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In this paper a two-phase lattice Boltzmann model, capable of handling large density jumps, is used to study the free surface jet impingement cooling in the non-boiling regime. The multiple-relaxation-time (MRT) collision operator is employed to enhance the numerical stability of the model at high Reynolds numbers. The capability of the outlined two-phase LB scheme in accurate capturing of the interface shape is assessed through relevant test cases. These include the oscillating drop, Rayleigh–Taylor instability, and Kelvin–Helmholtz instability. In addition to demonstrate the validity of the model in simulation of heat transfer the predicted distribution of the Nu number on the impingement plate are compared with those of analytical results. The validated numerical solver then is employed to study a planar liquid jet emanating from a nozzle into a quiescent gas and impinging the opposite plate. The influence of the jet Re number and liquid-to-gas density ratio on the instantaneous flow field, interface shape, and heat transfer rate is also examined. This work reports for the first time the simulation of an impinging free surface liquid jet using a two phase lattice Boltzmann model. [ABSTRACT FROM AUTHOR]

Analytical solutions play important roles in the understanding of fluid dynamics and heat transfer related problems. Some analytical solutions for incompressible steady/unsteady 2-D problems have been obtained in literature, but only a few of those are found under heat transfer conditions (which brings more complexities into the problem). This paper is focused on the analytical solutions to the basic problem of incompressible unsteady 2-D laminar flows with heat transfer. By using the traveling wave method, fluid dynamic governing equations are developed based on classical Navier–Stokes equations and can be reduced to ordinary differential equations, which provide reliable explanations to the 2-D fluid flows. In this study, a set of analytical solutions to incompressible unsteady 2-D laminar flows with heat transfer are obtained. The results show that both the velocity field and the temperature field take an exponential function form, or a polynomial function form, when traveling wave kind solution is assumed and compared in such fluid flow systems. In addition to heat transfer problem, the effects of boundary input parameters and their categorization and generalization of field forming or field evolutions are also obtained in this study. The current results are also compared with the results of Cai et al. (R. X. Cai, N. Zhang. International Journal of Heat and Mass Transfer, 2002, 45: 2623-2627) and others using different methods. It is found that the current method can cover the results and will also extend the fluid dynamic model into a much wider parameter ranges (and flow situations). [ABSTRACT FROM AUTHOR]

This paper introduces the historical background about the development of water based, ethylene glycol (EG) based and EG:water mixture nanofluids for the past 20 years. The primary consideration is to review the salient of research work related to EG:water mixture nanofluids and their applications. Nowadays, the fundamental studies of nanofluids are increasing rapidly for engineering applications. The determination of the forced convection heat transfer and pressure drop was reviewed for nanofluid flow in a tube. The experimental and numerical heat transfers of nanofluids were presented. A review of other relevant research studies is also provided. Substantial heat transfer literature has been studied on water based nanofluids used in the fundamental study for engineering applications. However, there are limited studies that use EG:water mixture nanofluids in evaluation of forced convection heat transfer. A number of research studies have been performed to investigate the transport properties of EG:water mixture nanofluids either in experimental or numerical approach. As the performance of EG:water mixture nanofluids could be verified through experimental studies, researchers have conducted the experimental works using several types of potential nanofluids. As a result, nanofluids have been used in certain engineering applications such as in automotive, transportation, cooling of electronics components, solar, and nuclear reactor coolant. [ABSTRACT FROM AUTHOR]

In this paper, swirling flow boiling of a dilute nanofluid (water and 0.1 vol% Fe 3 O 4 ) in an annulus with a twisted fin on the outside of the inner wall in the presence of transverse magnetic gradient has been numerically investigated, using a two fluid model and a control volume technique. The results indicate that, in the boiling of swirling flow, the rate of the heat transfer increases. This phenomenon can be attributed to the effect of centrifugal force on the liquid phase flow and also reduction of the conductive sub-layer thickness that exists on the heated wall. The effects of improved surface wettability induced by nanoparticle deposition during the boiling process are accounted. The results demonstrate that the modified liquid property due to the existence of nanoparticles in the liquid has a negligible effect on the boiling heat transfer performance with dilute nanofluids while the improved surface wettability plays an important role and leads to reduction of the void fraction and consequently, an increase of critical heat flux. Applying a transverse magnetic field causes augmentation of the centrifugal force and results in increased flow turbulence. Furthermore, in the presence of the magnetic field due to magnetic force, the bubble departure diameter is reduced and bubble detachment occurs faster. Therefore, the critical heat flux will be increased. Swirling flow boiling in the presence of magnetic field is strongly suggested in devices requiring high heat transfer rates. [ABSTRACT FROM AUTHOR]

Nanofluids have gained significant attention in recent years due their great potential for heat transfer enhancement . The heat transfer of nanofluids can be numerically studied using a single-phase or two-phase approaches. The first assumes that the fluid phase and nanoparticles are in thermal equilibrium and move with the same velocity, while the second requires more computational effort but provides the possibility of understanding the behavior of both fluid phase and solid particles in the heat transfer mechanism. This paper reviews various computational approaches to predict fluid flow and heat transfer characteristics of nanofluids. The characteristics of single-phase and two-phase (volume of fluid, mixture, Eulerian–Lagrangian and Eulerian–Eulerian) approaches have been analyzed and discussed systematically. Latest development and recent researches related to the computational nanofluids are also given. [ABSTRACT FROM AUTHOR]

Recently, nanofluids have been studied extensively by the researchers as a result of the developments in nanotechnology. It is essential for researchers to know nanofluids' physical properties in order to make calculations regarding their specific research topics. Determination of viscosity issue is an actual one due to its common usage in heat transfer and thermodynamics. In this study, graphite particles are selected to have nanofluid mixture with its base fluid of pure water. Their volumetric concentrations are varied from 0 to 2% in pure water. Once the stabilized nanofluid is prepared by a sonicator and ultrasonic bath, viscosity is measured by a viscosity meter for the temperatures ranging from 20 °C to 60 °C. Validation of the experiments have been done by means of the comparison of them with the 32 empirical correlations in the literature. Then, Artificial Neural Network (ANN) analyses have been performed in order to have better empirical correlation than those in the literature. Furthermore, detailed information on the preparation nanofluids, measurement of viscosity, a list of measured data, numerical model by Matlab software, and alteration of viscosity with temperature and concentration have been given in the paper. It was concluded that viscosity correlations in the literature can predict different types of nanofluids' viscosity although they have been derived using specific type and diameter of nano particles and their base fluids. [ABSTRACT FROM AUTHOR]

