Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2020"

Near-Field Radiative Heat Transfer Between Two α-MoO3 Biaxial Crystals

Abstract: The near-field radiative heat transfer (NFRHT) between two semi-infinite α-MoO3 biaxial crystals is investigated numerically based on the fluctuation–dissipation theorem combined with the modified 4 × 4 transfer matrix method in this paper. In the calculations, the near-field radiative heat flux (NFRHF) along each of the crystalline directions of α-MoO3 is obtained by controlling the orientation of the biaxial crystals. The results show that much larger heat flux than that between two semi-infinite hexagonal boron nitride can be achieved in the near-field regime, and the maximum heat flux is along the [001] crystalline direction. The mechanisms for the large radiative heat flux are explained as due to existence of hyperbolic phonon polaritons (HPPs) inside α-MoO3 and excitation of hyperbolic surface phonon polaritons (HSPhPs) at the vacuum/α-MoO3 interfaces. The effect of relative rotation between the emitter and the receiver on the heat flux is also investigated. It is found that the heat flux varies significantly with the relative rotation angle. The modulation contrast can be as large as two when the heat flux is along the [010] direction. We attribute the large modulation contrast mainly to the misalignment of HSPhPs and HPPs between the emitter and the receiver. Hence, the results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.

Framing the Impacts of Highly Oscillating Magnetic Field on the Ferrofluid Flow Over a Spinning Disk Considering Nanoparticle Diameter and Solid–Liquid Interfacial Layer

Abstract: This article communicates on the ferrofluid flow over a spinning disk in the presence of highly oscillating magnetic field. The flow is presumed to be unsteady. Ferrous nanoparticles are suspended within base medium water. This investigation reveals how presence and absence of oscillating magnetic field influence the hydrothermal basis of the flow. Also, the effects of particles diameter and solid–liquid interfacial layer have been precisely incorporated to reveal the thermal integrity of the system. Shliomis theory is introduced to frame the leading equations of the system. Resulting equations have been solved using innovative spectral quasi-linearization method (SQLM). Residual error analysis is included to explore the advantage of such computational scheme. The influence of dynamic parameters on the velocities and temperature is deliberated through graphs and tables. Several 3D pictures and contour plots are depicted to extract the key points of the flow. The results exhibit that heat transfer is reduced for nanoparticle diameter but amplifies for base liquid nanolayer conductivity ratio and elevated field frequency enhances the temperature. Relative magnetization reduces for high field frequency, but increases for angular displacement. SQLM exhibits an accurate computational scheme with fast convergence.

Heat Transfer Enhancement Analysis of Tube Receiver for Parabolic Trough Solar Collector With Central Corrugated Insert

Abstract: Tube receiver with central corrugated insert was introduced as the absorber tube of parabolic trough receiver to increase the overall heat transfer performance of the tube receiver for parabolic trough solar collector (PTC) system. The Monte Carlo ray tracing method (MCRT) coupled with finite volume method (FVM) was adopted to investigate flow characteristics and the heat transfer performance of tube receiver for parabolic trough solar collector system. The numerical results were successfully validated with the empirical correlations existing in the literature. The numerical results indicated that the introduction of the corrugated insert inside the absorber tube of the parabolic trough receiver can effectively enhance the heat transfer performance, where the average Nusselt number can be increased up to 3.7 times compared to the smooth absorber. While the overall heat transfer performance factor can be found to be in the range of 1.3–2.6. The results indicate that the heat transfer increases with increasing corrugated insert twist ratio and increases also with decreasing pitch between two corrugations.

Comparison of Single-Phase Convection in Additive Manufactured Versus Traditional Metal Foams

Abstract: Metal foams have been often used for thermal management due to their favorable characteristics including high specific surface area (SSA), high thermal conductivity, and low relative density. However, they are accompanied by shortcomings including the significant contact resistances due to attachment method, as well as the need for characterization of foam parameters such as pore diameter and SSA. Additive manufactured (AM) metal foams would eliminate the substrate/foam thermal resistance, decrease the need for pre-usage characterization, and allow for tailoring structures, while also taking advantage of the characteristics of traditionally manufactured foams. A commercial, aluminum foam (nominally 5 pores per inch (PPI), 86.5% porosity) was analyzed using X-ray microcomputed tomography, and a custom-designed metal foam based on the cell diameter and porosity of the commercial sample was subsequently manufactured. Reduced domain computational fluid dynamics/heat transfer (CFD-HT) models were compared against experimental data. Postvalidation, the flow behavior, effect of varying attachment thermal conductivities, and thermal performance were numerically investigated, demonstrating the usefulness of validated pore-scale models, as well as the potential for improved performance using AM metal foams over traditionally manufactured foams.

