等离子喷涂中纳米颗粒悬浮液的蒸发雾化动力学过程研究

Nanoparticle dynamics in suspension plasma spraying is complex, in basic fundamental research lags far behind the application, especially the mechanism of evaporating droplet atomization is unclear. This study is to build a nanoparticle dynamical model for suspension plasma spray based Euler - Lagrangian method, which includes three sub-models of droplet atomization, nanoparticles collision and melting model. Firstly, the KHRT model will be modified to consider the secondary atomization with droplet evaporation, and validated with experiments. Then, a high efficient collision model will be established to simulate particle collision and coalescence. Finally, the former two sub-models will be integrated into the nanoparticle movement and heat transfer model as a complete nanoparticle dynamics model. This research is aimed at: To investigate the mechanism of pneumatic atomization and evaporating atomization for a droplet; To give out the roadmap of how a suspension droplet is atomized under consideration of evaporation; To calculate the droplet atomization time and size distribution with different physical properties of the suspension type, the atomizing jet parameters and the incident mode; To identify the primary parameters of suspension droplet atomization; To simulate nanoparticle collision and coalescence, movement, heating and melting under different spray parameters; To analyze the effects of suspensions and particle properties as well as operating conditions on nanoparticles spatial and size distributions, melting degree and particle Reynolds number; To discuss the relationship between nanoparticles atomization, collision and melting process; To propose a set of indicator parameters for better suspension-plasma-sprayed nanocoatings.

等离子喷涂中，纳米颗粒悬浮液的蒸发雾化动力学基础研究远远落后于应用。本研究基于欧拉-拉格朗日方法，建立一套等离子射流中悬浮液雾化、纳米颗粒碰撞和熔化的动力学模型。该模型包括：基于KHRT雾化，建立带有液体蒸发破碎的雾化模型，并与实验进行对照；建立高效的颗粒碰撞算法，模拟纳米颗粒的碰撞和聚并；沿用现有的纳米颗粒运动和传热模型，并完成全套纳米颗粒动力学模型的整合。拟达到以下目标：研究液滴气动雾化和蒸发破碎的不同物理机理，结合实验研究得出带有蒸发破碎的液体雾化模型；计算不同悬浮液物性、雾化射流参数和入射方式下，液滴破碎所需的时间和粒径分布，找出决定雾化效果的主要参数；模拟不同参数下纳米粒子的雾化、聚并和熔化过程，分析悬浮液、粒子物性及喷涂工况对纳米颗粒的空间和粒径分布、熔化度和雷诺数的影响；讨论纳米颗粒雾化、碰撞与熔化过程的相互关系，提出优化纳米喷涂效果的指标参数，为工程应用提供理论依据。

等离子体喷涂是一项新兴的材料技术，其中，纳米颗粒悬浮液的蒸发雾化是重点和难点。本项目分别基于N-S方程和颗粒轨道法，建立了纳米等离子喷涂动力学程序，优化了粒子蒸发雾化模型和碰撞模型，完成了数值模拟系统的整合和验证。蒸发雾化的研究基于KHRT模型，改进了原有雾化模型只能考虑气动力驱动的雾化机理，即KH波动的局限性，加入了RT波动机制来模拟液滴的二次雾化。颗粒碰撞的研究基于TAB模型，考虑了碰撞对颗粒群的影响。项目分析了不同工况条件下单个液滴的参数变化，及液滴群在等离子射流场中的分布特性，讨论了颗粒的蒸发、雾化、碰撞、聚并和熔化等不同物理过程及其内在关系，归纳了影响等离子喷涂质量的主要因素。同时，考察了悬浮液的物性、等离子气体成分和基板位置等三个主要喷涂工况对粒子的平均粒径、温度、速度、质量分数等的影响，并根据研究结果给出了等离子喷涂工艺参数和操作条件的指导原则。