Influences of initial and transient combustor wall-temperature on thermoacoustic oscillations of a small-scale power generator

Abstract

Thermoacoustic oscillations are usually investigated for events during which the combustor wall-temperature is constant. Here, we experimentally demonstrate the influence of the transient combustor wall-temperature on the thermoacoustic oscillations of a power generation system for both cold- and warm-start conditions. The experiments are performed for fixed fuel-air equivalence ratios and both fixed and varying volumetric air flowrates. The former is varied between 0.7 and 1.4, and the latter changes between 50 and 130 standard liters per minute. A data acquisition system was used to simultaneously acquire the mixture pressure along with the mixture, the combustor wall-, and the exhaust temperature using a differential pressure transducer and three thermocouples, respectively. Long time-period analysis of the pressure signal suggests the pressure oscillations feature three modes. The dominant frequencies of these modes increase with increasing the fuel-air equivalence ratio; however, they are nearly insensitive to the air volume flowrate. Probability density function, power spectrum density, phase-space trajectory, and the Poincaré map of the pressure oscillations were obtained for a sliding window and for both cold- and warm-start conditions. The results suggest that, for the cold-start condition, the pressure signal transitions from chaotic to limit cycle, chaotic, bursting, and then limit cycle oscillations. For warm-start conditions, however, the pressure signal features a combination of both large- and small-amplitude limit cycle oscillations. For these conditions, as the combustor wall-temperature increases, the possibility of the large-amplitude oscillations occurrence decreases, and that for the small-amplitude oscillations increases. For all tested conditions, the estimated Helmholtz and Strouhal numbers decrease with increasing the Reynolds number, suggesting that the system dynamics is dominated by neither the acoustics nor the vortex-shedding.