Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2021"

Physics-Informed Neural Networks for Heat Transfer Problems

Abstract: Physics-informed neural networks (PINNs) have gained popularity across different engineering fields due to their effectiveness in solving realistic problems with noisy data and often partially missing physics. In PINNs, automatic differentiation is leveraged to evaluate differential operators without discretization errors, and a multitask learning problem is defined in order to simultaneously fit observed data while respecting the underlying governing laws of physics. Here, we present applications of PINNs to various prototype heat transfer problems, targeting in particular realistic conditions not readily tackled with traditional computational methods. To this end, we first consider forced and mixed convection with unknown thermal boundary conditions on the heated surfaces and aim to obtain the temperature and velocity fields everywhere in the domain, including the boundaries, given some sparse temperature measurements. We also consider the prototype Stefan problem for two-phase flow, aiming to infer the moving interface, the velocity and temperature fields everywhere as well as the different conductivities of a solid and a liquid phase, given a few temperature measurements inside the domain. Finally, we present some realistic industrial applications related to power electronics to highlight the practicality of PINNs as well as the effective use of neural networks in solving general heat transfer problems of industrial complexity. Taken together, the results presented herein demonstrate that PINNs not only can solve ill-posed problems, which are beyond the reach of traditional computational methods, but they can also bridge the gap between computational and experimental heat transfer.

Bioconvection of a Radiating Hybrid Nanofluid Past a Thin Needle in the Presence of Heterogeneous–Homogeneous Chemical Reaction

Abstract: The photocatalytic nature of TiO2 finds applications in medicinal field to kill cancer cells, bacteria, and viruses under mild ultraviolet illumination and the antibacterial characteristic of Ag makes the composition Ag−TiO2 applicable for various purposes. It can also be used in other engineering appliances and industries such as humidity sensor, coolants, and in footwear industry. Hence, this study deals with the analysis of the effects of magnetic field, thermal radiation, and quartic autocatalysis of heterogeneous–homogeneous reaction in an electrically conducting Ag−TiO2−H2O hybrid nanofluid. Furthermore, the gyrotactic microorganisms are used as active mixers to prevent agglomeration and sedimentation of TiO2 that occurs due to its hydrophobic nature. The mathematical model takes the form of partial differential equations with viscosity and thermal conductivity being the functions of volume fraction. These equations are converted to ordinary differential equations by using similarity transformation and are solved by RKF-45 method with the aid of shooting method. It is observed that the increase in the size of the needle enhances the overall performance of the hybrid nanofluid. Furthermore, the temperature of the hybrid nanofluid increases with the increase in volume fraction. It is observed that the friction produced by the Lorentz force increases the temperature of the nanofluid. It is further observed that the heterogeneous reaction parameter has more significant effect on the concentration of bulk fluid than the homogeneous reaction parameter.

A Modern Review on Jet Impingement Heat Transfer Methods

Abstract: Jet impingement cooling is considered as one of the most effective heat transfer enhancement techniques. The primary mode of heat transfer enhancement is due to the flow stagnation. The effectiveness of jet impingement as a cooling technique is well documented; however, the application of jet impingement to different problems has been hindered by inability of manufacturing methods to incorporate impingement systems easily into cooling designs. Impingement heat transfer effectiveness can be further improved by improving the jet strength by modifying the jet holes, enhancing surface features, or adding swirl. With an increased usage of this cooling technique and additional modifications in geometry to further enhance the heat transfer capacity to suit different applications, this paper provides a much-needed review of the advancements in the effectiveness of this cooling technique. A comprehensive look at impingement cooling over a variety of modifications and applications with a focus on improved manufacturing techniques impacting novel design and implementation is provided for a variety of heat transfer enhancement applications.

