An autonomous system for thermal convection of viscoelastic fluids in a porous layer using a thermal nonequilibrium model

Thermal convection of viscoelastic fluids saturating a horizontal porous layer heated from below is analyzed using a thermal nonequilibrium model to take account of the interphase heat transfer between the fluid and the solid. The viscoelastic character of the flow is considered by a modified Darcy’s law. An autonomous system with five differential equations is deduced by applying the truncated Galerkin expansion to the momentum and heat transfer equations. The effects of interphase heat transfer H on the thermal convection of viscoelastic fluids in a porous medium are analyzed and discussed. The results show that the weak interphase heat transfer tends to stabilize the steady convection.     

Unsteady thermal flow analysis in a heat regenerator with spherical particles

Heat regenerator occupied by regenerative materials improves thermal efficiency of regenerative combustion system through the recovery of sensible heat of exhaust gases. By using one-dimensional two-phase fluid dynamics model, the unsteady thermal flow of regenerator with spherical particles, were numerically analysed to evaluate the heat transfer and pressure drop and to suggest the parameter for designing heat regenerator. It takes about 7 h for the steady state in the thermal flow of regenerator, where heat absorption of regenerative particle is concurrent with heat desorption. The regenerative particle experiences small temperature fluctuation below 10 K during the reversing process. The thermal flow in heat regenerator varies with inlet velocity of exhaust gas and air, configuration of regenerator and diameter of regenerative particle. As the gas velocity increases with decreasing the cross-sectional area of the regenerator, the heat transfer between gas and particle enhances and pressure losses increase. As particle diameter decreases, the air is preheated higher and the exhaust gases are cooled lower with the increase of pressure losses. At the same exhaust gases temperature at the regenerator outlet, the regenerator length need to be linearly increased with inlet Reynolds number of exhaust gases. It is confirmed that inlet Reynolds number of exhaust gases should be introduced as a regenerator design parameter. Copyright © 2002 John Wiley & Sons, Ltd.     

双层玻璃空气层厚度变化对热阻影响的实验研究

针对目前对双层玻璃空气热阻最佳厚度利用解释模糊的问题,通过建立实验模型,对空气层分别为10～70mm等12组厚度的双层玻璃内表面温度等数据进行测量,按照有关规程中的方法进行理论分析,最终得出:当空气层厚度小于5mm时,传热过程以导热和对流换热为主;大于5mm时,以辐射换热为主;0～20mm时,导热率变化非常显著,25～70mm时,导热率变化较为缓慢,且在20mm左右出现一个峰值;工程设计时可以把空气层厚度控制在20mm左右,这样有利于发挥空气层热阻的作用。     

Modeling electrification of suspended particulate matter (SPM) in a two phase laminar boundary layer flow and heat transfer over a semi-infinite flat plate

The present paper envisages the effects of electrification of particles, volume fraction and diffusion of SPM on the boundary layer flow and heat transfer over a semi-infinite flat plate. Irrespective of the particle material density, it has been observed that the range of validity of the solution remains fixed and at x = 2.12 the nature of the profiles of the flow variables changes. Electrification of particles causes the particles to move faster for x < 1.0 and slows down for x > 1.0 but particle temperature increases with increase of M. It has been observed that heat transfer always occurs from the fluid to plate.     

Effect of contact number on heat extraction of particle material for hydrogen production

Hydrogen production by steam thermal reforming with waste heat of industrial particle material is of good prospects. But the insufficient research on the heat extraction traits of particulate matter has set up obstacles to its application. This paper focused on the effect of contact number, as it has not yet been fully studied. A series of steady-state numerical models based on a face-centered cubic packing was established to simulate the effect of contact number by removing selected particles. The results show that the contact number has a marked influence on the thermal resistance. As the contact number decreases by 12, the thermal resistance increases by 13.9∼31.9%. The removal of particles causes redistribution of heat transfer in different heat transfer modes. The heat transfer from the solid layer to the next layer decreases by 57.8% at most, and the radiation heat transfer between them is strengthened (36% at most). The effect of heat redistribution due to the removal of particles is significantly weakened after the heat flows through the first particle layer without removal operation. With these results, a better interpret of particulate matter heat extraction may be established.     

