Effect of Surface Topology on the Apparent Thermal Diffusivity of Thin Samples at LFA Measurements

Abstract

This paper deals with the problem of the influence of surface topography on the results of thermal diffusivity measurements when determined using the instantaneous surface heat source method, also called the pulse method. The analysis was based on numerical tests carried out using Comsol Multiphysics software. The results of experimental investigations on the actual material structure using an electron microscope, an optical microscope and a profilometer were used to develop a numerical model. The influence of the non-uniformity of the surface of the tested sample on the determined values of half-time of the thermal response of the sample’s rough surface to the impulse forcing on the opposing flat surface was determined by developing the data for simulated measurements. The effect of the position of the response data reading area on the obtained simulation results was also analyzed. The obtained results can be used to improve the accuracy of experimental heat transfer studies performed on thin-film engineering structures depending on the uniformity and parallelism of the material applied to engineering structures. The difference in half-life determination error results for various analyzed models can be as high as 16.7%, depending on the surface from which the responses of the heating impulse are read. With an equivalent model in which 10% of the material volume corresponds to the rough part as a single inclusion, hemisphere, the error in determining thermal diffusivity was equal to 3.8%. An increase in the number of inclusions with smaller weight reduces an error in the determination of thermal diffusivity, as presented in the paper.

The Possibility of Using the Finite Element Method for Determining Thermal Diffusivity on the Example of Nickel Using the Classic and The Modified Pulse Method

“…The influence of waviness is of great importance for calculations of heat transfer phenomena in thin films and in complex flow models used, among others, in aviation technology, e.g., rocket nozzles [10], engines [11] as well as in its heat conduction in microchannel flows [12], and heat exchangers [13,14] which is a frequently tackled problem of modern mechanical engineering. In the case of heat transfer studies based on determining the global temperature distribution on the surface, it is necessary to take into account the thermal properties of all layers in the analysis [15], in particular, if the thickness is a square function, as in the case of determining thermal diffusivity using the laser flash method (LFA -Laser Flash Analysis), where the thickness is given to the second power [16].…”