Insight into the combustion dynamics of ITA-driven swirl flames

TL;DR

This study investigates the combustion instability mechanisms in swirl flames driven by Intrinsic ThermoAcoustic effects, analyzing flow, flame, and acoustic interactions for methane and methane-hydrogen fuels, and develops a model to predict dominant instability frequencies.

Contribution

It provides new insights into ITA-driven combustion instability using experimental data and a simplified vortex dynamics model, highlighting differences between methane and methane-hydrogen mixtures.

Findings

Wider operating envelope for methane-hydrogen mixture.

Linear increase in instability frequency with airflow.

Periodic vortex shedding influences combustion stability.

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...