Large eddy simulation of flow and heat transfer in a 90 deg ribbed duct with rotation: Effect of Coriolis and centrifugal buoyancy forces

Results from large eddy simulations (LES) of fully developed flow in a 90 deg ribbed duct are presented with rib pitch-to-height ratio P/e = 10 and a rib height-to-hydraulic-diameter ratio e/D-h = 0.1. Three rotation numbers Ro = 0.18, 0.36, and 0.68 are studied at a nominal Reynolds number based on bulk velocity of 20000. Centrifugal buoyancy effects are included at two Richardson numbers of Ri = 12, 28 (Buoyancy parameter, Bo = 0.12 and 0.30) for each rotation case. Heat transfer augmentation on the trailing side of the duct due to the action of Coriolis forces alone asymptotes to a value of 3.7 +/- 5% by Ro = 0.2. On the other hand, augmentation ratios on the leading surface keep decreasing with an increase in rotation number with values ranging from 1.7 at Ro = 0.18 to 1.2 at Ro = 0.67. Secondary flow cells augment the heat transfer coefficient on the smooth walls by 20% to 30% over a stationary duct. Centrifugal buoyancy further strengthens the secondary flow cells in the duct cross-section which leads to an additional increase of 10% to 15%. Buoyancy also accentuates the augmentation of turbulence near the trailing wall of the duct and increases the heat transfer augmentation ratio 10% to 20% over the action of Coriolis forces alone. However it does not have any significant effect at the leading side of the duct. The overall effect of buoyancy on heat transfer augmentation for the ribbed duct is found to be less than 10% over the effect of Coriolis forces alone. Friction on the other hand is augmented 15% to 20% at the highest buoyancy number studied. Comparison with available experiments in the literature show excellent agreement.