Effect of Driven Sidewalls on Mixed Convection in an Open Trapezoidal Cavity With a Channel

Abstract: The mixed convection in an open trapezoidal lid-driven cavity connected with a channel is investigated in the present paper. Four different cases were considered depending on the movement of the cavity sidewalls. For case I, the left sidewall moves downward; for case II, the left sidewall moves downward and the right one moves upward; while for case III, only the right sidewall moves upward. A comparative case (case 0) is accounted when both sidewalls are assumed stationary. The base of the cavity is subjected to a localized heat source of constant temperature Th. The effects of Richardson number Ri and Reynolds number ratio Rer on the flow and thermal fields have been investigated. The results indicated that for cases I and II, the average Nusselt number increases with the increase of the Richardson number and Reynolds number ratio. Moreover, it was found that the maximum average Nusselt number occurs with case I. When the lid-driven speed is three times that of the inlet airflow velocity, the augmentations of the average Nusselt number compared with stationary walls are 163%, 158%, and 96% for cases I, II, and III, respectively.

Sensitivity of Thermal Predictions to Uncertain Surface Tension Data in Laser Additive Manufacturing

Abstract: To understand the process-microstructure relationships in additive manufacturing (AM), it is necessary to predict the solidification characteristics in the melt pool. This study investigates the influence of Marangoni driven fluid flow on the predicted melt pool geometry and solidification conditions using a continuum finite volume model. A calibrated laser absorptivity was determined by comparing the model predictions (neglecting fluid flow) against melt pool dimensions obtained from single laser melt experiments on a nickel super alloy 625 (IN625) plate. Using this calibrated efficiency, predicted melt pool geometries agree well with experiments across a range of process conditions. When fluid mechanics is considered, a surface tension gradient recommended for IN625 tends to overpredict the influence of convective heat transfer, but the use of an intermediate value reported from experimental measurements of a similar nickel super alloy produces excellent experimental agreement. Despite its significant effect on the melt pool geometry predictions, fluid flow was found to have a small effect on the predicted solidification conditions compared to processing conditions. This result suggests that under certain circumstances, a model only considering conductive heat transfer is sufficient for approximating process-microstructure relationships in laser AM. Extending the model to multiple laser passes further showed that fluid flow also has a small effect on the solidification conditions compared to the transient variations in the process. Limitations of the current model and areas of improvement, including uncertainties associated with the phenomenological model inputs are discussed.

Ballistic-Diffusive Heat Conduction in Thin Films by Phonon Monte Carlo Method: Gray Medium Approximation Versus Phonon Dispersion

Abstract: The gray medium approximation treating all phonons with an averaged and representative mean-free-path (MFP) is an often used method in analyzing ballistic-diffusive heat conduction at nanoscale. However, whether there exists a reasonable value of the average MFP which effectively represents the entire spectrum of modal MFPs remains unclear. In this paper, phonon Monte Carlo (MC) method is employed to study the effects of the gray medium approximation on ballistic-diffusive heat conduction in silicon films by comparing with dispersion MC simulations. Four typical ways for calculating the average MFP with gray medium approximation are investigated. Three of them are based on the weighted average of the modal MFPs, and the remaining one is based on the weighted average of the reciprocals of the modal MFPs. The first three methods are found to be good at predicting effective thermal conductivity and heat flux distribution, but have difficulties in temperature profile, while the last one performs better for temperature profile than effective thermal conductivity and heat flux distribution. Therefore, none of the average MFPs can accurately characterize all the features of ballisticdiffusive heat conduction for the gray medium approximation. Phonon dispersion has to be considered for the accurate thermal analyses and modeling of ballistic-diffusive heat transport. Our work could be helpful for further understanding of phonon dispersion and more careful use of the gray medium approximation. [DOI: 10.1115/1.4048093]

