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Abstract: A numerical study of pressure distribution and flow cross-over through the gas diffusion layer (GDL) in a PEMFC flow plate using a serpentine channel system has been undertaken for the case where the channel has a trapezoidal cross-sectional shape. The flow has been assumed to be 3-D, steady, incompressible and single-phase. The flow through the porous diffusion layer has been described using the Darcy model. The governing equations have been written in dimensionless form and solved by using the commercial CFD solver, FIDAP. The results obtained indicate that: (1) the size ratio, R, of trapezoidal cross-sectional shape has a significant effect on the flow cross-over. As R increases, the flow cross-over through GDL increases; (2) the ratio R also has a significant effect on the pressure variation in the flow field for both cross-over and no cross-over cases; (3) flow cross-over has a significant influence on the pressure variation through the channel, tending to decrease the pressure drop across the channel; (4) an increase in Re number can lead to a slight increase in the flow cross-over. [Copyright &y& Elsevier]

The topic of this paper is to couple the passive and active approaches to enhance the heat transfer performance in the inline tube bundle heat exchanger. Firstly, the k-kl-ω turbulence model and Kays turbulent Prandtl number model are validated by liquid metal cross flow tube bundle experiments and rectangular groove pulsating flow experimental results. Then the numerical studies were carried out for pulsating velocity amplitude of 0.5, frequency of 10, and Re of 20,000 with transverse and streamwise pitch-to-diameter ratios of 1.5, 1.65, and 1.8. The introduction of pulsating flow effectively improves the turbulence characteristics between the tube bundles, especially at the first three rows, where the velocity fluctuation is enhanced by about 1.5–2 times. The tube heat transfer performances are all improved. The performance evaluation criterion (PEC) variation ranges from 1.04 to 1.32, which signifies that the introduction of pulsating flow can enhance the comprehensive heat transfer performance. The per unit flow rate thermal entropy generation decreased by about 9.7–33.3 %, in addition, a phase difference between the transient thermal entropy production and pulsating velocity was found. The correlation equations about Nu and f factor within the scope of this paper are presented. • Liquid metal pulsating cross flow inline tube bundles are simulated. • Heat transfer performance and fluid flow structure characteristics are analyzed. • Transient and time-averaged irreversible losses are studied. • New correlations on Nu and f factors are given. [ABSTRACT FROM AUTHOR]

Dual compensation chamber loop heat pipe (DCCLHP) is a promising technology for future avionics cooling. In this work, the effect of orientations on the steady state and startup performance of a DCCLHP with kW class heat transfer capacity was investigated experimentally. Experimental results show that for high capacity DCCLHP, the variation of condenser performance at different orientations is a decisive factor in system performance, which could result in up to a 50 % decrease or 10 % increase in the heat transfer limit. In addition, the lack of condenser performance could also have a potential negative impact on the startup performance. By establishing a method to analyze the operating temperature, the authors provided an in-depth analysis of the related mechanisms. The effect of gravity could change the force condition and therefore the thickness of the condensate in the condenser, which in turn affects the condenser capacity and the working fluid temperature flowing into the compensation chamber. As a result, for practical applications, it is necessary to focus on the installation of the condenser to avoid the orientations that tend to cause the increase in the thickness of the condensate film, such as the condensate flow inside the condenser was opposite to gravity. The research content of this paper could deepen the understanding of LHP technology and contribute to the realization of application in acionics cooling in the future. [ABSTRACT FROM AUTHOR]

The effects of two-phase flow and radiative heat transfer have a significant impact on the internal thermal environment and thermochemical ablation process in aluminized solid rocket motor (SRM). However, these two effects were not revealed systematically in previous studies. In this paper, a multiscale methodology combined radiative properties of molten alumina particles at microscale calculated by Mie theory and the two-phase flow and radiative heat transfer at macroscale calculated by Eulerian-Lagrangian method coupling with discrete ordinate method is constructed within aluminized SRM. The convection and radiation are coupled with the chemical kinetics to predict the thermochemical ablation and thermal environment. The influence of two-phase fluid flow and thermal radiation on the internal thermal environment and thermochemical ablation of a SRM nozzle is analyzed quantitatively. Comparisons are made among the methodologies that models the alumina particles as gas species, considers the effect of two-phase flow and considers the effects of two-phase flow and radiative heat transfer. The results show that the erosion rate and total heat flux at the inner wall in the convergent section increase noticeably when considering the effect of two-phase flow. Meanwhile, the erosion rate in the divergent section also increases and the thermal environment is dependent on the particle size and aluminum content. Radiative heat transfer considerably enhances the total heat flux, especially in the convergent section. On the contrary, its impact on the erosion rate is very small except for the inner wall with very low temperature. [ABSTRACT FROM AUTHOR]

In this paper, the fluid flow behavior and cooling characteristics of a sweeping impingement jet were investigated. Also, the difference between free and confined impingement schemes was investigated. In addition, the conventional approach for the purpose of significantly reducing computational time by using a confined domain with a slip upper wall instead of an unconfined domain was evaluated. A 2D unsteady Reynolds averaged Navier-Stokes (URANS) simulation accompanied with the k-ω SST turbulence model is used in this study. The study has been conducted for a target wall with a constant heat flux of 3000 W/m2, jet-to-wall distance of 4, and a jet Reynolds number of 2500. The results show that the overall average cooling performance of the sweeping jet is better in the confined impingement scheme compared to that of the steady jet, while the steady jet is slightly better in the unconfined sweeping impingement scheme. Using a confined scheme with a slip upper wall does not reveal the complete thermal and flow behaviors, and the wall heat transfer distribution is very different from the unconfined domain. [ABSTRACT FROM AUTHOR]

This paper undertakes an experimental measurement of effective thermal conductivity, permeability, inertial coefficient, friction factor and wall convective heat transfer from octet truss lattice (OTL) structures as air-cooled compressive load-bearing light-weight heat sinks. Three aluminium alloy (AlSi10Mg) OTL structures of porosities 70.6%, 86.9% and 91.3% were fabricated using an additive manufacturing technique. Experimentally measured effective thermal conductivity, permeability, inertial coefficient, friction factor and wall Nusselt number for OTL structures are reported. Owing to high solid-to-fluid thermal conductivity ratio (∼4250), heat flows through the metal ligament and air independently and establishes steady-state, i.e. peripheral conduction coupling between solid metal and air is negligible. This is conferred from various theoretical and semi-empirical models from the literature. Friction factor and wall Nusselt number are characterised using a single length scale as against different length-scales often used in the literature. Flows through OTL structures seem to follow Forchheimer's flow law with negligible viscous contribution even at low pressure gradients. Further, the results are compared with previous analytical and experimental results for stochastic open-cellular foams. Owing to periodic nature of OTL structures, friction factor is lower than stochastic foams as mechanical dispersion effects are negligible. In conclusion, OTL structures are an excellent candidate for multifunctional applications where heat sinks are expected to bear compressive loads as well as dissipate heat while being lightweight. [ABSTRACT FROM AUTHOR]

Transpiration cooling using liquid water as coolant has been confirmed as a promising thermal protection method due to the huge phase change latent heat. A multi-region numerical strategy is developed and presented in this paper, to simulate directly the liquid transpiration cooling process within a wedge-shaped porous cone. An experiment was conducted to validate the numerical strategy. Using the validated strategy, five series of numerical simulations under different coolant mass fluxes were carried out. The numerical results revealed some interesting and valuable characteristics of fluid flow, heat absorption and phase change of liquid coolant in the porous cone and on the impermeable surface. When the liquid coolant is vaporized incompletely in the porous region, one can find two interesting phenomena: 1) A large amount of coolant flow towards the end of the porous cone, while only a small part of coolant can be transferred into the leading edge region; 2) An increase in inlet coolant mass flux causes a decrease of the coolant ejected from the leading edge. However, these phenomena are absent when the liquid coolant is vaporized completely in porous region. Another important phenomenon demonstrated in this work is an additional thermal protection effect on the downstream impermeable surface. This additional thermal protection effect increases with the inlet coolant mass flux, but at the price of a larger wastage of coolant phase change latent heat. [ABSTRACT FROM AUTHOR]

