Combustion instabilities of ultra-lean premixed H2/Air mixtures by prechamber hot jet ignition

Unstable combustion was observed in a dual chamber combustor that mimics heavy-duty gas engines utilizing prechamber jet ignition. Prechamber combustion generated a hot turbulent jet, which then ignited the ultra-lean H2/air mixture in the main chamber. Combustion instability was triggered for equivalence ratio, φ < 0.5. Thermoacoustic instability grew severe as the equivalence ratio of the main combustor reduced to the lean-burn regime, 0.22 < φ < 0.3. Simultaneous Schlieren and OH^∗ chemiluminescence were used to visualize the unstable flame propagation and to characterize the instability. Classical acoustic resonator analysis and pressure spectra showed the longitudinal mode of acoustic disturbance as the primary instability mode for all equivalence ratios. However, a leaner flame was affected more due to stronger coupling between heat release and the pressure field perturbation. Strain rate along flame edge showed an oscillating strain along pressure perturbation cycle. To model the combustion instability, firstly, a reduced order, 1D linearized Euler equations coupled with lower order combustion response functions were solved in the frequency domain to study and predict the fundamental modes of combustion instabilities in the system. Then 3D linearized Euler equations coupled with advanced combustion response functions were solved for the exact combustor geometry using a commercial finite element solver COMSOL. The 3D model accounted for mean flow effects, nozzle effects and advanced combustion response in analyzing instabilities. Based on the experimental and modeling results, a mechanism for thermoacoustic instability was proposed for ultra-lean premixed H₂/air combustion by a hot turbulent jet.