Showing papers in "Heat Transfer Engineering in 2016"
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TL;DR: In this article, a critical review of the theoretical, empirical, and numerical models for effective viscosity of nanofluids is presented, and different parameters affecting the visco-lasticity of nanoparticles such as nanoparticle volume fraction, size, shape, temperature, pH, and shearing rate are reviewed.
Abstract: The enhanced thermal characteristics of nanofluids have made it one of the most raplidly growing research areas in the last decade. Numerous researches have shown the merits of nanofluids in heat transfer equipment. However, one of the problems is the increase in viscosity due to the suspension of nanoparticles. This viscosity increase is not desirable in the industry, especially when it involves flow, such as in heat exchanger or microchannel applications where lowering pressure drop and pumping power are of significance. In this regard, a critical review of the theoretical, empirical, and numerical models for effective viscosity of nanofluids is presented. Furthermore, different parameters affecting the viscosity of nanofluids such as nanoparticle volume fraction, size, shape, temperature, pH, and shearing rate are reviewed. Other properties such as nanofluid stability and magnetorheological characteristics of some nanofluids are also reviewed. The important parameters influencing viscosity of nanofluid...
177 citations
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TL;DR: In this paper, heat transfer and pressure drop correlations for fully developed laminar Newtonian fluid flow in curved and coiled circular tubes are reviewed. The main purpose of this review paper is to provide researchers with a comprehensive list of correlations and concepts that they may need during their research.
Abstract: Heat transfer and pressure drop correlations for fully developed laminar Newtonian fluid flow in curved and coiled circular tubes are reviewed. Curved geometry is one of the passive heat transfer enhancement methods that fits several heat transfer applications, such as power production, chemical and food industries, electronics, environment engineering, and so on. Centrifugal force generates a pair or two pairs of cross-sectional secondary flow (based on the Dean number), which are known as the Dean vortices, and improves the overall heat transfer performance with an amplified peripheral Nusselt number variation. The main purpose of this review paper is to provide researchers with a comprehensive list of correlations and concepts that they may need during their research. The paper begins with an introduction to the governing equations and important dimensionless numbers for the flow in curved tubes. The correlations for developing flow in curved and coiled circular tubes are also presented. The main contr...
67 citations
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TL;DR: In this paper, the authors investigate an application of porous aluminum foam by using the local thermal equilibrium (LTE) and local thermal nonequilibrium (LTNE) heat transfer models.
Abstract: Aluminum foams are favorable in modern thermal engineering applications because of the high thermal conductivity and the large specific surface area. The present study aims to investigate an application of porous aluminum foam by using the local thermal equilibrium (LTE) and local thermal nonequilibrium (LTNE) heat transfer models. Three-dimensional simulations of laminar flow (porous foam zone), turbulent flow (open zone), and heat transfer are performed by a computational fluid dynamics approach. In addition, the Forchheimer extended Darcy's law is employed to evaluate the fluid characteristics. By comparing and analyzing the average and local Nusselt numbers, it is found that the LTNE and LTE models can reach the same Nusselt numbers inside the aluminum foam when the air velocity is high, meaning that the aluminum foam is in a thermal equilibrium state. Besides, a high interfacial heat transfer coefficient is required for the aluminum foam to reach a thermal equilibrium state as the height of the alumi...
52 citations
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TL;DR: In this paper, the authors combined both experimental testing and high-fidelity computer fluid dynamics (CFD) models to study various aspects of thermal management issues in battery systems, with an emphasis on comparing the operation of prismatic and cylindrical batteries under extremely hot environments.
Abstract: Lithium ion (Li-ion) batteries are promising as both alternative and auxiliary power sources in hybrid and electric vehicles. However, the reliable and efficient operation of the Li-ion batteries depends critically on effective thermal management, due to both the high operational thermal loads and the possible range operational conditions. This work therefore studied the issue of thermal management of battery systems under extreme hot conditions. This study combined both experimental testing and high-fidelity computer fluid dynamics (CFD). Due to the difficulty of conducting experiments under extreme conditions, controlled experiments were first conducted so that CFD models could be validated. The validated CFD models were then applied to study various aspects of thermal management issues in battery systems, with an emphasis on comparing the operation of prismatic and cylindrical batteries under extremely hot environments. The results presented include temperature distribution among cells, pump power requ...
