Heat Transfer Models And Thermal Conduction Properties Of Laminated Fiber Core Materials Of Vacuum Insulation Panels

Vacuum insulation panels(VIPs)are generally considered as super insulation materials,Due to high porosity of the core material and the high vacuum,VIPs’ thermal conductivity are extremely low and has been used for exterior insulation of buildings which have very high insulation requirements.Laminated glass fiber and glass wool are important fiber core materials of VIPs.Therefore,to understand the influential factors and the influential law of these two kinds of fiber core is of great significance to the VIPs production.In this paper,the microstructures of the two kinds of laminated fiber core materials were characterized by scanning electron microscopy.It was found that fibers in a layer of laminated fiberglass core materials are distributed randomly in the plane perpendicular to the thickness of the fiberglass layer.Meanwhile,fibers in a layer of laminated glass wool core materials are not only randomly distributed in the plane perpendicular to the thickness direction of the fiber layer,but also have certain random distribution in the thickness direction of the fiber layer.The diameter and length of fibers in the two kinds of core materials were measured by a digital micrometer and a vernier caliper.The pore size of the two kinds of laminated fiber core materials was determined according to the fiber distribution characteristics in fiber layers and the related core material parameters measured.Based on the the fiber structure tests,the structures of the two core materials are reconstructed.Two heat transfer models were proposed to calculate the equivalent thermal conductivity(ETC)of the two kinds of laminated fibers separately.The two models were tested by the experimental data and theoretical results provided by other researchers.The effects of temperature,pressure,fiber diameter,fiber volume fraction,fiber length and single-layer thickness on VIPs core ETC were studied.The results show that the equivalent thermal conductivities of the two kinds of laminated fiber core materials decrease with temperature,pressure and fiber diameter decreases.The ETCs of glass wool core materials decrease with the decrease of the thickness of single layer and the increase of fiber length.The ETCs of the two kinds of laminated fiber core materials do not change linearly with the increase of fiber volume fraction.They decrease firstly and then increase,which indicates that there exists an optimum fiber volume fraction leading the minimum thermal conductivity of core materials.The optimum volume fraction and minimum thermal conductivity of laminated fiber core materials are different when the temperature,pressure,fiber diameter,fiber length and thickness of single layer are different.The optimum volume fraction and minimum thermal conductivity of the core materials under different conditions were calculated in order to provide reference for VIPs manufactures.