基于大涡模拟的离心式喷嘴雾化过程研究

Process of Pressure Swirl Nozzle Atomization Based on Large Eddy Simulation

关键词:  离心喷嘴  内部流动特性  液膜破碎  液滴分布  大涡模拟

Key words:  Pressure swirl nozzle  Internal flow characteristics  Liquid sheet breakup  Droplet distribution  Large eddy simulation

摘要:

为实现离心式喷嘴雾化过程的精确数值仿真，探究喷嘴内部流动特性与外部液膜破碎形式，采用基于大涡模拟的仿真方法，对一种典型的四进口离心式喷嘴进行研究，仿真结果揭示了喷嘴内部相界面的振荡现象与外部液膜的破碎细节，并通过耦合流体体积法（VOF）与离散相模型（DPM），获得液滴粒径的空间分布特征。研究结果表明：在液体填充过程中，喷嘴内的气液相界面存在波动与褶皱，形状并不稳定，内部的空气芯直径呈现正弦模式的振荡变化，喷嘴出口液膜厚度沿周向分布不均，这些因素导致出口附近的液膜表面出现扰动。在不同的进口条件下，不稳定性导致液膜表面上的扰动波形式不同。进口压力为0.3MPa时，液膜破碎由开尔文-亥姆霍兹（K-H）不稳定性产生的轴向正弦波所导致，产生沿周向分布的环形液带；在0.7MPa下，液膜表面开始出现由瑞利-泰勒（R-T）不稳定性引发的周向扰动波；随着压力增加至1.1MPa，液膜的破碎则由R-T不稳定性主导，产生沿轴向分布的液带结构，随后在气动力与表面张力的作用下破碎成液滴。二次雾化破碎后，喷嘴外部截面内的粒径呈“单谷”分布，液滴平均粒径计算结果与实验的最大相对误差为5.1%，与实验数据吻合度较高。

Abstract:

In order to accomplish the accurate numerical simulation on the process of the pressure swirl nozzle atomization， the simulation method on the basis of large eddy simulation was applied to explore the internal flow characteristics of nozzle as well as the breakup form of the external liquid sheet. In this thesis， a research for typical pressure swirl nozzle with four inlets was conducted， the simulation results reveal the oscillation phenomenon of the nozzle’s internal phase interface and the breakup details of the external liquid sheet. Moreover， the spatial distribution characteristics of droplet size was obtained through coupling the method of volume of fluid with discrete phase model. The research results indicate as follows：in the process of liquid filling， there are fluctuations and folds at the gas-liquid interface in the nozzle， whose shape is unstable in nozzle. The diameter of air core oscillates in sinusoidal mode， and the thickness of liquid film at nozzle outlet is unevenly distributed along the circumference. These factors lead to the disturbance on the surface of liquid film near the nozzle outlet. Under different inlet conditions， the instability leads to different disturbance waves on the surface of liquid sheet. When the inlet pressure is 0.3MPa， the breakup of liquid sheet is caused by the axial sinusoidal wave produced by the Kelvin-Helmholtz instability， resulting in annular liquid ligaments which distributes along the circumference. Under the inlet pressure of 0.7MPa， the circumferential disturbance waves generated by Rayleigh-Taylor instability begin to appear on the surface of liquid film. With the increase of pressure to 1.1MPa， the breakup of the liquid film is dominated by Rayleigh-Taylor instability， which produces the liquid ligament structure along the axis. Subsequently， it breaks into droplets under the action of aerodynamic force and surface tension. After the secondary atomization and breakup， the droplet size in the outer section of the nozzle distributes in the form of ‘single valley’. The maximum relative error between the calculated average droplet size and the experimental results is 5.1%， which is in good accordance with the experimental data.