Effects of Gas Radiation on Thermal Performances of Single and Double Flow Plane Solar Heaters

In this paper, the thermal characteristics of single and double flow plane solar heaters with radiating working gas were analyzed and compared by numerical analysis for the first time. The laminar mixed convection gas flow in the heaters was numerically simulated by the CFD method using the finite volume technique. The set of governing equations included the conservation of mass, momentum and energy for the convective gas flows and the conduction equation for solid parts. Besides, the radiative transfer equation was solved by the discrete ordinate method for radiant intensity computation. From numerical results, the thermal efficiency of single flow heater found very sensitive to gas optical thickness, such that gas radiation always has positive influence on the performance of system. The efficiency increase about 50% was computed for optical thicknesses more than 2 in the test cases.  However, for double flow gas heater with less sensitivity to gas radiation and reciprocating trend for thermal efficiency with optical thickness, 15% increase in thermal efficiency was seen at the optimum optical thickness. Comparison between the present numerical results and those reported in literature, showed good consistency.