Assessment of the stabilization mechanisms of turbulent lifted jet flames at elevated pressure using combined 2-D diagnostics

The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH2O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities U-j and co-flow velocities U-c were examined. Data show that flames with U-c = 0.6 m/s stabilize much further away from the nozzle than those with U-c = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for U-c = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH4 Raman images shows that the flames with U-c = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For U-c = 0.6 m/s and relatively large bulk jet velocities (U-j = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of U-j = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and U-c = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.