Split Air Conditioning: Mean Age of Air

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Hi, I am working on a CFD ventilation analysis of an office room. I am particularly interested in simulating the Mean Age of Air under different scenarios.

This room has 4 split air conditioners, that I believe to have successfully simulated using velocity inlets and outlets boundary conditions. However, the problem I have is the calculation of the Mean Age of Air. This boundary condition takes the assumption that the air that goes through the velocity inlet is “new air” when it really is just recirculated air. You can see what I mean in the following image showing the result of the mean age of air for this simulation:

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I would also clarify that for this simulation I left a door as a pressure outlet for additional air circulation.

In this top view of the results, I can clearly see how the passive scalar (age of air)is coming out of the cassette split units (squares in the above drawing) have a value of 0 (blue). I believe the simulation is useful anyways, since it shows some areas of the office with bad air circulation.

Getting to point of my question. Can anyone think of a better way of simulating Split air conditioners for this Mean Age of Air analysis? I thought about defining a Momentum Source, and treating the splits like just a fan? But the most accurate representation would be a fan that also reduces air temperature. And I havent really found anything similar to this.

Thanks!

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Hello!

The approach with the momentum source would work , in the sense that the mean age would not be “reset” at the inlets. And technically you could use a power source with a negative power (which acts as a heat sink). You would still have to explicitly define some fresh air inlet, since you can’t have a simulation only with outlets.

In any case, this approach sounds much more complex, and it’s probably not something I would do, unless I am strictly interested in the mean age and nothing else.

Cheers

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Thanks Ricardo! The heat sink approach sounds good, but I bet it is hard to model correctly.
I will try it out and report back the results to this post. I will be simulating fresh air inlet with an open door as a pressure inlet boundary condition.

I know its increasing model complexity but this project is mainly focused on the ventilation design, and mean age of air. We need to show the office building owner how bad it is to have Split air conditioners recirculating air in a badly ventilated room. And how it can be improved significantly by adding a Dedicated Outdoor System.