In a reaction turbine, how does the velocity change as steam passes through moving blades?

• A. Increase
• B. Decrease
• C. Remain the same
• D. Increase then decrease

Answer: (B)

In a reaction turbine, energy is taken from the steam as it flows through the moving blades, so the steam ends the stage with less kinetic energy than it started with. The rotor blades are moving in the same direction as the flow, and as the steam expands and does work on those blades, its absolute velocity (the speed in the stationary frame) drops. Part of the enthalpy decrease goes into shaft work, leaving less kinetic energy to carry as speed, so the exit absolute velocity is lower than the entry velocity.

This is different from an impulse stage, where the jet’s speed is largely preserved and energy transfer comes mainly from changing the direction of the flow rather than reducing its speed.