1 Experimental and Numerical Investigation of Sealing Performance of Turbine Rim Seals

Performance of turbine rim seals was investigated for the upstream cavity. A rim seal configuration often adopted in actual engines was examined for both with overlap and with gap. Guide vanes (airfoil-shaped) and rotor blades (not airfoil-shaped) were installed to simulate the wake and the upstream potential respectively. CO2 concentration measurement was carried out for evaluating seal effectiveness. Three-dimensional unsteady CFD analysis was carried out to get information about the flow structure in the rim seal. It was shown that the presence of rotor blades degraded seal effectiveness whether there is overlap or gap. The effect of fin overlap showed adverse trend between the cases with and without rotor blades. In case with rotor blades, seal effectiveness was higher for the rim seal with gap. It may be related to the mainstream static pressure distribution generated by the interference between blade upstream potential and injected cooling air. NOMENCLATURE b : Outer radius of the cavity Cw : Non-dimensional flow coefficient m : Local mass flux P : Static pressure Pd : Dynamic pressure Pm : Mainstream static pressure Q : Volumetric flow rate r : Radial coordinate Rem : Mainstream Reynolds number Ret : Rotational Reynolds number Vax : Mainstream axial velocity : Seal effectiveness : Kinematic viscosity : Carbon dioxide concentration : Angular velocity of rotor disk Subscripts M : Mainstream C : Cooling air min : Minimum value INTRODUCTION As is well-known, mainstream gas ingestion into cavities around turbine disks are observed in aircraft and industrial gas turbine engines. Ingested gas raises the temperature in the cavity, and as a result the temperature of rotating parts such as disks, seals, etc. also rise and have significant damages to their lives. In order to avoid such kind of risk, secondary air bled off from compressor is supplied to the cavities as “purge air” and rotating and stationary parts (usually platforms of rotor blades and inner bands of nozzle guide vanes) are designed to form a radial or axial clearance seals at rim portion of the cavities. These efforts reduce the amount of mainstream gas ingested into the cavity and keep the temperature in the cavity low enough for rotating parts to meet their required lives. However, as secondary air has a negative impact on engine performance, cavity purge should be accomplished by as little air as possible. Therefore, it is very important to predict the amount of mainstream gas ingested into cavities as accurately as possible. There are several flow mechanisms that affect mainstream gas ingestion, including disk pumping, circumferential asymmetry of mainstream static pressure and interaction of stator vanes and rotor blades. Many comprehensive investigations have been carried out so far to get a better understanding of this phenomenon. Early works focused on the disk pumping effect that draws flow inward to the rotor-stator disk cavity to compensate the flow pumped outward in the boundary layer formed on the rotor disk surface. Bayley and Owen (1970) examined a shrouded rotor-stator system with a simple axial gap in a quiescent environment. They established a correlation for the minimum dimensionless cooling flow rate (Cw,min) required to prevent ingestion. Phadke and Owen (1983) also found a linear relationship between Cw,min and the rotational Reynolds number similar to that established by Bayley and Owen. Several workers have confirmed the influence of mainstream flow, particularly circumferential asymmetry of static pressure on cavity rim sealing. Abe et al. (1979), Dadkhah et al. (1991) and Hamabe and Ishida (1992) carried out experiments to measure mainstream gas ingestion by tracer gas technique. In these experiments, static pressure asymmetry was generated by nozzle guide vanes or other disturbances placed in the mainstream annulus. They agreed on the importance of the presence of mainstream flow. Chew et al. (1994) performed experiments on mainstream gas ingestion for a stator-rotor system with guide vanes. They varied the axial distance between vane trailing edge and rim seal gap and thus varied the magnitude of circumferential pressure asymmetry. Measured seal effectiveness was found to depend on the magnitude of circumferential pressure asymmetry at the seal gap. They also found that the rotational Reynolds number had little effect on seal effectiveness. Green and Turner (1994) examined mainstream gas ingestion for a stator-rotor system with both stator vanes and rotor blades. They found that the presence of rotor blades improved seal effectiveness. On the other hand, according to Bohn et al. (2000), it Copyright (c) 2003 by GTSJ Manuscript Received on March 28, 2003 Proceedings of the International Gas Turbine Congress 2003 Tokyo November 2-7, 2003