42 citations
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TL;DR: In this paper, an analysis of natural convection with heating source protruding from the nonheated lower surface of a rectangular enclosure has been performed using experimental apparatus of a two-dimensional particle image velocimetry system.
Abstract: Analysis of natural convection with heating source protruding from the nonheated lower surface of a rectangular enclosure has been performed using experimental apparatus of a two-dimensional particle image velocimetry system. Results obtained from the experiments are used to validate the numerical simulations. Extensive numerical simulation is carried out using in-house code based on the finite-volume method and the SIMPLE algorithm. Heat transfer and entropy generation are estimated numerically for a protruding heater of different perimeters and aspect ratios, Rayleigh number, and Prandtl number. It is found that the Rayleigh number, Prandtl number, and heater sizes have strong influence on the flow fields, thermal mixing, heat transfer characteristics, and entropy production rate in the enclosure. The analysis indicates that a high thermal mixing may not be the most favorable situation for achieving higher degree of temperature uniformity. The effect of Bejan number is discussed.
41 citations
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TL;DR: In this article, a triplex tube heat exchanger (TTHX) with phase-change materials (PCM) in the middle tube was used to power a liquid desiccant air-conditioning system.
Abstract: Thermal energy storage is critical for reducing the discrepancy between energy supply and energy demand, as well as for improving the efficiency of solar thermal energy systems. Among the different types of thermal energy storage, phase-change materials (PCM) thermal energy storage has gained significant attention recently because of its high energy density per unit mass/volume at nearly constant temperature. This study experimentally investigates the using of a triplex tube heat exchanger (TTHX) with PCM in the middle tube as the thermal energy storage to power a liquid desiccant air-conditioning system. Four longitudinal fins were welded to each of the inner and middle tubes as a heat transfer enhancement in the TTHX to improve the thermal performance of the thermal energy storage. The average temperature of the PCM during the melting process in the TTHX with and without fins was compared. The PCM temperature gradients in the angular direction were analyzed to study the effect of the natural convection ...
41 citations
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TL;DR: In this paper, a numerical study has been carried out to investigate the steady-state mixed convection around two heated horizontal cylinders in a square two-dimensional enclosure, where the cylinders are located at the middle of the enclosure height and the walls of the cavity are adiabatic.
Abstract: In this paper, a numerical study has been carried out to investigate the steady-state mixed convection around two heated horizontal cylinders in a square two-dimensional enclosure. The cylinders are located at the middle of the enclosure height and the walls of the cavity are adiabatic. Streamlines and isotherms are produced and the effects of cylinder diameter, Reynolds number, and Richardson number on the heat transfer characteristics are numerically analyzed. The average Nusselt number over the surface of cylinders and average nondimensional temperature in the enclosure are also presented. The results show that both heat transfer rates from the heated cylinders and the dimensionless fluid temperature in the enclosure increase with increasing Richardson number and cylinder diameter. However, the trend of average Nusselt number and nondimensional temperature variation is completely opposite when Reynolds number increases. In addition, by increasing the cylinders diameter and Richardson number, the left c...
36 citations
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TL;DR: In this paper, a numerical study of laminar forced convection of nanofluid flow over a backward facing step with a corrugated bottom wall in the presence of different shaped obstacles placed behind the step was performed.
Abstract: In this paper, a numerical study of laminar forced convection of nanofluid flow over a backward facing step with a corrugated bottom wall in the presence of different shaped obstacles placed behind the step was performed. The bottom corrugated wall of the channel downstream of the step is isothermally heated and the other walls of the channel and obstacle surface are assumed to be adiabatic. The governing equations are solved with a finite-element method. The influences of the Reynolds number (between 10 and 200), solid volume fraction of the nanoparticle (between 0 and 0.05), and obstacle type (circular, square, and diamond shaped) on fluid flow and heat transfer are numerically investigated. It is observed that among different obstacles, the diamond shaped obstacle provides better local heat transfer enhancement characteristic in the vicinity of the step compared to the circular or square obstacle at high Reynolds number. Heat transfer enhancement of 6.66% is achieved in terms of maximum values with a d...
