Applicability of theoretical models to the evaluation of thermal conductivity in microgravity

In several problems concerning heat transfer in liquids the thermal conductivity coefficient is required al different values of temperature and pressure, but often reliable experimental data are not available. In these conditions prediction methods are necessary to calculate the thermal conductivity coefficient. Both theoretical equations and empirical or semi-empirical equations were proposed by various authors, the first ones deriving from rigorous physical considerations, the second ones being based both on theoretical considerations and on experimental evidence. Theoretical equations should be obviously preferred, but often their use can be accepted with reasonable accuracy (+/-5%) for simple molecular structures only. Empirical or semi-empirical equations on the other hand seem to de more useful for engineering purposes because of their ease of use and their larger range of validity. In this work a review: of theoretical models is critically presented and explained with respect to their applicability in microgravity conditions. It will be shown that heat transfer phenomena in liquids are likely to be influenced by gravity forces only in a negligible way and how a set of experiments on pure fluids would be exceptionally useful both to enhance theoretical studies and to acquire neu experimental thermal conductivity data useful for engineering purposes.