基于树形自适应网格的旋流液膜雾化过程仿真

Simulation on Swirl Liquid Sheet Spray Process Based on Octree Adaptive Mesh Refinement

关键词:  旋流液膜  雾化破碎过程  自适应网格加密技术  Gerris

Key words:  Swirl liquid sheet  Spray and breakup process  Adaptive mesh refinement technique  Gerris

摘要:

为实现旋流液膜雾化过程的数值精确求解，基于Gerris采用的自适应网格技术和VOF方法，建立了一种模拟旋流液膜雾化过程的数值方法，分析了雾化破碎过程及三维雾场特征。研究结果表明:Gerris能够逼真地展示旋流液膜破碎成液丝、液丝进一步破碎成液滴全过程的细节特征，雾化破碎过程图像与实验拍摄的基本吻合；通过统计分析计算的带旋转速度的直射流雾化过程全场液滴粒径空间分布，与文献中实验测量值也吻合较好，分布曲线峰值对应的液滴直径的差值为1.8μm，相对误差为13.8%，表明建立的计算方法具有较高的准确性。另外，通过对旋流液膜破碎过程的精细仿真，对其有了更清楚的认识，液膜雾化过程中存在二次雾化现象，液丝在运动过程中受到气体力和表面张力的作用，开始断裂形成大液滴或液团，随着进一步运动收缩破碎成小液滴，液滴形状渐渐由不规则的柱形变成类球形。带旋转速度的直射流和空心旋流式锥形液膜的液滴空间分布存在不同，前者液滴在锥形区域内都有分布，而后者液滴只分布在锥形液膜两侧的环形区域。

Abstract:

In order to realize accurately numerical simulation of swirl liquid sheet spray process， the numerical method based on adaptive mesh refinement technique and VOF method in Gerris was established， which could simulate the spray process of swirl liquid sheet. The spray breakup process and 3D spray field characteristics were analyzed. The results show that the simulation of the breakup process of swirl liquid sheet in Gerris is very refined. The characteristics of the whole spray process are captured accurately， which consists of liquid sheet breaking up and disintegrating into ligaments or droplets. The computational results are basically in good agreement with the breakup process images of conical liquid sheet photographed in experiment. In the jet injector with rotational velocity， the spatial distribution of the droplet simulated in this article is also in good agreement with the experimental result in the literature by using statistical and analytical methods. The difference between the peak value of the distribution curves corresponding to the droplets diameters is 1.8μm， and the relative error is 13.8%， which shows that the numerical method established in this article is relatively accurate. In addition， the breakup process of swirl liquid sheet is understood more clearly by using refined simulation. The second spray phenomenon exits in the liquid sheet spray process. When moving downward， the ligaments start to disintegrate into many big droplets or liquid masses because of the effect of gas strength and surface tension. With big droplets or liquid masses further shrinking and breaking into small droplets， the shapes of droplets change gradually from irregular cylinder to approximate sphere. In the jet injector with rotational velocity and the hollow conical liquid sheet， the spatial distribution of droplets is different. The former is more uniform distribution in the cone area， while the latter is only distributed in the annular region located on both sides of the conical liquid sheet.