsteady flows at VR = 1, establishes the presence of standing waves. However, for VR = 2, the presence of degenerate pairs in the POD eigenspectrum ascertains the presence of travelling waves in the unsteady flows. The standing wave unsteady flow mode is found to yield the highest rate of mixing.
Babak Mosavati, Maziar Mosavati
Published: 1 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052606

Abstract:
The maintenance of uniform temperature distribution affects the efficiency in the most industrial applications. In the current study, a novel strategy has been developed for inverse radiative boundary design problems in radiant enclosures. This study presents the Backward Monte Carlo method to investigate the inverse boundary design of an enclosure composed of specular and diffuse surfaces. A new optimized Monte Carlo method is proposed to determine the temperature distribution of heaters to achieve desirable prescribed uniform heat flux on the design surfaces. The proposed approach is highly efficient and simple to implement with appropriate results. The evaluated heat fluxes on design surfaces and temperature distribution of heaters are compared with the case where the reradiating walls are assumed to be perfectly diffuse. In the proposed approach, for a specific range of specularity, the absorptivity of the reradiating surfaces does not affect the temperature distribution of heaters. Compared to the diffuse walls, the specular walls have more uniform temperature distribution and heat flux of heaters. This finding will provide insight into solar furnaces design to enhance temperature uniformity, making specular surfaces suitable in many industrial applications.
Published: 1 October 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052607

Abstract:
The performance of air-cooled generators can be improved only if they have efficient system designs for heat removal. An air-cooled generator is composed of a pair of coaxial cylinders, namely, a fixed outer cylinder (stator) and a rotating inner cylinder (rotor); the rotor has axial slits. In this study, we experimentally and numerically clarified the flow behavior and the heat transfer characteristics of rotating coaxial cylinders by simulating a salient-pole rotor in an air-cooled generator. The flow behavior in the slit between the salient poles was observed by using a high-speed video camera. We measured the temperature on the slit walls to investigate the heat transfer. The velocity fields and the heat transfer coefficient between the rotor and the stator were obtained via a numerical simulation. From the results, we experimentally and numerically observed the vortex structure in the slit. The local Nusselt numbers on the front-side wall of the slits near the impinging flow were higher than those on the back-side wall near the separated flow. The local Nusselt numbers on the front-side wall were high because the gap flow between the cylinders impinged on the front-side wall and promoted heat transfer. By contrast, the local Nusselt numbers on the back-side wall were low because a separated flow appeared near the back-side wall, where the hot fluid was retained, thereby causing the separated flow to disturb the heat transfer on the back-side wall.
Published: 30 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052581

Abstract:
The dry handling of bottom ash from coal-fired power plants has become more and more important in recent years, e.g. due to a lack of water availability at the location of power plants, or for environmental reasons. Thereby it is crucial that a sufficient cooling of the bottom ash can be ensured by the dry cooling air. Within this work, a numerical model for the assessment of heat transfer processes in dry ash conveyors is developed and implemented into Wolfram Mathematica. The model uses a newly introduced representative geometric quantity for the ash particle geometry. Moreover, in addition to the ash, the cooling air is considered as an own phase, for which a temperature solution is obtained. A numerical example, considering geometrical and operational data of an existing facility, shows that the main heat transfer between the ash and the cooling air takes place in the ash hopper, whereby convective heat transfer from ash to cooling air outweighs the effects from coke combustion and radiation from the boiler outlet area. The convective heat transfer in the ash hopper predominantly depends on the geometrical appearance, i.e. size and shape, of the particles, as well as on the grain density, and on the falling time/velocity. Conservatism of the calculation approach is indicated based on comparison of computed temperatures with measured data and literature values. The derived model can be used in future designs and projections of dry ash handling systems.
Published: 30 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052582

Abstract:
In this work, a tube with internal helical fins is analyzed and optimized from an entropy generation point of view. Helical fins, in addition to providing heat transfer enhancements, have the potential to level the temperature of the tube under non-uniform circumferential heating. The geometric parameters of helical fins are optimized under two different entropy-based formulations. Specifically, this work focuses on comparing the optimal design solution obtained through the minimization of total entropy and through the multiobjective optimization of the thermal and viscous entropy contributions when considered as two separate objectives. The latter quantities being associated with heat transfer and pressure drops, it is shown that, from a design optimization point of view, it is important to separate both entropies which are conflicting objectives.
Published: 30 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052580

Abstract:
The stability of nanofluid flow in a vertical channel packed with a porous medium is examined for the local thermal non-equilibrium state of the fluid, particle and solid-matrix phases. The effects of Brownian motion along with thermophoresis are incorporated in the nanofluid model. The Darcy-Brinkman model for the flow in a porous medium and three-field model, each representing the fluid, particle and solid-matrix phases separately, for temperature is used. A normal mode analysis is used to obtain the eigenvalue problem for the perturbed state, which is then solved using the Chebyshev spectral collocation technique. The critical Rayleigh number and corresponding wavenumber are presented graphically for the effect of different local thermal non-equilibrium parameters. It is noticed that the influence of LTNE parameters on the convective instability is significant.
Joseph Kangas, Li Zhan, , Harishankar Natesan, Kanav Khosla,
Published: 29 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052568