Abstract This paper is focused on the thermal buoyant flow stratification, generated by fire that occurred in a tunnel congested by an obstacle simulating a vehicle. The obstacle occupies about 0.31% of the tunnel cross section and is located at different distances from the floor. This case is considered for two main reasons: the first is that the remaining vehicles blocked upstream of the heat source cause a partial obstruction, the second reason is that the vehicle bottom actually has a certain distance from the tunnel floor. The obtained results showed that the increase of the forced airflow velocity has for results a loss of stratification. The obstruction existence effect in tunnel is reflected by smoke flow destratification upstream of the heat source fire. However, this effect is compensated downstream of the fire and there has been an improvement in the smoke flow stratification contrary to the case where the tunnel would be empty. Highlights • Thermal buoyant flow stratification induced by tunnel fire was investigated. • Effects of longitudinal imposed airflow and an upstream blockage was analyzed. • An increase of the longitudinal forced airflow velocity has for results a loss of stratification. • The obstacle presence within the tunnel facilitates the smoke flow destratification upstream of the heat source. [ABSTRACT FROM AUTHOR]

Abstract T-Junction experiments have been performed on the Fluid-Structure-Interaction (FSI) test facility at the University of Stuttgart to investigate the thermal fatigue effect in thermal mixing processes. The previous experimental works have been performed with different working conditions to discuss the influence of flow parameters (e.g. flow rate, flow temperature) on the mixing characteristics. The experiments have been performed with continuous and periodical mixing processes. 48 high-performance micro-thermocouples are installed in six measurement cross-sections close to T-junction to capture the near-wall fluid temperature. The temperature measurement data are analyzed and discussed with the viewpoint of their relevance for thermal fatigue. An unexpected mixing phenomenon, which is defined as the additional low frequencies, has been detected in the results of frequency analysis on the temperature data. Such kind of frequency with the value about 2.66 Hz can be found in most of our experiments. This similarity of the additional low frequency from the experimental work is summarized in this paper and compared with each other. With a small-scale experiment, this phenomenon is identified as harmonic oscillation of thermal stratification in pipe tangential direction, which has never been found in any of the previous T-junction investigations, nor in any literature on fluid mechanics. According to this phenomenon, novel flow patterns are introduced to categorize the mixing characteristics at horizontal T-junction. Among the patterns, the deflecting jet has the lowest possibility to initiate thermal fatigue in the mixing process at horizontal T-junction. With the results of the experimental and numerical investigations, the flow patterns of the thermal mixing processes at the FSI test facility can be summarized in the mixing envelope diagram, which presents the thermal mixing characteristics as the property of the specific horizontal mixing tee. Furthermore, once the mixing envelope has been drawn for a specific mixing tee, it can be used as operation guideline to reduce or avoid thermal fatigue close to the mixing tee. Graphical abstract Image 1 Highlights • The tangential oscillation of thermal stratification has been discovered from the temperature measurement in experiments. • A new classification of flow patterns has been summarized for thermal-mixing characteristics at horizontal T-junction. • Mixing envelope has been concluded for the flow patterns with the impact of the crucial factors. • Mixing envelope provides a solution to reduce or avoid thermal fatigue in the mixing flow at horizontal T-junction. [ABSTRACT FROM AUTHOR]

Abstract This paper presents numerical investigation to study the melting of Phase Change Material (PCM) partially filled in a vertical cylindrical tube. The top space of tube was filled with air to take into account the volumetric expansion of PCM. The natural convection inside PCM liquid phase was considered via the temperature dependency of density and gravitational force. The finite-volume method was adopted to discretize the conservation equations of mass, momentum, and energy. The enthalpy porosity formulation was employed to solve the energy equation in both regions of PCM, liquid and solid. In this multiphase system, the Air-PCM interactions have been treated using the Volume of Fluid model (VOF). The mathematical model is based on conjugate heat transfer in PCM subjected to a constant temperature at the external surface of cylindrical shell. The obtained results have been analyzed and compared with literature, and a good agreement was showed. Then, a parametric study was carried out to establish correlations for the liquid fraction and time of complete melting as a function of all dimensionless parameters that governing this problem, such as Fourier number, Grashof number, Stefan number, wall to PCM thermal diffusivity ratio, tube aspect ratio, shell-to-tube diameter ratio and dimensionless initial temperature. The results show that all parameters of the problem can really affect the phase change phenomena and consequently, affect the melting time. [ABSTRACT FROM AUTHOR]

Abstract In this paper, experimental and numerical studies of fluid flow and heat transfer characteristics of novel fish scale bioinspired structures on the bottom surface of microchannel to enhance heat transfer are presented. Deionized water is selected as the working fluid. A three-dimensional numerical model is developed to analyse conjugate heat transfer in the microchannel, and the model is validated with the experimental results obtained from a copper microchannel of hydraulic diameter of 193.5 μm and length of 20 mm for three different Reynolds numbers and a constant bottom heat flux of 50 W/cm2. Further the numerical model is extended to study the effect of several geometrical parameters on the thermohydraulic performance of microchannel. It is found that the bioinspired surface can enhance the convective heat transfer compared to the plain microchannel, whereas the pressure drop is found less for fish scale structure with dimensionless inclination height of 0.026. The friction factor for the fish scale inclination height of 0.026 reduces by maximum 5% and the Nusselt number increases by maximum 14% as compared to those quantities for the plain microchannel. Further increment in the inclination height increases the heat transfer rate and friction factor than the plain microchannel. The maximum value of the performance evaluation criteria is found as 1.75 at inclination height of 0.26 and Re = 1050. Correlations of Nusselt number and Poiseullie number are developed for this type of microchannel. Highlights • Novel bioinspired fish scale microchannel is designed and analysed. • Fish scale inclination angle of 1° shows reduction in ΔP than plain channel. • Higher number and angle of inclination of fish scales significantly enhances Nu. • The maximum overall performance for the novel microchannel is found as 1.75 • Dimensionless correlation is developed for Nu and fRe. [ABSTRACT FROM AUTHOR]

Abstract This paper is an extension of the previous work by Degiovanni & Remy (2014) [1], where it was analytically demonstrated that the appropriate representation of interfacial heat transfer consists in defining a generalized impedance Z. The latter is intrinsic to the flow and does not depend on the thermal boundary conditions. In this work, we demonstrate the pertinence of this representation for complex flow problems where a direct analytical formula of the impedance cannot be calculated or can only be presented in terms of complex equations that are not easily applicable. The general idea is to perform numerical experiments to estimate this impedance using an inverse method, thus obtaining Green's function. Numerical simulations are carried out using the software Fluent for each flow problem. We will show that by using the inverse method on the numerical results for a constant temperature boundary condition, we are able to calculate the intrinsic Green's function for each flow problem, from which we can accurately calculate the thermal characteristics for any other thermal boundary condition, particularly for a constant heat flux. [ABSTRACT FROM AUTHOR]

Abstract The advancements in the materials science and the ability of manufacturing new materials, make it possible for engineers to use the porous medium as a solid body that provide maximum surface contact with the fluid. In this paper, to increase the contact surface area and decrease the flow resistance, the solid tapered fins replace with porous one and its effect on convective heat transfer is investigated for different Reynolds number. To do this, first the heat transfer of the rigid tapered fins is studied and validated, then, the solid body is changed to the porous one and the influence of the Reynolds number was investigated on the heat transfer and pressure drop. The results show that in laminar flow the Nusselt number of the flow with porous medium is 33% higher and the pressure drop is 9.35% lower than the rigid one with the same conditions. Therefore the application of the porous medium at low velocities is much recommended but with increasing the velocity, the porous media is not preferable. Also, for the problem of flow around a bundle of porous tapered fins, an equation for Nusselt number is presented based on the Reynolds number. [ABSTRACT FROM AUTHOR]

