Experimental investigation of wall-film characteristics of methanol and ethanol sprays under varying flash-boiling intensities

With the ongoing downsizing of engines and the increasing adoption of direct injection systems, flash boiling and spray–wall impingement have become inevitable phenomena, particularly for methanol and ethanol, which are promising alternatives to gasoline. This study experimentally investigated the wall-film characteristics after spray impingement under varying flash-boiling intensities in a constant-volume chamber using high-speed imaging combined with the Refractive Index Matching (RIM) technique. Key parameters—the ambient-to-saturation pressure ratio (Pa/Ps), fuel temperature, and ambient pressure—were systematically varied. The results show that as Pa/Ps decreased from 1.0 to 0.15, sprays transitioned from non-flash to flare flash-boiling regimes, accompanied by spray collapse and droplet coalescence, leading to a morphological change in the wall film from uniform to converging curved structures. Increasing flash-boiling intensity markedly reduced the overall film area but caused localized thickening, which was more evident for methanol than for ethanol. Fuel temperature exhibited nonlinear effects: maintaining moderate superheating (approximately 313–343 K for methanol and ethanol) effectively reduced wall wetting, whereas excessive heating intensified spray collapse and film thickening. At low ambient pressures (0.03–0.01 MPa), flash boiling further accelerated film evaporation, leaving only minimal residues for alcohol fuels. These findings provide guidance for optimizing fuel temperature and injection strategies to minimize wall wetting and improve mixture formation in gasoline direct injection (GDI) engines.