6SigmaET Tips & Tricks: Modeling Vacuum Conditions

17 August, 2017

Planning to make your product out of this world? Or should I say take your product out of this world? Equipment designed to spend its lifetime in space can’t rely on natural or forced convection for cooling, instead relying on conduction – typically in the form of liquid cooling – to transfer heat to the outer casing and from there, radiation. As such, you need to incorporate vacuum conditions into your CFD models. Below are the steps you need to take to model vacuum conditions and correctly interpret results with 6SigmaET.

1. Set Cooling > Solution Control > Solution Scheme > Solution Type > Temperature Only. It’s pointless spending time calculating velocities and pressure, etc. for air that ‘doesn’t exist’.

2. Set Cooling > Air > Conductivity to 0 to stop conduction through the air.

3. Disable Cooling > Solution Control > Grid > Use Inflation. This setting is on by default to capture the flow boundary layer, so has no value in a temperature only solution. Disabling it reduces grid count and saves you time.

4. If the chassis boundaries are installed, you need to set an appropriate material and external environment on each of them to allow heat out of the model (which can be through conduction on the external side, if you specify a non-zero external heat transfer coefficient). If you have open boundaries, i.e. uninstalled chassis sides or a test chamber, you don’t need to worry about this step.

5. Enable the User Defined Heat Transfer Coefficients option (Solution Control > Solid Objects > Allow User Defined Heat Transfer). For all solid obstructions, in their properties sheet, set the Cooling > User Defined Heat Transfer > Fluid Heat Transfer Option to Fixed Coefficient, and make the coefficient value 0. This will guarantee no heat transfer into the air.

Note: The air temperature in vacuum cases can become undefined as it’s not linked to any part of the model – especially when fluid HTCs = 0 – which can be confusing. Usually it will stay cold at the initial value – but sometimes it will rise to be similar to the solid temperatures. Although the air might look warm, the zero conductivity and HTC will still mean there is actually no heat transfer. So you may need to be careful with the interpretation of the results. For the user defined heat transfer you need to leave the Solid heat transfer as ‘From Solution’ so it will calculate the correct conduction between the solids (depends on thermal conductivity and grid etc.).

6. In low conductivity solids, such as plastics, you will need a fine to model the correct heat spreading and temperatures (this is especially true for composites with planar isotropic conductivities). As conduction and radiation are the only heat transfer mechanisms, it’s important to get the conduction right.

By: Matt Evans, Product Engineer