Numerical simulation of heat transfer by natural convection in cavities of facade elements

Heat transfer by the natural convection of air layers within vertical, rectangular cavities with aspect ratios (A) of 20, 40 and 80 was investigated in relation to applications in building facade elements, such as insulating glazing units, double-skin facades, doors, etc. using a computational fluid dynamics (CFD) code. Boundary conditions were assumed to be isothermal hot and cold wall, and zero heat flux at bottom and top cavity surfaces. Rayleigh numbers were between 1000 and 106, i.e. flow was either laminar or turbulent, and a conduction, transition or boundary layer regime was applied. The study focuses on overall convective heat flow through the air layer. The average Nusselt numbers calculated as a function of Rayleigh numbers are compared with five correlations from the literature which are based mainly on experimental data. Except for one correlation, no calculated Nusselt number deviates more than 20% from the correlations. Deviation is even <10% for A=20. Therefore, this study improves the starting position for future applications of the code to more complex cases of facade elements, where less or even no experimental data are available in literature.