Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2022"

A Three-Dimensional Non-Newtonian Magnetic Fluid Flow Induced Due to Stretching of the Flat Surface with Chemical Reaction

Abstract: Three-dimensional (3D) flow of non-Newtonian liquid is studied in this analysis. Also, this paper is mainly focused on an incompressible magnetic liquid with moderate saturation magnetization and low Curie temperature. An extremely long, straight wire delivering an electric current generates a magnetic field that affects the fluid. To study heat and mass transport characteristics thermal radiation and chemical reaction impacts are considered. The pertinent flow expressions are reduced into ordinary differential equations (ODEs) through appropriate transformations. The obtained ODEs are solved by means of numerical method (Runge-Kutta-Fehlberg's fourth-fifth order method (RKF-45) algorithm with shooting technique). Results reveals that, the raising the value of Deborah number causes stress relaxation, which affects the flow characteristics of the fluid but improves heat transport. The improved values of radiation parameter advance the heat transport. The rise in values of chemical reaction rate parameter declines the mass transfer.

Spectral Simulation On the Flow Patterns and Thermal Control of Radiative Nanofluid Spraying On an Inclined Revolving Disk Considering the Effect of Nanoparticle Diameter and Solid-Liquid Interfacial Layer

Abstract: The current work enlightens the flow pattern and thermal scenario of nanofluid spraying on an inclined permeable rotating disk. The whirling disk is assumed to revolve with angular speed . The water-based alumina (Al2O3) nanofluid is considered as a functioning liquid. The nanofluid spraying is treated to be magnetically influenced and as well as thermally radiative. The perception of the nanoparticle diameter and solid-liquid interfacial layer is incorporated precisely at the nanolevel to observe the thermal variations of the nanofluidic motion. How the magnetic effect, permeability, and nanolayer affect the nanofluidic transportation is revealed in detail. The leading flow equations are altered nondimensional using apposite similarity translation and spectral quasi linearization method (SQLM) is instigated to tackle those multi-ordered nonlinear equations. Various three-dimensional figures, graphs, tables are described to detect and analyze the hydrothermal variations. The linear regression slope technique is addressed to extract the reduction or enhancement rate of heat transference. Also, the probable error is estimated statistically to assure that hydrothermal characteristic is correlated with physical parameters. The consequences indicate enhanced heat transport for nanolayer, but reduced heat transmission for nanoparticles' diameter. Thermal profile intensifies for thickness parameter and impermeable surface, whereas decreases for nanoparticles' diameter.

Opportunities in Nanoengineered Surface Designs for Enhanced Condensation Heat and Mass Transfer

Abstract: Recent advancements in surface nanoengineering have spurred intense interests in their implementation for enhancing condensation heat transfer. When appropriately designed, nanoengineered surfaces not only lead to highly efficient transport mechanisms not achievable with conventional dropwise condensation, they also demonstrate the possibility of augmenting condensation of low surface tension fluids widely used in industry. These advantages are further enhanced by the development of highly scalable nanofabrication methods, which enable the potential transition from laboratory-scale prototypes to real-world industrial applications. In this review, we discuss the progress, opportunities, and challenges of enhancing condensation heat and mass transfer with nanoengineered surfaces. This article provides an overview of the recent developments in micro/nanoscale coating and structure fabrication techniques and performs a thorough comparison of their condensation performance, elucidating the complex interfacial transport mechanism involved. Surface structuring methods that are durable, scalable and low-cost are essential attributes for large-scale industrial implementation. Here, the methods used to improve surface durability and demonstrations of nanostructure-enhanced meter-scale condensers are presented. Limitations are discussed and the potential techniques to overcome these challenges are summarized. Given the recent development of metal additive manufacturing technology and its growing relevance in manufacturing processes, we end this review by providing our perspectives on the opportunities in enabling surface nanostructuring of metal additive manufactured materials and the potential of nanometric-millimetric co-design optimization for the development of next-generation additively manufactured condensers.

