Numerical simulation of heat transfer through hollow bricks in the vertical direction

Abstract

This article focuses on the numerical modeling of the heat transfer in vertical cavities with small cross-sectional areas in hollow bricks heated from below by means of the computational fluid dynamics (CFD) analysis. The major aim is to specify the ratios between the equivalent thermal conductivities in vertical and horizontal directions (λ eq,v/λeq,h) for various types of hollow brick masonry. These ratios are not given by brick producers, though they are very important when assessing certain types of thermal bridges. This article presents the governing equations for CFD analysis, together with the main assumptions and boundary conditions. The validation of the FLOVENT CFD commercial code is also discussed, as are the effects of calculation mesh refinement. The results of the first analysis — vertical heat transfer in a single high cavity — show a strong influence of the cross-sectional area of the cavity on the natural convection. While the convective heat transfer for the heat flow in the downward direction is negligible for all considered cross-sectional areas, the natural upward convection disappears only for very high and narrow cavities. Such effects can also be seen in the results of the calculation of the ratio between the equivalent thermal conductivities in the vertical and horizontal directions for the model masonry or for actually produced hollow brick masonry. This ratio is smaller than 1.0 for downward heat flow and between 1.0 and 1.5 for upward heat flow in bricks with a small number of large cavities. By contrast, bricks with a large number of small cavities show almost the same ratio for both vertical directions of heat flow (from 2.2 to 2.7 depending on the actual honeycomb structure).