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.