Blow-off mechanisms of turbulent premixed bluff-body stabilised flames operated with vapourised kerosene fuels

Abstract

The lean blow-off (LBO) behaviour of unconfined lean premixed bluff-body stabilised flames with various fuels was investigated. Methane and vapourised ethanol, heptane, Jet-A, and an alternative alcohol-derived kerosene (Gevo) were used. OH* chemiluminescence (5 kHz), OH- and Fuel-PLIF (5 kHz), and CH2O-PLIF (10 Hz) were deployed. For all fuels, as the flame approached LBO fragmentation was observed downstream, the two sides of the flame merged at the axis, pockets of OH and CH2O were found in the recirculation zone (RZ), and eventually the individual fragments extinguished. The CH2O seemed to enter into the RZ from downstream early in the LBO process, with reactants following suit at times closer to LBO. During LBO, the integrated OH* signal decreased slowly to zero and the duration of this transition was ∼ 25 (d/UBO) in the methane and ethanol flames and ∼ 60 (d/UBO) in flames operated with heptane and the two kerosenes (where d is the bluff-body diameter and UBO the LBO velocity). This large difference could be due to re-ignitions of partially-quenched fluid inside the RZ during the LBO event. Additionally, for the same bulk velocity, the kerosene flames blow-off at higher equivalence ratios than the single-component fuelled flames, which is possibly due to the higher Lewis number and lower extinction strain rates of these fuels. The results suggest that the blow-off mechanism is qualitatively similar for each of the fuels; however, the complex chemistry associated with heavy hydrocarbons appears to result in a prolonged LBO event.