Experimental methodology and heat transfer model for identification of ignition kinetics of powdered fuels

A methodology for investigating and quantifying the thermal processes leading to ignition of rapidly heated metal powders was developed. The simple experiment involves observing ignition of a powder coated on the surface of an electrically heated filament and is well suited for a variety of powdered fuels. In an experimental case study, the ignition temperature of spherical Mg powder was detected optically at different heating rates. To interpret the results, a heat transfer model was developed for a multilayer powder coating on the heated cylindrical filament. The thermal contact resistance between particles was determined from the measured bulk thermal diffusivity of the powder considering the experimental particle size distribution. An Arrhenius type expression was used to describe the exothermic chemical processes leading to ignition with the pre-exponent as an adjustable parameter. For Mg, a pre-exponent value identified by matching the calculations with the experimental data was found to be 10¹⁰ kg/m² s. The match between the experimental and predicted temperatures and times of ignition was good for different heating rates, which validated the proposed heat transfer model and indicated that the developed methodology is practically useful.     

Forced Convective Heat Transfer in a Porous Plate Channel

INTRODUCTIONHeattransferenllancen1enttechniquesplayaveryimportantroleintllermalcontroltechnologies1lsedwithnlicroelectronicchips,powerfullasermirrors,aerospacecraft,thermalnuclearfusion,etc.Itiswidelyrecognizedthattl1eheattransfercanbein-creasedbyil1creasingthesurfaceareaincontactwiththecoolant.TuckermanandPease[1,2]pointedoutthatforlaminarflowinconfinedchannels,theheattransfercoefficientisinverselyproportionaltothewidthofthechannelsincethelimitingNusseltnum-berisconsta11t.Theybuiltawate…     

Time and spatial heat transfer performance around an isothermally heated sphere placed in a uniform, downwardly directed flow (in relation to the enhancement of latent heat storage rate in a spherical capsule)

The aim of this study is to reveal the temporal and spatial heat transfer performance of an isothermally heated sphere placed in a uniform, downwardly directed flow using a micro-foil heat flow sensor (HFS). A HFS, whose response time is about 0.02 s, was pasted on the surface of a heated copper sphere. Experiments were carried out using air with a Grashof number of 3.3 × 10⁵ and with several Reynolds numbers (Re) up to 1800. Three flow patterns appeared: a chaotic flow at Re<240; a two-dimensional steady separated flow at 240 Re<500, and a three-dimensional unsteady separated flow at Re 500. In addition, the instantaneous and time-averaged heat transfer performance around the sphere in each of the three regions was clarified. Next, enhancement of the latent heat storage rate of a solid phase change material (PCM) in a spherical capsule was performed. The flow around the spherical capsule, in which the solid PCM was filled and placed in a heated, upwardly directed flow, is the approximate adverse flow phenomenon around the heated sphere which was placed in a downwardly directed flow. In other words, the buoyant flow and the forced flow are in the opposite directions in these two cases. Tests of latent heat storage were run for two Reynolds numbers which represented different flow characteristics in the heat transfer experiments, Re=150 and 1800. Furthermore, copper plates were inserted into the solid PCM, of which thermal conductivity was considerably low, to enhance the latent heat storage rate for the two Reynolds number flows.     

Augmentation of heat transfer from a solid cylinder wrapped with a porous layer

In the present study, the heat transfer from a porous wrapped solid cylinder is considered. The heated cylinder is placed horizontally and is subjected to a uniform cross-flow. The aim is to investigate the heat transfer augmentation through the inclusion of a porous wrapper. The porous layer is of foam material with high porosity and thermal conductivity. The mixed convection is studied for different values of flow parameters such as Reynolds number (based on radius of solid cylinder and stream velocity), Grashof number, permeability and thermal conductivity of the porous material. The optimal value of porous layer thickness for heat transfer augmentation and its dependence on other properties of the porous foam is obtained. The flow field is analyzed through a single domain approach in which the porous layer is considered as a pseudo-fluid and the composite region as a continuum. A pressure correction based iterative algorithm is used for computation. Our results show that a thin porous wrapper of high thermal conductivity can enhance the rate of heat transfer substantially. Periodic vortex shedding is observed from the porous shrouded solid cylinder for high values of Reynolds number. The frequency of oscillation due to vortex shedding is dampened due to the presence of the porous coating. Beyond a critical value of the porous layer thickness, the average rate of heat transfer approaches asymptotically the value corresponding to the case where the heated cylinder is embedded in an unbounded porous medium.     

