On the break-up times and lengths of diesel sprays

Abstract

Measurements of the break-up zone characteristics are reported for simple orifice, direct-injection diesel injectors operating with commercial pumps and injecting into high pressure gas. The difficulty of using photography in this high density region leads to the use of indirect techniques for estimating break-up length. These include the computation of the overall void fraction of the spray, as a function of time, and curve fitting a new penetration rate correlation that incorporates break-up time as an adjustable empirical constant. A combination of theory and dimensional analysis is used to derive three interrelated equations for break-up time, break-up length, and characteristic velocity in the break-up zone. This latter velocity is less than the theoretical injection velocity due to the drag forces on the “intact liquid column” near to the injector orifice. The new equations are in excellent agreement with the present data, and they are also in good agreement with published data obtained from “break points” in penetration rate curves using an idealized constant fuel pressure, impulsively started injection apparatus. Published conductivity probe measurements of break-up length are consistently 35 percent of lengths predicted using the equations. This is expected as the conductivity probe technique measures the intact liquid length, which is shorter than the total break-up length.