Development of an acoustic diagnostic for determination of heat release rate perturbations in pulsated flames

Heat release rate fluctuations are the source of combustion instabilities that cause recurrent problems in many combustors. These quantities are difficult to measure and optical techniques are generally implemented. The initial validation of an alternative method is presented in the case of pulsated laminar premixed flames. The technique relies on determining the travel time of ultrasonic waves propagating through the flow. The link between heat release rate fluctuations and perturbations in the travel time of ultrasonic waves is established. Measurements made with this method are compared with optical measurements and an analytical model. A good agreement is obtained for the gain and phase of the transfer function between perturbations in the sound travel time and heat release rate disturbances when the flame is submitted to harmonic modulations of the flow. This work validates the principle of the proposed technique and lays the foundations for future developments.