Numerical and experimental investigation of heat transfer across semi-transparent horizontal screen layers

Estimating the total heat flux passing through layers of materials is commonly used to determine the performance and potential of thermal isolation for general building design. The thermal properties of building glazing have been studied theoretically and experimentally for several decade. In this study, an enhanced algorithm for calculating the heat transfer through semi-transparent screens was developed and validated via published data and in-site experiments in order to analyze thermal properties of different screen materials and their combinations under more realistic conditions. The proposed algorithm allows to simplify the linearization of a nonlinear system, which is solved by inverting a tridiagonal matrix. Overall heat transfer coefficients (U-values) for single and multiple thin-screens were calculated numerically and measured using the hot-box method. The building insulating materials such as glass, polymers, aluminium films, and selected commercial thermal screens were examined. The cross-validated results indicated good agreement between calculated, measured and previously published results demonstrating the abilities of the method to predict new data for materials and their combinations that have not yet been estimated. Moreover, the algorithm allows to adopt the experimentally validated results to real ambient conditions by applying relevant scaling methods. The following results demonstrate the insulation properties of various commercial thermal screens. It is found that U-values reduce by 30% and 15% for the combination of a glass screen with an additional layer of a transparent infra-red (IR) radiation absorbing screen and a polyethylene screen, respectively. In comparison, the contributions of an additional screen containing aluminium foil strips could reduce the U-value by about 80% and 70% for polyethylene and glass cover screen, respectively. It was confirmed that low-emissivity materials have a more significant effect on reducing the U-value than insulating materials with IR-absorbing properties. The approach is valid for investigating the performance of thermal insulation in different enclosures, glazing in buildings, or moveable thermal screens in greenhouses.

Automated summary

Estimating the total heat flux passing through layers of materials is commonly used in determining the performance of thermal isolation. An enhanced algorithm was developed in this study to analyze the thermal properties of different screen materials and their combinations. Results showed that combining different materials can effectively reduce the U-value, with low-emissivity materials having a significant impact on reducing the U-value.