Measurements of local mixture fraction of reacting mixture in swirl-stabilised natural gas-fuelled burners

Abstract

Local, time-dependent measurements of mixture fraction of the reacting mixture were obtained in a swirl-stabilised natural gas-fuelled, nominally non-premixed burner using the intensity of chemiluminescence from OH^∗ and CH^∗ radicals. The measurements quantified the mean, rms of fluctuations and probability density functions of local mixture fraction at the stabilisation region of the flame. In addition, the probability of flame presence and the degree of lean or rich versus stoichiometric reaction is reported. The burner was operated for three air flow Reynolds numbers (Re=18970, 29100 and 57600), at an overall equivalence ratio of 0.32, without and with imposed oscillations to the air flow of the burner at the resonance frequency of 350 Hz. Results show that combustion occurred in a partially premixed mode for all flow conditions, although fuel and air were injected separately in the reaction zone. The mean local mixture fraction was nearly stoichiometric at the base of the flame without imposed air oscillations, but with large fluctuations leading to around 80% of lean or rich reaction. The degree of non-stoichiometric reaction increased with axial distance from the burner exit and Reynolds number and lean reaction dominated. Imposed air oscillations led to lifted flames and increased the degree of non-stoichiometric reaction for Re=18970 and 29100, whereas the flame remained attached onto the injector for Re=57600 and little modification of the mixture fraction was observed.