---
title: "Gas turbine nozzle: principle and types"
subject: "Turbo Machines"
chapter: 4
key: "semester_7/turbo_machines:nozzle-principle-types"
source: https://www.bctengineer.com/q/semester_7/turbo_machines/nozzle-principle-types
site: BCT Engineering
---

# Gas turbine nozzle: principle and types

Turbo Machines · Chapter 4: Gas Turbine Nozzles

- Asked in: year not given, 5 marks
- Video: [Types of Nozzle -Convergent Nozzle - Divergent Nozzle - Convergent Divergent Nozzle](https://www.youtube.com/watch?v=vRyOQb8KGd4) — BEST MECHANICAL ENGINEERING
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/turbo_machines/nozzle-principle-types
- Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-4

## Question

Explain the principle of operation of a nozzle. Describe convergent, convergent-divergent and divergent nozzles with the variation of velocity, pressure and Mach number, and state where each is used in gas turbine engines.

## Short answer

Nozzle converts enthalpy to kinetic energy. Convergent nozzle accelerates subsonic flow to at most sonic at exit; used on subsonic engines. Convergent-divergent nozzle accelerates flow to supersonic in diverging section after sonic throat; used for high pressure ratio. Divergent nozzle accelerates supersonic flow; with subsonic entry it acts as a diffuser. Velocity increases, pressure decreases; Mach number rises in convergent, goes supersonic in divergent.

## Answer

A **nozzle** is a passage of varying cross-section that converts the enthalpy (pressure energy) of a fluid into kinetic energy. From the steady flow energy equation for an adiabatic nozzle:

$$
h_0 = h + \frac{C^2}{2} \;\Rightarrow\; C = \sqrt{2(h_0 - h)}
$$

and for isentropic flow of a perfect gas the area-velocity relation is

$$
\frac{dA}{A} = \frac{dC}{C}\left(Ma^2 - 1\right)
$$

#### Area-velocity relation
| Flow | Increasing velocity needs | Decreasing velocity needs |
|---|---|---|
| Subsonic ($Ma < 1$) | Area to decrease (convergent) | Area to increase (diffuser) |
| Supersonic ($Ma > 1$) | Area to increase (divergent) | Area to decrease |
| Sonic ($Ma = 1$) | Throat, minimum area | - |

#### Types
```text
 Convergent      Convergent-divergent    Divergent (supersonic)
 ---\            ---\       /---            /---
 ====>           ===> throat ====>        ====>
 ---/            ---/       \---            \---
```
1. **Convergent nozzle:** the area decreases; the exit velocity is at most sonic. If the back pressure falls below the critical value ($p_b/p_0 < 0.528$ for air), the nozzle is **choked**: the exit pressure stays at $p^*$ and the further expansion occurs outside the nozzle through shock and expansion waves. Used on subsonic and low-supersonic aircraft engines, and as the final nozzle of turbojets.
2. **Convergent-divergent (de Laval) nozzle:** the flow is subsonic in the convergent part, $Ma = 1$ at the throat, and supersonic in the divergent part, where the pressure continues to fall. Used when the pressure ratio is high: rocket engines, afterburning and supersonic jet engines.
3. **Divergent nozzle:** a divergent passage with supersonic entry (the divergent part alone) accelerates the flow further. With subsonic entry it acts as a **diffuser**, used in engine inlets and compressor diffusers.

In gas turbines the **turbine nozzle guide vanes** form a ring of convergent passages that accelerate the gas and turn it onto the rotor blades; the **propelling nozzle** produces the jet thrust.

## Check yourself

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

1. What does a nozzle do?
   A. Converts enthalpy (pressure energy) into kinetic energy
   B. Converts kinetic energy into pressure energy
   C. Adds shaft work to the fluid
   D. Adds heat to raise the stagnation temperature
   Answer: A. From h0 = h + C^2/2, the enthalpy drop appears as kinetic energy, C = sqrt(2(h0 - h)). (Key point: Nozzle principle)

2. For supersonic flow (Ma > 1), what area change increases the velocity?
   A. Decreasing area
   B. Constant area
   C. Increasing area (divergent)
   D. Area change has no effect
   Answer: C. dA/A = (dC/C)(Ma^2 - 1) is positive for Ma > 1 when dC is positive, so the passage must diverge. (Key point: Area-velocity relation)

3. A convergent nozzle with p_b/p_0 below 0.528 for air is choked. What happens?
   A. Exit pressure falls to the back pressure inside the nozzle
   B. The flow becomes supersonic inside the nozzle
   C. The flow reverses
   D. Exit stays at p* with Ma = 1 and the extra expansion occurs outside the nozzle
   Answer: D. A convergent nozzle can at most reach sonic speed at its exit; the remaining pressure drop happens outside through shock and expansion waves. (Key point: Choked convergent nozzle)

4. In a de Laval nozzle, where is Ma = 1 and what happens to pressure in the divergent part?
   A. At the throat; pressure keeps falling in the divergent part
   B. At the exit; pressure rises in the divergent part
   C. At the inlet; pressure stays constant
   D. At the throat; pressure rises in the divergent part
   Answer: A. Flow is subsonic converging, sonic at the minimum-area throat, then supersonic and still expanding (pressure falling) in the divergent part. (Key point: Convergent-divergent nozzle)

5. A divergent passage with subsonic entry acts as
   A. a nozzle
   B. a diffuser
   C. a choked throat
   D. a turbine nozzle guide vane
   Answer: B. Subsonic flow decelerates in an increasing area, so it is a diffuser, used in engine inlets and compressor diffusers. (Key point: Divergent passage with subsonic entry)

## Related in Turbo Machines

- [Convergent-divergent nozzle with efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acd-nozzle-efficiency-exit-area-numerical) — Turbo Machines, chapter 4
- [Stagnation properties and Mach number relations](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Astagnation-temperature-pressure-relations) — Turbo Machines, chapter 4
- [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
- [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 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

- [Convergent-divergent steam nozzle design](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Asteam-nozzle-numerical) — Fluid Machines, chapter 5
- [Steam nozzles and critical pressure ratio](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Asteam-nozzle-flow-critical-pressure) — Fluid Machines, chapter 5
- [Convergent versus divergent plate boundary](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aconvergent-versus-divergent-plate-boundary) — Engineering Geology I, chapter 2, asked 3×
- [Speed of sound and Mach number](https://www.bctengineer.com/question.md?key=semester_4%2Ffluid_mechanics_mechanical%3Amach-number-speed-of-sound-regimes) — Fluid Mechanics, chapter 9