Jul 29, 1999
Journal of Sound and Vibration
Abstract If the response of a flame to acoustic waves is not known, it must be deduced from measurements in a system for which the acoustic impedances can be inferred. Measurements have been made on a simple combustion system in which gaseous propane is mixed with air in a cylindrical chamber of variable length before flowing up a burner tube. At the end of the tube, the premixture flows around a centre-body, behind which a flame is stabilized. When placed in cylindrical combustion chambers of two different lengths, the flame generates self-excited acoustic oscillations. These have been measured with pressure transducers at three different points and with a system for collecting the UV radiation from the whole flame at a wavelength of chemiluminescent emission. The data demonstrate that the acoustic transmission through the premixture supply system and through the combustion chamber can be described by standard one-dimensional acoustic theory, provided an appropriate choice of mean speed of sound in the combustion chamber is made. This then allows the jump in the volumetric acoustic fluxes across the flame to be calculated, and it is shown, by examination of the magnitude and phase of the transfer function, that this jump is proportional to the UV emission over a range of frequencies up to 800 Hz (a frequency corresponding to wavelengths of the order of the combustion chamber length). This relationship is in accordance with thermo-acoustic theory for a “thin” flame, if the UV emission can be taken as proportional to the instantaneous rate of heat release. The observations therefore validate both the experimental technique and the applicability of the theoretical assumptions, and lay the foundation for a more detailed study of the structural response of the flame.