Abstract This paper presents the phenomenon of laminar turbulent transition simulated by modified algebraic intermittency model based on well known k- ω turbulence model by Wilcox for flows in parallel-plate channels. Results of simulations using described model were obtained for low Reynolds numbers 1000 ≤ Re ≤ 75000 with different turbulence intensity in inlet cross-section of the parallel-plate channel. Results were compared to and validated with existing data available in literature both experimental and theoretical. Implementation of described model with pre-processing and post-processing was made using open source software OpenFOAM. [ABSTRACT FROM AUTHOR]

Abstract An unusual type of humping defect with periodic humped region and perforated region may occur in high-speed full penetration GTAW, which deteriorates the homogeneity of weld property seriously. In this paper, the thermal behavior and fluid flow during full penetration humping (FP-humping) formation is revealed by a numerical investigation to explain the defect's physical mechanism. Self-adaptive double-ellipse distributions of arc heat flux, arc pressure, arc shear stress and electromagnetic force varying with transient free surface evolution are proposed. For the first time, the actual morphology of FP-humping in GTAW can be predicted numerically. The numerical results are well verified by experimental weld geometry and high-speed melt pool images captured by a color camera. The workpiece is melted under arc heat to form the melt pool, and its free surface is gouged significantly under strong arc forces. When the depth of gouging region approaches workpiece thickness, the bottom thin liquid layer in gouging region is easily disrupted to initiate FP-humping formation. The lateral channel becomes the only transfer channel for liquid metal backward flow. The transition region between lateral channel and rear part of melt pool will be necked, and solidified instantly without the arc heat. Both scaling model of thermal conduction and numerical results imply that the premature solidification of necked lateral channel is a predominant factor to influence the FP-humping formation. Highlights • The thermal behavior and fluid flow during GTAW FP-humping formation is studied by numerical modelling. • The detailed morphology of GTAW FP-humping can be predicted numerically for the first time. • The disruption of thin liquid layer at gouging region is the initial factor for FP-humping formation. • The premature solidification of necked lateral channel is the dominant factor to influence FP-humping formation. [ABSTRACT FROM AUTHOR]

Abstract: In the model presented in this paper the region around a single growing vapor bubble in nucleate boiling is subdivided into three parts: a small, ring-shaped zone between heating wall and bubble, called micro-region, the bubble itself, and its surrounding liquid, referred to as macro-region. As the micro-region is most important for heat transfer, a recently developed model put special emphasis on this region, and predicted heat transfer, bubble growth, and departure diameters of vapor bubbles for low to moderate heat fluxes fairly well. The current paper aims at modeling the macro-region in more detail than done before. For this purpose, Navier–Stokes equations for both vapor and liquid phase were solved with the aid of a finite element method. It turned out that the flow pattern in the liquid around a growing vapor bubble is determined by the movement of the bubble surface, but also by the vapor flow inside the bubble. The fluid in the macro-region transports cooler liquid towards the wall, thus increasing the heat transfer significantly. In regions farther away from the bubble, heat conduction prevails. Buoyancy was found to be of small influence compared to forced convection. Velocity and temperature fields, heat fluxes, bubble contours, and departure diameters were calculated for different fluids. The apparent contact angle is decisive for growth rate and departure diameter. Disturbances, caused by departing and ascending bubbles, are included into the calculations. [Copyright &y& Elsevier]

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air ( Pr = 0.71 ) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10 ≤ Re ≤ 45 and incidence angles, 0 0 ≤ α ≤ 180 0 . The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0 ≤ R i ≤ 2 . The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag ( C D ) , lift ( C L ) , and moment ( C M ) coefficients on the combined influence of α and R i , is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number ( S t ) are determined with respect to the various ranges of parameters considered in the present investigation. [ABSTRACT FROM AUTHOR]

This paper deals with the numerical investigation of heat transfer by mixed convection inside ventilated cavities with supply and exhaust slots, and filled with air under a steady and turbulent flow regime. Four configurations, rated A, B, C and D are considered here, according to the position of the inlet and outlet air ports: A, the inlet is on the top of the left vertical wall, while the outlet is on the bottom of the right vertical wall; B, the inlet is on the bottom of the left vertical wall and the outlet at the top of the opposite wall; C, the two slots are on the same side, i.e. the inlet is at the bottom and the outlet at the top of the left vertical wall, and D, the inlet is at the top and the outlet at the bottom of the left vertical wall. The bottom of the cavity is kept at a temperature T H and other walls are fixed at a temperature T C , with T H > T C . The cavity is provided with two slots: an inlet slot for introducing fresh air, and an outlet slot to extract hot air. The main aim sought here is to analyze the ventilation efficiency for temperature distribution, and fix the best configuration providing the thermal comfort targeted. We also address the influence of heating on the behavior of flow and thermal comfort, while considering different Rayleigh numbers ranging from 6.4 × 10 8 to 3.2 × 10 9 . Numerical studies have been yet devoted to these configurations, using RANS simulations. The RNG k-ε turbulence model has been adopted for the turbulence closure, and the set of governing equations was then numerically solved via the finite volume method. The SIMPLEC algorithm was associated to ensure the pressure-velocity coupling. In terms of results achieved, the configuration D provides a better ventilation effectiveness for temperature distribution ε T and ensures an even temperature in the occupied zone. As for configurations A and C, they maintain an acceptable level of heat and can be used in winter period to ensure good indoor air quality, while configuration B provides an efficiency close to unity and can be used to insure indoor air quality in temperate climate zones. [ABSTRACT FROM AUTHOR]

In this paper, a new method of estimation of a convective heat transfer coefficient on a hot spot is presented. First of all, a complete numerical model is developed in order to simulate the convection on a wall heated at a hot spot. Then, an analytical solution of the configuration is developed assuming simplified assumptions. Finally, an inverse method is performed to estimate the local convective heat transfer on a hot spot using noisy simulated data. The results of the estimation procedure are accurate and validate this new approach of convection measurement. • A 3D completed model of forced convection of a heated plate at a hot spot is developed. • An explicit analytical calculation using the Fourier and Hankel integral transforms. • Identification of the local convective coefficient is performed using simulated data. [ABSTRACT FROM AUTHOR]

In this paper, the pressure drops and interfacial heat transfer coefficient in metal foam with different pore density which was constructed by tetrakaidecahedron models were numerically studied under high velocity by Computational Fluid Dynamics (CFD). The velocity fields, temperature fields and pressure fields in the computational domain of metal foam were investigated. The equation of pore density and hydraulic diameter of metal foam was obtained. Influence of flow rate on the whole porous zone was also studied and obtaining the fitting formula to predict pressure loss in porous metal which was applied to the pore density range from 10PPI(Pore Per Inch)to 40PPI at high flow rate. Comparing with the previous literatures, the maximum error of pressure drop value was 12.23%. Meanwhile, the relationship between Reynolds number and interfacial heat transfer coefficient was studied, the results shown that the values of heat transfer coefficient increase with the rising of Reynolds number, but the lifting rate decreases gradually. And according to the data points, the numerical formula of interfacial heat transfer coefficient in porous metal was obtained which had a great correctness through contrasting with other data. As a result, the fitting formula could accurately predict the convection heat transfer coefficient of the metal foam interface and provide relevant data for the Thermal Non-equilibrium Model. • The heat transfer and fluid flow characteristics in foam metal under high Reynolds numbers were numerically studied based on tetrakaidecahedron model. • A new correlation for pressure drop in foam metal under high Reynolds numbers was obtained. • A new correlation for interfacial heat transfer coefficient in foam metal was obtained. [ABSTRACT FROM AUTHOR]