Falkner-Skan Problem for a Stretching or Shrinking Wedge with Nanoparticle Aggregation

Abstract: The Falkner-Skan problem for stretching or shrining wedge is generalized for nanoparticle aggregation effects. The model is developed in the presence of magnetic field, thermal radiation and suction/injection effects. For the inclusion of nanoparticle aggregation effects, modifications of Krieger-Dougherty model and Maxwell and Bruggeman models are used to predict effective viscosity and thermal conductivity of TiO2/EG nanofluid, respectively. These models are already tested experimentally in the past and are known to predict the true values for the TiO2/EG nanofluid with aggregated nanoparticles. The system of equations depicting the Falkner-Skan problem for wedge with nanoparticle aggregation effects is transformed via similarity transformations and solved via "bvp4c" function, which is accessible by MATLAB software. The validation of results is done through comparison of results with published literature and comparison of present results with "bvp5c" function and RKF-Shooting Technique. As suggested by the previously published experimental studies, it is observed that the nanoparticle aggregation effects are strong even when the nanoparticles concentration is low. The heat transmission rate of TiO2/EG nanofluid is seen higher with nanoparticles aggregation effects in comparison to its absence. The streamlines become denser and more intense with presence of magnetic field. The results of this study are applicable to a number of thermal systems, engineering and industrial process, which utilize nanofluid for cooling, and heating process.

Thermophysical Analysis of Microconfined Turbulent Flow Regimes At Supercritical Fluid Conditions in Heat Transfer Applications

Abstract: The technological opportunities enabled by understanding and controlling microscale systems have not yet been capitalized to disruptively improve energy processes. The main limitation corresponds to the laminar flows typically encountered in microdevices, which result in small mixing and transfer rates. This is a central unsolved problem in the thermal-fluid sciences, in what some researchers refer to as “quot;ab-on-a-chip and energy - microfluidic frontier”. Therefore, this work focuses on analyzing the potential of supercritical fluids to achieve turbulence in microconfined systems by studying their thermophysical properties. In particular, a real-gas thermodynamic model, combined with high-pressure transport coefficients, is utilized to characterize the Reynolds number achieved as a function of supercritical pressures and temperatures. The results indicate that fully-turbulent flows can be attained for a wide range of working fluids related to heat transfer applications, power cycles and energy conversion systems, and presenting increment ratios of O(100) with respect to atmospheric (subcritical) thermodynamic conditions. The underlying physical mechanism to achieve relatively high Reynolds numbers is based on operating within supercritical thermodynamic states (close to the critical point and pseudo-boiling region) in which density is relatively large while dynamic viscosity is similar to that of a gas. In addition, based on the Reynolds numbers achieved and the thermophysical properties of the fluids studied, an assessment of heat transfer at turbulent microfluidic conditions is presented to demonstrate the potential of supercritical fluids to enhance the performances of standard microfluidic systems by factors up to approximately 50x.

Optimization of the Thermal Performance of Three-Dimensional Integrated Circuits (3D ICs) Utilizing Rectangular-Shaped and Disk-Shaped Heat Pipes

Abstract: Rectangular-shaped and disk-shaped heat pipes, as innovative heat sinks, are investigated to optimize the thermal performance of three-dimensional integrated circuits (3D ICs) in this work. Finite volume numerical analysis is employed to carry out the simulation of the thermal performance of 3D ICs. Both rectangular-shaped and disk-shaped heat pipes substantially improved the overall thermal performance and reduced the hotspot temperatures by 7 K and 11 K on average, respectively. Furthermore, utilizing the rectangular-shaped or the disk-shaped heat pipe as the heat spreader in place of a solid copper heat spreader further optimizes the thermal performance by reduction of the junction temperatures 14 K and 16 K on average, respectively. These reductions are achieved while the weight of the set-up is also significantly reduced. The results indicate that the innovative flat-shaped heat pipes significantly optimize the thermal performance of 3D ICs. The model and results presented in this work aim to pave the way to markedly alleviate the thermal issues of the 3D ICs.

