The radiative heat exchange element has two causality strokes at the element.
Potential variables: e1: Bondgraphic effort variable of inflow f1: Bondgraphic flow variable of inflow, normalized positive for flows into the model e2: Bondgraphic effort variable of outflow f2: Bondgraphic flow variable of outflow, normalized positive for flows out of the model Parameters: sigma: Specific radiation constant (default value = 1 W/(m2*oK4) A: Radiating surface (default value = 1 m2)