Analysis of the impact of surface heat transfer on a new modification of the Angstroem’s method

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A new modification of the Angstroem’s method for thermal diffusivity measurement has been developed. This relies on the propagation of harmonic thermal waves with mean value equal to the ambient temperature. The diffusivity is evaluated by relatively simple processing of temperature data, acquired by infrared thermography. The evaluation is based on a mathematical model, in which the heat transfer coefficient at the specimen surface is assumed to be constant. This work is aimed at verifying that assumption. In particular, the effects of natural convection in air are investigated theoretically by numerical simulation. A strategy to improve the test procedure is finally outlined. A new technique for measurement of the thermal diffusivity has been developed, in collaboration between the DIMeC and the CNR-ITC Padova Section. The technique is a modification of the well-known Angstroem’s method, which relies on the steady-periodic propagation of thermal waves along a specimen to estimate the diffusivity of the tested material [1]. The novelty of the proposed modification is that harmonic thermal waves with mean value equal to the ambient temperature are induced in the specimen by means of a purposely developed source, based on the Peltier effect. This allows evaluating the diffusivity by relatively simple processing of surface-temperature data, acquired by infrared thermography. A cross comparison of the technique with other measurement methods gave promising results, showing the effectiveness of the approach to produce the thermal waves [2-6]. Accuracy and precision comparable with the standard flash method [7] were obtained. Further improvements seem to be possible, with especial regard to controlling the test conditions. In fact, the propagation of the thermal waves is affected by heat transfer at the surface of the specimen. The coefficient of heat transfer with the test environment, which includes the superposed effects of thermal radiation and air convection, is generally unknown, but its estimate is made unnecessary by the procedure adopted to evaluate the diffusivity. In the mathematical model on which the evaluation procedure is based, however, a homogeneous and constant value is assumed for the heat transfer coefficient. This is not so easy to accept, since strong buoyancy effects can be unset in the air around the specimen by the propagation of the thermal waves. Therefore, the assumption is verified in this work, investigating either radiation or convection. Particular attention is paid to the surface pattern of the convection coefficient, which is theoretically estimated along the thermal cycles by numerical simulation. A strategy to limit the start of buoyancy effects is finally outlined.