Thermoacoustic instabilities of highly premixed flames in combustors and boilers

Abstract

The attempt to reduce the formation of pollutants from combustion processes has led in recent years to the development of novel low-temperature combustion processes. Confined flames of this type exhibit a number of instability mechanisms, which can lead to unacceptable combustion noise or even to severe damage. The instability mechanisms, which appear in cases of technical interest, are reviewed and explained. It is shown that a realistic model for the description of the instability requires a proper representation of the contribution of the flame as the driving element. To date, nonlinear transfer functions have to be deduced from combustion experiments, but attempts to derive transfer functions numerically have already been published. In the past, modeling efforts were strongly focused on longitudinal modes. Although circumferential modes are of technical importance, they have not yet been fully understood. In this case, a complex interaction of the nonlinear flame with the linear acoustic system has to be taken into account and the transition between modes becomes more important. Since a satisfactory description of the interaction is not yet available, active instability control techniques have been proposed. Their limit in cases of technical interest will be discussed.