Control of combustion oscillations

Combinations of small quantities of fuel and imposed oscillations are shown to ameliorate combustion oscillations in ducted flows of premixed methane and air with equivalence ratios from the lean limit to stoichiometry. A sudden expansion from 51 to 80 mm stabilized the flames and the control arrangements were evaluated with an unconstricted exit and with a nozzle. The added fuel enhanced stability and reduced the amplitude of low-frequency oscillations associated with high strain rates and their tendency to modulate acoustic frequencies at high equivalence ratios. The imposed oscillations at harmonics of the acoustic frequencies moderated the latter. The two together caused substantial reductions in amplitude of combustion oscillations over a wide range of equivalence ratios and Reynolds numbers, limited only by the inability of acoustic drivers to operate with high-amplitude pressure signals. Related measurements in the absence of added fuel and imposed oscillations provided a basis for comparison. Three arrangements for injecting fuel were evaluated and the best involved radial injection of up to 5% fuel through four equally spaced holes a short distance from the sudden expansion: The spread of the improved stability with added fuel through the annular region of separated flow behind the expansion was remarkable, even with a single injection location. The injection of mixtures of fuel and air was examined and the resulting higher strain rates were found to render it less satisfactory. Combinations of active control with imposed oscillations and unsteady injection of fuel also proved to be unsatisfactory, partly due to increased strain rates and partly to the broadband low-frequency oscillations.