悬浮液冷喷涂的液滴蒸发及加速过程研究

Investigation on Evaporation and Acceleration Process of Droplets During Suspension Cold Spraying

摘要:

超细粉末的流动性差，导致其需要以悬浮液或气浮的方式输送，而冷喷涂时超细粉末悬浮液输送常选择在喷枪出口处，这限制了大面积稳定涂层的制备。针对该现状，采用基于离散相的Multicomponent模型，模拟了悬浮液液料在喷枪前端送料时液滴的蒸发及加速过程，并探讨了颗粒沉积的可行性。模拟结果表明，在不同压力和温度下，液滴沿喷枪轴线加速时，由于悬浮液液滴含量较少及与主气温度差值较大，导致悬浮液中的液相与预热气体之间发生强烈的传热传质，液相可在喷枪喉部前完全蒸发。对蒸发过程影响因素的研究结果表明：相同气体压力下随着预热温度的提升，悬浮液液滴蒸发速度更快；液滴直径越小，则完全蒸发所对应的喷枪位置越靠前；固含量越低，完全蒸发所对应的喷枪轴向位置越靠近喷枪喉部；气体压力和材料种类对液相蒸发过程无明显影响。采用幂函数拟合方式，对悬浮液液滴的蒸发过程进行拟合而得到拟合公式，其中pl值反映了液滴蒸发的变化趋势、x_c值的变化趋势基本反映了蒸发完全所对应的喷枪轴线位置的变化。对Cu悬浮液液滴蒸发加速的模拟结果表明：Cu颗粒与基体碰撞瞬间的速度值大于600 m?s^-1，与相关研究得到的Cu颗粒沉积速度范围610—630 m?s^-1相近，表明在喷枪前端注入的液料经过蒸发及加速后可以沉积到基体表面而形成涂层。

Abstract:

Due to the poor fluidity of ultra-fine powder， it needs to be transported by suspension or gas flotation. At present， the delivery of ultra-fine powder suspension in cold spraying is often selected at the outlet of the spray gun， which limits the preparation of large-area stable coatings. In view of the current research status， this study adopts the Multicomponent model based on discrete phases to simulate the evaporation and acceleration process of droplets when the suspension liquid is fed at the front of the spray gun， and discusses the feasibility of particle deposition. The simulation results show that when the droplets accelerate along the spray gun axis under different pressures and temperatures， due to the small droplet content of the suspension and the large temperature difference with the main gas， intense heat and mass transfer occurs between the liquid phase in the suspension and the preheated gas， the liquid phase can be completely evaporated before the throat of the spray gun. The research on influencing factors of the evaporation process shows that： under the same gas pressure， as the preheating temperature increases， the evaporation rate of the suspension droplets is faster； the smaller the droplet diameter， the closer the position of the spray gun to the nozzle inlet corresponding to complete evaporation； the lower the solid content， the closer the axial position of the spray gun to the throat of the spray gun corresponding to complete evaporation； gas pressure and material type have no obvious influence on the liquid phase evaporation process. The evaporation process of the suspension droplet is fitted using the power function fitting method. In the obtained fitting formula， the pl value reflects the changing trend of the droplet evaporation， and the changing trend of the x_c value basically reflects the complete evaporation process corresponding to changes in the position of the spray gun axis. The simulation results of the evaporation acceleration of Cu suspension droplets show that the impact velocity of Cu particles at the substrate is greater than 600 m?s^-1， which is similar to the deposition velocity range of 610—630 m?s^-1 for Cu particles obtained in related studies. Therefore， the simulation results of this study show that the liquid material injected at the front end of the spray gun can be deposited on the substrate to prepare coatings after evaporation and acceleration.