A Study on Surrounding Air Flow Induced by Diesel Sprays

A study of the mixing mechanism of fuel with surrounding air is necessary in order to clarify the combustion process. In this study, the flow field near non-evaporating diesel spray as well as spray surface were observed and analyzed using a Nd-YAG laser light sheet. A single shot fuel spray was injected into a high pressure vessel and photographed under double-pulse laser illumination. The images of dispersed particles in the vessel were processed and velocity vectors were obtained by the auto-correlation method.

Measured results showed temporal variation in the air movement around the spray. Just after the start of injection, air near the nozzle was pushed outward by the spray tip, after which the flow direction reversed. The air velocity ahead of spray tip was very low compared to fuel spray tip velocity. At a stable injection condition, air near the nozzle tip was pulled by the spray movement and flowed uniformly, and the spray-air boundary was smooth. Downstream from nozzle tip, the air flow generates turbulence along the spray, and spray surface becomes complicated by the interference between fuel and air. It seems that rather small amount of air was entrained near to the nozzle tip, and more air entertainment occurred in the irregular spray mid-section region and at the spray tip.