How do I Model a Fan Curve in SimScale?

This Knowledge Base article describes the steps to model a fan curve in SimScale.

A fan is a mechanical machine that is used to generate fluid flow. As the amount of flow increases, so does the pressure drop across the system, which affects the performance of the fan. Fan manufacturers provide a fan curve, which describes the relation between static pressure, power requirement, rotational speed, and efficiency values per-flow rate conditions. This information is very important for cooling purposes, which is why we need to model the fan curve when possible.

Approach

Fan curves are usually assigned as a boundary condition, either at the inlet as an intake fan or outlet surface of a fluid domain as an exhaust fan.

1. Intake Fan Condition

1.1 Inlet

An intake fan sucks air from the environment through the inlet into the domain.

1. Create a custom boundary condition by hitting the ‘+’ button next to Boundary conditions (BC).
2. Select ‘Custom’.
3. Define the values according to the following figure:

• (U) Velocity: ‘Pressure inlet-outlet velocity‘. This means that the velocity on the fan BC is dependent on the pressure difference. If the pressure difference between the inlet to the outlet is negative, the flow will leave the system (the reverse direction)
• (P) Modified pressure: ‘(P<fan>) Fan pressure‘. Using this setting, the user can upload a fan-curve.
• Flow direction: ‘In‘. This is an intake fan, therefore flow should come into the system.
• (P_t) Environmental total pressure: If the ‘Compressible‘ toggle on, assign the absolute pressure. Else, assign the gauge pressure.
• Temperature type: Inlet-outlet. If the fluid leaves the system, the temperature value at the inlet will be calculated by the solver. If the fluid enters the system, the user-defined Inlet value will be considered instead.

Now, press the graph icon to access the table view, where you can define the fan curve. You can either fill in the table manually or create a .csv file.

a. Manual Fan Curve Definition

You can directly fill in the table as presented in the following figure:

Once you have specified all the values, press ‘Apply’.

b. Upload a .CSV File Containing the Fan Curve Values

Click ‘Browse files’ to upload the .csv file.

The figure below shows how the .csv file should be made:

Make sure to have two columns: One representing the flow rate, and the other one the fan pressure. Assign the flow rate and pressure headers as well as the correct units to the corresponding columns. In the picture above, column A is the flow rate and column B is the fan pressure.

1.2 Outlet

Assign ‘Natural convection inlet/outlet’ boundary condition to all the outlets.

2. Exhaust Fan Condition

Exhaust fans are placed at outlets. As a result, they suck air from the domain and throw it into the environment. In other words, the air flowing from the inlet through the domain to the outlet, which can cool the electronic components.

We also need inlet and outlet boundary conditions for the exhaust fan situation and are presented below:

2.1 Inlet

The inlet is modeled as a naturally convective boundary with the Natural convection inlet/outlet BC (see section 1.2).

2.2 Outlet

Create a custom boundary condition again and define the following settings:

1. (U) Velocity: Pressure inlet-outlet velocity. This means that the velocity on the fan BC is dependent on the pressure difference. Depending on the inlet-outlet pressure difference, flow can either come into the system or leave it.
2. Flow direction: Out. This is an exhaust fan, therefore flow should leave through the fan.

The rest of the setup is exactly like presented in section 1.1.

Expected Outcome

The following image shows the flow direction and pressure distribution on a plane cut through the domain for the intake fan situation:

Since the fan blows the fluid into the system (see top), a positive pressure field is generated.

The following image shows the flow direction and pressure distribution on a plane cut through the domain for the exhaust fan situation:

Comparing figures 9 and 10, the flow direction has changed. The flow is being sucked out of the domain and, therefore, a negative pressure field is generated in the domain for the exhaust fan scenario.