Thermoacoustic coupling regions of premixed-flames in non-adiabatic tubes

Abstract

Self-induced thermoacoustic instabilities of premixed methane-air flames propagating in horizontal slender tubes are studied both theoretically and experimentally. First, modeling of responsive flames subject to heat-loss corrections, which yield realistic non-isothermal acoustics, is considered. In particular, we compare two coupling mechanisms proposed in the literature, velocity and pressure coupling. Their unstable behavior is controlled by different Flame Transfer Functions (FTF), which define the flame response to acoustic perturbations and are here coupled with a conductive heat-loss scenario in the aforementioned non-adiabatic tubes. Secondly, these mechanisms are characterized by distinct Acoustic Structure Functions (ASF), which are shown to provide the position of constructive coupling of the instabilities and their use as predictors of unstable regions is discussed. It is remarkable that heat losses have a significant impact on the theoretical predictions of the location of these physical mechanisms. Finally, a tube of variable length L and a fixed diameter D=10 mm with wall temperature control is used to exploit nearly viscous laminar flows induced by the propagating flame. The instability regions of each harmonic observed in the experiments are compared to the theoretical predictions. Excellent experimental agreement with the ASF predictions provides further evidence to validate the theoretical non-adiabatic model. These results provide additional evidence for velocity coupling mechanism to be responsible for the initial process that pairs the acoustic modes and the unsteady heat release rate of the flame in a combustion chamber.