Laminar flow and heat transfer in the entrance region of ribbed concentric annuli

An analysis is presented for simultaneously developing laminar flow and heat transfer in a concentric annulus. On the inner surface of the annulus, transverse ribs are added to interrupt the developing flow. The governing momentum and energy equations are solved numerically using a control-volume technique in the cylindrical coordinates. The inner surface of the annulus is maintained at a constant elevated temperature, while the outer surface is maintained at the same temperature as the inlet fluid. The resultant flow structure is presented for Reynolds numbers of 50, 250, and 500. The effect of the ribs on the developing Nusselt number distribution and local heat transfer is investigated. The local pressure gradient is also discussed. The parameters shown to affect the heat transfer are the rib height, rib spacing, Reynolds number and Prandtl number. For Pr = 0.7, the heat transfer from the extended surface is offset by the reduced local Nusselt number immediately upstream and downstream of each rib. Only for large rib heights, small rib spacings, and high Reynolds numbers is heat transfer significantly enhanced. For higher Prandtl number (Pr = 2.29 and 34.6) fluids, however, heat transfer is improved to a larger extent, over a wider range of rib heights and spacings.