28 citations
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TL;DR: In this article, the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina-water nanofluid over a wide temperature range (25°C-75°C).
Abstract: This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle co...
28 citations
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TL;DR: In this paper, the authors present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink, where the microchannel plate and heating block were divided by the partition wall for the local heat flux measurements.
Abstract: In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study was to experimentally establish how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux, and vapor quality. The copper microchannel heat sink contains 21 microchannels with 335 × 930 μm2 cross section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate was measured in the range of external heat fluxes from 50 to 500 kW/m2; the mass flux varied within 200–600 kg/m2-s, and pressure varied within 6–16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It showed that nucleate boiling is the dominant mechanism for heat transfer. A new model of flow boiling h...
27 citations
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TL;DR: In this article, an experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition.
Abstract: An experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition. To provide the applied nanafluids, MWCNTs are dispersed in heat transfer oil with mass concentrations of 0.05, 0.1, and 0.2 wt%. The Reynolds number varies between 100 and 4,000. Three tubes with hydraulic diameters of 11.9, 13.2, and 15.5 mm are applied as the test section in the experimental setup. Tubes are corrugated four times on the cross section; that is, there are four different helices around the tube. Depths of the corrugations are chosen as 0.9, 1.1, and 1.3 mm, and pitch of corrugation is 14 mm. The acquired data confirm the increase of heat transfer rate as a result of utilizing nanofluids in comparison with the base fluid flow. However, corrugating the tubes decreases the heat transfer rate at low Reynolds numbers. The highest increase in...
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TL;DR: In this article, the authors analyzed heat transfer and fluid flow of natural convection in inclined cavity filled with CuO-water nanofluid and differentially heated and found that the efficiency of heat transfer is improved by the addition of nanoparticles into base fluid; however, there is an optimum solid volume fraction that maximizes the heat transfer rate.
Abstract: This paper analyzes heat transfer and fluid flow of natural convection in inclined cavity filled with CuO–water nanofluid and differentially heated. Conservation of mass, momentum, and energy equations are solved numerically by a control volume finite-element method using the SIMPLER algorithm for pressure–velocity coupling. The Prandtl number is fixed at 7.02, corresponding to water. Aspect ratio and solid volume fraction are varied from 0.5 to 4 and from 0% to 4%, respectively. The inclination angle is varied from 0° to 90° and used as a control parameter to investigate flow mode-transition and the accompanying hysteresis phenomenon (multi-steady solutions). It is found that the efficiency of heat transfer is improved by the addition of nanoparticles into base fluid; however, there is an optimum solid volume fraction that maximizes the heat transfer rate. Numerical results show also that the diameter of solid particle is an important parameter that affects the heat transfer efficiency; its impact is mor...
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TL;DR: In this paper, thermal field measurements suggest lower and uniform server inlet temperatures for the case with contained aisle as compared to open aisle and for the fully provisioned case using active tiles as comp...
Abstract: Generally, passive perforated tiles are used in a data center and the supplied airflow rate is underprovisioned; thus, the balance of the server air requirement is met by the hot air in the room, resulting in higher server inlet temperatures. Full provisioning of the supplied airflow rate and containing the cold aisle is expected to minimize the hot air leakage in the cold aisle, resulting in uniform and lower server inlet temperatures. Thus, the supply air temperature can be raised, resulting in energy savings at the chiller plant. Supplying extra air can be achieved using active perforated tiles, having multiple fans installed on them. In this paper, the underprovisioned case using passive tiles and the fully provisioned case using active tiles are investigated for both open and contained aisle conditions. Thermal field measurements suggest lower and uniform server inlet temperatures for the case with contained aisle as compared to open aisle and for the fully provisioned case using active tiles as comp...
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TL;DR: In this article, a 3D numerical model is developed for prediction of heat and mass transfer coefficients on the surface of a rectangular moist object, where a computational fluid dynamics tool is used for flow analysis at three different higher velocities.
