| What are self-excited combustion oscillations? |
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In technical combustion and drive systems, under certain operating conditions, self-excited combustion oscillations can arise. These oscillations, also commonly known as pulsations or combustion instabilities, are characterized by severe pressure oscillations occurring at discrete frequencies. In low-power equipment such as vehicle or domestic heating systems, these pressure oscillations are most evident by the severe noise emissions they cause. In more powerful equipment, such as hot-blast stoves, process gas heaters, gas turbines and rocket drives, such high pressure amplitudes can be generated that the reciprocal loads these cause in combustion chambers and in upstream and downstream sections of plant can cause mechanical failure in this equipment. For example, in an atmospheric process gas heater, oscillation amplitudes of 0.5 bar can be reached, with amplitudes of up to 5 bar occurring in rocket combustion chambers.
As well as pressure oscillations, self-excited combustion oscillations in excess of the sound barrier can give rise to corresponding fluctuations in flow speed which in turn causes a sharp rise in thermal transfer to the walls of the combustion chamber. As well as mechanical loads, higher temperature loads can also occur, leading to the risk of heat causing the destruction of the combustion chamber. Often, adequate stabilization of the flame when high oscillation amplitudes occur does not arise, causing the flame to blow out or blow back. Essentially, any self-excited combustion oscillation may be the result of several physical mechanisms which, under appropriate conditions, may give rise to resonance. To enable these oscillations to arise independently, the acoustic pressure and thermal power induced oscillations need to x93encouragex94 each other. This calls for the presence of a feedback mechanism which excites thermal power oscillations in such a way that these can amplify the pressure oscillation. In most cases, this feedback is derived from the acoustic properties of the relevant combustion system. However, other mechanisms have also been observed. For example, structure oscillations in rocket motors gave rise to modulated fuel delivery to the combustion chamber, resulting in a fluctuation in heat-release during the combustion process. The vibrations resulting from these power fluctuations in turn amplified the structure oscillations, thereby closing the loop in the cycle. |