Experimental assessment of the effects of Prandtl number and of a guide vane on the thermal development in a ribbed square-ended U-bend

This is an experimental study of the thermal development in a stationary cooling passage which consists of two straight ducts of square cross section, with inclined ribs on two opposite sides connected by a square-ended U-bend. The upstream and downstream straight sections are 10 hydraulic diameters long. The ribs are of a square cross-section and oriented at 45° with the duct axis. The ratio between the rib height and the duct diameter, e/DHe/DH, is 0.1 and the rib spacing, P/eP/e, is 10. First, heat transfer data obtained using air as the working fluid are compared with experimental and numerical data from our earlier work, which were obtained using water, in order to explain how the flow develops through such passages and how the Prandtl number influences the thermal characteristics. Further heat transfer measurements and flow visualisation diagrams obtained with a 180° guide vane inserted in the U-bend demonstrate the changes caused to the flow and thermal development and also enable us to assess whether the inclusion of a 180° guide vane would improve the uniformity of the local coefficient of wall heat flux. Over a Prandtl number range from 0.7 to 5.9, it is shown that the main features in the local Nusselt number variation remain the same. At higher Prandtl numbers, when scaled through the Dittus–Boelter correlation, Nusselt number levels fall and also display a steeper variation within the rib intervals. Over the end wall, Nusselt number levels are found to be considerably higher over the upstream half, probably due to flow impingement. The introduction of a 180° guide vane is shown to result in a marked increase in the Nusselt number levels within the bend, especially along the right and end walls. There is, however, a severe attenuation of the Nusselt number along both ribbed walls after the bend.