In this paper, double-diffusive natural convection in an enclosure is introduced by an incompressible smoothed particle hydrodynamics (ISPH) method. Two different cases of an enclosure have been studied. In the first case, the non-Darcy model for natural convection and heat and mass transfer in an enclosure saturated with anisotropic porous media has been investigated numerically by a stabilized ISPH method. The second case including sloshing rod inside an enclosure filled with free fluid has been studied numerically by a stabilized ISPH method. In the ISPH algorithm, a semi implicit velocity correction procedure is utilized, and the pressure is implicitly evaluated by solving pressure Poisson equation. The results are presented with flow configurations, isotherms, concentration contours and average Nusselt and Sherwood numbers for different Darcy numbers from 10 − 4 to 10 − 2 , porosity values from 0.5 to 0.9, permeability ratio from 0.1 to 10, inclination angle of permeability from 0° to 90° and Rayleigh numbers from 10 3 to 10 5 . The results demonstrate the effects of the parameters such as Darcy number, porosity, permeability ratio and inclination angle in both of the heat and mass transfer rate and the flow regime. Adding sloshing rod with initial condition inside enclosure affects clearly in heat and mass transfer and the flow characteristics inside the enclosure. The results from this investigation are well validated and have favorable comparisons with previously published results. [ABSTRACT FROM AUTHOR]

This paper is related to a theoretical study to analyze the nanoclusters and their structural effects on the laminar forced convection heat transfer coefficients of Al 2 O 3 -water nanofluids in a circular tube. In this study, Eulerian/Eulerian two-phase model is used with regard to nanoclusters Brownian motion and their fractal structure. The hydrodynamic effects, nanofluid concentration, size, and fractal structure of nanoclusters were investigated. Modeling results were compared to the experimental data and it was shown that the numerical predictions agree well with the experimental data. Based on the results, convective heat transfer coefficient increases by increasing the nanoparticles concentration and reducing of the nanoclusters fractal dimension. The results clearly showed that Brownian motion has a significant effect on the convective heat transfer coefficient enhancement of Al 2 O 3 /water nanofluids . [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on the local and average condensation heat transfer in a single noncircular microchannel. Furthermore, it develops one dimensional model for annular condensation flow in a microchannel under variable heat flux condition. The condensate film thickness is calculated for each location in a microchannel including the effects of capillary number, Boiling number, contact angle, heat flux, vapor pressure, and hydraulic diameter. A comparative study shows that the present model well predicts the experimental data concerning local condensation heat transfer coefficient. The mean deviation between the local predictions of the theoretical model with the measurements for local heat transfer coefficient is 20%. It is found that the correlation of Quan et al. (2008) [19] gives the good predictions of the measurements with maximum deviation of 13% at high Reynolds number. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer and friction factor are experimentally studied in a double pipe heat exchanger which has an inner corrugated tube filled with various categories of twisted tapes from conventional to modified types which include perforated, V-cut and U-cut types. The twist ratios are 3, 5 and 7, the hole diameter ratio are 0.11 and 0.33, the width and depth ratio of the cuts vary from 0.3 to 0.6 and the Reynolds number is changed from 5000 to 15,000 of turbulent regime. All of these factors lead to the performance of more than 350 experiments. The results of the main parameters on heat transfer and pressure drop show that the Nusselt number and friction factor for all cases of twisted tape corrugated tube are more than the empty corrugated tube. Also the Nusselt number and friction factor for corrugated tube equipped with modified twisted tapes are higher than typical tapes except those of perforated types which lead to lower Nusselt number and friction factor. [ABSTRACT FROM AUTHOR]

A model of a first order leaf-like body (FOLLB), which is assembled by many elemental leaf-like bodies (ELLBs), is established in this paper by using Constructal theory. The maximum dimensionless heat transfer rate (DHTR) and the optimal structure of the FOLLB are obtained. It shows that there exist an optimal dimensionless cross-sectional area of the elemental vein, an optimal dimensionless thickness of the FOLLB, as well as an optimal number of ELLBs in the FOLLB which lead to the triple maximum DHTR of the FOLLB. The heat transfer performance (HTP) of the FOLLB becomes better with the increases in Biot number and the thermal conductivity ratio. When the optimal construct of the FOLLB was compared with that of the ELLB, it was found that the maximum DHTR of the FOLLB is increased by 69.83% for the same body volume, and the HTP of the body can be greatly improved. [ABSTRACT FROM AUTHOR]

In this work, a three-dimensional analysis is used to study the heat transfer and entropy generation inside a helically coiled tube-in-tube heat exchanger in laminar flow regime using two different types of nanofluids. Overall heat transfer coefficient, effectiveness, Nusselt number, and entropy generation of helically coiled tube-in-tube heat exchanger were investigated considering the nanoparticle volume concentrations between 0 and 2.0 vol.%. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The present contribution is a companion paper of Huminic and Huminic [1]. The numerical results reveal that the use of nanofluids in a helically coiled tube-in-tube heat exchanger improves the heat transfer performances. Thus, the maximum effectiveness was 91% for 2% CuO nanoparticles and 80% for 2% TiO 2 nanoparticles. Also, the increase of nanoparticles volume concentration leads to the Nusselt number increase and the reduction of the entropy generation due heat transfer effects, the viscous effects to entropy generation being negligible. [ABSTRACT FROM AUTHOR]

Numerical study of fluid flow and heat transfer within two types of liquid (liquid two-layer and oil–water emulsion) when subjected to microwave energy are discussed. In order to obtain the simulation of microwave heating from room temperature, a 2D comprehensive model was integrated with electromagnetic field, incompressible laminar flow and heat transfer. The effects of layered configuration, layered thickness and dispersed fraction of emulsions were investigated. Temporal profiles obtained using fiber optic sensors at four discrete points were compared with the simulated temperature profiles. The simulated outlet temperatures of liquid had a good agreement with experimental data within the maximum prediction error of 5%. The theory presented in this paper can be effectively used to explain fluid transport during microwave heating system using the rectangular waveguide. [ABSTRACT FROM AUTHOR]

