气流作用下同轴带电射流的不稳定性研究

通过对气体驱动同轴电流动聚焦的实验模型进行简化,开展了电场力和惯性力共同作用下同轴带电射流的不稳定性理论研究.在流动为无黏、不可压缩、无旋的假设下,建立了三层流体带电射流物理模型并得到了扰动在时间域内发展演化的解析形式色散关系,利用正则模方法求解色散方程发现了流动的不稳定模态,进而分析了主要控制参数对不稳定模态的影响.结果表明,在参考状态下轴对称模态的最不稳定增长率最大,因此轴对称扰动控制整个流场.外层气流速度越高,气体惯性力越大,射流的界面越容易失稳.内外层液-液同轴射流之间的速度差越大,射流越不稳定.表面张力对射流不稳定性起到促进作用.轴向电场对射流不稳定性具有双重影响:当加载电场强度较小时,射流不稳定性被抑制;当施加电压大于某一临界值时,轴向电场会促进射流失稳.临界电压的大小与界面上自由电荷密度和射流表面扰动发展关系密切.这些结果与已有的实验现象吻合,能够对实验的过程控制提供理论指导.

INSTABILITY STUDY OF AN ELECTRIFIED COAXIAL JET IN A COFLOWING GAS STREAM

Instability study of electrified coaxial jet coupling the electric and inertial forces is performed based on the simplified experimental model of gas-driven coaxial electro-flow focusing. Under the assumption that the fluids are inviscid, incompressible and irrotational, a triple-layer electrified fluid jet model is established and an analytical dispersion relation in the temporal regime is obtained. The dispersion equation is solved by the normal mode method, the unstable modes of the flow are calculated and the effects of mainly controllable parameters on the unstable modes are analyzed. The results indicate that the axisymmetric mode dominates the complete flow as the maximum growth rate of the axisymmetric mode is the largest among all unstable modes. As the velocity of outer gas stream increases, the inertial force can definitely promote the jet instability. The jet will become more unstable as the velocity difference between the inner and outer liquid jets increases. The surface tension also promotes the jet instability. The axial electric field has two-fold influence on the axisymmetric jet instabilities. There is a critical value for the axial electric voltage which is related to the free electric charge density at the interface and the perturbation propagations on the jet surfaces. The applied axial electric field can suppress the jet instability when its intensity is smaller than the critical value;otherwise, the applied axial electric field can promote jet instability. These results are in good agreement with the existing experimental results and can provide guidance on the process control of experiments.