Abstract:
Cryoprotective agents (CPAs) are routinely used to vitrify, attain an amorphous glass state void of crystallization, and thereby cryopreserve biomaterials. Two vital characteristics of a CPA loaded system are the critical cooling and warming rates (CCR and CWR), the temperature rates needed to achieve and return from a vitrified state respectively. Due to the toxicity associated with CPAs, it is often desirable to use the lowest concentrations possible, driving up CWR and making it increasingly difficult to measure. This paper describes a novel method for assessing CWR between the 0.4×105-107 °C/min in microliter CPA loaded droplet systems with a new ultra-rapid laser calorimetric approach. Cooling was achieved by direct quenching in liquid nitrogen, while warming was achieved by the irradiation of plasmonic gold nanoparticle-loaded vitrified droplets by a high-power 1064 nm millisecond pulsed laser. We assume "apparent" vitrification is achieved provided ice is not visually apparent (i.e. opacity) upon imaging with a camera during cooling or highspeed camera during warming. Using this approach, we were able to investigate CWR in single CPA systems such as glycerol, PG, and Trehalose in water, and mixtures of glycerol-trehalose-water and propylene glycol-trehalose-water CPA at low concentrations (20-40 wt%). Further, an phenomenological model for determining the CCRs and CWRs of CPA was developed which allowed for predictions of CCR or CWR of single component CPA and mixtures, providing an avenue for optimizing CCR and CWR and perhaps future CPA cocktail discovery.
Published: 24 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052508

Abstract:
Spray ponds offer significant advantages over mechanical draft cooling towers including superior simplicity and operability, lower preferred power requirements, and lower costs. Unlike a conventional spray pond in which spray nozzles are arranged in a flat bed and water is sprayed upward, the Oriented Spray Cooling System (OSCS) is an evolutionary spray pond design in which nozzles are mounted on spray trees arranged in a circle and are tilted at an angle oriented towards the center of the circle. Therefore, each nozzle is exposed to essentially ambient air as water droplets drag air into the spray region while the warm air concentrated in the center of the circle rises. Both of these effects work together to increase air flow through the spray region. Increased air flow reduces the local wet-bulb temperature of the air in the spray pattern, promoting heat transfer and more efficient cooling. The authors have developed analytical models to predict the thermal performance of the OSCS that are based on classical heat and mass transfer and kinetic vector relationships for spherical water droplets that rely only on generic experimental thermal performance data. The model is not limited in application with regard to spray pressure or nozzle spacing or orientation and is not limited by droplet size considerations. The paper compares the predicted performance of the OSCS with full-scale field test results that were measured in compliance with Nuclear Regulatory Commission requirements at the Columbia Generating Station where the ultimate heat sink is two OSCS.
Published: 24 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052509

Abstract:
This experimental study examines the use of planar laser Rayleigh scattering to measure instantaneous gas temperature distributions at different heights above the surface of an effusion cooled plate. An experimental test rig was used to model combustor conditions with a bulk crossflow temperature of 1500 K. Carbon dioxide was used as coolant at multiple blowing ratios ranging from 1.12 to 11.1. A "temperature-pegging" methodology was used to process Rayleigh light scattering images to create high resolution and accurate temperature images at heights of 2, 2.75, and 3.5 mm above the surface of a prototypical effusion plate. Measured temperature distributions were used to calculate root mean square (RMS) distributions, and were also converted to film effectiveness maps based on the upstream crossflow gas and effusion coolant temperatures. It is found that film cooling region spreads upstream with increasing effusion jet blowing ratio parameter. The root mean square (RMS) deviation of gas temperatures over each measurement plane show that the RMS fluctuations are low inside and outside the effusion film, but are high near the film edge. At a given height above the effusion panel, the RMS fluctuations decrease in the film region with increasing blowing ratio. Film effectiveness follows similar trends with high film effectiveness region expanding with increasing effusion jet blowing ratios.
Published: 24 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052510

Abstract:
Machine learning (ML) offers a variety of techniques to understand many complex problems in different fields. The field of heat transfer, and thermal systems in general, are governed by complicated sets of governing physics that can be made tractable by reduced-order modeling, and by extracting simple trends from measured data. Therefore, ML algorithms can yield computationally efficient models for more accurate predictions or to generate robust optimization frameworks. This study reviews past and present efforts that use ML techniques in heat transfer from the fundamental level to full-scale applications, including the use of ML to build reduced-order models, predict heat transfer coefficients and pressure drop, real-time analysis of complex experimental data, and optimize large-scale thermal systems in a variety of applications. The appropriateness of different data-driven ML models in heat transfer problems is discussed. Finally, some of the imminent opportunities and challenges that the heat transfer community faces in this exciting and rapidly growing field are identified.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052153

