Unexpected natural convection heat transfer for small Rayleigh numbers in external geometry

Recently, analysis of experimental data of heat transfer from submillimeter heaters at different levels of gravitational acceleration revealed a peculiar behavior of the natural convection Nusselt number (Kostoglou et al. (2011) [4]). In particular, it was found that no natural convection heat transfer appears for Rayleigh numbers smaller than a threshold value in external geometry which contradicts the common belief that this is possible only in internal (confined) geometry. This issue is studied further herein. Additional experiments are performed with higher resolution in the parameters' range and also with other liquids of varying thermophysical properties to clarify the situation. The analysis of the present data confirms the findings of the previous work and leads to a more detailed picture for the relation between Nusselt number and Rayleigh number. The relation found is against existing correlations and existing experimental evidence in literature (acquired at terrestrial conditions). Numerical simulations of natural convection in the employed external geometry are performed in order to explain the discrepancy between the behavior of the present measurements and existing correlations. It seems that the effect of varying gravitational acceleration is rather complex and cannot be accounted simply by a change in the Rayleigh number.