Low Turbulence Natural Convection In Triangular Enclosures

ABSTRACT

In many types of buildings, both single and multi-storey, including schools, offices, places of worship, factories and residential buildings, the attic space is often given paramount consideration because its thermal characteristics have great influence on the conditions of the space directly below it. A significant amount of the cooling load in residential and industrial buildings is the result of heat transfer across the ceiling from the attic. Also, in some rural areas in the tropics, agricultural products are sometimes kept in the attic either for drying or for storage. It is, therefore, necessary to have thorough knowledge of the fluid flow pattern and heat transfer characteristics of the attic space in realistic conditions. A long, air-filled, horizontal enclosure with isosceles triangular cross-section representing the actual-size attic of a pitched roof is considered for both summer and winter conditions under isothermal and isoflux cases for the former and isothermal case only for the latter. The enclosure extension in the direction perpendicular to the cross-section is more than double its width so that the flow and the heat transfer are taken to be two-dimensional. Three turbulence models, viz., the differential Reynolds stress, the shear stress transport, and the standard ke based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations subject to Boussinesq approximation are employed in the computational fluid dynamics (CFD) simulation technique adopted. Four aspect ratios representing different pitch angles of 180, 2216", 45°, and 60° are used with thermal boundary conditions that result in high Rayleigh number (Ra) range 10's Ras 10", considered to simulate a real size attic. Since reported experimental results on turbulent natural convection in triangular enclosures are unavailable, the solution procedure employed is validated by simulating the case of low turbulence natural convection in a differentially heated 50-10°C square enclosure with highly conducting horizontal walls for which there are recent experimental results.