Abstract:
Pool boiling around a heated cylinder having a diameter larger than the departure diameter of bubbles has been simulated numerically. Thermally uniform heat flux condition has been maintained at the outer surface of the cylinder, submerged at saturated water at atmospheric pressure. Using the volume of fluid type framework of liquid phase fraction in the domain, bubble life cycle around the horizontal cylinder has been analyzed to understand different stages of growth, sliding, merging prior to departure. An effort has also been made to characterize the bubble population, emerging from different sites over the cylindrical surface. The influence of cylinder inclination along its axis on these interfacial features has also been discussed using representative numerical simulation. Temperature profiles of the cylinder surface have been portrayed for both horizontal and inclined situations before presenting respective heat transfer coefficients.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052155

Abstract:
This article proposes the closed-form solution of the generalized non-Fourier model-based bioheat transfer equation (BHTE) in Cylindrical coordinates to understand the thermal behavior of living tissue heated by a pulsed laser. The axisymmetric living tissue exposed to the non-Gaussian temporal profile of laser heating has been considered to investigate the non-Fourier bioheat transfer phenomena. The closed-form solution of the generalized non-Fourier model-based BHTE with time-dependent thermal energy generation has been obtained through the finite integral transform (FIT) technique. The analytical solution was juxtaposed to the corresponding numerical solution in order to determine its reliability. The numerical solution of the aforementioned governing equation has been obtained by the finite volume method (FVM). The results of both analytical and numerical solutions have been verified using results given in published literature. Subsequently, the dual-phase-lag (DPL) model's findings were juxtaposed to those obtained using the hyperbolic and traditional Fourier models. The effect of different parameters like relaxation times corresponding to the temperature gradient and heat flux, metabolic energy generation, and blood perfusion on the resultant temperature distribution inside the axisymmetric living tissue exposed to pulsed laser heating has been discussed. The importance of this study might be found in various applications such as laser-based-photothermal therapy, melting of the surface of metal and alloys by laser heating.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051531

Abstract:
In honor of Professor John H. Lienhard's 90th Birthday his former students, colleagues, and friends join in wishing him a belated happy birthday.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052117

Abstract:
Thermofluid dynamics of an unconfined steady two-dimensional laminar jet impinging on an isothermal protruded heater is numerically studied for low jet inlet Reynolds number (Re) between 50 and 250. Results are shown for a range of impingement distances (h/W) between 1 and 10 for Prandtl numbers (Pr) 0.71 and 7.56. The volumetric entrainment increases with increasing h/w and decreasing Re. The reattachment distance of the wall jet appears to increase with Re and shows discernible deviation from the backward-facing step flow prediction for Re>150. Correlations are presented for average heater surface and sidewall Nusselt numbers as functions of Re and h/w for Pr=0.71 and Pr=7.56. In an overall convection dominant heat transfer, a relatively warmer and diffusion-dominated recirculation zone is identified adjacent to the sidewall with a low Nusselt number, which enhances significantly at Pr=7.56 when Re is increased above 100. At a low impingement distance, integrated kinetic energy flux shows greater magnitude in the impingement region but with a higher rate of decay. The integrated heat flux is greatly influenced by Re, and the effect is more pronounced at Pr=0.71. Self-similar behavior is observed for the velocity and heat flux profiles throughout the length in the developed region and for the temperature distribution over the heater surface. Both high Re and high h/w seem to adversely affect the self-similar behavior owing to a slower wall jet development.
Pedduri Jayakrishna, Ananda S. Vaka, Saurav Chakraborty, ,
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052204

Abstract:
An inverse heat transfer model based on Salp Swarm optimization algorithm is developed for prediction of heat flux at the hot faces of a mold in thin slab continuous casting. The industrial mold considered in this work is a funnel-shaped mold having complex arrangement of cooling slots and holes. Significant variations of heat flux along the casting direction, as well as across the width are observed. Subsequently, the obtained heat flux profile estimated by the inverse method is used to analyze the fluid flow and thermal characteristics of the solidifying steel strand inside the mold. Three different recirculatory zones are present due to molten steel flow, affecting the thermal and solidification characteristics significantly. The effect of these recirculatory flows on remelting phenomenon, and consequent formation of thinner shell at the mold outlet leading to quality control issues in the casting process have been discussed. Another practical issue of depression in the wide face shell thickness at the mold outlet has been identified, and its cause has been related to the location of the submerged entry nozzle and the high speed of the molten steel inflow.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052080

Abstract:
Harnessing concentrated high-flux solar energy to drive thermal processes over 1000 °C for fuel production and material processing has great potential to address environmental issues associated with fossil fuels. There is now also interest in solar thermal processing under extraterrestrial (e.g., lunar) conditions, which has the potential to provide materials and power for future space exploration and base construction with local resources as feedstock. In this review article, the recent progress on conventional solar thermochemical systems used for lunar production is reviewed. Important results are discussed to identify the applicability of existing devices and models at lunar conditions. Finally, the challenges ahead and promising directions are presented.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052116

