The GATE Grind

GATE 2026 AE – Question 28

Propulsion · Turbomachinery: Axial compressors, work and compression, degree of reaction, multi-staging · 1 mark · Multiple select

A multi-stage axial compressor can be operated at two points, A and B, both of which lie on the same speed line, as shown in the figure below. If $\eta$ is the isentropic efficiency of the compressor, select the statements that is/are TRUE.

A speed line on a map of pressure ratio $\pi$ against mass flow rate $\dot m$. The line rises to a maximum on the left, then falls steadily to the right. Point A is just to the right of the maximum and point B is further down the line to the right.
  1. $\eta_A > \eta_B$
  2. $\eta_B > \eta_A$
  3. In comparison to point B, point A is closer to the surge point
  4. In comparison to point A, point B is closer to the choke point

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (A) $\eta_A > \eta_B$; (C) In comparison to point B, point A is closer to the surge point; (D) In comparison to point A, point B is closer to the choke point

Explanation

Surge is at the low mass flow end of the speed line and choke at the high mass flow end. A is nearer the maximum of the line (the surge side) and B is nearer the choke side, so C and D hold. Towards the choke end the flow incidence becomes strongly negative and the efficiency falls, so A has the higher efficiency of the two.

Official GATE 2026 answer key: https://gate2026.iitg.ac.in/doc/download/2026/Keys/AE_Keys.pdf