ANA LYSIS OF THE FLOW AND THERMAL FIELDS IN BOBTAIL ROOFS HEATED FROM THE BASE WALL

Abstract

Analysis of airflow and thermal characteristics of attics of bobtail-shaped pitched roofs heated
through a horizontally suspended ceiling is numerically carried out in this study. Pitch angles
of 14o, 18o, 30o, and 45o within the standard pitch roof range are selected. The configuration
falls within Rayleigh number 3.19 x 105  Ra  2.04 x 107. A finite-volume CFD code was
used to solve the mass, momentum, and energy conservation equations governing the
problem. The results obtained indicate a strong influence on the shape and angle of the roof.
At lower roof pitches, the flow field is characterized by multiple counter-rotating vortices
asymmetrically arranged within the enclosures. Eight cells in the 14o enclosure were reduced
to five in the 45o roof pitch. The size and rotating strength of a vortex increase from the left
corner to the middle of the enclosures. At higher pitch angles, the vertical wall obstructed the
flow leading to a number of distorted cells. The maximum velocity within the aerodynamic
boundary layer along the base wall occurs at Y=0.02 with the values U=0.013 and U=0.028
in the 14o and 45o enclosures respectively. The thermal field portrays a convection system of
rising hot plumes from the base wall and descending cold jets from the inclined walls; all
enclosed by thin boundary layers along the walls. Graphical plots of velocity and temperature
variations along some cross-sections within the enclosures enable the prediction of some
important heat and flow parameters.