Question

The figure below shows the blading of the rotors of two different axial turbomachines under their typical operating conditions, labelled as Configuration I and Configuration II. Which of the following statements is/are TRUE?

• A. Configuration I corresponds to the rotor of a compressor and Configuration II corresponds to the rotor of a turbine
• B. Configuration I corresponds to the rotor of a turbine and Configuration II corresponds to the rotor of a compressor
• C. The rotor blades of the turbomachine in Configuration I move along the \(+\theta\) direction
• D. The rotor blades of the turbomachine in Configuration II move along the \(+\theta\) direction

Hint:

Compare blade camber: thin and lightly cambered means compressor (adverse pressure gradient), thick and heavily cambered means turbine (favourable pressure gradient); then check which face, pressure or suction, leads the +theta direction shown.

Answer 1

The Correct Option is A, D

Step 1: Compare Blade Shape and Camber.
The single most reliable way to tell a compressor rotor blade from a turbine rotor blade in a cascade picture is to look at how thick and how strongly cambered (curved) the blade is, because the two machines see opposite pressure gradients along the flow. In Configuration I, the blades are thin, aerofoil shaped sections with a modest amount of turning from inlet to exit. In Configuration II, the blades are noticeably more curved, wrapping through a much larger turning angle, and have a thicker, more cambered profile.

Step 2: Connect Blade Shape to the Pressure Gradient.
In a compressor, the flow moves against a rising static pressure (an adverse pressure gradient) as it passes through the rotor passage. An adverse pressure gradient makes the boundary layer prone to separation, so compressor blades must be kept thin and lightly cambered, turning the flow only a small amount per stage, to avoid stalling the blade passage. In a turbine, the flow accelerates through a falling static pressure (a favourable pressure gradient), which keeps the boundary layer attached even with strong curvature, so turbine blades can be thick and highly cambered, turning the flow through a much larger angle in a single stage.

Step 3: Assign the Configurations.
Configuration I's thin, lightly cambered blades match the compressor description from Step 2, so Configuration I is a compressor rotor. Configuration II's thick, strongly cambered blades match the turbine description, so Configuration II is a turbine rotor. This makes option (A) TRUE and option (B) FALSE.

Step 4: Determine the Direction of Rotor Motion.
A compressor rotor does positive work ON the fluid, so it must move with its concave, pressure side face leading the motion, the same way a fan blade pushes air ahead of its cupped face. A turbine rotor, on the other hand, is driven BY the fluid: the flow pressing on the blade's concave (pressure) face is what drives the rotor, so the blade is pushed along with its convex, suction side face leading, the pressure face trails behind. Reading the blade orientation against the marked \(+\theta\) direction in the figure, the Configuration I (compressor) blades have their pressure face oriented toward the \(-\theta\) side, so that rotor moves in the \(-\theta\) direction, not \(+\theta\); this makes option (C) FALSE. The Configuration II (turbine) blades have their suction face leading toward the \(+\theta\) side, so that rotor moves in the \(+\theta\) direction, making option (D) TRUE.

Final Answer:
\[ \boxed{\text{Configuration I is a compressor, Configuration II is a turbine and moves along } +\theta \text{, i.e. options (A) and (D)}} \]