Characteristics of spray and wall wetting under flash-boiling and non-flashing conditions at varying ambient pressures

Abstract

Flash-boiling in Direct Injection Spark Ignition (DISI) engines is very common. Flash-boiling atomization is one of the most effective ways to generate fully developed atomization and homogeneous mixtures. However, despite the positive effects of flash-boiling, this phenomenon may also lead to negative effects such as longer spray penetration, piston wall wetting, and increased soot emissions, due to increased interactions among the spray plumes which can result in spray collapse and the aforementioned problems. In this study, high-speed direct photography and Refractive Index Matching (RIM) were utilized to investigate the characteristics of n-hexane sprays and impingement using a constant volume vessel. Under low-pressure conditions, flash-boiling drives the collapsed spray to a quick impingement and small spray area. Through the volume distribution of the liquid fuel film acquired by RIM and the spray outline derived from the morphology of images, it was again confirmed that the spray collapse took place not only at a high degree of superheating, but also under conditions of high ambient pressure without flash-boiling. The spray collapse under high pressure conditions is characterized by a late-phase impingement and large spray-swept area. Varying fuel-film behavior was observed following spray-impingement. Under high-density conditions with spray collapse, the fuel film evaporated slowly after the injection, but the higher ambient pressures reduced the total impingement. At low-density conditions with flash-boiling, the resultant fuel-films evaporated more quickly, and reduced ambient pressures reduced the total fuel-film volumes, although flash-boiling could not completely inhibit spray impingement.