Cryogenic cycle power turbines optimised by hub contouring

Improvements in stage isentropic efficiency and reductions in stage total pressure loss are sought in a 1.5 stage axial turbine. This is representative of power generation equipment used in thermal power cycles, in a cryogenic plants, and in aeroderivative engines. The performance of cryogenic installations and of power plants can be enhanced by using turbines with a higher isentropic efficiency, by reducing the secondary flow losses. Secondary flow loss reduction is achieved by designing a non-axisymmetric end-wall for the turbine inlet stator hub. The approach is to use a novel guide groove to direct the pressure side branch of the horseshoe vortex away from the blade suction side, using a novel parametric end-wall hub surface definition. This delays the onset of the passage vortex and reduces its associated loss. The performance of the novel hub profile is compared with that from contouring the hub using representative industry best practice. For this, a three-dimensional steady RANS model with an axisymmetric hub is first validated against reference experimental measurements from RWTH Aachen. A Kriging surrogate model from the Alstom Process and Optimisation Workbench (APOW) is used to optimise the hub surface. Comparative CFD predictions with an optimised non-axisymmetric hub show a decrease in the stage total pressure loss coefficient and an increase in the stage isentropic efficiency at design and off-design. The potential benefits to a representative Liquefied Natural Gas (LNG) cryogenic cycle is assessed by thermodynamic cycle analysis. A 1.60 % increase in the axial turbine stage isentropic efficiency is predicted by using a hub contoured by the optimised guide groove compared to a 0.19 % increase by using the industry best practice. The higher turbine stage isentropic efficiency is predicted to enhance the performance of the LNG cryogenic plant by a 3.15 % rise in the Coefficient Of Performance.