Pressure Distortion Effects on Rim Seal Performance in a Linear Cascade

Rim seals are used to prevent the ingress of hot gas into the cavities beneath turbine platforms. As these cavities are not actively cooled, high-pressure air, known as purge flow, is taken from the compressor and introduced beneath the platform to prevent hot gas from penetrating through the gaps between stationary and rotating hardware. Improving the rim-seal geometry however, is made difficult by a lack of understanding of the salient fluid mechanics associated with this region. This study investigates both the impact of a vane-induced static pressure distortion as well as the influence of the pressure distortion of a downstream blade row on an engine-relevant rim seal in a stationary, linear cascade. Vane alterations resulted in minimal change to rim seal performance; however, adding the pressure distortion of a downstream blade row was found to disturb the trench flow resulting in poorer performance of the seal. INTRODUCTION Turbine inlet temperatures are rising to meet ever more demanding engine performance requirements. Such extreme temperatures well surpass the melting temperature of turbine component materials and therefore endanger uncooled hardware. Ingress of hot gas from the main gas path into the uncooled cavities beneath turbine platforms can risk the structural integrity of the disks. In a typical application, highpressure air from the compressor, which circumvents the combustor, is introduced into the under-platform cavities to prevent hot gas ingress and keep metal temperatures manageable. This flow will be referred to as purge flow. To reduce the amount of purge flow required to prevent ingress, a geometric feature known as a rim seal is placed below the platform between static and rotating airfoil rows. The rim seal reduces purge-flow requirements by creating a complicated flow path that discourages hot gas ingress. Effective rim seal geometries are desirable to minimize the amount of purge flow required to prevent hot gas ingress beneath the platform and to minimize aerodynamic stage losses. Minimizing purge requirements is made difficult due to the highly unsteady and complex flowfield that exists in the region in and around the rim seal. Many factors contribute to the complexity in the flowfield in the rim seal such as rotational effects, the high tangential velocity in the rim seal area resulting from the main gas path flow being turned by the vane, as well as main gas path pressure effects. In addition to these flow features, the potential field, as well as the endwall flows of nearby airfoils, disturb the flowfield at the seal exit. The geometry of a rim greatly influences how effective the seal is at preventing hot gas ingress and aerodynamic losses. A common seal geometry in modern gas turbines uses a series of radially overlapping geometric features to discourage ingress. An important aspect of this seal geometry is the axial distance that these features overlap, also known as the axial overlap. Increasing the axial overlap at the seal exit decreases the amount of hot gas ingress through the seal. Likewise, decreasing the width of the region where the purge flow mixes with the main gas path also improves sealing performance [1]. The impact of these geometry perturbations on the flowfield at the seal exit however is not fully described. Presented in this paper are sealing effectiveness and adiabatic endwall effectiveness data that describe the impacts of the upstream vane, downstream bluff bodies, and seal geometry on an engine-relevant rim seal in a linear cascade. These studies were conducted in a stationary, linear cascade to isolate the effects of the pressure distortion from other complicating factors such as rotation. Three-component laser Doppler velocimetry (LDV) was used to measure the mean and turbulent flowfields to identify the mechanisms through which seal performance was affected. Flowfield measurements were taken at the exit of the seal with and without downstream bluff Proceedings of ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition GT2016 June 13 – 17, 2016, Seoul, South Korea