Laser sheet dropsizing of evaporating sprays using simultaneous LIEF/MIE techniques

Abstract

Laser-induced-fluorescence/Mie-scattering (LIF/MIE) was proven to be a useful diagnostic for Sauter Mean Diameter (SMD) measurements in non-evaporating sprays. However, the measurement is not reliable for cases of an evaporating spray due to the interference of the fluorescence signal from the vapor phase. In this work, simultaneous Laser-induced-exciplex-fluorescence/Mie-scattering (LIEF/MIE) imaging techniques were proposed to obtain the SMD distribution of evaporating sprays. A special experimental condition was used to generate the flash-evaporating spray, which includes a fuel temperature of 60°C and an ambient pressure of 20kPa. Different from the conventional LIF/MIE technique, the combination of LIEF and Mie techniques allows eliminating the effect of tracer fluorescence from vapor phase in an evaporating spray. In addition, carefully selected tracers and specially designed filters were used to decrease the effects of variation on tracer concentration and temperature dependency of fluorescence intensity during evaporation. The numerical analysis based on geometrical optics approximation (GOA) and experimental analysis was conducted to determine the calibration coefficient K. Finally, SMD distribution of an evaporating spray measured by both LIEF/MIE and conventional LIF/MIE techniques was compared to PDI measurement. The results show that the SMD of the flash boiling spray obtained from LIEF/MIE is very close to those measured by PDI, while the results measured by LIF/MIE and PDI show a large deviation of around 40%. It indicates that the evaporation effect cannot be ignored for evaporating sprays.