格子Boltzmann方法对多孔介质复合通道内流动的研究

针对海绵、青砖和石膏3种典型的多孔介质,利用工业X-CT断层图像扫描技术进行处理,获得真实多孔介质几何特征的二维构造多孔介质。采用不可压缩单松弛格子Boltzmann方法进行数值模拟,通过改变通道内多孔介质模型和Re分析了多孔介质复合通道内流体的流动特性,从介观角度研究了多孔介质孔隙结构、孔隙率对复合通道内流场的渗流扰动的影响,为以后研究多孔介质复合腔体内热质交换提供一定参考。     

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。     

基于LBM的多孔介质无机复合相变材料储能特性北大核心CSCD

为了研究骨架形貌对无机复合相变材料(CPCM)相变储能特性的影响,基于格子玻尔兹曼方法,采用四参数随机生长法(QSGS)构造多孔介质骨架,建立随机分布的多孔介质CPCM相变模型,在此基础上,探究孔隙度(ε)、固相生长核分布概率(P_(c))、方向生长概率(P_(d))、瑞利数(Ra)对CPCM相变储能特性的影响。结果表明,ε越小,CPCM熔化时间越短,ε为0.70时的完全熔化时间相较于ε为0.90时缩短了23.63%。在相同ε(0.90)下,P_(c)增大或P_(d)减小,都有助于提高CPCM的熔化速度。Ra越大,自然对流强度越大,CPCM所需熔化时间越短,Ra为18000时CPCM所需熔化时间相较于Ra为1000时缩短了41.46%。本工作为研究多孔介质无机CPCM储能特性提供理论依据和参考。     

基于LBM的三角腔固液相变模拟

利用格子Boltzmann方法(LBM)模拟三角腔中的自然对流融化现象,分析瑞利数Ra、局部热源尺寸L及位置S对其相变换热和储能特性的影响.数值计算表明:①相变材料在全热边界三角腔内的完全融化时间随对流循环强度的增加呈现先增后减的趋势,临界Ra值为31000,当Ra大于31000时,融化进程和对流效应呈现正相关,低于3...     

空心喷雾油滴碰撞热多孔介质的数值研究

为加深对多孔介质发动机中均匀混合气形成的了解,用改进的KIVA-3V详细模拟了空心喷雾油滴碰撞热多孔介质的过程。在KIVA-3V中增加了油滴碰撞热多孔介质壁面的碰撞模型、传热模型及空心喷雾的线性不稳定性液膜破碎(LISA)模型。油滴与热壁的碰撞模型和传热模型经检验证明了其合理性。在简化多孔介质结构的基础上,在不同的环境压力及喷雾锥角下,模拟了空心喷雾与热多孔介质的相互作用。计算结果表明:油雾在碰撞到热多孔介质后,发生分裂的油束和多孔介质区域的高温,促使油滴实现快速蒸发并为油蒸汽与空气充分混合创造了前提。不同的空间压力及喷雾锥角直接影响到油滴在多孔介质中的分布。     

骨架导热特性对部分填充多孔介质复合腔体内传热影响的模拟研究

利用有限元数值方法对部分填充多孔介质复合腔体内的自然对流进行模拟研究,模拟利用两区域法来计算复合腔体内的自然对流,在多孔介质区域采用Forheimer-Brinkman-Darcy方程,在纯流体区域使用Navier-Stokes方程,重点研究了多孔介质骨架导热特性对自然对流的影响。研究发现,在一定范围内随着多孔介质导热性的增强,流体温度升高,部分填充多孔介质腔体内自然对流有一定程度的增强。     

伞喷油雾与热多孔介质相互作用的数值模拟

为深入认识多孔介质发动机中均匀混合气的形成,用改进的KIVA-3V详细模拟了伞喷油雾与热多孔介质之间的相互作用．在KIVA-3V中增加了油滴碰撞热多孔介质壁面的碰撞模型、传热模型．为检测数值模型的合理性,在Senda等人的实验条件下进行了数值计算．油束碰壁后油滴和油蒸气分布的数值计算结果与实验结果吻合较好．在简化多孔介质结构的基础上和不同的环境压力及喷雾锥角时,模拟了伞喷油雾与热多孔介质的碰撞过程．计算结果表明,伞喷油雾的喷雾锥角及空间压力对油滴在多孔介质中的分布有着很大的影响,在多孔介质厚度一定时,通过调节这些参数,能够形成均匀混合气．     

微型热光电系统多孔介质燃烧器性能的实验研究

为保证微型热光电动力系统能稳定、高效地工作,燃烧器壁面需有较高的温度,且分布均匀．对采用多孔介质结构的微型燃烧器进行了实验研究,分析了孔隙率、CH_4/O_2混合比等因素对燃烧器性能的影响．结果表明,采用多孔介质结构可以改善燃烧器内的燃烧传热过程;合理选择孔隙率和工况参数,可以优化燃烧器壁面温度分布,提高系统工作性能．     

应用热非平衡模型模拟梯度磁场作用下多孔介质方腔内空气热磁对流

梯度磁场可用来控制多孔介质内空气的自然对流传热过程.利用局部热非平衡模型对圆形截流线圈水平放置时三维多孔介质方腔内的空气热磁对流进行了数值研究.控制方程基本变量采用控制容积法离散,求解采用SIMPLE算法.计算过程中Ra为103～105,磁场力数γ为0～150、Da为10-7～100.获得了空气热磁对流的流场和温度场....     