Heat transfer enhancement technologies play an important role in research and industrial fields; thus, they have been widely applied to many applications as in refrigeration, automotive, aerospace, and process industry. For example, heat transfer can be passively enhanced by increasing the thermal conductivity of the working fluids, adopting nanofluids, or actively by employing impinging jets. In this paper a numerical analysis on confined impinging slot jets working with pure water or water/Al 2 O 3 based nanofluids is presented. The flow is laminar and a constant uniform temperature is applied on the target surface. The single-phase model approach has been adopted in order to describe the nanofluid behavior and different particle volume concentrations have been considered. Moreover, simulations have been performed for different geometric ratios in order to take into account the confining effects and Reynolds numbers. The behavior of the system has been analyzed in terms of average and local convective heat transfer coefficient, Nusselt number, and required pumping power profiles. Correlations for stagnation point and average Nusselt number for 100 ≤ Re ≤ 400, 0% ≤ ϕ ≤ 5% and 4 ≤ H / W ≤ 10 are provided. [ABSTRACT FROM AUTHOR]

In this paper, a novel design of heat collecting component for a flat plate solar collector is presented and a numerical study on this solar collector collection efficiency by computational fluid dynamics (CFD) method is conducted. The new heat collecting component consists of a corrugated upward surface and a flat downward surface. The significant influences of the tilt of solar collector, the mass flow rate of inlet water and the distance of air gap on the solar collector collection efficiency are investigated by CFD-simulation. Simulation results reveal that the optimum values of the three factors are the tilt of 10° for April to September, 30° to 35° for the rest months (in Guangzhou of China), the mass flow rate of inlet water 0.15 kg/s and the distance of air gap 20 mm, respectively. Finally, the maximum instantaneous efficiency of 85.1% and the total heat loss factor of 3.127 are obtained by fitting the instantaneous efficiency curve of this solar collector. [ABSTRACT FROM AUTHOR]

The mechanisms of heat transfer enhancement are used in many industrial applications. Several techniques have been promoted to enhance heat transfer rate and to decrease the size and cost of equipment especially the heat exchangers. In this paper, heat transfer coefficient and pressure drop for Al 2 O 3 /water nanofluid flow inside circumferential ribbed tubes with different rib dimensions have been experimentally and numerically studies. The nanoparticle size was set equal to 13 nm and the volume fractions from 0% to 3% were considered. The ribbed copper tubes tested in this investigation with inner diameter of 14.9 mm have the ranges: circumferential depth from 0.5 mm to 1.0 mm and axial pitch distance from 5 mm to 15 mm. The inlet temperature of turbulent nanofluid was 25 °C and the constant wall heat flux was 5,000 W/m 2 . Comparison of numerical data of ribbed tubes with plain tube shown that the heat transfer coefficient from 92% to 621% and friction factor from 25% to 241% compared to those obtained in smooth tube depending on the circumferential geometric parameters, mass velocity and thermal conductivity of the working fluid. [ABSTRACT FROM AUTHOR]

This paper experimentally investigates the heat transfer performance and characteristics of a heat sink with bio-mimetic oscillating foil. The oscillating foil is driven by a novel mechanism of grooved cam with linkage. The air flow is drawn into a rectangular duct through the motion of the bio-mimetic oscillating foil. Various heat sinks were set at the outlet of the duct to study the thermal performance. Two types of grooved cams were designed to obtain different angular velocity profiles for the oscillating foils. The angular velocity was specified as fast forward/slow backward and sinusoidal patterns. Foils with three flexural stiffness values were utilized in the study. The averaged velocity at the inlet was measured to estimate the Reynolds number and Strouhal number. The effects of configuration of heat sink design with oscillating foil were also under investigation. The junction temperature of the heat sink was measured to obtain the effective thermal resistance. Results show that the angular velocity profile and flexural stiffness of the oscillating foil have a significant effect on the thermal performance of the heat sink. [ABSTRACT FROM AUTHOR]

This paper focuses on designing an artificial neural network which can predict thermal conductivity and dynamic viscosity of ferromagnetic nanofluids from input experimental data including temperature, diameter of particles, and solid volume fraction. The experimental data were extracted and they were used as learning dataset to train the neural network. To find a proper architecture for network, an iteration method was used. Based on the results, there was no over-fitting in designed neural network and the neural network was able to track the data. ANN outputs showed that the maximum errors in predicting thermal conductivity and dynamic viscosity are 2% and 2.5%, respectively. Based on the ANN outputs, two sets of correlations for estimating the thermal conductivity and dynamic viscosity were presented. The comparisons between experimental data and the proposed correlations showed that the presented correlations were in an excellent agreement with experimental data. [ABSTRACT FROM AUTHOR]

This paper describes in detail the measurement of the overall heat transfer coefficient and phase transformation latent heat by modified pattern search method (PSM). By introducing the pattern search algorithm into finite element analysis, the measurement of heat transfer coefficients and phase transformation latent heat can be simplified as recording the temperature with time. Comparison of the measurements by PSM and temperature gradient method (TGM) shows that PSM can evaluate the overall heat transfer coefficient accurately. Experiments and simulations show that bainite transformation latent heat in 26NiCrMoV10-10 steel is about − 90.5 J/g, and the latent heat increases the surface temperature of a cylindrical specimen by a maximum of 76 °C. Error analysis shows that the highest difference between calculated temperature and measured temperature is 11.7 °C, which verifies that the heat transfer coefficient and phase transformation latent heat estimated by PSM are accurate, and that the pattern search method is reliable. [ABSTRACT FROM AUTHOR]

In this paper, the melting and solidification processes and the thermal conductivity enhancement of the phase change material (PCM) inside a sphere have been experimentally investigated to facilitate a greater understanding and improvement of the phase change heat transfer of PCM inside a sphere. The experiments are conducted by inserting a sphere filled with organic PCM into a constant temperature bath of hot/cold water. Through measuring and analyzing the temperature field inside the sphere under conditions of various initial and final temperatures, the thermal characteristics of melting and solidification processes of PCM are obtained. To determine the thermal conductivity enhancement effect of aluminum powder, the PCM-aluminum composite material in which the mass fractions of aluminum are 1% and 2%, respectively, is tested. And the experimental data of PCM-aluminum composite material are compared with those of pure PCM. Results indicate that the PCM in the upper part of the sphere melts faster than that in the lower part, and the PCM near the inner wall solidifies faster than that in the center of the sphere. The initial PCM temperature has little effect on the solidification process of the PCM inside the sphere when Stefan number ( Ste ) is small and Rayleigh number ( Ra ) is large. The thermal conductivity of the PCM inside the sphere is enhanced by adding aluminum powder. And the sedimentation of aluminum powder is more beneficial to accelerate the heat transfer of the whole sphere in the melting process, compared with the uniform diffusion of aluminum powder in the PCM. [ABSTRACT FROM AUTHOR]

