低Reynolds数雨滴表面流体滑移及重力效应对粒子清除的影响

Impact of velocity slip and gravitational effects on aerosol scavenging by raindrops at low Reynolds numbers

采用拉格朗日法求解了低Reynolds数雨滴以终端下落速度时对大气气溶胶粒子的捕集效率,为考察雨滴表面流体滑移效应,采用Hadamard-Rybczynski流场描述低Reynolds数下雨滴的绕流特征,数值计算中考虑了粒子的惯性碰撞与拦截捕集机理,分析和讨论了雨滴表面流体滑移及粒子重力效应作用对粒子捕集效率的影响.结果表明,若忽略雨滴表面流体滑移及粒子的重力效应,则数值计算结果与已有分析结果一致.当考虑雨滴表面流体滑移效应,则粒子捕集效率有一定程度增加,增加幅度与雨滴尺寸、粒子尺度及粒子密度等参量均有关.粒子所受的重力对雨滴捕集粒子的能力起削弱作用,其削弱程度依赖于粒子拦截参数(R)和Stokes数(St)的大小,在0.2<St<0.3区间,重力作用对粒子捕集的削弱作用最为显著.

Aerosol capture efficiency by a falling small raindrop at its terminal velocity with a low Reynolds number was obtained from trajectory calculation by using Lagrangian method. The flow field around the raindrop was described by Hadamard-Rybczynski flow considering the velocity slip effect at raindrop surfaces. The main capture mechanisms were considered to be inertial impaction and interception. The effects of velocity slip at raindrop surfaces and the gravity on particle capture efficiency by a single raindrop were discussed and analyzed by numerical simulations. The results showed that the calculated capture efficiencies in the absence of velocity slip at the raindrop surface and the gravitational forces on particles agreed reasonably well with previous theoretical studies. The calculated aerosol capture efficiency was enhanced with the consideration of the slip effect,and the increment of the efficiency was dependent on raindrop size,particle diameter and density,etc. With the influence of the gravitational effect,the capture capacity of the raindrop for particles was reduced along with interception parameter（ R） and Stokes number（ St）,and the capture efficiency minimum existed in the range of 0.2St0.3.