Direct Pore-Scale Simulations of Fully Periodic Unit Cells of Different Regular Lattices

Abstract: Abstract Open-cell metal foams are known for their superior heat dissipation capabilities. The morphological, pressure drop, and heat transfer characteristics of stochastic metal foams manufactured through traditional “foaming” processes are well established in the literature. However, employment of stochastic metal foams in next-generation heat exchangers is challenged by the irregularity in the pore- and fiber-geometries, limited control on the pore-volume, and an inherent necessity of a bonding agent between foam and the heat source. On the other hand, additive manufacturing (AM) is capable of printing complex user-defined unit cell topologies with customized fiber shapes directly on the substrates subjected to heat load. Moreover, the user-defined regular lattices are capable of exhibiting better thermal and mechanical properties than stochastic metal foams. In this paper, we present a numerical investigation on fully periodic unit-cells of three different topologies, that is, tetrakaidecahedron (TKD), rhombic-dodecahedron (DDC), and Octet with air as the working fluid. Pressure gradient, interfacial heat transfer coefficient, friction factor, and Nusselt number are reported for each topology. Rhombic-dodecahedron yielded the highest averaged interfacial heat transfer coefficient whereas Octet incurred the highest flow losses. Pore diameter, defined as the maximum diameter of a sphere passing through the polygonal openings of the structures, when used as the characteristic length scale for the presentation of Nusselt number and Reynolds number, resulted in a single trendline for all the three topologies.

Conjugate Free Convection Heat Transfer and Thermodynamic Analysis of Infrared Suppression Device with Cylindrical Funnels

Abstract: A convection system can be designed as an energy-efficient one by making a considerable reduction in exergy losses. In this context, entropy generation analysis is performed on the infrared suppression system numerically. In addition, results due to heat transfer are also shown. The numerical solution of the Navier-stokes equation, energy equation, and turbulence equation is executed using ANSYS Fluent 15.0. To perform the numerical analysis, different parameters such as the number of funnels, Rayleigh number (Ra), inner surface temperature, and geometric ratio are varied in the practical range. Results are shown in terms of heat transfer, entropy generation, irreversibility (due to heat transfer and fluid friction), and Bejan number with some relevant parameters. Streamlines and temperature contours are also provided for better visualization of temperature and flow field around the device. Results show that heat transfer and mass flow rate increase with the increase in Ra. Entropy generation and the irreversibility rise with an increase in the number of funnels and geometric ratio. Also, the Bejan number decreases with an increase in Ra and the number of funnels. A cooling time is also obtained using the lumped capacitance method.

Free Convection Heat Transfer From a Spherical Shaped Open Cavity

Abstract: A computational work is performed on laminar free convection from an isothermally heated spherical shaped open cavity with negligible wall thickness suspended in the air. Fluid flow and heat transfer characterization is elucidated in detail by solving governing differential equations (continuity, momentum, and energy) over wide ranges of dimensionless pertinent parameters, namely, Rayleigh number, 104 ≤ Ra ≤ 108; and height to diameter ratio, 0.15 ≤ h/D ≤ 0.95. The detailed behavior of thermal and flow fields is delineated by employing visualization techniques like temperature contours, velocity profiles, and velocity vectors for different Ra and h/D. The influence of Ra and h/D on the local and average Nusselt number is also predicted and it is observed that the average Nusselt number on both outer and inner surface decreases with the increase of h/D for a constant value of Ra. A suitable correlation for net average Nusselt number is obtained for the spherical shaped open vessel surface as a function of Ra, and h/D based on the computed data points which would be helpful for various academic and industrial operations.

Optimized Phonon Band Discretization Scheme for Efficiently Solving the Nongray Boltzmann Transport Equation

Abstract: Abstract The phonon Boltzmann transport equation (BTE) is an important tool for studying the nanoscale thermal transport. Because phonons have a large spread in their properties, the nongray (i.e., considering different phonon bands) phonon BTE is needed to accurately capture the nanoscale transport phenomena. However, BTE solvers generally require large computational cost. Nongray modeling imposes significant additional complexity on the numerical simulations, which hinders the large-scale modeling of real nanoscale systems. In this work, we address this issue by a systematic investigation on the phonon band discretization scheme using real material properties of four representative materials, including silicon, gallium arsenide, diamond, and lead telluride. We find that the schemes used in previous studies require at least a few tens of bands to ensure the accuracy, which requires large computational costs. We then propose an improved band discretization scheme, in which we divide the mean free path domain into two subdomains, one on either side of the inflection point of the mean free path accumulated thermal conductivity, and adopt the Gauss–Legendre quadrature for each subdomain. With this scheme, the solution of the phonon BTE converges (error < 1%) with less than ten phonon bands for all these materials. The proposed scheme allows significantly reducing the time and memory consumption of the numerical BTE solver, which is an important step toward large-scale phonon BTE simulations for real materials.

