Fem: Add radiation heat transfer (#13466)

* FEM: Add radiation heat transfer
* FEM: Add radiation heat transfer to CalculiX writer
* FEM: Update .inp files
* Fem: Rename heat flux ui members

---------

Co-authored-by: FEA-eng <59876896+FEA-eng@users.noreply.github.com>

src/Mod/Fem/femsolver/calculix/write_constraint_heatflux.py

@@ -54,6 +54,15 @@ def write_meshdata_constraint(f, femobj, heatflux_obj, ccxwriter):
             heatflux_obj.AmbientTemp,
             heatflux_obj.FilmCoef * 0.001
         )
+
+    elif heatflux_obj.ConstraintType == "Radiation":
+        heatflux_key_word = "RADIATE"
+        heatflux_facetype = "R"
+        heatflux_values = "{:.13G},{:.13G}".format(
+            heatflux_obj.AmbientTemp,
+            heatflux_obj.Emissivity
+        )
+
     elif heatflux_obj.ConstraintType == "DFlux":
         heatflux_key_word = "DFLUX"
         heatflux_facetype = "S"

src/Mod/Fem/femsolver/calculix/write_femelement_material.py

@@ -48,6 +48,9 @@ def write_femelement_material(f, ccxwriter):
             return True
         return False
 
+    f.write("\n** Physical constants for SI(mm) unit system with Kelvins\n")
+    f.write("*PHYSICAL CONSTANTS, ABSOLUTE ZERO=0, STEFAN BOLTZMANN=5.670374419e-11\n")
+
     f.write("\n{}\n".format(59 * "*"))
     f.write("** Materials\n")
     f.write("** see information about units at file end\n")