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-   -   super/sub sonic flows in laval nozzle (http://www.cfd-online.com/Forums/cd-adapco/54664-super-sub-sonic-flows-laval-nozzle.html)

Kalyan June 24, 2005 06:21

super/sub sonic flows in laval nozzle
 
I am currently working on supersonic flows and mass flow rates in Convergent-Divergent nozzle with StarCD 3.24. I am encountering some problems.

My nozzle is of dimensions

inlet dia : 9mm Outlet dia : 9mm throat dia : 3 mm throat length : 1mm total nozzle length :51mm

I am a beginner in Starcd and in Fluid dynamics stuff.

My design is a simple 2-d model (two conical sections meeting at the throat) with around 3000 elements.

boundaries are

pressure : 2 bar- 20 bar [ at one end of the nozzle]

pressure : 1 bar (constant all the time) [at another end of the nozzle]

rest of the cells as wall and i tried with symmetry BCīs too.

i have kept the density as a function dependent on temperature and pressure f(T,P) and viscosity as inviscid flow.

i have run with SIMPLE solver and a residual tolerance of 10e-5

As far as(from theory) i know that the mass flow rate should not increase after a certain pressure (after we reach mach 1).

but i see that from 2 bar - 20 bar the mass flow rate keeps on increasing.

i use the commands getc all dens integrate cset to get the mass flow rate

is there any problem with my model? can you please help me, why i cannot see the mass flow rate constant ...

can you please suggest me on the model, any help would be great

thanking you Kalyan Yalamanchili

Pauli June 24, 2005 10:05

Re: super/sub sonic flows in laval nozzle
 
If I understand your post, you're holding the downstream pressure constant & raising the upstream pressure.

I'm a bit rusty on supersonic flow. But I believe mass flow rate will continue to rise even after the nozzle chokes (Mach number = 1). I believe this is due to the increase in upstream density. Now if you hold the upstream pressure constant & continue to lower the downstream pressure, then mass flow rate will hit a limit at some pressure ratio. I don't remember the exact value.

kalyan yalamanchili June 24, 2005 10:28

Re: super/sub sonic flows in laval nozzle
 
thanx Pauli

i know that when the pressure is constant at the inlet and the pressure at the outlet is decreasing there will a choked flow...

are you sure that the inverse isīt true ...?

please confirm this .... if you have any links concerning this please do post them

thanx once again kalyan

Pauli June 24, 2005 11:04

Re: super/sub sonic flows in laval nozzle
 
Reference any text book on compressible flow or gas dynamics.

al August 17, 2005 17:58

Re: super/sub sonic flows in laval nozzle
 
try "nozzle" from http://www.aerorocket.com/products/products.html


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