Comprehensive Review of Heat Transfer Correlations of Supercritical CO2 in Straight Tubes Near the Critical Point: A Historical Perspective

Abstract: An exhaustive review was undertaken to assemble all available correlations for supercritical CO2 in straight, round tubes of any orientation with special attention paid to how the wildly varying fluid properties near the critical point are handled. The assemblage of correlations, and subsequent discussion, is presented from a historical perspective, starting from pioneering work on the topic in the 1950s to the modern day. Despite the growing sophistication of sCO2 heat transfer correlations, modern correlations are still only generally applicable over a relatively small range of operating conditions, and there has not been a substantial increase in predictive capabilities. Recently, researchers have turned to machine learning as a tool for next-generation heat transfer prediction. An overview of the state-of-the-art of predicting sCO2 heat transfer using machine learning methods, such as artificial neural networks, is also presented.

Microscale to Macroscale - Extending Microscale Enhancement Techniques to Large Scale Boiling Equipment

Abstract: Boiling is a multiscale phenomenon. Nucleation and rapid bubble growth at the heated wall provide a highly localized mechanism for heat transfer to the surrounding liquid. The liquid-vapor interface of the growing bubble supplies latent heat needed to evaporate the liquid and sustain the bubble activity. Although the boiling process is efficient in removing large amount of heat, further improvements are needed to increase the critical heat flux (CHF) as well as heat transfer coefficient (HTC) in many applications. Recent developments in enhancing boiling heat transfer have mainly focused on small-scale heaters, typically on the order of a centimeter, that are particularly relevant in electronics cooling application. Many of these developments are based on a fundamental understanding of the microscale processes of bubble nucleation, bubble growth and removal from the heater surface, and supply of liquid to the active nucleation sites. Some of these microscale enhancement techniques have set new records in heat dissipation (both CHF and HTC). This paper explores the potential of these microscale enhancement techniques in large-scale boiling equipment, such as boilers, reboilers and evaporators in power generation, refrigeration, air-conditioning, cryogenic, desalination, chemical, petrochemical, pharmaceutical and other industries.

Generalized Bio-Heat Transfer Model Combining with the Relaxation Mechanism and Non-Equilibrium Heat Transfer

Abstract: The heat transport within living biological tissue is a complicated process coupled various physiological activities, and its non-homogeneous inner anatomical structure leads to an essential difference from classical heat transfer. The generalized model of bio-heat transfer involving the relaxation mechanism as well as non-equilibrium heat transfer is first proposed to explore the heat transport properties within living biological tissues. Due to the volume averaging theory, the new local instantaneous energy equations of blood and tissue are constructed separately by implementing the phase lags, in which the delay effect between the heat flux and temperature gradient absent in existing generalized models is considered. The effective phase lags covering the delay effect and non-equilibrium effect are obtained on this basis. The detailed parametric study has been carried out to estimate the values of these effective phase lags and evaluate their contributions on heat transport within living biological tissues. The results states that the effective phase lags depend on the anatomical structure of tissues and its physical properties. The delay effect is dominated in general and has a higher temperature elevation than that induced by non-equilibrium effect only.

Thermosolutal Marangoni Bioconvection of a Non-Newtonian Nanofluid in a Stratified Medium

Abstract: Bioconvection due to the movement of microorganism cells is universal and affects many ecological and biological processes, including infection, reproduction, and marine life ecosystem. The impact of bioconvection is more significant in nanofluids. In the present problem, we investigate the Marangoni triply stratified bioconvective flow of Non-Newtonian (second grade) nanofluid with the presence of motile microorganisms over a permeable inclined plate. Impact of important second order effects namely, viscous dissipation, radiation and chemical reaction are analyzed in the problem which allows a set of similarity transformations to convert the governing PDEs into a coupled nonlinear DEs. Thereafter, the Runge-Kutta Fehlberg's numerical method is employed to find the solution of the DEs for some chosen values of different flow influencing parameters. The impact of crucial parameters on the velocity, temperature, nanoparticle volume fraction, motile density of microorganisms and on the quantities of physical interest namely local Nusselt number, Sherwood number, and local motile microorganism density number are illustrated through the plots and tables. It is found that the second grade fluid parameter indicates a prominent correlation with the Marangoni convection in bioconvective transport mechanism. Also, the Marangoni convection is significant in bioconvective flows for a large Peclet number.

