The Pulsating Arc Discharge As A Sound Source For Use In The Measurement Of Acoustic Stability Parameters In Operational Combustion Systems

A brief review of previous work on acoustic instabilities in combustion is presented, drawing the conclusion that a sound source of known strength located at the flame could provide an improved technique for determining the important acoustic stability parameters of working combustion systems. A theoretical and experimental investigation of the properties of a pulsating are discharge as a sound source is described, which indicates that such a source would be particularly suitable for that purpose. The investigation adds to and modifies the results of other workers on sound emission from gaseous discharge, specifically in interpreting the influence on sound output of the temperature of the discharge and the acoustic coupling between the source and the radiation field. A formula is derived giving the equivalent free-field acoustic strength of the are for use at low frequencies which is shown to accord with the experimental findings. The pulsating are source, used in a laboratory model combustion system, is capable of providing the acoustic impedance relevant to sound power emission and those transfer terms which describe any acoustic amplification brought about by the combustion process. The impedance measurements are shown to be in general agreement with theoretical prediction and an analysis of the system, based on a simple electrical analogue and using the measured transfer terms, is shown to give the correct stability boundaries for the system examined. The analysis, which differs from that of previous workers, treats the flame and the system as a single component in a feed-back loop; but distinguishes the specific contribution to stability conditions of local acoustic pressure and the volume velocity generated by the source process. Limitations of the technique are discussed and other uses for the are considered, including its operation as a suitable source for anti-sound.