Thermoacoustic response of fully compressible counterflow diffusion flames to acoustic perturbations

The goal of this research is to study the thermoacoustic response of diffusion flames due to their relevance in applications such as rocket engines. An in-house code is extended to solve the fully compressible counterflow diffusion flame equations, allowing for a spatially- and temporally-varying pressure field. Various hydrogen-air flames with a range of strain rates are simulated using detailed chemistry. After introducing sinusoidal pressure perturbations at the inlet, the gain and phase of various quantities of interest are extracted. As the frequency is increased, the gain of the temperature source term transitions from the perturbed steady flamelet value to a first plateau at intermediate frequencies, and finally to a second plateau at the highest frequencies. At these high frequencies, the gain of the integrated heat release decays to zero, underscoring the importance of compressibility. These three regimes can be identified and explained through a linearization and frequency domain analysis of the governing equations. The validity of the low Mach number assumption and importance of detailed chemistry are assessed.