Fluid Flow and Heat Transfer Characteristics of Clerestory-Shaped Attics Heated from Below

Abstract

In this study, a finite volume analysis of the aerodynamics and heat transfer in attics of a clerestory roof design with pitch angles 14o, 18o, 30o and 45o and Rayleigh number range 3 x 105≤ Ra ≤ 2 x 107 is carried out. The shape of the enclosure has strong influence on the structure of the flow and temperature fields. The flow field is characterised by counter-rotating vortices enclosed by aerodynamic boundary layers. The size, strength and direction of rotation of the cells are controlled by the forces propelling the thermal plumes and the cold jets. The reduction of the number and size of the counter-rotating cells and their formation within the enclosures provide an analogous reduction of the total heat transfer rate as the roof pitch angle increases. The velocity and temperature profiles across midheight and midlength of the enclosures enable the prediction of appropriate position in the attic with the right condition to place sensitive items. On the heat transfer, the relationship between the mean Nusselt number and the Rayleigh number is presented in form of a correlation. Results obtained in the study are of significance to building engineers engaged in the analysis and design of building attics and tropical agriculturalists for the control of produce drying rates. Keywords: Heat transfer, heated below, natural convection, pitch roof, clerestory-shaped