C-D Nozzle - Mach No. >1 at Throat

In summary: The Mach number rises to a value higher than one at the throat, then drops to subsonic speeds after (which is abnormal in itself).You are measuring the static pressure at the throat using a pressure sensor.
  • #1
Smith972
4
0
Hi,

As I understand, it is theoretically impossible to have a Mach No. higher than one at the throat of a Convergent Divergent Nozzle.

I'm currently working on some calculations, and while I've been told the calculations are correct, I'm getting abnormally high Mach Numbers at the throat.

For example, pressure I'm reading at the throat (4.31 Diameter) is 3.3Bar, temperature is 9 degrees celcius.

Using Velocity/Speed of Sound, I'm coming away with a Mach Number of over 1.2...

I'm assuming it's something to do with Shock Waves, but I really have no knowledge of why?
 
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  • #2
Can we have some more details about your setup and how you're getting the velocity? Is this a simulation, or is this a measured velocity at the throat?
 
  • #3
Sorry, I'd typed this up a bit quick...

Essentially I'm working on a nozzle apparatus,taking temperature, and pressure readings at the three taps. Then velocity, ambient pressure and temperature at the top.

upload_2015-2-12_18-46-6.png
upload_2015-2-12_18-46-35.png


The Mach number rises to a value higher than one at the throat, then drops to subsonic speeds after (which is abnormal in itself).
 
  • #4
How are you getting your mach number at the throat?
 
  • #6
Those numbers look odd - you don't have nearly enough temperature variation for your large pressure variation, for example. In addition, your method of obtaining mass flow rate seems prone to errors - specifically, how are you measuring V4? I wouldn't expect the velocity to be uniform across the cross section of region 4, so you'd have to have some way of averaging the velocity at a bunch of points.
 
  • #7
Measured the velocities via pressure sensor, we used the same pressure sensor at the centre of region 4.
Mass flow-rate's just mdot = pAV, as far as I'm aware? Is it the formula or the figures causing the error?
 
  • #8
Using a pitot-static measurement?

As for mass flow rate just being rho*A*V, yes, that's true, but I'm saying that I seriously doubt V is constant across the entire cross section, and you'd need some way to integrate/average the velocity profile to get a reasonably accurate value.
 
  • #9
How are you using the pressure measurement to get velocity? Are you just measuring the static pressure?
 

What is a C-D Nozzle?

A C-D Nozzle, also known as a Convergent-Divergent Nozzle, is a type of nozzle used in rocket engines to accelerate exhaust gases to supersonic speeds.

How does a C-D Nozzle work?

A C-D Nozzle has two sections - a converging section and a diverging section. The converging section narrows the flow of exhaust gases, increasing their velocity. The diverging section then expands the gases, further increasing their velocity and creating supersonic flow at the nozzle throat.

What is the importance of Mach No. >1 at the throat of a C-D Nozzle?

Mach No. >1 at the throat of a C-D Nozzle is important because it indicates that the exhaust gases are traveling at supersonic speeds. This is necessary for efficient propulsion in a rocket engine.

What happens if the Mach No. at the throat of a C-D Nozzle is less than 1?

If the Mach No. at the throat of a C-D Nozzle is less than 1, the exhaust gases are not traveling at supersonic speeds and the nozzle is not operating at its maximum efficiency. This can result in reduced thrust and performance.

What are some applications of C-D Nozzles?

C-D Nozzles are commonly used in rocket engines, jet engines, and supersonic aircraft. They are also used in some industrial processes, such as in gas turbines and steam turbines.

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