滴状模式下液桥形成及断裂的电流体动力学特性研究

对电场作用下微通道荷电液滴脱落过程中液桥形成及断裂的显微演变特征进行了可视化实验研究.借助时空分辨率较高的高速摄像技术精确捕捉了电场作用下液桥形成及断裂的界面演化过程,研究了液桥的界面结构变化及其断裂的动力学显微演变行为,获得了时间特征数、电邦德数及半月面形成角对液桥长度及断裂顺序的作用规律.实验结果显示,液桥断裂长度取决于黏度与表面张力之比,而受荷电弛豫时间的影响甚微,低电压工况下各实验介质液桥相对长度的变化并不明显,而在较高电压工况下相对液桥长度的增长速度加快.随着电邦德数的不断增加,液桥长度的变化在较高邦德数下更为明显且存在突变区,此时伴随着雾化模式的转变,表明液桥的突变恰恰是雾化模式过渡的信号.不同物性介质的射流过渡行为由于液桥上下游形成角的变化而存在较大差异.对于无水乙醇介质,电邦德数的增加使滴状模式首先过渡到纺锤模式,而对于生物柴油,滴状模式后会首先出现脉动模式而非纺锤模式. 

ELECTROHYDRODYNAMIC CHARACTERISTICS OF LIQUID BRIDGE FORMATION AT THE DRIPPING MODE OF ELECTROSPRAYS

A detailed visualization study on the evolution of Liquid bridge formation and fracture from a capillary is reported. By means of high-speed microscopy with high time-space resolution, special attention has been paid to the formation dynamics of the liquid bridge in the dripping mode, the change of interface structure and the fracture dynamics behavior of hydraulic bridge are studied, and the action rule of time characteristic number, electric Bond number and half-moon angle on liquid length and fracture order of liquid bridge is obtained. The results show that the fracture length of liquid bridge depends on the ratio of viscosity to surface tension, but is little affected by the relaxation time of charge. Under low voltage condition, the change of liquid bridge relative length for each experiment medium is not obvious, while the relative length of liquid bridge grows rapidly during high voltage condition. With the continuous increase of electric Bond number, the change of liquid bridge length is more obvious under the higher Bond number, and a mutation zone occur which shows the transition of the atomization model, this means the mutation of liquid bridge is a transition signal of atomization modes. With the changes of the formation angle of liquid bridge upstream and downstream, the transition behavior of jet flow in different physical media varies greatly. In the case of ethyl alcohol, the increase in the number of electric bonds causes the dropping mode to first transition to the spindle mode, while in the case of biodiesel, the pulsation mode rather than the spindle mode will first appear after the dropping mode. It is of great significance to reveal the transition law of charged micro fluidic atomization model and enrich the theory of electrostatic atomization.