Local Equivalence Ratio and Degree of Premixedness of Reacting Mixture in Swirl-Stabilised Natural Gas Fuelled Burners

Abstract

Local, time-dependent measurements of equivalence ratio of the reacting mixture and degree of non-stoichiometric (premixed) reaction were obtained in a swirl-stabilised natural gas-fuelled, nonpremixed burner using the intensity of Chemiluminescence from flame radicals. The measurements quantified the mean, rms of fluctuations and probability density functions of local equivalence ratio, heat release and probability of premixed, lean or rich, versus stoichiometric reaction at the stabilisation region of the flame. The burner was operated over a range of air flow Reynolds numbers from 18970 till 57600, overall equivalence ratio of 0.32 and imposed oscillations on the air flow to the burner at the resonance frequency of 350 Hz. The results show that the local mean equivalence ratio at the flame base was stoichiometric, but with large fluctuations to suggest that around 80% of the reaction was premixed either lean or rich. The degree of premixed reaction increased with axial distance from the burner exit and Reynolds number and the lean premixed reaction dominated. Imposed air flow oscillations lifted the flame and led to nearly fully premixed reaction.