Study of a Low Aspect Ratio Low Pressure Turbine Nozzle Guide Vane With High Outer Annulus Wall Slope

This paper presents the results of a combined experimental and CFD investigation of a first stage low pressure (LP) turbine nozzle guide vane (NGV). The configuration is characterized by a small number of low aspect ratio vanes in a strongly diverging annulus. The experimental part of the study has been conducted in a two-stage rotating rig environment with detailed area traverse measurements upstream and downstream of the NGV. The inlet whirl to the NGV has been varied by a row of pre-swirl vanes (PSV) located in the turbine rig inlet. The effect of inlet whirl angle on the development of the passage vortices and on the NGV pressure loss is assessed by using measurements and steady state 30 CFD predictions. The area traverse results, which neglect any mixing loss downstream of the traverse plane, show the traditional airfoil loss-loop characteristic with minimum loss at design incidence and increased loss at both positive and negative incidence. By considering the turbine overall performance, however, it is observed that the isentropic turbine efficiency increases for negative incidence and decreases for positive incidence to an extent that cannot be explained by the measured NGV total pressure loss only. This leads us to believe that the non-uniform flow field at the exit of the NGV generated by the wake and secondary flow impacts significantly on the loss generation further downstream in the turbine. Unsteady 3D CFD predictions of the NGV and the downstream rotor passage aerodynamics have been conducted in order to study the convection of the NGV wake and passage vortices through the rotor passage and the resulting unsteady loading on the rotor. The strongest interaction takes place in the hub region. The unsteady loading in the tip region is characterized by incidence variations which typically only affect the rotor unsteady pressure distribution in the leading edge region up to 40% axial chord. In the hub, however, the strong three-dimensional character of the flow forces large amplitude fluctuations to occur in the rotor unsteady pressure distribution at the throat region. These fluctuations affect the character and the level of diffusion on the suction surface and are therefore likely to have significant impact on the rotor loss. In striving to reduce manufacturing cost and engine weight by exploiting high outer annulus wall slope and smaller gap-to-chord ratios these types of effects are bound to become increasingly important. The application of advanced prediction tools, such as used in this paper, will inevitably play an important role in this development.