Experimental evaluation of Markstein-number influence on thermoacoustic instability

Laminar, premixed methane-air flames propagating through the annulus of a Taylor-Couette burner were studied experimentally. Such flames ignited at the top, open end of the burner and propagating downward toward the closed end are susceptible to the Darrieus-Landau hydrodynamic instability as well as to thermoacoustic instabilities that develop. Flame speeds measured during a stage of quasiplanar propagation near the middle of the burner attained through action of the primary thermoacoustic instability were found to agree well with laminar-flame speeds reported in the literature. Axial and circumferential velocity fluctuations of the flow field were measured during flame propagation using laser Doppler velocimetry (LDV), and critical acoustic-velocity amplitudes for saturation of the primary thermoacoustic instability and for the onset of the secondary thermoacoustic instability were determined. The influence of the equivalence ratio on the critical acoustic-velocity amplitude for the onset of the secondary thermoacoustic instability determined front the LDV measurements is shown to agree well with theoretical predictions. This influence is consistent with an increase in the stabilizing effect of flame stretch with increasing equivalence ratio for methane-air flames. The Markstein number increases with increasing equivalence ratio for these flames, resulting in growth rates of the secondary thermoacoustic instability for rich methane-air mixtures that are lower than those for lean mixtures with similar laminar-burning speeds. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.