Numerical and experimental study of forced convection in graphite foams of different configurations

Forced convection heat transfer in a channel with different configurations of graphite foams is experimentally and numerically studied in this paper. The physical properties of graphite foams such as the porosity, pore diameter, density, permeability and Forchheimer coefficient are determined experimentally. The local temperatures at the surface of the heat source and the pressure drops across different configurations of graphite foams are measured. In the numerical simulations, the Navier–Stokes and Brinkman–Forchheimer equations are used to model the fluid flow in the open and porous regions, respectively. The local thermal non-equilibrium model is adopted in the energy equations to evaluate the solid and fluid temperatures. Comparisons are made between the experimental and simulation results. The results showed that the solid block foam has the best heat transfer performance at the expense of high pressure drop. However, the proposed configurations can achieve relatively good enhancement of heat transfer at moderate pressure drop.     

An Investigation on Transport Properties Near the Wall in Porous Media by Fractal Models

A comprehensive investigation on the wall effects on the transport properties, permeability, thermal conductivity, and thermal dispersion conductivity is performed, based on the fractal models for these properties and the porosity variations near the wall in porous media. The results show that the fractal models for transport properties of porous media can provide good agreement with the conventional models in the region near the wall in porous media. This indicates that the fractal models for transport properties of porous media also hold in the region near the wall in porous media if the wall effects are taken into account.     

Fabrication of porous metal by selective laser melting as catalyst support for hydrogen production microreactor

To improve the hydrogen production performance of microreactors, the selective laser melting method was proposed to fabricate the porous metals as catalyst supports with different pore structures, porosities, and materials. The influence of the porous structures on the molecule distribution after passing through the porous metals was analyzed by molecular dynamics simulation. The developed porous metals were then used as catalyst supports in a methanol steam reforming microreactor for hydrogen production. Our results show that the porosity of the porous metal had significantly influence on the catalyst infiltration and the reaction process of hydrogen production. A lower degree of catalyst infiltration of the porous metal was obtained with lower porosity. A copper layer-coated stainless-steel porous metal with a staggered structure and gradient porosity of 80%–60% exhibited much larger methanol conversion and H₂ flow rate due to its better heat and mass transfer characteristic. Methanol conversion and H₂ flow rates could reach 97% and 0.62 mol/h, respectively. Finally, it was found that the experimental results were in good agreement with the simulation results.     

Modeling investigation of gradient electrolyte films deposited via convection–diffusion on porous electrode substrates

A numerical simulation based on 1-D upward deposition model has been carried out to investigate the deposition of electrolyte films with gradient microstructures via convection–diffusion process on porous electrode substrates. The simulation results concerning deposition dynamics and structural profiles of the gradient films are in a good agreement with the experimental data. The influences from the solution properties, substrate porosity and evaporation rate of solvent on the microstructural development of deposit layers have been studied by considering the deposition ability and diffusion coefficient of the solute species in porous substrate and the evaporation rate of solvent. It has been found that the concentration distribution in the porous substrate is mainly characterized by a rapid rise up to >10 times its initial value at the deposit–substrate interface and a decaying profile. The uniform deposit layer on the surface of porous substrate and the gradient layer stretching into the substrate are significantly controlled in growth dynamics by the deposition and diffusion abilities of solute for a given evaporation rate of solvent. Moreover, the solute's deposition ability appears to pose more influence on the thickening of top-deposit layer while the diffusion coefficient of solute is the main factor to control the depth development of the gradient layer inside the substrate.     

Porous Cu–Ni-YSZ cermets using CH4 fuel for SOFC

Thin and porous anodes need three important properties, high porosity, moderate catalytic ability, and no carbon deposition, when using hydrocarbon fuels for the operation of solid oxide fuel cells (SOFCs). Nickel-based (Ni/YSZ) cermets might perform serious carbon deposition using hydrocarbon fuels. Instead, many researches select Cu-based cermets to extend the application period. This study chose half replaced cermet (50%Cu–Ni in Y-doped ZrO₂), and verified the effects of H₂ formation and carbon deposition comparing to other two series, i.e. Ni-YSZ and Cu-YSZ. Three ingredients were homogeneously mixed and reduced, then sintered to the porosity of 54 ± 3%. The re-dox properties under various atmospheres and the permeability of made cermets have been investigated. The results showed that 50%Cu–Ni with 8YSZ performed moderate catalytic ability, low cracking rate for CH₄, and good electric conductivity of 1503 S cm⁻¹ at 650 °C. The sample is suitable for using CH₄ fuel for SOFCs.     