Abstract: Convective surface transfer coefficients data for a moist object are not easily available from the literature and this coefficient is usually assumed to be constant in most studies. A three-dimensional (3D) numerical model is developed for prediction of heat and mass transfer coefficients on the surface of the rectangular moist object. A computational fluid dynamics tool is used for flow analysis at three different higher velocities. The spatial distribution of heat and mass transfer coefficients is estimated at the surfaces of the object. The effect of constant and variable surface transfer coefficient is analyzed through transient 3D simultaneous heat and mass transfer solutions of convective drying. The numerical results are compared with experimental results. Good agreement was found with the experimental results when the numerical solution is considered with variable heat transfer coefficient. The numerical solutions with constant heat transfer coefficient gives overestimated results. A new set of Nu...
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TL;DR: In this article, a theoretical model is developed to optimize the absorption chiller for identifying conditions corresponding to maximum system coefficient of performance (COP) and minimum entropy generation rate, with the absorber having a dominant role in enhancing irreversibility in system.
Abstract: Thermodynamic optimization of single-stage water–LiBr absorption system was performed earlier both from first- and second-law points of view. However, a realistic comparison between the two approaches is essential to identify the superior one. Hence a theoretical model is developed to optimize the absorption chiller for identifying conditions corresponding to maximum system coefficient of performance (COP) and minimum entropy generation rate. Optimum generator temperature for minimum entropy generation is found to be lower than that corresponding to maximum COP, with absorber having a dominant role in enhancing irreversibility in system. Optimal generator temperature decreases with increasing evaporator temperature and decreasing condenser temperature. Thus, it is possible to identify optimum value for any combination of condenser and evaporator temperature from both the energy and entropy points of view. Concerned contour maps are presented. Adoption of an entropy-optimized map is suggested, owing to hig...
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TL;DR: In this paper, an experimental and numerical study to investigate the convective heat transfer characteristics of fully developed turbulent flow of a water-Al2O3 nanofluid in a circular tube is presented.
Abstract: In this paper an experimental and numerical study to investigate the convective heat transfer characteristics of fully developed turbulent flow of a water–Al2O3 nanofluid in a circular tube is presented. The numerical simulations are accomplished on the experimental test section configuration. In the analysis, the fluid flow and the thermal field are assumed axial-symmetric, two-dimensional, and steady state. The single-phase model is employed to model the nanofluid mixture and the k-ϵ model is used to describe the turbulent fluid flow. Experimental and numerical results are carried out for different volumetric flow rates and nanoparticles concentration values. Heat transfer convective coefficients as a function of flow rates and Reynolds numbers are presented. The results indicate that the heat transfer coefficients increase for all nanofluids concentrations compared to pure water at increasing volumetric flow rate. Heat transfer coefficient increases are observed at assigned volumetric flow rate for nan...
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TL;DR: In this article, a steady laminar flow of a Newtonian liquid through a microchannel subject to uniform heat flux, uniform peripheral temperature boundary conditions in the presence of viscous dissipation is investigated.
Abstract: Micro heat exchangers may achieve very high heat transfer coefficients thanks to their small dimensions and high area-to-volume ratio even in laminar flow, a highly desirable feature in many industrial applications. The main drawback of these devices is the high frictional losses—especially for liquid flows—that make viscous dissipation no longer negligible. In order to enhance heat transfer, modification of the channels’ cross-section is a viable strategy. In this work the fully developed steady laminar flow of a Newtonian liquid through a microchannel subject to uniform heat flux, uniform peripheral temperature boundary conditions in the presence of viscous dissipation is investigated. Entropy generation numbers and the constrained total heat transfer area performance evaluation criterion are employed to assess the influence of smoothing the corners of an initially rectangular cross section, with an aspect ratio ranging from 1 to 0.03 under four different types of geometrical constraints. The governing ...
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TL;DR: In this article, a modified general form of the two-phase flow multiplier, applicable both to boiling flow and condensation flow, is obtained, in which the nonadiabatic effects are considered.
