氮液滴在气流中的破碎和碰撞模拟

Numerical study on the breakup and collision of nitrogen droplets in high-speed gas flow

为揭示喷雾冷却中雾场氮液滴的行为,基于流体体积法(VOF)建立氮液滴的碰撞模型,对氮液滴在高速气流下的不同碰撞过程进行数值模拟,研究碰撞过程中液滴形态及气隙压力的变化情况.设定相同与不同液滴尺寸的液滴碰撞,以及对心与偏心液滴碰撞等4种不同条件,分析在各种碰撞条件下,碰撞初始条件改变对于液滴形态与后续液滴大小的影响.数值计算结果表明:高速气流下液滴发生碰撞时,其破碎形态主要由气流速度决定,而碰撞参数B主要决定液滴拉伸形成韧性带的方向;液滴接触时形成的气体间隙,其压力呈现先增大后减小的趋势;与对心碰撞过程相比,偏心碰撞时气隙压力会更快释放;相同尺寸对心碰撞的后续液滴分离情况最差,这是由于韧性带沿着气流方向,导致液膜没有明显破碎,对于后续液滴蒸发不利.

To obtain the droplet behavior in spray cooling,a CFD model based on volume of fluid（ VOF） approach is built to study the droplet collisions in the high-speed gas flow. The changes of droplet shape and gap pressure during the collision process are determined. Four different conditions are considered including the collisions between two droplets of same or different size,the center collision and eccentric collision. The impact of initial conditions on droplet morphology and the subsequent droplets size are discussed under various collision conditions. The results show that the breakup form mainly depends on the velocity of gas flow when droplets collide in high-speed gas flow.Collision parameter B mainly determines the direction of the elongated ligament. A gas gap is formed before droplets merge,and the pressure in the gap increases at the beginning of the collision and then decreases to gas pressure.Since the merged droplet can rotate during the eccentric collision process,pressure in the gas gap can release faster compared with center collision process. The droplet detachment is the poorest in the collisions between droplets of same size because the elongated ligament is in the direction of gas flow. Under this condition,there is no breakup of the liquid film,and therefore the droplet evaporation is worst.