Study on boiling heat transfer in liquid saturated particle bed and fluidized bed

An investigation on the effects of solid particles on boiling heat transfer enhancement is performed. The range of particle diameter is from millimeter to nanometer. The experimental results show that boiling heat transfer can be enhanced greatly by adding the solid particle into the liquid whether in fixed particle bed or in fluidized particle bed. The boiling enhancement is closely related to the particle size, the initial bed depth and the heat flux applied. The experiments show that boiling characteristics are greatly changed when a particle layer is put on the heated surface. The major effects of fixed particle bed on nucleate pool boiling heat transfer are the nucleation, bubble moving and thermal conductivity effect. A boiling heat transfer correlation is obtained to predict the boiling heat transfer coefficients in a liquid saturated porous bed. A volumetric convection mechanism of boiling heat transfer enhancement by fluidized particles is proposed. The calculated results from the model suggested in this paper agree reasonably with the experimental values.     

Development of new correlations for forced convection heat transfer during cooling of products

This paper presents the new, simple but powerful effective Nusselt–Reynolds correlations for estimating the effective convective heat transfer coefficients of spherical and cylindrical products cooled in water and air flows. In this respect, both experimental and theoretical works were obtained. In the experimental case, several spherical and cylindrical products, namely, tomatoes, pears and cucumbers were cooled in water and air flow and their centre temperature variations were measured. In the theoretical case, the effective convective heat transfer coefficients for the individual spherical and cylindrical products were determined using the centre temperature data in the present approach including Dincer's models. Therefore, the new Nusselt–Reynolds correlations were developed using the effective convective heat transfer coefficient values and a general diagram of Nu/Pr^1/3 against Reynolds number was drawn. This study indicates that the present effective Nu–Re correlations are capable of estimating the effective convective heat transfer coefficients of any spherical and cylindrical shaped products exposed to water and air cooling in practical applications in a simple and accurate manner.     

Mixed convection from an isolated spherical particle

A numerical study on mixed convection around a hot spherical particle moving vertically downwards in a still fluid medium has been made. The flow field is considered to be axisymmetric for the range of Reynolds number (based on the diameter and the settling velocity of the particle) considered. A third-order accurate upwind scheme is employed to compute the flow field and the temperature distribution. The form of the wake and the thermal field is analyzed for several values of Grashof number and the Reynolds number. The influence of buoyancy on drag and the rate of heat transfer are studied. At moderate Reynolds number, recirculating eddy develops in the downstream of the sphere. With the rise of surface temperature this eddy collapses and the fluid adjacent to the heated surface develops into a buoyant plume above the sphere. The increase in surface temperature of the sphere delays the flow separation. Our results show that the drag force and the rate of heat transfer strongly depend on Grashof number for the moderate values of Reynolds number. The conjugate heat transfer from the moving sphere is also addressed in the present paper. We have compared our computed solution with several empirical and asymptotic expressions available in the literature and found them in good agreement.     

A Thermal Nonequilibrium Approach to Natural Convection in a Square Enclosure Due to the Partially Cooled Sidewalls of the Enclosure

This article presents a numerical investigation of steady non-Darcy natural convection heat transfer in a square cavity filled with a heat-generating porous medium with partial cooling using a local thermal nonequilibrium (LTNE) model. Five different partial cooling boundary conditions and the fully cooled boundary condition are investigated under LTNE and local thermal equilibrium (LTE). The cooling portions of the left and the right sidewalls of the cavity are maintained at temperature T ₀ while the enclosure's top and bottom walls, as well as the inactive parts of its sidewalls, are kept insulated. The simulation results show that the placement order of wall cooling has a significant effect on the flow pattern and heat transfer rate. Compared with the fully cooled wall, the partially cooled wall of the cavity yielded a higher local Nusselt number for both fluid and solid phases. Under the same boundary conditions, the LTNE and LTE models can demonstrate significant differences in flow patterns and temperature fields. The total heat transfer rate increases with both Darcy number and Rayleigh number. Enhancement of interphase heat transfer coefficient (H) reduces the impact of Darcy number on the heat transfer rate of a porous cavity. Also, the total heat transfer rate of the porous medium decreases steadily with thermal conductivity ratio γ and interphase heat transfer coefficient H.     

Preliminary experimental study of a bio-inspired,phase-change particle capillary heat exchanger

In this study a new cooling concept using encapsulated phase-change particles flowing with water in a parallel-plate mini-channel is presented. This novel concept is inspired by the gas exchange process in alveolar capillaries, where red blood cells (RBCs) flow with blood plasma, yielding very high gas transfer efficiency. Another important characteristic of alveolar capillary blood flow, which is related to the high efficiency of the lungs, is the snug fitting of the RBCs into the capillaries. Hence, preliminary results of experimental tests using particles with diameter similar to the flow channel spacing flowing with water through a heated parallel-plate channel test module are presented and analyzed. The particles are octadecane paraffin (C₁₈H₃₈), a phase-change material, encapsulated in a thin melamine shell. The temperature distribution along the heated surface of the channel is measured for various water flow rates, with and without particles, and with different number of particles. Results are reported in terms of the channel heated surface average temperature and the average heat transfer coefficient, showing a sensible increase (over 20%) in the latter as compared to a clear (of particles) flow. There is strong evidence the increase in heat transfer efficiency to result from a combination of the extra mixing flow effect caused by the presence of particles in the flow and the phase-change effect caused by the EPCM inside the particles.     

