Temperature Imaging in an Unsteady Propane-Air Counterflow Diffusion Flame Subjected to Low Frequency Oscillations

Current emphasis in non-premixed turbulent combustion research is focused on the effects of hydrodynamic unsteadiness and transient effects in these flames. To address these effects, measurements of the temperature field in unsteady propane-air flames were made using planar laser-induced fluorescence of the hydroxyl radical to ascertain the effect of fluctuating hydrodynamics on flame temperature. Planar temperature measurements were made at four temporal locations within the 25 Hz velocity fluctuation as a function of initial steady strain rate and forcing amplitude. Results show that temperature in the propane flame is rather insensitive to initial strain rate for these weakly strained flames due to the balance between decreased heat release rate and reduced radiative losses resulting from diminished soot production as the strain rate is increased. The temperature is significantly influenced by the imposed velocity oscillation, however, which causes large fluctuations in the instantaneous strain rate. A decoupling of peak flame temperature and maximum PAH and excited CH concentration indicates significant transient effects resulting from the unsteady flow field even at these low oscillation frequencies.