The compositional structure of swirl-stabilised turbulent nonpremixed flames

Abstract

The spontaneous Raman–Rayleigh-LIF (laser–induced fluorescence) technique is used to make detailed, joint, single-point measurements of a range of reactive species in highly swirling, turbulent, nonpremixed flames. Such flows are of practical importance and involve highly complex fluid dynamics, the effects of which on the chemistry remain only vaguely understood. The LIF technique is used to measure the concentrations of OH, NO, and CO. Scatter plots are presented in mixture fraction space where each data point is colour coded to reveal its original radial location. Means and rms fluctuations of the mixing, temperature, composition and reactedness fields are also shown for flames of various fuels covering a range of swirl and Reynolds numbers. The complete data set which includes flow, mixing, temperature, and composition fields is made available on the web. It is found that, with increasing swirl number, the stability limits of the flames are broadened but the scatter plots show locally unburnt fluid samples occurring in flames further from global blow-off than nonswirling bluff-body-stabilised flames of similar fuel mixtures. Another novel feature of swirling flames is the detection of local nonburning or extinction within a recirculation zone which develops, at high swirl numbers, further downstream in the flow. It is found that most of the unburnt fluid samples originate from radial locations closer to the coflow air rather than the fuel jet. The means and rms fluctuations of the mixing fields confirm that swirl enhances mixing, especially in the primary recirculation zone and leads to a broadening of the jet.