Which statement best describes the fundamental difference between impulse and reaction turbines?

• A. Impulse turbines extract energy mainly from the momentum of steam striking the blades, while reaction turbines extract energy from pressure acting on the blades
• B. Impulse and reaction turbines are identical in operation
• C. Impulse turbines require low-velocity steam; reaction require high velocity
• D. Impulse turbines rely on magnetic interaction; reaction rely on electric fields

Answer: (A)

The fundamental distinction lies in how the blade gets its work from the steam. In an impulse turbine, the steam expands mostly in stationary nozzles to a high speed before hitting the rotor. The rotor blades then impart a deflection to that jet, transferring energy primarily through a change in momentum of the steam. The pressure on the blade surfaces remains nearly unchanged, so the work comes from momentum transfer.

In a reaction turbine, the steam continues to expand as it flows through both stationary and moving blades. The work is produced by the pressure forces acting on the blades as the steam expands across them, so energy transfer comes from a pressure difference on the moving blades.

So the best statement captures that impulse turbines extract energy mainly from momentum transfer, while reaction turbines extract energy from pressure acting on the blades.