천음속 고압 터빈 노즐의 3차원 끝벽 형상 최적화

Abstract

종횡비가 낮은 터빈의 이차 손실은 전체 손실의 반 이상을 차지하기도 한다. 비축대칭 끝벽 변형은 이차 손실을 저감시킬 수 있는 방법 중의 하나로, 통로 압력구배를 낮춰서 이차 손실의 원인인 이차 유동을 억제시킨다. 본 연구에서는 1단 고압 천음속 터빈의 노즐 허브와 슈라우드에 대해서 비축대칭 끝벽 변형 최적화를 수행하였다. 끝벽 변형 형상 생성을 위해서 제어점 기법을 사용하였다. 최적화는 Kriging 모델을 사용하여 근사모델을 생성하고, 이를 유전 알고리즘을 사용하여 최적해를 탐색하였다. 1단 효율 극대화를 목적함수로 사용하였으며, 목 면적을 유지하기 위해서 질량 유량을 구속조건으로 설정하였다. 허브와 슈라우드의 끝벽 변형 최적화를 서로 독립적으로 진행하고 전산해석을 수행하였으며, 추가로 둘을 동시에 적용한 경우도 전산해석을 수행하여 기본 형상과 비교하였다. 최적화된 끝벽 형상들은 타 연구들의 끝벽 변형 경향과 차이를 보였는데, 이는 본 연구의 최적화 형상은 목 충격파를 저감시키는 방향으로 진행하였기 때문이다. 두 끝벽 변형 형상을 동시에 적용 경우는 기본 형상보다 효율이 0.39%p 향상되었지만, 이는 슈라우드 끝벽 변형 형상만 적용한 경우보다 성능 개선이 되지는 않았다. 또 다른 한계점으로는 두 끝벽을 독립적으로 변형하고 합치니까 질량 유량이 설계 범위를 벗어나게 되었다. 본 연구에서 사용한 터빈 형상의 설계 범위를 만족하면서 끝벽 변형을 수행하기 위해서는 두 끝벽을 동시에 고려할 필요가 있다.|In low aspect ratio turbines, secondary loss can take over half of the total loss generated within the turbine. One methodology to reduce secondary loss is non-axisymmetric endwall profiling. Non-axisymmetric endwall profiling reduces the cross passage pressure gradient and weakens the strength of secondary flows, which are the sources of secondary loss. In the present study, an optimization of the non-axisymmetric endwall profiling using approximation model was conducted for the hub and shroud of the 1-stage high-pressure transonic turbine nozzle. The control point method was utilized to produce the endwall profile geometry. Kriging model was used to create an approximation model, and genetic algorithm was used to search for the optimum solution. The objective function was the stage efficiency and mass flow rate was set as a constraint in order to maintain the throat area. The optimum hub and shroud endwall profiles were achieved individually and simulated. Also, the two endwall profiles were simulated simultaneously, and the three simulations were compared to the baseline axisymmetric endwall case. The optimized endwall profiles showed different endwall deformation trends from those of the other studies, because the endwalls were more focused on eliminating the throat shock. The stage efficiency of the simultaneously simulated results showed a 0.39%p improvement, but this was no better than the single shroud endwall profile results. Another limitation shown was that combining the individually optimized endwall profiles made the mass flow rate exceed the design limit. Therefore, to achieve a higher performance within the design limits for turbines similar to the one used in the present study, profiling the endwalls simultaneously is suggested.; In low aspect ratio turbines, secondary loss can take over half of the total loss generated within the turbine. One methodology to reduce secondary loss is non-axisymmetric endwall profiling. Non-axisymmetric endwall profiling reduces the cross passage pressure gradient and weakens the strength of secondary flows, which are the sources of secondary loss. In the present study, an optimization of the non-axisymmetric endwall profiling using approximation model was conducted for the hub and shroud of the 1-stage high-pressure transonic turbine nozzle. The control point method was utilized to produce the endwall profile geometry. Kriging model was used to create an approximation model, and genetic algorithm was used to search for the optimum solution. The objective function was the stage efficiency and mass flow rate was set as a constraint in order to maintain the throat area. The optimum hub and shroud endwall profiles were achieved individually and simulated. Also, the two endwall profiles were simulated simultaneously, and the three simulations were compared to the baseline axisymmetric endwall case. The optimized endwall profiles showed different endwall deformation trends from those of the other studies, because the endwalls were more focused on eliminating the throat shock. The stage efficiency of the simultaneously simulated results showed a 0.39%p improvement, but this was no better than the single shroud endwall profile results. Another limitation shown was that combining the individually optimized endwall profiles made the mass flow rate exceed the design limit. Therefore, to achieve a higher performance within the design limits for turbines similar to the one used in the present study, profiling the endwalls simultaneously is suggested.