LASER BASED QUANTITATIVE IMAGING OF AIR ENTRAINMENT INTO EVAPORATING DIESEL SPRAY

A new laser-based method, which was developed by the authors for simultaneous and quantitative imaging of drop and vapor clouds in evaporating fuel sprays, was applied to the characterization of ambient gas entrainment into a Diesel spray in a high-pressure and high-temperature environment. The method was based on extinction of two wavelengths, that is, ultraviolet and visible laser lights through an α-methylnaphthalene fuel spray due to the absorption by the vapor and scattering by the drops. Subtraction of the transmissivity of the visible laser light from that of the ultraviolet laser light made it possible to image the distribution of the vapor cloud in the evaporating fuel spray. The distribution of the drop cloud was imaged by the transmissivity of the visible laser light. These images were processed to obtain the local vapor concentration, drop density, the excess air ratio and the amount of ambient gas entrained into the whole spray. The excess air ratio in the region of the spray before the impingement on the wall took a value around 0.2 or less except at the edge of the spray. The excess air ratio takes a larger value in the region of the spray along the wall after the impingement. The amount of fuel vapor in the spray was increased but the amount of drops was decreased with an increase in the impingement distance. The amount of ambient gas took a maximum, and the overall equivalence ratio took a minimum at a certain impingement distance.