This study presents an experimental investigation of flow boiling of a zeotropic mixture in a microchannel. In the recent years much attention has been paid to the possible use of fluorinated propene isomers for the substitution of HFC fluids, being in most cases high-GWP refrigerants. However, the available HFOs (hydrofluoroolefins) cannot cover all the air-conditioning, heat pump and refrigeration systems when used as pure fluids because their thermodynamic properties are not suitable for all operating conditions and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. In the present paper a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been studied. The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter is measured at a pressure of 14 bar, which corresponds to a bubble temperature of 26.2 °C. The flow boiling heat transfer coefficients measured in the present test section are discussed, with particular regard to the mass transfer mechanism in the microchannel. The new experimental data are also compared to flow boiling data of pure R1234ze(E) and pure R32 to analyze the heat transfer penalization due to the mass transfer resistance of the mixture. [ABSTRACT FROM AUTHOR]

In this paper, the concept of “smart” thermally insulating materials intelligently adapted to specific engineering configurations is established and extensively validated. Thermal insulation is achieved by local suppression of the momentum of the confined natural convection flow in the most critical regions, as determined by a linear stability analysis of the flow in the presence of implants of heterogeneous porous media. The implants are modelled by unconnected packed beds of equi-sized cylinders. The concept is based on a mesoscale approach in which the non-slip boundary conditions in the vicinity of the packed beds are explicitly imposed by utilizing the immersed boundary (IB) method. Two different patterns for the “smart” porous media are established, and their thermal insulation properties are quantified. It is shown that the optimized patterns for implants of heterogeneous porous media, occupying approximately only 5% of the overall volume, can drastically delay the steady-unsteady transition of the 2 D natural convection flow in a square differentially heated cavity with thermally perfectly conducting horizontal walls. In addition, it is demonstrated that the implants facilitate a consistent decrease in the heat flux through a cubic differentially heated cavity with all being thermally perfectly conducting lateral walls. [ABSTRACT FROM AUTHOR]

In this paper, a new dispersion model has been presented and evaluated for the thermal properties of nanofluids in flat tubes. First, by using the results of a multi-objective optimization, which has been implemented by combining the CFD techniques and optimization methods, a new dispersion model is presented for thermal properties of nanofluid in flat tubes, which of course is applicable to circular tubes by setting the tube flattening to zero. This model is a complete function of the important parameters of nanofluid flow in flat tubes, which include the tube flattening, flow rate, volume fraction of nanoparticles and the diameter of nanoparticles. In the next step, the capability and performance of the introduced model is evaluated through the numerical simulations of the nanofluid flow in flat tubes using FORTRAN programming language. The obtained results indicate that the presented dispersion model has a high capability in predicting the thermal properties of different nanofluid flows in flat tubes. [ABSTRACT FROM AUTHOR]

In this paper, the effects of magnetic field, nonlinear thermal radiation and homogeneous-heterogeneous quartic autocatalysis chemical reaction on an electrically conducting (36 nm) alumina-water nanofluid containing gyrotactic-microorganism over an upper horizontal surface of a paraboloid of revolution is presented. The case of unequal diffusion coefficients of reactant A (bulk-fluid) and reactant B (catalyst at the surface) in the presence of bioconvection is presented. In this article, a new buoyancy induced model for nanofluid flow along an upper horizontal surface of a paraboloid of revolution is introduced. The viscosity and thermal conductivity are assumed to vary with volume fraction and suitable models for the case 0% ≤ ϕ ≤ 0.8% are adopted. The transformed governing equations are solved numerically using Runge-Kutta fourth order along with shooting technique (RK4SM). Good agreement is obtained between the solutions of RK4SM and MATLAB bvp5c for a limiting case. The influence of pertinent parameters are illustrated graphically and discussed. It is found that at any values of magnetic field parameter, the local skin friction coefficient is larger at high values of thickness parameter while local heat transfer rate is smaller at high values of temperature parameter. [ABSTRACT FROM AUTHOR]

In thermoacoustics, stacks and regenerators are porous media where energy conversion takes place. Modelling full thermoacoustic devices with a CFD approach, in order to capture some nonlinearities, can be extremely expensive from a computational perspective compared to a standard linear approach used in the frequency domain. At the same time, macroscopic models for porous media developed for steady-state flows cannot be directly applied in oscillating flow conditions. Moreover, macroscopic models in the available literature for oscillating flows are inaccurate at high frequencies or require a closure coefficient to be determined numerically (with Direct Numerical Simulations) or experimentally. In this article, a time domain macroscopic model for heat and fluid flow is proposed based on the concepts of complex Darcy and Nusselt numbers in the linear regime. Such coefficients, introduced in the past to describe the oscillatory phenomena, have been used for the first time to build a CFD macroscopic model in terms of their real and imaginary parts. For two different porous media, a parallel plate and a transversal pin array, the developed macroscopic model is verified with the microscopic solution. Furthermore, for a transversal pin array stack, the proposed model is validated against experimental data from the available literature, showing a very good agreement. The findings of this paper can help to strongly reduce the computational costs of oscillatory flow simulations without prior direct numerical simulations of the porous core. [ABSTRACT FROM AUTHOR]

The transient heat transfer performance of passive residual heat removal heat exchanger (PRHR HX) with C-tube bundle directly influences nuclear reactor safety under accident condition. This paper presents an investigation of transient conjugate heat transfer and fluid flow phenomena and characteristics of scaled-down single C-tube submerged in water tank under atmospheric pressure at the initial period before saturation pool boiling occurs. Empirical calculation model, experimental setup and numerical model were established separately, and both numerical and experimental studies were carried out. The distributions of tube outlet temperature and water temperatures at horizontal and vertical monitoring points in tank were obtained. Detailed comparisons of the results show good agreements. Also, the change tendencies of heat flux, heat transfer coefficients and heat load were analyzed. The fluctuation of heat flux along normalized length from tube inlet firstly increases but finally tends to keep stable with the change of temperature difference and transformation of heat transfer mechanism. The heat transfer coefficient inside tube shows a slight downtrend along normalized length from tube inlet, but the heat transfer coefficient outside tube shows obvious change in the transient process. The total heat load continuously decreases with time. Furthermore, the volume fraction of vapor is relatively low during the subcooled boiling period. The two-phase flow enhances the agitation in water tank and the fluid velocity near tube outside wall is much larger. [ABSTRACT FROM AUTHOR]

While studies concerning heat transfer due to oscillating air (and few other gases) flow in metal foam are available, heat transfer due to oscillating water flow in metal foam has not been offered in the literature. This paper presents characteristics of heat transfer of oscillating water flow in commercial open-cell metal-foam pipe that were obtained experimentally, most likely for the first time. One main difference between gas and liquid flows in porous media is that dispersion is far more significant in the latter. Another difference is the length of the entrance region, which depends strongly on the Prandtl number. The trends in the cycle-averaged wall temperature, length-averaged wall temperature and cycle-averaged Nusselt number were similar to those for oscillating water flow in packed spheres and for oscillating air flow in aluminum, copper and graphite foams in a rectangular channel. For the higher flow frequency of the current study, the cycle-averaged Nusselt number was higher for higher flow displacement amplitude. [ABSTRACT FROM AUTHOR]

This paper presents a mathematical model and corresponding numerical results for a power-law fluid flowing in a channel partially filled with a homogeneous and isotropic porous medium. At the interface between the clear fluid and the porous material, a model for the stress jump condition takes into consideration the behavior of a power-law fluid. This study shows that the use of a modified permeability, K * , satisfactorily describes the friction factor of the flow for Re η ∗ ≤ 1 (Darcy regime). The mathematical modeling presented, supported by comparisons with analytical and numerical results, also shows that the form drag must be taken into account in the momentum equation, even for a power-law fluid. The mathematical modeling presented has been used to simulate Newtonian as well as power-law fluids flowing in both porous and unobstructed media. For a channel partially filled with porous material and under a fixed mass flow rate, results indicated that the pressure drop is a function of porosity, Darcy number, shear jump coefficient, β , and flow behavior index, n . [ABSTRACT FROM AUTHOR]

