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The present experimental study is concerned with heat transfer analysis of air flowing in solar air heater duct with a gap in V-rib with symmetrical gap and staggered ribs geometry. The investigated parameters are Reynolds number (Re) of 4000–14,000, relative roughness pitch (p/e) of 12, relative roughness height (e/Dh) of 0.043, angle of attack (α) of 60 deg, relative staggered rib pitch (p′/p) of 0.65, relative gap size (g/e) of 4, relative staggered rib size (r/e) of 4, relative gap position of additional gap in each symmetrical rib elements (d/w) of 0.65, relative gap size of additional gap in each symmetrical rib elements (g′/e) of 1, number of main gaps (Ng) of 1, 2, 3, 4, and number of additional gap (ng) varying from 1 to 5. Fourteen roughened absorber plates were tested. The maximum enhancement in Nusselt number (Nu) and friction factor (f) was 2.34 and 2.79 times that of smooth surface corresponding to the number of main gaps (Ng) of 4 with the number of additional gaps (ng) of 4. The performance of the gap in V-rib with symmetrical gap and staggered rib geometry has been compared with the existing latest V-rib geometry and smooth surface. The proposed gap in V-rib with symmetrical gap and staggered ribs geometry has a better performance than the existing latest V-rib geometry. The following correlations have been developed for heat transfer and friction factor in terms of roughness and operating parameters. Heat transfer:Nur=0.0073(Re)0.9788(Ng)0.2790(ng)0.0184exp[−0.1678(ln(Ng))2]exp[−0.0129(ln(ng))2] Friction factor:fr=0.0477(Re)−0.0678(Ng)0.5919(ng)−0.0562exp[−0.4922(ln(Ng))2]exp[−0.0487(ln(ng))2]

Artificial rib roughness is a convenient method for enhancing thermal performance of solar air heater. An experimental investigation has been carried out to study the thermo-hydraulic performance of solar air heater using W-shaped ribs on absorber plate, the roughened wall being heated while the remaining three walls being insulated. W-shaped ribs have been tested both pointing downstream (W-down) and upstream (W-up) to the flow. The roughened wall has relative roughness height (e/D h ) of 0.03375, relative roughness pitch (p/e) of 10 and duct aspect ratio (B/H) of 8. The airflow rate corresponds to Reynolds number between 2300 and 14 000. The heat transfer results have been compared with those for smooth duct under similar flow and thermal boundary condition. Thermo-hydraulic performance ratio of roughened absorber plate by providing W-shape roughness is in the range of 1.46-1.95 for W-down and 1.21-1.73 for W-up ribs. Friction factor and heat transfer coefficient correlations have been developed based on the law of wall similarity and heat-momentum transfer analogy.