等离子体湍射流中微纳米颗粒的运动和传热过程

发布时间：2018-06-19 19:20

本文选题：多相流 + 悬浮液等离子体喷涂　；参考：《浙江大学》2016年博士论文

【摘要】：等离子体喷涂是一项重要而复杂的表面处理技术,利用该技术可以生产出高质量的纳米结构涂层。等离子体喷涂以微纳米粉体作为喷涂材料,以等离子体射流作为加速和熔化粉体的气体介质。等离子体喷涂过程涉及到许多复杂现象,比如,等离子体射流的产生,颗粒的入射,等离子体流场和颗粒之间质量、动量和热量的传递等。等离子体喷涂的理论研究远远落后于该技术的发展,本文旨在研究微纳米颗粒在等离子体湍射流中的动力学和热力学行为以及流场与颗粒之间的输运现象;从机理上分析喷涂过程中的各种现象,研究影响喷涂过程的参数以期对喷涂过程进行优化。本文采用欧拉法和拉格朗日法相结合的理论,建立了高温等离体子射流中微纳米颗粒的受力、传热和相变的三维计算机模型。首先采用欧拉法求解流场,将等离子体湍射流场假设为多组分、可压缩、存在化学反应的理想气体,并用显式格式求解流场的瞬态过程。将计算得到的射流场温度与实验进行了对比,结果表明两者吻合良好,误差在10%以内。在已知流场信息和颗粒初始条件的情况下,采用基于拉格朗日描述的颗粒轨道法追踪不同颗粒的运动,用一维球形热传导模型求解液滴表面的对流传热、溶剂的蒸发和微米颗粒内部的热传导,采用微纳米颗粒多相流模型模拟多个微纳米粒子的受力、运动、雾化和碰撞等过程。利用该三维数值模型,论文深入研究了单颗粒及多颗粒的动力学行为。首先对等离子体喷涂中的悬浮微纳米颗粒进行了详细的受力分析,比较了单个颗粒主要受到的拖曳力、Saffman力和布朗力的量级,并通过分析Stokes数,验证了当前流场参数下颗粒绕过基板边界层的临界粒径。其次,考虑了多颗粒之间的碰撞以及颗粒与流场之间的双向耦合,定量分析了颗粒入射对流场速度和温度的影响,比较了微米颗粒群与纳米颗粒群之间不同的运动和分布特点,研究了湍流扩散对多颗粒分布的影响。论文对不同工况的喷涂过程进行了参数化研究。重点分析了输入功率和气体组分对流场以及颗粒参数的影响,研究了喷涂距离和基板尺寸对颗粒沉积效率的影响,比较了雾化入射和粉体入射两种入射方式对颗粒传热过程的影响,研究了入射位置、入射角度、液滴和聚合物尺寸对多颗粒释放百分数的影响。确定了有利于等离子体喷涂的工况参数。本文通过计算观察到了实验无法观测到的单颗粒和多颗粒的运动过程,全面的参数分析对喷涂过程有重要的理论指导意义。
[Abstract]:Plasma spraying is an important and complex surface treatment technology, which can be used to produce high quality nanostructured coatings. Plasma spray is made of micro and nano powder and plasma jet is the gas medium for accelerating and melting the powder. The plasma spray process involves many complicated phenomena, such as the generation of plasma jet, the incidence of particles, the mass of plasma flow field and particles, the transfer of momentum and heat, etc. The theoretical study of plasma spraying is far behind the development of this technology. The purpose of this paper is to study the kinetic and thermodynamic behavior of microparticles in the plasma turbulent jet and the transport between the flow field and particles. In order to optimize the spraying process, this paper analyzes the various phenomena in the process of spraying and studies the parameters that affect the process of spraying. Based on the theory of Euler method and Lagrangian method, a three-dimensional computer model of the force, heat transfer and phase transition of micro and nanocrystalline particles in high temperature isobaric jet has been established in this paper. The flow field is solved by Euler method. The plasma turbulent jet field is assumed to be a multicomponent compressible ideal gas with chemical reaction and the explicit scheme is used to solve the transient process of the flow field. The calculated temperature of the jet field is compared with the experimental results, the results show that the two agree well, and the error is less than 10%. In the case of known flow field information and particle initial conditions, the particle orbit method based on Lagrangian description is used to track the motion of different particles, and the one-dimensional spherical heat conduction model is used to solve the convection heat transfer on the liquid drop surface. The evaporation of solvent and the heat conduction in micron particles were used to simulate the force motion atomization and collision of multiple microparticles by using the multiphase flow model of micro- and nanocrystalline particles. Using the three-dimensional numerical model, the dynamic behavior of single particle and multi-particle is studied in depth. Firstly, the force of suspended microparticles in plasma spraying is analyzed in detail, and the order of magnitude of Saffman force and Brownian force is compared, and Stokes number is analyzed. The critical particle size of particles bypassing the boundary layer of the substrate is verified under the current flow field parameters. Secondly, considering the collision between particles and the bidirectional coupling between particles and flow field, the effects of velocity and temperature of particle incident flow field are quantitatively analyzed, and the different motion and distribution characteristics between micron particle group and nano-particle group are compared. The effect of turbulent diffusion on the distribution of multi-particles was studied. The parameterization of spray process under different working conditions was studied in this paper. The effects of input power, gas component flow field and particle parameters on the particle deposition efficiency are analyzed. The effects of spray distance and substrate size on particle deposition efficiency are studied. The effects of atomization incidence and powder incidence on the heat transfer process of particles were compared. The effects of incident position, incident angle, droplet size and polymer size on the percentage of multi-particle release were studied. The working conditions of plasma spraying were determined. In this paper, the motion processes of single or multiple particles which can not be observed in experiments are observed by calculation. The comprehensive analysis of parameters is of great theoretical significance to the spray process.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O53;O358

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