Effect of wall conduction on natural convection heat transfer in attic spaces / by Alaa Hussein Abdel Samad.

Abstract

In this study, steady natural convection heat transfer in trapezoidal enclosures is investigated numerically under winter-like and summer-like boundary conditions. These enclosures represent attic spaces with pitched roofs that are widespread in Lebanon. A two-dimensional model of the attic space is used to study the effect of the walls and ceiling conductivities on the natural convection heat transfer within the attic. During winter conditions the maximum height of the attic is considered to be 2 m with a span of 15 m. With the prevailing weather conditions in Lebanon, during winter the base wall is heated at 295 K whereas the vertical and inclined walls are exposed to an ambient wind stream at 276 K. On the other hand during summer conditions, the maximum height of the attic is considered to be 1 m with a span of 7.5 m. The base wall is cooled at 295 K while the vertical and inclined walls are exposed to an ambient air stream at 305 K. The thermal and geometrical conditions that are considered here lead to Rayleigh numbers in the order of 10⁹ and 10⁸ under winter and summer boundary conditions respectively. Using the two-dimensional model, different walls conductivities are studied for two ceiling assemblies having U-values of 3.12 and 0.49 W-m2°K, representing, respectively, conventional non-insulated and recommended roof by the Lebanese thermal standard. A three-dimensional model representing the attic space is also developed to study the actual flow during summer conditions. For computational analysis, turbulence is modeled using a low-Reynolds number k-omega model with the governing equations discretized using a finite-volume approach. The Semi-implicit method for pressure linked equation (SIMPLE) algorithm is employed to resolve the pressure-velocity coupling with the convection terms discretized using the second order upwind scheme. For every case studied, the average heat transfer rates are calculated and presented to underscore the differences. Moreover flow patterns and isotherms ar