Experimental study on flow fields of spray impingement under flash boiling conditions

Inside direct-injection spark-ignition engines, spray wall impingement is undesirable and unavoidable, while it would affect the fuel-air mixture and deteriorate combustion. Flash boiling spray presents a potential solution to this issue, but its impact remains uncertain. Therefore, this work focuses on the phase change impacts of flash boiling spray on the wall impingement process. Planar Mie scattering imaging technique obtained the macroscopic characteristics of the wall-impinging spray, and the velocity field was measured with time-resolved PIV. The impact of flash boiling on the flow characteristics of the impingement spray was examined. Additionally, the relationship between the quasi-impinging jet and the resulting wall jet was explored. This investigation found that under sub-cooled conditions, the concentrated and inclined spray impinging on the wall possesses higher momentum, leading to significant wall shear forces and prominent wall vortices. Under flash boiling conditions, the expanding plumes, with lower velocities, result in a wider impinging area with reduced and dispersed momentum. The quasi-impinging droplets exhibit various impingement angles, resulting in different small wall jets. These wall jets engage in complex interactions that hinder the formation of large-scale vortices while promoting the generation of numerous small-scale vortices that are smaller in area and weaker in intensity.