Modelling of Phase Change Heat Transfer System for Micro-channel and Chaos Simulation

The dynamic properties for the micro-channel phase change heat transfer system are studied by theoretical method combined with experiment. Liquid-vapour interface dynamic systems are obtained by introducing disjoining pressure produced by three phase molecular interactions and Lie algebra analysis. Experiments for 0.6 mm×2 mm rectangular micro-channel are carried out to obtain the pressure time serials. Power spectrum density analysis for these serials shows that the system is in chaotic state if the frequency is above 7.39 Hz. The result indicates that the high heat transfer performance of the micro channel phase change system may relate to the characteristics of chaos, The chaos attractor is drawn by the simulation of the obtained differential dynamic system under the conditions of our experiment.     

Fourier and Wavelet Transform Analysis of Pressure Signals during Explosive Boiling

The transient pressure in a liquid-pool during explosive boiling of acetone is measured by a micro-pressure-measuring system. The Fast Fourier transform and continuous wavelet transform methods are applied to investigate the frequency characteristics. The results show that the dominant frequency of the explosive boiling is 0-2 MHz, and the bubble cluster formed by numerous tiny bubbles departs twice. Analysis and discussions are also conducted to explain the bubble evolution during the explosive boiling.     

Heat Conduction and Characteristic Size of Fractal Porous Media

Based on fractal theory, two types of random Sierpinski carpets （RSCs） and their periodic structures are generated to model the structures of natural porous media, and the heat conduction in these structures is simulated by the finite volume method. The calculated results indicate that in a certain range of length scales, the size and spatial arrangement of pores have significant influence on the effective thermal conductivity, and the heat conduction presents the aeolotropic characteristic. Above the length scale, however, the influence of size and spatial arrangement of pores on the effective thermal conductivity reduces gradually with the increasing characteristic size of porous media, the aeolotropic characteristic is weakened gradually. It is concluded that the periodicity in structures of porous media is not equal to the periodicity in heat conduction.     

多孔介质内混溶流体间粘性指进现象的格子Boltzmann模型

提出一个模拟多孔介质内混溶流体间粘性指进现象的格子Boltzmann模型.采用双分布函数分别求解压力场和浓度场.在浓度场平衡态分布函数中引入与浓度扩散相关的参数.通过调节参数,使粒子碰撞过程中的松弛时间保持恒定.模拟了粘度相同的流体间的混相驱替问题.不同网格下的模拟结果均与解析解吻合良好,验证了模型的可行性.进一步研究粘度比和贝克莱数(Pe)对粘性指进现象的影响.结果显示,增大粘度比会促进"手指"的增长.当粘度比不变时,存在Pe的临界值.当Pe超过临界值时,"手指"前缘会出现分裂现象.对横向平均浓度场的研究显示,混合区域的长度随时间的变化分为两个阶段,它首先随着t^1/2成线性增长,然后随着t成线性增长.     

Numerical Simulation of Vapor Bubble Growth and Heat Transfer in a Thin Liquid Film

A mathematical model is developed to investigate the dynamics of vapor bubble growth in a thin fiquid film, movement of the interface between two fluids and the surface heat transfer characteristics. The model takes into account the effects of phase change between the vapor and liquid, gravity, surface tension and viscosity. The details of the multiphase flow and heat transfer are discussed for two cases： （1） when a water micro-droplet impacts a thin liquid film with a vapor bubble growing and （2） when the vapor bubble grows and merges with the vapor layer above the liquid film without the droplet impacting. The development trend of the interface between the vapor and liquid is coincident qualitatively with the available literature, mostly at the first stage. We also provide an important method to better understand the mechanism of nucleate spray cooling.     

A Lattice Boltzmann Kinetic Model for Microflow and Heat Transfer

In this paper, we propose a lattice Boltzmann BGK model for simulation of micro flows with heat transfer based on kinetic theory and the thermal lattice Boltzmann method (He et al., J. Comp. Phys. 146:282, 1998). The relaxation times are redefined in terms of the Knudsen number and a diffuse scattering boundary condition (DSBC) is adopted to consider the velocity slip and temperature jump at wall boundaries. To check validity and potential of the present model in modelling the micro flows, two two-dimensional micro flows including thermal Couette flow and thermal developing channel flow are simulated and numerical results obtained compare well with previous studies of the direct simulation Monte Carlo (DSMC), molecular dynamics (MD) approaches and the Maxwell theoretical analysis     

A Novel Kinetic Model of Liquid Nitrogen＇s Explosive Boiling at the Initial Stage

The liquid nitrogen＇s explosive boiling characteristics under transient high heat flux have attracted increasing attentions of researchers over the world due to its wide applications. Although some experiments have been performed, the process and the characteristics at the initial stage, especially within 1 ~s, have not been described reasonably yet. Based on the related experiments and theoretical analysis, a novel kinetic model combined with quasi-fluid idea is presented to analyse the characteristics of liquid nitrogen＇s explosive boiling at the initial stage. The results indicate that the model can appropriately describe the liquid nitrogen＇s explosive boiling. The behaviour and the heat transfer characteristics of a single bubble are very different from those of the bubble cluster, thus the behaviour of individual bubbles could not be directly applied to describe the explosive boiling process at the initial stage.     

