Abstract
Improving the off-design performance and turndown capability of power cycles for concentrated solar power (CSP) applications is critical considering possible heat source and cooling fluctuations. In this paper, the effect of different flow path geometrical parameters on the aerodynamic performance of large-scale axial turbines operating with supercritical carbon dioxide (sCO2) mixtures is evaluated at both design and off-design conditions. 3D steady-state multi-stage Reynolds-Averaged Navier Stokes equations Computational Fluid Dynamics (CFD) simulations are performed where the k-ω SST turbulence model is used. The number of stages, stator/rotor axial gap, leading-edge thickness, inlet wedge angle, and stagger angle are varied to evaluate their impact on the turbine operational flexibility, defined by the minimum acceptable ratio of mass flow rate to the design value. The ranges of variation are defined based on practical considerations and careful consideration of various design criteria such as the slenderness ratio and bending stresses. The results revealed that the stagger angle has the largest influence on the operational flexibility with a change of 17.1% in the minimum allowable part-load mass flow ratio corresponding to a change in the stagger angle between -5° and +5° from the reference design angle. This increase in the stagger angle resulted in an increase in the design point total-to-total efficiency of 2.3 percentage points. The leading-edge thickness has the least influence with a maximum change in the minimum part load mass flow ratio of 0.63% as obtained for the investigated range and a negligible change in the design point efficiency. The flow field investigations revealed that increasing the stagger angle leads to smaller separation regions caused by high incidence angles, despite the resulting increase in bending stresses.