Artificial Neural Networks in Radiation Heat Transfer Analysis

Abstract: In the Monte Carlo ray-trace (MCRT) method, millions of rays are emitted and traced throughout an enclosure following the laws of geometrical optics. Each ray represents the path of a discrete quantum of energy emitted from surface element i and eventually absorbed by surface element j. The distribution of rays absorbed by the n surface elements making up the enclosure is interpreted in terms of a radiation distribution factor matrix whose elements represent the probability that energy emitted by element i will be absorbed by element j. Once obtained, the distribution factor matrix may be used to compute the net heat flux distribution on the walls of an enclosure corresponding to a specified surface temperature distribution. It is computationally very expensive to obtain high accuracy in the heat transfer calculation when high spatial resolution is required. This is especially true if a manifold of emissivities is to be considered in a parametric study in which each value of surface emissivity requires a new ray-trace to determine the corresponding distribution factor matrix. Artificial neural networks (ANNs) offer an alternative approach whose computational cost is greatly inferior to that of the traditional MCRT method. Significant computational efficiency is realized by eliminating the need to perform a new ray trace for each value of emissivity. The current contribution introduces and demonstrates through case studies estimation of radiation distribution factor matrices using ANNs and their subsequent use in radiation heat transfer calculations.

On the Use of Thermal Energy Storage for Flexible Baseload Power Plants: Thermodynamic Analysis of Options for a Nuclear Rankine Cycle

Abstract: The intermittency of wind and solar energy can disrupt the dynamic balance utilities must maintain to meet fluctuating demand. This work examines the use of thermal energy storage (TES) to increase the operational flexibility of a baseload power plant and thus incentivize renewable energy and decarbonize the grid. A first and second law thermodynamic model of a nuclear power plant establishes the impacts of TES on the capacity factor and thermal efficiency of the plant. Four storage options, which are distinguished by the location within the cycle where steam is diverted for charging and whether discharge of the TES is via the primary or a secondary Rankine cycle, are considered. TES is compared to steam bypass, which is an alternative to provide baseload flexibility. TES is significantly better than steam bypass. The storage option with the greatest thermodynamic benefit is charged by diverting superheated steam at the outlet of the moisture separator/reheater (MSR) to the TES. The TES is discharged for peaking power through an optimized secondary cycle. TES increases the capacity factor as much as 15% compared to steam bypass at representative charging mass flowrates. The storage option that diverts steam from the steam generator to charge the TES and discharges the TES to the primary cycle extends the discharge power to a lower range and does not require a secondary cycle. In this case, the capacity factor and efficiency are as much as 8% greater than that of steam bypass.

Heat and Mass Transfer in the Food, Energy, and Water Nexus—A Review

Abstract: Engineering innovations—including those in heat and mass transfer—are needed to provide food, water, and power to a growing population (i.e., projected to be 9.8 × 109 by 2050) with limited resources. The interweaving of these resources is embodied in the food, energy, and water (FEW) nexus. This review paper focuses on heat and mass transfer applications which involve at least two aspects of the FEW nexus. Energy and water topics include energy extraction of natural gas hydrates and shale gas; power production (e.g., nuclear and solar); power plant cooling (e.g., wet, dry, and hybrid cooling); water desalination and purification; and building energy/water use, including heating, ventilation, air conditioning, and refrigeration technology. Subsequently, this review considers agricultural thermal fluids applications, such as the food and water nexus (e.g., evapotranspiration and evaporation) and the FEW nexus (e.g., greenhouses and food storage, including granaries and freezing/drying). As part of this review, over 100 review papers on thermal and fluid topics relevant to the FEW nexus were tabulated and over 350 research journal articles were discussed. Each section discusses previous research and highlights future opportunities regarding heat and mass transfer research. Several cross-cutting themes emerged from the literature and represent future directions for thermal fluids research: the need for fundamental, thermal fluids knowledge; scaling up from the laboratory to large-scale, integrated systems; increasing economic viability; and increasing efficiency when utilizing resources, especially using waste products.

Pressure Drop and Convective Heat Transfer in Different SiSiC Structures Fabricated by Indirect Additive Manufacturing

Abstract: The microstructure of porous materials has a significant effect on their transport properties. Engineered cellular ceramics can be designed to exhibit properties at will, thanks to the advances in additive manufacturing. We investigated the heat and mass transport characteristics of SiSiC lattices produced by three-dimensional (3D) printing and replication, with three different morphologies: rotated cube (RC), Weaire–Phelan (WPh), and tetrakaidecahedron (TK) lattices, and a commercially available ceramic foam. The pressure gradients were measured experimentally for various velocities. The convective heat transfer coefficients were determined through a steady-state experimental technique in combination with numerical analysis. The numerical model was a volume-averaged model based on a local thermal nonequilibrium (LTNE) assumption of the two homogeneous phases. The results showed that for TK and WPh structures, undesirable manufacturing anomalies (specifically window clogging) led to unexpectedly higher pressure drops across the samples and increased thermal dispersion. Compared to the TK and WPh structures the manufactured RC lattice and the random foam had lower heat transfer rates but also lower pressure drops. These lower values for the RC lattice and foam are also a result of their lower specific surface areas.

