Laminar Natural Convection in Attics of Rooftops with Depressed Walls

Abstract

Laminar natural convection in the attics of rooftops with depressed upper walls and heated base wall has been investigated numerically using a finite volume CFD package. Selected roofs referred to as Combination, Clerestory and Butterfly roofs are compared with standard isosceles triangular roof having the same pitch and base length. The results obtained show that the depressed wall distorts the multicellular air movement pattern within the attics. It compressed the cells, thereby reducing their sizes and damping their rotation. The depression brings the upper walls closer to the base wall resulting in further breakdown of the convection cells, distortion of the cell shape, modification of the velocity and temperature distribution, higher air pressure and increased heat transfer rate within the area under the depressed walls. Overall, the mean heat transfer rate along the base wall increases with the depth and length of the depressed wall. It is therefore recommended that roof designers should apply caution with depressed walls in order to minimize heat exchange across the ceiling.