An experimental study of combustion dynamics of a premixed swirl injector

Abstract

A comprehensive experimental study has been conducted on combustion instabilities in a premixed swirler injector system at various equivalence ratios, chamber pressures, and inlet air temperatures. A stability map has been determined to indicate the range of operating conditions conductive to the occurrence of instabilities. The amplitude of instabilities was found to be a strong function of the equivalence ratio, with pressure oscillations as high as 20% of the mean chamber pressure and unsteady velocities comparable to the mean flow values observed for equivalence ratios around 0.6. On the other hand, beyond an initial threshold value of inlet air temperature at which instability suddenly initiated, variations in inlet air temperature had minimal effect on the strength of instabilities. Measurements of steady and unsteady flame structures carried out using CH chemiluminescence and photographic imaging techniques indicate that the onset of instabilities can potentially cause significant alterations in flame structure, sometimes even causing near extinction of the flame during certain periods of the oscillation cycle. Coupled longitudinal oscillations were observed in the combustion chamber and upstream duct. Instability characteristics such as frequency, mode shape, and phase obtained through pressure measurements were found to be in excellent agreement with predictions from a linear acoustic analysis. A strong correlation was found between the heat release and pressure fluctuations near the dump plane, indicating a possible mechanism for creating and sustaining instabilities.