Investigation on flash boiling phase change characteristics of two-dimensional slit nozzles using backlit light attenuation with multi-component fuels

Flash boiling atomization is an important topic in fuel preparation and fuel processing with the emergence of alternative fuels or zero-carbon fuels. The phase change of the fuel is crucial in determining the atomization performance since the phase change both affects the primary breakup and the evaporation of the fuel sprays. Typically, it is considered that the superheat index and injector nozzle designs are critical parameters in determining flash boiling spray behavior and tendency. However, the definition of superheat index is not applicable to multi-component fuels that are more commonly seen in fuel applications, such as gasoline and diesel fuels. Furthermore, no consensus has been reached on the mechanisms of flash boiling atomization. This investigation aims to address some of the key issues with optically transparent, two-dimensional slit nozzles with varied geometries. A two-dimension expansion chamber is connected to the nozzle so that the phase change occurred in the near-field downstream of the nozzle exit can be clearly visualized as well. A single-component fuel of pentane and three multi-component fuel surrogates were tested under different fuel temperatures and high-speed measurements were conducted to reveal the two-phase flow patterns and characteristics. The results show that a modified superheat index using the bubble point pressure is well-suited for multi-component fuel applications. The light attenuation in the expansion chamber correlates very well with the phase change characteristics theoretically and experimentally, that could be used as an important basis to develop a quantitative measurement technique for near-field flash boiling spray atomization.