An Acoustic Methodology to Measure Heat Release Rate Fluctuations From Unsteady Laminar Flames

Abstract

This paper presents first validations of an alternative method to optical diagnostics to evaluate unsteady heat release rate disturbances. The technique is based on the determination of the propagation time of ultrasonic waves crossing a reacting flow. A train of pulses is synthesized and transmitted to the space by a tweeter. This signal is captured before and after its passage through the flame. The cross-correlation of the two signals yields a narrow compressed pulse response with a main lobe corresponding to the time lag between the incident and transmitted signals. The technique is examined in two generic laminar configurations. For open flames featuring buoyancy effects, it is possible to link the rate of change of fluctuations of the sound travel time dΔt′/dt to heat release rate disturbances Q˙′. For pulsated confined flames, the situation is more complex and a transfer function must be defined between perturbations in the sound travel time Δt′ and heat release rate disturbances Q˙′. Measurements are compared to predictions using different techniques. A general agreement is obtain in term of phasing and amplitudes. This study shows that the proposed technique is very sensitive to small perturbations in the heat release rate and provides the basis for further developments oriented towards more practical configurations.