Thermal Analysis and Thickness Optimization of Two-Cavity Thermal Insulator in Buildings

• DOI: 10.1007/s13369-017-2854-y
• Publication Date: 2017-10-24T03:26:00Z
• Authors: Mohammed Alnahhal; Mohamed Elnaggar; Ezzaldeen Edwan
• Keywords: 0211 other engineering and technologies; 02 engineering and technology

This work presents an analytical and computational study on the use of two separated air cavities, located in common walls and roofs, as an application for thermal insulation in buildings. The aspect ratios of the two cavities were varied from 10 to 100. Firstly, the thermal analysis is performed, under the assumption that air in the two cavities is motionless, to determine the influence of cavity aspect ratio. The results show that the thermal diffusivity of buildings’ wall and roof components is sensitive to the presence of first and second air cavities as well as to the variation of cavity aspect ratio. In addition, the natural convection thermal analysis across the two separated air cavities, for isothermal case, was performed to assess the influence of aspect ratio on various natural convection parameters. The results show that the Rayleigh number decreases with small range of aspect ratio, but remain nearly constant in high range of aspect ratio. The optimum aspect ratio of the two separated air cavities (i.e., optimum cavities thicknesses) is determined. At each air cavity, the optimum cavity aspect ratio will be at a location where Nusselt number $$Nu = 1$$ . At this location, the natural convection heat transfer rate approximates pure conduction heat transfer rate, and thus, the cavities can work optimally as thermal insulators. The results revealed that the optimum aspect ratios for the two wall and two roof air cavities are 50 and 52.6 corresponding to optimum thicknesses of 0.02 and 0.019 m, respectively.