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Multiflow Analysis

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Old   August 19, 2015, 02:20
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Hello,
I am using Solidworks 2012 Flow Simulation to solve an internal flow problem for an avionics enclosure. I have a simple rectangular enclosure filled with avionics boxes on a common wall and am interested in how air enters from an inlet on one side of the enclosure, flows around the boxes as it migrates through the enclosure, then exits through a fan on the opposite side. I have no problem setting up and running this solution using a static pressure inlet BC and fan outlet or volumetric flow outlet BC.

However, now I want to create more realistic conditions inside the enclosure. Several of these avionics boxes have their own cooling fans where flow is sucked into one side of the box and flows through heat sink fins and then exits the fan. My issue is how do I set up boundary conditions for the box internal to the enclosure? I have a flow within a flow problem and SW doesn't seem to have the capability to solve this. If I try to use lids it views these as solids and will not allow fluid to be input or exit from the internal enclosure volume.
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Old   August 19, 2015, 04:39
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Hi Tasco,

If I understand you correctly, you are trying to setup an internal-internal analysis?

This is not necessary at all. The specification of internal and external is only in order to consider only the flow in an enclosed environment like a box or a larger compartment if you are ok to simplify the model by ignoring the external influence or simplifying it by specifying an heat transfer coefficient to the external environment and therefore avoid any external calculation. It doesn't matter how many smaller boxes you have in that enclosure, just leave the openings to these enclosures open and the fluid will flow into those as well.

An external flow specification is necessary if you want to consider the external volume also. That doesn't mean you cannot have any internal flow in an external analysis. It just means that the fluid domain outside of the enclosed space is also modeled.
For example you can have a pipe with lids on inlet and outlet to specify an internal hot liquid flow and if you activate heat conduction you can then see in the external fluid domain which you specify as air the natural convection around the pipe.

In your case simply do the internal flow in the larger enclosure and do not model any lids over the openings of any smaller enclosure so the fluid can pass through these openings on its own. Only if you want to know how much mass flow goes through these opening you can model the lids but then you have to deactivate them in the component controls of SWFS or FloEFD. This makes the "solid" lids non-existing for the flow and it can pass right through. If you then apply for example a surface goal for mass flow or other flow parameters on these lids, you can measure the flow through it for example to see if there is an uneven flow distribution in the smaller enclosures you have in the larger one. This might lead to different temperatures in the smaller avionics boxes even though they are of the same type, just because the airflow is different and therefore the cooling performance. But make sure to use a local mesh on the opening to resolve them finer for a higher accuracy of the goal values. Otherwise there might be deviations due to the coarse mesh in the openings.

So SWFS or FloEFD is very well capable to handle internal flow. Internal is internal no matter how many more boxes are in the internal space.

Boris
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Old   November 24, 2015, 14:57
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Hi Boris,

Thank you for your suggestions. You were right, this is an internal-internal analysis because I have fans internal to a surrounding enclosure with a fan itself. After more careful review, I was able to solve this problem by setting up a fan model with a Pressure vs flow rate table in the engineering database.

I then created lids for each of my fan openings and then under the Flow Simulation tab selected insert fan which enabled me to select the inner face of a specific lid and define the type of fan. There are 3 choices; 1) External Inlet Fan, 2) External Outlet Fan and 3) Internal Fan.

I used External outlet for my single fan on the exterior enclosure. I used External Inlet for one fan that pulled air from outside this external enclosure. For all the fans mounted on avionics boxes inside the enclosure, I used an Internal fan type because air was being pulled from inside the enclosure. One has to be careful to set the correct inlet pressure and temperature condition for each fan as well.

By using this approach, I was able to create fluid flow into the enclosure via openings, which migrated from one side of the enclosure and out the other end. Along the way, this air encountered obstructions like avionics boxes, with some of these having their own fans which pulled air from inside the enclosure and pushed it through heat sink fins to remove the heat load. This heated air went back into the enclosure where it was removed by the migrating air. Overall I ended up with 1 external inlet fan, 1 external outlet fan, and 6 internal fans.

Thanks again for responding so expediently. I meant to respond much earlier, but had to move onto 2 other short deadline projects after finishing this one.

Best Regards,

Tim
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Old   November 24, 2015, 23:20
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Hi Tasco,

I'm glad everything worked out so well.
May I ask which company you work for since you do avionics cooling?
You can also reply in a PM if you don't want to mention your company here.

Regards,
Boris
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Old   November 25, 2015, 11:55
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Hi Boris_M, unfortunately I cannot due to company policy.
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Old   November 30, 2015, 08:15
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No problem, I understand.
I was just curious as I studied aerospace engineering.
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