Modeling premixed combustion-acoustic wave interactions: A review

The interactions between acoustic waves and a premixed combustion process can play an important role in the characteristic unsteadiness of combustion devices. In particular, they are often responsible for the occurrence of self-excited, combustion-driven oscillations that are detrimental to combustor life and performance. A tutorial review is provided of current understanding of these interactions. First, the mutual interaction mechanisms between the combustion process and acoustic, vorticity, and entropy waves are described. Then, the acoustic-flame interaction literature is reviewed, primarily focusing on modeling issues. This literature is essentially organized into four parts, depending on its treatment of 1) linear or 2) nonlinear analyses of 1) flamelets or 2) distributed reaction zones. A sizeable theoretical literature has accumulated to model the unsteady response of the laminar flame structure, for example, the burning rate response to pressure perturbations. However, essentially no serious experimental effort has been performed to critically assess these predictions. As such, it is difficult to determine the state of understanding in this area. On the other hand, good agreement has been achieved between well-coordinated experiments and theory describing the interactions between inherent flame instabilities and acoustically induced flow oscillations. Similarly, both the linear and nonlinear kinematic response of simple laminar flames to acoustic velocity disturbances appear to be well understood, as evidenced by the agreement between surprisingly simple theory and experiment. Other than kinematic nonlinearities, additional potential mechanisms that introduce heat release-acoustic nonlinearities, such as flame holding, or extinction, have been analyzed theoretically, but lack experimental verification. Unsteady reactor models have been used extensively to model combustion processes in the distributed reaction zone regimes. None of these predictions appears to have been subjected to direct experimental scrutiny. It is unlikely that this modeling approach will be useful for quantitative combustion response calculations, due to their largely heuristic nature and the difficulty in rationally modeling the key interactions between reaction rate and the global characteristics of the combustion region, such as its volume. Several areas in need of work are particularly highlighted. These include finite amplitude effects, modeling approaches for interactions outside of the flamelet regime, turbulent flame wrinkling effects, and unsteady vortex-flame interactions.