The thermal conductivity of liquids—IV: Temperature dependence of thermal conductivity

Abstract

The thermal conductivity of benzene, toluene, m-xylene, carbon tetrachloride, liquid paraffin, and nitrobenzene was measured at several temperatures using a guarded horizontal parallel plate apparatus formerly described. The thickness of the liquid layer was varied within the range in which the measured values are not perceptibly increased by convection according to the results of a former investigation. It was found that the dependence of the thermal conductivity and of its temperature coefficient on the thickness of the liquid layer are in fair agreement with those values which are obtained by calculating the contribution of the radiation to the heat transfer in liquids with small ir-absorption. These values can be calculated by using an absorption diagram of the liquid as has been shown formerly. The precision of the measured thermal conductivity values is estimated at about ±0·5 per cent. The uncertainty of the calculated temperature coefficients is about ±0−1 × 10 −6 Wcm −1degC −2. The effective thermal conductivity of poorly absorbing liquids like toluene can vary at room temperature by about 5–7 per cent because of a radiative component depending on apparative conditions. The temperature coefficient of thermal conductivity can vary by about 20–30 per cent. A study of measurements published by other authors in the last 20 years shows that nearly all the values of thermal conductivity and more than half of their temperature coefficients are within an interval given by the variability of the radiative component. The more recently measured values of thermal conductivity seem to be on an average somewhat smaller than the earlier values. We suppose that more efforts are made now to avoid convection.