Radiation heat transfer

Just general ramblings if any use at all ...

It is difficult enough trying to determine the correct loads for static analyses. Determining heat transfer coefficients, emissivities and thermal resistances as inputs is very difficult. Yes, using CFD helps (another bag of assumptions) and so does a thermal imaging camera and other experimentation to see how good your estimates for inputs are to calibrate the model better for next time.

In the 'old days' one would use the convection condition in Mechanica to iteratively approximate a radiation load (a technique I think should only work properly for spherical problems,,, but we ignored that otherwise we couldn't do anything). In Simulate we cannot have co-existing (on the same geometry) convection and radiation loads and I wonder whether in the background the convection condition is still used like this but now the s/w deals with the iterating on your behalf; which is good.

A lot of studies are to find out what happens/include the effects of something remote being heated by a radiator. For example, an electronic component that dissipates power in an enclosure; how hot will the case get? We can't do this in Simulate unless we estimate the heat load on the case and input it directly into the case. Simulate radiation doesn't travel through space though an emissivity can be a function of temperature.

We end up being pragmatic. Thermal problems often become a case of 'bounding' likely ranges of answers by tinkering with the input coefficients because we can't find the emissivity for a particular paint for example. So we take a D.O.E kind of approach and amongst the variables try e=0.1 then e=1.0 and see what happens. We find that a lot of the time it is sinking the heat by conduction that overwhelms the other heat transfer mechanisms. It is then down to how good the thermal contact is between components ... it is usually pretty poor.

I would like to be able to shine the sun at the tarmac on a zero wind day and see how hot it gets inside something mounted under a wing. But we always want more for our money.