Free convection flow and heat transfer within attics in cold climate

The transitional free convection flow and heat transfer within attics in cold climate are investigated using three-dimensional numerical simulations for a range of Rayleigh numbers from 103 to 106 and height-length ratios from 0.1 to 1.5. The development process of free convection in the attic could be classified into three stages: an initial stage, a transitional stage and a fully developed stage. Flow structures in different stages including transverse and longitudinal rolls are critically analyzed in terms of the location and strength of convection rolls and their impacts on the heat transfer. The transition to unsteady flow and asymmetry flow in the fully developed stage is discussed for the fixed height-length ratio 0.5. Various flow regimes are given in a bifurcation diagram in the parameter space of Rayleigh numbers (102<Ra<107) for height-length ratios (0.1<A<1.5). The time series of heat transfer rate through the bottom wall is quantified for different height-length ratios. The overall heat transfer rate for the low Prandtl fluid (Pr=0.7) could be enhanced based on three-dimensional flow structure.