The Pulsed Electric Discharge As An Acoustic Probe For Combustion Chamber Diagnostics

A brief review is given of combustion as an acoustic source with special reference to the acoustic stability of working combustion chambers. A theoretical model is developed of the pulsed arc discharge as an acoustic source under free field conditions following a review of previous work. A specific feature of the model is that account is taken of the fractional heat loss to the electrodes due to thermal conduction. The experimentally determined acoustic emission from the discharge is shown to follow the trends predicted by the model. A theoretical model is also developed of the acoustic impedance 'seen' by the pulsed arc at the fundamental resonant frequency in a working combustion chamber. The model is based on electrical transmission line theory and takes into account the radiation resistance of the chamber ends, the mean temperature inside the chamber, and the acoustic impedance of the burner and fuel supply system. The model is further developed to include the effects of ceramic fibre material used as an end termination. Lt shows the way that the acoustic impedance 'seen' by the source is altered by the additional acoustic resistance introduced to the system and shows how the resistance and reactance of the material may be determined from relatively simple acoustic pressure measurements. An experimental system is described whereby the relevant electrical power to the arc is derived from the Fourier series of the periodic power pulse. This enables the relevant source strength to be determined easily and from which the impedance at system resonance may be calculated. The performance and limitation of the pulsed arc as an acoustic transducer for stability diagnosis are discussed.