Diverging Solutions—Internal Fans

Issue

A model with one or more internal fan material models diverges.

Solution

The internal fan material model is usually assigned to a cylindrical (or other simple shape) to simulate the presence of a fan device without modeling complex geometry. Inputs are either a fixed flow-rate or an operating curve (flow/pressure). There are two typical causes of divergence when using internal fan materials:

 1. Insufficient Mesh 

Internal fans should be meshed with at least three layers of elements across the thickness of the part. The inlet and outlet surfaces should have a quality mesh resolution. The Use Uniform button on the Mesh Sizes dialog is useful for ensuring an adequate mesh distribution.

2. Excessive Fan Curve Oscillation 

Internal fans defined with a fan curve may encounter large swings in pressure and flow rate results. For example, if the solver computes a high flow rate, the result will be an increased pressure drop across the system. This larger pressure drop causes the solver to select a lower flow rate from the fan curve on the next iteration. The lower flow rate, in turn, causes the system resistance to drop, causing the solver to again select a higher flow rate from the fan curve.

The solution is to reduce the Manual Solution Control on pressure and velocity. This has been found to greatly reduce these oscillations, and allows the solver to more directly arrive at the actual fan operating point.

To reduce the Solution Control values:

1. Right-click off the model, and select Solve...
2. Click the Solution control button on the Control tab.
3. On the Solution Controls dialog, uncheck Enable (in the Intelligent solution control group).
4. Move the Velocities and Pressure sliders to 0.2.
5. Check Enable again, and click OK to close the dialog. 

Usually 0.2 is a sufficient value, although in some extreme cases, the value may have to be reduced to 0.05.