EXPERIMENTAL INVESTIGATION ON COMBUSTION INSTABILITIES IN A LEAN PREMIXED BLUFF BODY COMBUSTOR PRODUCING ACOUSTICALLY LONG FLAME

The global and local characteristics of thermoacoustic instability occurring in a bluff body combustor which produces an acoustically non-compact flame (the Helmholtz number H-e = kL(flame), between 0.29 to 0.60) are reported. Over the flow conditions employed (Mach number of 0.05-0.13 and equivalence ratio of 0.60-0.85), the unstable flames are observed for a short combustor (CL=0.66 m) with partially-closed combustor exit and a long combustor (CL=1.22 m) with open combustor exit. The dominant peaks of pressure oscillation are found over a frequency range of 150-230Hz for the former and a frequency range of 200-280Hz for the latter. The discrepancy between the estimated and measured frequency is far greater for the short combustor than for the long combustor, which is attributed to the relatively longer flame length for the short combustor with respect to the combustor length than that for the long combustor, suggesting the relative flame length with respect to the characteristic wavelength is an important factor in determining the instability frequency and hence the prediction of thermoacoustic instability. The phase-averaged chemiluminescence image sequence during one period of pressure oscillation shows that the flame length changes significantly, responding to the pressure fluctuation during unstable combustion. Further analysis shows that for unstable flames, a region of flame where the local heat release fluctuation is out of phase with respect to the local pressure fluctuation and a region of flame where the local heat release fluctuation is in phase with respect to the local pressure fluctuation coexist over the flame length. The peak values of both damping and growing acoustic energy source increase as the Mach number increases, but the total acoustic source term becomes positive and its total value increases as the instability becomes strong.