Which blade type operates primarily by impulse action?

Which blade type operates primarily by impulse action?

• A. Impulse
• B. Reaction
• C. Impulse-reaction
• D. Mixed

Answer: (A)

This question tests how energy is transferred from steam to the rotor through blade action. Impulse blades operate by changing the momentum of the steam with little to no pressure rise across the blade surfaces. In an impulse turbine, steam expands in fixed nozzles to a high velocity and then strikes and is deflected by the blades. The thrust on the blade comes mainly from this momentum change (an impulse), so most of the energy transfer is through impulse action rather than a lift produced by pressure differences. Reaction blades, by contrast, rely on a pressure rise in the flow as it passes through the moving blades, with energy transfer arising from both pressure forces and a flow acceleration. Mixed or impulse-reaction blades combine both effects but are not primarily impulse. So the blade type that operates primarily by impulse action is the impulse blade.

This question tests how energy is transferred from steam to the rotor through blade action. Impulse blades operate by changing the momentum of the steam with little to no pressure rise across the blade surfaces. In an impulse turbine, steam expands in fixed nozzles to a high velocity and then strikes and is deflected by the blades. The thrust on the blade comes mainly from this momentum change (an impulse), so most of the energy transfer is through impulse action rather than a lift produced by pressure differences.

Reaction blades, by contrast, rely on a pressure rise in the flow as it passes through the moving blades, with energy transfer arising from both pressure forces and a flow acceleration. Mixed or impulse-reaction blades combine both effects but are not primarily impulse.

So the blade type that operates primarily by impulse action is the impulse blade.