Estudo experimental de uma abordagem transiente para estimação simultânea de propriedades térmicas dependentes da temperatura em materiais metálicos

Abstract

This work describes an experimental and straightforward technique towards the simultaneous estimation of temperature-dependent thermal conductivity, k, and specific heat, cp, in samples of different metallic materials, ranging from 20°C to more than 150 °C. Two thermal models based on transient nonlinear heat conduction across the metallic samples are applied. In the first one, one-dimensional (1D), the whole upper surface is heated by a constant heat flux and the other surfaces are kept insulated. The second one, three-dimensional (3D), is only partially subject to a constant heat flux on the upper side, and insulated on the other surfaces. The imperfect contact at the heater-plate interface in both cases causes contact resistance effect, which is considered as a reducing agent on heat flux. This enabled one better assess the heat diffusion effects, increasing the sensitivity, and addressing a more realistic case study. Through sensitivity analysis, it is possible to obtain prior information about estimation feasibility and establish all experimental aspects. D-optimality-based sensitivity analysis was used to determine the best location to collect the measurements so that data from a single thermocouple were sufficient to identify these thermal properties. Thermal analysis has been performed in 304 and 316 autenitic stainless steels, 1045 carbon steel and in hardmetal composite WC10Co. The direct problem was solved in COMSOL Multiphysics, obtaining the temperature field from known initial and boundary conditions. The Levenberg–Marquardt (L-M) method is employed to provide the solution to an inverse heat conduction problem capable of simultaneously evaluating the temperature-dependent thermophysical properties using transient temperature measurements at room temperature. Nonlinear Function Specification Method is used to confirm the reliability of the inverse estimation technique by using the achieved outcomes to recover the heat flux imposed on the test plate. Furthermore, the statistical study into confidence bounds and comparison with literature reveal the robustness of the results. Finally, the accuracy of the developed approach is investigated through the analysis of the errors deriving from experimental and numerical procedures. It is verified that there is no significant variation in the dispersion of the data obtained for specific heat, maintaining 0.5% confidence bounds regardless of the material. For thermal conductivity, a strong influence of sensitivity is verified, with confidence bounds ranging from 1 to 6%, with the lowest value corresponding to the 3D thermal model, which provides greater sensitivity for the parameter. Furthermore, error analysis indicates that the relative uncertainty of the estimation process is around 6%.