Influence of Heat Losses on Thermo-Acoustic Coupling Frequencies*

A simplified model to predict the coupling frequencies of self-induced thermo-acoustic instabilities of premixed flames propagating from the open to the closed end of a tube is here addressed in the presence of heat-losses. Non-adiabatic tubes affect the coupling frequency by modifying the temperature distribution of burned gases and thereby the acoustic modes.

Experimental validation of the model is achieved thanks to a novel apparatus that controls the combustion chamber wall temperature through a recirculating thermal bath. A slender tube of diameter D=10 mm is used to burn methane-air mixtures. It is observed that different acoustic modes exhibit preferential regions of instability. In sufficiently long tubes, L ~ 100 cm, the coupling between the flame and the acoustic modes leads to a first-harmonic excitation near the ends of the tube. Contrarily, in short combustion chambers, L ~ 30 cm, the interaction destabilizes the fundamental acoustic mode at the mid region. For intermediate lengths, superposition of both modes is reported following a progressive shift for varying lengths. The experimental results of excited frequency and coupling regions are obtained via high-speed imaging and audio recording analyses, which provide great agreement with the theoretical model.