Insight into the combustion dynamics of ITA-driven swirl flames

Abstract: In this article, we examine the flame, flow, and acoustic coupling of Intrinsic ThermoAcoustic (ITA) driven combustion instability using a high-shear swirl injector in a model combustor. The combustor is operated using pure methane and methane-hydrogen mixture as fuels with air in a partially premixed mode. In this study, the airflow rate is varied by keeping the fuel flow rates constant, corresponding to the Reynolds number range of 8000-19000 and a fixed thermal power of 16 kW. Acoustic pressure in the combustion chamber, high-speed OH$^*$, CH$^*$ chemiluminescence images, high-speed particle image velocimetry, and steady exhaust gas temperature are measured for scrutiny. The combustor shows non-monotonic variations in the acoustic pressure amplitude for both fuels. A wider operating envelope and relatively large amplitude acoustic fluctuations are observed for the methane-hydrogen mixture compared with pure methane. Both methane and methane-hydrogen mixtures exhibit a linear increase in dominant instability frequencies with airflow, indicating the ITA-driven combustion instability. A low-order network model is developed that incorporates a simple $n-\tau$ flame response. It reproduces the observed dominant instability frequency by taking in the convective time delay from the experiments that substantiates the ITA-driven instability. Spatio-temporally resolved flame and flow dynamics show periodic axisymmetric vortex shedding from the outer shear layer and its subsequent interaction with CRZ during large amplitude acoustic oscillations. At relatively low amplitude, the vortices shed from both inner and outer shear layers convect downstream at different velocities without interacting with each other. The periodic vortex shedding process across the operating condition is further examined using a simplified model representing vortex dynamics...