Micro-channel two-phase critical flow is encountered in the chemical industry and power engineering, such as critical flow leakage/release and two-phase micro-channel heat sinks. Realistic modelling of micro-channel two-phase critical flow deserves enough attention. The paper aims to review the existing two-phase critical flow models and to evaluate the application to simulate the micro-channel critical flow. Choking and critical flow criteria of the single-phase flow and two-phase flow are discussed, and the pressure gradient near the critical cross-section is compared. The development as well as the key characteristic of the critical flow model is drawn on. A benchmark of critical flow models against the experimental data for the micro-channel critical flow, especially for the pressure profile, has been carried out to check the ability of the models to accurately reproduce the micro-channel critical flow, as well to reasonably describe the fluid flow and interfacial transfer. The constitutive relations of existing separated flow models are further compared and evaluated depending on the analysis of the fluid flow and interfacial transfer for two-phase flow to assess constitutive relations. Additionally, the bubble nucleation and metastable liquid property are included in separated flow models to explore the boiling delay in the micro-channel critical flow. • A review and benchmark study of two-phase critical flow models are conducted. • Choking and critical flow criteria in a micro-channel are discussed. • Constitutive relations of the separated flow model are compared and evaluated. • The boiling delay in micro-channel critical flow is explored. [ABSTRACT FROM AUTHOR]

In this paper we investigate the transient MHD laminar free convection flow of nano-fluid past a vertical surface. The vertical surface is considered porous and stretched under acceleration. Four different types of water based nano-fluid are considered in this investigation where copper (Cu), copper oxide (CuO), aluminum oxide (Al 2 O 3 ), and titanium dioxide (TiO 2 ) are the nano-particles. The boundary-layer forms of the governing partial differential equations (momentum and energy equations) are transformed into highly nonlinear coupled ordinary differential equations (ODEs) using similarity technique. The ordinary differential equations are solved numerically using a fourth order Runge-Kutta method based shooting technique. For some special cases, an excellent agreement is observed between the current results and the results available in the existing literature. The effects of different parameters: the nanoparticle volume fraction ( φ ), unsteadiness parameter ( A ), magnetic parameter ( M ), buoyancy parameter ( λ ), suction parameter ( f w ) and different types of nanoparticles on the fluid velocity component ( f ′ ( η ) ) , temperature distribution ( θ ( η )), the skin friction coefficient ( C f R e x 1 / 2 ) , and the local Nusselt number ( N u x R e x − 1 / 2 ) are presented graphically and discussed in details. The results illustrate that selecting Al 2 O 3 and Cu as the nanoparticle leads to the minimum and maximum amounts of skin friction coefficient absolute value, and also Cu and TiO 2 nanoparticles have the largest and lowest local Nusselt number. [ABSTRACT FROM AUTHOR]

If a gel is formed when the flow of waxy crude oil within a pipeline is stopped for a period of time, the required restart pressure may be high and the line is costly to operate. The conventional method of predicting the restart pressure for gelled crude oil assumes a constant yield stress across and along the pipe section; this often leads to excessive over sizing of pump and piping system. Many researches highlighted that the presence of gas voids upon cooling of waxy crude may have an impact on the yield stress of gelled crude. This paper describes the use of Magnetic Resonance Imaging (MRI) to investigate the formation behaviour of gas voids within a gelled crude oil sample from a field in the South China Sea. Scanning of gelled crude at selected temperatures was performed following cooling in the circular pipe section within an experimental flow loop rig. Gas voids within the range of 7–12% were observed in the gelled samples resulted from different cooling temperatures and cooling rates. The cooling temperature and cooling rates were observed to influence significantly the location and volume of gas voids formed within the gelled samples. Higher cooling rates resulted in higher volume of gas voids close to pipe wall while lower cooling rates resulted in more gas voids located around core of the pipe. [ABSTRACT FROM AUTHOR]

Numerical investigation of heat transfer and fluid flow in a single pass counter flow chevron corrugated-plates plate heat exchanger considering nanofluids (CeO2and Al2O3) as homogeneous mixtures has been presented in this paper using the Commercial CFD software, ANSYS FLUENT. The required thermophysical properties of the nanofluid were measured and used in the CFD model through UDF (User Defined Function) commercial CFD software ANSYS/FLUENT. Individual optimum concentration of CeO2/water and Al2O3/water nanofluids yield maximum heat transfer improvement has experimentally determined and then CFD simulation has been done with those concentrations to obtain the temperature, pressure, and velocity fields. The results of numerical simulation were compared with experimental data in order to verify the accuracy of the homogeneous model. Validation of the CFD model suggests that considering nanofluid a homogeneous mixture, simulation can be performed to predict the plate heat exchanger performance with reasonable accuracy. CFD simulation shows that corrugation pattern of the plate develops turbulence and vortices of fluid which results in high heat transfer rates. [ABSTRACT FROM AUTHOR]

Fireside metal temperature is quite important in the safety evaluation of boiler water-cooled wall. While little literature reported the accurate temperature calculation model for spiral water-cooled wall. This paper proposes a coordinate transformation method for heat reallocation in the spiral water-cooled wall temperature calculation. A computational fluid dynamics (CFD) model based on a 600 MW tangentially coal-fired boiler is used to provide the original heat flux distributions under various boiler loads. The reallocated heat distribution directly maps with the flow path of the spiral water-cooled wall. The combination of the heat reallocation model and thermal-hydraulic model is realized in MATLAB platform. The calculated temperature distributions at the outlet of the spiral water-cooled wall agree well with the in-situ data, and the maximum relative errors under 100% BMCR load and 75% THA load are 2.7% and 3.2%, respectively. The numerical results show that the working fluid flow rates of the divided loops are almost equal and the maximum metal temperatures of the spiral water-cooled wall are 732.1 K, 710.4 K, 760.9 K and 792.9 K under 100% BMCR, 75% THA, 50% THA and 35% BMCR loads, respectively. The local overheating is likely to occur in low boiler load. This model is intended to improve the metal temperature calculation method of the spiral water-cooled wall, which could benefit the safety monitoring of the boiler under variable loads. • Heat reallocation for spiral water wall was conducted by coordinate transformation. • The reallocated heat distribution mapped with the flow path of the spiral water wall. • Coupled modeling of combustion and hydrodynamics was used in temperature calculation. • The working fluid flow rates of the spiral water wall loops were almost equal. • The local overheating of spiral water wall was likely to occur in low boiler load. [ABSTRACT FROM AUTHOR]