In this paper, laminar forced convective heat transfer of water/alumina nanofluids in a straight microchannel is studied numerically using CFD modeling. In the first part, single-phase and two-phase mixture models have been used for prediction of hydrodynamics and heat transfer parameters of nanofluids in a simple microchannel heat sink. The CFD predictions were compared to the experimental data and it was concluded that the two-phase approach gives better predictions compared to the single-phase model. The effects of ribs turbulators on the fluid flow and heat transfer performance of microchannel were investigated in the second part. The effects of geometrical characteristics of the ribs were studied, and the results showed that the Nusselt number and friction coefficient of nanofluids in the ribbed microchannel are higher than those of simple microchannel, and this enhancement increased with increasing the width of the ribs. [ABSTRACT FROM AUTHOR]

Based on constructal theory, the construct of a three-dimensional cylindrical heat sources with convection heat transfer is investigated in this paper. The heat source model is composed of several cylindrical heat sources surrounded with a cylindrical fin or a conical fin. The heat source model not only exists in the heat generating electronic component surrounded with a fin to enhance heat transfer, but also in the heat generating fuel rod with zircaloy cladding. For the specified volumes of the fin and the heat sources, this construct is optimized by taking the minimization of the dimensionless hot spot temperature as optimization objective. The results show that the hot spot temperature of the model is effectively reduced after constructal optimization, which will provider some theoretical guidelines for the optimal design of heat source system. [ABSTRACT FROM AUTHOR]

In this paper, the characteristic variational multiscale (C-VMS) method is proposed to solve the nonstationary conduction–convection problems. The stability analysis is carried out using the energy estimate method. Compared with the standard variational multiscale (VMS) method, the C-VMS method does not need nonlinear iteration. Finally, some numerical examples are given, which show that the C-VMS method is efficient, reliable and can save a lot of CPU time for this problem, besides, it can deal with the high Rayleigh number. [ABSTRACT FROM AUTHOR]

This paper reviewed the application of nanofluids in vehicle engine cooling system. So far, nanoparticles have been used in engine oil, transmission oil, and radiator coolant to enhance heat transfer removal from vehicle engine. The heat transfer performance of nanofluids has been reported to perform better compared to pure fluid. This review focused on the experimental and numerical studies by previous researchers and their suggested amount of nanoparticles for optimum performance in vehicle engine cooling system. Finally, the conclusions and important summaries were presented according to the data collected. [ABSTRACT FROM AUTHOR]

This paper provides a review about the major work done on helical baffles to improve the performance of shell and tube heat exchangers. Some of the major factors affecting the performance of shell and tube heat exchanger are discussed. A comparison between segmental baffles and helical baffles is also presented to show that helical baffles are more advantageous than segmental baffles. In most cases, discontinuous, folded, sextant helical baffles, 40° baffle inclination angle as well as low baffle spacing will give the best results when integrated in some combination, whereas continuous helical baffles eliminate dead regions. Moreover, sealing strips are more likely to improve the performance of shell and tube heat exchangers with continuous helical baffles. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer characteristics and flow visualization of the hydrocarbon refrigerant R-600a during flow boiling inside horizontal smooth tube are investigated. For this purpose the well instrumented experimental apparatus including pump, flow meter, pre-heaters, test evaporator, by-pass and condenser was designed, fabricated and installed in order to measure the experimental data on flow boiling heat transfer and flow pattern of hydrocarbon refrigerant. The test section was a plain copper tube with an inside diameter of 8.7 mm and a wall thickness of 0.41 mm. The experimental tests were carried out varying: (i) the refrigerant mass fluxes within the range of 130–380 kg/m 2 s; (ii) the vapor qualities up to 0.70; and (iii) the heat fluxes from 10 to 27 kW/m 2 . The results indicate that, intermittent and annular flow patterns are the dominant flow patterns in the ranges of operating parameters of the present study. Also, the flow boiling heat transfer coefficient increased as the mass velocity increased in the fixed vapor quality. Moreover, heat transfer coefficient increased by the increase of vapor quality as the mass velocity was fixed. Most of the flow boiling heat transfer and flow pattern prediction methods have been developed for HFC and HCFC and CFC refrigerants and there are no general methods which can be applicable for all fluids. Therefore the present study experimental data for flow boiling heat transfer coefficient and flow pattern were compared with some existing models in order to find out which method could predict the convective boiling heat transfer and flow pattern characteristics of R-600a as a hydrocarbon refrigerant more accurately. The results indicate that Kattan–Thome–Favrat [23] flow map was closer to the flow patterns observed in this study. In addition, Gungor–Winterton [8] correlation had the best consistency with the present study experimental heat transfer data. [ABSTRACT FROM AUTHOR]

In this paper, natural convection heat transfer and fluid flow within a single slope solar still (SSSS) are investigated numerically. To enhance the productivity and performance of this system, the differences of bladed and non-bladed solar stills in terms of vortices resulted from natural convection heat transfer are presented. It is assumed that the flow within the still is two-dimensional, steady and laminar. The numerical method is based on SIMPLEC algorithm and is used for discretization and solving continuity, momentum, energy and concentration conservation equations. The blades are installed on the non-insulated walls of solar stills in two different ways; downward and upward. Results are posed for different blade locations, Rayleigh and Nusselt numbers. Moreover, results depict that exploiting a blade can augment system's performance substantially in comparison with more than one. Comparison of the present numerical approach and well-known published empirical correlations proves the acceptable accuracy of this study. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation on heat transfer performance and flow fields of different nanofluids flows through elliptic annulus in a laminar and turbulent flow regimes. The three-dimensional continuity, Navier–Stokes and energy equations are solved by using finite volume method (FVM) and the SIMPLE algorithm scheme is applied to examine the effects of laminar and turbulent flow on heat transfer characteristics. This study evaluates the effects of four different types of nanoparticles, Al 2 O 3 , CuO, SiO 2 and ZnO, with different volume fractions (0.5–4%) and diameters (25–80 nm) under constant heat flux boundary condition using water as a base fluid were used. The Reynolds number of laminar flow was in the range of 200 ≤ Re ≤ 1500, while for turbulent flow it was in the range of 4000 ≤ Re ≤ 10,000. The results have shown that SiO 2 –water nanofluid has the highest Nusselt number, followed by ZnO–water, CuO–water, Al 2 O 3 –water, and lastly pure water. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number. It is found that the glycerine–SiO 2 shows the best heat transfer enhancement compared with other tested base fluids. [ABSTRACT FROM AUTHOR]