Thermosolutal Marangoni Impact on Bioconvection in Suspension of Gyrotactic Microorganisms Over an Inclined Stretching Sheet

Abstract: Microorganism cells movement in the fluid is universal and affects many ecological and biological processes, including infection, reproduction, and marine life ecosystem. There are many biological and medical applications that require an understanding of the transport process in nanofluids containing a suspension of microorganism. The present problem deals with the bioconvection of Casson nanofluid containing a suspension of motile gyrotactic microorganisms over an inclined stretching sheet in the presence of thermal radiation, viscous dissipation, and chemical reaction and magnetic field. At the surface, the influence of the thermosolutal Marangoni convection and suction/injection impact are considered. The governing equations are solved numerically by using fourth-order Runge–Kutta–Fehlberg method with shooting technique. The impact of the major pertinent parameters on the velocity, temperature, nanoparticles concentration, and density of the motile microorganism is illustrated graphically. Finally, the correlations of various crucial parameters on skin friction, local Nusselt number, Sherwood number, and local motile microorganism density number are displayed through the graphs and tables.

Thermal Transport in Polymers: A Review

Abstract: In this article, we review thermal transport in polymers with different morphologies from aligned fibers to bulk amorphous states. We survey early and recent efforts in engineering polymers with high thermal conductivity by fabricating polymers with large-scale molecular alignments. The experimentally realized extremely high thermal conductivity of polymer nanofibers are highlighted, and understanding of thermal transport physics from molecular simulations are discussed. We then transition to the discussion of bulk amorphous polymers with an emphasize on the physics of thermal transport and its relation with the conformation of molecular chains in polymers. We also discuss the current understanding of how the chemistry of polymers would influence thermal transport in amorphous polymers and some limited, but important chemistry-structural-property relationships. Lastly, challenges, perspectives and outlook of this field are presented. We hope this review will inspire more fundamental and applied research in the polymer thermal transport field to advance scientific understanding and engineering applications.

Heat Transfer Enhancement Feature of the Non-Fourier Cattaneo-Christov Heat Flux Model

Abstract: Cattaneo-Christov heat flux model was proposed to remedy the weakness of the traditional Fourier heat flux model in order to maintain the finite travel time of heat. The literature is replete with numerical studies to understand the heat transfer enhancement property. The present effort is to provide a mathematical rigor and to analytically demonstrate why the new model should act towards cooling and thus, in the way of enhancing the heat transfer rate from the surfaces. The derived and presented formulae here prove this assertion through comparison with a few selected examples from the open literature.

Status, Challenges, and Potential for Machine Learning in Understanding and Applying Heat Transfer Phenomena

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.

New Correlations for Flow and Conjugate Heat Transfer With Surface Radiation Characteristics of a Real-Scale Infrared Suppression System With Conical Funnels

Abstract: The consistent and accurate prediction of fluid flow and heat transfer characteristics in an infrared suppression (IRS) device is challenging due to the complex nature of the flow features. The cool ambient air intake and subsequent mixing of hot exhaust gas from the engine in the cargo/naval ships are done inside the IRS system. The objective is to propose correlations for mass entrainment and outlet temperature for the IRS device with conical funnels. The mass intake rate and funnel exit temperature are determined by a set of relevant operating and geometric parameters, such as Reynolds number, nozzle exhaust temperature, the number of funnels, and funnel overlap. In this study, the funnel walls are conducting with finite wall thickness, and the surface radiation is taken into consideration. Numerical simulations are performed for the real-scale IRS unit by solving the mass, momentum, energy, and radiation equations in the computational domain surrounding the system. Nonlinear regression analysis of the data is carried out using the Levenberg and Marquest (L–M) method to achieve an empirical correlation of mass intake ratio and outlet temperature ratio. The proposed correlation for mass intake ratio is valid within ±6%, and that of outlet temperature is valid within ±5% of the numerical data. The valid ranges for correlations are 6×105≤ nozzle Reynolds number ≤3×106; 2 ≤ number of funnel ≤ 5; −0.325 ≤ funnel-overlapping height ≤ 0.25; and 1.33 ≤ nozzle exit temperature ≤ 2.

