Design And Analysis Of Variable Geometry Turbine With Large Meridional Expansion

To adapt to the changing environments and power requirements, the modern high-performance turbine cannot always fixed in the design point. Compared with the conventional fixed geometry, variable geometry turbine which controlling the guide vane throat area to adjust the mass flow can achieve a higher efficiency without changing the rotation speed. Due to the end wall's large expansion, the efficiency of the large meridional expansion turbines will be in decline seriously as the adjustable guide vane ratites. And the tip clearance is too large and has a dramatic changes at the variable conditions, which also cause huge losses.This paper investigates a variable geometry turbine which has great diffused angle casing.Based on a five-stage meridional expansion turbine, the flow structure in the vane flow field and gap clearance is analyzed through numerical simulation. A new step casing contour which is using in the meridional expansion turbine is promoted and optimized in this paper. The results show that the step casing contour can weaken the tip leakage flow and the effect gradually decreases as the step angle becomes smaller. Compared with the prototype, the new end wall casing can ensure the constant gap clearance when the vane ratites. The efficiency decreased only 0.87% after adding the gap. The step casing contour has a good performance in the design point.Based on the optimized step casing end wall contour, single-stage and multi-stage off design condition characteristic was investigated. Through the numerical simulation, the influence of guide vane throat area on the adjacent blades and next stages was calculated and analyzed. The result shows that with the opening of guide vane angle, the mass flow and the power is increasing for the first stage. Also, the tip leakage flow is reducing. The efficiency increased first ,and decreased after the angle is greater than 4°. Off design condition characteristic of the five stage is similar with the first stage, and the efficiency decreasing at the design point is smaller. The optimized step casing end wall contour has shown an excellent performance in the off design conditions.