T型通道内冷热流体混合过程的热波动大涡模拟

通过大涡模拟(LES)对T型通道内冷热流体混合过程的流动与传热情况进行了数值模拟,获得了混合区域内的瞬时速度和瞬时温度,通过时均值和均方根值来描述速度和温度的平均大小和波动程度.数值结果表明,在主管下游离主管和支管交汇中心不远处区域内速度和温度波动最为剧烈.该研究揭示了混合过程热波动的流动与传热本质,获得了温度波动强度与频率,对预测热疲劳和判断易发生区域具有指导意义.     

多孔介质对冷热流体横向射流混合过程热波动的影响

运用流固耦合方法建模,应用FLUENT计算软件平台对填充有多孔介质的T型连接方形管道内冷热流体横向射流混合过程的流动和热传递进行大涡模拟,采用了Smagorinsky-Lilly亚格子模型,获得了瞬时速度和温度分布.结果表明,填充多孔介质能够有效减少T型连接管道中冷热流体横向射流混合的温度和速度波动.固体骨架的导热率较...     

浮升力对T型管道内冷热流体混合过程热波动的影响

利用Fluent软件对T型管道内冷热流体混合过程进行了大涡模拟,考虑了温差引起的浮升力,获得了混合过程的瞬时温度和瞬时速度,通过定义时均值和均方根值来描述温度和速度的平均大小和波动程度.数值模拟结果表明,理查德森数对冷热流体的混合有着重要的影响:当理查德森数为负值且绝对值越大,则温度和速度的波动也就越大;相反,当理查德...     

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。     

共轭梯度法求解竖直环隙间多孔介质的传热反问题

提出一种反演多孔介质热物性参数的一般数值模式。采用有限体积法对竖直环隙间多孔介质模型进行离散化,通过SIMPLE算法获得竖直环隙间多孔介质传热系统的温度场和流体的速度场。通过构建误差信息最小化函数,建立反演模型。利用共轭梯度法对单宗量和多宗量的多孔介质热物性参数进行了反演,并讨论了待反演参数的初始猜测值、温度测点数、温度测量误差等对反演结果的影响。     

不同动量比对湍流穿透现象的影响

在核电工业中,T型管是一种常见的管道结构.在稳压器波动管道的T型接口处,由于主支管动量比很大,会产生湍流穿透现象.针对T型管不同动量比冷热流体混合过程中的湍流穿透现象,使用数值方法进行了研究,通过合理划分网格及设定时间步长,采用大涡模拟方法获得了管内流体的瞬时速度与温度.结果表明:当湍流穿透现象发生时,支管内速度与温度...     

多孔介质对太阳池传热传质影响的研究

通过实验与数值模拟相结合的方法研究在太阳池底部增设多孔介质水层对太阳池热盐双扩散及储热量、稳定性等的影响.在实验室研究了多孔介质对盐扩散的抑制作用.在海边沙滩建造了两个小型太阳池,测量了有无多孔介质太阳池的温度分布和盐度分布并进行数值模拟,模拟值与实验值吻合较好.计算了有无多孔介质太阳池盐梯度的分布、池子稳定性情况以及多孔介质对太阳池的储热量的影响.结果表明,在太阳池底部加设多孔介质水层可以提高太阳池LCZ温度.多孔介质水层有利于提高太阳池的储热量,有利于抑制盐分向上扩散,可以节省太阳池的盐资源消耗,并有利于提高非对流层的盐梯度和稳定性.     

