Mixed-Flow Hydroturbine

a reaction hydraulic turbine of the propeller type in which the flow of water through the runner is first radial and then axial relative to the turbine axis. The water passes from the scroll casing of the turbine to the guide apparatus of the stator. The draft tube is usually curved.

A mixed-flow turbine has the highest optimum efficiency of all types of hydraulic turbines. However, the performance characteristic of mixed-flow turbines is not as flat as that of adjustable-blade turbines. Therefore, mixed-flow turbines are inferior to adjustable-blade turbines with respect to power production in hydraulic power plants where the head varies greatly and the number of units is small. The cavitational properties of mixed-flow turbines are far superior to those of adjustable-blade turbines.

Since the blades of a mixed-flow turbine are rigidly attached to the upper and lower rims, the turbine has high mechanical strength and can be used at heads of up to 600 m. For heads in the range of 45 to 150 m, mixed-flow turbines are efficient in hydraulic power plants where head variations are small and the number of turbine units is large. Only mixed-flow hydraulic turbines are used for heads exceeding 150 m.

Mixed-flow turbines with a high specific speed are used for low heads, and those with a low specific speed are used for high heads. As of 1974, the largest mixed-flow hydroturbines in terms of both power and runner diamater were installed in the USSR at the Krasnoiarsk Hydroelectric Power Plant. The runner diameter of these turbines is 7.5 m, and their power exceeds 500 megawatts; they are designed for operation at a maximum head of about 100 m.