冷喷涂中,粒子撞击基板时的速度对涂层形成起到至关重要的作用,而适当的喷涂距离可以使喷涂粒子获得较高的速度.利用商用CFD软件Fluent模拟了喷嘴外超音速流场的分布情况,分析了粒子自身条件对冲击速度的影响,同时提出了确定粒子与基板最佳距离的方法.分析发现,小尺寸、低密度以及不规则形状的粒子容易受到喷嘴外产生的膨胀波、斜激波和基板前的弓形激波影响.随着能量的耗散或当粒子穿过弓形激波时,这些粒子的速度明显下降.另外,在基板与喷嘴之间存在一个最佳距离,它的确定需要两个必要条件：一是基板处的气流必须保持较低的速度%The impact velocity of particles plays an important role in coating formation of cold spray.A proper standoff distance from the nozzle exit to the substrate surface can ensure sprayed particles to achieve relatively high impact velocities.Numerical simulations were conducted by using a CFD software,Fluent,to predict the supersonic flow field outside the nozzle.Based on the simulation results,the effect of particle conditions on impact velocity is studied and also the way to determine the optimal standoff distance is proposed.It is found that particles with small size,low density or irregular shape can be significantly affected by the dilatational waves,oblique shock waves and bow shock waves.With the energy dissipation or passing through the bow shock waves,these particles are decelerated sharply.Moreover,there exists an optimal standoff distance for different conditions.The determination of the optimal standoff distance requires two essential considerations： One is that the substrate must be located at the place where the carrier-gas velocity is relatively low to weaken the strength of the bow shock waves;the other is that particles should be accelerated twice by dilatational waves before entering the bow shock waves,which means there must be two groups of dilatational waves in front of the bow shock waves.
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