CFD (computational fluid dynamics) investigation on Nusselt number and friction factor of solar air heater duct roughened with non-uniform cross-section transverse rib

• A transverse periodic rib based on a new concept, non-uniform cross-section rib, investigated using CFD.
• Non-uniform cross-section rib results in higher Nusselt number than uniform cross-section rib at higher Reynolds number.
• Non-uniform cross-section rib results in lower friction factor than uniform cross-section rib.

A 3-dimensional CFD (computational fluid dynamics) investigation has been carried out to study the heat transfer and friction characteristics of solar air heater duct roughened with periodic transverse rib. The selected rib roughness is a new concept; it has non-uniform cross-section in the form of saw-tooth. For comparison, transverse ribs with uniform cross-section of circular, square and trapezoidal have also been investigated. The Nusselt number and friction factor have been determined for Reynolds number range of 3000–15,000. The k-ε turbulence model was selected for analysis. The non-uniform cross-section saw-tooth rib was found to result in higher Nusselt number than uniform cross-section ribs for Reynolds number above 7000 due to reduced low heat transfer area downstream of the rib caused by disruption in re-circulations. The maximum enhancement in Nusselt number for duct roughened with saw-tooth rib and trapezoidal rib was 1.78 and 1.50 respectively. The friction factor was found to be lower for saw-tooth rib as compared to uniform cross-section ribs investigated. The maximum enhancement in friction factor for duct roughened with saw-tooth and trapezoidal rib was 2.49 and 3.58 respectively.