DEVELOPMENT AND VALIDATION OF A FLASH BOILING MODEL FOR SINGLE-COMPONENT FUEL DROPLETS

Abstract

In a series of flash boiling submodels in the present work, droplet external surface flashing, bubble internal vaporization, and droplet breakup outcome submodels are proposed and adopted. In the droplet external surface flashing submodel, an analytical solution to the transient temperature distribution of the inside of a flash boiling droplet has been derived, and the external surface fuel flashing rate can be determined from the thermal conduction energy flux from the inside of the droplet as well as the thermal convection energy flux at the interface between the droplet surface and the surrounding gas. In the bubble internal vaporization submodel, both aerodynamic pressure difference driven bubble growth and heat transfer between the liquid and vapor bubble are considered. The Rayleigh–Plesset equation is used for the aerodynamic force to control the growth rate of a cavitation bubble. In the droplet breakup outcome submodel, the sizes and velocities of resulting droplets are derived, and the disruption energy has been analytically derived. Using the present flash boiling model, the flash boiling of a single-component single droplet was simulated, showing qualitatively correct droplet-bubble related parameters. The simulated bubble growth in a water droplet has been well validated with available experimental data. A single-hole flash boiling iso-octane spray was simulated using different approaches. Comparison results show that the present flash boiling model has a much better capability to capture the flash boiling characteristics than a normal evaporation model. Another single-hole flash boiling iso-octane spray was simulated and compared with experimental data, showing that both the spray shape and the penetration match the experimental data very well. Finally, the present model was successfully applied in capturing the flash boiling phenomenon in a practical gasoline engine.