Adaptive Computations to Pressure Profile for Creeping Flow of A Non-Newtonian Fluid with Fluid Non-Constant Density Effects

Abstract: The sperm density through the cervical canal plays a dynamic part in promoting the pregnancy progressions of organisms. Therefore, this study aims to probe the combined effects of concentration and temperature-dependent density on the creeping flow of Carreau nanofluid in the cervical canal as the first look in this direction. Chemical reaction and Hall effects are considered. The system of a physical model is simplified /streamlined using appropriate transformation (d«1). The system that describes the fluid model is recurrence/ rearranged with aid of AST by a computer program using Mathematica 13.1.0. Solutions are offered via sketches on the pressure profiles. Besides, graphs of streamlined are achieved in dissimilar values of the non-constant density of the fluid. To get accurate results and approve the validation of the proposed technique, a comparison with Noreen et al [3] is obtained and seems to be very good. The results indicate that high values of non-constant density parameters impose a pressure gradient in the cervical canal, which supports the sperm to be more energetic in ovum fertilizing.

Heat and Mass Transfer Correlations for Staggered Nanoporous Membrane Tubes in Flue Gas Crossflow

Abstract: In this study, the heat transfer, mass transfer and pressure drop imposed by the crossflow ceramic nanoporous tubes in Transport Membrane Condenser (TMC) have been studied numerically within wide ranges of tube diameters (4.57-7.62 mm), number of rows (2-24 rows), and Reynolds number (170-8900), under flue gas condensation. The flue gas was consisting of one condensable water vapor (H2O) and three non-condensable gases (CO2, O2, and N2). The turbulent flow of the flue gas mixture was modeled by the shear stress transport SST k-? turbulence model. A hybrid/mixed condensation model written in user defined functions (UDFs) was employed to calculate the water vapor condensation rate. Numerical results with condensing flue gas are compared to available correlations for single-phase Nusselt number and pressure drops in the literature. It was found that except for selected conditions, the single-phase correlations were noticed to differ from the TMC numerical results. Empirical TMC correlations for heat transfer and pressure drops with respect to condensation rate, number of rows, and the nanoporous membrane geometrical properties were derived thereby. The derived correlations for TMC show a good agreement with numerical data for all investigated parameters and can predict the 96% of the convective Nusselt number, overall Nusselt number, and friction factor inside the TMC within ±10%, ±10% and ±15% respectively. The effects of key parameters on the heat transfer, mass transfer, and pressure drops are illustrated and discussed in detail.

Revisiting the Schrage Equation for Kinetically-Limited Evaporation and Condensation

Abstract: The Schrage equation is commonly used in thermofluid engineering to model high-rate liquid-vapor phase change of pure fluids. Although shortcomings of this simple model were pointed out decades ago and more rigorous models have emerged from the kinetic theory community, Schrage's equation continues to be widely used. In this paper, we quantify the accuracy of the Schrage equation for evaporation and condensation of monatomic and polyatomic fluids at the low to moderately high flux operating conditions relevant to thermofluid engineering applications. As a high-accuracy reference, we numerically solve a BGK-like model equation for polyatomic vapors that has previously been shown to produce accurate solutions to the Boltzmann transport equation. We observe that the Schrage equation overpredicts heat/mass fluxes by ~15% for fluids with accommodation coefficients close to unity. For fluids with smaller accommodation coefficients, such as water, the Schrage equation yields more accurate flux estimates. We find that the Mott-Smith-like moment methods developed for liquid-vapor phase change are much more accurate than the Schrage equation, achieving heat/mass flux estimates that deviate by less than 1% (evaporation) and 4% (condensation) from the reference solution. In light of these results, we recommend using the moment-method equations instead of the Schrage equation. We also provide: tables with our high-accuracy numerical data for evaporation of any fluid and condensation of saturated water vapor; engineering equations fit to our data; and code for moment method calculations of evaporation and condensation.

