---
title: "Compressor and turbine performance characteristics"
subject: "Turbo Machines"
chapter: 3
key: "semester_7/turbo_machines:compressor-turbine-performance-characteristics"
source: https://www.bctengineer.com/q/semester_7/turbo_machines/compressor-turbine-performance-characteristics
site: BCT Engineering
---

# Compressor and turbine performance characteristics

Turbo Machines · Chapter 3: Gas Turbine

- Asked in: year not given, 3+3 marks
- Video: [Performance Characteristics of Centrifugal and Axial Air Compressors by Dr. Dussa Govardhan](https://www.youtube.com/watch?v=x6ODx084e_4) — Institute of Aeronautical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/turbo_machines/compressor-turbine-performance-characteristics
- Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-3

## Question

Explain the performance characteristics of a gas turbine compressor and turbine. What are surge and choking? Compare centrifugal and axial flow compressors for use in gas turbines.

## Short answer

Compressor characteristic: pressure ratio vs. corrected mass flow with constant speed lines. Surge is low-flow stall causing flow reversal and pressure pulsation; choking occurs at high flow when Mach number reaches 1 limiting mass flow. Turbine characteristic: expansion ratio vs. corrected flow; once choked, flow parameter constant. Centrifugal compressors have higher pressure ratio per stage, lower cost, wider operating range; axial compressors offer higher efficiency and flow capacity, smaller frontal area.

## Answer

#### Compressor characteristic
The performance map plots the pressure ratio against the corrected mass flow $\dot m\sqrt{T_{01}}/p_{01}$ for lines of constant corrected speed $N/\sqrt{T_{01}}$, with islands of constant isentropic efficiency.

```text
 p02/p01
    |           surge line
    |          /
    |         *-----.  N3
    |        /       \
    |       *-----.   |
    |      /       \  |  N2
    |     *-----.   | |
    |    /       \  | |  N1
    |   /         | | |
    +---------------------------- m*sqrt(T01)/p01
      each speed line starts at the surge line (*) and
      drops almost vertically at its right end (choke)
```

- **Surge:** at low flow the blades stall, the delivery pressure collapses and the flow reverses; the cycle repeats with violent pulsations. The surge line bounds the left side of the map.
- **Choking:** at high flow the Mach number reaches 1 at some section (rotor inlet or a throat), the mass flow cannot increase further, and the speed lines become vertical.
- The operating line must lie between the surge and choke limits, in the region of high efficiency.

#### Turbine characteristic
The map of expansion ratio against $\dot m\sqrt{T_{01}}/p_{01}$ for different values of $N/\sqrt{T_{01}}$ shows that once the nozzle (or stage) is choked, the flow parameter becomes constant and independent of speed. The curves for different speeds therefore merge at high expansion ratio. The efficiency is flat over a wide range, so turbines are less sensitive than compressors to off-design operation.

#### Centrifugal and axial compressors
| Point | Centrifugal | Axial |
|---|---|---|
| Pressure ratio per stage | 4 to 8 (single stage) | 1.1 to 1.4 |
| Number of stages | 1 to 2 | 5 to 20 |
| Efficiency | 75 to 85% | 85 to 92% |
| Frontal area | Large | Small |
| Flow capacity | Smaller | Larger |
| Operating range | Wide, rugged | Narrow, surge-prone |
| Cost, FOD tolerance | Low cost, tolerant | Costly, sensitive |
| Use | Small engines, APUs | Large engines, aircraft |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a compressor map plot?
   A. Pressure ratio against corrected mass flow for lines of constant corrected speed
   B. Power against time
   C. Temperature against entropy only
   D. Efficiency against pipe length
   Answer: A. Pressure ratio vs m sqrt(T01)/p01 at constant N/sqrt(T01), with efficiency islands. (Key point: Compressor map)

2. What is surge?
   A. Mach number reaching 1 and flow cannot increase
   B. Excess efficiency at high speed
   C. At low flow, blades stall, delivery pressure collapses and flow reverses with violent pulsations
   D. Fuel flooding
   Answer: C. Surge occurs at low flow on the left of the map, bounded by the surge line. (Key point: Surge)

3. What happens at choking?
   A. Flow reverses
   B. Mach number reaches 1 at a section, so mass flow cannot increase and speed lines become vertical
   C. Pressure ratio doubles
   D. Efficiency becomes 100%
   Answer: B. Choking limits mass flow at the right end of each speed line. (Key point: Choking)

4. Which comparison is correct?
   A. Axial has higher pressure ratio per stage
   B. Centrifugal has higher efficiency and small frontal area
   C. Axial has 1.1-1.4 per stage but higher efficiency, small frontal area and larger flow; centrifugal is rugged and cheaper
   D. Both are identical
   Answer: C. Axial compressors need many stages but are more efficient and suit large engines; centrifugal ones are rugged, wide-range and low-cost. (Key point: Centrifugal vs axial)

## Related in Turbo Machines

- [Axial gas turbine stage design calculation](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aaxial-turbine-stage-gas-numerical) — Turbo Machines, chapter 2
- [Actual gas turbine cycle with component efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aactual-brayton-cycle-numerical) — Turbo Machines, chapter 3
- [Gas turbine power from isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Agas-turbine-power-isentropic-efficiency) — Turbo Machines, chapter 1
- [Convergent nozzle: choking and mass flow](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aconvergent-nozzle-choking-mass-flow) — Turbo Machines, chapter 4
- [Gas turbine engine components and arrangement](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Agas-turbine-engine-schematic-components) — Turbo Machines, chapter 3
- [Gas turbine combustion chamber: requirements and types](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acombustion-chamber-requirements-types) — Turbo Machines, chapter 3

## Related in other subjects

- [Impulse and reaction turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-reaction-performance) — Hydro Power, chapter 7
- [Two-shaft gas turbine, pressure ratio 7:1](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-two-shaft-7-1) — Power Plant Equipment, chapter 6, asked 2×
- [Gas turbine 800 kW with regenerator](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-800kw-regenerator) — Power Plant Equipment, chapter 6, asked 2×
- [Ideal gas turbine 100 kW: pressure ratio, work, efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aideal-gas-turbine-100kw-pressure-ratio) — Fundamental of Thermodynamics & Heat Transfer, chapter 6