Heat transfer and fluid flow analysis of PCM-based thermal energy storage concept for double pass solar air heater

Solar air heaters (SAHs) are flat plate collectors used for drying applications without causing any negative impact to the environment. However, SAH suffers from two drawbacks: first is the low convective heat transfer due to formation of boundary layer along the absorber plate, and second is the low thermal efficiency resulting from the lack of solar energy available after sunshine hours. The present study aims to examine the heat transfer and friction factor characteristics of double-pass SAH using two composite roughness geometries: perforated baffles and semicylindrical PCM tubes. The study is conducted at design parameters: blockage ratio (eb/Hd) of 0.4 to 0.8 and tube radius ratios (rt/Hd) of 0.2 to 0.5 under different Reynolds numbers (Re) from 3000 to 13,000. The maximum Nusselt number (Nu) and friction factor (f) are determined to be 200 at Re of 13,000 and 0.629 at Re of 3000, respectively, for the constant value of rt/Hd as 0.3 and eb/Hd as 0.8. In the range of parameters analyzed, maximum increase in Nu and f is determined to be 5.08 times and 65.80 times, respectively, over the smooth duct. Furthermore, correlations are derived for Nu and f with mean absolute error of 4.7% and 3.8%, respectively.