Heat Transfer Coefficient, Pressure Gradient, and Flow Patterns of R1234yf Evaporating in Microchannel Tube

Abstract: This paper presents the heat transfer coefficient, pressure gradient, and flow pattern of R1234yf in a microchannel tube. Both heat transfer coefficient and pressure gradient are presented against real saturation pressure, while flow pattern captures at the exit of data points are presented in the same plot. The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg m−2s−1, heat flux from 0 to 6 kW m−2, vapor quality from 0 to 1, and inlet saturation temperature from 10 to 30 °C. Comparing the correlations to the HTC measurements at very low quality (about 0.1), Gorenflo, D., and Kenning, D. (2010, Pool Boiling, in: VDI Heat Atlas, 2nd ed, Springer, pp. 757–788) agree with the results. As vapor quality increases, pressure gradient increases. The adiabatic pressure gradient is a strong function of mass flux and saturation pressure (temperature). Flow patterns of R1234yf are also affected by mass flux and saturation pressure. The heat transfer coefficient is a strong function of mass flux and heat flux. The saturation temperature has a smaller effect on HTC in the condition range (10 – 30 °C). Under the test range, the accelerating pressure drop is insignificant compared to friction. Comparing to the results, Mishima, K., and Hibiki, T. (1996, “Some Characteristics of Air-Water Two-Phase Flow in Small Diameter Vertical Tubes,” Int. J. Multiph. Flow, 22(4), pp. 703–712) and Muller-Steinhagen, H., and Heck, K. (1986, “A Simple Friction Pressure Drop Correlation for Two-Phase Flow in Pipes,” Accessed March 1, 2018)., 20, pp. 297–308.) have small mean absolute error (MAE) to predict local pressure gradient. For the heat transfer coefficient, Sun, L., and Mishima, K. (2009, “An Evaluation of Prediction Methods for Saturated Flow Boiling Heat Transfer in Mini-Channels,” Int. J. Heat Mass Transf, 52(23–24), pp. 5323–5329) and Gungor, K. E., and Winterton, R. H. S. (1986, “A General Correlation for Flow Boiling in Tubes and Annuli,” Int. J. Heat Mass Transf, 29(3), pp. 351–358) have an MAE less than 30%.

The Past and Future of the Monte Carlo Method in Thermal Radiation Transfer

Abstract: The history and progress in Monte Carlo methods applied to radiative energy transfer are reviewed, with emphasis on advances over the past 25 years. Unresolved issues are outlined, and comments are included about the outlook for the method as impacted by the advances in massively parallel and quantum computers.

On the Characterization of Lifting Forces During the Rapid Compaction of Deformable Porous Media

Abstract: In a recent paper, Wu et al. (2005, "Dynamic Compression of Highly Compressible Porous Media With Application to Snow Compaction, "J. Fluid Mech., 542, pp. 281-304) developed a novel experimental and theoretical approach to investigate the dynamic lift forces generated in the rapid compression of highly compressible porous media, (e.g., snow layer), where a porous cylinder-piston apparatus was used to measure the pore air pressure generation and a consolidation theory was developed to capture the pore-pressure relaxation process. In the current study, we extend the approach of Wu et al. to various porous materials such as synthetic fibers. The previous experimental setup was completely redesigned, where an accelerometer and a displacement sensor were employed to capture the motion of the piston. The pore-pressure relaxation during the rapid compaction of the porous material was measured. The consolidation theory developed by Wu et al. was modified by introducing the damping effect from the solid phase of the porous materials. One uses Carman-Kozeny's relationship to describe the change in the permeability as a function of compression. By comparing the theoretical results with the experimental data, we evaluated the damping effect of the soft fibers, as well as that of the pore air pressure for two different porous materials, A and B. The experimental and theoretical approach presented herein has provided an important methodology in quantifying the contributions of different forces in the lift generation inside porous media and is an extension of the previous studies done by Wu et al.

