The effects of nozzle geometry and equivalence ratio on a premixed reacting jet in vitiated cross-flow

Abstract

The effects of nozzle geometry, jet equivalence ratio (ϕj), and momentum flux ratio (J) on the flow field, stability characteristics, and flame topology of a premixed ethylene–air jet injected transverse to a vitiated cross-flow are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to simulate the environment of a secondary combustor in a staged combustion system. The dependence of the flame liftoff height on J suggests that at these conditions the flame stabilization process is flame-propagation controlled rather than autoignition assisted. A circular nozzle and high aspect ratio slotted nozzle of identical exit area were investigated for jet to cross-flow momentum flux ratios ranging from 5 to 65 for jet equivalence ratios of up to ϕj = 5.0. High-speed particle image velocimetry was utilized to study the time-averaged flow field and OH* chemiluminescence was used to capture time-averaged and instantaneous features of the flame behavior. The nozzle geometry was determined to have a significant effect on RJICF flame stability, with substantially expanded blow-out limits for the slotted nozzle. Enhanced operability of the high aspect ratio slotted nozzle was shown to be attributable to the substantially larger and stronger recirculation zone on the leeward side of the jet when compared to the circular nozzle. This area is characterized by a more disperse region of elevated vorticity levels, resulting in the entrainment of more hot combustion products with a longer residence time in the recirculation zone, which in turn provides a stronger and more stable ignition source to the oncoming, unburned reactants. A correlation for the isothermal JICF trajectory was modified to account for gas expansion effects and found to satisfactorily capture the jet trajectory for both the non-reacting and reacting slotted nozzle. The jet trajectory was demonstrated to be independent of ϕj, whereas the jet flame penetration decreases as ϕj increases, indicating that where the flame situates itself is determined by the local mixture concentration rather than changes in the flow field. Nevertheless, ϕj was also found to have an effect on the mean flow field, with slightly higher magnitudes of reversed flow velocity and an increase in mean recirculation zone length observed as the fuel content of the jet is increased.