The Effect of Metal Foam Thickness on Jet Array Impingement Heat Transfer in High-Porosity Aluminum Foams

Abstract: High-porosity metal foam (MF) is a popular option for high-performance heat exchangers as it offers significantly higher heat transfer participation area per unit volume compared to other convection enhancement cooling methods. Further, metal foams provide highly tortuous flow paths resulting in thermal dispersion assisted by enhanced mixing. This paper presents experimental and numerical studies and the detailed underlying physics of jet array impingement onto high-porosity (ε∼0.95) thin aluminum foams. The jet and foam configurations were designed for the maximum utilization of the foam area for heat transfer and reduced penalty on the pumping power requirement. Three different pore density foams were tested with three different array-jet impingement configurations. The minimum possible thickness for each pore density was tested, viz., 5 pores-per-inch (PPI): 19 mm, 10 PPI: 12.7 mm, and 20 PPI: 6.35 mm. The baseline case for these foam-based jet impingement configurations was the corresponding configuration of orthogonal jet impingement onto a smooth heated surface, where the distance between the jet-issuing plane and the heated surface was maintained at the foam thickness level. In general, thinner foams facilitated greater jet penetration and increased foam volume usage, resulting in higher heat transfer rates for a given pore density, especially when combined with jet configurations with larger open areas. Finally, we evaluated the thermal hydraulic performance for different foam configurations and the optimum value of a given PPI was found to be at an intermediate rather than the lowest foam thickness.

Heating Protocol Design Affected by Nanoparticle Redistribution and Thermal Damage Model in Magnetic Nanoparticle Hyperthermia for Cancer Treatment

Abstract: Recent micro-CT scans have demonstrated a much larger magnetic nanoparticle distribution volume in tumors after localized heating than those without heating, suggesting possible heating-induced nanoparticle migration. In this study, a theoretical simulation was performed on tumors injected with magnetic nanoparticles to evaluate the extent to which the nanoparticle redistribution affects the temperature elevation and thermal dosage required to cause permanent thermal damage to PC3 tumors. 0.1 cc of a commercially available ferrofluid containing magnetic nanoparticles was injected directly to the center of PC3 tumors. The control group consisted of four PC3 tumors resected after the intratumoral injection, while the experimental group consisted of another four PC3 tumors injected with ferrofluid and resected after 25 min of local heating. The micro-CT scan generated tumor model was attached to a mouse body model. The blood perfusion rates in the mouse body and PC3 tumor were first extracted based on the experimental data of average mouse surface temperatures using an infrared camera. A previously determined relationship between nanoparticle concentration and nanoparticle-induced volumetric heat generation rate was implemented into the theoretical simulation. Simulation results showed that the average steady-state temperature elevation in the tumors of the control group is higher than that in the experimental group where the nanoparticles are more spreading from the tumor center to the tumor periphery (control group: 70.664.7 C versus experimental group: 69.262.6 C). Further, we assessed heating time needed to cause permanent thermal damage to the entire tumor, based on the nanoparticle distribution in each tumor. The more spreading of nanoparticles to tumor periphery in the experimental group resulted in a much longer heating time than that in the control group. The modified thermal damage model by Dr. John Pearce led to almost the same temperature elevation distribution; however, the required heating time was at least 24% shorter than that using the traditional Arrhenius integral, despite the initial time delay. The results from this study suggest that in future simulation, the heating time needed when considering dynamic nanoparticle migration during heating is probably between 19 and 29 min based on the Pearce model. In conclusion, the study demonstrates the importance of including dynamic nanoparticle spreading during heating and accurate thermal damage model into theoretical simulation of temperature elevations in tumors to determine thermal dosage needed in magnetic nanoparticle hyperthermia design. [DOI: 10.1115/1.4046967]

Wettability Effects on Falling Film Flow and Heat Transfer Over Horizontal Tubes in Jet Flow Mode

