The free and forced response of a propane/air diffusion flame

An experimental investigation into the relationship between acoustic forcing and coherent structures in propane/air diffusion flames is presented. The relationship between acoustic forcing and coherent structures is established with the view to applying controlled acoustic feedback to improve mixing rates, and hence combustion efficiency. Coherent structures are sensed using the laser Schlieren technique, and the detector signals are correlated with the acoustic pressure signal (excitation component) under a variety of excitation conditions. Impulse response estimates for the acoustic pressure to Schlieren response transfer function are obtained by a PRBS based cross correlation technique. The flow response waveforms were processed further using minimum mean square error identification and homomorphic deconvolution to resolve the propagation delay of the large scale structures. By combining simple excitation waveforms and the technique of signal averaging it is possible to identify, at least in a qualitative sense, some of the non-linear characteristics of the flow process. Impulsive, square wave, pseudo-noise and sinusoidal signals were used to excite the flow at varying levels of excitation. The results obtained by each method were consistent with a frequency selective saturation mechanism with the knee of saturation occurring at an overall sound pressure level of around 60 dB. These observations appeared to be consistent with velocity induced vortex shedding caused by the acoustic characteristics of the combustor. Mathematical modelling of the acoustic characteristics of the combustor demonstrated the validity of the velocity induced vortex shedding hypothesis. Application of Schlieren feedback significantly altered the flow structures within the diffusion flame. Schlieren feedback results demonstrated a large reduction in small scale turbulence and a concentration of flow energy into the coherent resonant structures. The frequency of resonance of the flow with feedback is dependent upon the position of the laser Schlieren detector relative to the nozzle exit plane. Centre-line temperature profiles show that both acoustic forcing and Schlieren acoustic feedback result in improved mixing rates. In the main it seems that the outer structure has greatest effect upon mixing rates as excitation at low frequencies gave the greatest increase in centre line temperature. Inner structure forcing and feedback also results in improved mixing although the temperature improvement is not as large.