The Influences of Property Variations on Natural Convection from Vertical Surfaces

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Abstract
The objective of this paper is to show the influences of property variations in natural convection. Heat transfer from a vertical isothermal, heated surface to gaseous nitrogen is experimentally investigated. The ambient temperature, T∞, is varied in order to cover a large range of the Rayleigh number and also to enable the generation of large values of this parameter. The range 80 K < T∞ < 320 K results in Rayleigh numbers between 107 and 2 × 1010 for the 0.28 m model. By using a cryogenic environment, large ratios of the absolute temperature of the wall to the ambient temperature, Tw/T∞, are generated without the results being masked by radiative heat transfer. The range 1 < Tw/T∞ < 2.6 is investigated. Variable properties cause dramatic increases in heat transfer rates in the turbulent regime, and virtually no influence is seen in the laminar regime. The results obtained correlate extremely well with the addition of a single parameter Tw/T∞.

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