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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.
Siebers et al. (Mon,) studied this question.