The Influence of Blade Sweep Technique in Linear Cascade Configuration

Abstract

One key issue in the advanced aerodynamic optimization of turbomachinery involves the application of 3D blade design techniques. The complex shape of the resulting blades is often a combination of simple techniques such as sweep. Such a blade arrangement is often imposed to the designer by structural constrains, space reduction needs, diameter optimization or spanwise blade loading control. This work aims to study the aerodynamic effects produced on turbine passages by blade sweep; with this term we refer to a configuration where the flow mainstream direction and the blade stacking axis are not orthogonal. A linear cascade of turbine blades, obtained by stacking the same profile with a sweep angle of 20 degrees, was investigated in a blow down facility at an isentropic downstream Mach number of 0.65. Standing the low aspect ratio of the cascade, the blade was built by simply shifting in axial direction the 2D profile originally used in the reference prismatic blade. The choice to build the swept blade keeping the same 2D section parallel to the incident flow was considered taking into account the blade low aspect ratio. Measurements were performed by means of blade surface pressure taps and five holes probe traversing downstream of the cascade; oil and dye flow visualizations were also performed to study the effects on the secondary vortices evolution inside of the passage. Moreover, a commercial CFD code was applied to provide information on the flow field all along the passage. The same profile was already extensively investigated both by measurements and CFD calculations [1, 2] in order to clarify the effects of blade lean and bowing. This additional paper gives a final contribution addressed to deeply understand the aerodynamic effects produced on turbine cascade flow field by the separate application of each one of the typical 3D design techniques. Results from both the experimental and computational investigations are presented and discussed in the paper where a phenomenological approach has been preferred. Measurements of the blade surface pressure distribution, performed at several blade heights, support the analysis of the pressure field inside of the passage which is mainly based on numerical results. In particular, the paper shows the influence of pressure contours shape on streamwise vorticity inside and downstream of the passage focusing the attention on secondary structures. The downstream vorticity field is then discussed together with the loss distribution in the same region to provide a more exhaustive description.