A method for determining the in-plane thermal diffusivity of planar samples was constructed. The time-dependent temperature field of the sample heated at one edge was measured with an infrared camera. The temperature fields were averaged for different times over a narrow strip around the center line of the sample, and the temperature profiles for varying time were fitted by a solution to a corresponding one-dimensional heat equation. Heat losses by convective and radiative heat transfer were both included in the model. Two fitting parameters, the thermal diffusivity and the effective heat-loss term, were obtained from time-dependent temperature data by optimization. The ratio of these two parameters was also extracted from the steadystate temperature profile. The method was found to give good and consistent results when tested on copper and aluminum samples.
The in-plane thermal conductivity of porous sintered bronze plates was studied both experimentally and numerically. We developed and validated an experimental setup, where the sample was placed in vacuum and heated while its time-dependent temperature field was measured with an infrared camera. The porosity and detailed three-dimensional structure of the samples were determined by X-ray microtomography. Lattice-Boltzmann simulations of thermal conductivity in the tomographic reconstructions of the samples were used to correct the contact area between bronze particles as determined by image analysis from the tomographic reconstructions. Small openings in the apparent contacts could not be detected with the imaging resolution used, and they caused an apparent thermal contact resistance between particles. With this correction included, the behavior of the measured thermal conductivity was successfully explained by an analytical expression, originally derived for regular structures, which involves three structural parameters of the porous structures. There was no simple relationship between heat conductivity and porosity.
The transient fin model introduced recently for determination of the in-plane thermal diffusivity of planar samples with the help of infrared thermography was modified so as to be applicable to poor heat conductors. The new model now includes a temperature-dependent heat loss by convective heat transfer, suitable for an experimental setup in which the sample is aligned parallel to a weak, forced air flow stabilizing otherwise the convective heat transfer. The temperature field in the sample was measured with an infrared camera while the sample was heated at one edge. The symmetric temperature field created was averaged over the central fifth of the sample to obtain one-dimensional temperature profiles, both transient and stationary, which were fitted by a numerical solution of the fin model. One of the fitting parameters was the thermal diffusivity, and with a known density and specific heat capacity, the thermal conductivity was thus determined. The test measurements with tantalum samples gave the result (57.5 ± 0.2) W · m −1 · K −1 in excellent agreement with the known value. The other fitting parameter was a temperature-dependent heat loss coefficient from which the lower limit for the temperature-dependent convection coefficient was determined. For the stationary state the result was (1.0 ± 0.2) W · m −2 · K −1 at the temperature of the flowing air, and its temperature dependence was found to be (0.22 ± 0.01) W · m −2 · K −2 .
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