Role of a Sinusoidal Wavy Surface in Enhancement of Heat Transfer Using Turbulent Dual Jet

Abstract: In this paper, the role of sinusoidal wavy surface in enhancing the heat transfer is numerically studied. The heat transfer characteristics are studied for two thermal boundary conditions of the wavy wall. To assess the effect of wavy wall, the amplitude is varied between 0.1 and 0.7 and number of cycle from 4 to 12 at an interval of 0.1 and 1, respectively. In order to see the effect of offset ratio, it is varied between 3 and 15 at an interval of 2. The Reynolds number (Re) and Prandtl number (Pr) are set to 15, 000 and 0.71, respectively, for all the numerical simulations. It is found that the maximum average Nusselt number (Nuavg) depends not only on the amplitude and number of cycle but also on the offset ratio. Overall, 23.27% in maximum heat transfer enhancement is achieved with reference to the plane wall surface. An approximately linear decrement in maximum Nuavg is observed when offset ratio increases. The results indicate that Nuavg increases with an increase in the amplitude of sinusoidal wavy surface up to N = 8 and almost follows the linear trend up to N = 7. It is also found that Nux is always on the higher side as compared to the corresponding case of a plane wall surface when N = 4, irrespective of the offset ratio. With an increase in N, Nux fluctuates about the result of plane wall surface after the initial increase because of the obstruction. The amplitude of the fluctuation increases with an increase in the number of cycle N, which indicates that fluid accelerates and decelerates gradually owing to the presence of trough and crest. Also, it is worth noticing that for some cases, there is a decrease in the heat transfer rate as compared to the plane wall case. Therefore, it is concluded that the increase in the surface area does not necessarily result in an increase in the heat transfer rate.

Experimental Investigation of Composite Phase Change Material Heat Sinks for Enhanced Passive Thermal Management

Abstract: Phase change materials (PCMs) are effective at storing thermal energy and are attractive for use in electronics to smooth temperature peaks during periods of high demand; however, the use of PCMs has been somewhat limited due to the poor thermal properties of the materials. Here, we propose a design for a tunable composite PCM heat sink for passive thermal management in electronic systems and develop an improved test platform to directly compare performance between different designs and PCMs. The composite design leverages high conductivity pathways, which are machined into aluminum heat sinks, and back-filled with PCMs. Two package sizes are considered with several internal fin structures. All designs are evaluated using a test platform with realistic power profiles, controlled interfacial loading, and in situ temperature measurement. The composite PCM heat sinks are benchmarked against solid aluminum packages of the same size. This study focuses on three commercially available PCMs. Performance is evaluated based on (1) the time it takes the test heater chip below each composite PCM package to reach the cut-off temperature of 95 °C and (2) the period of a full melt-regeneration cycle. A range of heat fluxes are considered in this study spanning 6.8–14.5 W cm−2. The isokite design with PlusICE S70 extends the time to reach 95 °C by 36.2% when compared to the solid package, while weighing 17.3% less, making it advantageous for mobile devices.

An Experimental Study on Segmented-Encapsulated Electrode Dielectric-Barrier-Discharge Plasma Actuator for Mapping Ice Formation on a Surface: A Conceptual Analysis

Abstract: A novel design of the dielectric barrier discharge (DBD) actuator/sensor is proposed for mapping the location of icing on a surface. The new design uses segmentation of the embedded electrode of the DBD actuator. Segmented DBD actuator/sensor devices were fabricated and experimentally tested in terms of mechanical, thermal and sensing abilities. The sensing capability of the new actuator was analyzed experimentally. Stationary and dynamic icing tests were conducted and the electrical characteristics of the DBD were measured. A parametric study on the effect of the electrode dimensions on the degree of sensitivity of the device was performed. Experimental results show that by using a segmented configuration it is possible to sense the onset of ice formation and also to detect its location. Furthermore, it is possible to detect the initiation of the melting process and measure the time for the water/ice to be completely expelled from the surface. It is also shown that the segmented actuator has better deicing performance in comparison to the conventional actuators. It is also shown that the thermal and active flow control abilities are not compromised by the segmented configuration and thus this device may perform deicing, ice formation and location detection and active flow control. [DOI: 10.1115/1.4048252]