Spatiotemporal Evolution of Temperature During Transient Heating of Nanoparticle Arrays

Abstract: Nanoparticles (NPs) are promising agents to absorb external energy and generate heat. Clusters of NPs or NP array heating have found an essential role in several biomedical applications, diagnostic techniques, and chemical catalysis. Various studies have shed light on the heat transfer of nanostructures and greatly advanced our understanding of NP array heating. However, there is a lack of analytical tools and dimensionless parameters to describe the transient heating of NP arrays. Here we demonstrate a comprehensive analysis of the transient NP array heating. Firstly, we develop a set of analytical solutions for the NP array heating and provide a useful mathematical description of the spatial-temporal evolution of temperature for 2D, 3D, and spherical NP array heating. Based on this, we introduce the concept of thermal resolution that quantifies the relationship between minimal heating time, NP array size, energy intensity, and target temperature. Lastly, we define a set of dimensionless parameters that characterize the transition from confined heating to delocalized heating. This study advances the understanding of nanomaterials heating and guides the rational design of innovative approaches for NP array heating.

Weakly Nonlinear Stability of Thermosolutal Convection in an Oldroyd-B Fluid-Saturated Anisotropic Porous Layer Using a Local Thermal Nonequilibrium Model

Abstract: The two-temperature model of local thermal nonequilibrium(LTNE) is utilized to investigate a weakly nonlinear stability of thermosolutal convection in an Oldroyd-B fluid-saturated anisotropic porous layer. The anisotropies in permeability, thermal conductivities of the porous medium and solutal diffusivity are accounted for by second order tensors with their principal directions coinciding with the horizontal and vertical coordinate axes. A modified Darcy-Oldroyd model is employed to describe the flow in a porous medium bounded by impermeable plane walls with uniform and unequal temperatures as well as solute concentrations. The cubic Landau equations are derived in the neighbourhood of stationary and oscillatory onset using a modified perturbation approach and the stability of bifurcating equilibrium solutions is discussed. The advantage is taken to present some additional results on the linear instability aspects as well. It is manifested that the solutal anisotropy parameter also plays a decisive role on the instability characteristics of the system. It is found that the stationary bifurcating solution transforms from supercritical to subcritical while the oscillatory bifurcating solution transforms from supercritical to subcritical and revert to supercritical. The Nusselt and Sherwood numbers are used to examine the influence of LTNE and viscoelastic parameters on heat and mass transfer, respectively. The results of Maxwell fluid are outlined as a particular case from the present study.

A Compact WSGG Formulation to Account for Inhomogeneity of H2O-CO2 Mixture in Combustion Systems

Abstract: An alternative weighted-sum-of-gray-gas (WSGG) model is proposed to deal with conditions that are common in oxy-?red combustion. The method proposes a single set of pressure-based absorption coefficients that account for different mole fraction ratios (MR) of H2O-CO2, thus requiring no further interpolation, which in turn brings not only less uncertainty into the model, but also simplifies its use. The HITEMP-2010 spectral database along with the line-by-line (LBL) integration is employed to generate a set of accurate total emissivities from which the WSGG coefficients are fitted. The fitting procedure employs a novel formulation to account for the MR dependence, leading to a more compact set of WSGG correlations when compared to the alternatives available in the literature. As oxy-?red combustion usually occurs in two distinct scenarios, dry- and wet- ?ue-gas-recirculation (FGR), the paper also proposes two other sets of coefficients intended to support the MR ranges corresponding to these specific conditions. Comparisons made against the benchmark LBL integration and other WSGG models, for one- and three-dimensional calculations, show the satisfactory level of accuracy of the proposed sets of correlations. In particular, the three-dimensional test case illustrates that the new model is also applicable to conditions observed in air-fuel combustion.

