# Fan for Air Recirculation Model

Hi Simscale Team

I have spent the past week slowly building up a model of the HVAC system in a room. The simulation chosen is “convective heat transfer”. Yesterday, following lots of reading up on the forum, I got my first simulation to run fairly successfully (I think).

I would like to progress the model to include air recirculation fans. My question is, can I model the airflow of these recirculation fans (which are not ducted) using boundary conditions “velocity inlet” and “velocity outlet” on an object surface without specifying the temperature of the velocity inlet. In other words, these fans won’t provide any cooling and I would like the inlet temperature to equal the outlet temperature. Apologies if this has been covered before but I couldn’t find anything on the forum.

Hey @sdoep and thanks for the nice & detailed description of your problem!

Indeed there would be a way. Instead of creating/modeling an explicit fan geometry you can use the so called momentum source approach - if you already know the dimensions and output velocity, simple use this approach which is IMHO the fastest and easiest way if you want to test some designs inside of your HVAC system.

Best,

Jousef

Hello sdoep,

In this scenario I think an advanced concept called a momentum source might work? This is without seeing your simulation and understanding what your goal is. A momentum source is basically a fan providing momentum to the air without providing any heat energy.

I have used momentum sources to model circulation fans within large structures and it works well. I have used them with diverters and ventilation systems. Let me know what you think.

Best,
Jake

Hi,
normally the output speed has also a rotation-compoment. Is it possible to include this Rotation? Thank you very much for your Answer.
I have done my first project of a Test-Fan to understand the capabilities od CFD and will make some investigations for the flow after an Fan or compressor.
Thanks a lot for for your helping hand.
Kind regards Johannes

Hello Johannes,

With a momentum source, there is no rotational component to the output. If you are seeking to study the more indepth flow a MRF rotational zone can be used in conjunction with fan, compressor, or turbine blades. This is would require a flow zone volume around the component you want to spin. More can be found about the MRF zones here https://www.simscale.com/drone-simulation-rotating-zones/
Let me know what you think.

Best,
Jake

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