Numerical investigations on fully-developed mixed turbulent convection in dimpled parabolic trough receiver tubes

The fully-developed mixed turbulent convective heat transfer characteristics in dimpled tubes of parabolic trough receiver are numerically studied at a certain Reynolds number of 2 × 10⁴and different Grashof numbers ranged from 0 to 3.2 × 10¹⁰to produce substantial surface heat transfer augmentations with relatively small pressure drop penalties. The Boussinesq approximation is applied, in which variations in fluid properties other than density are ignored. The Realizable k-ε two-equation turbulence model with enhancement wall treatment is adopted. The influences of outer wall heat flux distributions and dimple depth on flow resistance and heat transfer rate are illustrated and analyzed. The results indicate that the average friction factor and Nusselt number in dimpled receiver tubes under non-uniform heat flux (NUHF) are larger than those under uniform heat flux (UHF). In most cases, the comprehensive performance of dimpled receiver tube under NUHF is also better than that under UHF. The deep dimples (d/D_i = 0.875) are far superior to the shallow dimples (d/D_i = 0.125) at a same Grashof number.