Experimental Evaluation of Natural Convective Fluid Flow Phenomenon in Compound Parabolic Concentrating (CPC) Solar Collector Cavities
Abstract
In a CPC solar collector with a selectively coated absorber, natural convection within the cavity is the dominant mode of heat loss from the hotter absorber to the cold aperture cover and reflector walls. It is felt that accurate knowledge of the values of critical Rayleigh number (Rac) signifying the onset of natural convective flow of cavity air can lead to the design of suitable convection breakers at specific locations within the cavity to substantially suppress this loss and thus enhance the performance of the CPC solar collectors. This paper reports the results of an experimental investigation into natural convective air flow within the direct flow type CPC solar collector cavity simultaneously tilted between longitudinal angles of 0 ≤ θ ≤ 10º and transverse angles of 0≤ ф ≤ 40º. A general contention that the results and correlations obtained for natural convection in regular shaped (rectangular or cylindrical annuli) cavities are sufficient to describe that in the cavity of a CPC solar collector is being challenged here. It is concluded that the employment of the results obtained for regularly shaped enclosures to describe the natural convection phenomena in CPC solar collector cavities will be misleading.