Cavitation in a Single Hole Nozzle and Mechanism of Liquid Jet Atomization

通过对压力雾化器喷嘴和液体射流界面空化现象的高速同步观测，研究了空化诱导雾化的机理。同时，还测量了二维喷嘴中的液体速度，以研究空化、湍流和雾化之间的关系。喷嘴几何形状的变化对空化的影响和常规的空化数作为喷嘴中的流型的指标的有效性也被检查。建立了一个基于界面跟踪法、三种粒子跟踪法、基于多流体模型的平均法、大涡模拟和修正的Rayleigh-Plesset方程的混合集成的数值模型，用于预测喷嘴和液体射流中的湍流空化流动，并对喷嘴中的空化流动进行了数值模拟。(1)当喷嘴中出现空化时，空化云在出口附近脱落并崩溃，从而产生强烈的湍流。当云迹从喷嘴出来时，强烈的湍流诱导韧带的形成。(2)空化区厚度随喷嘴上游截面积与喷嘴截面积之比的增大而增大。(3)我们可以预测空化的发展，空化数，占的流动收缩和摩擦压降的影响。(4)云和韧带形成之间的因果关系不仅适用于二维喷管，而且适用于圆柱形喷管。(5)气泡和液体的速度分布，以及使用所提出的模型预测。

The mechanism of cavitation-induced atomization is studied based on high-speed simultaneous visualizations of cavitation in nozzles of pressure atomizers and liquid jet interfaces. Liquid velocity in a two-dimensional nozzle is also measured to investigate the relationship among cavitation, turbulence and atomization. The effects of the change in nozzle geometry on cavitation and the validity of the conventional cavitation numbers as an indicator of flow patterns in nozzles are also examined. A numerical model based on the hybrid integration of an interface tracking method, three kinds of particle tracking methods, an averaging method based on a multi-fluid model, Large Eddy Simulation and a modified Rayleigh-Plesset equation for predicting turbulent cavitating flows in a nozzle and liquid jets is developed, and numerical simulation of a cavitation flow in a nozzle is carried out As a result, the following conclusions are obtained.(1) When cavitation is developed in a nozzle, cavitation clouds are shed and collapse near the exit, which produces strong turbulence. When the trace of the clouds comes out of the nozzle, the strong turbulence induces ligament formation.(2) The thickness of cavitation zone increases with the ratio of the cross-sectional area upstream of the nozzle to that of the nozzle.(3) We can predict the development of cavitation using the cavitation number which accounts for the effects of the flow contraction and frictional pressure drop.(4) The causal relationship between the cloud and the ligament formation holds not only in the two-dimensional nozzle but also in the cylindrical nozzle.(5) Bubble and liquid velocity distributions are well predicted using the proposed model.