壁面覆盖部分多孔介质方腔自然对流流动的数值模拟

多孔介质壁面封闭腔体的自然对流在生产实际中有重要的应用。针对左侧部分多孔介质壁面方形封闭腔体,基于有限元法对封闭腔体的自然对流换热进行了数值模拟,得到了在不同Ra、孔隙率条件下腔体内空气的温度分布、速度分布。结果表明:随着Ra的增大,腔体内的流场及温度场发生了明显的变化,多孔介质壁面孔隙率的变化对腔体的流动换热的影响很小。     

部分填充多孔介质复合腔体内自然对流的二维PIV实验研究

利用PIV技术对部分填充多孔介质腔体内的二维流动进行测试试验,采用三维打印技术构造高孔隙率的球体结构作为多孔介质模型,搭建了二维PIV测试试验台对部分填充多孔介质复合腔体内自然流动进行了实验测试。通过实验结果与模拟结果的对比分析,发现在多孔介质中间断面位置,因两侧端面导致的三维效应可忽略不计,可以作为二维流场的最佳测试断面。     

多孔介质回热微燃烧器预混燃烧特性研究

设计了多孔介质回热徽燃烧器,对微燃烧器内H2/Ak的预混燃烧特性进行了实验研究和数值模拟,实验结果表明,当过量空气系数1.0＜α＜3.0时,微燃烧器具有较高的燃烧效率,出口烟气温度和较低的燃烧热损失率,且燃烧热功率P越高,α越大,热损失率越小.当P=100 W时,其出口烟气温度最高可达到1 232 K,当α=3.0时,燃烧效率仍达到96.85％,而热损失率仅为14.87％.数值模拟结果表明,由于采用了回热夹层和多孔介质回热结构,有效地回收了热量损失,使得微燃烧器具有良好的热性能.证明设计的多孔介质回热微燃烧器是一种燃烧效率高、热损失率低的微燃烧器.     

多孔介质局部非热平衡模型研究综述

简述了局部热平衡模型和局部非热平衡模型的基本概念,并分别对局部热平衡模型的有效性问题和局部非热平衡模型的边界条件进行了讨论,从流态、多孔介质骨架材料和研究尺度3个角度对局部非热平衡模型在多孔介质热质传递领域的应用进行了系统的总结。针对低导热系数内的局部非热平衡效应,提出了一种基于局部非热平衡模型的等效球简化法,该方法较为准确地获取了低导热系数固体和流体的温度。为多孔介质局部非热平衡模型的使用和进一步研究提供借鉴。     

内插多孔介质微通道热沉的数值模拟

以水为流动介质,在微通道内添加堆叠金属丝网多孔介质,采用局部非热平衡假设和双能量方程模型,分析了内插不同目数金属丝网的微通道在层流状况下的传热与阻力特性,并采用数值计算方法对微通道热沉进行了数值模拟。结果表明,在微通道内插入多孔介质能显著提高热沉的对流换热系数、降低加热面平均温度,但阻力增加较大,且当插入的金属丝网目数为100目时,微通道热沉的对流换热系数较大,与填充其他目数金属丝网相比阻力增加较小。     

多孔介质燃烧室内湍流流动及燃油喷雾的数值模拟

以多孔介质发动机为背景,用数值模拟方法考察气缸内加入多孔介质蓄热体后对燃烧室内湍流流场及混合气形成的影响.计算基于Antohe和Lage的适用于多孔介质的k-e模型,其中引入了Darcy项和Forchheimer项.对燃油喷雾在自由空间和多孔介质组成的燃烧室内的流场进行了数值计算.计算结果表明,多孔介质对燃油液滴与空气的混合过程具有重要的影响,多孔介质内的流场作为一个局部流场和整个燃烧室内流场发生相互作用.Darcy项和Forchheimer项均对湍能起阻尼作用,从而降低多孔介质内的湍能水平.这种阻尼作用随多孔介质渗透率的减小而增大.为检验数值模型的合理性,针对文献中的实验条件进行了相应的数值计算,计算结果与实验结果吻合良好.     

生物质热燃气多孔介质直燃机理与脱焦特性研究

为研究含焦油的生物质热燃气在多孔介质中的燃烧机理与焦油燃烧脱除特性,采用固相实体颗粒堆积法模拟多孔介质,通过分析燃烧过程中反应器内温度、热流密度以及反应动力学速率等参数场的分布特征,揭示了当量比对生物质热燃气多孔介质燃烧过程的显著影响作用.研究表明,焦油燃烧脱除过程中直接氧化反应速率高是决定焦油出口浓度小、转化率高的关...     