As an effective technology to reduce flow resistance, drilling holes on the surface of vortex generators had been studied extensively. In this paper, thermal performance of punched rectangular winglet vortex generators (PRWVGs) in turbulent regime (Reynolds number was in the range of 9,090 ∼ 21,210) was studied experimentally and numerically. Air was applied as the working fluid. The PRWVGs were inserted into the tube with three attack angles (α = 30 ° , 45 ° , 60 °). The punched holes were set in three different positions (g = 3 m m , 5 m m , 7 m m) in the vertical direction and at three different hole heights (d = 1 m m , 3 m m , 6 m m). The performance of PRWVGs was quantified by the heat transfer coefficient ratio (h / h 0), the friction factor ratio (f / f 0), and a combined thermal enhancement factor (T E F) of the two. The results revealed that h / h 0 as well as f / f 0 values of PRWVGs compared with those of planar VGs (without holes) were reduced. The flow and temperature fields of different cases were obtained by numerical simulations, and the experimental results were verified by analyzing the fluid flow behaviors. The results showed that the jet flow (defined as a jet emanating from the punched hole of a VG), would enhance the heat transfer efficiency in the recirculation area. And the T E F value of VGs was improved by punching holes on the surface of VGs. However, the shape of the jet flow depended on the position of the hole. When the position of the hole was lower in the vertical direction, the jet flow was effective in increasing its T E F value. Additionally, the size of the hole had a large effect on the T E F value. Besides, entropy generation analysis showed that PRWVGs were thermodynamically advantageous. When d = 1 m m , w = 5 m m , the Bejan number reached the minimum. Under this condition, the proposed PRWVGs performed best in practice. When the height of the hole was d = 1 m m , T E F was the maximum, which was 1.25 at α = 45 ° and R e = 9,090. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the hydraulic and thermal fields of a 29 nm CuO nanoparticle–water nanofluid with various volume fractions, 0.24%, 1.03% and 4.5% flowing inside a rectangular microchannel heat sink under both laminar and turbulent conditions. The isothermal and heated tests are conducted for Reynolds number up to ≈5000 and to ≈2500, respectively. For a given fluid flow rate experimental results show an increase of the pressure drop and the friction factor with respect to water. This increase can be as high as 70%, 25%, and 0–30%, respectively, for the 4.5%, 1.03%, and 0.24% particle volume fractions. Although the laminar-to-turbulent transition was observed at nearly the same critical Reynolds number Re c ≈ 1000 for water and the tested nanofluids, this value of Re c is clearly lower than that corresponding to a smooth surface microchannel. Results show a slight heat transfer enhancement with respect to water for nanofluids with low particle volume fractions, 0.24% and 1.03%, while for the 4.5% fraction a clear decrease of heat transfer was found. In general, the nanofluid overall energetic performance, defined by the heat transferred/pumping power ratio, remains lower than that of water for a given Reynolds number. This ratio decreases with an augmentation of the particle volume fraction. [ABSTRACT FROM AUTHOR]

Abstract: Propagation of uncertainty through the physical model has been investigated in the present paper by solving two specific simple stochastic problems using the Non-Intrusive Spectral Projection method. The uncertain parameters are described by either a Gaussian or a LogNormal probability distribution function. For each of the problems, the stochastic and the deterministic mean solutions have been compared and the respective confidence intervals have been obtained. For the deterministic problems, the confidence intervals have been estimated using both one-dimensional and multi-dimensional bound methods. From the results it has been observed that the differences between the stochastic and the deterministic mean solutions are apparent only when large uncertainties are introduced in the random variables. For both the specific problems, considered in the present study, the confidence intervals for the stochastic problems have been exactly predicted by the deterministic limits when uncertainty is introduced only in one of the parameters. For more than one uncertain parameters, the multi-dimensional bound method produces better agreement with the stochastic confidence intervals than the one-dimensional bound method. The findings are expected to be applicable to problems in heat and mass transfer with similar characteristics or input–output relations. [Copyright &y& Elsevier]

Abstract: In the present paper, a numerical model of spot pulsed current GTA welding for partially and fully penetrated weld pools is presented. Heat transfer and fluid flow in the weld pool driven by the combination of electromagnetic force, buoyancy force, surface tension gradient and latent heat are included in our model. A new formulation of the electromagnetic problem is introduced to take into account eddy current in the weld pool. The shape of the free deformable surface under the action of pulsed arc force is also handled after the magneto-hydrodynamic calculation. The numerical model was applied to 304 stainless steel welding. We compare the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. Experimental study is conducted to compare our numerical prediction with welding macrographies. It shows a good agreement of the model. [Copyright &y& Elsevier]

Abstract: Fluid flow and heat transfer characteristics in a channel with staggered porous blocks were numerically studied in this paper. The Navier–Stokes and Brinkman–Forchheimer equations were used to model the fluid flow in the open and porous regions, respectively. Coupling of the pressure and velocity fields was resolved using the SIMPLER algorithm. The local thermal equilibrium model was adopted in the energy equation to evaluate the solid and fluid temperatures. The effect of Darcy number, Reynolds number, porous block height and width on the velocity field were studied. In addition, the effects of the above parameters as well as the thermal conductivity ratio between the porous blocks and the fluid on the local heat transfer were analyzed. The pressure drops across the channel for different cases were discussed. The results show that the flow behavior and its associated local heat transfer are sensitive to the variation of the above parameters. It is predicted by the present study that an increase in the thermal conductivity ratio between the porous blocks and the fluid results in significant enhancement of heat transfer at the locations of the porous blocks. [Copyright &y& Elsevier]

This paper focuses on evaluating the effects of flow rate and temperature changes of hot and cold fluid flows in multistream multi-fluid plate-fin heat exchangers with a single-component two-phase flow in the middle heat exchanger. This three-fluid heat exchanger included three layers with offset strip and wavy fin configurations. In the adjacent heat exchanger with offset strip fins, hot and cold fluids were flowing, while the middle heat exchanger with wavy fins contained a single-component two-phase flow (condensed vapor). The Nusselt number, friction factor, the thermal performance factor, and the heat transfer surface contribution of the middle heat exchanger with each of the adjacent heat exchangers, were obtained using the measured experimental data. The results indicate that the two-phase Reynolds number was between 500 and 1800, and the vapor quality varied in the range of 0.17–0.56. Also, the heat transfer surface contribution of the adjacent cold and hot fluids was between 0.18 and 0.5. The two-phase flow in the wavy channel has a wavy and stratified pattern. The examinations showed that raising the Reynolds number of hot and cold fluids, respectively, led to maximum 13.8 % and 9.5 % of reduction in the vapor quality, 8.8 % and 9 % of increase in the two-phase Nusselt number, and 18 % and 12 % of reduction in the average value of friction factor. The domain change in the two-phase vapor quality, the two-phase fluid heat transfer surface contribution, and the flow pattern determination parameters including the dimensionless mass parameter and the Lockhart-Martinelli parameter was 33.67 %, 80.81 %, 32.72 %, and 19.09 %, respectively. As the Reynolds number of the cold fluid was changed, the variation domain of these parameters was higher compared to when the Reynolds number of the hot fluid was altered. Increasing the dimensionless temperature of the hot and cold raised the vapor quality by 12–16 %. The experimental results revealed that by decreasing the temperature of adjacent fluids and increasing their Reynolds number, the thermal performance factor of the two-phase flow was increased. • The effects of Re and temperature changes were studied in a multi-flow plate-fin heat exchanger with a two-phase flow. • The three-fluid heat exchanger included three heat exchangers with offset strip and wavy fin configurations. • The two-phase flow in the wavy channel had a wavy and stratified pattern. • Increasing the Reynolds number of the cold and hot fluid flows increased two-phase Nusselt number. • To achieve a better thermal performance, the adjacent flows should have lower temperatures and higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

Indoor air stability is a general term that refers to the indoor air's tendency to encourage or discourage its initial inertia motion. In stable conditions where vertical temperature gradients are positive, would prevent the vertical motion of fluid flows, thus the initial inertia motions are maintained, whereas in unstable conditions where vertical temperature gradients are negative, fluids experience intensive convection, thus the initial inertia motions are more easily to get disturbed. This study experimentally and numerically investigated the dispersion of exhaled contaminants and body thermal plume in a displacement-ventilated room integrated with unstable and stable conditions. Two real-human subjects participated in the full-scale experiments. The effects of participants' relative position with reference to the ventilation vents and of ventilation rates on the flow field were examined. Results show that the thermal stratification associated with the displacement ventilation system was disturbed by unstable conditions, resulting in a uniformly distributed contaminants and thermal fields, therefore a reduced exposure level; whereas the thermal stratification was enhanced by stable conditions so that the transport of contaminants and thermal more inclined to follow their initially released direction and led to a relatively high exposure level in the breathing microenvironment. Air distribution in unstable conditions, when compared with stable conditions, was more sensitive to the relative position between the participants and the vents. Increasing the ventilation rate could greatly reduce the contaminant level of unstable conditions but may not achieve as good results in stable conditions. This paper has practical significance in controlling the transmission of exhaled contaminants and preventing the spread of infectious diseases. [Display omitted] • Stable air with positive temperature gradients exerts directional effect on fluid. • Unstable air with negative temperature gradients develops turbulent instability. • Thermal stratification gets enhanced in stable air but disturbed in unstable air. • The relative position of people with vents matters more in unstable air. • Increasing ventilation rates is not an optimal way to reduce cross-infection. [ABSTRACT FROM AUTHOR]

