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converging diverging nozzle

Posted May 6, 2015, 5:00 p.m. EDT Version 4.4 7 Replies

Mehlam Jupiterwala
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Hello
I am a student in Stonybrook University .
I am trying to design a bell nozzle for a rocket application .
Exit Mach number of nozzle is 3 . assuming isentropic equations with inlet to converging section is P=300psi and T=3400 K (which is also the exit of combustion chamber)
I have the dimensions of the nozzle from analytical calculations.
Since flow is supersonic , throat should have M=1.

I used Heat transfer in fluid module , and defined inflow and outflow heat flux boundary conditions , But some thing is wrong . can some one help me do design the problem and specify BC's suitably.
7 Replies Last Post May 11, 2015, 2:24 p.m. EDT
Jim Freels mechanical side of nuclear engineering, multiphysics analysis, COMSOL specialist
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Posted: 10 years ago May 6, 2015, 5:09 p.m. EDT
Have you used the compressible flow option within the CFD module. You will need that unless you model with your own equations using the PDE solver.
Have you used the compressible flow option within the CFD module. You will need that unless you model with your own equations using the PDE solver.
Mehlam Jupiterwala
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Posted: 10 years ago May 6, 2015, 7:05 p.m. EDT
Yes i am using turbulent K-epsilon model which says compressible (M<0.3). i don't know how corrects the results will be.
but prior to that i want to learn how to define the BC conditions.
if you can guide me to any tutorial or procedure . Bcz i think i am not doing that correct.

Yes i am using turbulent K-epsilon model which says compressible (M
Jim Freels mechanical side of nuclear engineering, multiphysics analysis, COMSOL specialist
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Posted: 10 years ago May 7, 2015, 11:28 a.m. EDT
Sounds like you are not using the compressible flow physics. The M<0.3 is actually an incompressible algorithm, but adapted for minor compressible flows. For your application, you are well above M=.3 (as you said yourself, M=1 in the throat). This set of physics will not work. You will need fully compressible flow which is supported in the CFD module. there is actually a good example in the COMSOL model library. Look up
CFD module->high mach number flow->sajben diffuser. Spatlart-Allmaras and k-e turbulence models are supported.
Sounds like you are not using the compressible flow physics. The Mhigh mach number flow->sajben diffuser. Spatlart-Allmaras and k-e turbulence models are supported.
Mehlam Jupiterwala
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Posted: 10 years ago May 7, 2015, 2:02 p.m. EDT
sajben diffusor is a good model to benchmark , thanks for referring.
Although the Mach number in throat section doesn't go to M=1 , hence no super sonic effect.
I would try to change the design and parameters and c if i get good results and will revert back.

Thanks.
sajben diffusor is a good model to benchmark , thanks for referring. Although the Mach number in throat section doesn't go to M=1 , hence no super sonic effect. I would try to change the design and parameters and c if i get good results and will revert back. Thanks.
Mehlam Jupiterwala
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Posted: 10 years ago May 11, 2015, 2:10 p.m. EDT
Hi sajben diffusor is a subsonic model . whenever i try to apply super sonic boundary condition at the exit .
model shows an error of missing variable .
can you guide me to a model having supersonic BC's ??
Hi sajben diffusor is a subsonic model . whenever i try to apply super sonic boundary condition at the exit . model shows an error of missing variable . can you guide me to a model having supersonic BC's ??
Jim Freels mechanical side of nuclear engineering, multiphysics analysis, COMSOL specialist
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Posted: 10 years ago May 11, 2015, 2:17 p.m. EDT
This would be a good question to ask COMSOL tech support.

Are you using the CFD module, and are you running compressible flow option ?

This would be a good question to ask COMSOL tech support. Are you using the CFD module, and are you running compressible flow option ?
Mehlam Jupiterwala
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Posted: 10 years ago May 11, 2015, 2:24 p.m. EDT
yes it is a compressible flow .
I am using the same module as sajben diffuser. "High Mach flow - turbulent flow:- spalarts allmara "
yes it is a compressible flow . I am using the same module as sajben diffuser. "High Mach flow - turbulent flow:- spalarts allmara "

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