Relationship Between the Nonlocal Effect and Lagging Behavior in Bioheat Transfer

Abstract: Lots of generalized heat conduction models have been developed in recent decades, such as local thermal nonequilibrium model, phase lagging model, and nonlocal heat conduction model. But no attempt was made to prove which model is better (or worse) than others, or whether there is a certain relationship between these different models. With this inspiration, we establish the nonlocal bioheat transfer equations with lagging time, and the two and three-temperature bioheat transfer equations with considering all the carrier's heat conduction effect are also constructed. Comparing the two (or three)-temperature equation model with the nonlocal bioheat transfer models with lagging time, one may obtain: the lagging time of temperature gradient τtand the nonlocal characteristic length λq in the space derivative items of heat flux have the same effect on heat transfer; when the heat transport occur among N energy carriers with considering the conduction effects of all carries, the heat transfer processes are dependent upon the high-order effect of τqN-1, τtN-1 and λt(2N-1) in nonlocal dual phase lag bioheat transfer model. This phenomenon is very important for biological and medical systems where numerous carriers may exist on the cellular level.

Convective Cooling of Compact Electronic Devices Via Liquid Metals With Low Melting Points

Abstract: The increasingly high power density of today's electronic devices requires the cooling techniques to produce highly effective heat dissipation performance with as little sacrifice as possible to the system compactness. Among the currently available thermal management schemes, convective liquid metal cooling provides considerably high performance due to its unique thermal properties. This paper first reviews the studies on convective cooling using low-melting-point metals published in the past few decades. A group of equations for the thermophysical properties of In-Ga-Sn eutectic alloy is then documented by rigorous literature examination, following by a section of correlations for the heat transfer and flow resistance calculation to partially facilitate the designing work at the current stage. The urgent need to investigate the heat transfer and flow resistance of forced convection of low-melting-point metals in small/mini-channels, typical in compact electronic devices, is carefully argued. Some special aspects pertaining to the practical application of this cooling technique, including the entrance effect, mixed convection, and compact liquid metal heat exchanger design, are also discussed. Finally, future challenges and prospects are outlined.

Entropy Generation Analysis and Cooling Time Estimation for a Rotating Vertical Hollow Tube in the Air Medium

Abstract: An objective function combining the first and second laws of thermodynamics has been employed to delineate the thermodynamic performance on mixed convection around a vertical hollow, rotating cylinder within the laminar range with the variation of Rayleigh number (104 ≤ Ra ≤ 108), Reynolds number (ReD < 2100), and aspect ratio (1 ≤ L/D ≤ 20). Entropy generation in the system is predominantly triggered by heat transfer in comparison to fluid friction. The irreversibility incurred progressively increases with an increase in Ra and ReD. The variation pattern of (I/Q)Rotation/(I/Q)Non−Rotation has been demonstrated to find out the optimized regime where heat transfer is maximum within the laminar range. The contribution of fluid friction irreversibility toward total irreversibility rises abruptly with an increase in ReD for all cases of L/D and Ra. To demonstrate this study's thermodynamic characteristics, the static temperature contours as well as the contours of entropy generation have been represented pictorially. The estimation of cooling time has been reported by using the method of lumped capacitance.

Combined-Hole Film Cooling Designs Based on the Construction of Antikidney Vortex Structure: A Review

Abstract: Film cooling is one of the most efficient and widely used cooling methods for high-temperature components. The interaction between the film cooling jet and main flow creates the counter-rotating vortex pair (CRVP), which enhances the mixing between coolant and hot stream and lifts the coolant film off the protected surface. The desire to overcome the unfavorable effects of CRVP and thus efficiently improve cooling effectiveness promotes various new combined-hole designs for film cooling. In this review paper, a summary of previous progress on film cooling and a special focus on recent literature related to the combined-hole film cooling designs with less difficulty in machining are provided. The underlying mechanisms of the enhancement in cooling effectiveness and film coverage due to antikidney vortex structure by combined holes are analyzed. Some perspectives on future prospects are finally addressed.

