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

Abstract : This paper presents a new model of the the transient temperature at a point situated at a distance 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 = 15 kg m) 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%. 1. Introduction A variant of the hot wire method consists in placing in the medium to characterize a thermocouple parallel to the heating wire and to record its temperature. The measurement of this temperature makes it possible to estimate the thermal conductivity and the thermal diffusivity of the medium. This transient method called the parallel hot wire method was the object of the standard ISO 8894-2 [1] concerning the determination of the thermal conductivity of refractory materials. The device of the parallel hot wire associated with the estimation method defined by the standard was used to characterize various types of materials among them: refractories [2-3], polymers [4], soils [5] and insulating materials [6].
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Yves Jannot, Alain Degiovanni. An improved model for the parallel hot wire: Application to thermal conductivity measurement of low density insulating materials at high temperature. International Journal of Thermal Sciences, Elsevier, 2019, 142, pp.379-391. ⟨10.1016/j.ijthermalsci.2019.04.026⟩. ⟨hal-02139120⟩

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