Turbulent heat transfer under conditions of buoyancy-opposed mixed convection in an annulus

An experimental investigation is reported of heat transfer under buoyancy-influenced conditions to water flowing downwards through a vertical passage of annular cross-section having a heated core and an adiabatic outer casing. The Reynolds Number was varied from 20000 to 1000 and the Grashof Number from 2.12 x 10(8) to 2.58 x 10(9). For conditions of negligible buoyancy influence the results were fitted by the equation Nu=0.024Re(0.8)Pr(0.4). As Reynolds number was reduced and Grashof number increased so as to produce buoyancy-influenced conditions, heat transfer was enhanced as a result of additional turbulence being produced. It was found that the influence of buoyancy on heat transfer could be described very satisfactorily by an equation based on a simple semi-empirical model of mixed convection.