Low-frequency combustion instability

J.-M. Samaniego, B. Yip, T. Poinsot, and S. Candel. Low-frequency combustion instability mechanism in a side-dump combustor. Combust. Flame, 94(4) 363-381, 1993. 

The existence of low-frequency combustion oscillations superimposed on the primary instability has been reported by De Zilwa et al. [9] for premixed gaseous systems. As noted earlier, the present pressure and CH-photo-diode measurements also reveal low-frequency oscillations at around 12 Hz. This low-frequency mode is due to the cavity between the acoustically-closed upstream inlet end and the constricted exit nozzle of the air-delivery inlet that leads to a bulk mode oscillation in the flame. The existence of these oscillations can be clearly seen in Figs. 15.2, 15.5, and 15.6, where the time-variations in the peak pressure and... 

When an energetic material burns in a combustion chamber fitted with an exhaust nozzle for the combustion gas, oscillatory combustion occurs. The observed frequency of this oscillation varies widely from low frequencies below 10 Hz to high frequencies above 10 kHz. The frequency is dependent not only on the physical and chemical properties of the energetic material, but also on its size and shape. There have been numerous theoretical and experimental studies on the combustion instability of rocket motors. Experimental methods for measuring the nature of combustion instability have been developed and verified. However, the nature of combustion instability has not yet been fully understood because of the complex interactions between the combustion wave of propellant burning and the mode of acoustic waves. 

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. 

Low Frequency Noise Sources Combustion roar and instability Fan noise... 

Combustion instability noise can occur if too much steam or air is added in the base of the flame. So, overaerating the waste gas stream in a flare, causes the flame to periodically lift from the flare tip. This periodic lifting and reattachment of the flame from the flare tip is the mechanism that drives the low frequency rumbling noise. Usually, combustion instability noise can... 

Combustion instability within a furnace is characterized as a high amplitude, low frequency noise often resembling the puffing sound of a steam locomotive. This type of noise can create significant pressure fluctuations within a furnace that can cause damage to the structure and radiate high noise levels to the surroundings. 

Otherwise called pulses, induced flow instabilities at relatively low frequencies (<5 Hz) have been shown to improve heat transfer and have a positive effect on fouling. An analogy can be seen in pulse combustion systems, where the effect is to break down steady-state conditions and to create instantaneous changes in the fluid velocity and direction. The effect on fouling has been likened to coughing . 

Similar phenomena have also been observed in the combustion of composite propellants. Eisel (El) has observed that there is a unique frequency-pressure relation in a low-pressure region where nonacoustic instability results. He speculates that this preferred frequency is related to the periodic appearance and depletion of the aluminum particles on the propellant surface. High-speed pictures confirm the periodic sluffing of aluminum, but its relation to the preferred frequency is still not clear. 

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