A New Apparatus for Study of Stabilities in Horizontal Air Layers Heated from Below
An apparatus has been designed and built for studies and experimental evaluation of heat transfer by simultaneous conduction, convection and radiation. It consists of a rigid enclosure with two horizontal parallel plates separated by an air gap which was heated from below. Any preselected temperature difference, of up to 100°C across the plates could be maintained at a steady state. The apparatus was operated as a conduction-radiation cell. Using an energy balance and a calibration curve, “net” radiative heat transfer was evaluated and “emittance” of plain surfaces and “apparent emittance” of surfaces with V-notches was calculated. The results obtained were compared with these measured independently on the same samples by C.S.I.R.O. using optical techniques. It was found that a maximum difference of ±0.2% exists. To ensure that conduction and radiation only were present, a method similar to that used  to determine the onset of convection, was applied. Results obtained for “critical pressure” and “critical Rayleigh number” were of greater accuracy then theirs, due to the use of a small thin and very sensitive heat flux sensor of low thermal capacity.
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