Numerical Consideration of LTNE and Darcy Extended Forchheimer Models for the Analysis of Forced Convection in a Horizontal Pipe in the Presence of Metal Foam

Abstract: The intent of the current research work is to emphasize the computational modeling of forced convection heat dissipation in the presence of high porosity and thermal conductivity metallic foam in a horizontal pipe for different regimes of the fluid flow for a range of Reynolds number. A two-dimensional physical domain is considered in which Darcy extended Forchheimer (DEF) model is adopted in the aluminum metallic foam to predict the features of fluid flow and local thermal nonequilibrium (LTNE) model is employed for the analysis of heat transfer in a horizontal pipe for different flow regimes. The numerical results are initially matched with experimental and analytical results for the purpose of validation. The average Nusselt number for fully filled foam is found to be higher compared to other filling rate of metallic foams and the clear pipe at the cost of pressure drop. As an important finding, it has been observed that the laminar and transition flow gives higher heat transfer enhancement ratio and thermal performance factor compared to turbulent flow. This work resembles numerous industrial applications such as solar collectors, heat exchangers, electronic cooling, and microporous heat exchangers. The novelty of the work is the selection of suitable flow and thermal models in order to clearly assimilate the flow and heat transfer in metallic foam. The presence of aluminum metal foam is highlighted for the augmentation of heat dissipation in terms of PPI and porosity. The parametric study proposed in this work surrogates the complexity and cost involved in developing an expensive experimental setup.

Thermofluid Characteristics on Natural and Mixed Convection Heat Transfer From a Vertical Rotating Hollow Cylinder Immersed in Air: A Numerical Exercise

Abstract: Numerical investigations are performed on natural and mixed convection around stationary and rotating vertical heated hollow cylinder with negligible wall thickness suspended in the air. The fluid flow and heat transfer characterization around the hollow cylinder are obtained by varying the following parameters, namely, Rayleigh number (Ra), Reynolds number (ReD), and cylindrical aspect ratio (L/D). The heat transfer quantities are estimated by varying the Rayleigh number (Ra) from 104 to 108 and aspect ratio (L/D) ranging from 1 to 20. Steady mixed convection with active rotation of hollow vertical cylinder is further studied by varying the Reynolds number (ReD) from 0 to 2100. The velocity vectors and temperature contours are shown in order to understand the fluid flow and heat transfer around the vertical hollow cylinder for both rotating and nonrotating cases. The surface average Nusselt number trends are presented for various instances of Ra, ReD, and L/D and found out that the higher rate of heat loss from the cylinder wall occurs at high Ra, low L/D (short cylinder) and high ReD.

Pool Boiling of R514A, R1224 yd(Z), and R1336mzz(E) on a Reentrant Cavity Surface

Abstract: This paper quantifies the pool boiling performance of R514A, R1224 yd(Z), and R1336mzz(E) on a flattened, horizontal Turbo-ESP surface for air-conditioning applications for heat fluxes between roughly 10 kWm−2 and 100 kWm−2. R514A, R1224 yd(Z), and R1336mzz(E) are replacements for R123 and R245fa. All of these replacement refrigerants had measured boiling heat fluxes that were larger than that for R123 for most heat fluxes. For example, for heat fluxes between 10 kWm−2 and 80 kWm−2, R514A, R1224 yd(Z), and R1336mzz(E) exhibited average heat fluxes that were 30%, 57%, and 13% larger than that for R123 for a saturation temperature of 277.6 K. For the same comparison done at a saturation temperature of 298.2 K, the average heat flux for R514A was roughly 43% larger than that for R123. A pool boiling model, that was previously developed for pure and mixed refrigerants on the Turbo-ESP surface, was compared to the measured boiling performance. The model predicted the measured superheats of the mixed refrigerants and the single-component refrigerants to within ± 0.7 K and ± 0.45 K, respectively.

