Influence of enhanced heat transfer in pulsating flow on the damping Characteristics of resonator rings

Rocket thrust chambers are prone to thermoacoustic instabilities. Apart of the structural loads induced by pressure fluctuations, considerably enhanced heat transfer has been repeatedly observed under pulsating flow driven by unstable combustion. To increase stability and extend the operation margin of the engine, the application of resonator rings is common practice. This thesis aims at providing a more fundamental understanding of the functionality of resonator rings and their sensitivity to gas temperature inhomogeneities possibly caused by the aforementioned enhanced heat transfer. To truly evaluate the functionality of the resonators and provide a complete picture of their stabilizing influence, a linear thermoacoustic stability prediction method is presented. This low-order acoustic network approach is capable not only of handling three-dimensional acoustic modes, but also of accounting for the essential driving and damping mechanisms, giving special attention to the resonator ring, and allowing parametric studies. It is shown that the inhomogeneity in the gas temperature can indeed reduce the performance of the resonators and might lead to the destabilization of the engine. Furthermore, the mechanisms that lead to enhanced heat transfer in pulsating flow induced by acoustic waves are also investigated through a series of configurations of increasing complexity. Firstly, a low-order analytical model for the convective heat flux through a wall of finite thickness is given, that accounts for heat transfer coefficient and bulk flow temperature imposed pulsations. In subsequent steps, computational fluid dynamic approaches are employed to study the response of the laminar and turbulent boundary layers, and resulting heat transfer, to bulk flow velocity pulsations. An acoustically compact LES approach is followed, allowing for management of the turbulent case with an incompressible solver under admissible computational costs. The method is extended into a weakly-compressible formalism to account for temperature-dependent properties and imposed acoustic pressure fluctuations. These investigations give a qualitative order of the magnitude of the enhancement for a wide range of pulsation parameters.