Diesel fuel jet lift-off stabilization in the presence of laser-induced plasma ignition

Abstract

The mechanisms affecting lift-off stabilization at diesel conditions were investigated by laser-igniting a diesel fuel jet upstream of its natural lift-off position. Single-nozzle fuel sprays penetrating into an optically accessible constant-volume chamber were ignited using laser-induced plasma formation both prior to natural autoignition or after a quasi-steady lift-off length was established. Fuel sprays ignited readily, with reaction kernels growing in connected regions. After laser-ignition, the lift-off persists upstream of the natural lift-off position for a substantial period of time indicating that upstream ignition has a strong influence on lift-off stabilization. While not discounting the role of flame propagation downstream of the ignition event, these results show that upstream ignition sites can start a chain of events that effectively controls lift-off. Lift-off eventually returns to its natural position, but only after injection times that are too long for practical engines. The time of return to the natural position depends upon the relative distance of the laser-ignition site to the natural lift-off length. A theory for fuel jet lift-off stabilization based on flame propagation into pure fuel-ambient reactant streams fails to predict the long upstream stabilization away from the natural lift-off length because turbulent velocities are higher in upstream regions of the fuel jet. Likewise, upstream lift-off stabilization by autoignition of pure reactants (no mixing with combustion products) fails because of cooler temperatures and shorter residence times. A potential mechanism explaining the transient lift-off response to laser-ignition is offered based on turbulent mixing with high-temperature combustion products found at the jet edges.