Flow Characterisation of Flames in an Acoustically Excited Chamber

Flame-acoustic wave interactions have been studied widely in the combustion community; however the whole physicochemical mechanism is still not clear. The present research aims to further investigate the unfinished areas and focus on the acoustic excitation effect on flow and flame characteristics. An experimental approach has been adopted, and the results have been analysed and discussed. PIV diagnostic system and high speed schlieren visualisation system have been applied and built together with a signal synchronisation system which enables the phase-locked observation. The acoustic field and the interactions between acoustic waves and flames in an acoustically excited tube were studied in detail. The acoustic wave induced by external excitation and its effect on both cold jet fuel flow and jet flames have been investigated. Results indicate that the flame behaviour is affected mostly by the variation of the excitation frequency. The infrasound (Frequency < 20 Hz) was observed to have less influence on air motion. The natural flame flicker was suppressed by the forcing infrasound because of the excitation effect on the fuel jet. In the case of harmonic frequencies at which a standing wave is formed, the ambient air and flame were less affected by the excitation in the velocity node area. In contrast, the cold fuel jet, flame and ambient air experienced large velocity variations in the anti-node area. At non-resonant frequencies, which are between the 1st and 2nd harmonic frequencies (65 Hz to 220 Hz), the flame pattern and luminosity were very different in each excitation case. The frequency analysis of flames has shown that flame/acoustic coupling behaviour results in a complex nonlinear coupling effect. The excitation frequency may be coupled with the sub-harmonic frequencies and the flame flickering frequency. These frequencies were then found to couple with each other and create complex nonlinear frequency couplings. It can be seen that the surrounding air, cold jet fuel flow and flames are strongly affected by the excitation frequencies, phase angles and the nozzle location relative to the tube, and the coupling effect creates complex flame dynamics.