On the aerothermochemistry of recirculating premixed flames with and without swirl

This paper describes recent progress in the analysis of the nature of turbulent premixed flames stabilised behind an axisymmetric baffle, which are of fundamental interest in the development of new and cleaner propulsion combustion systems. The work includes the use of laser-based diagnostics for velocity and temperature measurements, which are extended to the analysis of turbulence statistics, including the energy spectrum and typical length scales in a reacting shear layer. The results provided experimental evidence of the extension of the flamelet regime beyond the Klimov-Wiliams criterion. Arguments based on the shape of the weighted joint probability distributions of axial velocity and temperature fluctuations show that the counter-gradient nature of heat flux is derived from large departures from the local mean values. The influence of swirl in strongly sheared disc-stabilised flames is analysed and the results obtained show that swirl attenuates the rate of turbulent heat transfer due to the decrease of the temperature gradients across the reacting zone, but does not alter the existence of a large zone of flame characterised by non-gradient scalar fluxes.