Discussion
An impeller with backward curved blades :
Answer: C
An impeller with backward curved blades has a drooping head-discharge characteristic.
With backward curved blades (outlet angle less than 90°), the whirl velocity at outlet falls as the flow rises, so the theoretical head decreases steadily with discharge. This drooping characteristic is stable, and the power drawn reaches a maximum and then falls, so the motor cannot be overloaded. That is why nearly all centrifugal pumps use backward curved blades.
A rising head-discharge characteristic belongs to forward curved blades, and radial blades give a flat theoretical one. The options about reverse rotation and an unbalanced rotor describe nothing specific to the blade shape.
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