Literature shows that there is no comprehensive study available on convective heat transfer in a scroll compressor. In this paper, a two-dimensional model has been developed to study the fluid flow and heat transfer in the working chamber of the scroll compressor. The unsteady continuity, momentum and energy equations for the gas flow in the scroll chamber were formulated. The curvilinear, moving and deforming scroll chamber was modeled employing the body-fitted coordinate grid by applying a “two-boundary” algebraic method. The computation accounted for the effect of simultaneous change in the instantaneous volume (volumetric size) and the moving boundary (geometrical shape) of the scroll chamber. With the aid of an available CFD code, the convective heat transfer has been calculated using a standard k–#x03B5; turbulence model with a log-law wall function. Results show that available empirical correlations with lumped parameter approach are inadequate in predicting heat transfer within the scroll compressor chamber. This is due to higher heat transfer as a result of the re-circulating flow, in particular, during the later period of the compression process. Results showed that gas properties in the chamber, except the pressure, are highly spatially distributed. [Copyright &y& Elsevier]

Data from core analyses, such as residual oil saturation and relative permeabilities, are of great importance for proper exploitation of the petroleum resources. Such quantities are typically determined through interpretation of data acquired during some flooding experiment. In such determinations, the absolute permeabilities are typically represented by a single average value, i.e., the core is assumed homogeneous and isotropic. Recent studies, however, show that the validity of such assumptions can be questioned. When using such assumptions analyzing flooding data, the derived relative permeabilities will depend on the actual core sample heterogeneity, i.e., the variation and distribution of the absolute permeability in the core. A better option would therefore be to determine the absolute and relative permeabilities simultaneously from the data, thereby accounting for heterogeneity effects. In this paper we describe and test a method for such determinations, and discuss some results. [Copyright &y& Elsevier]

This paper reports the results of our studies on the performance of thin film evaporation, mostly used in the evaporator of an ultra-thin vapor chamber using micropillar arrays. The dry-out heat flux and evaporator surface temperature have been used as the performance metrics. A numerical model has been formulated to solve for the fluid flow and phase change heat transfer considering the capillary pumping action (wicking action) using a cell-by-cell forward approach and validated with results reported in the literature. The solutions of the model equation are then utilized to study the effects of geometry and arrangement of the micropillars within the vapor chamber and to gain further insights into the interplay of the various forces. It is seen that these variations strongly influence the pumping action as well as the shape of the liquid vapor interface thereby impacting the pressure field, flow rate and the dry-out heat flux. A conical shape of the micropillar (having the same volume as that of other shapes) has a significant impact on the dry-out heat flux e.g., reducing the top diameter from 10 to 8 μm results in an 18% reduction. For a rectangular arrangement of the micropillars, the performance is quite sensitive to the transverse and axial spacing. In the range of 30–40 μm of axial and transverse spacing, the dry out heat flux can be altered by as much as 20%. Higher transverse pitch with lower axial pitch is found to be a better combination for a given overall geometry and number of pillars. • Development of experimentally validated numerical model to predict the performance of thin-film evaporation in thin vapor chambers using micropillar arrays. • Sensitivity of dry-out heat flux and temperature variation for cylindrical micropillars in a rectangular array. • Effect of pillar geometry, axial and transverse spacing studied and found to have significant impact on performance. • Conical profile of the micropillar of same volume affects the dryout heat flux. • For rectangular arrangement of the micropillars , the performance is very sensitive to the transverse and axial pitch and was found to vary. As much as 20% in the range of 30-40 μm. [ABSTRACT FROM AUTHOR]

Natural convection around a single horizontal cylinder has been extensively studied for heat transfer characteristics, but when coupled with fluid flow, the laminar-to-turbulent transition in buoyant plume poses severe challenge on modelling fidelity and physical interpretability. To address this challenge, this paper conducts detailed verification, validation and analysis of high-fidelity large eddy simulation (LES) for an unconfined horizontal cylinder in water with a Rayleigh number of 8 × 107, complemented by Reynolds-Averaged Navier-Stokes (RANS) computations. To the best of the authors' knowledge, this is the first LES study with acceptable accuracy as validate by experimental data for buoyant plume above a single horizontal cylinder. It is found that LES is far more sensitive to mesh resolution and boundary condition setting than RANS. An alarm is set for the use of periodic condition in LES on lateral boundaries of computational domain due to its inferior accuracy than RANS with transition SST model. The finely-tuned LES with pressure condition on lateral boundaries shows satisfactory agreement with experimental data in terms of heat transfer on cylinder surface and buoyant plume velocity, based on which new physical insight on transitional behavior of thermal plume is obtained. It is found that after leaving the cylinder, buoyant plume is laminar and accelerates, subject to work input from buoyancy force, while its temperature keep decreasing due to heat loss to atmosphere. Flow instability appears first in upward velocity at a streamwise Grashof number of 1.5 × 108, where transition to turbulence onsets. Thermal plume continues to accelerate until it begins to sway horizontally, where energy dissipation into turbulence becomes the major loss mechanism of mean flow energy. Thus, cross-stream diffusivity is augmented notably due to turbulent stresses, leading to smoothing of transverse velocity distribution and reduction of transversely-averaged mean kinetic energy. Transversely-averaged turbulent kinetic energy keeps increasing in transitional regime until the streamwise Grashof number reaches 7 × 109 and then declines to approach an asymptotic value, signifying the end of transition. Overall speaking, buoyancy work dominates the change in mean flow energy, while mean shear outweighs buoyancy in producing turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

In this paper, the efficiency regarding using helical-coiled tubes having corrugated wall is numerically investigated. The fluids of interest flow in laminar regime and they are in the Reynolds number of 2000–8000 and Dean number ranging from 496 to 2806. After validating a plain coil-tube with available data in the literature, effects concerning the Reynolds number, tube diameter, corrugated height and pitch on the heat transfer and pressure drop are studied. Results shows that Nusselt number increases with Reynolds number while relative Nusselt number decreases with it. Higher corrugated height and lower corrugated pitch enhances the heat transfer with further friction factor. The thermal efficiency of these cases of interest are all greater than unity and it increases by twice with decreasing tube diameter by half. It rises by increasing corrugated height, decreasing tube diameter and lowering corrugated pitch. Moreover, a Nusselt number correlation is developed which is based on numerical results, being a function of Dean number, corrugated height and pitch where it is stronger function of corrugated height compared to corrugated pitch. • Fluid flow and thermal characteristic of water inside coiled tube was studied. • The effect of tube diameter, corrugation height and pitch was explored. • Corrugated wall augments significantly the heat transfers and thermal performance. • Corrugation height is more effective compered to corrugation pitch. • Maximum thermal performance of 3 was obtained by implemented geometry. [ABSTRACT FROM AUTHOR]

