The thermal conductivity of mineral liquid dielectrics

Accurate knowledge of the thermophysical properties of dielectric liquids is essential to give a datum upon which to establish the probable thermal performance of apparatus using any proposed substitute. The thermal conductivity of a selection of natural mineral oils used in transformers and cables has been measured by the transient line source method. The interesting observation emerges that in a range of viscosity (at a specific temperature) studied, the thermal conductivity increases with decreasing viscosity. The importance of accurate evaluation of thermophysical properties and of their variation with temperature is shown by comparing calculated and measured wall temperatures in a tubular heat exchanger. By making measurements in a steady state parallel plate thermal conductivity apparatus and of radiation absorption properties, the “Poltz” radiation error associated with the apparatus when it is used with diathermanous liquids is demonstrated.