Abstract: The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability on the film flow behavior and its associated surface heat transfer in the jet-flow mode. Volume of fluid (VOF) based simulations are carried out for horizontal tubes with different surface wettabilities. The wettability of the tube surfaces is represented using the Kistler's dynamic contact angle model. Surface wettability effects ranging from superhydrophilic to superhydrophobic are studied by varying the equilibrium contact angle from 2 deg to 175 deg. Two different liquid mass flow rates of 0.06 and 0.18 kg/m-s corresponding to the inline and staggered jet flow modes are studied. Results are presented in terms of the liquid film thickness, the contact areas between the different phases (solid–liquid and liquid–air), and the heat transfer coefficient or Nusselt number. The resistance imposed by the increasing contact angles inhibits the extent of the liquid spreading over the tube surface, and this, in turn, influences the liquid film thickness, and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle increased from 2 deg to 175 deg. The local distributions of the Nusselt number over the tube surface are strongly influenced by the flow recirculation in the liquid bulk.

High Performance Loop Heat Pipe With Flat Evaporator for Energy-Saving Cooling Systems of Supercomputers

Abstract: Two high performance loop heat pipes (LHPs) are developed for direct cooling of the chips in supercomputer. The two LHPs using flat evaporator are: one called water-cooling LHP and another one called air-cooling LHP. The working fluid of LHP is ammonia. The water-cooling LHP can work well at a heat load up to 663 W and air-cooling LHP can work well at a heat load up to 513 W. The two LHPs applying to the real computer servers are realized and tested. The server test results with water-cooling LHP have shown that the operating temperature of central processing units (CPUs) can be controlled to about 67 °C to ensure the reliable operating and acceptable level for electronic chips, even at condenser-cooling water temperature of 40 °C with low water flowrate of 0.055 m3/h. The server test results with air-cooling LHP have shown that the operating temperature of CPUs can be controlled to about 51 °C even at condenser-cooling wind temperature of 30 °C with wind flowrate of 41.88 m3/h.

Evaporation Momentum Force and Its Relevance to Boiling Heat Transfer

Abstract: As a liquid evaporates into its vapor, the vapor phase leaves at a higher velocity than the approaching liquid and exerts a net momentum force on the evaporating interface. This force is especially relevant in the contact line region where liquid temperature is higher than the bulk liquid, and local saturation temperature is reduced due to curvature effects. These factors result in an increased evaporative flux resulting in higher evaporation momentum force that can influence the interface motion and bubble trajectory. This force provides a new mechanism for enhancing boiling heat transfer by altering the individual bubble trajectory. In microchannels, it can lead to flow instability. These effects are critically evaluated in this paper and their relevance to bubble growth and heat transfer phenomena during pool and flow boiling is presented. Two nondimensional groups K1 and K2, respectively, representing the ratio of evaporation momentum force to inertia and surface tension forces, have been used in modeling heat transfer and interface motion. Evaporation momentum force has been successfully applied in modeling critical heat flux (CHF) in pool and flow boiling, analyzing instability during flow boiling in microchannels, controlling individual bubble motion, and enhancing CHF and heat transfer coefficient (HTC) during boiling on flat surfaces as well as tubular geometries.

Benchmark Solutions of Three-Dimensional Radiative Transfer in Nongray Media Using Line-by-Line Integration

Abstract: Nongray gas radiation calculations are conducted for four three-dimensional benchmarks using line-by-line (LBL) integration with the up-to-date high-resolution spectroscopic database HITEMP 2010. The radiative transfer equation (RTE) is solved using the finite volume method (FVM) over each wavenumber interval of the spectrum. A detailed mesh quality analysis assured the mesh independence of the solution. Accurate results for distributions of volumetric radiative heat source term and wall radiative heat flux are provided for four cases: (i) an isothermal pure water vapor medium at 1000 K; (ii) an isothermal and nonhomogeneous H2O–N2 mixture at 1000 K; (iii) a nonisothermal and homogeneous CO2–H2O–N2 mixture; and (iv) a nonisothermal and nonhomogeneous CO2–H2O–N2 mixture. These data can be useful to assess the accuracy of gas radiative property models.