支管结构对T型管内流体混合过程热波动影响

通过CFD计算流体软件FLUENT,分别对支管无附加结构及支管有附加结构直管、渐扩管、减缩管四种结构的T型管内冷热流体混合过程进行了大涡模拟,获得了管道内部的瞬时温度。将各结构温度云图与速度矢量图、无量纲时均温度及无量纲均方根温度进行了对比。数值结果表明,附加结构的添加使管内流体流型由冲击射流变为偏转射流,显著减小了T型管壁上的温度波动;缩管结构的无量纲均方根值比其他附加结构更小,表明缩管结构更适合用以减小管壁的温度波动。     

主支管动量比对T型管道流体混合过程影响

采用大涡模拟(large-eddy simulation,LES)的方法对T型管道内主管与支管不同动量比的流体混合过程的流动情况进行了数值模拟,采用时均值和均方根值来描述速度的平均大小和波动强度。通过改变主管和支管的速度比即动量比,将流体分为三类:碰撞射流、偏射流和壁面射流,研究其对速度的平均值和波动的影响,并研究其所反映的惯性力对流动的影响。该研究揭示了流体混合过程中动量比对波动的影响规律,对预测和校核管壁疲劳失效具有重要的指导意义。     

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。     

Linear analysis of an aerothermal instability occurring in diffusion-controlled premixed catalytic combustion

The onset of the unstable temperature distribution which may appear in plane axisymmetric catalytic burner is studied. The instability mechanism is assumed to be related with the temperature dependence of the viscosity, thus with the pressure drop in the porous combustor, governed by Darcy’s law. An area e.g., of lower temperature (dark zone) characterised by a smaller value of the kinetic viscosity gives rise to a locally increased gas flow mass velocity, the pressure drop remaining constant over the burner cross-section. The locally increased mass velocity produces an enhanced cooling of the area, whereby heat conduction from the hotter surrounding area tends to restore a homogeneous temperature distribution. A linear analysis of this thermal instability mechanism is carried out and yields a simple analytic solution for the state of neutral stability.     

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。     

Flow through a porous medium in the presence of a magnetic field

Unsteady free convective flow of an electrically conducting fluid through a porous medium bounded by an infinite vertical and porous plate is considered. The free stream velocity of the fluid vibrates about a mean constant value and the temperature of the plate is constant. The influences of the permeability parameter and the magnetic parameter on the velocity field are discussed.     

Multimode heat transfer in cyclic flow reversal combustion in a porous medium

Experimental and numerical studies of combustion and multimode heat transfer in a porous medium, with and without a cyclic flow reversal of a mixture through a porous medium, were performed. Parametric studies were done in order to understand combustion characteristics such as maximum flame temperature and radiative heat flux using a one‐ dimensional conduction, convection, radiation and premixed flame model. The porous medium was assumed to emit and absorb radiant energy, while scattering is ignored. Non‐local thermodynamic equilibrium between the solid an d gas is taken into account by introducing separate energy equations for the gas and the solid phase. As a prelimina ry study, the combustion regime was described by a one‐step global mechanism with an internal heat source uniformly dist ributed along the reaction zone. The effects of the flame position, cyclic flow reversal, period of the cyclic flow rever sal, the optical thickness and the flow velocity on the burner performance were clarified by a rigorous radiation analysis. Th e model was validated by comparing the theoretical results with the experiments. It was shown that, for maximizing the fl ame temperature and the net radiative heat flux feedback, the flame should be stabilized near the centre of the po rous medium with a cyclic flow reversal, the period of which should be as small as possible. A high optical thickness prod uced a high flame temperature and a high net radiative feedback. Also, a high flow velocity at low period of the cyclic f low reversal of mixture yielded a high value of both the flame temperature and the net radiative feedback. Thermal structure predictions in terms of the gas‐phase and the solid‐phase temperature distributions along the axis of the combustor show good agreement with the experimental ones. Copyright © 1999 John Wiley & Sons, Ltd.     

