Numerical Modeling of Spray Formation under Flash-boiling Conditions 2020-01-0328
Flash boiling occurs in sprays when the ambient gas pressure is lower than the saturation pressure of the injected fuel. In the present work, a numerical study was conducted to investigate solid-cone spray behaviors under various flash-boiling conditions. A new spray cone angle correlation that is a function of injection parameters was developed and used for spray initialization at the nozzle exit to capture plume interactions and the global spray shape. The spray-breakup regime control was adjusted to enable catastrophic droplet breakup, characterized by Rayleigh-Taylor (RT) breakup, near the nozzle exit. The model was validated against experimental spray data from five different injectors, including both multi-hole and single-hole injectors, with injection pressure varying from 100 to 200 bar. Different fuels, including iso-octane, n-heptane, n-pentane, ethanol, and n-butanol, were investigated under a wide range of flash-boiling conditions, in which flash boiling was induced by high injected fuel temperature, ranging from 323 to 493 K, and/or low ambient gas pressure, ranging from 0.1 bar to atmospheric. It is found that flash boiling can significantly increase the spray cone angle near the nozzle exit, causing spray spreading and obvious plume interactions, which strongly influence the global spray shape. For flash boiling induced by high fuel temperature, penetration length tends to decrease because of enhanced liquid breakup and vaporization. For flash boiling induced by low ambient gas pressure, the reduced resistance from the ambient gas and enhanced droplet breakup lead to competing effects on spray penetration. By using the modified breakup regime control and spray cone angle correlation, the model shows good agreement with experimental data in capturing spray spreading, plume interactions and global spray shape in all simulated cases. The modeling approach proposed in this paper is expected to be universally applicable to all solid-cone flash-boiling sprays.