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UBC Theses and Dissertations

Experimental study of thermoacoustic oscillations inside a small-scale power generator combustor Heydarlaki, Ramin


Thermoacoustic (pressure and heat release rate) oscillations can be detrimental to the operation of engineering equipment ranging from large-scale gas turbine engines (hundreds of mega watts) to small-scale power generators (few kilo watts). The focus of this thesis is thermoacoustic oscillations occurring in the combustor of a small-scale power generator. In this combustor, circulation of exhaust gas around reacting gas is utilized to increase the combustor thermal efficiency and is facilitated by using a stagnation wall. Two experimental setups are utilized. The former is a close replication of the engineering equipment, and the latter is a simplified version of the equipment, but it provides optical accessibility. In the experiments, the fuel-air equivalence ratio and air flowrate are varied from 0.7to 1.4 and 50 to 180 SLPM, respectively. For the experiments related to the first setup, pressure and temperature measurements are performed. For the second setup, both pressure and flame chemiluminescence measurements are acquired. For the first setup, the characteristics of thermoacoustic oscillations while the wall-temperature increases and how the initial wall-temperature influences these characteristics are investigated. It is shown, that for cold-start conditions, the pressure transitions from chaotic to limit cycle, chaotic, bursting, and then limit cycle oscillations. For warm-start conditions, however, the pressure features a combination of both large- and small-amplitude limit cycle oscillations. For the second setup, contributions of different types of thermoacoustic modes to Rayleigh gain is investigated. Results show that, for fuel-air equivalence ratio of 0.7, thermoacoustic oscillations feature natural structural (21 Hz) and acoustic (61 Hz) modes. However, oscillations related to fuel-air equivalence ratio of 0.9 feature intrinsic modes near 240 and 290 Hz, in addition to structural and acoustic modes. It is demonstrated that acoustic and structural modes mostly contribute to the growth of the thermoacoustic oscillations. However, generally, intrinsic modes contribute to mitigation of the thermoacoustic oscillations by providing negative acoustic energy source term mainly near the flame holder and stagnation wall. To the best knowledge of the author, the contributions of structural and intrinsic modes to Rayleigh gain have not been investigated in the past.

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