Planform structure and heat transfer in turbulent free convection over horizontal surfaces

This paper deals with turbulent free convection in a horizontal fluid layer above a heated surface. Experiments have been carried out on a heated surf ace to obtain and analyze the planform structure and the heat transfer unde r different conditions. Water is the working fluid and the range of flux Ra yleigh numbers (Ra) covered is 3x10(7)-2x10(10). The different conditions c orrespond to Rayleigh-Benard convection, convection with either the top wat er surface open to atmosphere or covered with an insulating plate, and with an imposed external flow on the heated boundary. Without the external flow the planform is one of randomly oriented line plumes. At large Rayleigh nu mber Ra and small aspect ratio (AR), these line plumes seem to align along the diagonal, presumably due to a large scale flow. The side views show inc lined dyelines, again indicating a large scale flow. When the external flow is imposed, the line plumes clearly align in the direction of external flo w. The nondimensional average plume spacing, Ra-lambda(1/3), varies between 40 and 90. The heat transfer rate, for all the experiments conducted, repr esented as Ra-delta T(-1/3), where delta(T) is the conduction layer thickne ss, varies only between 0.1-0.2, showing that in turbulent convection the h eat transfer rates are similar under the different conditions. (C) 2000 Ame rican Institute of Physics. [S1070-6631(00)00903-X].