In order to explore the similarities and differences of the loss characteristics of carbon dioxide and steam in turbine cascades

the flow characteristics of the two kinds of working fluids in stator cascade and stage were studied by numerical methods. And the optimal Mach number for efficient operation of turbine stage under subsonic condition was obtained. The results show that with the increase of Mach number

the flow loss first increases and then decreases. When the Mach number is lower

the diffuser has a large range and is easy to backflow

which makes the wall boundary layer thicken and separate

and increases the overall flow loss. When the Mach number is higher

the strength of the secondary vortex in passage is larger

and the shock wave will be generated near the trailing edge of blades. The reason for the larger flow loss is the secondary flow and shock wave. Compared with steam

the dynamic viscosity of carbon dioxide is slightly higher

and its density is about twice that of steam. Under the same Mach number condition

the mainstream velocity is lower

the boundary layer is thicker

and the overall loss is larger. When the Mach number is lower than 0.30

the total-total efficiency of turbine stages with carbon dioxide is lower. While the Mach number is higher than 0.50

the efficiency of carbon dioxide is slightly higher than that of steam. When the optimal outlet Mach number of balde is about 0.60

the efficiency of both is the highest. The research results will provide a reference for further improving the design level for axial flow turbines of steam and carbon dioxide

and further understanding the loss characteristics of different medium in turbine stage.