Why does surface film coefficient (aka heat transfer coefficient) not equal Heat Flux/(Ts-Tref) ?

Simon Wilkinson

Hello all,

The heat transfer coefficient, which is referred to as the ‘surface film coefficient’ in AcuSolve, does not seem to be linked to the heat flux in accordance with Newton's law of cooling i.e. Heat Transfer Coefficient=Heat Flux/ΔT .

To use an example, one of my simulations predicts a heat flux of around 500 W/m^2 on one element of the part surface, and for the same element predicts a surface film coefficient of 10W/m^2K. According to Newtons law of cooling this would require the temperature difference of 50K. However, such a large temperature difference is not possible in my model as I have set temperature at the inlet and wall temperatures to 453K and the part temperature to 443K. I have also set my reference temperature to 453K. The maximum possible temperature difference in the model is therefore 10K, so the surface film coefficient cannot be not determined by dividing the heat flux by the temperature difference. Dividing the heat flux through by 10K the air-part temperature difference of 10K, gives 50W/m^2K, which is more in agreement with some experimental observations we've made.

So my questions are: How are the surface film coefficient and the heat flux calculated, and why are they not linked through Newton's law of cooling? Which should be used to calculate the convective heat transfer?

Thanks in advance for any help,
Simon

acupro

This is certainly one of the more interesting aspects of AcuSolve.  The reported surface-film-coefficient has nothing to do with the thermal solution.  It is based purely on the turbulent flowfield/boundary layer in the results - based on a self-similarity solution.  Thus, one needs to be very careful when using this output and comparing to other data and/or using it to continue with other types of solutions.

It is likely better to calculate the heat transfer coefficient using the calculated Q, local (or integrated) wall temperature, and a consistent reference temperature, as you've already noted.

Bottom line - the heat flux is calculated correctly using the energy equation, but the surface-film-coefficient is based purely on the turbulent flow solution.