Developing flow and transport above a suddenly heated horizontal surface in water

Abstract

The results of this experimental investigation are observations and conclusions, determined by transport measurements and flow visualization, regarding the development of convection above an instrumented, horizontal surface (with side walls) in an extensive water ambient subjected to a step in electrical energy generation. The initial mode of heat transfer was concluded to be conduction, since the measured plate surface temperature closely agreed with one-dimensional transient conduction theory. Departures from the theoretical conduction solution were an indication of the onset of convective motion. When the heated layer became sufficiently thick, a wave-like instability was observed, followed by fluid motion, with nearlyspherically-shaped ‘heat bubbles’ (thermals) rising randomly, increasing in size, with some assuming a mushroom shape and breaking away from the bulk of the heated fluid. Thereafter, both the local and spatially averaged plate surface temperatures were seen to be time dependent. Another observation was the presence of wispy, swaying ‘convection columns’ which meandered to-and-fro on the surface. The first visual convective instability was seen to precede the departure of the measured surface temperature from the conduction solution. As the low ambient temperature range was approached, the density extremum effect reduced Nu Ra − 1 3 by as much as about 50%. It is concluded that transport above a heated, horizontal surface in an extensive water ambient is inherently time dependent; initially largely because of‘heat bubbles’ breaking away from the conduction layer, and later because of the continual movement of ‘convection columns’.