This paper deals with thermosolutal convection in an inclined enclosure inserted with heat-generating porous blocks under the influence of magnetic field. The general Brinkman-extended Darcy model is adopted to formulate the fluid flow in the enclosure. An extensive series of numerical simulations is investigated in the range of parameters 0 ≤ Ha ≤ 150, 103 ≤ Ra ≤ 107, −10.0 ≤ N ≤ 10.0, 10−9 ≤ Da ≤ 10−1, 0.1 ≤ Kr ≤ 10 and −90° ≤ Φ ≤ 90°. Streamline, isotherms, isoconcentrations and masslines are produced to illustrate the fluid, heat and moisture flow structures. It is founded that overall Nusselt number is an increasing function of Ra , N , Da and Kr in the vertical enclosure, while decreasing with Ha. The permeability and thermal conductivity of porous blocks have no significant effect on moisture transfer rate. The Nusselt and Sherwood curves for different inclination angle are presented to be parabolic, and the maximum are near at Ф = −10° and 30°, respectively. In addition, correlations of the overall Nusselt and Sherwood numbers depending on thermal Rayleigh number, buoyancy ratio, Hartmann number and inclination angle have been obtained, which are beneficial to determine heat and moisture transfer rates in electrical devices. • Heat and moisture transfer processes occurred in cooling of multiple electronic devices • Disconnected porous blocks with internal energy generation are inserted in the enclosure. • Overall Nusselt number increases with Ra , N , Da and Kr , while decreases with Ha. • Permeability and thermal conductivity of porous blocks hardly influence the moisture transfer rate. • Correlations of the overall Nusselt and Sherwood numbers have been obtained. [ABSTRACT FROM AUTHOR]

The main objective of the study was to present the Chilton-Colburn analogy in relation to mass and heat transfer processes occurring in minichannels. Because of the so-called scaling effects being in channels of small geometry the validation of the analogy between transfer processes in the case of convective fluid flow through the mninichannels seems to be necessary. The paper provides an overview of the experimental results of mass and heat transfer measurements performed under analogical conditions. The investigations concerned transfer processes in minichannels of different geometry. Channels with circular and rectangular cross section of small hydraulic diameter have been discussed. The ratio of minichannel length to the hydraulic diameter has been taken into account in the considerations. The analysis has been provided on the basis of the available literature results and the own author's investigations on mass transfer in short circular minichannels and preliminary results of mass transfer in long square channels of the mini heat exchanger. In order to validate the Chilton-Colburn analogy between mass and heat transfer the cases of transfer processes in minichannels which best meet the conditions of geometrical similarity have been discussed. The considered cases of heat transfer were experimentally studied with the use of thermal balance method while the mass transfer processes were investigated using the limiting current technique. On the basis of the analysis performed no definite answer was found on confirm Chilton-Colburn analogy applied in minichannels. Both convergence and divergence of results were noted. The discrepancy between thermal results and the results received from Chilton-Colburn analogy may be due to various reasons. One of them is a difference in the geometry of tested channels. Further research in conditions of full similarity of the channels geometry is expected. Another problem concerns thermophysical properties of the fluids used in investigations. The application of gases and liquids in thermal and mass experiments respectively, may cause a deviation from the mass/heat transfer analogy in minichannels. Measurement uncertainty also plays an important role in the discussion on the analogy between mass and heat transfer processes during the fluid flow through the channels of small characteristic dimensions. The total validation of mass/heat transfer analogy in minichannels in the form of Chilton-Colburn formula requires further experimental research. • Limiting current technique is an useful method for mass/heat transfer measurements. • Correlations on mass transfer can be transferred to thermal analogical problems. • Mass and heat experiment results in minichannels differ from those for conventional channels. • Mass/heat transfer analogy in minichannels requires further study in full similarity conditions. [ABSTRACT FROM AUTHOR]

This paper explores the performance effects of a novel small-scale multiple vortex generators (MVG) on heat transfer deterioration (HTD) mitigation and pressure drop using supercritical CO2 (sCO2) as the working fluid. The HTD mitigation and pressure drop are examined using an overall performance factor, R = ΔHTD/(Δp/Δp 0). Numerical calculations are first conducted to investigate the effects of single conventional vortex generator's (VG) configurations (rectangular, triangular and trapezoidal). The results indicate that under the same aspect ratio, rectangular VG has the largest area facing the fluid flow and the largest angle of attack which induces the strongest longitudinal vortices leading to its highest value of R. Then, the morphological properties (density and height) of five pairs of evenly distributed rectangular VGs in an MVG are varied while their uniformity is maintained. With the increase in MVG's height, the channel's heat transfer coefficient (HTC) profiles increase significantly at the locations where MVG is installed while the opposite trend is found for R. The density and nonuniformity of the MVG have the highest performance effects on HTD mitigation, with R of more than 18% and 11% for the density of 2.5 and non-uniform MVG respectively. Analysis reveals that as the density of MVG increases, the fluid recirculation zone behind each VG diminishes correspondingly, causing a greater vortex intensity mixing by swirling flows which enhances the downstream production of turbulence kinetic energy (TKE), thus weakening the buoyancy forces and leading to HTD mitigations. In addition, by further increasing the MVG density beyond 2.5, it was found that the HTC decreases while the pressure drop increases, with the wall temperature peaks stabilizing at 178 °C, which demonstrates the breakdown of Reynold's analogy. Increasing the number of VGs in an MVG increases its delay and broadening effects on the wall temperature peaks by further delaying the boundary layer recovery caused by interaction of longitudinal vortices generated by each VG, but has relatively little effects on R. Also, analysis further reveals the presence of buoyancy-induced flow oscillations near the channel walls, which are attenuated by the VG and MVG effects. Overall, MVG offers superior performance than convectional single VGs in terms of HTD mitigation, and the results presented here can be employed as a reference guide in the design of highly efficient, safe and reliable supercritical heat exchanger systems. • A novel small scale MVG offers superior mitigation effects on HTD to the conventional single VGs. • The density and nonuniformity of the MVG have the highest performance values, R among all cases studied. • Increasing the height of MVG increases the channel's HTC profiles at the MVG locations but has a decreasing effect on R. • The number of VGs in an MVG causes wall temperature peaks delay and broadening but has relatively little effects on R. [ABSTRACT FROM AUTHOR]

The present paper numerically investigates the laminar, two-dimensional and horizontal flow around a horizontal isothermal rotating cylinder under the buoyancy effect. To study the buoyancy effect, a range of Grashof numbers from 0 to 12 × 104 is used, and to check the effect of the cylinder rotation, the ratio of the rotational speed to the free-stream velocity between −4.5 and 4.5 is applied. The simulation is carried out at the Reynolds number of 200 and the Prandtl number of 0.7. To simulate the fluid flow, the Navier-Stokes equations are numerically solved using a finite-volume scheme. Results show that the interaction of the main stream with the rotation and buoyancy can alter the flow field and heat transfer characteristics such as flow pattern, surface pressure distribution, lift and drag coefficients, and Nusselt number. The clockwise (cw) rotation decreases the amplitude of oscillations in the flow compared to the counterclockwise (ccw) rotation. In ccw rotation, as the Grashof number increases, the mean lift coefficient increases; while in cw rotation, increase in the Grashof number reduces the mean lift coefficient. Moreover, in terms of Grashof numbers ranging between 8 × 104 and 12 × 104, the Nusselt number is almost unchanged. [ABSTRACT FROM AUTHOR]

Fouling is one of the customarily happened, fundamental and expensive problems in heat transfer systems. It causes the reduction in heat transfer performance of the heat exchangers, energy loss and also, damage to the apparatus. Effect of fluid flow rate on the salt deposition was complicated in the previous researches. In this paper, a comprehensive rigorous investigation was performed to make clear the effect of fluid flow rate of calcium sulfate aqueous solution on the heat transfer coefficient and fouling resistance in subcooled flow boiling condition. Large numbers of experiments performed at different flow rates (2.5–11.5 l/min), solution concentrations (1.75–2.20 g/l), fluid bulk temperatures (55–75 °C), and heat fluxes (8–95 kW/m2). Chen model, Gungor and Winterton model, and Kandlikar model approved pure water experimental data. It was found that increasing the heat flux increased the fouling rate and consequently, sharply decreased the heat transfer coefficient. It was also found that increase in the flow rate may cause both increase or decrease of the fouling layer, which depends on the applied heat flux. At low heat fluxes, deposition is mass transfer controlled, and increasing Re enhances the fouling rate while at high heat fluxes the deposition is chemical reaction controlled, and incresing Re decreases the fouling rate. [ABSTRACT FROM AUTHOR]