Determination of thermal accommodation coefficients on CaSiO3 and SiO2 using molecular dynamics and experiments

The thermal accommodation coefficient alpha has been assumed, although lacking any experimental proof, to be near unity for most gases so far, which denotes no influence. However, it plays a contributing role in the field of the effective thermal conductivity of highly porous insulation materials based on SiO2 or CaSiO3 as it is shown in this work. Besides, this work investigates a possible influence on alpha for Ar, N-2 , He within parameters like temperature, roughness and contamination as this has not been examined on such materials so far. More importantly, it answers the question whether the assumption of alpha = 1 is valid. By using a parallel plates device, very similar to the guarded-hot-plate, following EN 12667 it was possible to determine alpha on a dense CaSiO3. It occured that the assumptions alpha = 1 (for Ar, N 2 ) and alpha = 0.3 (for He) are valid for measurements near room temperature. Further, physical adsorption was found to in-crease alpha. The determination of the influence of roughness has been started showing an interesting effect, but it still remains an open topic. In a collaborative study molecular dynamics (MD) simulations were performed showing a strong equivalence of alpha between SiO2 and CaSiO3. These results can be considered a lower limit of alpha as neither roughness nor adsorption processes have been included in the simulation. Therefore, any deviations between experiments and MD could be considered as an apperance of physical adsorption. (c) 2021 The Authors. Published by Elsevier Ltd.

Automated summary

The thermal accommodation coefficient alpha is assumed to be near unity for most gases, but this study shows its contribution to the effective thermal conductivity of highly porous insulation materials. The study investigates the influence of parameters like temperature, roughness, and contamination on alpha for Ar, N-2, He, and validates the assumptions used for calculations. The results also show the effects of physical adsorption and provide insights into the equivalence of alpha for SiO2 and CaSiO3.