The strong influence of heat losses on the accurate measurement of thermal diffusivity using lock-in thermography (original) (raw)
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Accurate measurements of the thermal diffusivity of thin filaments by lock-in thermography
Journal of Applied Physics, 2010
In lock-in Í‘modulatedÍ’ thermography the lateral thermal diffusivity can be obtained from the slope of the linear relation between the phase of the surface temperature and the distance to the heating spot. However, this slope is greatly affected by heat losses, leading to an overestimation of the thermal diffusivity, especially for thin samples of poor thermal conducting materials. In this paper, we present a complete theoretical model to calculate the surface temperature of filaments heated by a focused and modulated laser beam. All heat losses have been included: conduction to the gas, convection, and radiation. Monofilaments and coated wires have been studied. Conduction to the gas has been identified as the most disturbing effect preventing from the direct use of the slope method to measure the thermal diffusivity. As a result, by keeping the sample in vacuum a slope method combining amplitude and phase can be used to obtain the accurate diffusivity value. Measurements performed in a wide variety of filaments confirm the validity of the conclusion. On the other hand, in the case of coated wires, the slope method gives an effective thermal diffusivity, which verifies the in-parallel thermal resistor model. As an application, the slope method has been used to retrieve the thermal conductivity of thin tubes by filling them with a liquid of known thermal properties.
Measurement of thermal diffusivity of solids using infrared thermography
Materials Letters, 2008
We report measurement of thermal diffusivity of solid samples by using a continuous heat source and infrared thermal imaging. In this technique, a continuous heat source is used for heating the front surface of solid specimen and a thermal camera for detecting the time dependent temperature variations at the rear surface. The advantage of this technique is that it does not require an expensive thermal camera with high acquisition rate or transient heat sources like laser or flash lamp. The time dependent heat equation is solved analytically for the given experimental boundary conditions. The incorporation of heat loss correction in the solution of heat equation provides the values of thermal diffusivity for aluminum, copper and brass, in good agreement with the literature values.
Thermal-Diffusivity Measurement in Low Thermal-Conductivity Solids by a Transient Heating Method
International Journal of Thermophysics, 2012
A simple method for thermal-diffusivity measurement in low thermalconductivity solids is proposed on a basis of the analytical solution of the heat diffusion equation for a solid plate with uniform continuous heating at one of its surfaces. The method involves combined measurements of the temperature evolution at both the front (illuminated) and rear surfaces of the sample in both the thermally thin and thick regimes. The principal advantage of the method is its independence on a knowledge of the convection and radiation heat loss coefficient, and hence, the non-necessity of performing measurements in blackbody similar samples under vacuum conditions. If these conditions are achieved, the thermal conductivity and specific (volume) heat capacity could also be achieved.
Measurement of the Thermal Diffusivity of Solids with an Improved Accuracy
2003
A photothermal radiometry technique is being developed at the NPL with the goal of improving the accuracy of thermal diffusivity measurements. The principle is to perform a laser-induced thermal experiment while simultaneously making accurate measurements of the experimental boundary conditions. A numerical three-dimensional heat diffusion model based on thermal transfer functions has been developed to account for the measured boundary conditions. The thermal diffusivity is determined from the experimental data by a nonlinear, least-squares fit to the model. Experiments carried out on pure metals at 900 K demonstrate good agreement between the theoretical predictions and experimental data, and uncertainties of about 1.5% for the thermal diffusivities of platinum, titanium, and germanium were obtained.
SUMMARY: The monotonic heating regime method for determination of thermal diffusivity is based on the analysis of an unsteady-state (stabilised) thermal process characterised by an independence of the space-time temperature distribution on initial conditions. At the first kind of the monotonic regime a sample of simple geometry is heated / cooled at constant ambient temperature. The determination of thermal diffusivity requires the determination rate of a temperature change and simultaneous determination of the first eigenvalue. The eigenvalue is found from a relationship between the synchronous temperatures measured at the surface and at the middle of specimen, which is expressed by eigenfunctions in the analytical solution. According to a characteristic equation the first eigenvalue is a function of the Biot number defined by a surface heat transfer coefficient and thermal conductivity of an analysed material. Knowing the surface heat transfer coefficient and the first eigenvalue ...
Some peculiarities of the heat transfer through a sample that is heated by the superficial absorption of light energy under continuous uniform illumination are discussed. We explain, using a different approach to that presented in a recent article published in this journal (Salazar et al 2010 Eur. J. Phys. 31 1053-9), that the front surface of a thick sample reaches a higher equilibrium temperature than a thin one, a fact that may be against some people's first intuition. From the analytical solution of the problem, we obtain a condition necessary to apply a very thermally thin sample approximation, i.e. to neglect a temperature gradient across a sample. From the analysis of the heat transfer through both a thermally thin and a thermally thick sample, we suggest an inexpensive experiment suitable for measuring the thermal diffusivity of low thermal conductivity solids. Both the theoretical analysis and the suggested experiment can be suitable for students and teachers dealing with heat transfer problems, thermal characterization techniques and/or partial differential equations at undergraduate and graduate levels.
2014
The paper concerns two methods of inverse problem solution for the equation of heat conduction, what allows to determine the thermal diffusivity of the materials using pulsed infrared thermography. Both methods are related to finding the time dependence of the temperature of an infinite plate surface, when opposite surface of the plate was heated by a short heat pulse. This dependence is compared with the time evolution of the temperature of the rear plate surface measured by the means of an infrared (IR) camera. Such comparison allows to extract, from experimental data, the information about thermal diffusivity of the tested material.