As key technologies to improve the performance of steam turbines, various types of high performance seal, such as active clearance control (ACC) seals and leaf seals, have been developed by Mitsubishi Heavy Industries, LTD (MHI). Moreover, a new seal concept has also been developed, which remarkably reduces the leakage flow through the blade tip/base clearance by using an aerodynamic approach.
The main concept of this technology is to control and utilize the vortex structure in the cavities of the labyrinth seal by optimizing the cavity geometry. In the optimized geometry, locally-controlled flow on the upstream side of the fin tip causes a strong contraction of the leakage flow and reduces the discharge coefficient significantly. This concept allows for a remarkably reduced leakage flow, whilst keeping clearance at the same level as that of conventional set-up, and thus the risk of contact between rotating and stationary parts low. Therefore, this technology realizes reliable and durable seals with high performance. In addition, it is possible to keep manufacturing cost at the same level as conventional seals, since the structure of this seal is basically the same as the conventional one.
In the development of this technology, a parametric study using Computational Fluid Dynamics (CFD) was carried out to optimize the cavity geometry. The verification test was carried out for the optimized geometry under the real steam conditions. From the results, we confirmed that the optimized geometry reduced the discharge coefficient by up to 30%, compared to conventional seals.