Efficiency of biradial impulse turbines concerning rotor blade angle, guide-vane deflection and blockage

Self-rectifying impulse turbines have two sets of guide vanes symmetrically located on each rotor side. Aerodynamic losses resulting from the inherent misalignment between the rotor outflow and the outlet guide vanes penalize these turbines' efficiency. This paper presents the aerodynamic design of a radial guide-vane system for self-rectifying biradial impulse turbines and the relation between guide-vane flow deflection and blockage, rotor blade angle and turbine efficiency in design conditions. The system comprises two concentric rows of constant-thickness vanes. The design method solves a multi-objective optimization problem that maximizes deflection while minimizing outflow blockage, returning a Pareto optimal set of guide vanes. A subset of these results is used to configure and assess multiple turbine geometries. Data are obtained numerically with a RANS solver. The new guide-vane system improves flow deflection over reference designs based on aerofoil-section vanes with identical blockage. A turbine efficiency of 68.2% is achieved for a guide -vane deflection of 67.3 degrees, blockage factor of 0.61 and rotor blade angle of 35 degrees. The new design leads to an estimated reduction of 58% on the stagnation pressure losses in the outlet guide vanes.