Turbulent Natural Convection on Upward and Downward Facing Inclined Constant Heat Flux Surfaces

Abstract

Local heat transfer data were obtained for turbulent natural convection on vertical and inclined upward and downward facing surfaces. The test surface consisted of a 1.83 m (6 ft) wide × 7.32 m (24 ft) high plate with a constant heat flux obtained by electrical resistive heating of a metal foil on the surface. The tests were conducted in air for modified Grashof numbers up to 1015. Measurements were made of the local surface temperature for this constant heat flux condition, for the plate inclined at angles from 30 deg to the vertical (upward facing, unstable) through the vertical to 80 deg to the vertical (downward facing, stable). The results show the location of the transition to be a function of the plate angle. For the unstable case, the transition length decreases as the plate angle increases from the vertical while for the stable case the position of transition increases with the angle from the vertical. The laminar data for both orientations are correlated as: Nux=0.55(Grx*Pr)0.20 in which the gravity is the component along the surface, g cos θ. The turbulent natural convection data are correlated quite well by the relation: Nux=0.17(Grx*Pr)0.25 In the turbulent case the correlation is independent of angle for the unstable case, whereas for the stable case the data correlate best when the gravity is modified by cos2 θ, where θ is measured from the vertical. Thus, there is a significant influence of angle on the convective heat transfer for the stable turbulent region.