高速气流中液滴变形破碎的实验研究

Experimental investigation of the deformation and breakup of a droplet in high-speed flow

通过高速摄影测试技术获得了液滴的3种典型变形破碎模式:振荡变形、剥离破碎和爆炸式破碎.定量研究了不同破碎模式下液滴形态的变化过程,发现在液滴演化的初始阶段3种工况的液滴增长曲线几乎重合.当液滴展向宽度增长到初始直径的3倍左右时,液滴受气流剪切作用开始剥离出颗粒.在液滴形态的发展过程中爆炸式破碎与剥离破碎模式具有一定的相似性.实验结果在一定程度上支持了Theofanous提出的"爆炸式破碎模式非独立存在"的观点,符合以作用机理为依据的液滴破碎模式分类.采用平面激光诱导荧光技术(Planar Laser Induced Fluorescence,PLIF)研究了液滴破碎雾化过程中液滴内部微小颗粒的分布,发现气流速度的增加增强了背风面反向涡对的强度.气流的剪切和夹带对液滴的破碎起着主导作用.

Experiment on the droplet deformation and breakup is performed in high-speed flow passaged by shock wave. Vibration deformation, shear stripping breakup and catastrophic breakup are obtained in the droplet breakup experiment. The deformation and breakup of droplet are recorded by high-speed photography. The morphologies of droplet in different breakup modes are analyzed quantitatively including the evolution of length and width respectively identified in streamwise and spanwise direction. At early time the development curves of droplets are almost overlapped indicating that droplets in three conditions are compressed by the flow in the same manner. As a result of the shear and stripping of flow, droplet breaks into fragments when the spanwise width increases to approximately three times of initial diameter.There is some similarity between shear stripping breakup and catastrophic breakup in the temporary evolution which indicates that catastrophic breakup mode has the same mechanism with shear stripping mode somehow. The analysis of experiment results supports the viewpoint that catastrophic breakup is not an independent mode. The results agree well with the types of breakup which are put forward by Theofanous based on breakup mechanisms. Thanks to the planar laser-induced fluorescence（PLIF） measurement, the distribution of atomized particles is visualized which shows that the increase of flow speed enhances the strength of vortex pair in the wake. The vorticity makes the area more unstable and accelerates the mixing between ambient air and atomized liquid. By the sufficient contact with hot air produced by shock wave, liquid in droplet is vaporized rapidly and becomes a bulk of droplet particles which are more easily to be blew off.Flow shear and stripping play an essential role in the breakup and atomization.