Thermoacoustic oscillations

Depending on the geometry of the dewar neck and of the other gas-filled tubes connecting cold with warm parts, the excitement of thermoacoustic oscillation is possible above the helium bath. 

CH levels are plotted. The low-frequency (12 Hz) flame movement makes the control of the combustion oscillations difficult due to changes in the heat-release distribution from one cycle of thermoacoustic instability (at 213 Hz) to another. It is evident in Fig. 15.5 that there is a time lag (of the order of 2-5 ms) between the peak in the pressure oscillation and the peak in the CH level. The variability in the time lag is presumably associated with the cycle-to-cycle variations in the flame movement. 

Figure 15.8 shows the time-averaged distributions of heat releeise with and without control. With strong oscillations and no control, the flame is anchored to the flame holder. Thus, the pressure anti-node is associated with high heat release, a condition that will promote thermoacoustic coupling. With control, the flame is slightly lifted, and heat release at the flame holder or pressure anti-node is considerably lower. 

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