A simple laboratory flash apparatus for thermal diffusivity measurement: Modeling and application for composite material based on clay and straw

In this work. we have developed and tested a flash apparatus for thermal diffusivity measurement of building materials. After the test on homogeneous materials, this very inexpensive apparatus was then used to estimate the thermal diffusivity of some local materials based on clay and straw. To estimate this quantity, the measured temperature rise at the rear face of the sample was processed by solving analytically the transient heat equation based on a one-dimensional model with time-dependent convective boundary conditions. The obtained mathematical model takes into account the temporal shape f(t) of the heat pulse used to excite the front sides of the samples as well as four unknown parameters ie: the thermal diffusivity, the two Biots numbers related to the heat losses on the rear and the front sides and the maximum adiabatic temperature. For this purpose, the excitation form performed via a flashlight was experimentally measured and f(t) properly modeled by a first square form followed by a short exponential waning phase attributed to the thermal inertia of the used halogen lamps. Finally, the simultaneous estimation of the remaining unknown parameters was carried out by minimizing the sum of the quadratic differences between the mathematical model and the measured thermogram on the back faces of the samples. The precision and accuracy of the apparatus results were also examined by measuring the thermal diffusivity of several homogeneous building materials. The standard deviation in thermal diffusivity for the studied samples doesn’t exceed 4.72 %. Then, five samples from clay mixed with different mass fractions of straw are prepared and their thermal diffusivities are measured. The thermal conductivity and the heat capacity of these composite materials are experimentally achieved and their values are reported for comparative purposes. The thermal characterization shows a decrease of about 27.5 % in thermal diffusivity, 43.32 % in thermal conductivity and 4.40 % in heat capacity versus the increase of the straw mass fraction.