【作者】 刘汉涛；

【导师】 刘谋斌； 苏铁熊； 常建忠；

【作者基本信息】 中北大学 ， 机动武器系统工程， 2012， 博士

【摘要】 介观尺度通道内的多相流动涉及复杂的动力学特性，仅靠理论简化分析、实验半经验的方法以及引入许多经验关系式的数值模拟方法，很难准确描述该复杂流动中的颗粒受力、动量能量的相间传递、相间分界面的变化，影响人们对该流动真实机理的理解和掌握。本文基于任意拉格朗日－欧拉（ALE）方法模拟惰性颗粒两相流动，采用有限元方法数值求解流场的N-S方程，并增加联立求解能量方程，应用牛顿定律追踪颗粒运动，并通过积分颗粒表面的粘性应力和压力获得颗粒的受力，从而实现了对颗粒两相流运动的真正直接数值模拟。通过Delaunay－Voronoi法生成非结构化的三角形单元网格，颗粒移动时，通过求解Laplace方程得到网格移动速度，当单元网格严重变形时网格将重新划分，以确保网格质量。控制方程通过Galerkin法离散，流体与颗粒动量方程通过推导生成一种弱解形式，这样颗粒和流体间相互作用的力和扭矩就不必专门加以计算。颗粒位置的更新将由其速度决定，时间步长由颗粒的速度和加速度来自动调整，方程的非线性部分由牛顿迭代求解，线性部分由GMRES算法来求解。用该方法模拟颗粒的沉降及颗粒溶解引起的相变及相间分界面的形状变化，在介观尺寸上阐明该复杂流动中颗粒的运动规律、相间分界面的移动变形、颗粒相相互作用形成的结构以及传热和流动的相互作用机制，得到颗粒两相流的一些新特征。通过以上方法的研究得到了以下主要结论：（1）热对流引起了流场流动的变化和不对称，颗粒在热流体中沉降，热对流产生的力加速了冷颗粒的运动，尾部形成了涡脱落；颗粒在冷流体中沉降，热对流产生的力阻碍了冷颗粒的运动，尾部形成了羽流；颗粒溶解引起的颗粒表面形态的变化引起了颗粒的横向摆动，并使颗粒沉降速度发生了变化。（2）双颗粒在等温流体中，经历了拖曳、亲和、翻滚后，最终分别在一侧通道壁附近稳定沉降；双颗粒在热流体中沉降，热对流引起了颗粒沉降时的水平方向的摆动，出现了周期的拖曳、亲和、翻滚现象，且颗粒趋于分散；双颗粒在冷流体中沉降，热对流使颗粒的沉降保持稳定，未出现拖曳、亲和、翻滚现象，且颗粒趋于聚集；与等温条件下颗粒沉降相比，溶解双颗粒沉降时，颗粒运动轨迹、颗粒间的相互作用除与颗粒质量相关外，还与对流引起的颗粒尾迹、涡的脱落有关。（3）椭圆颗粒在等温流体中沉降，当椭圆长轴与x坐标轴垂直时，椭圆颗粒要发生旋转，运动行为的变化和摆动；椭圆颗粒在热流体中沉降，颗粒最终以通道中心线为平衡位置在水平方向上周期性摆动；热颗粒在冷流体中沉降，椭圆颗粒在一侧通道壁发生周期性摆动，沉降速度及角速度也出现周期性变化。另外，基于连续力学的网格方法通常很难捕捉边界滑移、热扰动等的影响，也难以适用于含多尺度特性的各种问题，而分子动力学时间及空间尺度通常局限于纳秒和纳米级。有鉴于此，本文基于无网格粒子耗散粒子动力学方法(dissipativeparticle dynamics, DPD)，采用四次方光滑函数构造了远程排斥近距吸引的保守力势函数，对液气两相流动进行了模拟。并完善了DPD方法中的运动颗粒受力、扭矩计算方法、运动控制方程、DPD参数，对固液两相流动进行了计算，阐明DPD方法在介观尺度上研究颗粒两相流动的可行性。对Y型通道内的流动过程，通过调整流固粒子间作用力系数比a_w/ a_f、粒子注入速率、驱动外力等因素，模拟分析了流体在表面张力、重力以及流体与固壁相互作用下的流动过程及流动模式。研究了流动过程形成的不同的多相系统界面和接触线动力学特征。对流体绕流三维球体进行了耗散粒子动力学计算，并与经典关联式进行了对比验证。研究表明，在一定雷诺数范围内，DPD方法能准确的计算出阻力系数，在较大雷诺数时，由于流体的动力学参数及流体压缩性导致计算结果出现差异。低雷诺数时的颗粒沉降计算结果与直接数值模拟结果一致，表明DPD方法对颗粒两相流动的研究具有可行性。更多还原