The Effect of Surface Thermal Radiation on Heat Transfer in a Ventilated Cavity

Abstract: In this work, experimental and numerical results were obtained to analyze the effect of surface thermal radiation on heat transfer by mixed convection in a ventilated cavity. Experimental temperature profiles were obtained at six different depths and heights consisting of 14 thermocouples each. Five turbulence models were evaluated against experimental data. The radiative heat transfer model was solved with the discrete ordinate method. The effect of thermal radiation on experimental heat transfer coefficients is significant; it increases between 87% (Re = 30,372 and Ra = 3.04 × 1011) and 110% (Re = 6021 and Ra = 2.27 × 1011), when the emissivity of the walls increases from 0.03 to 0.98.

The Thermohydraulic Characteristics and Optimization Study of Radial Porous Heat Sinks Using Multi-Objective Computational Method

Abstract: The use of porous media (PM) to improve conductive heat transfer has been at the focus of interest in recent years. Limited studies, however, have focused on heat transfer in radial heat sinks (RHSs) fully and partially saturated porous media with a different arrangement. As a development of the above-mentioned investigations, this research, therefore, addresses the ability of radial porous heat sink solutions to improve the thermohydraulic characteristics and reduce the effect of the second thermodynamics law. The response surface methodology (RSM) technique with ansysfluent-cfd is utilized to optimize the thermohydraulic features and the total entropy generation by the multi-objective optimum design for different design parameters such as porosity (Ø), inlet temperature (Tin), and applied heat flux (Q) simultaneously after achieving the optimum porous media arrangement related to the flow direction. The results show that, in terms of the flow direction, the optimum radial porous heat sink of the 100%PM model is recognized as providing the best results and the best option (fully saturated porous media). Moreover, a significant agreement between the predicted and numerical simulation data for the optimum values is also seen. The optimum and undesirable designs of the thermohydraulic features, the total entropy generation, and the optimum thermal management are detected in this investigation.

Augmentation of Pure Mixed Convection Heat Transfer in a Non-Newtonian Power-Law Fluid Filled Lid-Driven Trapezoidal Cavity With Double Rotating Cylinders

Abstract: In this study, a numerical investigation on mixed convection inside a trapezoidal cavity with a pair of rotating cylinders has been conducted. Three different power-law fluid indexes (n = 1.4, 1.0, and 0.6) have been considered to model different sets of non-Newtonian fluids. Four separate cases are considered based on the rotational orientation of the cylinders within the cavity. In the first two cases, the cylinders rotate in the same direction, i.e., both counterclockwise (CCW), and both clockwise (CW), whereas, in the other two cases, cylinders rotate in opposite directions (CW–CCW and CCW–CW). Simulations have been carried out over a broad range of Reynolds number (from 0.5 to 500) and angular speeds (a dimensionless value from 0 to 10). The average Nusselt number values at the isothermal hot inclined cavity surface are determined to evaluate heat transfer performance in various circumstances. Streamlines and isotherm contours are also plotted for a better understanding of the effects of different cases for various parameters on thermal and fluid flow fields. It is found that the Nusselt number varies nonlinearly with different angular speeds of the cylinders. The combined effect of the mixing induced by cylinder rotation and viscosity characteristics of the fluid dictates the heat transfer in the system. Predictions from the numerical investigation provide insights into the sets of key parametric configurations that have a dominant influence on the thermal performance of the lid-driven cavity with double rotating cylinders.

