Theoretical Research On Liquid Nitrogen Jet Atomization Under Thermodynamics

High Reynolds number wind tunnel tests play a vital role in the design and development of large aircraft.Liquid nitrogen spray is currently one of the most economical and effective way to increase the wind tunnel Reynolds number by reducing the temperature in the wind tunnel.The atomization characteristic of liquid nitrogen spray directly determines the uniformity of the flow and temperature field in the wind tunnel.Therefore,a deep understanding of the atomization process is the prerequisite for improving the heat transfer performance and flow field uniformity of the spray cooling process.Although a lot of work has been done in understanding the atomization mechanism,the coupling and the interaction of thermal and aerodynamic effects in cryogenic fluids still need to be further explored.Aiming at the process of liquid nitrogen jet atomization and phase change,a crossscale theoretical study of the primary breakup and secondary atomization is carried out in this paper.The effects of aerodynamic and thermal effects on the movement of high and low speed jets and the drop with different Weber numbers(We)is explored.The influence mechanism of the phase change on the breakup process is clarified.The main content is summarized as follows:1.Linear stability analysis of the primary breakup of low-speed jet.To explore the primary breakup process of the low-speed jet,a linear stability analysis method is used to establish the relationship between the instability and the breakup of the jet.Subsequently,a theoretical calculation model is constructed for the primary breakup of the low-speed jet.Main parameters include the wavelength of the most unstable wave of and the diameter of broken droplets are obtained.2.Direct numerical simulation analysis of the primary breakup of high-speed jet.To describe the evolution of phase interface caused by the primary breakup of the highspeed jet under the influence of phase change,a direct numerical model with the volume of fluid method(VOF)and the interface shift algorithm is constructed.Based on the numerical model,the morphological evolution of the jet primary breakup is analyzed and the size distribution regularities of the tiny droplets formed by atomization is obtained.The calculation results show that the phase change influences the spatial distribution of the primary breakup droplets and changes the surface deformation process.3.Kinetic analysis of droplet flash breakup during secondary atomization.A bubble micro-explosion model is established to describing the flashing atomization which is based on the bubble motion equation in the droplet.The TAB model is used to describe the aerodynamic effect.A criterion for predicting the dominant mode of secondary atomization is proposed,and the influence of physical parameters such as environment pressure on the atomization process is analyzed.The results show that the flashing effect is stronger at lower environment pressure,while the surface tension and viscosity have negligible effect.4.Study on the evaporation process of droplet surface during secondary atomization.Numerical research is carried out on the secondary atomization of different We number droplets include the surface evaporation process.Taking into account the difference in the flow and phase change characteristics of different We number droplets,a source term diffusion method is established to capture the effect of phase change on flow field at low We number.The evaporation rate is calculated based on the interface temperature gradient.The separation method of mass transfer and interface localization is combined with sharpening of the interface temperature distribution.Several examples are used to verify this model and compare with the interface shift method.The results show that the source term diffusion method performs better than the interface shift method but requires higher resolution.In addition,the influence of phase change on the atomization morphology of droplets decreases with the increase of We number,and the evaporation rate of droplets increases significantly at high We number.