Combustion Dynamics: Advancing Knowledge and Coping with Applications
A considerable research effort has concerned combustion dynamics issues. This work initially motivated by problems encountered during the early development of rocket propulsion has been focused during the last period on issues raised by gas turbines. To reduce NOx emissions these machines now operate in a lean premixed combustion mode. In the new combustor architectures, the flames are more compact, the power density is higher, and the damping rate is reduced a combination of factors that promotes instabilities. Under certain conditions this leads to unacceptable levels of pressure oscillations in the system and this has become an essential issue in the design of these systems. Much of the early understanding of combustion dynamics phenomena has its origin in research carried out by Luigi Crocco, his students and colleagues during his years at Princeton. This group understood that instabilities in rocket engines were caused by the delay between injection and combustion and derived the sensitive time lag theory which accounted in a clever but somewhat empirical way for the fact that the delay was sensitive to operating parameters. Much progress has been accomplished since that time in the physical understanding and identification of the fundamental processes giving rise to combustion dynamics phenomena and self sustained oscillations. This has been based on well-controlled experiments, novel theoretical ideas and high performance computing. This lecture proposes a review of issues, progress and current research in this broad field. It will cover the following topics: (1) Historical background and combustion dynamics fundamentals (introductory material intended for a general audience), (2) Perturbed flame dynamics experiments and theory, (3) Flame describing function unified framework for the nonlinear analysis of self-sustained oscillations, determination of limit cycle amplitudes, mode switching and triggering, (4) Dynamics of swirling flames, (5) Annular combustor dynamics and azimuthal modal coupling (see Fig.1). The last topic will be emphasized because the annular geometry is used in many practical systems of importance like jet engines and gas turbines. In general, these combustors are equipped with multiple injectors. This situation is investigated by making use of novel experimental configurations, new low order models and comprehensive large eddy simulations.