Thermal performance evaluation of continuous and truncated semi-circular rib turbulators in a rectangular duct

This study investigates the effect of different semi-circular rib arrangements on the heat transfer and friction characteristics of a rectangular duct. Six semi-circular rib configurations, ranging from continuous to hybrid patterns, were fabricated on the bottom wall of the test section to evaluate their thermo-hydraulic performance in comparison with a smooth duct. Experiments were conducted over a Reynolds number range of 13,500–35,000 and relative pitch ratios (P/e) of 8, 10, and 12.5. The experimental results were validated using well-established correlations, including the Dittus-Boelter correlation for the Nusselt number and the Blasius equation for the friction factor, and were further verified through Computational Fluid Dynamics (CFD) simulations. The results showed that the Nusselt number ratio varied from 1.44 to 2.57, while the friction factor ratio ranged from 2.20 to 4.51. The thermo-hydraulic performance parameter ranged from 1.08 to 1.69 compared with the smooth duct. The experimentally obtained Nusselt numbers deviated from the Dittus-Boelter correlation by 5.65%-12.06%, whereas deviations from the Abdel-Moneim correlation ranged from −2.48% to 0.62%. The friction factor predicted by the Blasius equation differed from the experimental values by 2.13%-7.81%. CFD simulations performed for the optimum rib configuration (P/e = 8) showed good agreement with the experimental results, with deviations within ±10% for the Nusselt number and ±14% for the friction factor. Uncertainty analysis indicated that the uncertainty in the friction factor ranged from 4.41% to 10.20%, while that in the Nusselt number varied between 4.64% and 9.31%. Among the six rib configurations, the hybrid semi-circular rib arrangement with P/e = 8 exhibited the best thermo-hydraulic performance, achieving enhancement ratios between 1.47 and 1.69 with only a moderate increase in friction losses. Regression-based correlations developed from the experimental data predicted 92.22% of the Nusselt number values and 93.33% of the friction factor values within a deviation of ±15%. The proposed correlations are applicable within the ranges of 13,500 ≤ Re ≤ 35,000 and 8 ≤ P/e ≤ 12.5.