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

  • Thread starter Thread starter Smith972
  • Start date Start date
  • Tags Tags
    Mach Nozzle
Click For Summary

Discussion Overview

The discussion centers around the theoretical and practical aspects of achieving a Mach number greater than one at the throat of a convergent-divergent nozzle. Participants explore calculations, measurements, and potential anomalies in observed Mach numbers, with a focus on experimental setups and methodologies.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant asserts that it is theoretically impossible to have a Mach number greater than one at the throat of a convergent-divergent nozzle, yet reports calculations yielding a Mach number of over 1.2.
  • Another participant requests clarification on the setup and method used to obtain velocity, questioning whether the data is from a simulation or actual measurements.
  • A participant describes their experimental setup, noting that the Mach number exceeds one at the throat and then drops to subsonic speeds downstream, which they find abnormal.
  • Concerns are raised about the accuracy of the Mach number calculation, particularly regarding the temperature variation relative to pressure variation.
  • Participants discuss the method of measuring mass flow rate and the potential for errors in velocity measurement across the nozzle's cross-section.
  • Questions arise about the use of pressure measurements to derive velocity, specifically whether static pressure is being measured and how it relates to the velocity calculation.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the observed Mach numbers and the methodologies used to obtain them. There is no consensus on the reasons for the high Mach number readings or the accuracy of the measurements.

Contextual Notes

Participants note potential limitations in the experimental setup, including assumptions about uniform velocity across the nozzle's cross-section and the relationship between pressure and temperature in their calculations.

Smith972
Messages
4
Reaction score
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 celsius.

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?
 
Engineering news on Phys.org
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?
 
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).
 
How are you getting your mach number at the throat?
 
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.
 
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?
 
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.
 
How are you using the pressure measurement to get velocity? Are you just measuring the static pressure?
 

Similar threads

Is there a pressure ratio limit for choked flow through a c-d nozzle?
  • · Replies 10 ·
Replies
10
Views
3K
Is it possible to have a Mach number greater than 1 at the throat?
  • · Replies 18 ·
Replies
18
Views
23K
Solving Long Mach Nozzle Shape Problem for MBMS
  • · Replies 6 ·
Replies
6
Views
3K
Air flow in Convergent divergent nozzle
  • · Replies 2 ·
Replies
2
Views
12K
Isentropic Relations for Real Gas in Converging-Diverging nozzle
  • · Replies 1 ·
Replies
1
Views
4K
Hobby rocket engine experiments
  • · Replies 5 ·
Replies
5
Views
2K
Converging-Diverging Nozzle
  • · Replies 3 ·
Replies
3
Views
3K
Graduate Optimal Size of Rocket Engine Nozzle for Vacuum Performance
  • · Replies 26 ·
Replies
26
Views
4K
Calculating Mass Flow Rates and Mach Numbers in a Convergent-Divergent Nozzle
  • · Replies 7 ·
Replies
7
Views
4K
Archived Filling an Empty Tank: Calculating Pressure & Temp
  • · Replies 2 ·
Replies
2
Views
2K