Influence of the Air Gap Layer Thickness on Heat Transfer Between the Glass Cover and the Absorber of a Solar Collector

A numerical study is carried out to evaluate the thermal performances of a solar collector. As it is well known, that the thermal losses of such systems are mainly of a convective nature, the study is concentrated in particular on the features of natural convection that is activated in the air domain delimited by the upper glass and the lower absorber of the solar collector. The efficiency of such a system depends essentially on both the temperature difference and the distance between the absorber and the glass. Since the temperature difference remains an uncontrolled variable (because it depends on the incident heat flux), while the distance between the glass and the absorber can be "controlled" at the design stage, we focus specifically on the latter, i.e. the effect of the thickness of the air gap on the resulting properties of buoyancy convection (essentially in terms of emerging structure and heat transfer coefficient).