An improved model for the parallel hot wire: Application to thermal conductivity measurement of low density insulating materials at high temperature

This paper presents a new model of the transient temperature at a point situated at a distance d of the heating wire, in a parallel hot wire measurement device. A preliminary theoretical study shows the limits of the estimation method proposed by the standard ISO8894-2. First, we proposed an optimal processing method, based on the model used in the standard, to improve the estimation of the thermal conductivity. Then, a new model based on the quadrupolar formalism is developed, it takes into account: the mass and the radius of the heating wire, the thermal contact resistance between the heating wire and the sample and the mass of the thermocouple. A theoretical study shows that this model enables a precise estimation of the thermal conductivity of a large range of materials and that it makes also possible to obtain an estimation of the volume heat capacity. An experimental study has been realized using a very low density material (polystyrene with ρ=15kgm−3) at ambient temperature and a reference material (Silcal 1100) at temperatures varying from 200 °C to 1000 °C to validate the results of the theoretical study. Compared to known thermal properties of these two materials, the thermal conductivity was estimated with a deviation lower than 3.4% and the volume heat capacity was estimated with a deviation lower than 10%.