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?
Show 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.
Configuration I corresponds to the rotor of a compressor and Configuration II corresponds to the rotor of a turbine
Configuration I corresponds to the rotor of a turbine and Configuration II corresponds to the rotor of a compressor
The rotor blades of the turbomachine in Configuration I move along the \(+\theta\) direction
The rotor blades of the turbomachine in Configuration II move along the \(+\theta\) direction
Show Solution
The Correct Option isA, D
Solution and Explanation
This question checks two separate skills: reading blade camber to tell a compressor rotor from a turbine rotor, and reading the sense of blade motion off a cascade sketch using the marked $+\theta$ direction.
(A) Configuration I is a compressor, Configuration II is a turbine: Compressor blades work against a rising, adverse, pressure gradient along the passage, which limits how much they can turn the flow before the boundary layer separates, so they stay thin and only lightly cambered, close to what Configuration I shows. Turbine blades work with a falling, favourable, pressure gradient, which lets them stay attached even with a large turning angle, so they are thick and heavily cambered, close to what Configuration II shows. This statement is TRUE.
(B) Configuration I is a turbine, Configuration II is a compressor: This is simply the reverse assignment of (A), and since (A) is correct, (B) must be FALSE.
(C) Configuration I blades move along $+\theta$: A compressor rotor blade does work on the fluid, so it advances with its concave, pressure surface leading the motion, pushing the air ahead of it. In the sketch, that pressure surface of the Configuration I blades faces the $-\theta$ side, so the rotor turns toward $-\theta$, not $+\theta$. This statement is FALSE.
(D) Configuration II blades move along $+\theta$: A turbine rotor blade is driven by the fluid pushing on its concave, pressure surface from behind, so it moves with its convex, suction surface leading. In the sketch, the Configuration II blades have that suction surface facing the $+\theta$ side, matching the marked direction, so this statement is TRUE.
So Configuration I is a compressor rotor, thin, lightly cambered blades for an adverse pressure gradient, and Configuration II is a turbine rotor, thick, heavily cambered blades for a favourable pressure gradient, that physically rotates along $+\theta$.
A blade's pressure (concave) face leads the motion when the blade drives the fluid (compressor); its suction (convex) face leads when the fluid drives the blade (turbine).
