The measurement of the thermal diffusivity of a thin layer in the direction of its plane is usually a difficult operation. The standard ‘‘flash technique’’ is very appropriate for diffusivity measurement in the direction of the thickness of the sample but adaptations of this method to in-plane measurements remain very sensitive to the position and form of heat excitation and temperature sensors. The new procedure proposed here consists of applying any geometrically nonuniform heat impulse on the front face of the sample and recording the entire transient temperature image on the rear face thanks to an infrared camera. The influence of axial diffusion can be avoided for periods much longer than the axial diffusion characteristic time. Integral transforms on the radial space variables (Fourier transform) are very suitable for treating the temperature field and to estimate radial diffusivity. The main advantage of this method is to avoid any experimental precaution (no knowledge of the geometrical form of the excitation - replacement of the sensor positioning by an image calibration). Furthermore, the considerable number of data produced by the camera is processed using a statistical approach. The validation of the method is made on a homogeneous sample by comparison between the in-plane direction measurements (obtained with the present procedure) and the thickness direction measurements (obtained by the classical flash technique).
This paper presents a method dedicated to thermal conductivity measurement of thin (a few millimeters thickness) insulating and super-insulating materials. The method is based on the measurement of the temperature at the center of a heating element inserted between two samples, with the unheated surface of the samples maintained constant. A 3D model of the heat transfer in the system has been established and simulated to determine the validity conditions of a 1D model to represent the center temperature. This 1D model was then used to realize a sensitivity analysis of the center temperature to the different parameters. The conclusion is that the thermal conductivity may be estimated with a good precision for all insulating materials from a simple steady state measurement and that the thermal capacity may also be estimated from transient recording of the temperature with a precision increasing with the value of the thermal capacity of the samples. It has then been shown that a device with two samples of different thickness improves the precision of the estimation of the thermal capacity. These conclusions are validated by an experimental study on polyethylene foam and PVC samples leading to an estimation of their thermal properties very close to the values measured by other classical methods (deviation < 5%).
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