Influence of system buoyancy and hydraulic diameter on turbulent mixed convection in a rectangular duct

We investigated buoyancy-aided, turbulent mixed-convection heat transfer in a vertical rectangular duct. In order to achieve highly buoyant conditions, mass transfer experiments instead of heat transfer ones were performed based on the analogy between heat and mass transfer. Reynolds numbers ranged from 2,700 to 25,450 and Grashof numbers were 3.30 x 10(5) and 5.15 x 10(6). The results of turbulent forced-convection experiments were consistent with previous studies. Notably, at the same axial position, local heat transfer was affected by downstream flow for different heated lengths, and heat transfer was affected by the hydraulic diameter. Five distinct flow regimes were observed based on buoyancy coefficients. Particle image velocimetry measurements revealed that variations in local heat transfer were closely related to the amount of turbulence produced, which was affected by differences in velocity between buoyant and forced flows. Turbulent mixed-convection heat-transfer correlations are proposed based on analytical models and parametric studies of the influence of system buoyancy and hydraulic diameter. (C) 2021 Elsevier Ltd. All rights reserved.