AdvancedTopic 78 of 105Thermodynamics & Fluids
Isentropic nozzle flow
Converging-diverging nozzles accelerate gas to supersonic speed.
In depth
In a converging nozzle flow chokes when exit reaches Mach 1 at the critical pressure ratio 0.528 (air); maximum mass flow ṁ=ρ*A*a*. A converging-diverging (de Laval) nozzle accelerates to supersonic speed in the diverging part. Used in rockets and supersonic wind tunnels.
Key points
- Area-Mach relation
- Design vs off-design pressure
- Underexpanded and overexpanded jets
ExampleA rocket nozzle expands exhaust from 7 MPa chamber to near-vacuum.
What is Isentropic nozzle flow?
Converging-diverging nozzles accelerate gas to supersonic speed.
Why is it important to understand isentropic nozzle flow?
Converging-diverging nozzles accelerate gas to supersonic speed, which is the basis of rocket engines and high-speed wind tunnels.
What makes a nozzle supersonic?
Subsonic flow accelerates in the converging part, and it reaches sonic speed at the throat, then accelerates further in the diverging part.
Why does a converging nozzle alone not go supersonic?
The flow can only reach Mach 1 at the throat of a converging nozzle, so a diverging section is needed to go faster.
How is Isentropic nozzle flow used in practice?
Typical applications include designing the nozzle of a rocket engine for a required thrust, sizing a supersonic wind tunnel nozzle for a test section and choosing the exit area of a steam nozzle in a turbine stage.
What should I learn before Isentropic nozzle flow?
Study "Mach number and speed of sound" first (topic 77 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Isentropic nozzle flow?
Area-Mach relation. Design vs off-design pressure. Underexpanded and overexpanded jets.
Can you give an example of Isentropic nozzle flow?
A rocket nozzle expands exhaust from 7 MPa chamber to near-vacuum.
What comes after Isentropic nozzle flow?
The next topic is "Normal shock waves", which builds directly on this one.
How difficult is Isentropic nozzle flow?
It is classed as Advanced in Thermodynamics & Fluids (topic 78 of 105). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Thermodynamics & Fluids.