Heat Transfer and Fluid Flow of Natural Convection over Horizontal-Heated Plate. High-Rayleigh-Number Flow.

Abstract

The natural convection over an upward-facing horizontal plate was investigated experimentally. Special interest was directed at high-Rayleigh-number flow ranging up to Ra*=1015. Test plates of 850 mm and 1500 mm width were adopted to attain such high Rayleigh numbers and they were heated with uniform heat fluxes. Water was used as the working fluid. The fluid flow over the plate and the surface temperatures of the plate were visualized with dye and liquid crystal thermometry, respectively. Local heat-transfer coefficients of the plate were also measured. The result shows that the following four regions appear over the plate at the distance x from the plate edge : 1) the laminar boundary layer region, 2) the streaky, transitional flow region, 3) the turbulent region and 4)impinging flow region. It is found that the second region covers a considerable portion of the plate and that the local heat-transfer coefficients there decrease monotonously with x. Thus, the overall heat-transfer coefficient of the plate still depends on the width of the plate even for the highest Rayleigh numbers.