Experimental investigation of heat transfer and fluid flow in solar air heaters using transverse wire rib roughness with variable gaps

Solar air heaters are widely used in low-temperature applications such as drying, heating, etc. The growth of the laminar sub-layer under the absorber plate in conjunction with flowing air leads to lower convective heat transfer and, thereby, low performance. In this present work, the conventional solar air heater is reconfigured using transverse wire ribs with variable gaps to break the laminar sub-layer. The experiment was performed under actual outdoor weather conditions in Jagdalpur, India. The important design and control variables used are relative roughness height (e/D) of 0.043, relative roughness pitch (p/e) of 10, gap width (g) of 4 mm, rib roughness diameter (e) of 2 mm, the number of gaps (Ng) varying from 1 to 4 (in 4 steps) and Reynolds number in the range of 2000–16,000 (in 8 steps). The experimental result indicates that the maximum Nusselt number and friction factor are achieved for Ng = 2 compared to all the gaps. Ng = 2 configuration outperforms both continuous rib and smooth duct configurations. The findings show that the maximum augmentation in heat transfer and fluid friction compared to smooth ducts is 3.04 and 3.03, respectively. The optimal parameters identified as p/e = 10, e/D = 0.043, e = 2, g = 4, and Ng = 2 resulted in the highest enhancement in heat transfer across all cases studied. The heat transfer found in the present study outperforms that of prior investigations in the field of solar air heaters, which utilized similar rib configurations but did not incorporate variable gaps.