A Note on the Comparative Analysis Between Rectangular and Modified Duct Heat Exchanger

Abstract: The plate fin heat exchangers usually have either rectangular or triangular shaped flow passage. In comparison to triangular flow passage, rectangular flow passage gives comparatively higher heat transfer at the cost of higher pumping power. In the present investigation, flow passage is modified by rounding the corner of triangular passage to investigate the heat and flow characteristics of air flowing through it. Comparison of performance between modified and rectangular flow passage has also been presented and discussed. The radius of curvature of the rounded corner has been kept constant with value of 0.49 times duct height (H). The dimple was also fabricated at the inner side of the flow passage and arranged in rectangular array. Distance between them was defined by two different dimensionless parameters, relative transverse width (x/h), and relative streamwise length (z/h), whereas, dimensionless height of the protrusion is defined by relative dimple height (h/D). Noticeable increment in both heat transfer and friction factor has been observed by modifying the duct corners and 2.98 times increment in Nusselt number resulted due to dimples in modified duct for h/D, x/h, and y/h value of 0.44, 10, and 10, respectively, in comparison to smooth duct at Reynolds number of 19,500. For similar combination of roughness parameters, highest frictional penalty was estimated with value of 4.46 times that of the smooth duct at Reynolds number of 4400. Additionally, the comparative assessment of heat transfer enhancement (Nuenh), frictional penalty (fpenalty), and thermohydraulic performance index (THPi) has also been carried out to understand the suitability of round cornered duct. In comparison to protruded rectangular duct, 28% higher THPi is obtained in modified duct under similar conditions.

Condensation and Wetting Behavior on Hybrid Superhydrophobic and Superhydrophilic Copper Surfaces

Abstract: A novel hybrid superhydrophobic and superhydrophilic copper surface was fabricated using a lift-off process to integrate the benefits of dropwise and filmwise condensation together. The superhydrophilic surface was comprised of microflower like CuO and nanorod Cu(OH)2 with a diameter in the range of 200–600 nm and the superhydrophobic surface was fabricated by chemical modification with Cytop on the hierarchically structured surface of CuO/Cu(OH)2. Wetting condition effect on the hybrid surface was investigated experimentally with a high-speed camera attached to a microscope and an environmental scanning electrical microscope (ESEM). Out-of-plane droplet jumping motion on superhydrophilic region and gravity effect on the droplet motion were examined. Experiment results showed that effective heat transfer coefficients of hybrid superhydrophobic and superhydrophilic surfaces were improved as compared with those of pure superhydrophobic surface. Comparison results between two hybrid surfaces with 2 and 4 mm pattern pitches indicated that the distance reduction between two neighboring superhydrophilic areas can enhance the condensation performance because short distance can promote the microcondensate coalescence and droplets removal.

Computational Fluid Dynamics Studies on Unstable Oscillatory Direct Contact Condensation of Subsonic Steam Jets in Water Cross-Flow

Abstract: In the last decade, researchers working on direct contact condensation (DCC) have focused their attention on studying the effect of liquid cross-flow, in contrast to the conventional stagnant liquid pool condensers. Currently, the major applications of DCC in liquid cross-flow include the sterilization process of milk and the mixing of oxygen-rich turbine drive gas with liquid oxygen (LOX) at the booster turbopump exit of a typical staged combustion cycle-based rocket engine. In this work, attempt has been made to develop and validate a two-fluid two-phase model for predicting the complex phenomena of steam injection into a cross-flow of subcooled water. A correlation for interaction length scale has been developed for DCC cases. The correlation includes the effect of all the critical operating parameters such as liquid subcooling, steam mass flux, and liquid velocity, which hitherto has not been available in the literature. The unstable nature of steam plumes has been investigated, and critical Weber numbers for predicting stable to unstable transition in a DCC cycle have been computed. The associated pressure and temperature oscillations due to unstable nature of plume have been studied. The critical design parameters for direct contact condenser such as the heat transfer coefficients and dimensionless vapor penetration lengths have been quantified and analyzed.

Asymmetric Conduction in an Infinite Functionally Graded Cylinder: Two-Dimensional Exact Analytical Solution Under General Boundary Conditions

Abstract: This paper focuses on using an analytical method to obtain an exact solution for a long cylindrical vessel made of functionally graded materials (FGMs). Heat conduction equations are assumed to be in both radial and circumferential directions. The conduction coefficients are considered as different power-law functions of the radius. The general linear boundary conditions are adopted to make the solution applicable to the full range of problems. The obtained solution is successfully validated. Through solving illustrative test examples, the effects of material constants and boundary conditions on temperature distribution are studied. The obtained formulation can be utilized for tailoring of FGM based on the actual sophisticated thermal boundary conditions in the production process. The current analytical findings can help to manage the temperature distribution in FGMs which is an essential parameter in controlling the thermal stresses.

