Developing fluid flow and heat transfer in a channel partially filled with porous medium

A three-dimensional computational model is developed to analyze fluid flow in a channel partially filled with porous medium. In order to understand the developing fluid flow and heat transfer mechanisms inside the channel partially filled with porous medium, the conventional Navier–Stokes equations for gas channel, and volume-averaged Navier–Stokes equations for porous medium layer are adopted individually in this study. Conservation of mass, momentum and energy equations are solved numerically in a coupled gas and porous media domain along a channel using the vorticity–velocity method with power law scheme. Detailed development of axial velocity, secondary flow and temperature field at various axial positions in the entrance region are presented. The friction factor and Nusselt number are presented as a function of axial position, and the effects of the size of porous media inside the channel partially filled with porous medium are also analyzed in the present study.     

Periodic laminar flow and heat transfer in a channel with 45° staggered V-baffles

A numerical investigation has been carried out to examine periodic laminar flow and heat transfer characteristics in a three-dimensional isothermal wall channel of aspect ratio, AR = 2 with 45° staggered V-baffles. The computations are based on the finite volume method, and the SIMPLE algorithm has been implemented. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 100 to 1200. To generate two pair of main streamwise vortex flows through the tested section, V-baffles with an attack angle of 45° are mounted in tandem and staggered arrangement on the lower and upper walls of the channel. Effects of different baffle heights on heat transfer and pressure drop in the channel are studied and the results of the V-baffle pointing upstream are also compared with those of the V-baffle pointing downstream. It is apparent that in each of the main vortex flows, a pair of streamwise twisted vortex (P-vortex) flows can induce impinging flows on a sidewall and a wall of the interbaffle cavity leading to drastic increase in heat transfer rate over the channel. In addition, the rise in the V-baffle height results in the increase in the Nusselt number and friction factor values. The computational results reveal that the optimum thermal enhancement factor is around 2.6 at baffle height of 0.15 times of the channel height for the V-baffle pointing upstream while is about 2.75 at baffle height of 0.2 times for the V-baffle pointing downstream.     

Flow visualization of fluid flow through a U-bend channel with periodical high-permeability porous blocks

The issue about the heat transfer enhancement in a 180-deg turned channel has attracted many attentions. The heat transfer capacity is influenced strongly by the behavior of the fluid flow. This work experimentally investigated the fluid flow characteristics in a 180-deg round turned channel with discrete aluminum-foam blocks. The air was used as coolant. Several aluminum-foam blocks (0.90 porosity and 10 PPI pore density) were installed discretely in the 180-deg round turned channel with the square cross section. Four kinds of test section were employed to perform the flow visualizations. The results indicate that the arrangement of aluminum-foam blocks impacts on the flow pattern remarkably. The present results can be used to explain the pressure drop characteristics of such aluminum-foam channel, and be benchmarking by other investigators who would like to solve this problem numerically.     

Three-dimensional numerical simulation of fluid flow with phase change heat transfer in an asymmetrically heated porous channel

Fluid flow with phase change heat transfer in a three-dimensional porous channel with asymmetrically heating from one side is numerically studied in this paper. The “modified” Kirchhoff method is used to deal with the spatial discontinuity in the thermal diffusion coefficient in the energy equation. The velocity and temperature fields, as well as the liquid saturation field on the heated section of the wall with different Peclet and Rayleigh numbers are investigated. The results show that the liquid flow bypasses the two-phase zone, while the vapor flows primarily to the interface between the sub-cooled liquid zone and the two-phase zone. An increase in the Peclet number decreases the two-phase region while an increase in the Rayleigh number helps to spread the heat to a larger region of the domain. The distribution of the liquid saturation on the heated section of the wall indicates that the minimum liquid saturation increases with the increase of both the Peclet and Rayleigh numbers.     

Numerical simulation of turbulent fluid flow and heat transfer characteristics of heated blocks in the channel with an oscillating cylinder

In this study, numerical simulations have been carried out to investigate the influence of transient flow field structures, and the heat transfer characteristics of heated blocks in the channel with a transversely oscillating cylinder. To solve the interaction problems between liquid and solid interface in the simulations, a Galerkin finite element formulation with Arbitrary Lagrangian–Eulerian method (ALE) is adopted.The main parameters in the study are Reynolds numbers (Re = 800–8000), dimensionless oscillating frequencies (F = 0.1–0.4), dimensionless amplitudes (L = 0.05–0.4). The results of numerical simulations show that the oscillating cylinder induces the flow vibration. This phenomenon disturbs the flow and thermal fields in the channel flow, and the heat transfer rate in the channel would be enhanced. Furthermore, the resonance effect of channel flow and oscillating cylinder can be observed as the oscillating frequency of the cylinder approach to the vortex shedding frequency. Due to the phenomenon of resonance in the channel flow, the heat transfer rate is enhanced more remarkably. In the studied ranges, the results show that the optimum dimensionless cylinder oscillating frequency and dimensionless amplitude value are 0.21 and 0.1 and that the heat transfer from heated blocks is enhanced as the oscillating frequency of the cylinder is in a lock-in region.     