Super-high heat flux removal using sintered metal porous media

Introduction Recently there have been a demand for the technique to efficiently and steadily cool down extremely high heat flux of over 10 MW/m2, to fulfill not only the needs for plasma facing components in nuclear fusion reactors, but also the needs associated with sophisticated computers or downsizing of such devices as high-density laser equipment and power devices. However, existing cooling techniques in such high heat loading environment are basically based on high speed and highly subc…     

Modelling of Non-Darcy Flows through Porous Media Using Extended Incompressible Smoothed Particle Hydrodynamics

In this article, a novel numerical method is presented for the simulation of non-Darcy flows through porous media by the incompressible smooth particle hydrodynamics (ISPH) method with a predictor-corrector scheme. In the ISPH algorithm, a semi-implicit velocity-correction procedure is used and the pressure is obtained by solving the pressure Poisson equation. The key point for the application to non-Darcy flows is to include porosity and drag forces of the medium (the Darcy term and the Forcheimer term) in the ISPH method. Unsteady lid-driven flow, natural convection in non-Darcy porous cavities, and natural convection at a porous medium–fluid interface are examined separately by our extended ISPH method. The results are presented with flow configurations, isotherms, and average Nusselt numbers for different Darcy numbers from 10^?4 to 10^?2, porosity values from 0.4 to 0.9, and Reynolds/Rayleigh numbers. The flow pattern and rate of heat transfer inside the cavity are affected by these parameters. The results demonstrate the important effect of the Darcy number on both the heat transfer rate and the flow regime. The results from this investigation are well validated and compare favorably with previously published results.     

The effect of porous insert position on the enhanced heat transfer in partially filled channels

In the present work, the effect of porous insert position on enhanced heat transfer in a parallel-plate channel partially filled with a fluid-saturated porous medium was studied. The fully-developed laminar flow and convective heat transfer in the channel were simulated using Lattice Boltzmann Method (LBM). The walls of the channel were subject to a uniform constant temperature. The flow field and thermal performance of the channel were investigated and compared for two configurations: first the porous insert was attached to the channel walls, and second the same amount of the porous material was positioned in the channel core. Comparing the results of the present study to the analytical solutions, a reasonable agreement was observed. The effects of various parameters like Darcy number, porous medium thickness, etc. on the conduit thermal performance were investigated in both channel configurations. It was found that the position of the porous insert has significant influence on the thermal performance of the channel.     

Turbulent heat transfer past a sudden expansion with a porous insert using a nonlinear model

This work presents numerical investigations for turbulent flow and heat transfer in a backward-facing step with and without porous inserts. Two classes of the model were employed, namely linear and nonlinear turbulence closures. The entire set of transport equations was discretized by means of the control volume method and the system of algebraic equations obtained was relaxed using the SIMPLE (Semi Implicit Pressure-Linked Equations) method. Results were first validated against the experimental data and the simulations follow experimental values and trends. Computations further indicated that when using the porous insert, the size, shape, and length of the recirculating region were drastically reduced in addition to being pushed toward the channel exit, leading eventually to a complete bubble suppression for thicker inserts. A more permeable medium gave better results in quickly suppressing the circulatory motions. By including porous inserts in the channel, turbulence generated due to the shear inside the recirculating region was damped, whereas high levels of k were concentrated within the permeable structure. Large variations for the skin friction factor along the bottom wall were also smoothed out by placing inserts, spanning from a typical distribution for an unobstructed back-step flow to a standard parallel channel flow distribution as the inserts got ticker. On the other hand, at the upper wall, flow pushed toward the top surface gave rise to a sudden increase of the skin friction factor, which was later stabilized downstream the flow. Heat transfer analysis followed showing damping for Nu at the bottom wall as the thickness of the porous substrate was increased. Overall, the thickness of the insert played a dominant role in changing the final flow and heat transfer characteristics rather than the porosity or permeability of the porous material. Finally, this work indicated that the sudden increase of Nu around the reattachment point, known to be undesirable in many practical situations for causing additional thermomechanical loads on the surface, may by avoided by the use of a porous obstacle past the back-step.     

Periodic homogenization for heat,air, and moisture transfer of porous building materials

ABSTRACT

In this work, a macroscopic model of hygrothermal transfers in porous building materials was developed, using periodic homogenization, where the air infiltration was added to the classical mass and energy balance equations written at the microscopic scale. The corresponding infiltration, hygric, and thermal input parameters were carefully identified. Numerical calculations of thermal and diffusion tensors were performed on a representative concrete elementary cell. Further, the diffusion tensor was compared to the equivalent experimental results available in the literature, and its sensitivity to the water content variations and porosity has been evaluated on the concerned elementary cell.     