Experimental and Numerical Investigations on the Heat Transfer of Film Cooling With Cylindrical Holes Fed With Internal Coolant Cross Flows

Abstract: The heat transfer coefficient of cylindrical holes fed by varying internal cross-flow channels with different cross-flow Reynolds numbers Rec is experimentally studied on a low-speed flat-plate facility. Three coolant cross flow cases, including a smooth case and two ribbed cases with 45/135-deg ribs, are studied at Rec = 50,000, and 100,000 with varying blowing ratios M of 0.5, 1.0, and 2.0. A transient liquid-crystal (LC) measurement technique is used to determine the heat transfer coefficient. At lower M, the heat transfer enhancement regions are asymmetrical for the smooth and 45-deg cases. The asymmetrical vortex is more pronounced with increasing cross-flow direction velocity, resulting in a more skewed distribution at Rec = 100,000. Conversely, the contours are laterally symmetric in the 135-deg case at varying Rec. A fork-shaped trend with a relatively high heat transfer coefficient appears upstream, and the increases in the heat transfer in the 135-deg cases are lower than those in the 45-deg cases. As M increases to 2.0, the vortex intensity increases, resulting in a stronger scouring effect upstream, especially at large Rec. The range and degree are affected by Rec at M = 2.0. The core of the heat transfer enhancement is skewed to the −Y side for both cases.

Study of Magnetic Field Effects on the Oxygen Transfer in Liquid Lead Cavity Flow Using the Lattice Boltzmann Method

Abstract: Natural convection and oxygen transfer characteristics in square cavity subjected to the magnetic field are studied numerically. Oxygen transfer in liquid metals has attracted much attention because it can decrease the corrosion rate of steel in contact with liquid metals. In advanced reactors, liquid lead has been utilized as an effective coolant. As indicated by many research reports that corrosion could be decreased by controlling the proper level of oxygen in the liquid lead. In this method, oxygen needs to mix in liquid lead homogenously and rapidly to produce a protective oxide layer. In this study, the impact of the magnetic force on oxygen transfer in a rectangular container is studied using the lattice Boltzmann method (LBM). Three different Schmidt numbers (Sc) and Hartmann numbers (Ha) have been simulated in this study. Some useful results are obtained such as an adverse effect was found that heat/mass transfer rates are decreased when Ha number is increased. In addition, the existence of an applied magnetic field has caused a significant increase in the required time to reach desired oxygen concentration and needs to be controlled in operation to have a faster distribution of the oxygen in the domain.

Thermodynamic Analysis of Microchannel Heat Sink With Cylindrical Ribs and Cavities

Abstract: Heat transfer improvement in microchannel heat sink (MCHS) has been a challenge, because it increases the power requirements for the fluid flow. In the present study, MCHS with different wall, geometric, and design configurations of cylindrical ribs and cavities are simulated to investigate their effect on thermal and hydrodynamic performance of MCHS using a laminar flow having Reynolds number in the range from 100 to 1000. The wall configurations include; base wall cylindrical ribs (BWCR), side wall cylindrical ribs (SWCR), and all wall cylindrical ribs (AWCR). Moreover, the geometric configurations involve different AWCR cases having rib spacings (Sfr) of 0.4 mm, 0.8 mm, 1.2 mm, and 0.4 mm staggered arrangement. Furthermore, the design configurations include; AWCR, all wall cylindrical cavities (AWCC), and all wall cylindrical ribs and cavities (AWCRC) with constant Sfr = 0.4 mm. The performance of various channels with flow disruptors is analyzed in terms of friction factor (f) and Nusselt number and then compared with smooth channel in terms of thermal enhancement factor (η). Based on the first law of thermodynamics, thermal resistance (Rth) is used to investigate the resistance of any configuration to flow of heat comparing at same pumping power. Moreover, the second law of thermodynamics is applied to quantify the rate of entropy generation (S˙gen) and transport efficiency (ηt) for MCHS. The results show that although the MCHS with all wall ribs has a lower value of η than the base wall and side wall ribs; however, it has the maximum value of ηt and minimum value of Rth and S˙gen; thus, indicating that η is not the only performance criteria for the selection of MCHS.

