Impact of Large Tip Clearance on Aerodynamic Performance in a Linear Turbine Cascade

Abstract

Small turbomachines like turbopumps have relatively large tip clearances due to tolerance limits in manufacturing. However, only a few studies so far have been conducted on this topic and understanding of large tip clearance impact on turbine blade performance is lacking. Based on Lee [1]s experiment, this research presents numerical study about the large tip clearance in a turbine cascade. Tip clearance varies from 1% to 20% based on the blade chord like experiment and CFD simulation was validated by comparison with Lee [1]s experiment. This research focused on variation of aerodynamic loss for large tip clearance in detail. Overall loss at downstream increased up to 10% tip clearance and then decreased over 10% tip clearance. Decreasing overall loss for large tip clearance is different result from previous researches for small tip clearances. From the breakdown of the overall loss, it was found that variations of tip leakage loss and secondary loss were the main reason of the decreasing overall loss for large tip clearance. Thus, by detailed investigation about tip leakage loss and secondary loss, this research explains the reason of decreasing overall loss at downstream for large tip clearance. Even though the overall loss at downstream decreased for large tip clearance, stream-thrust averaged loss increased continuously with increasing tip clearance. The reason of continuously increasing stream-thrust averaged loss was also explained in this research.

터보펌프와 같은 소형터보기계는 제조공정 상 공차의 한계로 인해 상대적으로 큰 팁 간극(10%~)을 가진다. 하지만 작은 팁 간극과 달리 큰 팁 간극에 대해 지금까지 많은 연구가 수행되지 않았다. 이 논문은 이효성 [1]의 실험을 바탕으로 CFD 해석을 하였고, 큰 팁 간극에서 공력손실의 변화에 대해 자세히 살펴보았다. 하류에서의 전체 공력손실은 팁 간극 비율 10% 까지 증가를 하다가 10% 이상에서는 감소하는 경향을 보여준다. 하류에서의 전체 공력손실을 분해해본 결과, tip leakage loss 와 secondary loss 의 변화가 큰 팁 간극에서의 전체 손실 변화의 주 원인임을 확인하였다. 이 논문은 큰 팁 간극에서의 tip leakage loss 와 secondary loss의 변화 원인을 CFD 해석을 통해 자세히 분석하였다. 비록 하류에서의 전체 손실은 10% 이상의 큰 팁 간극에서 감소하지만, stream-thrust averaged loss 는 오히려 팁 간극이 증가함에 따라 계속적으로 증가하는 모습을 보여주었고, 이에 대한 원인과 이것의 의미에 대해서도 이 논문에서 자세히 다루었다.