Most of gas turbines are operated by the type of dry premixed combustion to reduce NOx emission and economize fuel consumption. However, in this type of operation, combustion-induced instability brought failure problems caused by high pressure and heat release fluctuations. Even though there have been lots of studies since Lord Rayleigh to understand this instability mechanism and to control the instabilities, none of them is able to explain the instability mechanism clearly.
In order to understand the instability phenomena, a simple experimental study with dump combustor was conducted at the moderate pressure and ambient temperature conditions. From this model gas turbine combustor, self-excited instabilities at the resonance mode (200Hz) of combustion chamber and bulk mode (10Hz) were occurred and observed at the three points of view; pressure, heat release and equivalence ratio which are acquired by peizo-electric transducer, HICCD camera and acetone LIF respectively.
From these results, we could explain the instability mechanism clearly using the time scale analysis which can be explained by the propagation of pressure wave to the upward of mixture stream and convectional transfer of the equivalence ratio fluctuation induced by this pressure fluctuation. Furthermore, from heat release images, we could know heat release rate fluctuations qualitatively, and thus we found that the time derivative of heat release rate is more significant parameter than the heat release rate when we consider the instability mechanism.
Finally instability mechanism can be explained by the feedback system coupling among the pressure fluctuation, heat release rate fluctuation, flow velocity fluctuation of a mixture and equivalence ratio fluctuation. Furthermore, we can get the other detailed conclusions. Firstly, the period of feedback loop cycle is determined by the characteristic time scale which plays an important role in combustion instability mechanism. Secondly, the dominant elements of feedback system coupling mechanism can be differed according to the operating conditions.
근래 대부분의 가스터빈은 NOx 등의 유해배기가스를 줄이기 위해 건공기 예혼합 연소방식을 이용하고 있다. 그러나 이러한 방식은 연소에 기인한 진동을 일으키며, 이는 열과 압력의 변동으로 인한 연소실과 터빈 블레이드의 기계적 파괴를 일으킨다. 이러한 연소진동의 기구를 이해하기 위하여 덤프형의 연소기를 제작하고, 상온 상압의 조건에서 실험을 하고, 자기발생의 불안정성을 일으키는 것을 관찰하였으며, 이는 운전조건에 따라 연소기의 공명모드와 시간 지연 분석법으로 설명되는 모드를 구분할 수 있었다. 압력센서, 고속 카메라, 아세톤 레이저 유도 형광법을 이용하여 압력, 열발생율, 당량비 변동의 측면에서 연소 불안정성을 관찰하였다. 그 결과 대류시간 음향시간 등의 특성시간을 정의한 시간지연분석법을 통해 불안정성 기구를 이해할 수 있었다. 또한 이 시간스케일이 연소 진동의 현상을 지배하는 중요 변수이고, 운전 조건에 따라 각기 다른 불안정성 기구에 의해 설명되는 것을 알게 되었다.