Transitional flow patterns behind a backstep with porous-based fluid injection

The flow structure downstream of a backstep with mass injection from a porous base was analyzed both qualitatively and quantitatively in the transitional flow regime of Re_h = 2009–3061. By increasing the wall injection velocity ratio gradually, four distinct flow patterns, shifted from pattern A to B, C and D, were categorized. Pressure distributions of these patterns were dominated by the wall injection velocity ratio, and various downstream-flowing tendencies were produced correspondingly. The effect of flow stabilization by decreasing the Reynolds number became more prominent if the wall injection velocity ratio was increased. Due to the existence of a shear layer, a large value of the Reynolds stress was measured near the tip of the step in pattern A. Once the wall injection was initiated, the local strength of Reynolds stress at the same location was decreased. By increasing the wall injection velocity ratio, the region with decreased level of Reynolds stress extended gradually from the tip of backstep to the streamwise location x = 0.45X_r. The turbulent kinetic energy in pattern A was mostly contributed by the horizontal fluctuation of flow near the backstep in the recirculation zone, and the region with maximum horizontal fluctuation was found to evolve toward the base as the flow moves downstream. However, the weighting of vertical fluctuation became more significant as the wall injection velocity ratio increased.     

Heat and mass transport on MHD flow over semi-infinite moving porous plate with chemical reaction effect

The consequences of chemical reaction, on unsteady magnetohydrodynamic heat and mass transport laminar flows of a viscous, electrically conducting with heat-generating or absorbing fluid enclosed through a semi-infinite absorbent plate has been premeditated. The plate is assumed to be in motion with a constant velocity within the path of fluid flow. A homogeneous magnetic field performs at right angles to the absorbent surface; it is absorbing the fluid with a suction velocity varying with a certain instant of time. The nondimensional governing equations for the present configuration are solved systematically utilizing harmonic and nonharmonic terms. Graphical consequences for the velocity, temperature, and concentration profiles together supported by the investigative solutions are displayed and discussed computationally. The resulting velocity is reducing by an augment in the strength of the magnetic field and Prandtl number, whereas it is enhancing by growing in the permeability of the porous medium. The temperature delivery is reduces by an escalating heat source parameter and occurrence of fluctuation. It is significant to note that the temperature increases notably with growing the radiation absorption parameter. The influences of the chemical reaction and Schmidt number reduced the concentration in the entire fluid medium.     

3D numerical study on syngas production in a structured packed bed with connected pellets

Fuel-rich partial combustion in porous media is numerically studied at the pore scale with 3D staggered arrangement of connected pellets. The chemistry is treated with detailed chemical mechanism and the solid-to-solid radiation is taken into account. The numerical results are validated against experimental results and indicates that 3D pore scale numerical modeling gives reasonable syngas components. Considerable heat recirculation in the preheating zone is predicted by 3D pore-scale model. Root mean squares (RMS) is used to quantitatively estimate the local spatial variations of the velocity and gas temperature. It is shown that the distribution of RMS of gas temperature along flow direction is basically wave-like shape and the spatial variation of gas velocity reaches 78% of the average velocity.     

Analysis of heat transfer with liquid-vapor phase change in a forced-flow fluid moving through porous media

This study focuses on the experimental analysis of transient-regime heat transfer with liquidvapor phase change in a fluid as it flows through a porous media composed of small bronze spheres. Three distinct zones can be observed: liquid, two-phase and superheated vapor. The boundaries between these zones are determined using temperature and pressure fields. An N-shaped profile is observed for the temperature values along the main flow axis. The first local maximum value on the temperature curve corresponds to the boundary between the liquid zone and the two-phase zone. When a local minimum temperature exists, it corresponds to the boundary between the two-phase and the vapor zones. A finite element numerical simulation is used to predict the saturation field, which is numerically determined from the boundaries of the two-phase zone and of the experimental temperature field. The liquid and vapor pressure fields are then deduced for all three phase zones of the porous medium.     

A reciprocal flow filtration combustor with embedded heat exchangers: numerical study

A reciprocal flow filtration combustor with embedded heat exchangers is numerically studied. In this system the combustion of methane and air mixture is stabilized in a transient porous media combustor by periodical switching the direction of the flow. Two heat exchangers are placed in the terminal sections of the porous matrix, constraining the reaction in the central insulated zone. The predicted temperature profile inside the reactor has a typical trapezoidal shape. The central plateau temperature ranges between 1300 and 1600 K as the equivalence ratio varies from 0.15 to 0.7 and the filtration velocity from 15 to 45 cm/s. The efficiency spans the range of 50-80% being higher for higher equivalence ratios and filtration velocities.