Experimental investigation of thermo-hydraulic performance of a solar air heater duct equipped with multiple wavy S-shaped rib roughness

The present work reports an experimental investigation of the thermo-hydraulic performance of a solar air heater (SAH) duct equipped with a multiple wavy S-shaped (MW-S) rib roughness on the absorber plate. The experiments are conducted in a rectangular duct with an aspect ratio of W/H = 11 over a Reynolds number range of 3000–14000. The investigation focuses on the effect of the MW-S rib arc angle, α = 30 ° , 45 ° , and 60 ° , while the rib height, rib pitch, secondary wave amplitude, and wavelength are maintained at fixed values ( e = 3.0 mm, p = 25.0 mm, a = 0.9 mm, λ = 18.75 mm). A conventional single S-shaped rib (identical e , p and arc angles, but without secondary waviness) is also tested under identical conditions to isolate the contribution of the multi-scale waviness. The smooth duct serves as the experimental reference. For the present duct, W = 330 mm, H = 30 mm and D h = 55.0 mm. Electrical heat input is held constant at Q in = 400 W ( V e = 200 V, I = 2.0 A) for every Reynolds number. Each operating point is recorded at least three times after steady state. A digital differential-pressure transducer (range 0–100 Pa, accuracy ± 0.25 Pa) is used for the low pressure drops of the smooth duct, while a U-tube manometer remains available for the higher roughened-duct readings. The smooth-duct Nusselt number increases from 10.8 at Re ≈= 3000 to 38.5 at Re ≈= 14000, whereas the MW-S configurations produce Nusselt numbers of 11.5–84.1 and the single S-rib configurations produce 16.3–65.4, depending on arc angle. The corresponding friction factors of the MW-S configurations are 0.0205–0.0444, compared with 0.0181–0.0291 for the smooth duct. Using the experimentally measured smooth-duct values in the constant-pumping-power criterion, the calculated THP ranges from 953 to 2.042 over all investigated MW-S cases. The highest cal-culated THP is 2.042 for α = 30 ° at Re ≈= 14000. Relative to the single S-rib, the MW-S geometry yields a further Nusselt-number increase of 5–30% at the higher Reynolds numbers while the friction penalty remains comparable or slightly lower, producing a net THP advantage of up to 36.4%. Energy-balance ratios Q u /Q in lie between 71% and 82% across the data set. The smooth-duct comparison with the Dittus–Boelter and Blasius correlations shows deviations of 6.2–11.2% below (Nu) and 31.3–37.9% below (friction factor); the residual friction-factor offset is attributed mainly to the low absolute pressure-drop levels, which are comparable with the transducer resolution (the offset is not removed by a rectangular-duct correction). The proposed MW-S geometry therefore provides substantial measured heat-transfer enhance-ment, accompanied by an associated friction penalty that is more than offset by the thermal gain under the constant-pumping-power criterion for Re ≥ 5000.