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. 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. High-speed chemiluminescence imaging also shows that high-temperature self-ignition occasionally 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. 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. Fuels with shorter ignition delays generally produce shorter lift-off lengths.
The experimental results show that the ignition quality of a fuel affects lift-off. Experiments were performed in the same optically-accessible combustion vessel as the previous lift-off research.
This paper shows experimental evidence that ignition processes affect diesel lift-off stabilization. However, several effects did not correlate with the gas-jet more » scaling laws, suggesting that other mechanisms could be important to lift-off stabilization at 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. 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. This distance is referred to as flame lift-off length. The reaction zone of a diesel fuel jet stabilizes at a location downstream of the fuel injector once the initial autoignition phase is over. The trends observed may eventually help explain effects of parameters such as injection pressure and orifice diameter on emissions. The more » combined data suggests that a systematic evolution of the relationship and interaction between various processes in a DI diesel spray has been occurring over time, as injection pressures have been increased and orifice diameters reduced as part of efforts to meet emissions regulations. The lift-off length increase with increasing orifice diameter, however, is different than the independence of lift-off length on orifice diameter noted for gas jets An important overall observation was made by considering the lift-off length data in conjunction with data from recent investigations of liquid-phase fuel penetration and spray development. The observed lift-off length increase was linearly dependent on injection velocity, the same dependency as previously noted for gas jets. Lift-off lengths determined from images of natural light emission at 310 nm show that as either injection pressure (i.e., injection velocity) or orifice diameter increase, the lift-off length increases. At this location, natural light emission at 310 nm is dominated by OH chemiluminescence generated by high-temperature combustion chemistry. The results show that natural light emission at 310 nm provides an excellent marker of the lift-off length. The lift-off length experiments were conducted in a constant-volume combustion vessel under quiescent, heavy-duty DI diesel engine conditions using a Phillips research grade No.2 diesel fuel. The effects of injection pressure and orifice diameter on the lift-off length of a direct-injection (DI) diesel spray (defined as the farthest upstream location of high temperature combustion) were investigated using a natural light emission imaging technique.