# Radiation boundary condition (heat transfer)

I am looking for reference on how to implement nonlinear boundary conditions. Specifically, I am interested in implementing a radiation boundary condition for heat transfer with the FEM:

$-k \frac {\partial T} {\partial n} =F (T^{4} - T_{\infty}^{4})$

with $T$ temparature, $F$ is some factor, $k$ the thermal conductivity.

An ideal reference has the theory and a simple (1D/2D) example and possibly some code to try/verify. I have looked at Finite Element Analysis for Heat Transfer (Hou-Cheng Huang) but for the price the review was not favorable. Maybe some knows of some lecture nodes, a deal ii or fenics implementation I could look at?

This is not particularly difficult once you realize that the nonlinear boundary condition simply yields a nonlinear term in the weak formulation. Let's assume that you want to solve the steady state problem, i.e., $$-k \Delta T = f \qquad \text{in} \ \Omega\\ -k \frac{\partial T}{\partial n} = F(T^4 - T_\infty^4) \qquad \text{on} \ \partial\Omega.$$ You convert this into the weak form by multiplying the equation by a test function and integrating by parts, which yields $$(\nabla \varphi,k\nabla T)_\Omega -(\varphi,k\partial_n T)_{\partial\Omega} = (\varphi,f)_\Omega$$ Now realize that you can substitute the boundary condition in the second term on the left to obtain $$(\nabla \varphi,k\nabla T)_\Omega +(\varphi,F T^4)_{\partial\Omega} = (\varphi,f)_\Omega + (\varphi,F T_\infty^4)_{\partial\Omega}$$ You then only have to implement a nonlinear solver for this problem -- but that works the same way as for all other nonlinear problems and you can find many references for this. (Among them are my video lectures on nonlinear problems.)
• Thanks. I have a non-linear solver working just fine, also for linear Robin BCs. But what I miss to understand is how I should linearize the $(\varphi,F T^4)_{\partial\Omega}$ term. I have seen your video lectures (great!) but could not find anything about linearization of boundary terms. – user21 Jan 10 '18 at 15:10
• It works exactly the same as for nonlinear domain terms. You write down the residual of the weak form (which now has a nonlinear boundary term, instead of a nonlinear domain term) and compute its derivative via the $\lim_{\varepsilon\rightarrow 0}$ process. – Wolfgang Bangerth Jan 10 '18 at 16:58