SIMULTANEOUS MEASUREMENT OF THE THERMAL CONDUCTIVITY AND THERMAL DIFFUSIVITY OF SINGLE AND TWO-PHASE MATERIALS USING THE TRANSIENT HOT WIRE METHOD

The thermal conductivity and thermal diffusivity of single and two-phase materials were measured as a function of temperature and pressure using the transient hot wire method. The mathematical basis for the use of this method for measurements on single-phase samples is well documented. Thermal property measurements on two-phase samples (powders) are more complex to analyze than those of single-phase samples. Heat transfer mechanisms in powders include conduction through the solid phase, conduction through the gas phase, along with heat transfer between the phases at any point in the material. Because of this complex means of heat transfer, thermal properties of powders are not true properties but are effective properties which account for all these heat transfer mechanisms. A mathematical model based on volume averaging was developed to predict measured values of the effective thermal properties of powdered materials. Modeling of the thermal properties of powders included samples of glass beads, powdered oil shales, and powdered coals. The model accurately predicted thermal properties of these samples which varied as a function of pressure over several orders of magnitude. This work provides an understanding of thermal property measurements of powders and of heat transfer mechanisms in general.