Investigation of methane steam reforming in planar porous support of solid oxide fuel cell

Adopting the porous support in integrated-planar solid oxide fuel cell (IP-SOFC) can reduce the operating temperature by reducing thickness of electrolyte layer, and also, provide internal reforming environment for hydrogen-rich fuel gas. The distributions of reactant and product components, and temperature of methane steam reforming for IP-SOFC were investigated by the developed physical and mathematical model with thermodynamic analysis, in which eleven possible reaction mechanisms were considered by the source terms and Arrhenius relationship. Numerical simulation of the model revealed that the progress of reforming reaction and the distribution of the product, H₂, were influenced by the operating conditions, included that of temperature, ratio of H₂O and CH₄, as well as by the porosity of the supporting material. The simulating results indicate that the methane conversion rate can reach its maximum value under the operating temperature of 800 °C and porosity of ε = 0.4, which rather approximate to the practical operating conditions of IP-SOFC. In addition, characteristics of carbon deposition on surface of catalyst were discussed under various operating conditions and configuration parameters of the porous support. The present works provided some theoretical explanations to the numerous experimental observations and engineered practices.     

Combined radiation and convection heat transfer in a porous channel bounded by isothermal parallel plates

Combined radiation and convection heat transfer in a porous medium confined between gray isothermal parallel plates is investigated. The medium is absorbing, emitting and scattering. Cases of boundaries at temperatures higher or lower than the medium are considered. In the porous medium, the boundary effect on the fully developed laminar velocity field as proposed by Kaviany is accounted for. For various values of the extinction coefficient, the scattering albedo, the conduction-radiation parameter and the boundary emissivity, Nusselt number, temperature and heat flux distributions are found for the range of values including the extreme limits of the porous medium shape parameter (PMSP), γ=(W²φ/K)^1/2, where W is the channel width, φ the porosity and K the permeability. For the lower limiting value of the PMSP γ, the effect of the porous medium is negligible and the situation approaches that of Poiseuille flow. For this limiting case, results from the present work are compared with those available in the literature. For medium to high values of the PMSP γ, for the purpose of comparison, some results are presented in tabular form. Radiation is found to have a significant effect on various parameters studied. The discrete transfer method was used for the solution of the radiative part of the energy equation. An iterative finite difference scheme was used to solve the energy equation.     

柴油机颗粒捕集器内颗粒沉积结构的实验研究

基于可视化单通道实验系统,采用激光位移测量方法,对柴油机颗粒在陶瓷捕集器内的沉积过程进行了在线测量.研究结果发现,颗粒沉积过程分为4个阶段:深床期、长树期、搭桥期和颗粒层期;颗粒层厚度的增加在长树期迅速增长,而后在颗粒层期呈缓慢增加的趋势.颗粒层过滤期颗粒层的孔隙结构受过滤速度的影响,过滤速度增大,颗粒层的渗透系数和孔隙率减小,形成的颗粒层致密.实验结果证实了当Pe<1时,渗透系数和孔隙率随Pe的增大而减小,且变化显著;当Pe>1时,渗透系数和孔隙率随Pe数的变化趋于平坦.     

Numerical investigation of heat transfer enhancement on a porous vortex-generator applied to a block-heated channel

The numerical simulation is used to obtain the unsteady laminar flow and convective heat transfer in the block-heated channel with the porous vortex-generator. The general Darcy–Brinkman–Forchheimer model is adopted for the porous vortex-generator. The parameters studies including porosity, Darcy number, width-to-height ratio of porous vortex-generator and Reynolds number have been explored on heat transfer enhancement and vortex-induced vibration in detail. The results indicate that heat transfer enhancement and vortex-induced vibration increase with increasing Reynolds number and width-to-height ratio. However, the porosity has slight influence on heat transfer enhancement and vortex-induced vibration. When Darcy number is 10^?3 or 10^?4, installing a porous vortex-generator with B/h = 1.0 improves overall heat transfer the best along heated blocks, and has a strong reduction of vortex-induced vibration.     

Optimization of Thermal Insulation Performance for the Porous Materials

ABSTRACT

Porous materials are widely used in porous media filtration, membrane separation, catalyst substrates, solid fuel cells, insulation, and other fields. When the porous material used in the field of insulation, heat transfer characteristics become its most important performance parameters. The heat transfer characteristics of porous material is a complex issue affected not only by solid elements and porosity, it is also affected by composite structures. Therefore, how to optimize the heat transfer properties of porous materials is a problem to be urgently solved. In this paper, the numerical method is used to study the effects of pore size, pore shape, pore connectivity, porosity and so on. It is found that pore shape, pore connectivity and gas conductivity have great impacts on the heat transfer of porous materials. The effect of pore arrangement is very little. The design optimization of porosity is affected by porous material mechanical property.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Flow control with electrode bank arrangements by electrohydrodynamics force for heat transfer enhancement in a porous medium

