Wei, Y., Li, T., Wang, B., and Shi, W., "Simultaneous Measurement of the Flame Lift-Off Length on Direct Injection Diesel Sprays Using High Speed Schlieren Imaging and OH Chemiluminescence," SAE Technical Paper 2017-01-2307, 2017.
Lift-off length is defined as the distance from injector hole to the location where flame stabilized on a high injection pressure direct injection (DI) diesel spray. In this paper we used the high-speed (40 kHz) Schlieren and time-averaged OH chemiluminescence imaging technique to simultaneously measure the flame lift-off locations on a DI diesel spray in an optically accessible and constant-volume combustion vessel. The time-resolved development of the diesel spray acquired from the high-speed Schlieren imaging system enabled us to observe the instantaneous spray structure details of the spray flames. The OH chemiluminescence image obtained from a gated, intensified CCD video camera with different delay and width settings was used to determine the quiescent lift-off length. Experiments were conducted under various ambient temperatures, ambient gas densities, injection pressures and oxygen concentrations. From the Schlieren images of the injecting procedure, along the axis of the spray from the injector hole, a distinct expansion located in the downstream, after which the edge of the spray changed to transparent gradually and the refractive index gradients started to diminish. This point was defined as the flame location of high-speed Schlieren and the length from it to the injector was employed as the lift-off length. It is found that the Schlieren imaging lift-off length fluctuated within a certain range during the injection process, and the average lift-off length of quiescent conditions was also measured based on the OH chemiluminescence image. The results of the high-speed Schlieren imaging coincided well with the lift-off length results acquired from the CCD results, indicating high-speed Schlieren imaging technique can be used to measure the flame lift-off length on DI diesel spray.