Study On The Stability Of Liquid Jet In Coaxial Swirling Compressible Gas And Cavitation Bubble Within A Single Droplet

Liquid jet is a common phenomenon in every field of engineering technology and natural world. The injection, breakup and atomization processes of fuels are essential for raising the combustion efficiency and reducing the environmental pollution of diesel engines, gasoline direct injection engines and gas turbines. In fact, the breakup and atomization process of fuel jets is the stability problem of liquid jets, and it is a hot issue in internal combustion engines.In this dissertation, the breakup and atomization of liquid jets in a coaxial swirling compressible air stream were studied by the methods of analysis and numerical simulation, respectively. Research works could improve the mathematical models of liquid jet stability and bubble stability within a droplet, and deeply understand the mechanism of the breakup and atomization of liquid jets.The dispersion equation and its solving method corresponding to temporal mode and spatial mode on the stability of liquid jet under supercavitation were presented. The effects of swirling gas, fluids compressibility and cavitation bubbles on the stability of liquid jet were investigated, respectively. The calculation formula of droplet size was improved. Then, the comparative study on the temporal mode and spatial mode stability of liquid jet under supercavitation was carried out, and the affecting parameters on the difference between them were analyzed. The effects of swirling gas, fluids compressibility and cavitation bubbles on the stability of liquid jet, and the differences between temporal mode and spatial mode stability of liquid jet were determined clearly.A dispersion equation describing the thermal effect on the stability of three-dimensional cylindrical liquid jet in a coaxial swirling compressible air stream was established by the use of linear stability analysis. The mathematical model and its solving method were verified, and then the thermal stability of liquid jet in a coaxial swirling compressible air stream was analyzed. The temperature gradient in liquid jet and the temperature differences between jet and surrounding gas on the spatial stability of liquid jet were determined clearly.The disturbance governing equation sand boundary conditions of the liquid jet and the ambient gas for the first and the second order were derived by the use of nonlinear stability analysis. And the second order dispersion equation which describes the stability of liquid jet in a coaxial swirling compressible air stream was derived. On the basis of the established dispersion equation, the spatial evolution and breakup of liquid jet were compared under linear and nonlinear stability theories, and the nonlinear stability of liquid jet in a coaxial swirling compressible air stream was studied. The spatial evolution and breakup characteristics of liquid jet under two kind of stability theories, the effects of swirling gas, fluids compressibility and cavitation bubbles on spatial evolution and breakup of liquid jet were determined clearly.A new dispersion equation by considering the effect of fluids viscosity which describes the stability of cavitation bubble within the diesel droplet was established, and the analytical solution was solved. The bubble stability was analyzed from the perspective of stresses. Furthermore, based on the modified bubble breakup criterion, the breakup time and the breakup radius of cavitation bubble are studied deeply in this paper. The effects of fluids viscosity on cavitation bubbles, the effects of inertial force, compressibility force, internal hydrodynamic force, external hydrodynamic force, and surface tension force on the bubble stability, the breakup characteristic of cavitation bubble within the diesel droplet were determined clearly.Spherically symmetric bubble expansion and collapse within the diesel droplet were simulated numerically based on the volume of fluid (VOF) method, and the control mechanism and affecting parameters of bubble growth process were analyzed. The bubble collapse process and the variations of characteristic parameters in bubble collapse process were studied. In addition, bubble collapse model was improved by considering the phase change, and the collapse process of cavitation bubble considering the phase change was analyzed based on the modified bubble collapse model. The control mechanism of bubble growth process, the effects of parameters including surface tension coefficient, droplet viscosity and droplet density on bubble growth process, the variations of collapse pressure and volume, rebound pressure and volume, and the collapse process of cavitation bubble considering the phase change were determined clearly.