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What is the difference between impulse turbines and reaction turbines?

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Rows of white wind turbines with spinning blades stand on a dry, grassy landscape at a wind farm near Tehachapi, California. — turbine Wind turbines near Tehachapi, California.

The primary difference between impulse and reaction turbines lies in how they extract energy from a fluid to drive the rotor.

Impulse Turbines

In an impulse turbine, the fluid's potential energy is converted into kinetic energy through a nozzle before it reaches the rotor. The high-velocity jet strikes the blades (or buckets) directly, driving the rotor without a pressure drop across the blades themselves. Because there is no pressure change within the rotor passages, the fluid velocity relative to the blades remains essentially constant.123

Reaction Turbines

In a reaction turbine, the rotor is driven by the reaction force created as the fluid accelerates through the runner while its pressure drops. Unlike impulse designs, a significant portion of the energy extraction occurs within the moving blades, where the pressure decreases from the inlet to the exhaust. This pressure drop exerts both tangential forces, which turn the rotor, and axial forces that often require a thrust bearing to manage.452

Due to their design, reaction turbines can operate across a wider range of heads and flow rates compared to impulse turbines.4