GATE 2022 AE – Question 14
The point of maximum entropy on a Fanno-curve in a Temperature-Entropy (T-s) diagram represents the
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Correct answer: (C) sonic Mach number
Explanation
A **Fanno flow** is steady, adiabatic flow in a constant-area duct with friction. On a T-s diagram the Fanno line is a curve with a **maximum entropy point**.
- The upper branch is subsonic and the lower branch is supersonic.
- Friction always increases the entropy of the flow (second law), so on either branch the state moves **toward the point of maximum entropy**.
- At the maximum-entropy point the Mach number is exactly **1** (the sonic state, "choked" flow).
So the point of maximum entropy represents the **sonic Mach number** (option C).