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

[Smith972]

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?

 

[cjl]

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?

 

[Smith972]

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.

The Mach number rises to a value higher than one at the throat, then drops to subsonic speeds after (which is abnormal in itself).

 

[cjl]

How are you getting your mach number at the throat?

 

[Smith972]

Mach No. = Velocity/Speed of Sound

I've attached the Excel file I'm using

https://www.dropbox.com/s/los1hgjurq7i3dn/GQztHN8eturu2QyxB2uX.xls?dl=0

 

[cjl]

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.

 

[Smith972]

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?

 

[cjl]

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.

 

[RandomGuy88]

How are you using the pressure measurement to get velocity? Are you just measuring the static pressure?