This paper addressed new and unique information on the influence of local thermal non-equilibrium (LTNE) effects on heat exchange and entropy generation during thermal-free convection in a non-Darcy porous domain. The simplest configuration was considered, which consisted of a square cavity in which the steady-thermal-free convection motion is generated by maintaining the vertical sidewalls at different and constant temperatures. The Darcy-Brinkman-Forchheimer design has been adopted. The governing equations, which include the two energy equations for the fluid and solid phases, along with momentum equations in their dimensionless form, are solved using the finite volume computational approach. Due to the application of two different heat transport equations under the LTNE modeling technique, sources of entropy generation owing to thermal diffusion in the fluid phase and in the solid phase in addition to the entropy generation owing to viscous dissipation were identified. Maps of the distribution pattern of LTNE sources were also visualized. Furthermore, to assess and compare the thermal diffusion irreversibility in the two components of the porous structure, for the first time, we introduced and examined a new parameter (Ty), which is the ratio of the thermal entropy generation rate in the fluid phase to that in the solid phase. [ABSTRACT FROM AUTHOR]

On course to maximization and optimization of a two-phase closed thermosyphon, this paper expounds on multiple impactful factors in an experimental and numerical setting. Among various fluids, 1-Butanol with 6% concentration, due to having the largest difference in surface tension with water, engendered the most favorable outcome. Response Surface Method has been employed to develop two correlations for efficiency and the thermal resistance of the thermosyphon paving the way for more accurate quantifications without necessarily running costly experiments. Other studied factors based on heat transfer coefficients of evaporator and condenser have been analyzed. The resultant numbers suggest an increase up to roughly 40% and 9% by changing power input and fluid, respectively. Also, enhancements falling within the range of 16 to 20% with the employment of 6% water-Butanol were observed by manipulation of filling ratio. However, coolant mass flow rate influence was limited to less than 10% when exposed to a 40% jump in flow rate if the working fluid was the same, implying its poor dominance in thermal performance of thermosyphon. As 1-Butanol proved superior among enhancing determinants, two numerical cases based on this factor were analyzed, confirming experimental findings while concurrently revealing distinctive phase-change patterns arising from the self-rewetting fluid. • Effect of different self–rewetting fluids on TPCT performance has been investigated. • The 1-Butanol, 1-Pentanol, and 1-Hexanol have been selected as self–rewetting fluids. • Effect of heat input, FR and coolant flow rate on TPCT performance have been studied. • Water/1-Butanol 6% has augmented the h e by 8.6% at 250 W comparing with pure water. • Numerical simulation has been done for water and Water/1-Butanol 6% charged TPCT. [ABSTRACT FROM AUTHOR]

The impingement cooling is used to protect turbine components under high thermal load. This paper conducts a numerical study of heat transfer characteristics of turbine leading edge with lamilloy cooling structure. Target surface curvature and jet-to-surface spacing under various crossflow Reynolds number conditions are considered. Meanwhile, the target surface is dimpled to enhance heat transfer capability. Besides, staggered film holes are considered in the computational domain. The results show that the crossflow induces a counter rotating vortex pair that greatly increase the near wall turbulence and local heat transfer. The vortex pair forms the bound for a U-shaped high heat transfer region caused by the jet impingement. The target surface curvature tends to enlarge the relative area of the U-shaped region. The increased crossflow strength weakens the jet cooling effect, the total heat transfer performance is therefore decreased. Large jet-to-surface spacing enhances the impingement cooling when crossflow is weak but the total heat transfer will not recover as crossflow Reynolds number get higher compared with small and medium spacing cases. • Computational fluid dynamics (CFD) models used to simulate turbine leading edge internal heat transfer. • Impingement cooling configured with dimpled surface and film holes. • Compound influence of jet-to-surface spacing, surface curvature under crossflow. [ABSTRACT FROM AUTHOR]

Hydrofluoroolefins are environment-friendly working fluid and used as replacements for hydrofluorocarbons. A plate heat exchanger with a zeotropic mixture as working fluid has been widely used as a condenser in various thermodynamic systems. However, only very few studies investigated the zeotropic mixture condensation in the plate heat exchanger, and none of these studies addressed mixtures based on hydrofluoroolefins. In this paper, the hydrofluoroolefin mixture condensation in a plate heat exchanger were experimentally tested, and the characteristics of heat transfer and pressure drop were analyzed. We tested the condensation of R1234ze(E)/R1233zd(E) with four different mass fractions at wide ranges of temperatures and flow rates. The test results suggest the R1234ze(E)/R1233zd(E) mixture with the 0.352/0.648 mass fraction presents a 54% maximum degradation in heat transfer coefficient, compared with results obtained using the ideal mixing rule of pure fluids. A modified Silver-Bell-Ghaly model and a pure-fluid correlation can predict the heat transfer and pressure drop data with a mean absolute percentage deviation of 4.1% and 16.1%, respectively. In addition, for the same temperature glide, the heat transfer coefficient and frictional pressure drop of the R1234ze(E)/R1233zd(E) are up to 13.5% and 5.4% higher than those of the R134a/R245fa, respectively. [ABSTRACT FROM AUTHOR]

