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How do reaction water turbines use a pressure drop and accelerating water flow to turn a runner?

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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.

In a reaction turbine, the forces that drive the rotor are generated by the reaction of an accelerating water flow within the runner as the pressure drops.1

Unlike impulse turbines, where energy extraction occurs primarily through the kinetic energy of a high-velocity jet, reaction turbines utilize both pressure and velocity changes. As water passes through the turbine, a portion of its potential energy is converted into kinetic energy within the inlet casing and control gates. As the water flows through the runner, it undergoes a simultaneous pressure drop and acceleration. This change in velocity exerts a force on the runner blades, causing the rotor to turn.1

The principle is analogous to a rotary lawn sprinkler, where the reaction to the emerging water jet drives the rotor in the opposite direction. Because these turbines rely on the pressure differential across the runner, they are typically enclosed and operate over a wide range of heads and flow rates.12