Abstract:
The fabrication of porous metallic composite coating on the heating surface to improve pool boiling heat transfer (BHT) performance has received significant attention in recent years. In this work, Cu–GNPs nanocomposite coatings, which were prepared on a copper substrate using various current densities through a two-step electrodeposition technique, were used as heating surfaces to study the pool BHT performance of refrigerant R-134a. The surface morphology, elemental composition, thickness, surface roughness, and porosity of prepared Cu–GNPs nanocomposite coatings are studied and presented in detail. All Cu–GNPs nanocomposite coated surfaces exhibited improved boiling performance compared to the plain Cu surface. The heat transfer coefficient (HTC) values for Cu–GNPs nanocomposite coated Cu surfaces prepared at 0.1, 0.2, 0.3, and 0.4 A/cm2 were improved up to 1.48, 1.67, 1.82, and 1.97, respectively, compared with the plain Cu surface. The enhancement in the HTC is mainly associated with the increase in surface roughness, active nucleation site density, and micro/nanoporosity of the heating surface.
Published: 22 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052197

Abstract:
Optimization of thermal performance processes using genetic algorithm (GA) combined with some commercial software or other soft computing methods like artificial neural networks are common in many heat transfer applications with the exception of battery thermal management. In this article, a novel and innovative approach for single-objective optimization using GA combined with in-house developed finite volume method (FVM)-based code is investigated. Three important thermal and fluid flow performance parameters of modern electric vehicle Lithium–ion battery cells, namely, average Nusselt number (Nuavg), friction coefficient (Cf,avg), and maximum temperature (T¯max) are optimized. The operating parameters considered for optimization include heat generation term (S¯q), Reynolds number (Re), conduction-convection parameter (ζcc), aspect ratio (Ar), and spacing between the cells (W¯ff) varying in some selected range. Optimization in case of internal flow between the battery cells and external flow over the battery cell is performed. Computational time taken by the combined GA and FVM code for 5, 10, 15, and 20 iterations in case of internal and external flow is also presented. From the complete optimization analysis, it is found that for higher charging/discharging rates at which the heat generation is very high, T¯max can be kept within the safe limit, Nuavg to maximum and Cf,avg to a minimum with a slight compromise in pumping power requirement to circulate the coolant in internal flow. For external flow analysis, Re and ζcc in a selected medium range will provide optimized thermal and fluid flow situations.
Dasith Liyanage, , , Abheek Basu, Madeleine Du Toit,
Published: 17 September 2021
Journal of Heat Transfer; https://doi.org/10.1115/1.4052436

Abstract:
High-temperature laser-scanning confocal microscopy (HT-LSCM) has proven to be an excellent experimental technique through in-situ observations of high temperature phase transformation to study kinetics and morphology using thin disk steel specimens. A 1.0 kW halogen lamp, within the elliptical cavity of the HT-LSCM furnace radiates heat and imposes a non-linear temperature profile across the radius of the steel sample. This local temperature profile when exposed at the solid/liquid interface determines the kinetics of solidification and phase transformation morphology. A two-dimensional numerical heat transfer model for both isothermal and transient conditions is developed for a concentrically solidifying sample. The model can accommodate solid/liquid interface velocity as an input parameter under concentric solidification with cooling rates up to 100 K/min. The model is validated against a commercial finite element analysis software package, Strand7, and optimized with experimental data obtained under near-to equilibrium conditions. The validated model can then be used to define the temperature landscape under transient heat transfer conditions.
Published: 10 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051764

Abstract:
This paper considers the three-phase lag (TPL) bioheat model to study the phase change phenomena in skin tissue during cryosurgery. The considered TPL model is based on the model of thermo-elasticity, i.e., the combination of the rate of thermal conductivity and new phase lag (τv) due to thermal displacement. An effective heat capacity-based numerical algorithm is established to solve the nonlinear governing equation for biological tissue freezing. Space and time derivatives appearing in the mathematical model are approximated using the radial basis function (RBF) and finite difference method (FDM), respectively. The impact of three nonclassical models, single-phase lag (SPL), dual-phase lag (DPL), and TPL, on the freezing process is studied. The effects of phase lags involved in the models on freezing are also part of this study.
Published: 10 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051882

Abstract:
Lasers and laser heating have a wide variety of applications such as spectroscopy, laser welding, laser cutting, and even biological applications like tumor irradiation and surgery. Theoretical modeling of laser heating has proven to be quite difficult, and classical heating equations have shown to be inaccurate due to the large temperature gradients created by the laser heating. Furthermore, the commonly used Fourier's Law assumed the speed for a thermal wave to propagate as infinite; this is unrealistic in any medium and especially in domains with slow propagation speeds such as biological media and in fast nano/microscale heating applications. This study helps fill some of the gaps in accurate model of laser heating by presenting unique 1D and 2D models of the analytically solved Dual-Phase-Lag heating equations which can much more accurately describe the temperature of such interactions in both the temporal and spatial domains.
Published: 10 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052198

