旋涡流动中微液滴颗粒的滑移效应

The slip effect of micro-droplets in Rankine vortex

本文利用改进的相间滑移矩方法,数值研究了兰金涡在加入微液滴颗粒后的演变,注重考察相间滑移效应的影响.相间滑移矩方法分别考虑气相与液滴弥散相的运动过程,通过颗粒分布函数各阶矩方程的源项来表达相间质量、动量与能量的传递,从而实现气液两相间的速度滑移效应的模拟.研究结果表明,当向兰金涡中注入液滴微粒后,兰金涡的结构会在液滴滑移效应的扰动下发生一系列的演化,注入微粒的半径不同,演化过程也不相同.当注入微粒的半径较小时,滑移效应较小,微粒会在滑移效应下做离心运动,最终在涡核外呈圆环状分布;当注入微粒的半径足够大时,滑移效应增强导致流动失稳,涡核内产生负涡量,在其作用下颗粒呈现"旋臂"结构而快速向外迁移.最后,本文考察了微液滴颗粒由于凝结增长对旋涡演化和滑移效应的影响,发现微液滴的凝结增长导致更多的气相动量转移到液滴相,使得微液滴与气相的滑移驰豫过程变短,微液滴的跟随性增强,从而使得微液滴与气相的平均滑移速度递减更快.

The method of moments with interphase slip is simplified to numerically study the interaction between nanoscale droplets and gas flow in a Rankine vortex. In the method, the flow of gas phase and the moments of the droplets size distribution are calculated respectively and the transfers of momentum and heat between the gas and the droplets which are expressed by the moments are introduced into the governing equations as the source terms. In this way, the velocity slip between the gas phase and the droplet phase is captured by the present method. Then, the method is used to simulate the flow of Rankine vortex where the water droplets with constant radius are injected into the vortex core. The results show that the vortex is disturbed and the droplets move outward due to the effect of the velocity slip between the phases. The larger the droplet radius, the stronger the velocity slip effect. When the radius of the injected droplets is small, the vortex keeps stable and the droplets aggregate as a ring outside the core of the vortex. When the droplets radii are large enough, the vortex is not stable any more due to the strong slip effect and negative vorticity is induced in the core of the vortex. The droplets form a swing arm and spiral out of the vortex. At last, the effects of the growth of droplets on the vortex evolution and interphase slip are also investigated. The grown droplets spiral out and the position of maximum mean radius also moves outward with time. In the vortex core, condensation stops after a short time because no more vapor is supplied. With the growing process, more momentum is transferred from the gas phase to the droplets and the slip velocity between droplets and gas vanishes more quickly.