Performance and size comparison of two-stage and three-stage axial turbines for a nitrogen gas Brayton cycle coupled with a sodium-cooled fast reactor

•Blade sizes and velocity triangles were determined from mean-line design method.•Flow analyses were numerically performed based on three-dimensional blade designs.•Reducing number of turbine stage was possible with little loss of cycle efficiency.•Fewer turbine stage increase aerodynamic loss owing to higher Mach number. A nitrogen (N2) Brayton cycle is considered as an alternative to the conventional steam Rankine cycle to overcome essential risk during pressure boundary rupture accidents with sodium-water reaction in a steam-generator of a sodium-cooled fast reactor (SFR). The objectives of this study are to estimate the turbine performances after proposing three-dimensional (3-D) blade designs and to suggest an option of detailed turbine design for N2 Brayton cycle-SFR applications. A free-vortex design method was applied in the 3-D design. Three candidates, including two- and three-stage designs, were selected from the Smith charts. Aerodynamic losses were investigated and compared through computational fluid dynamics analyses for the cases selected above. According to the results, rotor blade with milder curvature increased proportions of secondary and tip leakage losses among the total aerodynamic loss. When comprehensively considering the turbomachinery size and cycle thermal efficiency, the size of the two-stage turbine could be reduced to under 55% of the three-stage turbine size, resulting in no significant decrement of the cycle efficiency.