Hydraulic Machines MCQ with answer and solution

Concept

In reaction turbines, the total head is available in the form of both pressure energy and kinetic energy, and pressure changes occur continuously as water passes through the guide mechanism and the runner.

Formula

Degree of Reaction (R) = (Pressure drop in runner) / (Total pressure drop in the stage)

Step-by-step solution

1. Identify the type of turbine: Reaction Turbine. 2. Recall that in reaction turbines, the casing is completely filled with water. 3. Note that the guide vanes act as nozzles to increase velocity (reducing pressure). 4. Note that the runner blades are shaped such that a pressure difference is maintained across them to produce lift/rotation, leading to further pressure reduction.

Explanation

In a reaction turbine (like Francis or Kaplan), the water enters the runner under pressure. As it flows through the guide vanes, a portion of the pressure energy is converted into kinetic energy, and as it passes through the runner, the remaining pressure energy is gradually converted into kinetic energy and then into mechanical work, causing a pressure drop in both components.

Why the other options are wrong

A: Pressure change is not limited to the runner; it starts in the guide vanes. B: This describes an impulse turbine where the nozzle is the only place where pressure drops. D: Pressure must change to convert fluid energy into mechanical work in any hydraulic turbine.

Shortcut

Impulse turbine = Pressure change only in nozzle; Reaction turbine = Pressure change in both guide vanes and runner.

Common mistake

Confusing reaction turbines with impulse turbines (like Pelton wheels), where pressure remains atmospheric throughout the runner.