Diffusers: Characterization for CFD
A diffuser for an HVAC supply can be comprised of a lot of very small details (e.g., holes, slots, vanes) with respect to the room that it is supplying, which makes it a good candidate for simplification through a more generic characterization.
Identify Goals of Characterization
At the supply from the HVAC system, the air has already been conditioned and is entering the space at a known flow rate and temperature. The purpose of the diffuser is to distribute the air by adjusting the throw pattern, so it has a significant impact on the flow fields around it.
Defining Characterization Method
Knowing that the flow is an inlet to the system, the diffuser can be characterized using simpler geometry with boundary conditions. The angle (i.e. vector) of the incoming flow will be controlled using component velocity boundary conditions.
Gathering the Necessary Inputs
If diffuser performance specifications are available, hand calculations can be used to capture the component velocities given some of the following information:
- Angle of flow leaving diffuser
- Incoming flow rate
- Area that the flow will be coming from
For example, assuming a flow rate of 100 cfm, an area of 2 square feet and a throw angle of 30 degrees from horizontal:
- Vy = Q/A = 100/2 = 50 fpm
- Vx = Vy/tan(theta) = 50/tan(30) = 86.6 fpm
If specifications are not available, a simple test model, with the diffuser explicitly modeled can be simulated. The results from the simulation are then used to determine the inputs needed for the diffuser characterization.
The preferred method of obtaining the component velocities is to position a Plane in the middle of the diffuser and then use an XY-Plot to see the velocities.
A simple validation study comparing the actual and characterized versions of the diffuser model can be used to verify that the inputs and assumptions are correct. Note that the characterized version will rarely match the actual version with exact precision; the objective here is to only get a reasonably accurate representation.
When the characterization technique is used correctly, a direct comparison of the two versions should reveal similar flow velocity profiles.