60011 - Dynamical Characterization of Thermoacoustic Oscillations in a Hydrogen-Enriched Partially Premixed Swirl-Stabilized Methane/air Combustor 

To minimize NOx emissions and respond to changing energy demands, operators of land based gas turbine engines that produce electricity are interested in hydrogen-enriched natural gas combustors. Varying power requirements can be met by altering the fraction of hydrogen in the fuel flow rate to the combustor while maintaining lower turnover times. In this study, we systematically analyze the effect of hydrogen addition in a partially premixed swirl stabilized methane/air combustor. The burner is based on the widely studied “PRECCINSTA” geometry. The equivalence ratio and thermal power are held constant while the fraction of hydrogen in the fuel is varied from 0% to 80% in steps of 10%.

The combustor is found to exhibit a variety of dynamical states when operated over a range of thermal power (P = 10, 15, 20 kW) and equivalence ratios. The acoustic pressure fluctuations in the combustor are measured using a microphone sampling at a rate of 100 kHz, while the simultaneous global heat release rate fluctuations are obtained from OH* chemiluminescence images acquired at 10 kHz. We use tools from dynamical systems and synchronization theories to characterize each dynamical state observed.
When the thermal power is low (P = 10 kW), we observe intermittent oscillations characterized by long epochs of low amplitude aperiodic fluctuations and short epochs of large amplitude periodic fluctuations, for most of the cases of hydrogen enrichment. However, for an intermediate thermal power of 15 kW, the subharmonic mode is excited, resulting in a low amplitude period-2 limit cycle oscillations (LCO). During this state, hydrogen addition leads to the modal interaction between the subharmonic mode and its fundamental mode, resulting in switching between period-2 and period-1 oscillations. Successive increments in hydrogen fuel fraction cause the dominant frequency to pull in and lock to the fundamental mode, resulting in high amplitude period-1 LCO. With an even higher amount of hydrogen in the fuel, the combustor reverts back to intermittent oscillations comprised of period-1 and aperiodic oscillations. For a high thermal power (P = 20 kW), the system shows switching between period-1 and period-2 oscillations for lower hydrogen content in the fuel. For a higher hydrogen content, we notice the occurrence of period-2 LCO followed by period-1 LCO. These observations suggest that, the system prefers to excite subharmonic mode for lower hydrogen composition and fundamental mode for higher hydrogen composition in the fuel.
We also study the coupled behavior of the acoustic pressure and heat release rate oscillations in the combustor for the aforementioned dynamical states. During the state of period-2 LCO, we observe a 2:1 frequency-locking, where two cycles of the pressure signal lock with one cycle of the heat release rate signal. In contrast, during the state of period-1 LCO, we observe a 1:1 frequency-locking, where both the pressure and heat release rate signals repeat their behavior after every cycle. To conclude, our study demonstrates that the addition of hydrogen in the fuel can lead to increased modal interaction, resulting in the occurrence of several rich dynamical states.