가스터빈과 증기터빈이 조합된 복합발전설비는 열효율이 높고, 신속한 기동정지로 부하추종성이 우수하며, 가스를 연료로 사용하므로 상대적으로 공해물질 배출이 낮은 등의 장점과 세일가스 붐이 맞물리면서 전 세계적으로 건설이 증가하였다. 전 세계에 단일 Model로 가장 많이 설치된 가스터빈은 GE사의 7FA Model이다. 2017년 6월 현재 전 세계에 약 717대, 한국에 약 40대가 운전되고 있다. 가스터빈은 Daily Start & Stop 운전으로 압축기, 연소기, 터빈 부품손상이 많이 발생하나 최근에는 Hot Gas Path Parts, Thermal Barrier Coating, Cooling 등의 기술발전으로 손상이 현저히 감소하고 있다.
본 연구에서는 가스터빈 고온부품 중 GE 7FA 가스터빈의 1단 Bucket Platform을 대상으로 초기 Model인 7FA.01과 개선된 Model인 7FA.03의 1단 Bucket Platform 손상해석을 CFD로 수행하였다. 초기 Model인 7FA.01 Bucket에 비해 개선된 Model인 7FA.03 Bucket Platform은 Shank에 Cooling Hole 1EA, Platform 표면에 Cooling Hole 10EA가 추가되었고 Platform Pressure Side Mid Span End부에 Scallop이 적용되었다.
손상해석은 열유동 및 열응력해석 상용 프로그램인 ANSYS 17.2를 활용하였으며 1단 Bucket의 Modeling을 위한 경계조건 설정은 GE 7FA.01 & 03 가스터빈 실제운전 Data와 EPRI의 OEM Design data를 활용하였다.
열유동 해석결과 초기 Model인 7FA.01 Bucket Platform은 Bucket Leading Edge에서 형성된 Horseshoe Vortex와 Bucket Pressure Side에서 Suction Side로 형성되는 Passage Vortex로 인해 Platform 표면에 주 유동에 의한 열 영향이 크게 미치는 것을 확인하였다. 개선된 Model인 7FA.03 1단 Bucket은 Cooling Path 개선으로 Platform 표면온도가 7FA.01 Model에 비해 낮아졌으며, Platform Pressure Side에서 Suction Side로 형성되는 Passage Vortex도 약화되어 Platform 표면에 미치는 열 영향이 억제되는 것으로 확인되었다.
7FA.01 & 03 1단 Bucket Platform 열응력 해석결과 7FA.01 1단 Bucket Platform의 Pressure Side 표면온도가 7FA.03 1단 Bucket에 비해 높았으나 7FA.01 및 7FA.03 Model의 1단 Bucket Platform 모두 Material의 최대 항복응력보다 낮아 열응력에 의해 손상되지 않는 것으로 판단된다.
제작사가 제시한 7FA.01 1단 Bucket Platform 손상 Mechanism은 Cyclic 운전 중 Platform 표면온도가 상승되어 TBC가 탈락되면 Bucket Material 속으로 산소가 침투하여 산화층을 형성하고 산화층 경계면으로부터 균열이 발생하는 것이다.
본 연구를 통해서 7FA.01 Model 1단 Bucket Leading Edge에서 형성된 Horseshoe Vortex와 Platform Pressure Side에서 Suction Side로 형성되는 Passage Vortex로 인해 Platform으로 열전달이 크게 작용하여 온도가 상승하고 TBC가 탈락되면 Platform이 손상될 수 있다는 가능성을 확인하였다. 가스터빈 Bucket 등 고온부품 손상을 줄이기 위해서는 Material, TBC, Cooling 기술의 발전과 Aero-design 강화를 통한 열유동 안정화 등의 기술개발 노력을 지속하여야 할 것으로 판단된다.

Combined cycle power plant that combines gas turbine and steam turbine has high thermal efficiency and excellent load followability due to rapid start & stop. The construction of combined cycle power plant is increasing worldwide because the pollution emission is relatively low and the shale gas boom is engaged. The most installed gas turbine in the world as a single model is GE''s 7FA model. As of june 2017, there are about 717 units in the world and about 40 units in korea. The gas turbine cyclic operating type such as daily start & stop causes many damage of compressor, combustor and turbine parts. Recently, The damage has been remarkably reduced due to the development of technologies such as hot gas path parts, thermal barrier coating and cooling.
In this study, the GE 7FA first stage bucket platform among the gas turbine hot gas path parts was analyzed by CFD for the first stage 7FA.01 and the improved stage 7FA.03. The 7FA.03 bucket platform, which is an improved model compared to the initial model 7FA.01 bucket, has added cooling hole 1EA to the shank, cooling hole 10EA to the platform surface and scallop to the platform pressure side mid span end.
Damage analysis utilized analysis 17.2, a commercial program for thermal flow and thermal stress analysis. The boundary conditions for modeling of the first stage bucket were made using GE 7FA.01 & 03 gas turbine actual operation data and EPRI''s OEM design data.
As a result of the heat flow analysis, 7FA.01 bucket platform which is the initial model, confirmed that the heat effect was greatly affected by the main flow on the platform surface introduced by the horseshoe vortex formed at the bucket leading edge and passage vortex formed from bucket pressure side to the suction side together. Improved model 7FA.03 first stage bucket has lowered the surface temperature of the platform compared to 7FA.01 model due to the improvement of cooling path and restricted the thermal effect to the platform surface due to weaken passage vortex formed from platform pressure side to suction side respectively.
Results of thermal stress analysis of 7FA.01 & 03 first stage bucket platform, the pressure side surface temperature of 7FA.01 first stage bucket platform was higher than that of 7FA.03 first stage bucket. It is verified that both the first stage bucket platform of 7FA.01 and 7FA.03 models aren''t damaged by thermal stress because the stress is lower than the maximum yield stress of the material.
The 7FA.01 first stage bucket platform damage mechanism proposed by the manufacturer suggests that when the TBC is spalled due to the rise of the platform surface temperature during the cyclic operation, oxygen penetrates into the bucket material to form an oxide layer and finally cracks from the oxide layer interface.
Through this study, horseshoe vortex formed at first stage bucket leading edge of 7FA.01 model and passage vortex formed from platform pressure to suction side greatly contributed to the heat transfer to the platform. When the temperature rises, TBC can be spalled and the platform is damaged eventually. In order to reduce the damage of hot gas path parts such as gas turbine bucket, it is necessary to continue the efforts to develop technologies such as material, TBC, cooling technology and heat flow stabilization by strengthening the aero-design.