Time-varying behaviour of turbulent swirling nonpremixed flames

This paper presents the results of experimental investigations into the instabilities of turbulent swirling nonpremixed flames. The unconfined flames considered here possess features similar to those found in practical combustors. Extensive time-mean (space-resolved) flow field and composition data already exist in these flames. Results presented herein seek to complement those data with time-resolved measurements acquired using high-speed imaging of laser Mie scattering and shadowgraphs. Mie scattering measurements are designed to identify instabilities specific to the fuel jet that is geometrically centred on the bluff body. Conversely, shadowgraphs reveal the behaviour of the fuel jet as well as a recirculation zone that stagnates on the face of the bluff body. Results from these two imaging techniques are augmented with observations from the frequency spectra of velocity measurements. Overall, the results indicate that two modes of instability exist in these flames: (i) a precession mode in the centre (fuel) jet and (ii) a puffing mode characterised by an expansion and collapse of the toroidal recirculation zone which forms on the face of the bluff body. © 2006 The Combustion Institute.