Competing Contributions of Structural, Acoustic, and Intrinsic Modes to Thermoacoustic Oscillations

ABSTRACT

Structural, acoustic, and intrinsic modes contributions to thermoacoustic oscillations of a small-scale power generator combustor are investigated experimentally. Simultaneous $\mathrm{O}{\mathrm{H}}^{\ast }$ chemiluminescence and pressure data were acquired at frequencies of 5 and 50 kHz, respectively, and are used to study the above coupling. Experiments were performed for fuel-air equivalence ratios of 0.7 and 0.9, igniter rod locations of −5, 1.5, 2.5, and 5 mm, and air volumetric flowrates of 120, 140, 160, and 180 SLPM, generating 32 experimental conditions. The results show that, depending on the tested condition, turbulent methane-air premixed flames were stabilized either on a torus-shaped flame-holder (generating Bunsen flames) or on both the flame-holder and an igniter rod (generating M-shaped flames). At fuel-air equivalence ratio of 0.7, two dominant peaks near the structural and acoustic modes are detected. In addition to structural and acoustic modes, two intrinsic thermoacoustic modes are also detected for conditions pertaining to the fuel-air equivalence ratio of 0.9. It is shown that acoustic and structural modes mostly contribute to positive Rayleigh gain values. However, generally, intrinsic modes contribute to negative values of the Rayleigh gain. The negative Rayleigh gain values associated with the intrinsic thermoacoustic modes are detected near the exit of the burner as well as the stagnation wall. This suggests that, for the utilized power generator combustor, compared to structural and acoustic modes, existence of intrinsic thermoacoustic modes are desired.

KEYWORDS:

• Thermoacoustic oscillations
• Rayleigh gain
• OH^* chemiluminescence
• intrinsic thermoacoustic mode
• structural mode

Acknowledgments

The authors would like to acknowledge the Natural Sciences Engineering Research Council (NSERC) of Canada for financial support through the Collaborative Research and Development grant (CRDPJ 525890-18).