Buildings are the largest energy consumer, responsible for about 40% of the overall primary energy consumption in the United States. In this sense, reducing building energy consumption is of great importance to save energy, reduce greenhouse gases emission, and save fossil fuels. Among the various components of building energy consumption, heating, air-conditioning, and illumination makes the most contributions. Therefore, it is a critically important to develop thermal insulation materials.

Gaseous thermal conductivity is the major channel for thermal transport in most building insulation materials. Therefore, the key to improving the R-value of building insulation materials is to reduce the thermal conductivity of gases. Often, for the building insulation materials, to reduce the thermal conductivity of gases, the pore size is often sufficiently small for gases to reach their subcontinuum regime. In the subcontinuum regime, the gas thermal conductivity is a function of accommodation factor between gases and solids. Therefore, determining the accommodation factor between gases and solids are a key part to understand and reduce the thermal conductivity of thermal insulation materials. However, in the past, most of the studies were focused on the interface between gases and metals, while the interface between gases and building insulation materials, e.g., polymers, has not been extensively studied yet.

In this work, we study the accommodation coefficient between various gases and polymers by using molecular dynamics simulations. The gases we study include the prototype gases, helium (He) and argon (Ar), as well as the commonly used gases in building thermal insulation materials, air (N~2~ and O~2~) and CO~2~. The polymer material is chosen as amorphous polystyrene (a-PS), with chemical formula (C~8~H~8~)~n~, which is a commercial thermal insulation material used in buildings. We find that the accommodation coefficient ranges from 0.5-1, instead of the commonly believed 1. We expect the results are also roughly applicable to other polymers that are composed of C and H since the accommodation coefficients are mainly determined by the van der Waals interaction between polymers’ C/H atoms and the gas molecules and this van der Waals interaction has little dependence on which polymer it is, i.e., whether the C/H atoms belong to -CH~3~, -CH~2~-, -CH=, or -C~6~H~5~. This argument is examined by an amorphous polyethylene (C~2~H~4~)~n~ and He system, which is found to have similar accommodation coefficient with that of a-PS and He. Nevertheless, we do not exclude the possibility that some polymers may have different accommodation coefficient for the same gas, which is worth further study in the future.


Revealing Energy Accommodation Between Gases and Polymers for High-Efficiency Thermal Insulation

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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