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.     

Lattice Boltzmann Simulation of Natural Convection in a Square Cavity with Linearly Heated Wall(s)

In this study, the lattice Boltzmann method is used in order to investigate the natural convection in a cavity with linearly heated wall(s). The bottom wall is heated uniformly and the vertical wall(s) are heated linearly, whereas the top wall is insulated. Investigation has been conducted for Rayleigh numbers of 10³ to 10⁵, while Prandtl number is varied from 0.7 to 10. The effects of an increase in Rayleigh number and Prandtl number on streamlines, isotherm counters, local Nusselt number and average Nusselt number are depicted. It has been observed that the average Nusselt number at the bottom wall augments with an increase in Prandtl number.     

MHD Turbulent and Laminar Natural Convection in a Square Cavity utilizing Lattice Boltzmann Method

In this study, the lattice Boltzmann method was used to solve the turbulent and laminar natural convection in a square cavity. In this paper a fluid with Pr = 6.2 and different Rayleigh numbers (Ra = 10³, 10⁴,10⁵ for laminar flow and Ra = 10⁷, 10⁸,10⁹ for turbulent flow) in the presence of a magnetic field (Ha = 0, 25, 50, and 100) was investigated. (Results show that the magnetic field drops the heat transfer in the laminar flow as the heat transfer behaves erratically toward the presence of a magnetic field in a turbulent flow. Moreover, the effect of the magnetic field is marginal for a turbulent flow in contrast with a laminar flow.The greatest influence of the magnetic field is observed at Ra = 10⁵ from Ha = 0 to 100 as the heat transfer decreases significantly.     

Non－Darcian and Anisotropic Anisotropic Effects on Natural Convection in Horizontal Porous Media Enclosure

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Non—Darcian and Anisotropic Effects on Natural Convection in Horizontal Porous Media Enclosure

Natural convection heat transfer in a horizontal enclosure filled with anisotropic porous media,being isothermally heated at bettom and cooled at top while the vertical walls being adiabatic,is numerically studied by applying the Brinkman model-a modified form of Darcy model giving consideratioin to the viscous effect.The results show that:(1)a larger permeability ratio(K^*) causes a lower flow intensity in the enclosure and a smaller Nusselt number,all Nusselt numbers approach unity in the limit of K^*→∞;a larger thermal conductivity ratio(λ^*) causes a stranger distortion of isotherms in the enclosure and a higher flow velocity near the walls,all the Nusselt numbers approach unity in the limit of λ^*-→0,the permeability and thermal conductivity ratios generally have opposing effects on the Nusselt number.(2) an increasing Darcy number decreases the flow intensity and heat tansfer rates,which is more significant at a lower permeability ratio.In particular,with K^*≤0.25,the Nusselt number for Da=10^-3 would differ from that of Darcy flow up to an amount of 30%,an analysis neglecting the non-Darican effect will inevitably be of considerable error.     

基于格子Boltzmann方法的饱和土壤渗流与传热数值模拟

本文利用随机多孔介质生成算法重构了与真实土壤外貌相近的多孔介质几何结构。通过引入不可压耦合双分布格子Boltzmann模型（lattice Boltzmann model ,LBM）对孔隙尺度下单相饱和土壤渗流和传热进行了模拟。着重讨论了不同渗流压差、孔隙率、土壤固体颗粒尺寸分布对流动与传热的影响。结果表明：土壤渗流速度与渗流压差呈线性单调递增关系,平均温度随渗流压差增加而增大,但温升速率逐渐减缓;当孔隙率增大时,渗流速度增加,且当孔隙率大于0.58时,对流换热作用迅速增强,土壤温升速率显著加快;对于相同孔隙率,当土壤固相颗粒尺寸较大时,流动出现典型优先流效应;随着土壤固相颗粒尺寸减小,土壤温度变化逐渐趋于平缓,平均温度降低。     

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.     

Lattice Boltzmann simulation on flow fields connected with multiple side‐channels

For this study, using the lattice Boltzmann method (LBM), we conducted flow‐field analyses in which two straight channels were mutually connected with multiple side‐channels. Results showed that calculated flow fields can be categorized into three types of flow pattern depending on flow field geometry and flow conditions. The following typical flow patterns were identified: case 1, the incoming flow passes uniformly through the side channels; case 2, the flow passes preferentially through the side channel in the inlet and the outlet; and case 3, the flow passes mainly through the side channel of the outlet side. Results also indicate that these flow patterns depend on two dimensionless parameters: the ratio of permeability of the side channels to the channel width, and the Reynolds number. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(2): 96–104, 2007; Published online in Wiley InterScience ( www. interscience.wiley.com ). DOI 10.1002/htj.20143