Thermal and water transfer in PEMFCs: Investigating the role of the microporous layer

The objective of this work was to investigate experimentally the effects of the microporous layer (MPL) within a PEMFC. The experiments consisted in measuring, at the anode and at the cathode, the average temperature of the electrodes using small platinum wires, heat fluxes using heat flux sensors and water fluxes by means of water balance for two builds of cell; one with porous layers and MPL and another without MPL. Three thermal configurations related to the imposed temperature of the plates were studied. The measurements put forward a new role of the microporous layer on heat transfer. Indeed, the MPL implies an increase of the electrodes temperature by adding a thermal resistance. This higher temperature enables to avoid the saturation at the electrodes and improve the water removal towards the flow field plates. In addition, the effective thermal conductivity of microporous layer, a key parameter for the analysis of heat transfer in the fuel cell, was estimated in situ.     

Effects of porosity and thickness of porous sheets on heat transfer enhancement in a cross flow over heated cylinder

An experimental study is carried out to investigate the thermal impact of wrapping an aluminum porous sheet over a circular tube in a heat convection configuration. The experimental apparatus consists of a heated horizontal cylinder with a constant heat flux. The cylinder is then covered with porous sheets of different thicknesses. The tube is exposed to a cross flow of air at different speeds which corresponds to different Reynolds numbers. The effect of the added porous layer on the pressure loss over the cylinder was also investigated. It is observed that heat transfer is greatly enhanced with the addition of the porous layer. Also, the addition of the porous layer doesn’t appear to increase the pressure loss.     

Investigation of fluid flow and heat transfer in a vertical channel heated from one side by PV elements, part I - Numerical Study

The impetus of this paper is to analyse numerically the fluid flow and heat transfer characteristics of buoyancy-driven convection between two vertical parallel walls, heated from one side. Both convection and radiation heat exchanges are considered as the heat transfer mechanisms by which the thermal energy is transferred into the air. A steady-state two-dimensional model is used for the analysis. Numerical results are derived for a channel of 6.5 m in height and different widths of the channel. Various heat fluxes are considered in order to show the effect of the input heat on the heat transfer across the air layer. Detailed studies of the flow and thermal fields in the air are presented in order to explore the thermal behavior of air in the channel. Velocity and temperature profiles of the outlet air and the surface temperature of the heated and insulated wall are presented. In Part II of this paper the findings from an experimental study are reported.     

One-dimensional analysis of supersonic two-stage HVOF process

The one-dimensional calculation of the gas/particle flows of a supersonic two-stage high-velocity oxy-fuel (HVOF) thermal spray process was performed. The internal gas flow was solved by numerically integrating the equations of the quasi-one-dimensional flow including the effects of pipe friction and heat transfer. As for the supersonic jet flow, semi-empirical equations were used to obtain the gas velocity and temperature along the center line. The velocity and temperature of the particle were obtained by an one-way coupling method. The material of the spray particle selected in this study is ultra high molecular weight polyethylene (UHMWPE). The temperature distributions in the spherical UHMWPE particles of 50 and 150μm accelerated and heated by the supersonic gas flow was clarified.     

Natural convection and radiation heat transfer in a vertical porous layer with a hexagonal honeycomb core (Part 1: numerical analysis)

Combined heat transfer characteristics were obtained numerically for three-dimensional natural convection and thermal radiation in a long and wide vertical porous layer with a hexagonal honeycomb core. We assumed that the porous layer was both homogeneous and isotropic. The pure Darcy law for fluid flow and Rosseland's approximation for radiation were used. The numerical methodology was based on an algebraic coordinate transformation technique and the transformed governing equations were solved using the SIMPLE algorithm. The effect of radiation on the heat transfer characteristics was investigated over a wide range of radiation numbers and temperature ratios for two Darcy-Rayleigh number values (Ra^* = 100 and 1000) and for a fixed aspect ratio of H/L = 1. The results are presented in the form of combined radiation and convection heat transfer coefficients and are compared with the corresponding values for pure natural convection. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 278–294, 1999