Integrated Aerodynamic and Structural Blade Shape Optimisation of Axial Turbines Operating With Supercritical Carbon Dioxide Blended With Dopants

Abstract

Within this study, the blade shape of a large-scale axial turbine operating with sCO2 blended with dopants is optimised using an integrated aerodynamic-structural 3D numerical model, whereby the optimisation aims at maximising the aerodynamic efficiency whilst meeting a set of stress constraints to ensure safe operation. Specifically, three candidate mixtures are considered, namely CO2 blended with titaniumtetrachloride (TiCl4), hexafluorobenzene (C6F6) or sulfur dioxide (SO2), where the selected blends and boundary conditions are defined by the EU project, SCARABEUS. A single passage axial turbine numerical model is setup and applied to the first stage of a large-scale multi-stage axial turbine design. The aerodynamic performance is simulated using a 3D steady-state viscous computational fluid dynamic (CFD) model while the blade stress distribution is obtained from a static structural finite element analysis (FEA). A genetic algorithm is used to optimise parameters defining the blade angle and thickness distributions along the chord line while a surrogate model is used to provide fast and reliable model predictions during optimisation using genetic aggregation response surface. The uncertainty of the surrogate model represented by the difference between the surrogate model results and the CFD/FEA model results is evaluated using a set of verification points and found to be less than 0.3% for aerodynamic efficiency and 1% for both the mass flow rate and the maximum equivalent stresses. The comparison between the final optimised blade cross-sections have shown some common trends in optimising the blade design by decreasing stator and rotor trailing edge thickness, increasing stator thickness near the trailing edge, decreasing rotor thickness near the trailing edge and decreasing the rotor outlet angle. Further investigations of the loss breakdown of the optimised and reference blade designs are presented to highlight the role of the optimisation process in reducing aerodynamic losses. It has been noted that the performance improvement achieved through shape optimisation is mainly due to decreasing the endwall losses of both stator and rotor blades.

A Comparison of Flow Path Designs for Axial Turbines Operating with Pure CO2 and CO2 Mixtures

“…Furthermore, the mean-line design tool has been verified against CFD simulation results for a 130 MW axial turbine operating with 80%CO 2 /20%SO 2 mixture. A good agreement was obtained between the mean-line model and CFD results where a maximum difference in the mass flow rate and total-to-total efficiency of 0.5% and 1.0% was achieved respectively [32]. For the same turbine design, the blade has also been evaluated using finite element analysis to ensure mechanical stresses are within the specified limits (i.e., the maximum stress is less than 260 MPa).…”

“…Ultimately, this demonstrates that using the bending stress limit applied within the mean-line design model results in feasible turbine geometries from a mechanical design perspective. Further improvements to the 3D blade geometry can be achieved using blade shape optimisation to match the cycle operating conditions alongside improving the turbine performance [32]; however, such analysis is beyond the scope of the present study.…”