骨架发热多孔介质通道内单相流阻力与换热特性数值模拟

以Fluent 6.3为平台,采用局部非热平衡模型,对紊流及紊流过渡区范围内骨架发热多孔介质竖直通道内的非达西强制对流换热进行了数值模拟。采用三维N-S方程及标准k-ε湍流模型描述多孔介质内的流动,详细研究了孔隙有效雷诺数Re(400Re2000),表面热流密度q(q=5、30和90 kW/m2)和冷却剂入口温度Tin(Tin=20、50和80℃)的变化对多孔介质流道内流动阻力及换热特性的影响。结果表明:低热流密度下,表面热流密度的变化对流动阻力和换热系数的影响很小;小球直径对换热系数的影响显著,且随着雷诺数的增加而增加;换热系数随冷却剂入口温度的增加而减小。     

热渗耦合作用下U型埋管换热器的数值模拟

针对土壤耦合热泵地下U型换热埋管,建立了管内流体以及换热器周围土壤热渗耦合物理数学模型。所建模型考虑了U型管的实际形状,土壤考虑为饱和多孔介质,管内湍流流动采用R ea lizab le k-ε模型。采用F luen t软件对模型进行模拟计算,得到了管内流体以及周围土壤温度分布。分析了土壤中水的渗流对传热过程的影响,并对考虑渗流作用时不同土壤物性对单根U型垂直埋管换热器周围土壤温度场进行了模拟计算与分析。     

结构化填充床内非平衡性的数值研究

利用孔隙尺度介观方法对有序排列多孔介质小球的二维填充床进行数值研究。在考虑小球内部的热传导和小球表面的辐射换热基础上,研究空气流入填充床内部时,固体和气体之间热的非平衡性以及局部和整体之间的流动特性。数值计算与实验结果吻合较好。结果表明:在填充床内部,相邻小球之间和整个区域的不同位置均存在热的非平衡现象。当空气流速从0.23～0.63 m/s时,最高温度逐渐降低,但是高温区域的范围逐渐扩大。同时,无量纲速度在填充床内呈阶梯式抛物线变化,其值在0.0～8.3,速度非平衡特性较明显。     

Analytical investigation of heat transfer enhancement in a channel partially filled with a porous material under local thermal non-equilibrium condition

Forced convection through a channel partially filled with a porous medium is investigated analytically in the present work. Thermally developed condition is considered and the local thermal non-equilibrium model is utilized to obtain the exact solutions of both fluid and solid temperature fields for flow inside the porous material as well as for flow in the clear region. Nusselt number is obtained in terms of the porous insert thickness (S), porosity (?) as well as pertinent parameters such as thermal conductivity ratio (k), Biot number (Bi), and Darcy number (Da). The values of S by which the temperature difference between the two phases approach to zero, for different values of Bi, k, and Da number are obtained. It is found that three mechanisms affect the Nu number i: clear fluid conduction ii: internal heat exchange in the porous medium iii: channeling effect in the clear flow. The value of S, which yields the highest Nu number is found to vary linearly from 0.8 to 0.97 as the value of Da decreases from 10⁻³ to 10⁻⁷. At the expense of reasonable pressure drop the optimum thickness of porous material in order to enhance the heat transfer rate is found S = 0.8.     

The onset of Lapwood-Brinkman convection using a thermal non-equilibrium model

The stability of a horizontal fluid saturated sparsely packed porous layer heated from below and cooled form above when the solid and fluid phases are not in local thermal equilibrium is examined analytically. The Lapwood-Brinkman model is used for the momentum equation and a two-field model is used for energy equation each representing the solid and fluid phases separately. Although the inertia term is included in the general formulation, it does not affect the stability condition since the basic state is motionless. The linear stability theory is employed to obtain the condition for the onset of convection. The effect of thermal non-equilibrium on the onset of convection is discussed. It is shown that the results of Darcy model for the non-equilibrium case can be recovered in the limit as Darcy number Da → 0. Asymptotic analysis for both small and large values of the inter phase heat transfer coefficient H is also presented. An excellent agreement is found between the exact solutions and asymptotic solutions when H is very small.     

A boundary layer analysis of heat transfer by free convection from permeable horizontal cylinders of elliptic cross-section in porous media using a thermal non-equilibrium model

This work uses a thermal non-equilibrium model to study the free convection boundary layer flow driven by temperature gradients near a permeable horizontal cylinder of elliptic cross-section with constant wall temperature in a fluid-saturated porous medium. A coordinate transformation is used to obtain the nonsimilar boundary layer equations. The transformed boundary layer equations are then solved by the cubic spline collocation method. Results for the local Nusselt numbers are presented as functions of the porosity scaled thermal conductivity ratio, the heat transfer coefficient between solid and fluid phases, the transpiration parameter, and the aspect ratio when the major axis of the elliptical cylinder is vertical (slender orientation) and horizontal (blunt orientation). An increase in the porosity scaled thermal conductivity ratio or the heat transfer coefficient between the solid and fluid phases increases the heat transfer rates. Moreover, the use of suction (positive transpiration parameter) tends to increase the heat transfer rates between the porous medium and the surface.     