A Comparative Modeling Study of Thermal Mitigation Strategies in Irreversible Electroporation Treatments

Abstract: Irreversible electroporation (IRE), also referred to as nonthermal pulsed field ablation (PFA), is an attractive focal ablation modality for solid tumors and cardiac tissue due to its ability to destroy aberrant cells with limited disruption of the underlying tissue architecture. Despite its nonthermal cell death mechanism, application of electrical energy results in Joule heating that, if ignored, can cause undesired thermal injury. Engineered thermal mitigation (TM) technologies including phase change materials (PCMs) and active cooling (AC) have been reported and tested as a potential means to limit thermal damage. However, several variables affect TM performance including the pulsing paradigm, electrode geometry, PCM composition, and chosen active cooling parameters, meaning direct comparisons between approaches are lacking. In this study, we developed a computational model of conventional bipolar and monopolar probes with solid, PCM-filled, or actively cooled cores to simulate clinical IRE treatments in pancreatic tissue. This approach reveals that probes with integrated PCM cores can be tuned to drastically limit thermal damage compared to existing solid probes. Furthermore, actively cooled probes provide additional control over thermal effects within the probe vicinity and can altogether abrogate thermal damage. In practice, such differences in performance must be weighed against the increased time, expense, and effort required for modified probes compared to existing solid probes.

Impact of Thermal Source-sink Arrangements On Buoyant Convection in a Nanofluid-Filled Annular Enclosure

Abstract: The present investigation is devoted to analyze the buoyancy-driven flow behavior and associated thermal dissipation rate in a nanofluid-filled annular region with five different single source-sink and three different dual source-sink arrangements along the vertical surfaces. The remaining region on the vertical boundaries and horizontal surfaces are kept adiabatic. Numerical simulations have been performed by solving the dimensionless model equations by employing finite difference method. To analyze the impacts of the type of nanofluid, nanoparticle volume fraction, Rayleigh number, size and arrangement of sources and sinks, the results are graphically represented through streamline and isotherm contours, thermal profiles, average Nusselt number and cup-mixing temperature. Comparison of the results of current investigation reveal fairly good agreement with the existing benchmark results. The results showed that identifying an optimum location and length of source-sink with a proper selection of other control parameters can lead to enhanced thermal transport and thermal mixing in the enclosure. Also, the calculated enhancement ratio of the heat dissipation rate enhances with an increment in nanoparticle concentration.

Role of Three-Dimensional Swirl in Forced Convection Heat Transfer Enhancement in Wavy-Plate-Fin Channels

Abstract: The influence of swirl flow on enhanced forced convection in wavy-plate-fin cores has been investigated. Three-dimensional computational simulations were carried out for steady-state, periodically developed flow of air (Pr ~ 0.71) with channel walls subject to constant-uniform temperature and flow rates in the range 50 = Re = 4000. The recirculation that develops in the wall troughs and grows to an axial helix is scaled by the Swirl number Sw. As Sw increases, tornado-shaped vortices appear in the wave trough region mid-channel height, then extend longitudinally to encompass majority of the flow channel. As shown by the local wall-shear and heat transfer coefficient variations, the boundary-layer thinning upstream of the wave peak assists to intensify the momentum and heat transfer. However, the flow recirculation in wave trough impedes the local heat transfer at low Sw due to flow stagnation but promotes it at high Sw because of swirl-augmented fluid mixing. Swirling flows also create pressure drag that contributes substantively to the overall pressure loss. Its proportion grows as Sw, corrugation severity, and fin spacing increases to as much as 80% of the total pressure drop. The fin-wall curvature-induced secondary circulation nevertheless produces significantly enhanced convection, and more so in flows with higher Sw. It is quantified by Ff (or j), which is seen to increase log-linearly as fin corrugation aspect ratio and/or fin spacing ratio increases; the influence of cross-section aspect ratio is found to be marginal.

Numerical Investigation of Thermal-Hydraulic Performance of U-Tubes Enhanced with Ellipsoidal 45° Dimples

