Numerical Study Of The Dimethyl Ether Droplet Breakup Process In Flash Boiling Spray

The atomization of droplet in flash boiling spray is greatly affected by the bubble nucleation. growth and breakup process. In order to investigate the mechanism of flash boiling spray, the bubble growth and breakup process in superheated flash boiling spray are theoretically studied in detail.In this paper, a simplified bubble growth model has been developed according to the qualitative analysis of the temperature profile in the thermal boundary layer surrounding the vapor bubble. This model is based on the research of one-dimensional bubble growth model under superheated conditions. This model is validated by the experiment data of vapor bubble growth in superheat water and the numerical solution of dimethyl ether (DME) vapor bubble growth using the one-dimensional bubble growth model respectively. The comparison shows excellent accuracy and less computational cost than one-dimensional bubble growth model. The model is used to simulate the process of DME bubble growth under superheated conditions and the influences of physical parameters and initial state of system on DME bubble growth are studied significantly. The results show that according to curve of time vary interface acceleration, the process of DME bubble growth can be divided into three sections:surface tension controlled section, transition section and heat transfer controlled section. Bubble growth is greatly affected by surface tension in surface tension controlled section and greatly affected by thermal diffusivity in the heat transfer controlled section, but bubble growth is greatly affected by the both above in the transition section.In order to investigate the influence of nucleation and bubble growth on the breakup process of droplet under flash boiling condition, a bubble-droplet system is developed. The breakup of bubble-droplet system is attributed to aerodynamic instability and the unstable bubble growth according the relative motions exist. The breakup models of these two breakup modes are established respectively and the coupling effect between these two breakup modes is ignored. A modified TAB model is developed for the aerodynamic breakup, the results show that the bubble makes the system more unstable. the deformation grows faster with increasing void fraction, and this faster growth results in the decrease in the breakup time, the drop size decreases significantly with increasing void fraction. For the breakup caused by the unstable bubble growth. a dispersion equation which can describe the instability of bubble growth is derived based on the linear instability analysis method. The results show that the disturbance growth rate is primarily determined by the Weber number of bubble growth and the void fraction, a system with a larger Weber number and a larger volume ratio has a larger disturbance growth rate which indicates that the system is more unstable.