基于图像处理的高频脉动撞击雾化特性研究

Atomization Characteristics of High-Frequency Pulsatile Impinging Jets Based on Image Processing

为研究高频脉动下撞击式喷嘴的雾化特性,采用水力扰动装置产生喷前压力扰动,由高速摄影对动态的喷雾场进行背光拍摄,采用普通长焦距镜头拍摄宏观的喷雾场,采用微距镜头拍摄微观的喷雾场。为解决高频脉动喷雾场瞬态液滴粒径测量困难的问题,基于图像处理建立了雾场瞬态液滴粒径捕捉测量方法,获得了高频脉动喷雾场局部区域的Sauter平均直径(SMD)随时间的变化规律。从宏观来看,自然喷雾场的液滴空间分布比较均匀,而高频脉动喷雾场则出现了稠密区与稀疏区交替分布的现象。宏观雾场灰度值较小的区域对应稠密喷雾区,从微观来看,该区域的液滴行为更加复杂,发生了液滴碰撞融合、二次破碎等一系列复杂物理过程。雾场空间出现复杂的液滴行为,导致液滴的粒径分布与空间分布发生改变,对燃烧释热产生重要影响。自然喷雾场的SMD随时间的变化表现出白噪声特性,高频脉动喷雾场的SMD随时间的变化表现出周期性特征,并且周期性变化的频率与施加的强迫扰动频率一致。

In order to investigate the atomization characteristics of impinging jet injector under high-frequency pulsatile conditions,hydro-mechanical pulsator was used to generate pre-injection pressure perturbations. Backlit images of dynamic atomization field were captured by high-speed camera. An ordinary lens with long-distance focus was used to capture macroscopic atomization field,and a microscopic lens was used to capture microscopic atomization field. To solve the difficulty of the measurement of transient droplet diameter in high-frequency pulsatile atomization field,a method for capturing and measuring transient droplet size was established based on image processing. The variation rule of Sauter mean diameter(SMD)of droplets in local regions of high-frequency pulsatile atomization field with time was obtained. From a macro perspective,droplet distribution in space of natural atomization field is rather uniform. However,for the high-frequency pulsatile atomization field,the phenomenon of alternating distribution of dense and dilute region occurs. Regions with small grayscale values in macroscopic atomization field correspond to dense spray regions. From a microscopic perspective,droplet behaviors in these regions are more complex. Droplet collision and fusion,secondary breakup and other complicated physical processes occur in these regions. The appearance of complicated droplet behaviors in these regions leads to variation of droplet diameter and distribution in space,and it causes significant effects on combustion heat release. Variation of SMD in natural atomization behaves white noise characteristics. However,variation of SMD in high-frequency pulsatile atomization field behaves periodic characteristics and the frequency is consistent with that of the imposed forced perturbation.