Accurate and simultaneous measurement of the thermal conductivity and thermal diffusivity of liquids using the transient hot-wire method

Abstract

A procedure for simultaneously obtaining values of the thermal conductivity and thermal diffusivity of liquids by the transient hot-wire (THW) method is described. The practical model of the THW method is described by a transfer function having a feedback loop. All transient data obtained by integration during a finite interval were assigned to a particular instantaneous time. The concept of the reflectivity of a thermal wave at a container wall was also investigated. The capability of the system, including the computing algorithm, was confirmed by measuring the thermal conductivity and thermal diffusivity of toluene and n-heptane within the temperature range 0 °C to 50 °C at atmospheric pressure using two hot-wire cells under different configurations of bridge-circuit (constant-voltage or constant-current sources). Furthermore, heat capacities were derived from both thermal properties and densities, and the consistency of the measurements was examined. Empirical equations representing thermal conductivity and thermal diffusivity of both toluene and n-heptane were derived as functions of temperature. The uncertainty of coefficients derived from the thermal conductivity equation is estimated to be 0,4 % to 0,5 % (with a coverage factor kp = 2 (i.e. p = 95 %)), whereas that for thermal diffusivity is about 4 % (kp = 2).