In this paper transient condensation and heat transfer been investigated on a printed circuit board (PCB) and a surface mounted (SMD) capacitor during a vapour phase reflow soldering (VPS) process. Classic VPS processes provide rapid heating with high heat transfer coefficient rates due to condensation that forms a continuous liquid film on the surfaces. Our aim was to develop a simple and fast computable method that, instead of relying on static descriptions, can reproduce the transient dynamics of the process. It was defined as an inverse heat conduction problem (IHCP) and proposed a solution based on finite difference modelling (FDM) with an alternating direction implicit (ADI) scheme. The layer dynamic was described with a parametric differential equation, and it was coupled to the concentrated and distributed thermal models of the PCB and the capacitor. The parameter values were determined by fitting to measured data. It was found that the heat transfer coefficient is inversely proportional to the layer thickness, and it changes over time according to the dynamic properties of layer growth. It was corroborated that the heat transfer depends on the geometry and thermal properties of the heated surfaces in VPS ovens. The presented method shows good agreement with the validation data. The new method provides a significant update over previous modelling approaches describing the condensation-based heating of the VPS process. [ABSTRACT FROM AUTHOR]

In practice, nanofluids' thermal conductivity and viscosity are the most important parameters in engineering applications. Viscosity affects pumping performance. Theoretical viscosity correlations are widely used in numerical studies. However, existing correlations show an underestimation of the actual viscosity compared to the measurement results. Although many nanofluid viscosity correlations have been developed, there is no generally accepted correlation. This paper reviews the theoretical, numerical, and experimental viscosity correlations and proposes a new correlation based on an analysis of approximately 1200 experimental and 4000 theoretical data tested for about 50 types of nanofluids in the temperature range 273–333 K and particle diameters 2–300 nm. The studied volume fraction range for the nanofluids was up to 10%. Existing correlations take into account the impact of up to two to three parameters The new viscosity correlation is proposed to predict the effective viscosity of nanofluids based on regression analysis of theoretical and experimental viscosity results, and it considers several factors that significantly affect the effective viscosity of nanofluids, such as nanoparticle diameter, density, temperature, types of nanoparticles, and base fluid. [ABSTRACT FROM AUTHOR]

In this paper, the flow behavior in a rectangular channel with miniature riblets (MRs) is numerically investigated. Specially, the MRs are arranged at the bottom of the channel along streamwise direction, and the height of MRs is much less than the thickness of boundary layer. In addition, the heat transfer feature in such a channel is probed as well. The numerical results indicate that the MRs play a role like a "wall" which effectively restrains the momentum exchange of adjacent speed streaks. In addition, it's found that the MRs can regulate the vortices of their surroundings. Furthermore, the flow structure in channel is extracted with the dynamic modal decomposition (DMD), and the frequency of the dominate mode is obtained, which demonstrates the flow stability in channel is improved by the MRs. Therefore, the drag reduction can be achieved consequently. It's also found that the local drag and heat transfer characteristics present similar features with the distance from MRs. Compared with the rectangular channel without MRs, the rectangular channel with MRs shows excellent drag reduction feature, and drag reduction rate is up to 17.19%, which is obtained at the expense of a degree deterioration in heat transfer performance. Furthermore, the comprehensive performance coefficients of various channel with MRs are all less than 1, which illustrates that the MRs is a device that is beneficial to drag reduction but to heat transfer enhancement. • Miniature riblets can restrict the motion of speed streaks in the spanwise. • Streamwise vortex become more orderly for the presence of miniature riblets. • Dynamic mode decomposition is used to show flow stability. • Velocity gradient vector is used to explain the mechanism of drag reduction. • Miniature riblets help to obtain drag reduction but heat transfer improvement. [ABSTRACT FROM AUTHOR]

This paper presents novel treatments for the Ellis non-Newtonian nanofluids flow over a rotating cone in the presence of motile oxytactic microorganisms. These treatments are depending on solutions of the stress tensor equations at specific values of the power-law index α (α = 0,0.5,1, 2) and introducing the stress as expressions in terms of the velocity gradients. Here, four systems of the governing equations for each value of α are presented and the non-similar forms for these systems are obtained. An efficient finite difference method based on the Blottner technique is applied to solve the converted systems. Various important impacts are examined in these simulations such as, magnetic field, non-linear radiation, viscous dissipation, heat generation and Joule heating. Wide ranges of the non-similar parameters are considered, namely, 0 ≤ ξ ≤ 10, 0 ≤ η ≤ 35. The major results revealed that, the skin friction gets higher values at the Ellis case α = 2 while the Newtonian nanofluids α = 1 gives the highest heat transfer rate. Both the velocity and temperature gradients are diminishing as the bioconvection Rayleigh number is enhanced. [ABSTRACT FROM AUTHOR]

This paper presents the flow boiling heat transfer coefficient and pressure gradient measurements of R32 and R1234yf mixtures in a microchannel tube at three concentrations: 15/85, 50/50, and 85/15 by mass. R32/R1234yf mixture has a temperature glide curve that the 15/85 R32/R1234yf mixture has low-temperature glide (0.5 K) while the opposite concentration (85/15) has the highest glide (7 K). The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg-m−2 s−1, heat flux from 0 to 6 kW-m−2, and vapor quality from 0 to 1. Mass flux has a strong effect on both HTC and dP/dz. Heat flux affects HTC but not dP/dz. In addition, the results are compared to pure fluid to discuss the effect of temperature glide. When the glide is higher, the HTC is lower and flatter. At fixed quality, as R1234yf concentration increases, HTC of the mixture decreases slightly first and then increases dramatically. Furthermore, two experiment methods are compared: evaporation in one pass and in one test section. The non-equilibrium in two-phase flow affects concentration gradient in the two phases and further affects the HTC of mixtures with high-temperature glide. • Heat transfer coefficient and pressure gradient of R32/R1234yf are measured. • Three mixtures with different compositions are studied. • Non-equilibrium in two-phase flow affects heat transfer coefficient measurements. • Heat transfer coefficient is lower when the mixture has a larger temperature glide. [ABSTRACT FROM AUTHOR]

