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In this study, combined thermal radiation and natural convection heat transfer from glass and plastic cover flat solar collectors is examined by varying tilt angle and cover materials. The flat-plate solar collector tilt angle is varied from 0º to 45º. The performance of glass, lexan, and acrylic cover materials is investigated. Numerical simulations have been performed for various solar collector thicknesses exposed to external ambient temperature and wind heat transfer coefficient. Continuity, momentum and the energy equations, along with the Boussinesq approach, are solved with the finite volume method using the SIMPLE algorithm. The cover temperature and the top loss coefficient are calculated for each cover material, collector tilt angle and bottom plate temperature, wind heat transfer coefficient and external ambient temperature. The flow and temperature field are obtained, and the mean convection and radiation Nusselt numbers are calculated for the bottom plate. The analytically and numerically computed glass cover temperatures are found to be in perfect agreement. The top loss coefficient of the plastic cover is lower than that of the glass cover. It is determined that with increasing heat input from the bottom plate, the top loss coefficient and the mean cover material temperature increase linearly. As the external ambient temperature increases, the top loss coefficient and the cover material temperature do not present any significant change.

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Three dimensional combined natural convection and thermal radiation in an inclined cubic enclosure with pins attached to the active wall is investigated numerically. The vertical opposing walls are heated and cooled while the other walls are assumed to be adiabatic. The governing flow, momentum equations and the radiative transfer are solved using Fluent® 6.3 CFD software. In the discretization of the convection terms, the second order upwind scheme and for the solution algorithm SIMPLE is used. The cubic enclosure is filled with air and the flow is considered to be laminar. The properties of air are assumed to be constant except for the density variation for which the Boussinesq approximation is used. The surface to surface (S2S) heat model is used as the radiation transfer model. The computations are performed for Rayleigh number in the range 10 3 ≤Ra≤10 6 and for the surface emissivity (ε) 0≤ε≤1 while the inclination angle is varied 0°≤φ≤75°. The mean Nusselt number for convection and radiation transfer were evaluated as a function of Rayleigh number, emissivity and inclination angle and for some cases, the fluid flow and the temperature distributions were analyzed. The results showed that the mean total and radiative Nusselt number increases monotonically with increasing Ra number and the surface emissivity.