• Energy Research

Abstract Highly three-dimensional and complex flow structure within the tip gap of an axial flow turbine is a substantial source of aerodynamic loss and heat transfer due to the interaction between the tip leakage vortex, secondary flows and the main passage flow. Most contemporary shroudless high pressure (HP) turbine designs employ squealer tips for durability, structural, aerodynamic design and heat transfer reasons. The present research deals with the influence of squealer width and height on the aerothermal performance of a HP turbine blade. In this study, four different squealer heights and seven squealer width values are investigated using a computational approach for an axial turbine blade depicting an E 3 “Energy Efficient Engine” design. The specific HP turbine airfoil under investigation is identical to the rotor tip profile of the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University. Numerical calculations are performed by solving the three-dimensional, steady and turbulent form of the Reynolds-Averaged Navier-Stokes (RANS) equations. A two-equation turbulence model, Shear Stress Transport (SST) k-ω is used in the present set of calculations. The current numerical predictions show a very good agreement with the extensive aerodynamic measurements obtained in the nozzle guide vane passages of AFTRF. The results indicate that determining proper squealer width and height is crucial to obtain better aerothermal performance in the form of reduced aerodynamic loss and heat transfer to the tip platform. Extensive numerical analysis within the tip gap reveals that increasing squealer height and reducing squealer width increases cavity volume leading to enlarged vortical structures near the pressure side and suction side of the cavity. Because of this enhanced vortical activity in the tip cavity, a blockage to the incoming pass-over flow is introduced and as a result tip leakage mass flow rate is reduced. While the tip leakage flow rate tends to decrease with increased height and reduced width, there is a strong effect from the squealer width and height combination due to the presence of complex interactions in the tip gap region. From a heat transfer point of view, decreasing squealer width and increasing squealer height noticeably reduces the overall Nu ‾ on the blade tip platform. Nu ‾ on the cavity floor, blade tip and squealer side walls are reduced depending on the increasing height and decreasing width values.

Abstract Highly three-dimensional and complex flow structure in the tip gap between a blade tip and the casing leads to significant inefficiency in the aerodynamic performance of a turbine. The interaction between the tip leakage vortex and the main passage flow is a substantial source of aerodynamic loss. The present research deals with the effect of groove type casing treatment on the aerodynamic performance of a linear turbine cascade. Grooved casings are widely used in compressors in order to improve the stall margin whereas limited studies are available on turbines. In this study, various circumferential grooves are investigated using the computational approach for a single stage axial turbine blade. The specific HP turbine airfoil under numerical investigation is identical to the rotor tip profile of the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University. The carefully measured aerodynamic flow quantities in the AFTRF are used for initial computational quality assessment purposes. Numerical calculations are obtained by solving the three-dimensional, incompressible, steady and turbulent form of the Reynolds-Averaged Navier–Stokes (RANS) equations. A two-equation turbulence model, Shear Stress Transport (SST) k-ω is used in the present set of calculations. Current results indicate that groove type casing treatment can be used effectively in axial turbines in order to improve the aerodynamic performance. Detailed flow visualizations within the passage and numerical calculations reveal that a measurable improvement in the aerodynamic performance is possible using the specific circumferential grooves presented in this paper.

Abstract Tip clearance is a crucial aspect of turbomachines in terms of aerodynamic and thermal performance. A gap between the blade tip surface and the stationary casing must be maintained to allow the relative motion of the blade. The leakage flow through the tip gap measurably reduces turbine performance and causes high thermal loads near the blade tip region. Several studies focused on the tip leakage flow to clarify the flow-physics in the past. The “squealer” design is one of the most common designs to reduce the adverse effects of tip leakage flow. In this paper, a genetic-algorithm-based optimization approach was applied to the conventional squealer tip design to enhance aerothermal performance. A multi-objective optimization method integrated with a meta-model was utilized to determine the optimum squealer geometry. Squealer height and width represent the design parameters which are aimed to be optimized. The objective functions for the genetic-algorithm-based optimization are the total pressure loss coefficient and Nusselt number calculated over the blade tip surface. The initial database is then enlarged iteratively using a coarse-to-fine approach to improve the prediction capability of the meta-models used. The procedure ends once the prediction errors are smaller than a prescribed level. This study indicates that squealer height and width have complex effects on the aerothermal performance, and optimization study allows to determine the optimum squealer dimensions.