Internal fans simulate an axial momentum source within the interior of the geometry. Fans can have a constant flow rate, or the flow rate can vary with a head-capacity curve so that the fan operating point depends on the pressure drop through the device.
Internal axial fans should not be placed on an external boundary. Likewise, it is not good practice to apply boundary conditions to any surface of an internal fan material. Doing so may cause convergence difficulties and will affect the flow rate reported in the summary file. If an internal fan contacts an external boundary, it is better to either create an extension onto the fan inlet (so that the boundary condition is not applied directly to the fan) or simply use an external fan boundary condition instead of an internal fan material.
This feature allows internal fans (and blowers) to be dependent on a temperature within the model. The fan will run as long as this “trigger temperature” is above (or below) a pre-defined cut-off. When the temperature at the thermostat location is below (or above) the cut-off, respectively, the fan will not run.
Check the Thermostat box on the Internal Fan/Pump Materials task to expand the Internal Fan Thermostat controls. This dialog allows for specification of a Trigger temperature and a thermostat location.
The average temperature on the surface is used as the sensing temperature. (Any surface in the model can be used.) While this dialog is open, the interface allows for the selection of a surface. Only one surface can be used as a sensing surface, so selecting a new surface will update the selection list. The surface ID is written in the space called “Location,” and the surface in the model is highlighted.
The Default material database contains at least one instance of every material type. A convenient way to create a new material is to use a Default material as an example. Because these materials are read-only, use the Material Editor to copy the original into a custom database, and modify the copy.
Enter velocity profile data in the table. Values for Radius and Axial Velocity are required. Values for Swirl Velocity and Radial Velocity are optional. Alternatively, data can be read in from a comma-separated file (“.csv”). Data can be prepared in an Excel spreadsheet and saved to a “.csv” format.
In some instances, certain fans such as large industrial units deliver a non-standard velocity distribution. When several such fans are present, the default uniform velocity distribution provided by the internal fan material does not adequately predict the flow profiles and the interaction between the fans. This information, however, is required for a complete understanding of the overall flow distribution throughout the enclosure.
The Velocity Profile flow variation method allows the specification of the velocity profile for an internal fan. It provides a mechanism to apply the velocity distribution computed from a detailed rotating region fan analysis to a simple geometric representation of that fan in a subsequent system-level analysis.
A velocity profile distribution can be computed from a separate rotating region analysis by creating a radial line of monitor points from the center to the outer edge of the fan. These monitor points should be created prior to running the analysis so that a time history of velocity is generated.
Create the line of monitor points along a Cartesian axis, if possible. This will greatly facilitate determining the radial position of each point. In the example shown above, the points all have the same y and z coordinates, and the origin is at the center of the fan. The radial position of each point is its x coordinate.
If the fan is not centered about the global origin, specify the fan flow direction by selecting a surface on the fan that is normal to the flow direction (instead of selecting a Global coordinate direction). For the radius value of each point in the profile, specify the global coordinate.
After the analysis is complete (so that the velocity values are converged on a time averaged basis), save the velocity components for each monitor point from the Convergence Motor table into an Excel spreadsheet, and save as a “.csv” file.
The slip factor is the ratio of the flow rotational speed to the fan blade rotational speed. Due to inefficiencies in the fan, slip can cause the tangential velocity of the flow to be slower than that of the fan blades. Autodesk Simulation CFD will determine the tangential velocity of the flow by multiplying the slip factor by the specified fan rotational speed.
The default slip factor is 1.0. This will cause the rotational speed of the flow to be the same as the rotational speed of the fan. The permitted range of slip factor values is between 0 and 1. Values outside of this range are not allowed by the User Interface.