Generalized thermal resistance and its application to thermal radiation based on entransy theory

Entransy is a novel concept developed in recent years to measure the transport ability of heat in heat transfer processes. In order to analyze and optimize thermal radiation process more conveniently and intuitively, we develop the entransy-theory-based method to the field of thermal radiation problems to define the generalized thermal resistance for multi-dimensional steady state thermal radiation systems, with a consistent dimension of those for heat conduction and convection systems proposed in previous studies. Based on it, the minimum generalized thermal resistance principle for thermal radiation is developed and its equivalence with the entransy dissipation extremum principle can be concluded by defining the concept of heat flux weighted average temperature difference for thermal radiation systems. We point out that these two principles emphasize particularly on the optimization of the mean performance for heat transfer processes. A steady state thermal radiation process in an enclosure with three opaque surfaces is taken as an example to illustrate the applicability and reliability of generalized thermal radiation resistance when it is used to optimize thermal radiation problems. Finally, a comparison between the principles of minimum generalized thermal resistance and minimum entropy generation is discussed to show that the latter might not be appropriate to optimize some thermal radiation problems.