Abstract: In this paper a method developed earlier by the authors is applied to calculations of pressure drop and heat transfer coefficient for boiling flow and condensation flow with account of nonadiabatic effects for some recent data collected from the literature. The first effect, the modification of interface shear stresses in an annular flow pattern, is considered through incorporation of the so-called “blowing parameter.” The mechanism of modification of shear stresses at the vapor–liquid interface for such case is presented in detail in the paper. In the case of annular flow it contributes to thickening and thinning of the liquid film, which correspond to condensation and boiling, respectively. There is also another influence of the wall heat flux, where it is influencing the bubble nucleation in the case of the bubbly flow pattern. As a result, a modified general form of the two-phase flow multiplier, applicable both to boiling flow and condensation flow, is obtained, in which the nonadiabatic effects are ...
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TL;DR: In this article, the effects of various vortex generator configurations on the heat transfer and flow friction characteristics are investigated by numerical method, and the results show that the intensity of heat transfer can be greatly increased with decreasing the location and attack angle of vortex generator, and with increasing height of vortex generators, accompanying with the increase of pressure drop.
Abstract: In this study, the effects of various vortex generator configurations on the heat transfer and flow friction characteristics are investigated by numerical method. Compared with common-flow-down configuration, the Nusselt numbers of common-flow-up configuration increase by 2.7–2.9% in the range of studied Reynolds number, while the friction factors reduce by 7.8–10.0%. A comparative study of the effects of location of axial dimension, location of spanwise dimension, attack angle, and length and height of vortex generator on fin performance is conducted. The results show that the intensity of heat transfer can be greatly increased with decreasing the location and attack angle of vortex generator, and with increasing height of vortex generator, accompanying with the increase of pressure drop. The Nusselt number and friction factors first increase and then decrease with increasing length of vortex generator. The parameters of the vortex generator fin-and-tube heat exchanger are optimized by the Taguchi method...
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TL;DR: In this article, the authors investigated the heat transfer enhancement from a single heated circular cylinder in laminar pulsating cross-flows and found that the enhancement increased up to 2.10, and an empirical correlation between convection heat transfer coefficient and pulsating flows was developed.
Abstract: Experiments were designed to investigate the heat transfer enhancement from a single heated circular cylinder in laminar pulsating cross-flows. Several parameters were explored, including Strouhal number between 0.18 and 2.80, Reynolds number between 205 and 822, and pressure amplitude prms between 40 Pa and 276 Pa. It was observed that the heat transfer enhancement factor increased up to 2.10, and an empirical correlation between the convection heat transfer coefficient and pulsating flows was developed. Results found that the heat transfer enhancement factor decreases with Strouhal number. Increasing Reynolds number was found to have a negative impact on the heat transfer enhancement factor when the pulsating frequency is relatively low, but with a higher pulsating frequency, an increasing Reynolds number increases the heat transfer enhancement factor. Larger pressure amplitude was found to continuously produce larger heat transfer enhancement factor when both Strouhal number and Reynolds number are kep...
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TL;DR: In this paper, the experimental and theoretical determination of the effective thermal conductivity of three magnesium oxide (MgO) nanoparticles of different sizes dispersed in glycerol was presented.
Abstract: This paper presents experimental and theoretical determination of the effective thermal conductivity of three magnesium oxide (MgO) nanoparticles of different sizes dispersed in glycerol. The glycerol-based nanofluids were prepared at volume fractions ranging from 0.5% to 4% and no surfactant. The nanoparticles were dispersed and deagglomerated for 2 hours using an ultrasonic probe. The effective thermal conductivity of nanofluids was measured from 20°C to 45°C using a thermal conductivity analyzer. The experimental results show an increase in the thermal conductivity of MgO–glycerol nanofluids with increasing volume fraction of nanoparticles. The thermal conductivity ratio is unaffected as the temperature increases. In the given volume fraction and temperature range, the thermal conductivity ratio of MgO–glycerol nanofluids decreases with increasing particle size. The obtained experimental data were also compared with some existing theoretical and empirical models that may work for glycerol-based nanoflu...
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TL;DR: In this paper, a review of spray heat transfer at large Reynolds numbers and surface boiling is presented, including some recent results summarizing the effects of various injection parameters, such as coolant and surface temperatures, water and air flow rates, and injection conditions.
