Heat release rate fluctuations are the source of combustion instabilities that causerecurrent problems in many combustors. These quantities are difficult to measure andoptical techniques are generally implemented. The initial validation of an alternativemethod is presented in the case of pulsated laminar premixed flames. The technique relieson determining the travel time of ultrasonic waves propagating through the flow. The linkbetween heat release rate fluctuations and perturbations in the travel time of ultrasonicwaves is established. Measurements made with this method are compared with opticalmeasurements and an analytical model. A good agreement is obtained for the gain and phaseof the transfer function between perturbations in the sound travel time and heat releaserate disturbances when the flame is submitted to harmonic modulations of the flow. Thiswork validates the principle of the proposed technique and lays the foundations for futuredevelopments.

The paper is concerned with an extension of the classical relation between the flame speed and the curvature-flow stretch, valid only for high Lewis numbers (diffusively stable flames). At low Lewis numbers the corresponding flame-flow system suffers short-wavelength instability, making the associated initial value problem ill-posed. In this study the difficulty is resolved by incorporation of higher-order effects. As a result one ends up with a reduced model based on a coupled system of second-order dynamic equations for the flame interface and its temperature. As an illustration the new model is applied for description of diffusively unstable stagnation-point flow flames.