流体流过带折流板通道时的流动及传热数值研究

对常物性流体在通道内的周期性充分发展层流流动和换热特性进行了二维数值计算分析。所研究的通道是由两平行平板布置于中心线位置的一系列折流板构成。平行平板保持温度恒定，折流板则分成完全导热和绝热两种情况，对不同几何参数，Re数和Pr数下的流动和换热性能进行了数值研究。文章还给出了系统流函数图和局部换热系数分布情况。     

Flow and heat transfer of couple stress fluid in a porous channel with expanding and contracting walls

In this paper, an incompressible laminar flow of a couple stress fluid in a porous channel with expanding or contracting walls is considered. Assuming symmetric injection or suction along the uniformly expanding porous walls and using similarity transformations, the governing equations are reduced to nonlinear ordinary differential equations. The resulting equations are then solved numerically using quasilinearization technique. The graphs for velocity components and temperature distribution are presented for different values of the fluid and geometric parameters.     

泡沫陶瓷多孔介质内流体流动及表面传热模拟

气隙扩散蒸馏脱盐技术利用具有大比表面积的多孔介质作为蒸发器,海水在多孔介质内部流动并在表面蒸发,多孔介质起到了强化液体蒸发的作用;但由于多孔介质结构极其复杂,很难使用传统的实验技术从微观水平观测到多孔介质孔隙通道内流体的流动状态以及传热现象。针对此问题采用计算机数值模拟方法,拍摄实际碳化硅泡沫陶瓷CT图片,构建三维模型进行有限元模拟分析。结果表明,多孔介质内流体会优先通过较大孔隙通道。流体在多孔介质表面向环境空气的散热量随孔隙密度增大而增大,孔隙密度从10提高至30 PPI,散热量提高约1.43倍。进口热流体与环境空气温差越大,向环境的散热量越大,孔隙密度在30 PPI条件下,进口热流体温度从49.38增加至68.67 ℃,散热量提高近2.07倍。     

Analysis of staggered tube bundle heat transfer to vertical foam flow

In general heat transfer intensity between solid surface and coolant (fluid) depends on three main parameters: heat transfer coefficient, size of heat exchange surface and temperature difference between surface and fluid. Sometimes the last two parameters (surface size and temperature difference) are strictly limited due to the process or technological requirements, and only increase of heat transfer coefficient is allowed. Simplest way offering sufficient increase in heat transfer rate (heat transfer coefficient as well) is to go from the laminar fluid flow regime to the turbulent one by increasing flow velocity. In many cases it helps despite such disadvantages like more complicated fluid supply system, rise of fluid flow mass rate and growth of energy usage for pumping. But in some cases, for example, in space application, in nuclear engineering, etc. there is not allowed to use high flow velocity of coolant – gas (due to vibration danger) or to apply high mass rate of coolant – liquid (due to limitation concerning weight or mass). One of the possible solutions of that problem could be the usage of two-phase flow as a coolant. An idea to use such two-phase coolant for heat removal from the solid surface is not new. Boiling liquid (water especially), gas flow with liquid droplets and other two-phase systems are widely used for heat and mass transfer purposes in various industries like food, chemical, oil, etc. An application of such two-phase coolants has lot advantages; high value of heat transfer coefficient is one of the most important. Unfortunately nothing is ideal on the Earth. Restrictions on vibration, on coolant weight (or mass rate); necessity to generate two-phase flow separately from the heat removal place; requirements on very low coolant velocities and other constraints do not allow using such type of two-phase coolant for purposes which were mentioned above (space application especially). As a possible way out can be usage of the statically stable foam flow produced from gas (air) and surfactant solutions in liquid (water). Our previous investigations [J. Gylys, Hydrodynamics and Heat Transfer under the Cellular Foam Systems, Technologija, Kaunas, 1998] showed the solid advantages of that type of two-phase coolant, including high values of heat transfer coefficient (up to 1000 W/m² K and more), low flow velocities (less than 1.0 m/s), small coolant density (less than 4 kg/m³), possibility to generate foam flow apart from the heat removal place, etc.This article is devoted to the experimental investigation of the staggered tube bundle heat transfer to the vertical upward and downward statically stable foam flow. The investigations were provided within the laminar regime of foam flow. The dependency of the tube bundle heat transfer on the foam flow velocity, flow direction and volumetric void fraction were analyzed. In addition to this, the influence of tube position in the bundle was investigated also. Investigation shows that the regularities of the tube bundle heat transfer to the vertical foam flow differ from the one-phase (gas or liquid) flow heat transfer peculiarities. It was showed that the heat transfer intensity of the staggered tube bundle to the foam flow is much higher (from 25 to 100 times) than that for the one-phase airflow under the same conditions (flow velocity). The results of the investigations were generalized using criterion equations, which can be applied for the calculation and design of the statically stable foam heat exchangers with the staggered tube bundles.     

Micropolar flow in a porous channel with high mass transfer

In this research the injective micropolar flow in a porous channel is investigated. The flow is driven by suction or injection on the channel walls, and the micropolar model is used to describe the working fluid. This problem is mapped into the system of nonlinear coupled differential equations by using Berman's similarity transformation. These are solved for large mass transfer via Optimal Homotopy Asymptotic Method (OHAM). Also the numerical method is used for the validity of this analytical method and excellent agreement is observed between the solutions obtained from OHAM and numerical results. Trusting this validity, effects of some other parameters are discussed.     