Abstract: Sprays used for enhanced heat transfer are reviewed, starting from the spray characteristics, measurement methods, and spray dynamics, to spray heat transfer. Some results for spray heat transfer at large Reynolds numbers and surface boiling are also presented, including some recent results summarizing the effects of various injection parameters. It is recommended that basic principles of heat transfer be used to integrate various effects, such as coolant and surface temperatures, water and air flow rates, and injection conditions, into a concise form so that the results can be generalized and be applied to a large range of conditions.
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TL;DR: In this article, a three-dimensional computational fluid dynamics model is developed to predict the thermal and electrical performance of a water-cooled concentrated photovoltaic (CPV) system.
Abstract: In this paper, a three-dimensional computational fluid dynamics model is developed to predict the thermal and electrical performance of a water-cooled concentrated photovoltaic (CPV) system. Based on the good agreement between the numerical results and experimental data from literature, an attempt was made to improve this system performance. Indeed, as the developed model is able to predict the thermal behavior of the different system components, many hot spots were detected in the cell module. In order to avoid this disadvantage while promoting solar cell cooling, the number of water cooling pipes of the CPV module was first increased and then a rectangular channel was employed. Numerical simulation results indicate the potential of the different modified systems for reducing these hot spots and the CPV module temperature, thus providing increased electrical and thermal efficiencies. The optimum design, which presents a solar cell temperature of 315.15 K and respectively a thermal and combined (thermal p...
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TL;DR: In this article, the film cooling effectiveness and the jet exit conditions and the associated vortex structure downstream of the injection hole for both a circular exit shaped hole and an elliptical exit shape were numerically investigated for the blowing ratios of 0.25, 0.5, 1, and 1.5.
Abstract: The film cooling effectiveness and the jet exit conditions and the associated vortex structure downstream of the injection hole for both a circular exit shaped hole and an elliptical exit shaped holes were numerically investigated for the blowing ratios of 0.25, 0.5, 1, and 1.5. Four turbulence models, including the standard, RNG, realizable k − ϵ, and Reynolds-stress model, in combination with three near-wall approaches, were used for the present simulations. It was found that the predicted results using the realizable k − ϵ model combined with the standard wall function were in better agreement with the available experimental data from the literature. Further, the results indicate that the circular exit shaped hole improved the centerline and laterally averaged adiabatic effectiveness, particularly at high blowing ratios. Finally, adding the sister holes provided a notable decrease in the strength of the counterrotating vortex pairs, where the highest effectiveness was achieved for the circular exit sha...
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TL;DR: In this article, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing.
Abstract: The lifetime of a droplet deposited on a hot plate decreases when the temperature of the plate increases, but above the critical Leidenfrost temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapor between the droplet and the substrate, which plays a double role: First, it thermally insulates the droplet from the plate, and second, it allows the droplet to “levitate.” The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water droplets ranging in size from 8 to 24 μl and the behavior of the droplets was recorded using high-speed digital photography. The Leidenfrost point was found to vary by up to 120°C for pillar spacings from 10 to 100 μm. If the droplet is placed on the boundary bet...
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TL;DR: In this article, two heat-conducting solid circular cylinders of identical shape are placed within the enclosure and simulations are performed for a range of controlling parameters such as the Richardson number (1 to 10), Hartmann number (0 to 50), and dimensionless rotational speed ( 0 to 5), keeping the Reynolds number based on the lid velocity fixed as 100.
Abstract: Numerical analysis of hydromagnetic mixed convective transport in a differentially heated vertical lid-driven square enclosure is performed in this article. Two heat-conducting solid circular cylinders of identical shape are placed within the enclosure. The cylinders may be either stationary or rotating about their centroidal axes. Two different combinations of cylinder rotations are considered: one when the top cylinder rotates and the bottom one is kept stationary, and the other when the bottom cylinder rotates and the top one is kept stationary. Simulations are performed for a range of controlling parameters such as the Richardson number (1 to 10), Hartmann number (0 to 50), and dimensionless rotational speed (0 to 5), keeping the Reynolds number based on the lid velocity fixed as 100. Furthermore, three different Prandtl numbers, 0.02, 0.71, and 7, are considered. The flow and thermal fields are analyzed through streamline and isotherm plots for various Hartmann and Richardson numbers and rotational s...
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TL;DR: In this article, the effect of geometrical placement of the cylinders on the resulting velocity and temperature fields in the laminar free convection regime was studied by considering six asymmetric locations of the two cylinders.
