Relationship Between Ignition Processes and the Lift-Off Length of Diesel Fuel Jets

Abstract

The reaction zone of a diesel fuel jet stabilizes at a location downstream of the fuel injector once the initial autoignition phase is over. This distance is referred to as flame lift-off length. Recent investigations have examined the effects of a wide range of parameters (injection pressure, orifice diameter, and ambient gas temperature, density and oxygen concentration) on lift-off length under quiescent diesel conditions. Many of the experimental trends in lift-off length were in agreement with scaling laws developed for turbulent, premixed flame propagation in gas-jet lifted flames at atmospheric conditions. However, several effects did not correlate with the gas-jet scaling laws, suggesting that other mechanisms could be important to lift-off stabilization at diesel conditions. This paper shows experimental evidence that ignition processes affect diesel lift-off stabilization. Experiments were performed in the same optically-accessible combustion vessel as the previous lift-off research. The experimental results show that the ignition quality of a fuel affects lift-off. Fuels with shorter ignition delays generally produce shorter lift-off lengths. In addition, a cool flame is found upstream of, or near the same axial location as, the quasi-steady lift-off length, indicating that first-stage ignition processes affect lift-off. High-speed chemiluminescence imaging also shows that high-temperature self-ignition occasionallymore » occurs in kernels that are upstream of, and detached from, the high-temperature reaction zone downstream, suggesting that the lift-off stabilization is not by flame propagation into upstream reactants in this instance. Finally, analysis of the previous lift-off length database shows that the time-scale for jet mixing from injector-tip orifice to lift-off length collapses to an Arrhenius-type expression, a common method for describing ignition delay in diesel sprays. This Arrhenius-based lift-off length correlation shows comparable accuracy as a previous power-law fit of the No.2 diesel lift-off length database.« less