This paper describes a new experimental technique for determining the stability margin of combustors, which is a measure of how close they are to becoming unstable. The stability margin is determined from measurements of transfer functions G_(q/p) and G_(p/q), which describe the response of the combustion process heat release to driven flow oscillations and the response of the combustor flow to the excitation of reaction rate oscillations, respectively. For limit cycle oscillations to occur, the product of these two nonlinear transfer functions must equal unity, and the sum of their phases must equal zero. The two transfer functions were measured in a developed small-scale combustor setup and were used to determine its stability margin for a range of frequencies. In this study, a siren was used to drive flow oscillations and a fuel injector actuator was used to generate reaction rate oscillations. Comparisons of the determined stability margin of the combustor with observations of its stability characteristics show that the proposed technique can indeed determine the stability margin of the combustor. The results indicate that the transfer function G_(q/p) is amplitude dependent and dominates the nonlinear processes that determine limit cycle amplitude. Therefore, determination of this transfer function by the developed technique will enable combustor designers to predict the amplitude of the limit cycle oscillations in the combustor should it become unstable.
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