# Need help defining a momentum source for a diffuser inlet

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March 2, 2012, 14:23
Need help defining a momentum source for a diffuser inlet
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Join Date: Jun 2010
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Hi all,

I am trying to simulate a variable vane diffuser inlet using a momentum source method. The method works as follows:

(from Srebric, J. and Chen, Q. 2002. “Simplified numerical models for complex air supply diffusers,” HVAC&R Research, 8(3), 277-294.)

This momentum method de-couples momentum and mass boundary conditions for the diffuser in a CFD simulation. The diffuser is represented in the CFD study with an opening that has the same gross area, mass inflow, and momentum flux as a real diffuser. This model enables specification of the source terms in the conservation equations over the real diffuser area. The air supply velocity U0 for the momentum source term is calculated from the mass flow rate, mdot , and the diffuser effective area A0: U0=mdot/(A0*rho)

I am having trouble understanding how to implement the method.

So here are the steps I took:
1. Defined diffuser volume as a subdomain, with inlet side connected to the duct, and with outlet side connected the main volume.
2. Calculate the mass flow rate on the inlet side.
3. Outlet side. This is where I'm having trouble understanding what to do. How do I define a momentum source which would properly represent the air supply velocity?

Any suggestions would be appreciated.

I am attaching an image with proper implementation of the method in a different paper. The image shows where momentum source was applied, but unfortunately has no details whatsoever how exactly they did it so it's fairly useless for this discussion.
Attached Images
 Momentum method.png (42.6 KB, 75 views)

Last edited by serezhkin; March 2, 2012 at 14:40.

 April 2, 2013, 12:20 #2 New Member   Ma Jianjun Join Date: Apr 2013 Posts: 1 Rep Power: 0 Hi,I am now facing the same problem,have you worked it out?