What size of MRF should be used for axial fans (e.g. Underhood cooling fans)?
Underhood environment has become very congested; therefore, the MRF used to model the cooling fans has become very small. A common question: How small can I make the MRF?
Some facts about MRF:
1. Non-axisymmetric stationary walls cannot touch or be inside the MRF. Otherwise, it will produce false velocities.
2. If an axisymmetric stationary wall does touch and is inside the MRF (e.g. part of the shroud), you must explicitly declare is as stationary in absolute reference frame in the wall boundary conditon. This includes wall-shadows as well.
MRF in general works OK provided that the flow is either (1) circumferentially uniform at the interface or (2) if circumferential non-uniformities in the flow exist, but the flow must primarily passes one way through the interface.
In the real underhood situation, the fan is inserted partially or fully inside the cylindrical portion of the shroud. Since the cylindrical portion of the shroud is axi-symmetric, we can make its walls touch the cylindrical portion of the MRF interface. So the cylindrical portion of the MRF would be a wall. It will have wall and wall-shadow - the one which is adjacent to the MRF cell zone must be declared as stationary in absolute reference frame. Making the cylindrical portion of the shroud as wall will prevent any flow passing it.
If that portion of the shroud is not fully axi-symmetric in reality, one could - in the preprocessor- make it to be. By doing this we are moving away from reality, but make sure that the modification is not significant.
In the case of a plane shroud, which is typically used in the experiment when the fan curve is obtained, there is a significant recirculation near the fan tip. If you increase the radius of the MRF boundary away from the recirculation, you will see better prediction. But in the circular faces of the MRF boundary, the flow is only one directional - there are no recirculation at any of these two circular faces. So the accuracy is not degraded as you decrease the MRF size in the axial direction.