Local and average natural convection Nusselt numbers for a uniformly heated, shrouded or unshrouded horizontal plate

Experiments were performed to determine the local and average natural convection heat transfer characteristics for a uniformly heated, upward-facing horizontal plate shrouded by a parallel adiabatic surface situated above the plate. Three parameters were varied during the experiments, including the dimensionless height of the interplate gap (the no-shroud case serving as the upper limit), the dimensionless heating rate (the Rayleigh number), and the configuration of the surroundings (absence or presence of an obstruction and of supplemental insulation). Air was the heat transfer medium. The local Nusselt number was highest at the edge of the heated plate and decreased along the plate surface; at the center of the plate, the Nusselt number was about one-third of that at the edge. The local Nusselt number distributions, when normalized by the corresponding average Nusselt number, were found to be universal (i.e. independent of the parameters) for gap heights above a threshold value. Both the local and average Nusselt numbers were depressed at small gap heights but rebounded rapidly with increasing gap height. A critical gap height was found such that for values exceeding it, the average Nusselt number was independent of the presence or absence of the shroud.