Combustion Oscillations Extinction Control

Naturally occurring pressure oscillations in a model combustor were quantified with heat release around 100 kW as functions of geometric and flow parameters and actively controlled by the oscillation of fuel flow. Amplitudes up to 10 kPa were dominated by the quarter-wave frequency of the duct length upstream of an annual ring, depending on the equivalence ratio of the annular flow and decreased as ring and a step were moved closer. Active control of pressure oscillations was sensitive to the location and addition of oscillated fuel and was more effective with oscillations of fuel in the main flow. Experiments with opposed flames showed that forced flame extinction depended on the total duration of pulsation and ranged from a few milliseconds to almost a second. Extinction times increased quasi-exponentially with decreasing amplitude and increasing frequency of oscillation for diffusion flames but were a non-monitonic function of the frequency inpremixed flames, with the longest extinction times corresponding to higher frequencies. The characteristics of the opposed flames were represented by the concept of a mixinglet with the width of the mixing layer a function of the sum of the bulk, the turbulent the periodic strain and the mixinglet assumed to be randomly convected. The results reproduced measured trends in the mean an rms of the scalar fluctuations and the dissipation in non-combusting, periodically forced, opposed jet flows and include extinction times in opposed flames that increased exponentially with frequency and decreasing amplitude again in accord with experiment.