Determining choked flow

[n3scz]

It is possible to determine whether air flow from a nozzle is choked (Ma=1) based only on the pressure and temperature before the exit and the area of the converging nozzle?

 

[zdas04]

Yes. See faq378-1201

David

 

[ione]

Well my answer is you need another parameter. You should also be capable of measuring the mass flow rate (in chocked flow condition the mass flow rate is independent from downstream pressure).

To establish whether flow is chocked or not it is necessary to check whether the ratio between absolute upstream and downstream pressure is equal or greater than [ ( k + 1 ) / 2 ] ^[k / ( k - 1 )], being “k” the specific heat ratio of the gas. So upstream and downstream pressure are required.
Now if you can measure the mass flow rate you can check whether it matches with the value given by the equation in the FAQ quoted by zds04. If the answer is yes then the flow is chocked.

 

[davefitz]

I think the 2 properties of a choked nozzle are (a) lowering the downstream pressure will not inrease the mass flow thru the nozzle and (b) sound waves will not propagate upstream thru the nozzle.

The mass flow is also affected by the nozzle geometry and molecular wt , so some sort of discharge coeficient needs to be addended to the relationship listed above by ione. In particular, sharp angles in the vicinity of the nozzle can cause "oblique shockwaves" to form and restrict the flow.

 

[FOURe]

Hi there:

Here a few plots dealing with the general nozzle performance:

​

Also, here are a few online calculators for compressible flow:

http://www.engineering-4e.com/calc5.htm

Thanks,

Gordan

PS

I have to point out that I agree with ione and davefitz -- they are correct, you need another piece of information ...

http://www.engineering-4e.com

 

[zdas04]

FOURe,
That is the fourth or fifth time you've posted that exact set of unreadable graphs. I guess I'm missing where they apply to the question asked other than you are getting pay-per-click revenue from people who follow your link. I have no problem with that, but the link really should apply to the question.

David

 

[bchoate]

n3scz
Beychok's thread FAQ378-1201 referenced by zdas04 gives a full explanation of 'choked' flow. If the P(1)/P(2) ratio for an orifice is in the range 1.7 - 2.1 for most gases, the conditions are there for 'choked' flow. Since the velocity through the orifice at choked flow is fixed at Ma 1, the flow is established also:

1129 ft/s = cft/s / sft (orifice)

From the volumetric flow and the gas conditions the mass flow rate can be calculated. The upstream and downstream pressures, orifice area (size), velocity of sound, and the k for the gas were needed for the calculations. Process conditions are needed for mass flow rate calculation.

'Choked flow' is a bit of a misnomer. The velocity is choked. Mass flow is not. Increasing the upstream pressure or decreasing the temperature will increase the density and increase the mass flow. For an orifice with ambient conditions upstream and a vacuum downstream, both mass flow and velocity are choked. The following reference provides a basic description of choked flow.

{

http://www.therebreathersite.nl/04_Links/Downloads/Choked.pdf}

 

[FOURe]

zdas04:

Thank you for your reply and remarks with respect to my input.

In my opinion, when dealing with nozzle operation, a temperature vs entropy diagram could be used to indicate the nozzle expansion process. Also, there should be some kind of presentation of Tstagnation/Tstatic as well as pstagnation/pstatic ratio values given as a function of the Mach Number.

Furthermore, a reference to some kind of online calculator for ideal nozzle expansion case should be provided to allow easy and quick evaluation of output values for the given input values so that participants can verify what the outcome is and get in agreement what is going on when it comes to the engineering equations ...

Apparently, my good intentions are not welcomed here.

Thanks,

Gordan

http://www.engineering-4e.com