Abstract:
Mechanistic models developed to predict partial nucleate boiling are not adequate for fully developed nucleate boiling due to differences in the prevailing heat transfer governing mechanisms. In place of the mechanistic model, several empirical correlations and semimechanistic models have been proposed over the years for the prediction of fully developed nucleate boiling as presented in this study but they are unsuitable for use in computational fluid dynamics (CFD) code. Recently, the simulation of fully developed nucleate boiling has become much more practical because of advancement in a computational method that involves the coupling of the interface capturing method (for slug bubbles) with the Eulerian multifluid model (for dispersed spherical bubbles). Nonetheless, there is a need for a mechanistic closure law for the fully developed nucleate boiling phenomenon that would complement this advancement in CFD. Toward this end, a mechanistic wall heat flux partitioning model for fully developed nucleate boiling is proposed in this study. This model is predicated on the hypothesis that a high heat flux nucleate boiling is distinguished by the existence of a liquid macrolayer between the heated wall and the slug or elongated bubbles, and that the macrolayer is interspersed with numerous high frequency nucleate small bubbles. With this hypothesis, the heat flux generated on the heated wall is partitioned into two parts: conduction heat transfer across the macrolayer liquid film thickness and evaporation heat flux of the microlayer of the nucleating small bubbles. The proposed model is validated against experimental data.
, W. H. Azmi
Published: 10 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051763

Abstract:
The dispersion of nanoparticles in conventional heat transfer fluids has been proven to improve the performance of the fluids. However, the study on the heat transfer performance of hybrid nanofluids in the mixture of water and green bioglycol (BG) is limited in the literature. This paper presents the heat transfer performance and friction factor of green BG-based TiO2–SiO2 nanofluids. The TiO2 and SiO2 nanoparticles were dispersed in the mixture of 60:40 water: bioglycol (W/BG) and prepared at various concentrations up to 2.5% and composition ratios of 20:80. The experimental study on forced convection heat transfer was done under turbulent flow at constant heat flux for operating temperature of 70 °C. The heat transfer enhancement increased significantly with volume concentrations. The maximum heat transfer enhancements of the TiO2–SiO2 nanofluids at bulk temperature of 70 °C were observed to be up to 67.81% for 2.5% volume concentration. A slight friction factor escalation of the nanofluids was observed with 12% maximum increment. New correlations were developed to estimate the Nusselt number, and friction factor with average deviations of less than 4.3%. As a conclusion, the employment of the ecofriendly coolant nanofluids in improving thermal performance is proven and applicable for turbulent forced convection heat transfer applications.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051980

Abstract:
In this contribution in honor of the late Prof. E. M. Sparrow, we reflect on the pioneering work of Sparrow and Gregg on the development of similarity solutions for laminar film condensation on a vertical plate. Dhir and Lienhard, using this work as a basis, developed a generalized solution for isothermal curved surfaces on which gravitational acceleration varied along the surface and for variable gravity situations. Subsequently, nonisothermal surfaces were also considered. These studies were publisher earlier in the Journal of Heat Transfer.
, Faezeh Masooomi, Philipp Schimmels, , James Klausner,
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052081

Abstract:
Pelletized magnesium manganese oxide shows promise for high temperature thermochemical energy storage. It can be thermally reduced in the temperature range between 1250 °C and 1500 °C and re-oxidized with air at typical gas-turbine inlet pressures (1–25 bar) in the temperature range between 600 °C and 1500 °C. The combined thermal and chemical volumetric energy density is approximately 2300 MJ/m3. The rate at which a thermochemical storage module can be charged is limited by heat transfer inside the solid packed bed. Hence, the effective thermal conductivity of packed beds of magnesium-manganese oxide pellets is a crucial parameter for engineering Mg-Mn-O redox storage devices. We have measured the effective thermal conductivity of a packed bed of 3.66 ± 0.516 mm sized magnesium manganese oxide (Mn to Mg molar ratio of 1:1) pellets in the temperature range of 300–1400 °C. Since the material is electrically conductive at temperatures above 600 °C, the sheathed transient hot wire method is used for measurements. Raw data is analyzed using the Blackwell solution to extract the bed thermal conductivity. The effective thermal conductivity standard deviation is less than 10% for a minimum of three repeat measurements at each temperature. Experimental results show an increase in the effective thermal conductivity with temperature from 0.50 W/m °C around 300 °C to 1.81 W/m °C close to 1400 °C. We propose a dual porosity model to express the effective thermal conductivity as a function of temperature. This model also considers the effect of radiation within the bed, as this is the dominant heat transfer mode at high temperatures. The proposed model accounts for microscale pellet porosity, macroscale bed porosity, pellet size, solid thermal conductivity (phonon transport), and radiation (photon transport). The coefficient of determination between the proposed model and the experimental results is greater than 0.90.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4052084