【Abstract】 Multiphase flows in meso-scale channels involve complicated dynamic behaviors.Analytical theories with simplified assumptions and experimental observations withsemi-empirical empirical formula are usually difficult to portray the forces acting onparticles, the energy and momentum transfer of the fluid-solid system and the changesof interphase boundary. Therefore they significantly influence the understanding theinherent true mechanism of meso-scale multiphase flows.Based on the study of isothermal inertial particle sedimentation, we use afinite-element method to solve the initial value problem for the sedimentation ofparticles in a vertical channel. The algorithm is based on an Arbitrary Lagrangian-Eulerian technique(ALE). The fluid motion is computed from the conservation laws.Momentum balance are governed by the Navier-Stokes(N-S) equations, while theenergy conservation is controled by a convection-diffusion equation, in which thenatural convection is taken into account through thermal expansion. Particles moveaccording to the equations of motion of a rigid body under gravity and hydrodynamicforces arising from the motion of the fluid. An unstructured mesh of triangularelements is generated by the Delaunay-Voronoi method. As a particle moves, the meshalso moves and deforms according to a mesh velocity which is determined by aLaplace equation. When the elements become severely distorted, a re-mesh procedureis carried out to restore mesh quality. The equations are discretized using Galerkinformulation. The solid and fluid momentum equations are combined into one weakform. In this way, the forces and torques acting on the particles are balanced, and thereis no need to compute the forces and torques explicitly. The positions of the particlesare updated according to their velocities, and the time step is automatically adjustedaccording to their velocities and accelerations. The nonlinear part of the governingequation is solved by Newton iteration and the linear parts are solved using thepreconditioned GMRES algorithm. This paper aims to simulate the sedimentation andthe interphase deformation by melting, to investigate forces, trajectory, and to abtainflow field of one or more particles, obtain mechanism of particle-particle interaction with different structures and flow patterns. The main conclusions are as follows:(1) Thermal convection induces asymmetric of the flow field. For cold particles,downward thermal convection accelerates up the sedimentation, The vortex sheddingalso contributes to the movement style of the particles; During the sedimentation of hotparticle in the cold fluid, the warm wake forms a strong upward thermal plume, the hotlayer of fluid next to the particles carry upward momentum, therefore, the hot particleshave smaller vertical velocities, lateral and angular velocity; The interface deformationof melting particles makes the particle oscillate and change the sedimentation velocity.(2) The drafting, kissing and tumbling (DKT) scenario is found during thesedimentation of two isothermal particles. The two particles will settle steadily near thewall finally. For the sedimentation of two cold particles settling, periodic DKTscenario appears and the cold particles tend to disperse; The drafting, kissing andtumbling scenario was not found in the sedimentation of two hot particles and hotparticles tend to aggregate; Compared with isothermal sedimentation, the vortexshedding, mass losing by melting and morphology change the sedimentation velocityand trajectory.(3) The isothermal elliptical particle rotates from parallel to vertical to the x-axisduring the process, and displays weak and somewhat irregular lateral oscillations aboutthe centerline. Elliptical particle in hot fluid develops a regular lateral oscillation alongthe centerline finally. The elliptical particle in cold fluid moves away from centerline,and then develops a regular lateral oscillation about an off-center equilibrium positionwith a periodic velocity and angular velocity.Besides, Grid-based numerical methods within the frame of continuum mechanicsare usually difficult in capturing inherent flow physics such as boundary slip andthermal disturbance. They are also not valid to problems with multiple scale physics.In contrast, molecular dynamics (MD) is practical only on extremely small time scales(nanoseconds) and length scales (nanometers) even if the most advancedhigh-performance computers are used. In this paper, a modified dissipative particledynamics is used, which employs an interaction potential with short-range repulsion and long-distance attraction, and enables the simulation of multiphase fluid flowprocesses. Further more, we studied and improved numerical techniques to calculateforce and torque on solid particles and the governing equation of motion as well as therelated parameters. The sedimentation of a particle is later investigated with DPDmethod, In this way, we demonstrated the effectiveness of the DPD method inmodeling particle-fluid two-phase systems.The multiphase flow through a Y shape mesoscopic channel is simulated bydissipative particle dynamics with this new potential function with differenta_w/a_f afratios of interaction strength coefficients of fluid-fluid and fluid-wall particles,rate of particles injection, external force. The results show that the new method iscapable of simulating the flow process and flow pattern. The flow past athree-dimensional sphere within two parallel plates is also studied with comparisons toclassical results. The results show that the DPD method can predict drag coefficientaccurately while Re is less than 100. When Re is bigger than 100, the results deviatefrom analytical values, mainly due to the fluid compressibility. The sedimentation of asolid sphere ball is studied and compared with the result of DNS, The results show thatit is feasible to simulate particle-fluid two-phase systems using DPD method.更多还原