Influence of Temperature Characteristics of the Upper Surface on Heat Transfer in a Solar Collector

An interferometric technique was used to study coupled convective heat transfer for an inclined solar collector. The collector was simulated by an inclined, closed rectangular cavity which was heated isothermally at the bottom plate and cooled by natural convection at the top plate. Both plates had dimensions 457.2 mm wide and 424.2 mm long. Infinite interferograms, for temperature field visualization, and finite interferograms for analysis were obtained. The parameters studied were Rayleigh number, aspect ratio, and angle of inclination. The Rayleigh number was varied from subcritical to 6 × 105 and the aspect ratio was changed from 6.68 to 33.4. The angle of inclination was 15° and 60° to the horizontal. The temperature on the cold plate was found to increase with distance from the lower boundary. Temperature reversal occurs in the cavity and first appears in the corner regions. The temperature gradients were obtained and thus the local heat transfer coefficients were calculated indicating a strong dependence from side boundaries. In comparison with reports where both surfaces were isothermal a significant difference in the dependence of average Nusselt number on Rayleigh number was found. At high Rayleigh numbers the average Nusselt number is less dependent on Rayleigh number for the cavity with a non-isothermal top surface than for the isothermal top surface.