Abstract:
The passing of Professor Ephraim M. Sparrow on August 1, 2019 left a large void in the worldwide heat and mass transfer community. To many in the field, he was an intellectual giant, who famously was the "most cited" mechanical engineering professor in the Science Citation Index for many decades. Many scholars, young and old, have encountered Professor Sparrow's multitude of works over a wide range of topics in standard academic textbooks, reference books, and eminent heat transfer journals. To his colleagues and students, 'Eph' was more than a giant in the field of heat transfer, he was an incredible mentor and a lifelong friend who was passionate about the multifaceted area of thermal science and never stopped wanting to learn. To celebrate Eph's multitude of contributions, a memorial symposium was held at the University of Minnesota, where he had spent his entire professional academic career.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051322

Abstract:
The thermoconvective instability of the parallel vertical flow in a fluid saturated porous layer bounded by parallel open boundaries is studied. The open boundaries are assumed to be kept at constant uniform pressure while their temperatures are uniform and different, thus forcing a horizontal temperature gradient across the layer. The anisotropic permeability of the porous layer is accounted for by assuming the principal axes to be oriented along the directions perpendicular and parallel to the layer boundaries. A linear stability analysis based on the Fourier normal modes of perturbation is carried out by testing the effect of the inclination of the normal mode wavevector to the vertical. The neutral stability curves and the critical Rayleigh number for the onset of the instability are evaluated by solving numerically the stability eigenvalue problem.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051465

Abstract:
Since their introduction in the early 1990s, pulsating heat pipes (PHPs) have received a lot of attention due to their obvious advantages such as their geometrical simplicity, and their potential for high-heat flux applications even without power consumption. Although numerous investigators have studied PHPs over the last three decades, there still exist a few controversial issues on fundamental characteristics and several technical problems in practical applications. To put the finishing touches to the controversial issues and to shed light on the technical problems, recent advances in PHPs are critically reviewed in this paper. The results of this critical review are classified into two categories: (i) fundamental aspects of PHPs and (ii) practical aspects of PHPs. The first category focuses on reviewing the current state-of-the-art fundamental characteristics of PHPs. The second category summarizes the technical problems that are resolved for utilizing PHPs in practical applications. This review paper would help researchers or engineers who are working on or utilizing PHPs.
, Vladimir Solovjov
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051499

Abstract:
The influence of real gas radiation on the thermal and hydrodynamic stability of a two-dimensional layer of radiatively participating H2O and/or CO2 heated from below is investigated. The nongray radiation effects of the two species are treated rigorously using a global spectral approach, the Spectral Line Weighted-sum-of-gray-gases model. The phenomena are explored by solving the full coupled laminar equations of motion, energy, and radiative transfer from the low-Rayleigh number, pure conduction-radiation regime through the onset of buoyancy-induced flow to the developed Bénard convection regime. The evolution of the thermal, hydrodynamic, and radiative heating fields is studied, and the critical Rayleigh number is characterized as a function of participating gas species mole fraction, average layer gas temperature, layer thickness, wall emissivity, and total pressure. It is found that participating radiation in the medium has the effect of stabilizing the layer, delaying transition from a stable conduction regime to buoyancy-induced flow. The development of convective flow and temperature, along with the radiative heating are presented. It is found that the critical Rayleigh number in the radiatively participating gas layer can be more than an order of magnitude higher than the classical convection-only scenario. The onset of instability is found to depend on the species mole fractions, average gas temperature in the layer, wall emissivity, layer thickness, and total pressure. Generally, all other variables being the same, H2O has a greater stabilizing influence on the layer than CO2.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051999

Abstract:
To improve the reliability and efficiency of power electronics, their thermal management must be further enhanced. Next-generation electronics systems are predicted to dissipate more heat as die size shrinks and power levels increase. Traditional air-cooling approaches usually provide insufficient performance or require heavy and bulky heat sinks to achieve adequate thermal management. To address this problem, a novel air cooled vertically enhanced manifold microchannel system (VEMMS) was developed. While minimizing the footprint required on the printed circuit board, the system offers efficient thermal management in a conformal scheme that accommodates the associated power electronics and their electrical connections. This work describes the manufacturing process of the air-cooled VEMMS heat sink and its experimental characterization and thermo-fluidic performance. Good agreement was obtained between the test results and numerical predictions. Using air at ambient conditions, thermal resistance of 2.6 K/W was achieved with a single-sided cooling architecture with a <1.5 cm2 footprint and <2 cm3 total heat sink volume. A full-bridge electrical power density of ∼84 kWe/L and overall direct current (DC–DC) converter power density of ∼20 kWe/L were achieved at reasonable flow rates and pressure drops using commercially available miniature electric fans.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051785

