Experimental study on the droplet characteristics in the spray tip region: Comparison between the free and impinging spray

Abstract In this work, we compared the microscopic characteristics of the free and impinging spray tip using double-pulse laser particle image analysis technique. Droplet population density, diameters, and velocities were experimentally resolved for both the free and impinging spray at three spray tip penetration positions (STP = 32,37 and 42 mm) under three injection pressures (10, 20 and 30 MPa). Results showed that firstly, wall impingement will delay the arrival timing of the spray tip at a specific spray tip penetration location. The droplets were less populated in the impinging spray compared to the free spray especially at downstream positions. The population density between the impinging and the free spray is more profound at higher injection pressure. Secondly, for fixed injection pressure, more small droplets are observed in upstream region after impingement, while this effect becomes weak at downstream positions. The droplet diameter distribution of the impinging spray is less sensitive to the increase of injection pressure, compared to the free spray. Sauter mean diameter significantly reduces at higher injection pressure for both the free and impinging spray. Finally, the smaller droplet velocity was observed in the impinging spray due to the dissipation of spray momentum during interaction with the wall plate. Based on the droplet size and velocity, gas phase Weber number was further calculated. Compared to the free spray, the Weber number distribution of droplets in the impinging spray concentrates in significantly smaller values at all injection pressures and observation locations.