Modeling Swirl Diffuser in Airpak

How to model Swirl Diffuser in Airpak?
In order to model the swirl diffusers, the first thing to make note of is whether the underfloor plenum will be modeled or not.

Lets assume that the underfloor plenum is not being modeled in the analysis. In other words, the cfm per diffuser is known (flow through the diffuser is assumed). The diffuser catalog will provide the performance data inputs (vertical throw, terminal velocity and horizontal spread) for the diffuser.

In Airpak, use the diffuser macro to generate a grille diffuser (preferred since most underfloor diffusers are circular) based on the volumetric flow rate, vertical throw and corresponding terminal velocity for the diffuser. The result will be a circular fan object and a cylindrical source object. The missing piece on this diffuser is the swirl component. Paste an internal fan (with a linear fan curve - pressure is 0 for entire range of flow rates - 0 cfm - 10,000 cfm should be a wide enough range for all practical purposes) with the defined swirl (magnitude or rpm) on top of the volumetric source object. The volumetric flow rate alongwith the swirl will define the horizontal spread from the diffuser. It is recommended that the swirl specification be calibrated on a simple setup (one diffuser) first before using it for the actual model.

For the case where the underfloor plenum is being modeled, please follow the entire procedure described above. However, since the flow rate at the supplies is known but the flow through each diffuser is not known, please deactivate the 'fans' generated by the diffuser macro and replace them with free openings (no boundary condition definition - equivalent of an interior boundary in FLUENT).

Please note that the procedure described above is based on the capabilities available in the Airpak tool and is a good approximation at best. In reality, the vertical throw from the diffuser would be influenced by the swirl component itself.

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