Atomization and vaporization for flash-boiling multi-hole sprays with alcohol fuels

The spray structural changes and vaporization processes for flash-boiling multi-hole sprays over a broad range of superheated conditions were investigated using Mie-scattering and Laser-induced-exciplex-fluorescence (LIEF) optical techniques. The fuel property effects were examined by characterizing n-hexane, methanol and ethanol fluids over a wide range of conditions consistent with that found in today’s spark-ignition-direct-injection (SIDI) engines. The macroscopic spray structure was quantified using spray penetration, spray-plume width and normalized distance between spray plumes. These structural parameters were correlated to the ratio of the ambient pressure to saturation pressure (Pa/Ps) that represents the superheated degree. Three continuous regions were identified by quantifying the spray transformation with increasing superheated degree; namely the non-flash-boiling, transitional flash-boiling and flare flash-boiling regions. Two critical values of Pa/Ps were identified, where the flash-boiling and spray collapsing transitions occurred at Pa/Ps values of 1.0 and 0.3, respectively. The evaporation process was examined using the LIEF optical technique for n-hexane, providing the relative vapor quantity throughout the spray transformation process. The correlations of the spray structural change and extent of vaporization with increasing superheated degree provided good insight into the mechanisms responsible for the observed behaviors during flash-boiling conditions.