# Neumann Boundary Condition for thermal radiation between two bodies

Posted 4 months ago
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 Hi All,I'm trying to formulate and solve a pde which models thermal radiation between two bodies using a Neumann boundary condition.I've created a mesh and want to capture the thermal radiation heat transfer between element 106 and element 19 in the boundary mesh wireframe plot below.I've modelled convective BCs as follows: GconvPistComb = NeumannValue[hPistComb*(TPistHeat - temp[z, r]), ElementMarker == 106]; GconvBlkCool = NeumannValue[hCoolWater1*(TWater1 - temp[z, r]), ElementMarker == 75] The pde with just these included looks like: pde = {1/r D[-kMesh r D[temp[z, r], r], r] + D[-kMesh D[temp[z, r], z], z] == GconvPistComb + GconvBlkCool If I were modelling the thermal radiation from ambient gas to element 19 I would use the following Neumann BC in which TAmb is a pre-defined fixed value: GradAmbTest = NeumannValue[Epsilon Sigma (TAmb - temp[z, r])^4, ElementMarker == 19]; How can I replace TAmb with an expression that represents the solved temperature field for Element 106?