Abstract: This study aims to numerically investigate thermal-hydraulic performance augmentation of ellipsoidal 45o dimpled U-tubes with various bend curvatures, subjected to constant external heat flux (q″=10kW/m2), for a range of Reynolds numbers (5000 ≤ Re ≤ 30000). Three smooth U-bends with curvatures radii of 0.695Dh, 1.5Dh, and 2.0Dh, where Dh is hydraulic diameter of smooth tube, are used in both smooth and enhanced tubes. A comparison of thermal-hydraulic characteristics of dimpled and smooth U-tubes is carried out using steady-state Reynolds Averaged Navier Stokes simulations. The analysis shows that the performance of dimpled U-tube is superior to smooth tube for all bend curvatures. The swirl flow patterns generated by the dimples induce early flow reattachment in the post bend section of the tube which enhances its heat transfer rate. The dimpled U-tube having the shortest curvature radius significantly alters Dean vortices which leads to substantial improvement in its heat and flow performances. Contrary to smooth U-tube, the average thermo-hydraulic performance of the shortest radius of curvature of dimpled U-tube is enhanced by 21.4% while other curvature radii (1.5Dh, and 2.0Dh) dimple U-tubes enhances the performance by 10.7% and 8.9%, respectively.

A New Approach to Solve Inverse Boundary Design of a Radiative Enclosure With Specular–Diffuse Surfaces

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.

A Steady-State Energy-Based Monte Carlo Method for Phonon Transport with Arbitrary Temperature Difference

Abstract: A steady-state Monte Carlo scheme is developed for phonon transport based on the energy-based deviational phonon Boltzmann transport equation (PBTE). Other than tracking trajectories and time evolution of each packet in the transient methods, this steady-state method determines the paths of energy packet from being emitted to the steady state through statistics of scattering propability. By reconsidering and developing the periodic heat flux boundary condition, we extend the capability of this method to systems with arbitrary temperature difference. This steady-state energy-based Monte Carlo (SEMC) method has been verified by comparisons with predictions with those by the previous discrete-ordinates method, the analytical solution, and transient MC methods for phonon transport in or across thin films. The present SEMC algorithm significantly improve the computational efficiency for a steady phonon transport process, instead of time evolution by a transient simulation.

Modeling of Supercritical Co2 Shell-and-Tube Heat Exchangers Under Extreme Conditions. Part 2: Heat Exchanger Model

Abstract: Heat exchangers play a critical role in supercritical CO2 Brayton cycles by providing necessary waste heat recovery. Supercritical CO2 thermal cycles potentially achieve higher energy density and thermal efficiency operating at elevated temperatures and pressures. Accurate and computationally efficient estimation of heat exchanger performance metrics at these conditions is important for the design and optimization of sCO2 systems and thermal cycles. In this paper (Part II), a computationally efficient and accurate numerical model is developed to predict the performance of STHXs. Highly accurate correlations reported in Part I of this study are utilized to improve the accuracy of performance predictions, and the concept of volume averaging is used to abstract the geometry and reduce computation time. The numerical model is validated by comparison with CFD simulations and provides high accuracy and significantly lower computation time compared to existing numerical models. A preliminary optimization study is conducted and the advantage of using supercritical CO2 as a working fluid for energy systems is demonstrated.

MHD Natural Convection of Water in An L-Shaped Container Filled with An Aluminium Metal Foam

Abstract: The current research employed computational simulation to assess magnetohydrodynamics (MHD) natural convection in an L-shaped container with metal foam, which has a variety of engineering applications such as electronic systems, heat exchangers, solar collectors and nuclear energy. As a result, the originality of this study is employing numerical simulation together with response surface methodology (RSM) to investigate the optimal natural convection in an L-shape container filled with metal foam with and without MHD. The influence of varied aspect ratios (AR), tilted angles of the container (θ), Hartmann number (Ha) and porosities (F) on the cooling rate concerning average Nusselt number (Nuave), Nusselt number enhancement (NNE), surface temperature (Ts) and entropy generation (S) were evaluated. According to the findings of this study, no effect of the MHD and θ on the Nuave, Ts and S except for porosity of 0.9. Furthermore, the Nuave enhances while the Ts and S reduce as the aspect ratio of horizontal (ARh), vertical (ARv), and Darcy number (Da) increases. The multi-objective optimisation methodology for the highest desirability is accomplished as ARv, ARh and Da are 0.9, 0.9 and 10-1 respectively. In this case, the maximum increase in NNE was 26.7 times with the greatest reductions in Ts and S being 59% and 97% respectively compared to the unfavourable design data. Thus, this combination investigation of the CFD and RSM yields a novel approach and valuable recommendations for the optimum cooling L-shaped container design.