Three-Dimensional Navier-Stokes Analysis of Tip Clearance Flow in Linear Turbine Cascades

Abstract

A three-dimensional viscous flow code was used to study the tip clearance flow in linear turbine cascades. Two test cases with detailed measurements were used for validation. The flow properties inside the tip gap, such as static pressure, velocity, and angle, were compared with the data. The blade loading at different span locations and the downstream flowfield properties, such as loss and angle, were also compared with the data. The clearance flow phenomena, such as leakage vortex, changing of loss distribution, and flow underturning, were predicted by the numerical code. This code not only can be used as an analysis tool to study tip clearance effects in real turbine blade rows but also can be used as a design tool to design the turbine components. HE effort expended to develop a modern gas turbine engine is substantial. A significant portion of this effort is devoted to the preliminary design of aerodynamic components and subsequent modification by iteration to achieve the final design that meets the required engine performance goals. With the rapid growth in computer technology, computational fluid dynamics (CFD) codes are used as an analysis tool to help engineers understand the flow features in turbine components. Use of CFD codes that can accurately predict operation char- acteristics of turbomachiner y components will result in con- siderable reduction of development effort and provide in- creased performance. The validation of a CFD code is necessary to use it as a design tool. To evaluate the performance of the CFD code, reliable experimental data are needed. The data should provide detailed information for flow features in turbine components. Unshrouded turbine rotors operate with a tip clearance between the blade tip and the casing. The fluid that leaks between the blade tip and casing causes large efficiency losses. The tip leakage flow also changes the loss distribution, exit flow angle, and loading. Therefore, it is important for the turbomachinery designer to understand and predict the tip clearance flow phenomena. Hah,1 Pouagare and Delaney,2 and Dawes3 predicted tip clearance flow using a three-dimensional Navier-Stokes code; however, all compressor blade rows are calculated with the assumption of zero thickness on the blade tip. They used one grid line and periodic conditions for the flow inside the tip clearance region. This model is satisfactory for very thin air- foils, such as those found at the tip of a transonic compressor rotor. However, thick airfoils, found in turbine blades, re- quire the full flowfield resolution in the tip gap region. Moore and Moore4 predicted the tip clearance flow in a linear turbine cascade with the three-dimensional incompressible Navier- Stokes code. However, the flow in turbomachiner y compo- nents is usually compressible. In this study, a numerical code that solves three-dimensional, compressible Navier-Stokes