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Documentation

How to Set the Right Thermal Wall Conditions in CHT Without Using Radiation?

Objective

SimScale offers a variety of options to define the thermal situation on a surface. This article does not account for radiative effects, or cover radiation. An article about radiative walls will be published soon.

Approach

The following schematic draws the thermal situation at a wall: Based on the temperature difference of the wall and the environment as well as on the flow situation at the wall, heat is transferred from the outside to the wall or the other way round.

thermal situation at a wall simscale
The thermal situation at a wall

How to set up the conditions now depends on the given parameters – meaning the physical boundaries you already know.

setting in a wall boundary condition simscale
Setting in a wall boundary condition

If you are wondering about the first two options, have a look at how to make a choice for (U) velocity and the Wall Turbulence.
The next item concerns the thermal settings. There are four options:

  1. External Wall Heat Flux
  2. Fixed Value
  3. Zero Gradient
  4. Turbulent Heat Flux

As 1 and 4 are related to each other as they allow the user to define an actual heat flux. Strictly speaking, 2 and 3 are not physically valid, but they are simplifying a simulation and can help improving convergence and shorten calculation time. And in most simulations, the error is neglectable.

Fixed Value

The option fixed value means that the surface will have the same temperature during the whole simulation.

Zero Gradient

You chose the option zero gradient if you want your wall to be adiabatic, which means that there will be no heat transfer through the wall. Hence this wall will be considered as a perfect insolator.

External Wall Heat Flux and Turbulent Heat Flux

You should choose one of these if you are interested in the heat transfer due to an initial temperature difference in your system, or how the thermal situation in your system will change due to a heat transport you define. The following figure shows what to choose in different situations:

Choices within the temperature condition SimScale
Choices within the temperature condition
A. Derived

With this option, you need to define the flow situation outside your wall by defining the heat transfer coefficient.
Furthermore, you can add:

  • a virtual layer and specify its thickness and thermal conductivity.
  • an additional heat source, which would apply if you have eg an electrical wire or another kind of heat source inside the wall.

The heat transfer is a result of your inputs.

options within derived heat flux
Options within derived heat flux
B. Fixed Heat Flux & C. Flux Heat Source

You define the heat flux and the initial temperature of the surface. Based on the dimensions of the surface, the total amount of heat being transferred will be calculated.

options within flux heat source simscale
Options within Flux heat source
D. Power Heat Source

You define the heat power and the initial temperature of the surface. The difference to B. and C. is that you do not specify a heat flux, but the heat power, which means that the dimensions of your surface will not have an impact anymore.
If you select more than the surface in this boundary condition, every surface will produce the specified heat.
In other words: If you create a Turbulent Heat Flux BC, you specify 5W there and you select three surfaces, each surface will produce 5W. We are not specifying the total heat produced by all the surfaces selected, but the heat produced by each one.

Note

If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

Last updated: July 7th, 2020

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