Combustion dynamics and instabilities: Elementary coupling and driving mechanisms

Elementary processes that can be involved in the development of combustion instabilities in gas turbine combustors are described. The premixed mode of combustion is considered more specifically because it is used in most advanced gas turbine systems. The processes envisaged portray the combustion dynamics of real systems, but they are analyzed in simple laboratory configurations. Among the many possible interactions, the most relevant mechanisms are those that generate fluctuations in heat release or induce pressure perturbations. Some typical paths are highlighted to help in the understanding of the multiple links that can exist between elementary processes. Processes involving acoustic/flame coupling, unsteady strain rates, flame response to inhomogeneities, interactions of flames with boundaries, and flame/vortex interactions are specifically examined. For each process, a driving or a coupling path is proposed relating heat release fluctuations to acoustic variables in certain cases or leading from acoustic variables to heat release fluctuations in other cases. Stress is also put on characteristic time lags, which are key parameters in the triggering and development of instabilities. Well-controlled experiments illustrate the many possibilities and can serve to guide the modeling effort and to validate computational tools for combustion dynamics.