Mixed-Convection Flow and Heat Transfer in the Entry Region of a Horizontal Rectangular Duct

Entry-region hydrodynamic and thermal conditions have been experimentally determined for laminar mixed-convection water flow through a horizontal rectangular duct with uniform bottom heating. Direct heating of 0.05 mm stainless steel foil was used to minimize wall conduction, and the foil was instrumented to yield spanwise and longitudinal distributions of the Nusselt number. Flow visualization revealed the existence of four regimes corresponding to laminar forced convection, laminar mixed convection, transitional mixed convection, and turbulent free convection. The laminar mixed-convection regime was dominated by ascending thermals which developed into mushroom-shaped longitudinal vortices. Hydrodynamic instability resulted in breakdown of the vortices and subsequent transition to turbulent flow. The longitudinal distribution of the Nusselt number was characterized by a minimum, which followed the onset of mixed convection, and subsequent oscillations due to development of the buoyancy-driven secondary flow.