Numerical and Experimental Investigation of Rim Cavity Sealing Performance in a Turbine Stator Well

One of the most important tasks of gas turbine secondary air system is to seal stator-rotor cavities in order to prevent hot gas ingestion into disk spaces. Minimizing this sealing flow by an effective rim sealing design reduces secondary air consumption and contributes to machine efficiency and power. Understanding the flow behavior in rim seal area, in interaction with main flow, is vital for improving the design. In this paper, numerical and experimental investigation of rim cavity sealing performance in a turbine stator well in MAPNA heavy-duty gas turbine MGT-70 is presented. Numerical investigations are performed using 1D models and 3D CFD simulations. Using an in-house flow network simulation tool, a 1D model of the secondary air system is set up. This lumped model is used for simulating the overall flow behavior and distribution. Details of the flow are investigated using high fidelity 3D CFD. Rim cavity sealing path, together with mainstream flow, including upstream and downstream vanes and blades, are modeled considering all geometrical details. The interaction of main and secondary flows in rim area is processed and an approach is proposed to evaluate the sealing effectiveness using CFD simulations. Furthermore, some tests are conducted in one of MGT70 upgraded units. Pressure and temperature sensors are installed in secondary air system and main flow path, with particular attention to rim cavity area. The test results are adequately utilized to experimentally investigate the overall rim cavity sealing performance, monitor hot gas ingestion, and also validate and tune the numerical simulations. INTRODUCTION The main role of secondary air system (SAS) in a gas turbine is to provide sufficient flows for turbine cooling and sealing requirements. One of the most critical zones to be sealed by SAS is the unavoidable gaps between rotor and stator rows in turbine section. Insufficient sealing flow in stator-rotor cavities results in hot gas ingestion into disc spaces and may impose penalties on component life. On the other hand, consuming a high amount of secondary flow for sealing reduces the machine efficiency and power. An efficient rim cavity sealing design is therefore an important issue for turbine performance and life. The most important questions raised in rim seal design are the followings (Sangan et al. 2013):  For a given rim seal geometry, how much sealing air is required to ensure a sufficient sealing?  In the event of ingestion, how much hot gas penetrates to the wheel-space?  How does the ingested hot gas affect the temperatures in the inner components? Many studies have been conducted in order to understand the flow behavior near rim seals. The most important objective of the studies is to find the sufficient amount of sealing flow for different configurations and conditions. The extensive researches started back in 1980s when Phadke and Owen tested several seal configurations in different conditions. The ingestion was tracked by pressure measurements and a correlation was used to model the minimum sufficient flow (Phadke and Owen, 1983a). Same authors later presented a formulation for calculating the minimum sufficient flow in externally induced (EI) ingestions (Phadke and Owen, 1983b). Over the years, different methods have been investigated to better understand and evaluate the hot gas ingestion to stator-rotor gaps. Reichert and Lieser used temperature measurement as an indicator for ingestion (Reichert and Lieser, 1999). Palafox et al. used gas concentration measuring tools to investigate the sealing performance (Palafox et al., 2013). Many rim cavity test rigs have been built to investigate the ingestion phenomena in different rim seal configurations and flow conditions. Most of them are equipped with pressure, temperature and also gas concentration measurements to evaluate the sealing performance by different methods (Coren et al. 2010, Mirzamoghadam et al. 2017, Clark et al. 2017 and Scobie et al. 2017).