Abstract: Laminar free convection heat transfer in power-law fluids from two side-by-side cylinders (one hot and one cold) confined in a square duct has been studied numerically in the two-dimensional flow regime. For a fixed value of the ratio of cylinder radius to size of the enclosure, the effect of geometrical placement of the cylinders is studied on the resulting velocity and temperature fields in the laminar free convection regime by considering six asymmetric locations of the two cylinders. In particular, extensive results reported herein span the range of conditions of Grashof number, 10 to 105; Prandtl number, 0.7 to 100, thereby yielding the range of the Rayleigh number as 7 to 107; power-law index, 0.3 to 1.8; and the relative positions (dimensionless) of the cylinders with respect to the centerline, –0.25 to 0.25. The heat transfer characteristics are analyzed in terms of the local Nusselt number along the surfaces of the two cylinders and the enclosure walls. Overall, the average Nusselt number shows a...
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TL;DR: In this paper, the effect of Prandtl number on the laminar natural convection heat transfer to Newtonian fluids in a square enclosure consisting of one hot circular cylinder and one cold circular cylinder is explored.
Abstract: This study explores the effect of Prandtl number on the laminar natural convection heat transfer to Newtonian fluids in a square enclosure consisting of one hot circular cylinder and one cold circular cylinder. The walls of the square enclosure are maintained isothermal and at the same temperature as the cold cylinder and the fluid medium. The governing partial differential equations have been solved numerically over the following ranges of conditions: Grashof number, 10 to 105; Prandtl number, 0.7 to 100 (or the range of Rayleigh numbers as 7 to 107); and relative positioning of the cylinders, −0.25 to 0.25. However, the ratio of the radius of the cylinder to the side of the enclosure is held fixed at 0.2. Extensive results on the streamline and isotherm contours, the local Nusselt number distribution, and the average Nusselt number are discussed to delineate the influence of Grashof and Prandtl numbers on them for a given location with respect to the horizontal center line. The surface-averaged Nusselt ...
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TL;DR: In this article, the effect of using fly ash nanofluid obtained from fly ash, which is comprised of various types of metal oxides in varying ratios, on improving the performance of a parallel flow (PFCTHE) or a cross-flow (CFCTHE), concentric tube heat exchanger was evaluated.
Abstract: This study experimentally demonstrated the effect of using a nanofluid obtained from fly ash, which is comprised of various types of metal oxides in varying ratios, on improving the performance of a parallel flow (PFCTHE) or a cross-flow (CFCTHE) concentric tube heat exchanger. Fly-ash nanofluid/water and water/water hot/cold working fluids were used for monitoring the differences in the performance of the heat exchangers. The Triton X-110 dispersant was used in the study to produce 2% (wt) concentration of the fly-ash nanofluid via direct synthesis. The heat exchanger is of the double-pipe type with hot water flowing through the central tube while cooling water flows through the annular space. A double-pipe heat exchanger with co-current or countercurrent flow was utilized along with all ancillary equipment and instrumentation for the determination of overall heat transfer coefficients and heat transfer coefficients during turbulent flow. An improvement of 31.2% and 6.9% was determined in the study for t...
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TL;DR: In this article, a feed-forward back-propagation neural network was used in conjunction with a genetic algorithm to perform an exhaustive search and estimate the dimensions of the heat sink cavity for which the melt time and hence the thermal performance of a heat sink is maximum.
Abstract: Transient three-dimensional simulations are done for a phase-change material (PCM)-based heat sink subjected to constant heat flux of 1.59 kW/m2 with commercially available FLUENT software. Numerically obtained temperature–time histories are matched up with experimental results obtained earlier by our group to determine the overall heat transfer coefficient from the geometry that reconciles the differences between simulations and experiments. For a constant power level and fixed volumes of heat sink material (aluminum) and PCM (n-eicosane), simulations were done for 30 different geometric configurations to determine the time to reach complete melting. A feed-forward back-propagation neural network was then used in conjunction with a genetic algorithm to perform an exhaustive search and estimate the dimensions of the heat sink cavity for which the melt time and hence the thermal performance of the heat sink is maximum.