多孔介质内交变流动与换热的格子Boltzmann研究

针对制约脉冲管制冷机效率提高的蓄冷器内交变流动与换热问题,本文使用格子Boltzmann方法计算并分析了多孔介质内交变流动与换热特性。结果表明,交变流动压力波振幅对速度与压力和温度间的相位差的影响很小;通道界面处速度与压力的相位差在某一孔隙率下有一极小值;多孔介质内最大阻力系数与雷诺数的关系可以为设计蓄冷器时填料结构的...     

Approximate Analytical Solutions for a Class of Laminar Boundary-Layer Equations

A simple and efficient approximate analytical technique is presented to obtain solutions to a class of two-point boundary value similarity problems in fluid mechanics. This technique is based on the decomposition method which yields a genera/analytic solution in the form of a convergent infinite series with easily computable terms. Comparative study is carried out to show the accuracy and effectiveness of the technique.     

致密多孔介质中气体渗流的格子Boltzmann模拟

为了能用格子Boltzmann方法来正确地刻画致密多孔介质中微尺度流动问题,对单通道模型进行推广,并将其应用于孔隙群里气体渗流的数值模拟.通过对部分具有代表性的多孔介质中的实际流动问题进行模拟,研究渗透率与平均压力和Knudsen数之间的相互关系.基于理论分析及相关文献中的试验结果,验证模拟结果,为用格子Boltzmann方法深入研究气体渗流问题奠定基础.     

Series Solution for Unsteady Boundary-Layer Flows Due to Impulsively Stretching Plate

The third-order nonlinear partial differential equation modelling the unsteady boundary-layer flows caused by an impulsively stretching fiat plate is solved by using the Adomian decomposition method （ADM）. The ADM yields analytic solution in the form of a rapidly convergent infinite series with easily computable terms. The series solution using the ADM for the unsteady flows is presented for the first time.     

基于格子Boltzmann方法的多孔介质流动模拟GPU加速

利用NVIDIA CUDA平台,在GPU上结合稀疏存贮算法实现基于格子Boltzmann方法的孔隙尺度多孔介质流动模拟加速,测试该算法相对基本算法的性能.比较该算法在不同GPU上使用LBGK和MRT两种碰撞模型及单、双精度计算时的性能差异.测试结果表明在GPU环境下采用稀疏存贮算法相对基本算法能大幅提高计算速度并节省显存,相对于串行CPU程序加速比达到两个量级.使用较新构架的GPU时,MRT和LBGK碰撞模型在单、双浮点数精度下计算速度相同.而在较上一代的GPU上,计算精度对MRT碰撞模型计算速度影响较大.     

Lattice Boltzmann方法模拟多孔介质Klinkenberg效应

格子Boltzmann数值模拟方法是研究复杂的多孔介质结构特别是Klinkenberg效应的有效方法之一，对处理复杂边值问题尤其有效，用格子Boltzmann方法研究了气流穿越多孔介质问题，并将数值计算结果与实验结果进行了比较，结果表明格子Boltzmann方法是数值模拟气流穿越多孔介质问题的有效方法之一。     

Transition to Film Boiling in Microgravity： Influence of Subcooling

The transition process to film pool boiling in microgravity is studied experimentally aboard the Chinese recoverable satellite S J-8. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Small, primary bubbles are formed and slid on the surface, which coalesce with each other to form a large coalesced bubble. Two ways are observed for the transition from nucleate to film boiling at different subcoolings. At high subcooling, the coalesced bubble with a smooth surface grows slowly. It is then difficult for the coalesced bubble to cover the whole heater surface, resulting in a special region of transition boiling in which nucleate boiling and local dry areas can coexist. In contrast, strong oscillation of the coalesced bubble surface at low subcooling may cause rewetting of local dry areas and activation of more nucleate sites, resulting in an abrupt transition to film boiling.     

