Prediction of turbulent heat transfer in concentric annuli with outer rough wall

The effect of surface roughness on heat transfer in concentric annular ducts with the roughness elements only on the outer wall are analytically studied on the basis of the experiment for the fully developed flow. Three annuli having different radii of the inner cores, Ri = 25.4, 38.2 and 54 mm with a fixed outer tube, Ro=97 mm, are used over a range of Reynolds number between 15,000 and 62,000. The roughness element on the outer wall consists of the roughness height of 2 mm and the roughness pitch to height ratio (P/e) of 2. The resulting heat transfer is discussed by means of the radius ratio, Reynolds number, and heating boundary conditions; i. e., 1) the inner wall heated with the outher insulated, 2) the outer wall heated with the inner insulated, and 3) both the walls heated with different heat flux.