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Experimental Study on Flame Structure and Combustion Dynamic Characteristics in a Low NOx Model Gas Turbine Combustor
저 NOx 모형 가스터빈 연소기에서 화염구조 및 연소동특성에 대한 실험적 연구

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Authors
김민기
Advisor
윤영빈
Major
공과대학 기계항공공학부
Issue Date
2016-02
Publisher
서울대학교 대학원
Keywords
Gas Turbine CombustorCombustion InstabilityFlame Vortex StructureCentral Recirculation Zone (CRZ)Inlet Mixing SectionMulti-Position Dynamic Pressure SensingCanted InjectionLiquid Column TrajectoryBreakup LengthOptimum Combustion Tuning
Description
학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2016. 2. 윤영빈.
Abstract
There has been increased demand in recent years for low NOx gas turbines to meet stringent emissions goals by operating in a lean, premixed combustion and an advanced combustion system for aero gas turbine engine such as the Rich-Burn, Quick-Mix, Lean-Burn (RQL) combustor. Unfortunately, detrimental combustion instabilities are often excited within the combustor when it operates under lean and rich equvalence ratio conditions, degrading performance and reducing combustor life. To eliminate the onset of these combustion instabilities and develop effective approaches for their control, the mechanisms responsible for their combustion oscillation chartacteristics and jet spray mechanism of secondary quick mix zone must be understood. The main objective of this study was conducted to identify the secondary spray jet mechanism for the turbulent quick mixing zone and combustion instability characteristics in a swirl-stabilized and partially premixed model gas turbine combustor, with the attention focused on the effect of the various fuel-air mixing section geometries, fuel-air mixture velocities and effect of the formation of recirculation zones and vortex interaction on the combustion instability characteristics. Lastly, for the confirmed the this experimental study and analyzed mechanisms, we investigated an optimized operating strategy and developed a combustion tuning methodology for the GE 7FA+e DLN-2.6 (DLN : Dry Low NOx) ground state gas turbine engine used for power generation operated by Korea Western Power Co. Ltd. at Seo-Incheon power plant, Incheon, Republic of Korea.
The flame recirculation zone is very important, as it can modulate the fuel flow rate and may be the source of instability, plus its flame structure has a major impact on heat release rate oscillation and flame stabilization. This study addresses structural characteristics of natural gas flames in a lean premixed swirl-stabilized combustor with attention focused on the effect of the formation of recirculation zones and vortex interaction on the combustion instability. To improve our understanding of the role of the recirculation zone and vortex combustion instability, the flame structure was investigated for various mixture velocities, equivalence ratios and swirl numbers. The optically accessible combustor allowed for the laser diagnostics of Particle Image Velocimetry (PIV) measurement, while OH chemiluminescence was used to characterize the flow structure under both cold flow conditions and hot flow combustion conditions. and heat release oscillation rate with the use of a high-speed ICCD camera under both stable and unstable flame conditions. Multi-channel dynamic pressures were also measured at the same time to investigate characteristics of the combustion phenomenon. We also observed fundamental longitudinal type of combustion instability characteristics related to the instability of thermo-acoustics. The result suggests that the formation of the recirculation zone is strongly related to the occurrence of combustion instabilities.
The effect of fuel-air mixture velocity on combustion instability characteristics have been investigated by measuring the flame structure, dynamic pressure mode and phase. The swirling CH4 - air flame was investigated with an overall equivalence ratio of 1.2 to lean blowout limit and dump plane velocity of 30 ~ 70 m/s. Phase locking analysis was performed to identify structural changes at each phase of the reference dynamic pressure sensor under conditions of instability. At an unstable condition, flame root size varies a lot compared to stable condition which is because of air and fuel mixture flow rate changes due to combustor pressure modulation. After this structural change, local extinction and re-ignition occur and it can generate a feedback loop for combustion instability. This analysis suggests that pressure fluctuation of combustion causes deformation of flame structure and variation of flame has a strong effect on combustion instability. In this section, we observed two types of combustion instability characteristics related to the instability of both the thermo-acoustics and flame vortex interaction mode.
To investigate the instability characteristics of combustor geometry. The combustor and inlet mixing section length was varied in order to have different acoustic resonance characteristics from 800 to 1680 mm of the combustor and 470, 550 and 870 mm of the inlet mixing section. In this study, we observed two dominant instability frequencies. Lower instability frequencies were obtained around 240 ~ 380 Hz, which were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were observed around 410 ~ 830 Hz. These were related to the secondary longitudinal mode in the combustion chamber and the secondary quarter wave mode in the inlet mixing section. These second mode instability frequency characteristics are coupled with the conditions of the combustor and inlet mixing section acoustic geometry. Also, these higher combustion instability characteristics include dynamic pressure oscillation of the inlet mixing section part, which was larger than the combustor section. As a result, combustion instability was strongly affected by the acoustically coupling characteristics of the combustor and inlet mixing section geometry, which is called the plenum.
The effects of variable angled injection characteristics for quick mixing zone such as liquid column trajectory and breakup length has been experimentally studied in liquid jets injected into subsonic crossflow. With water as fuel injection velocity and injection angle were varied to provide of jet operation conditions. The pulsed shadowgraph photography with highly resolution and PLLIF (Planar Liquid Laser Induced Fluorescence) measurements were used for determined the liquid column trajectory and breakup length of angles spray. As the result, this research has been shown that liquid column trajectories and liquid column breakup length were spatially dependent on air-stream velocity, fuel injection velocity, various injection angle, and normalized injector exit diameter. Furthermore, the empirical formula of liquid column trajectories and breakup length has been some different of drag coefficient results between forward injection and reverse angle injection in subsonic crossflow.
In the Appendix section, the optimum combustion control of real gas turbine combustor was introduced. On the basis of a MARK-Ⅵ system, the optimized tuning for operation of a DLN-2.6 combustor was studied for the maintenance of a GE 7FA+e gas turbine at a Seo-Incheon combined cycle power plant. Also, the optimum combustion control system was created by all of measuring the inlet and outlet combustion data of the GE 7FA+e gas turbine.
Language
English
URI
https://hdl.handle.net/10371/118493
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Mechanical Aerospace Engineering (기계항공공학부)Theses (Ph.D. / Sc.D._기계항공공학부)
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