Active flow control with electrohydrodynamics (EHD) force in the channel flow has been numerically investigated for enhancing heat transfer. This study focuses on the effect of electrode bank arrangements and the number of electrodes on corona wind and fluid flow for heat transfer onto a porous medium. Aligned and staggered configurations of electrode banks are compared. The numerical results show that electric field intensity depends on electrical voltage and the number of electrodes. Shear flow is increased with larger numbers of electrodes and in the aligned configuration, resulting in the enhancement of vortex strength. The swirling flow from staggered configurations spread wider than that of aligned configurations, but the aligned configuration produced more turbulence. In addition, the temperature distribution in the channel flow is increased with increasing numbers of electrodes. With the effect of swirling flow, airflow above the porous sample surface is faster leads the heat to more transfer to the porous sample surface. This causes the temperature of porous medium to increase rapidly so the convective heat transfer coefficient on porous medium surface is increased. Finally, the modified case of the numerical results is validated against the experimental results. The experimental flow visualization is based on the incense smoke technique, in order to verify the accuracy of the swirling flow pattern subjected to the electric field. It is shown that the comparison results in both techniques are in good agreement.     

Numerical investigation of multi-layered porosity in the gas diffusion layer on the performance of a PEM fuel cell

A mathematical model is developed to investigate the influence of porosity configurations in the gas diffusion layer (GDL) of the cathode on the electrochemical performance characteristics of a 3-D high-temperature proton exchange membrane (PEM) fuel cell. Four different non-uniform porosity configurations are defined through step functions and analyzed with uniform porosity case. The results are presented in terms of the cell performance characteristics viz. Current density, power density, vorticity magnitude, oxygen molar concentration, overpotential, and total power dissipation density. Our study reveals that oxygen molar concentration, current density, power density are found to be maximum when the stepwise porosity in GDL decreases in the streamwise direction. However, these parameters observed to be the least when the stepwise porosity in GDL increases along the streamwise direction. Additionally, the highest total power dissipation density is observed when the porosity in GDL varies across cross-stream wise direction among other configurations considered. However, it is found to be the least when porosity varies in a streamwise direction. The overpotential becomes the least when stepwise porosity decreases in the streamwise direction although the same is found to be maximum when the porosity in GDL increases along the streamwise direction. The performance is found to be optimal when porosity is maximum at cathode gas channel inlet and GDL-cathode gas channel interface.     

Lattice Boltzmann method for nanofluid forced convection heat exchange in a porous channel with multiple heated sources

Abstract

The nanofluid forced convection heat exchange in a porous channel within three heated blocks was numerically investigated using the Nonorthogonal multiple-relaxation time lattice Boltzmann method (MRT-LBM). The effects of various parameters such as nanoparticle volume fraction (?), Darcy number (Da) on heat exchange performance and flow phenomena were analyzed when the Pecklel number (Pe), the Prandtl number (Pr), and porosity (ε) were 25, 5.829 and 0.3, respectively. The outcome showed that the mean Nusselt number (Nu) on the surface of heated sources remarkably improved by adding nanoparticles. Furthermore, the forced convection heat exchange of the fluid flow in the mainstream area and the heat conduction in the liquid retention zone had a conspicuous influence on the heat-transfer properties. It is worth noting that the forced convection heat transfer of the fluid flow dominates heat exchange. The simulation showed that the average surface Nusselt number on the heated blocks and the heat exchange performance declined with the increase of the Darcy number.     

Thermal buckling and postbuckling responses of geometrically imperfect FG porous beams based on physical neutral plane

Abstract

Considering the third-order shear deformation and physical neutral plane theories, thermal postbuckling analysis for functionally graded (FG) porous beam are performed in this research. The cases of shear deformable functionally graded materials (FGM) beams with initial deflection and uniformly distributed porosity are considered. Geometrically imperfect FG porous beams with two different types of immovable boundary conditions as clamped–rolling and clamped–clamped are analyzed. Thermomechanical nonhomogeneous material properties of the FG porous beam are assumed to be temperature and position dependent. FG porous beams are subjected to different types of thermal loads as heat conduction and uniform temperature rise. Heat conduction equation is solved analytically using the polynomial series solution for the one-dimensional condition. The governing equilibrium equations are obtained by applying the virtual displacement principle. Assuming von Kármán type of geometrical nonlinearity, equilibrium equations are nonlinear and are solved using an analytical method. A two-step perturbation technique is used to obtain the thermal buckling and postbuckling responses of FG porous beams. The numerical results are compared with the case of perfect FGM Timoshenko beams without porosity distribution based on the midplane formulation. Parametric studies of the perfect/imperfect FG porous beams for two types of thermal loading and boundary conditions are provided.