Transition to Instability in Segmented Rijke Tube

Thermoacoustic instabilities can arise in systems where unsteady heat release is favorably coupled with acoustic pressure oscillations. A modified Rijke tube with segments having different sections is found to have lower threshold levels of heat addition rate needed for exciting the fundamental acoustic mode of the tube. Experiments indicate that about 115 W of heating power is required to produce sound in the 90-cm-long segmented tube, while over 230 W is needed in 60-cm-long and 90-cm-long tubes with constant-area cross sections. One of the causes for lowering the threshold of heat addition rate is due to a significant reduction of the resonator natural frequency. The presented results suggest an importance of including the system geometry details into analysis of practical devices prone to thermoacoustic instabilities.