Flame Lift-Off on Direct-Injection Diesel Sprays Under Quiescent Conditions 2001-01-0530

Ambient gas temperature and density, injection pressure, and orifice diameter effects on the flame lift-off length on a direct-injection (DI) diesel spray under quiescent conditions were experimentally investigated. The impacts of the observed lift-off length variations on air entrainment upstream of the lift-off location, soot formation, and the relationship between fuel vaporization and combustion were also examined. The research was conducted in a constant-volume combustion vessel using a common-rail fuel injector and a Phillips research grade #2 diesel fuel.

The lift-off length measurements show that lift-off length decreases with increasing ambient gas temperature or density, and increases with increasing injection pressure or orifice diameter. The sensitivity of lift-off length to a change in either temperature or density was non-linear, with the sensitivity to either parameter decreasing as it increased. The increase in lift-off length with increasing injection pressure was linearly dependent on injection velocity. Estimates of the air entrainment upstream of the lift-off length, show that the amount of fuel-air pre-mixing upstream of the lift-off length is dependent on the combined effects of many of the above mentioned parameters on air entrainment and the lift-off length. Coupling the air entrainment estimates with soot incandescence measurements indicates that there is a strong link between the fuel-air mixing upstream of the lift-off length and soot formation. Soot incandescence was observed to decrease as the amount of fuel-air premixing upstream of the lift-off length increased, with no significant soot incandescence (i.e., no significant soot) being observed when enough air was entrained to reduce the average equivalence ratio at the lift-off length to a value less than approximately two. Finally, comparison of the lift-off lengths with previously measured liquid lengths indicates that the relationship between fuel vaporization and combustion is strongly affected by ambient gas and injector parameters.