Flow reversal and entropy generation due to buoyancy assisted mixed convection in the entrance region of a three dimensional vertical rectangular duct

The laminar buoyancy assisted mixed convection flow and heat transfer, as well as entropy generation in the entrance region of a three dimensional vertical rectangular duct are investigated numerically. The duct is symmetrically heated with uniform wall temperature and of finite length. The effect of buoyancy parameter (Gr/Re) on the velocity and temperature profiles is discussed in detail for 0 ⩽ Gr ⩽ 2 × 105, 50 ⩽ Re ⩽ 300 and Pr = 0.7, taking into account axial diffusion of momentum and energy. The fluids in the vicinity of the four corners between neighboring walls are accelerated by the assisted buoyancy, whereas those at the centerline of the duct are decelerated. The local entropy generation due to heat transfer and fluid friction is strongly influenced by the buoyancy induced flow reversal phenomenon. For a constant Reynolds number, the global entropy generation due to fluid friction increases sharply with increasing buoyancy for Gr/Re > 100, while the global entropy generation due to heat transfer is not sensitive with the variation of buoyancy force. The effect of buoyancy on local and average Nusselt number, and Bejan number, is also investigated.