A cooling channel model with semi-circular sidewall ribs in a rectangular heat generation body (HGB) is considered in this paper. With fixed volumes of the cooling channel and ribs, constructal design of the cooling channel is implemented with the objective of weighted sum function composed of the heat transfer and flow performances. Multi-objective optimization is further conducted by considering the two performance indexes. The results reveal that the quadratic minimum weighted sum function of the rectangular HGB can be gained by choosing the optimal width-to-height ratio of the cooling channel and the optimal radius of the sidewall ribs. The weighted sum function is augmented by 8.5% after constructal optimization. The comprehensive heat dissipation performance of the rectangular HGB is elevated by employing the suitable number of the sidewall ribs and mass flow rate of the coolant. The smallest deviation index is 0.267, which is gained by the LINMAP decision method, and the corresponding optimal construct can be selected as the design scheme of the cooling channel in a rectangular HGB. Multi-objective optimization provides a Pareto optimal solution set for the comprehensive performance of the rectangular HGB, which provides the heat dissipation designers of various electronic devices with new guidelines. • Constructal design of a cooling channel with semi-circular sidewall ribs in a rectangular heat generation body. • Linear weighted sum of maximum temperature difference and pumping power is minimized. • Multi-objective optimization based on NSGA-II is also conducted and Pareto frontier is obtained. • It provides theoretical guidelines for different design requirements of the electronic devices. [ABSTRACT FROM AUTHOR]

The low energy conversion efficiency of the plasma synthetic jet actuator (PSJA) and the misfiring problem caused by the low gas backfill rate have hindered the application of the PSJA in supersonic aircraft for fast response aerodynamic control. Although the hybrid synthetic jet actuator (HSJA) based on the PSJA has the potential to solve these problems, the heat and mass transfer mechanism of hybrid synthetic jets is still unclear. According to the capacitive discharge process, a novel three-dimensional center energy deposition model was proposed in this paper. The accuracy of the three-dimensional center energy deposition model was verified by comparing it with the experimental results. Considering the damped oscillation of the discharge intensity, the effect of nonconstant center energy deposition on the heat and mass transfer inside the cavity was further discussed. The simulation results show that the electrodes have an impact on the creation and evolution of the vortices inside the cavity. And the three-dimensional nonconstant center energy deposition model reveals the real heat and mass transfer mechanism of the HSJA, which provides the theoretical foundation for improving energy conversion efficiency and gas backfill rate of the HSJA. [ABSTRACT FROM AUTHOR]

Flash evaporation spray is an efficient heat transfer method. Changes in the microstructure and wettability of the cooling surface have important effects on its heat transfer. In this paper, four kinds of cooling surfaces, including a smooth copper surface, a square-column microstructured surface, a superhydrophilic smooth surface, and a superhydrophilic square-column surface, were prepared by chemical corrosion and laser etching. Using liquid nitrogen as the cooling medium, we conducted transient flash evaporation spray experiments on these four types of cooling surfaces. The experimental results show that surface superhydrophilic treatment can substantially enhance the flash evaporation spray of liquid nitrogen. It can more than double the cooling rate for both smooth copper surfaces and square-column microstructured surfaces. In addition, the combination of a microcolumn structure and superhydrophilicity on the cooling surface can provide double reinforcement for the liquid nitrogen flash evaporation spray. Its average cooling rate can reach 131,046 °C/min, and the maximum heat flux is 173 W/cm2. This study sheds some lights on heat transfer mechanism of superhydrophilic microstructure on the flash evaporation spray and contributes to the development of a new method for cell vitrification. [ABSTRACT FROM AUTHOR]

The paper presents a critical review of the performance evaluation criteria (PEC) developed by Webb (Webb, 1981). These criteria evaluate the benefits that could be obtained in heat exchangers if their surfaces and inserts are designed to enhance the heat transfer, particularly in two-fluid heat exchangers in single-phase flow. This study revealed that the imposed constraint for fixed pumping power in the comparison of the augmented channel with the reference one is a major obstacle to achieving greater benefit. Accordingly, the elimination of the constraint of fixed pumping power and the objective of reduced pumping power decreases substantially the criteria of Webb. A new constraint related to the fixed augmentation entropy generation number has been involved replacing the constraint of fixed pumping power. The pumping power can be left to increase until the augmentation entropy generation number is less than unity. The maximum benefit can be obtained when the augmentation entropy generation number reaches the value of unity. The use of greater in tube heat transfer enhancement (with the use of compound heat transfer enhancement techniques instead of simple ones), and with an enhanced outer tube surface of the augmented exchanger is a useful instrument to increase the benefits, only if the augmentation entropy generation number does not cross the frontier of unity. • Enhanced heat transfer surface • Reduction of the performance evaluation criteria • The role of the augmentation entropy generation number • Benefits [ABSTRACT FROM AUTHOR]

The thermal lattice Boltzmann method can be adopted to simulate cavitation bubble collapse in heating or cooling systems. The numerical results satisfy Laplace's law and are consistent with temperature solutions derived from the Rayleigh–Plesset equation. In this paper, in order to study the effects of wall temperature on a collapsing bubble, a calculation model for a cavitation bubble near the heated/cooled wall is established. The influence mechanism of the micro-jet and the cavitation bubble itself on the solid-wall heat transfer, and the thermodynamic behavior characteristics of the cavitation bubble collapse near the wall are obtained. Adjusting the wall temperature T w also affects the thermal effects of cavitation. Furthermore, A dimensionless temperature parameter η is introduced to study the heat transfer intense of the model. The influence of λ , Δ p and R 0 on the heat transfer intense are studied and analyzed. The results show that the optimal λ , Δ p , R 0 and T w values can be used to enhance heat transfer and realize heating or cooling treatment of different surfaces. • Analysis of cavitation bubble in a heating or cooling system is presented by a thermal lattice Boltzmann method. • The influence mechanism of the micro-jet and the cavitation bubble itself on the solid wall heat transfer. • The changes of Tw under different initial wall temperature are analyzed. • A dimensionless parameter η is introduced to study the heat transfer intense, which is related to the λ , Δ p and R 0. • The optimal λ , Δ p , R 0 and T w are used to realize heating or cooling treatment of different surfaces. [ABSTRACT FROM AUTHOR]

