The impulse turbine works on the principle that

• A. Kinetic energy can be converted to mechanical energy
• B. Steam expansion in the moving blades creates a force impulse
• C. The high pressure creates unbalanced forces on the blades
• D. High velocity steam exerts force upon the rotating blades due to change in momentum

Answer: (D)

Impulse turbines generate work from the momentum change of the steam as it is deflected by the rotating blades. The nozzle first accelerates the steam to a very high velocity, and when this jet strikes the moving blades, its direction and speed change. This change in the steam’s momentum transfers an impulse to the blade, producing torque and extracting energy. The fundamental driver is this momentum exchange, which is why the statement about high velocity steam exerting force on the blades due to a change in momentum is the best description.

Other ideas are related but not the core mechanism. The energy source is the jet’s kinetic energy, but the essential process in an impulse turbine is the momentum change during deflection. Expansion and high pressure effects described in the other options are characteristic of different turbine concepts or are not the primary driving mechanism in an impulse turbine.