Abstract:
Sustained ocular drug delivery systems are necessary for patients needing regular drug therapy since frequent injection is painful, undesirable, and risky. One type of sustained-release systems includes pellets loaded with the drug, encapsulated in a porous shell that can be injected into the vitreous humor. There the released drug diffuses while the physiological flow of water provides the convective transport. The fluid flow within the vitreous is described by Darcy's equations for the analytical model and Brinkman flow for the computational analysis while the drug transport is given by the classical convection–diffusion equation. Since the timescale for the drug depletion is quite large, for the analytical model, we consider the exterior surrounding the capsule to be quasi-steady and the interior is time dependent. In the vitreous, the fluid-flow process is relatively slow, and meaningful results can be obtained for small Peclet number whereby a perturbation analysis is possible. For an isolated capsule, with approximately uniform flow in the far field around it, the mass-transfer problem requires singular perturbation with inner and outer matching. The computational model, besides accommodating the ocular geometry, allows for a fully time-dependent mass-concentration solution and also admits moderate Peclet numbers. As expected, the release rate diminishes with time as the drug depletion lowers the driving potential. The predictive results are sufficient general for a range of capsule permeability values and are useful for the design of the sustained-release microspheres as to the requisite permeability for specific drugs.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051321

Abstract:
An experimental study of bubble growth from submerged orifice plates in pools of water is carried out to scale and correlate the effects of surface wettability and orifice diameter D0 on ebullience. Measurements of bubble growth on surfaces with nine different contact angles (38 deg ≤ θ ≤ 128 deg) and varying air flow rates (1–300 ml/min) were made using high speed videography and image processing. In the static or constant-volume regime, below a critical contact angle θc, the bubble base remains attached to the orifice, and the equivalent departure diameter Db is independent of θ. On the other hand, above the critical contact angle, the bubble base spreads on the surface resulting in larger Db. For θ > θc, Db is strongly dependent on θ and increases with it. Using minimum energy method, it is shown that the wettability effects can be scaled and correlated by a modified capillary length, defined as a function of the Laplace length and contact angle. The proposed correlation provides predictions of Db that agree with experimental data of this study as well as those available in the literature to within ±15%. Moreover, for a hydrophobic surface when D0 > twice the modified capillary length, the bubble grows inside the orifice; for a hydrophilic surface, this scales with twice the capillary length, and effect of θ is not seen.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051320

Abstract:
We develop a two-dimensional model for the transient diffusion of gas from the cavities in ridge-type structured surfaces to a quiescent liquid suspended above them in the Cassie state to predict the location of the liquid vapor-interface (meniscus) as a function of time. The transient diffusion equation is numerically solved by a Chebyshev collocation (spectral) method coupled to the Young–Laplace equation and the ideal gas law. We capture the effects of variable meniscus curvature and, subsequently, when applicable, movement of triple contact lines. Results are presented for the evolution of the dissolved gas concentration field in the liquid and, when applicable, the time it takes for a meniscus to depin and that for longevity, i.e., the onset of the Cassie to Wenzel state transition. Two configurations are examined; viz., one where an impermeable membrane pressurizes the liquid above the ridges and one where hydrostatic pressure is considered and the top of the liquid is exposed to noncondensible gas.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051323

Abstract:
Nanobubbles are typically classified as gas/vapor phase cavities in an aqueous solution with a characteristic length of approximately 100 nanometers (nm). The theoretical lifetime of these nanobubbles has been estimated to be less than ∼1 μs at a diameter of 100 nm based upon the Young-Laplace pressure, but experimental observations have been reported that indicate that they may exist for many hours, or even days. These nanobubbles can be generated by a number of different methods, such as solvent exchange, pressure and/or temperature variations, chemical reactions, or through the electron beam radiolysis of water. The imaging methods utilized to observe these nanobubbles have evolved from low temporal resolution/high spatial resolution, using atomic force microscopy (AFM); or low spatial resolution/high temporal resolution, using optical microscopy (X-rays); or finally, high spatial/high temporal resolution using more recent electron microscopy techniques. A review of the various methods utilized in the nucleation of nanobubbles and the different imaging technologies utilized, along with a summary of the most recent experimental and theoretical investigations of the dynamic behavior and processes of these nanobubbles, including nanobubble growth, nanobubble collapse, and nanobubble coalescence, are presented, discussed and summarized.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051056

Abstract:
Boiling heat transfer suffers deteriorations under subatmospheric conditions, which can be attributed to a shortage of viable nucleation sites at declining pressures. In this work, the possibility of enhancing low-pressure saturated boiling of water using a combination of wettability patterning and structural modifications was experimentally explored. The copper test surface, comprised of an array of circular “dimples” (0.3 mm in depth, 0.5 mm in diameter, and 3.0 mm in pitch), was spray-coated by polytetrafluoroethylene (PTFE) coatings so as to form a matching biphilic pattern with the surface cavities. The resulting dimpled biphilic surface showed appreciable heat transfer enhancement—with a maximum 60% increase of the average heat transfer coefficient of nucleate boiling compared with a flat biphilic surface—down to about 9.5 kPa. Further lowering the pressure to 7.8 kPa, however, was found to lead to diminished performance gains. The visualization study of the bubble departure dynamics revealed signs of additional vapor trapping of the hydrophobic-coated cavities, which can induce uninterrupted bubble regeneration with zero waiting time and explain the qualified enhancement of subatmospheric boiling. Thanks to a potential secondary pinning of contact line inside the hydrophobic cavities, incomplete bubble detachment could prevail at somewhat lower pressures than was otherwise possible without the dimple structure, leaving behind significantly more vapor residues. However, the vapor-trapping capacity was found to decrease with pressure, which provided clues with regard to the reduced efficacy of the surface at even lower pressures.
, , S. Vengadesan
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051507