Numerical simulation of turbulent fluid flow and heat transfer characteristics in heat exchangers fitted with porous media

Numerical simulations have been carried out to investigate the turbulent heat transfer enhancement in the pipe filled with porous media. Two-dimensional axisymmetric numerical simulations using the k–? turbulent model is used to calculate the fluid flow and heat transfer characteristics in a pipe filled with porous media. The parameters studied include the Reynolds number (Re = 5000–15,000), the Darcy number (Da = 10^?1–10^?6), and the porous radius ratio (e = 0.0–1.0). The numerical results show that the flow field can be adjusted and the thickness of boundary layer can be decreased by the inserted porous medium so that the heat transfer can be enhanced in the pipe. The local distributions of the Nusselt number along the flow direction increase with the increase of the Reynolds number and thickness of the porous layer, but increase with the decreasing Darcy number. For a porous radius ratio less than about 0.6, the effect of the Darcy number on the pressure drop is not that significant. The optimum porous radius ratio is around 0.8 for the range of the parameters investigated, which can be used to enhance heat transfer in heat exchangers.     

Forced Convective Heat Transfer in a Porous Plate Channel

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

Transient behavior of fluid flow and heat transfer with phase change in vertical porous channels

The transient behavior of flow boiling in vertical porous channels is numerically studied in this paper. The velocity and temperature fields under different parameters for both aiding and opposing flows are investigated. Distinctly different flow and heat transfer features are observed in the comparison of aiding and opposing flows. An analysis of liquid saturation along the heated wall indicates that the minimum liquid saturation for aiding flow is located at the tail of the heated section, whereas for opposing flow, it is within the heated section and shifts upstream with the increase of Rayleigh number and decrease of Peclet number.     

A study on entropy generation and heat transfer in a magnetohydrodynamic flow of a couple-stress fluid through a thermal nonequilibrium vertical porous channel

The effect of local thermal nonequilibrium (LTNE) on the entropy generation and heat transfer characteristics in the magnetohydrodynamic flow of a couple-stress fluid through a high-porosity vertical channel is studied numerically using the higher-order Galerkin technique. The Boussinesq approximation is assumed to be valid and the porous medium is considered to be isotropic and homogeneous. Two energy equations are considered one each for solid and fluid phases. The term involving the heat transfer coefficient in both equations renders them mutually coupled. Thermal radiation and an internal heat source are considered only in the fluid phase. The influence of inverse Darcy number, Hartmann number, couple-stress fluid parameter, Grashof number, thermal radiation parameter, and interphase heat transfer coefficient on velocity and temperature profiles is depicted graphically and discussed. The entropy generation, friction factor, and Nusselt number are determined, and outcomes are presented via plots. The effect of LTNE on the temperature profile is found to cease when the value of the interphase heat transfer coefficient is high, and in this case, we get the temperature profiles of fluid and solid phases are uniform. The physical significance of LTNE is discussed in detail for different parameters' values. It is found that heat transport and friction drag are maximum in the case of LTNE and minimum in the case of local thermal equilibrium. We observe that LTNE opposes the irreversibility of the system. The corresponding results of a fluid-saturated densely packed porous medium can be obtained as a limiting case of the current study.     

Impingement heat transfer of staggered arrays of air jets confined in a channel

Impingement heat transfer of circular air jets confined in a channel was experimentally investigated. The impingement plate was exerted with a constant surface heat flux. Five jets, including one center jet and four neighboring jets, in staggered arrays were considered. The considered jet Reynolds number (Re) was in the range 5000–15,000; the jet height-to-jet diameter ratio (H/d) was in the range 1.0–4.0; the jet spacing-to-jet diameter ratio (S/d) was in the range 4.0–8.0; the jet plate width-to-jet diameter ratio (W/d) was in the range 6.25–18.75. Two jet plate length-to-jet diameter ratio (L/d), 31.7 and 83.3, were individually arranged. For the center jet with a specific Reynolds number, its stagnation Nusselt number was found to linearly increase with the jet Reynolds number of the four neighboring jets. For all the five jets with the same Reynolds number, the correlation result shows that the stagnation Nusselt number of the center jet is proportional to the 0.7 power of the Re and the −0.49 power of the W/d. A weak dependence of the stagnation Nusselt number on H/d, S/d and L/d was found.     

Peristaltic flow of a Williamson fluid in an inclined asymmetric channel with partial slip and heat transfer

The present article deals with the peristaltic flow of a Williamson fluid in an inclined asymmetric channel. The relevant equations have been modeled. Analysis has been carried out in the presence of velocity and thermal slip conditions. Expressions for stream function, temperature, pressure gradient and heat transfer coefficients are derived. The solutions are compared with the existing available results in a limiting sense. Numerical integration has been performed for pressure rise per wavelength. Plots are presented and analyzed for various embedded parameters into the problem. Comparison between the solutions is also shown.