Internal Fans with Fan Curves

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.