Working with fan curves

In the Flow Boundary Condition dialog box, the Fan Curve option in the Mode list lets you apply a fan curve to the boundary condition. Fan curves define flow boundary condition performance in terms of the relationship between fan flow rate and static pressure rise across the fan. During the solve, the software predicts the operating point of the flow boundary condition; the intersection of the system and fan curves. You can use the Fan Curve option with all types of Flow Boundary Condition, except for Opening, Convective Outflow, and Static Pressure.

You can define a fan curve:

• By selecting a pre-defined fan curve using manufacturer-supplied data in the Fan Catalog.

• By defining a fan curve field or expression.

Using the Fan Catalog

You can specify a flow boundary condition using a pre-defined fan curve from the fan catalog. The fan catalog contains records that define characteristics, including fan curves, for a wide variety of standard fans. The database is organized hierarchically by fan manufacturer, family, model and part number. Once you select a fan from the fan catalog, the software creates the appropriate fan curve and applies it to the flow boundary condition.

The fan catalog is stored in the fan.xml file.

Note:

The fan.xml file must be stored in the same folder as the other Simcenter 3D Thermal/Flow, Simcenter 3D Electronic Systems Cooling, and Simcenter 3D Space Systems Thermal catalog files. The location of the catalog folder is, by default, the [software_installation]\NXCAE_extras\tmg\if\catalogs folder or the folder specified by the MAYA_TMG_CATALOG_DIR environment variable.

Manually defining fan curves

A fan curve describes the relationship between fan flow rate and static pressure rise across the flow boundary condition. The volume flow rate is the independent variable which determines the dependent variable, the local pressure on the fan surface. Pressure on the fan is determined by the flow solver which controls the volume flow rate.

• If the flow boundary condition is operating in free air with zero back pressure then the flow rate is at a maximum. The maximum flow rate point lies on the X-axis, at zero pressure rise across the fan.

• If the back pressure on the flow boundary condition is too high, the flow boundary condition stalls and the flow rate through the flow boundary condition diminishes. The point on the Y-axis, at zero flow rate, represents the stall pressure point.

When you define the fan curve in the Create Table Field dialog box, you must define the maximum flow rate at zero pressure rise and the stall pressure point at zero flow. You can add any number of additional points between these two limits.

You can define a fan curve in one of the following ways:

• As a field.

• As an expression that references the fan curve field as follows: (scale factor) x (fan curve field) x (dimensionless spatial distribution). For example, suppose you create a fan curve field named fan curve(1) and a dimensionless spatial field named spatial(1). To include the functional relationship in the formula for the fan curve expression, type:2.5*fd(“fan curve(1)”)*fd(“spatial(1)”)

Understanding fan curves

A fan curve describes the relationship between fan flow rate and the fan static pressure. The fan's static pressure is defined as the difference between the static pressure at the fan exhaust and the total pressure at the fan intake. The following is an example of a fan curve and system curve relationship.

• The point Pso represents the fan operating in a stall condition. That is, at no flow and maximum back pressure on the fan.

• The point Fro represents the fan operating in free air. That is, operating at its maximum flow rate with no back pressure.

You can find the fan curve operating point summary information in the solver log file and in the corresponding fan curve graph (PNG file) saved in the run directory. The Solution Monitor provides you with this information from the solver during the analysis.

See Solution Monitor and Fan curves graph for more information.

Note that the fan curve relationship cannot contain a positive slope between two points. You should remove any inflections from the actual fan curve defined for Pre/Post. This helps ensure solution stability. The operating point represents the intersection of the fan curve and the enclosure system (pressure loss) curve.

At each iteration, the flow rate is updated according to the static pressure of the flow boundary condition calculated by the solver and the fan curve relationship. The new flow rate is calculated as:

New flow rate = (F) * Fan curve flow rate + (1–F) * Old flow rate

Where F is the relaxation factor and is equal to 1 by default.

If the software has difficulty converging (oscillating), then you should use a smaller relaxation value to improve the stability of convergence.

• If the fan static pressure is negative, the flow boundary condition operates at its maximum flow rate. The flow rate never exceeds Fro.

• If the fan static pressure is larger than Pso, then the flow boundary condition cannot operate. That is, the flow rate is zero.

In both cases, the software issues a warning message to indicate these abnormal operating conditions.

Testing and rating fans

For information on testing and rating fans, refer to AMCA Standard 210–85 and the ASRAE Standard 51–1985.

How do I

Create a Flow Boundary Condition

Learn more

Flow Boundary Condition types

Modeling fan swirl

Modeling heat loss or gain in a recirculation loop

Flow boundary condition modeling

External Conditions

Fan Speed Controller

Inputs to expressions

Look up more details

Auto-generated expressions

Customizing catalogs

Quick links

Simcenter 3D Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal boundary conditions

Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal

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Pre/Post video examples

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Working with fan curves, Simcenter 3D 2021.1 Series

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