The flame on a high injection pressure direct-injection (DI) diesel spray under quiescent conditions stabilizes at a location downstream of the fuel injector. The distance from the injector to the location of stabilization is referred to as the flame “lift-off” length (or height). Air entrained into a diesel spray upstream of the flame lift-off length will mix with the injected fuel. The air and fuel premixed upstream of the lift-off length are believed to react immediately downstream of the location of flame lift-off. Recent measurements suggest that as much as 20% of the air required to burn the fuel injected is entrained prior to the flame lift-off length for typical, moderate-load, heavy-duty DI diesel conditions. These results imply that combustion at the flame lift-off location will play a pivotal role in diesel combustion and emission formation processes. However, our current understanding of how various parameters affect the lift-off length, and the resulting fuel-air premixing, is extremely limited under diesel conditions.
This paper presents a line-of-sight, time-averaged OH-chemiluminescence imaging technique for determining the flame lift-off length on a DI diesel spray. The technique uses a gated, intensified CCD video camera. The image acquisition and analysis, the repeatability of lift-off length measurement, the sensitivity of the measurement to camera gain, and the effect of time varying ambient conditions on the application of the technique are discussed. Also discussed are the spectral character, the spatial distribution, and the instantaneous versus time-averaged nature of the natural light emission at the flame lift-off location. The investigations were carried out in an optically accessible, constant-volume combustion vessel.