Temperature-Dependent Viscosity Effects on Non-Darcy Hydrodynamic Free Convection Heat Transfer from a Vertical Wedge in Porous Media

An analysis is presented to investigate the effect of temperature-dependent viscosity on free convection flow along a vertical wedge adjacent to a porous medium in the presence of heat generation or absorption. The governing fundamental equations are transformed into the system of ordinary differential equations using scaling group of transformations and are solved numerically by using the fifth-order Rung-Kutta method with shooting technique for various values of the physical parameters. The effects of variable viscosity parameter on the velocity, temperature and concentration are discussed. Numerical results for the problem considered are given and illustrated graphically.     

考虑双向耦合效应的格子Boltzmann气固两相湍流模型

在流体粒子概率密度函数输运方程中考虑颗粒对流体的反作用力,发展了考虑双向耦合效应的LB气固两相流模型,引入Smagorinsky亚格子模型模拟高雷诺数气相流场.对经典后台阶气固两相流动进行模拟,气相和颗粒相速度分布与实验结果进行比较,发现考虑双向耦合效应的LB气固两相流模型结果明显优于单向耦合结果.进一步研究不同惯性颗粒在流场中的弥散特性,小颗粒(St~O(0.1))对流体的跟随性较好,在流场中分布较为均匀;而St~O(1)的颗粒难被流场涡卷吸进入涡内,呈现倾向性弥散现象;大颗粒(St~O(10))由于自身惯性进入流场涡,在流场中分布较为均匀.     

Heat and mass transfer analysis on the flow of a second grade fluid in the presence of chemical reaction

The laminar flow problem of convective heat transfer for a second grade fluid over a semi-infinite plate in the presence of species concentration and chemical reaction is investigated. The governing equations are transformed into a dimensionless system of three non-linear coupled partial differential equations. These equations have been solved analytically subject to the relevant boundary conditions by employing a homotopy analysis method (HAM). It is noted that for the arising system, the HAM performs extremely well in terms of efficiency and simplicity. The influence of dimensionless pertinent parameters on the velocity, temperature and concentration fields has been examined carefully.     

格子Boltzmann方法模拟自然对流作用下融化传热过程

建立自然对流作用下融化的格子Boltzmann双分布函数模型,根据非线性对流扩散方程的格子Boltzmann模型理论提出一个新的表征融化温度场的分布函数演化方程,并通过变松弛时间方法处理固液两相变热物性传热问题.应用模型对热传导融化及自然对流融化特别固液变热物的融化过程进行模拟.模拟结果与分析解、经典的关联式结果吻合较好,模型的正确性得到了验证.模拟结果表明,自然对流对融化传热过程有着重要的影响,此外固相热传导也对融化传热、融化速率及固液两相温度分布都有一定影响.     

Implementation of Multi-GPU Based Lattice Boltzmann Method for Flow Through Porous Media

The lattice Boltzmann method (LBM) can gain a great amount of performance benefit by taking advantage of graphics processing unit (GPU) computing, and thus, the GPU, or multi-GPU based LBM can be considered as a promising and competent candidate in the study of large-scale fluid flows. However, the multi-GPU based lattice Boltzmann algorithm has not been studied extensively, especially for simulations of flow in complex geometries. In this paper, through coupling with the message passing interface (MPI) technique, we present an implementation of multi-GPU based LBM for fluid flow through porous media as well as some optimization strategies based on the data structure and layout, which can apparently reduce memory access and completely hide the communication time consumption. Then the performance of the algorithm is tested on a one-node cluster equipped with four Tesla C1060 GPU cards where up to 1732 MFLUPS is achieved for the Poiseuille flow and a nearly linear speedup with the number of GPUs is also observed.     

Simulating Solid-Liquid Phase-Change Heat Transfer in Metal Foams via a Cascaded Lattice Boltzmann Model

A cascaded lattice Boltzmann (CLB) model is constructed for simulating heat transfer in metal-foam-based solid-liquid phase change materials (PCMs). The present model captures the phase interface implicitly via the enthalpy methodology, and to avoid iterations in simulations, the CLB equation of the PCM employs the enthalpy as the basic evolution variable through modifying the cascaded collision process. Numerical results demonstrate the effectiveness and practicability of the CLB model for investigating heat transfer in solid-liquid PCMs with metal foams. The effects of the inertial coefficient, permeability and porosity on the melting process are investigated. The results indicate that the empirical correlations of inertial coefficient and permeability based on packed beds overestimate the melting rate at high porosities. Moreover, the porosity has significant impact on phase-change processes. The melting rate increases as the porosity of the metal foam decreases since heat conduction through high thermal conductive metal foam dominates the total heat transfer.