直流互击式喷注单元雾化特性准直接数值模拟

Quasi-Direct Numerical Simulation on Atomization

关键词:  射流撞击雾化  雾化特性  树形结构  自适应加密  VOF  撞击波

Key words:  Impinging jets atomization  Atomization characteristics  Tree-structure  Adaptive mesh refinement  Volume of fluid  Impact wave

摘要:

为实现直流互击式喷注单元雾化过程的数值求解并探究结构及工作参数对雾化特性的影响规律，基于一种树形自适应加密算法与流体容积(Volume-of-Fluid，VOF)方法实现了雾化过程的准直接数值模拟。计算得到了两股射流由喷射到撞击形成液膜，液膜进一步破碎形成液丝、液滴的全过程，获得了液膜的破碎长度、液滴的Sauter平均直径、雾化频率等雾化特性参数。通过将典型算例的计算结果与试验数据进行对比验证了计算的有效性，给出了数值求解精度。探讨了撞击波的形成机理，分析了雾场液滴的尺寸分布规律，撞击夹角、孔径比、射流速度、动量比对雾化特性的影响规律。结果表明:所采用的算法可以实现多相、多尺度雾化过程的数值求解；撞击波的形成是由于两股射流撞击时惯性力的不完全对称导致的；雾场液滴的尺寸分布近似服从Rosin-Rammler分布；撞击夹角的增大与射流速度的提高导致液膜的破碎长度减小，液滴的平均粒径减小，撞击夹角增大雾化频率呈减小的趋势，射流速度提高雾化频率呈增大的趋势；动量比为1而孔径比不为1时会形成凹形液膜，雾场存在一定程度的偏斜；动量比主要影响雾场的偏斜程度。

Abstract:

In order to realize numerical simulation of impinging jets atomization and to investigate the effects of geometry and working parameters on atomization characteristics，Quasi-Direct Numerical Simulation of atomization process was carried out based on tree-structure adaptive mesh refinement algorithm and VOF(Volume-of-Fluid) method. The whole process of a pair of jets from ejecting to liquid sheet formation and breakup of liquid sheet into ligaments and droplets was obtained. The information of breakup length of liquid sheet and Sauter mean diameter of droplets and atomization frequency were also obtained. The simulations were validated by comparing numerical results of typical cases with experimental data. The simulation precision was also given. The formation mechanism of impact wave was explored. Diameter distribution of droplets and the effects of impinging angle，ratio of diameter，jet velocity，ratio of momentum on atomization characteristics were also analyzed. The results show that the multi-phase，multi-scale process of atomization can be numerically simulated with the algorithm used in the paper. The formation of impact wave is the result of uncomplete balance of inertia force when impinging happens. The diameter distribution of droplets approximately obeyed Rosin-Rammler distribution. The increase of impinging angle and jet velocity will lead to decrease of breakup length of liquid sheet and mean droplet diameter. However，the increase of impinging angle will lead to decrease of atomization frequency while increase of jet velocity will lead to increase of atomization frequency. When ratio of momentum is one and ratio of diameter is not one，there forms hollow liquid sheet and to some extent the atomization field also declines. The ratio of momentum mainly affects decline degree of atomization field.