Numerical and Experimental Investigation of Phase Change Heat Transfer in the Presence of Rayleigh–Benard Convection

Abstract: This research investigates the melting rate of a phase change material (PCM) in the presence of Rayleigh–Benard convection. A scaling analysis is conducted for the first time for such a problem, which is useful to identify the parameters affecting the phase change rate and to develop correlations for the solid–liquid interface location and the Nusselt number. The solid–liquid interface and flow patterns in the liquid region are analyzed for PCM in a rectangular enclosure heated from bottom. Numerical and experimental results both reveal that the number of Benard cells is proportional to the ratio of the length of the rectangular enclosure over the solid–liquid interface location (i.e.,, the liquified region aspect ratio). Their effect on the local heat flux is also analyzed as the local heat flux profile changes with the solid–liquid interface moving upward. The variations of average Nusselt number are obtained in terms of the Stefan number, Fourier number, and Rayleigh number. Eventually, the experimental and numerical data are used to develop correlations for the solid–liquid interface location and average Nusselt number for this type of melting problems.

Nonlinear Radiative Squeezed Flow of Nanofluid Subject to Chemical Reaction and Activation Energy

Abstract: This study explores the flow of magnetized nanomaterials between two parallel disks. Novel aspects of activation energy and nonlinear thermal radiation characterized the heat and mass transfer. Nonlinear system of ODEs is obtained via proper variables. Homotopic scheme determines the convergence interval of governing expressions. Plots have been interpreted in order to examine how the temperature and concentration are influenced by various physical variables. Further, surface drag forces and heat and mass transfer rates are computed numerically and analyzed. Our computed analysis depicts that the influence of squeezed and magnetic parameters have reverse effects on temperature.

Reflection of Thermoelastic Waves From the Insulated Surface of a Solid Half-Space With Time-Delay

Abstract: This paper is devoted to study the reflection of thermoelastic plane waves from the thermally insulated stress-free boundary of a homogeneous, isotropic and thermally conducting elastic half-space. A new linear theory of generalized thermoelasticity under heat transfer with memory-dependent derivative (MDD) is employed to address this study. It has been found that three basic waves consisting of two sets of coupled longitudinal waves and one independent vertically shear-type wave may travel with distinct phase speeds. The formulae for various reflection coefficients and their respective energy ratios are determined in case of an incident coupled longitudinal elastic wave at the thermally insulated stress-free boundary of the medium. The results for the reflection coefficients and their respective energy ratios for various values of the angle of incidence are computed numerically and presented graphically for copper-like material and discussed.

Unsteady Flow and Heat Transfer Characteristics of Primary and Secondary Corrugated Channels

Abstract: Direct numerical simulations (DNS) have been performed using open source computational fluid dynamics (CFD) package openfoam to investigate the unsteady flow and heat transfer characteristics of primary and secondary corrugated wavy channels for fixed values of wavelength and amplitude by varying Reynolds number (Re). Computations are carried out by considering only one module of the wavy channel and applying periodic boundary conditions to reduce the time for computations. Numerical method has been validated thoroughly against the numerical and experimental results reported in the literature. Steady as well as unsteady flow characteristics in the primary and secondary corrugated channels have been elaborated and explained with the help of streamlines, velocity contours, isosurfaces of velocity, and Q value. Temperature and Nusselt number contours are presented to illustrate the heat transfer characteristics of wavy channels.

Thermal Exchange of Glass Micro-Fibers Measured by the 3ω Technique

Abstract: In this work, we propose an experimental setup to measure the thermal conductivity and specific heat of a single suspended glass fiber, as well as the thermal contact resistance between two glass fibers. By using optical lithography, wet and dry etching and thin film deposition, we prepared suspended glass fibers that are coated by niobium nitride (NbN) thin film used as room temperature thermal transducer. By using the 3ω technique, the thermal conductivity of glass fiber was measured to be 1.1 W m−1 K–1 and specific heat 0.79 J g−1 K–1 around 300 K under vacuum conditions. By introducing exchange gas into the measurement chamber, influence of the gas on the heat transfer was studied, and the convection coefficient h for all the measurement ranges from a pressure of 0.01 hPa to 1000 hPa, over more than five orders of magnitude, has been obtained. By adding a bridging glass fiber on top of two other suspended glass fibers, it was possible to estimate the thermal contact resistance between two glass fibers Rc in the range of 107–108 K W–1.