In this paper, the effect of an unheated starting length, L 0 varying between 0 and 30 cm, on the burning and heat transfer characteristics over a horizontal Poly (methyl methacrylate) flat plate is experimentally investigated under wind velocities ranging from 0.5–2.0 m/s. The flame images, flame standoff distance, and Nusselt number are measured or calculated. In comparison to the wind velocity, the effect of an unheated starting length is of first-order importance in influencing the flow, heat transfer, and combustion over a solid fuel surface. The unheated segment introduces unique streamwise streaky flames. A critical condition is found to exist at L 0 = 10 cm, where the spanwise wave and lattice structures are observed in the flame, causing the standoff distance and Nusselt number to increase markedly beyond the theoretical prediction. When the unheated segment exceeds a certain length, the increased unheated length effect becomes negligible. Furthermore, the unheated segment shortens the laminar forced convection region and broadens the turbulent buoyant convection region. The study provides very crucial information regarding the effects of an unheated segment on the transitioning mechanism of a wind-driven flame which might be useful and important for fire research. [ABSTRACT FROM AUTHOR]

In this paper, the heat transfer characteristics of a tissue-like capillary porous media are investigated, using the realistic and volume averaging methods. For this purpose, a cylindrical medium is introduced with counter-current vascular network, including longitudinal parallel vessel pairs and numerous interconnect capillaries. The energy equations of solid and fluids phases are derived based on one realistic and two volume averaging methods. Accordingly, the thermal behavior of this capillary porous media was modeled in three cases: constant temperature of solid phase, uniform moderate heat generation in solid phase, and local intensive heat generation in solid phase. The comparison of results for various lengths and densities of interconnect capillaries indicates the inaccuracy of volume averaging method, especially if the supply and return fluids are considered as single integrated phase. Thus, despite much anatomical dissimilarities between the present medium and real living tissues, the occurrences of such errors can be expected if the volume averaging method is used. In addition, the predicted temperature of solid phase at the end of various interstitial heating processes shows that the constant delivery of focused energy in shorter time can increase the maximum temperature of solid phase and its heat affected zone. [ABSTRACT FROM AUTHOR]

In this paper, the finite element method (FEM) and experiment were adopted to evaluate the thermal performance of a multi-chip COB LED lamp based on the cold spray technology. The results show that the junction temperature can be limited under 110 °C. Compared to an aluminum-substrate LED module pressed on the heat sink, the soldering structure of a copper-substrate LED can reduce the junction temperature. The junction temperature of the LED module soldered on the heat sink is only 97.3 °C, while one of the pressing structures is 103.5 °C. It is further found that the chip gaps and the thickness of the copper-circuit layer have significant effect on the heat spread. Increasing the chip gaps and the thickness of the copper-circuit layer can effectively reduce the junction temperature and improve the LED lamp's thermal performance. [ABSTRACT FROM AUTHOR]

Recent works have examined the thermal and dynamical aspects of natural convection occurring in tilted hemispherical cavities whose disk constitutes the hot active wall. The influence of some important aspects on the natural flows have been considered such as the Rayleigh Number, the disk inclination angle with respect to the horizontal plane and the thermal boundary condition on the disk (Dirichlet, Neumann). Combinations of these parameters have been treated in steady state as well as is transient regime. Correlations of Nusselt–Rayleigh type (steady state) and Nusselt–Rayleigh–Fourier type (transient regime) that allow quantifying the heat transfer are proposed in these surveys. The objective of this paper is to synthesize and summarize these works in order to facilitate their implementation in engineering applications. [ABSTRACT FROM AUTHOR]

In the present paper the combination of three-dimensional isothermal and two-dimensional non-isothermal Lattice Boltzmann Method (LBM) are employed to simulate the nucleate pool boiling phenomenon. In order to validate the proposed model, rising bubble phenomenon is simulated afterward the boiling process is investigated by employing a function for heat transfer. The investigation is compared with other numerical works and is found to be in very good agreement. The effects of the parameters including contact angle, heat flux and heater length on departure dimensionless time and departure diameter of bubble are studied. The results show that departure diameter of bubble increases due to the increase of contact angle and decrease of gravity force. Formation, motion and breakup of the bubbles are also investigated as results of gravity force and heat flux. Furthermore, it is worthwhile pointing that out departure diameter of bubble increases as the heat flux increases and increasing of the heater length has more effect in comparison to heat flux on bubble growth. Transient regime is shown by increasing the contact angle and the small bubbles vanishing while rising upward at high gravity force. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical model for studying the effects of shrinkage induced flow on the growth rate of binary alloy dendrites. An equivalent undercooling of the melt is defined in terms of ratio of the phase densities to represent the change in dendrite growth rate due to variation in solutal and thermal transport resulting from shrinkage induced flow. Subsequently, results for dendrite growth rate predicted by the equivalent undercooling model is compared with the corresponding predictions obtained using an enthalpy based numerical method for dendrite growth with shrinkage. The agreement is found to be good. [ABSTRACT FROM AUTHOR]

In this paper, the convective heat transfer of the heat transfer oil-copper oxide nanofluid flow in horizontal smooth and microfin tubes is investigated experimentally. Using a flow control system, the flow regime is always laminar and the wall temperature is constant by using a steam tank. Pure heat transfer oil and nanofluid with the weight concentrations of 0.5%, 1% and 1.5% are used as working fluids. The results are in good agreement with the classic correlations for the pure fluid flow. Based on the results, combination use of nanoparticles and the microfin tube leads to the heat transfer enhancement up to 230%, in comparison with the base fluid flow in the smooth tube. The results are useful in the prediction of the heat transfer rate and the pressure drop in nanofluid flows. [ABSTRACT FROM AUTHOR]

The geometry and heat transfer effects on heat pipes embedded heat sinks-cooling system are investigated in the present paper. In the forced convection system, two different heat pipe geometrical shapes of L and U configurations are taken into account. This study adopts a versatile superposition method and least-square estimators with thermal resistance network analysis to design and experiment their geometry and heat transfer effects under different fan speeds and heat source areas. The results suggest that the characteristics of system performance under varying speeds and areas are significantly different from those under altering speeds and areas. When the thermal performances of these two configurations are 0.04 °C/W of U-shaped heat pipes at 78.85 W, and L-shaped heat pipes are lowest 1.04 °C/W at 34 W, respectively, the lowest thermal resistances of the representative L- and U-shaped heat pipe–heat sink thermal modules are respectively 0.25 °C/W and 0.17 °C/W under twin fans of 3000 RPM and 30 × 30 mm 2 heat sources. The result of this work is a useful thermal management method to facilitate rapid analysis and has provided a useful insight into the design of heat pipe cooling systems. [ABSTRACT FROM AUTHOR]