In-depth wood temperature measurement using embedded thin wire thermocouples in cone calorimeter tests

Abstract

In-depth temperature measurements of wood samples using thermocouples are commonly performed in fire research at all scales. However, the size or the type of thermocouples or how they are placed in the sample can lead to the measured temperature being significantly different. In this work, an original method combining thin wire thermocouples with a meticulous implantation, was used to provide an accurate measurement of the in-depth temperature of wood samples during their degradation. Wood samples were thus equipped with both embedded thin wire thermocouples (0.1 mm diameter) and sheathed thermocouples (1 mm diameter) installed perpendicularly or in parallel to the conductive heat flux to compare the results between the two kinds of measurements. Afterwards, samples were exposed to the cone calorimeter at different heat fluxes (from 16.5 kW.m−2 to 93.5 kW.m−2) in order to measure both in-depth temperature and mass loss during the degradation. The smallness and high flexibility of thin embedded wire thermocouples enabled in-depth temperature measurement without affecting the mass loss measurement. This is usually not possible when using sheathed thermocouples, because of their high stiffness. Results showed that it is crucial to rigorously control the thermocouple implantation, as an underestimation up to was observed for thermocouples put near the surface exposed to the heat flux. There are several reasons for this difference: the heat sink due to conductive heat flux through the thermocouple body, a poor contact between the sensor and the sample and uncertainty about the actual position of the thermocouple. The char front evolution deduced from temperature measurement by wire thermocouples was found to comply with direct measurements of the char layer performed on samples. Finally, the charring rate obtained from in-depth temperature as a function of the heat flux was found to follow an affine law.