18 February 2019
- Last edited 18 February 2019
Thank you for the detailed description of the environment downstream of the fan. It looks to me like you need to know the airflow characteristics of the radiator in order to get to the point in the system where the measurements are made. You said the airflow is measured 100 mm downstream of the radiator, and I see the pressure gauge is also downstream of the radiator and upstream of the louvre bank.
For example, if the louvres are adjusted so that the pressure in the box after the radiator is 200 Pa, what is the pressure in the plenum after the fan, before the radiator: is it 300 Pa, 400 Pa, or what?
It would be best to add a pressure gauge in this space and measure it directly. If you build a CFD model of the fan exhausting into a plenum before the radiator, you would assign a Pressure Outlet boundary condition, which means you need to know that value. Do you have a tachometer on the fan? Another input to the CFD model is the rpm of the fan blades.
When there is a hub with fan blades in a CFD model, a cylinder of air slightly larger than the size of the blades is cut from the larger volume of air that goes out to the walls of the fan ring, at a diameter half way betweeen the tip of the blades and the fan ring. This cylinder and the two flat faces before and after the blades becomes a sliding interface between a mesh that rotates with the blades and a mesh that is stationary. That is what I created in the attachment. It requires specifying the rpm of the blades as an input to the model. A zero pressure inlet and a positive pressure outlet boundary condition is used at the entrance and exit to the fan ring and the fan ring is a wall boundary condition.
The geometry I constructed is easily edited to make a smaller cylinder around the fan blades and pull the enclosure to match the fan ring diameter. The two ends of the enclosure can be pulled to the correct dimensions of the fan ring, upstream and downstream of the hub. However, the sketch shows a large motor body in the inlet flow path. That body could be modeled in later model to assess the motor body contribution to the flow.
A larger model would have the larger area of the radiator inlet surface. An even larger model could use a porous block to represent the radiator and have louvres on the backside box.