Thermal transport analysis for natural convection in a porous corrugated rhombic enclosure

A numerical study of fluid flow and heat transfer, applying natural convection is carried out in a porous corrugated rhombic enclosure. A uniform heating source is applied from the bottom boundary wall while the inclined side walls are maintained to a constant cold temperature and the top corrugated wall is retained at insulated condition inside the enclosure. The heat transfer and flow features are presented for a wide spectrum of Rayleigh numbers (Ra), 10⁴ ≤ Ra ≤ 10⁶, and Darcy numbers (Da), 10^?3 ≤ Da ≤ 10^?2. The number of undulations (n) for the top and bottom walls have been varied from 1 to 13 keeping the amplitude of undulation fixed. It is revealed that the characteristics of heat transfer are conceivably modulated by changing the parameter of the undulation number on the enclosure walls, specifically at the bottom and top. The influencing control of n in altering the heat transfer rate is felt maximum on the left wall and minimum for the right wall, and there is a strong interplay between Ra and Da together with n on dictating the heat transfer characteristics. The critical value, where heat transfer rate is observed as maximum is at n = 11 and thereafter the values decrease.     

Double-diffusive natural convection in a porous medium under constant heat and mass fluxes

Double-diffusive natural convection in a rectangular fluid-saturated porous medium has been studied analytically and numerically. The analysis reveals that there is a range of buoyancy ratios N in which one obtains two types of solutions or oscillating convection. In the case of 0.4 < N < 1.0, there exist two analytical solutions when R_c = 100 and Le = 30. In that case, two solutions, temperature-dominated and concentration-dominated solutions, are calculated when the aspect ratio is small. It is found that the oscillation is due to a temporal formation of a two-roll flow pattern in the cavity when the aspect ratio is sufficiently large. The oscillation of time-dependent Nusselt number and flow patterns are shown. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 255–265, 1999     

A thermal nonequilibrium model to natural convection inside non-Darcy porous layer surrounded by horizontal heated plates with periodic boundary temperatures

This article displays a numerical investigation on natural convection within non-Darcy porous layer surrounded by two horizontal surfaces having sinusoidal temperature profiles with difference in phase and wave number. The Darcy–Brinkman–Forchheimer model and local thermal nonequilibrium condition have been employed. Simulations have been performed for wide ranges of inertia coefficient (10^–4 ≤ Fs/Pr* ≤ 10^–2), thermal conductivity ratio (0.1 ≤ K _r ≤ 100), phase difference (0 ≤ β ≤ π), modified Rayleigh number (200 ≤ Ra* ≤ 1000), wavelength (3 ≤ k ≤ 12), and nondimensional heat transfer coefficient (0.1 ≤ H ≤ 100). Results demonstrate that Nusselt number highly relies on Fs/Pr*, K _r, β, Ra*, and k as compared to H. A considerable enhancement in fluid, solid, and overall Nusselt numbers has been observed with diminishing Fs/Pr* and β and increasing k, K _r, and H. The raising in β has a significant impact on Nu for smaller k and this effect is almost ignored when k > 12. The increase in Ra*, K _r, β, and H and decrease in Fs/Pr* and k acts to reduce the severity of nonequilibrium zone and increase the size of thermal equilibrium zone. The influence of H on nonequilibrium area is more evident than K _r.     

Combined effect of magnetic field and thermal dispersion on a non-darcy mixed convection

This paper is devoted to investigate the influences of thermal dispersion and magnetic field on a hot semi-infinite vertical porous plate embedded in a saturated Darcy-Forchheimer-Brinkman porous medium. The coefficient of thermal diffusivity has been assumed to be the sum of the molecular diffusivity and the dynamic diffusivity due to mechanical dispersion. The effects of transverse magnetic field parameter (Hartmann number Ha), Reynolds number Re (different velocities), Prandtl number Pr (different types of fluids) and dispersion parameter on the wall shear stress and the heat transfer rate are discussed.     

The thermal modeling of a matrix heat exchanger using a porous medium and the thermal non-equilibrium model

Heat transfer and fluid flow characteristics through a porous medium were investigated using numerical simulations and experiment. For the numerical simulations two models were created: a two-dimensional numerical model and a Fluent™ computational fluid dynamics (CFD) porous media model. The experimental investigation consisted of a flow channel with a porous medium section that was heated from below by a heat source. The results of the numerical models were compared to the experimental data in order to determine the accuracy of the models. The numerical model was then modified to better simulate a matrix heat exchanger. This numerical model then generated temperature profiles that were used to calculate the heat transfer coefficient of the matrix heat exchanger and develop a correlation between the Nusselt number and the Reynolds number.