The stabilization of turbulent flames, together with the emission of combustion products, is the most important problem of the gas turbine combustor. Combustion instability is a serious obstacle for the lean premixed combustion of gas turbines and can even cause fatal damage to the combustor and entire system. Thus improved understanding of mechanisms of combustion instability is necessary for the design and operation of gas turbine combustors. In the present study, in order to understand the instability phenomena, an experimental study was conducted at the moderate pressure and ambient temperature conditions. From this model gas turbine combustor, various types of combustion instabilities were observed with respect to the change of equivalence ratio, fuel flow conditions and fuel injection location. 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 piezo-electric transducer, High speed Intensified Charge Coupled Device (ICCD) camera and acetone Laser Induced Fluorescence (LIF) respectively. It is found that the pressure fluctuation of turbulent combustion in a lab-scale combustor is highly related to the vortex structure and the equivalence ratio fluctuations due to fuel flow modulation and unmixedness of the fuel and air. These results suggest that the causes of heat release rate fluctuation are not only equivalence ratio fluctuation due to the fuel and air flow modulation but also large vortical motion of the mixture flow and the unmixedness of the fuel and air. Among the causes of heat release rate fluctuation, the coherent vortex structures have a dominant effect on pressure fluctuation of combustion instability.
Particularly in the unchoked fuel flow condition, it is found that the oscillation time of combustion instability is highly related to the convection time of the fuel, which is defined as the time taken by fuel from its inlet to the dump plane. To investigate the modulation of fuel flow at fuel injection holes, in experimental method, after acquired the mixture of the fuel and air using the high speed solenoid valve and gas sampling cylinder and we could analyze quantitatively by using a gas chromatography. Besides in numerical method, we use the commercial program to know that how the fuel injection condition modulated with respect to the pressure, velocity of fuel flow, and mass fraction at choked and unchoked condition. To reduce the effect of modulation of fuel flow, Helmholtz type resonator was used to apply to the channel of the burner because the equivalence ratio could be modulated due to the unburned mixture flow in the channel of this burner. After we attached the Helmholtz type resonator at the channel to make the mixture flow of this burner or at the fuel injecting holes, the effect and the performance could be checked. The experimental and numerical methods were used to verify the result. Therefore it is found that there are some methods to decrease the effect of modulation of fuel flow.
These results provide the combustion instability mechanism of the model gas turbine combustor and if these causes of combustion instability are properly considered they can help to design the more improved gas turbine combustors.
본 연구에서는 실제 가스터빈 연소기를 모사한 덤프형태의 모델 연소기를 사용하여, 혼합실 (mixing chamber)을 이용한 완전 예혼합 (fully premixed) 조건과 연소실 바로 직전에서 산화제 유동쪽으로 연료를 분사하는 부분 예혼합 (partially premixed) 조건으로 실험장치를 구현하였다. 부분 예혼합되는 경우는 하류의 압력변동에 영향을 받지 않는 초우크 연료유동 (choked fuel flow: 이하 “초우크 조건”으로 표기함) 조건과 압력변동에 영향을 받는 언초우크 연료유동 (unchoked fuel flow: 이하 “언초우크 조건”으로 표기함) 조건으로 구별하여 실험함으로써 열발생률 변동을 초래하는 당량비 변동을 연료유량의 변조에 의한 당량비 변동과 연료와 공기 혼합기의 혼합정도 (unmixedness)에 의한 당량비 변동으로 구분하였다. 또한 유동에 의해 생성된 재순환영역의 와동 효과도 고려하여, 열발생률 변동의 원인이 연소 불안정성에 어떤 영향을 미치는지를 밝히고자 한다.
그리고, 가스터빈 제작자와 운전자에 의해 심각하게 보고되고 있는 저주파수 (~10 Hz) 대역의 연소 불안정성에 대한 발생 메커니즘을 규명하고, 그 발생 메커니즘을 뒷받침할 만한 실험적, 수치해석적 결과를 정량적으로 도출하고, 특히 저주파수 대역의 연소 불안정성을 효과적으로 감쇄하거나 그 운전영역을 좁힐 수 있는 실용적인 방법으로 버너의 혼합기가 유동하는 채널 (channel)에 헬름홀츠 공진기 (Helmholtz resonator)를 부착하거나, 연료 분사구 입구에 헬름홀츠 공진기를 부착하여 그 효과와 성능을 확인하고자 한다. 이를 위해 실험과 수치계산을 통해 저주파수 대역의 불안정한 연소모드의 발생영역을 좁힐 수 있는 구체적인 방안을 제안하고자 한다.
궁극적으로 본 연구를 통해 가스터빈과 같은 실용 연소기에서 발생하는 불안정한 연소모드의 발생 및 증가, 지속, 감소기구를 관찰하여, 이러한 현상에 관한 원인을 명확히 밝히고, 제어할 수 있는 방법을 모색하고자 한다.
