Flow and Heat Transfer in a Ribbed Converging-Diverging U-Duct Under Rotating and Non-Rotating Conditions

A numerical study based on steady Reynolds-averaged-Navier-Stokes (RANS) with the shear stress transport (SST) turbulence model was performed to investigate the flow and heat transfer in a ribbed U-duct with a rectangular cross section that converges linearly in the radially outward direction and diverges linearly in the radially inward direction under rotating and nonrotating conditions. Parameters examined include rotation number (Roi = 0, 0.0219, 0.0336, 0.0731), Reynolds number (Rei= 46,000, 100,000, and 154,000), and the duct's taper angle (alpha = 0 deg and 1.41 deg) under conditions relevant to gas turbines used for electric power generation. Results obtained show that increasing the taper angle from 0 deg to 1.41 deg, which appears negligibly small, significantly increases both the friction coefficient and the Nusselt number whether there is rotation or not. With rotation at Roi = 0.0336 and Rei = 100,000, the maximum increase in the average friction coefficient and Nusselt number was found to be 41.7% and 36.6%, respectively. Without rotation at Rei = 46,000, those are 11.5% and 14.7%, respectively.