GT 2006-91020 INVESTIGATION OF 3 D UNSTEADY FLOWS IN A TWO-STAGE SHROUDED AXIAL TURBINE USING STEREOSCOPIC PIV AND FRAP — PART II : KINEMATICS OF SHROUD CAVITY FLOW

This paper presents an experimental study of the behavior of leakage flow across shrouded turbine blades. Stereoscopic particle image velocimetry and fast response aerodynamic probe measurements have been conducted in a low-speed twostage axial turbine with a partial shroud. The dominant flow feature within the exit cavity is the radially outward motion of the main flow into the shroud cavity. The radial migration of the main flow is induced by flow separation at the trailing edge of the shroud due to a sudden area expansion. The radially outward motion is the strongest at mid-pitch as a result of interactions between vortices formed within the cavity. The main flow entering the exit cavity divides into two streams. One stream moves upstream towards the adjacent seal knife and reenters the main flow stream. The other stream moves downstream due to the interaction with the thin seal leakage flow layer. Closer to the casing wall, the flow interacts with the underturned seal leakage flow and gains swirl. Eventually, axial vorticity is generated due to these complex flow interactions. This vorticity is generated by a vortex tilting mechanism and gives rise to additional secondary flow. Due to these fluid motions combined with a contoured casing wall, three layers (the seal leakage layer, cavity flow layer, and main flow) are formed downstream of the shroud cavity. This result is different from the two-layer structure which is found downstream of conventional shroud cavities. The seal leakage jet formed through the seal clearance still exists at 25.6 percent axial chord downstream of the second rotor. This delay of complete dissipation of the seal leakage jet and its mixing with the cavity flow layer is due to the contoured casing wall. Time-averaged flow downstream of the shroud cavity shows the upstream stator’s influence on the cavity flow. The time-averaged main flow can be viewed as a wake flow induced by the upstream stator whose separation at the shroud trailing edge induces pitchwise non-uniformity of the cavity flow. INTRODUCTION Shrouds are used to prevent pressure-driven leakage flow across the turbine blade tip. Nevertheless, there is still a gap between the stationary (casing wall or stator shroud) and rotating parts (rotor shroud or stator hub), and flow leaks through the gap. To reduce the leakage flow, labyrinth seals are often employed. Much work has been done to clarify how the labyrinth seal affects flow field not only within the seal cavity but also downstream of the blade passage. To quantify loss with a simple model, Denton [1] made a simplified description of the cavity flow that the meridional velocity of the leakage jet undergoes dissipation inside the shroud cavity while the swirl of the leakage flow remains unchanged. Thus, there is a mismatch of flow angle between the leakage and the main flows when the two flows interact. The mismatch causes a shear layer which is a source of loss [1, 2]. According to Wallis et al. [3], a strategy for reducing the losses associated with seal leakage flow is to minimize the entropy generated during the interaction between the seal leakage and the main flow. They added airfoil shaped turning devices on top of the shroud surfaces to turn the leakage flow through the shroud cavity, but actually found performance degradation. Also, the seal leakage flow causes negative incidence on the subsequent stator, thus intensifying the passage vortex formed inside the subsequent stator [4-6]. Song and Song [7] developed an analytical model to predict flow response to labyrinth seals in an axial turbine stage with a shrouded rotor. In their control volume approach, they assumed that the leakage flow is radially inward at the exit of the cavity and the leakage and the main flows remain separate downstream. Thus, downstream of the shroud cavity, there are two flow layers—the main flow and the leakage flow which is retarded in the axial direction and tangentially underturned. It was not until recently that the flow within the seal cavity was experimentally investigated in detail. Pfau et al. [8] made detailed measurements of the flow within the shroud cavity in an annular turbine cascade. They found a pitchwise variation in static pressure within the shroud cavity as well as radially outward and inward motions of fluid at the exit cavity. They inferred that the pitchwise non-uniformity of the flow within the cavity is attenuated as the leakage flow passes through three seal knives, presumably due to the mixing in the cavities. Also, they observed a radially stratified axial velocity distribution and a toroidal vortex within the exit cavity. Pfau et al. [9]