Numerical Simulation of Thermal Diffusivity Measurements With the Laser-Flash Method to Evaluate the Effective Property of Composite Materials

Abstract: The laser-flash method (LFM) is a technique commonly used to measure thermal diffusivity of homogeneous and isotropic materials, but can also be applied to macroscopically inhomogeneous materials, such as composites. When composites present thermal anisotropy, as fiber-reinforced, LFM can be used to measure the effective thermal diffusivity (αeff) in the direction of heat flux. In this work, the thermal behavior of composites during thermal diffusivity measurements with the LFM was simulated with a finite element model (FEM) using a commercial software. Three composite structures were considered: sandwich layered (layers arranged in series or parallel), fiber-reinforced composites, and particle composite (spheres). Numerical data were processed through a nonlinear least-square fitting (NL-LSF) to obtain the effective thermal diffusivity of the composite. This value has the meaning of “dynamic effective thermal diffusivity.” Afterward, the effective thermal conductivity (λeff) is calculated from the dynamic effective thermal diffusivity, equivalent heat capacity, and density of the composite. The results of this methodology are compared with the analytically calculated values of the same quantity. This last assumes the meaning of “static effective thermal conductivity.” The comparison of the dynamic and static property values is so related to the inhomogeneity of the samples, and a deviation of the temperature versus time trend from the analytical solution for the perfectly homogeneous sample gives information about the lack of uniformity of the sample.

Filmwise Condensation From Humid Air on a Vertical Superhydrophilic Surface: Explicit Roles of the Humidity Ratio Difference and the Degree of Subcooling

Abstract: The process involving heat and mass transfer during filmwise condensation (FWC) in the presence of noncondensable gases (NCG) has great significance in a large variety of engineering applications. Traditionally, the condensation heat transfer is expressed in the literature as a function of the degree of subcooling—reckoned as the difference between the ambient dry bulb temperature and the condenser wall temperature. However, in the presence of NCG, there exists a finite gradient of vapor mass fraction near the condenser plate, which directly influences the vapor mass flux to the condenser surface, thus limiting the condensation rate. The effects of both these influencing thermodynamic parameters, i.e., the degree of subcooling and the difference of humidity ratio (between the freestream environment and on the condenser plate), have been characterized in this work both experimentally and through a mechanistic model. The vapor mass flux during condensation on a subcooled vertical superhydrophilic surface under free convection regime is experimentally measured in a controlled environment (temperature and humidity) chamber. The mechanistic model, based on the similarity of energy and species transports, is formulated for the thermogravitational boundary layer over the condenser plate and tuned against the experimental results. Further, the model is used to obtain comprehensive data of the condensate mass flux and condensation heat transfer coefficient (CHTC) as functions of the salient thermal operating conditions over a wide parametric range. Results indicate that humidity ratio difference has a more pronounced influence on the condensation mass transfer rather than the degree of subcooling. Regime maps of condensate flux and CHTC show how these can be explicitly identified in terms of the degree of subcooling and humidity ratio difference, regardless of the prevailing thermal and humidity conditions at the freestream and the condenser plate. The mechanistic model thus lends to the development of empirical correlations of condensate mass flux and CHTC as explicit functions of these two parameters for easy use in practical FWC configurations.

Anisotropy and the Onset of the Thermoconvective Instability in a Vertical Porous Layer

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 wave vector 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.

Pore-Scale Simulation on Pool Boiling Heat Transfer and Bubble Dynamics in Open-Cell Metal Foam by Lattice Boltzmann Method

Abstract: Based on the newly developed geometrical model of open-cell metal foam, pool boiling heat transfer in open-cell metal foam, considering thermal responses of foam skeletons, is investigated by the phase-change lattice Boltzmann method (LBM). Pool boiling patterns are obtained at different heat fluxes. The effects of pore density and foam thickness on bubble dynamics and pool boiling heat transfer are revealed. The results show that “bubble entrainment” promotes fluid mixing and bubble sliding inside metal foam. Based on force analysis, the sliding bubble is pinned on the heating surface and cannot lift off completely at high heat flux due to the increasing surface tension force. Pool boiling heat transfer coefficient decreases with increasing pore density and foam thickness due to high bubble escaping resistance.