This paper is based on findings resulting from ASHRAE Research Project RP-1517.

Combustion oscillations are the result of thermoacoustic instabilities. Sound produced from the flame is reflected from the combustion chamber into the mixture chamber. This disturbs both the mixture flow rate and composition, intensifying the sound produced by the volumetric expansion and contraction of the flame. The ratio of the sound pressure to the volume velocity or source strength at the flame is known as the acoustic impedance. Acoustic impedance is a very useful quantity for both assessing whether combustion oscillations are likely to occur and making system modifications to avoid them. Principles for both measuring and simulating the acoustic impedance are summarized in this paper. The impedance is determined both experimentally and via simulation for three different combustion chambers. The methods presented permit the inclusion of structural coupling fromthecombustion chamber. Additionally, a method for determining the impedance above the plane wave cutoff frequency of the boiler is documented.