Off-design performance assessment of an axial turbine for a 100 MWe concentrated solar power plant operating with CO<inf>2</inf> mixtures

Abstract

This paper presents an investigation of the aerodynamic performance of a 130 MW axial turbine operating with a CO2/SO2 mixture using a mean-line off-design performance model; where the validity of this model has been confirmed through verification against results from the literature and computational fluid dynamic (CFD) simulations. This analysis also includes assessing the impact of varying the number of stages on the part-load operation. Additionally, the application of similitude theory to non-dimensionalise performance characteristics is validated by assessing the performance of the same turbine with different working fluids, mixture compositions, and rotational speeds. The mean-line performance model applied throughout this study is based on the Aungier loss model, whilst a multi-stage, Reynolds averaged CFD model is employed to assess the 3D flow behaviour using the 𝑘 − 𝜔 𝑆𝑆𝑇 turbulence model. Significant deviations in total-to-total efficiency were observed between the mean-line and CFD results during part-load operation, especially at lower mass flow rates. These deviations can reach up to 18% when the blade Mach number exceeds the design point by 12%. This is attributed to flow separation, which is evident from the CFD simulations, and the mean-line loss model fails to predict. From a purely aerodynamic standpoint, the turbine can operate at part-load conditions up to 88.5% of the design flow coefficient based on the CFD results and achieve an efficiency of 80.2%. It was also found that increasing the number of stages from 4 to 14 can improve the off-design total-to-total efficiency by approximately 7.7% at 93% of the design flow coefficient. This demonstrates that increasing the number of stages enhances turbine performance at both design and part-load operations. Finally, the similitude scaling laws formulated using real-gas equation of state were found to remain valid for all the mixtures, molar compositions, and operating conditions considered.