Axial turbines are gaining prominence in supercritical carbon-di-oxide (S-CO2) Brayton cycle power blocks. S-CO2 Brayton cycle power systems designed for 10 MW and upwards will need axial turbines for efficient energy conversion and compact construction. The real gas behavior of S-CO2 and its rapid property variations with temperature presents a strong challenge for turbomachinery design. Applying gas and steam turbine philosophies directly to S-CO2 turbine could lead to erroneous designs. Very little information is available in the open literature on the design of S-CO2 axial turbines. &nbsp;In this paper, design of a 10 MW axial turbine for a simple recuperated Brayton cycle waste heat recovery system is presented. Three repeating stages with nominal stage loading coefficient of 2.6 and flow coefficient of 0.32 were designed. An axial turbine mean-line design method tuned to S-CO2 real gas fluid medium is discussed. 3D blade design using commercial turbomachinery design software Axstream is also described. The turbine was designed for intel temperature of 818.15 K, pressure ratio of 2.2, rotational speed of 12000 rpm and mass flow rate of 104.5 kg/s. 3D CFD simulations were carried out using the commercial RANS solver ANSYS CFX 2020 R2 with SST k-ω turbulence model for closure. S-CO2 was modelled as real gas with Refrigerant Gas Property tables generated over the appropriate pressure and temperature ranges using NIST Refprop database. CFD studies were carried out over a range of mass flow rates and speeds covering the design and several off-design conditions. The performance maps generated using CFD simulations of the turbine are shown here. It was observed that the turbine produced 11 MW power at the design condition passing the required mass flow. The CFD studies also showed that the turbine could be operated with excellent efficiency of 89% at the design condition. Very conservative loading and flow coefficients were used during the design. The design has potential for further optimization to obtain improved performance and also for reducing the number of stages to two.

Preliminary Aerodynamic Design of a S-CO2 Axial Turbine

Paper Proceedings: https://asmedigitalcollection.asme.org/GTINDIA/GTINDIA2021/volume/85536

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