Dynamic modeling of micro- and nano-sized particles impinging on the substrate during suspension plasma spraying

题目：悬浮等离子体喷涂过程中微纳米颗粒撞击基板的动力学模拟目的：研究微纳米颗粒在流场中的运动和传热特性，确定颗粒绕流的临界尺寸以及微纳米颗粒合适的喷涂距离。创新点：1．建立微纳米颗粒的受力和运动模型；2．推导颗粒粒径和斯托克斯数的关系，确定颗粒绕流的临界尺寸；3．确定适于微纳米颗粒的喷涂距离。方法：1．通过颗粒运动和传热的三维模型，模拟颗粒在等离子体流场中的运动和传热过程；2．对流场采用欧拉法进行求解，对颗粒采用拉格朗日法进行求解；3．动态追踪颗粒的轨迹和空间分布，从而得到颗粒的速度、温度和空间分布。结论：1．布朗力会影响纳米颗粒的分布；现有模型可以很好地模拟微纳米颗粒的行为。2．可以用斯托克斯数和粒径表征微纳米颗粒绕流的临界尺寸；当前工况下，临界粒径约为800nm。3．基板会影响流场结构和颗粒的空间分布；在当前研究中，得出有利于纳米颗粒沉积的喷涂距离约为50mm；对微米颗粒来说，喷涂距离应适当增大。4．微纳米颗粒的空间分布呈现不同的特点；纳米颗粒的分布区间更大，布朗力对纳米颗粒的作用比对微米颗粒更为显著。5．微纳米颗粒的运动和传热过程呈现不同的特点；纳米颗粒的惯性和热容小，因此它们的速度和温度变化更迅速。

Suspension plasma spraying （SPS） can be utilized to manufacture finely structured coatings. In this process, liquid suspended with microor nano-sized solid particles is injected into a plasma jet. It involves droplet injection, solvent evaporation, and discharge, acceleration, heating, and melting of the solid particles. The high-speed and high-temperature particles final- ly impact on the substrate wall, to form a thin layer coating. In this study, a comprehensive numerical model was developed to simulate the dynamic behaviors of the suspension droplets and the solid particles, as well as the interactions between them and the plasma gas. The plasma gas was treated as compressible, multi-component, turbulent jet flow, using Navier-Stokes equations solved by the Eulerian method. The droplets and solid particles were treated as discrete Lagrangian entities, being tracked through the spray process. The drag force, Saffman lift force, and Brownian force were taken into account for the aerodynamic drag force, aerodynamic lift force, and random fluctuation force imposed on the particles. Spatial distributions of the micro- and nano-sized particles are given in this paper and their motion histories were observed. The key parameters of spray distribution, including particle size and axial spray distance, were also analyzed. The critical size of particle that follows well with the plasma jet was deduced for the specified operating conditions. Results show that in the downstream, the substrate influences the flow field structure and the particle characteristics. The appropriate spray distances were obtained for different microand nano-sized particles.