Abstract:
Internal cooling of the gas turbine blade is critical for the durability of the blade material. One of the ways to accomplish this is by passing coolant through serpentine passages roughened with surface elements to enhance the heat transfer. In the present study, the traditional square rib (SQ-rib) placed normal to the flow direction is modified to a backward-facing step rib (BS-rib) and a forward-facing step rib (FS-rib). Large-eddy simulation (LES) is carried out for a square duct at Reb = 20000. Results show that the modified rib shapes result in substantial increase in heat transfer over the square rib with only a marginal increase in flow losses. The BS-rib shape produces the highest heat transfer augmentation followed by the FS-rib. The overall heat transfer augmentation for the BS-rib and FS-rib is 18% and 10% larger than the SQ-rib, respectively. Thermal-hydraulic performance is enhanced by 15%.
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051672

Abstract:
Chemical vapor deposition (CVD) is a widely used manufacturing process for obtaining thin films of materials like silicon, silicon carbide, graphene, and gallium nitride that are employed in the fabrication of electronic and optical devices. Gallium nitride (GaN) thin films are attractive materials for manufacturing optoelectronic device applications due to their wide band gap and superb optoelectronic performance. The reliability and durability of the devices depend on the quality of the thin films. The metal-organic chemical vapor deposition (MOCVD) process, which uses compounds that contain metals and organic ligands as precursors in a CVD reactor, is a common technique used to fabricate high-quality GaN thin films. The deposition rate and uniformity of thin films are critical to a successful and useful process. These are determined by the thermal transport processes and chemical reactions occurring in the reactor, and are manipulated by controlling the operating conditions and the reactor geometrical configuration. In this study, the epitaxial growth of GaN thin films on sapphire (Al2O3) substrates is carried out in two commercial MOCVD systems: a vertical rotating disk MOCVD reactor and a close-coupled showerhead MOCVD reactor. The surface morphology and crystal quality of GaN thin films have been examined using atomic force microscopy (AFM) and scanning electron microscope (SEM). This paper focuses on the composition of the precursor and the carrier gases since earlier studies have shown the importance of precursor composition. The results show that the flow rate of trimethylgallium (TMG), which is the main ingredient in the process, has a significant effect on the deposition rate and uniformity of the films. Also, the carrier gas plays an important role in deposition rate and uniformity. Using hydrogen as a carrier gas enhances the quality of the thin film but a lower deposition rate occurs on the wafer surface. On the other hand, a high flow rate of pure nitrogen gas improves the growth rate of the film. However, it decreases the uniformity of the film and promotes carbon contamination on the wafer surface. Thus, the use of an appropriate mixture of hydrogen and nitrogen as the carrier gas can improve the deposition rate and quality of GaN thin films.
Wenjing Ning, Jun Ma, Cheng Jiang, , ,
Published: 8 September 2021
Journal of Heat Transfer, Volume 143; https://doi.org/10.1115/1.4051784

Abstract:
The loop heat pipe (LHP) is a passive heat sink used in aerospace and electronic devices. As the core component of the LHP, the physical property parameters of porous wick directly affect the overall performance of the LHP. In this paper, the performance of the porous wick is improved by adjusting the pore size, thereby improving the performance of the LHP. The nickel-based double-pore porous wicks are prepared by T225 nickel powder and NaCl particles, and the pore size of the porous wicks can be changed by different cold pressing force (30 kN, 40 kN, 50 kN, and 60 kN). The effects of different cold pressing force on the porosity, permeability, and other physical property parameters are studied when the ratio of pore former is 20 wt.%. In the end, we select the cold pressing force of 30 kN to prepare the porous wick of the LHP. Then the effects of constant load and variable load of the heat transfer performance under different placement elevations are studied. The results show that the heat load range is 10 W–100 W, the minimum evaporator thermal resistance is 0.424 K/W, and the minimum LHP thermal resistance is 0.598 K/W. When β = 0 deg, there is a “backflow” phenomenon at the initial stage of low thermal load. With the increase of thermal load, the “backflow” duration decreases until it disappears, and the startup time becomes shorter. The thermal resistances of the evaporator and LHP decrease rapidly and then slowly increase. When β = –90 deg, the LHP appears to demonstrate “reverse startup” phenomenon.
Page of 216
Articles per Page
by
Show export options
  Select all
Back to Top Top