Ansys Icepak Airflow Results

[mjjmecheng]

Good morning! Quick question for the gurus out there.

I have been working on an Icepak (v.14, Ansys 2013) simulation for a chassis design. In the design, there is one fan that pulls air from inlets in a computer chassis through a number of channels and exhausts the air out the back of the unit. For the time being, I am only interested in airflow and have turned of any thermal calculations in the model. I have deactivated gravity for the time being as well.

My issue is that I have performed numerous simulations using different gridding techniques and though my results always appear nearly the same, I seem to have one fundamental issue. When I go through and tally up the airflow through the 6 inlet ducts that I have, the sum is ~15% different from what Icepak tells me that my fan operating point is. For example, my exhaust fan's operating point is 31cfm while the sum of my inlets is only 26cfm. Am I misunderstanding Icepaks definition of the fan's operating point? I don't seem to have any leaks in my model.

Air @ 160F. Elevation = sea level. Gravity off. Ambient air pressure = 1atm.

Thank you for the help!

 

[mjjmecheng]

Any thoughts? This issue has got me stumped...

I'm using 'grills' at the different air inlets with 0 resistance to monitor the airflow in, though I've tried using thin 'fluid blocks as well'. I can't understand why the the fan's operating point would differ from what the airflow in the unit is.

 

[tkordyban]

You are right to be worried if the air flow through the system does not balance out. The mass flow in and out should be the same. Of course there will always be some small discrepancies in a numerical solution, but it has to balance out better than your example. The first things I would check are (please forgive me if you already know these):

1. Is the solution actually converged? The continuity residual (mass balance) should be below 1e-3.
2. Try increasing the mesh density on the vent openings and the fan itself. A coarse mesh can cause mass flow balance errors. After all, Icepak is only adding up the flow through each face to get the total. If there is a large variation in velocity across the face of each vent, then you need a fine mesh to capture it accurately.

If that does not help, pack up your model and send it to ANSYS/Icepak customer support. They are helpful at answering these questions.

 

[mjjmecheng]

1. Is the solution actually converged? The continuity residual (mass balance) should be below 1e-3.
2. Try increasing the mesh density on the vent openings and the fan itself. A coarse mesh can cause mass flow balance errors. After all, Icepak is only adding up the flow through each face to get the total. If there is a large variation in velocity across the face of each vent, then you need a fine mesh to capture it accurately.

I appreciate the input. The model has not fully converged, though my residuals are all in the realm of 1e-4. The pressure and velocity monitor points appear to have reached steady-state as well. Initially, I did have some issues with my meshing as I accidentally left 'stair-stepping' on in the mesh controls. However, after I refined my mesh (currently at ~13.5 million cells), the results appear to be very similar.

I put in a help ticket with Ansys, hoping to get an answer soon, but I'm still looking for ideas.

Thanks again!