Large Eddy Simulation of Flow and Heat Transfer in a Staggered 45° Ribbed Duct and a Rotating 90° Ribbed Duct

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Virginia Tech

For the past several years there has been great effort in the analysis of internal duct cooling. The steady increase in power output and thermal efficiency requirements for gas turbine engines has called for significant advancement in turbine blade internal duct cooling technology. Numerical analysis of turbulent duct flow has been largely limited to Reynolds Averaged Navier-Stokes (RANS) simulations. This is because of the low computational requirements of such calculations relative to Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). However, the tides have started to turn in favor of LES, partly because of the exponential increase in computer hardware performance in recent years.

Three conference papers make up the contents of this thesis. LES is performed for fully developed flow and heat transfer in a staggered 45º ribbed duct in the first paper. The rib pitch-toheight ratio P / e is 10 and a rib height to hydraulic diameter ratio h e / D is 0.1. The Reynolds numberbased on the bulk flow rate and hydraulic diameter is 47,300. The overall heat transfer enhancement obtained was a factor of 2.3, which matched experimental data within 2%. The surfaces of highest heat transfer enhancement were the ribbed walls and the outer wall.

Results from LES of an orthogonally rotating 90º ribbed duct are presented in the second paper for rotation numbers: Ro = 0.18, 0.35 and 0.67. The Reynolds number is 20,000. The P / e and h e / D were the same as in the first paper. Turbulence and heat transfer are augmented on the trailing surface and reduced at the leading surface. Secondary flows induced by Coriolis forces, increase heat transfer augmentation on the smooth walls. Finally, the third paper studies the same flow conditions of the second paper and goes further by including effects of centrifugal buoyancy forces using LES. Two buoyancy numbers are studied: Bo = 0.12 and 0.29. Centrifugal buoyancy does not have a large effect on leading side augmentation ratios for all rotation numbers, but increases heat transfer significantly on the trailing side.

In all papers, mean flow and heat transfer results compare well with experimental data.

Internal Cooling, Heat--Transmission, Large Eddy Simulation, Computational fluid dynamics, Gas Turbine