Unraveling the initial flash boiling spray formation at the same superheated index achieved by altering ambient pressure and fuel temperature independently

摘要

When fuel is injected into a combustion chamber where the ambient pressure is lower than the saturation pressure of the fuel, flash boiling occurs. Recently, the flash boiling phenomenon has been investigated by many researchers due to its improved atomization performance on the spray. Recent engineering studies about flashing jet behaviors have correlated the macroscopic structure of flash boiling sprays solely as a function of ambient-to-saturation pressure ratio (P_a/P_s) regardless of fuel temperature, ambient pressure, and even fuel types. However, it is questionable if a superheated condition, which is achieved either by increasing the fuel temperature T_f(P_s) or decreasing the ambient pressure (P_a), would generate identical spray characteristics. In this research, a comparative study was conducted for various P_a/P_s conditions to unravel how the effect of increasing fuel temperature and decreasing ambient pressure to achieve a certain superheated condition appears differently on the flash boiling spray characteristics. To do this, first, bubble growth rates were predicted through the Rayleigh-Plesset equation. Then, the initial dispersion angle and droplet size distribution were analyzed using an X-ray imaging technique. The prediction results of the Rayleigh-Plesset equation showed that the bubble growth rate can be higher in the increased T_f case compared to the decreased P_a case at the same superheated indices. X-ray images revealed that the case of increasing T_f caused larger initial flow dispersion and smaller droplet sizes. These indicated that increasing T_f and decreasing P_a could generate different spray characteristics in particular conditions that need to be paid attention for the spray characterization.