A numerical study of the effect of a below window hot air vent on the convective heat transfer rate from a cold window covered by a top-down, bottom-up plane blind system

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

When heating systems are in use during cold winter months hot air from a floor-mounted vent located below a window will often flow over the cold window. The nature of the air-flow pattern over the window tends to be changed by the presence of the vent flow thus altering the rate of convective heat transfer to the window. The effect of the vent flow on the convective heat transfer rate from the window will be influenced by the presence of a blind system over the window. In recent times top-down, bottom-up plane blind systems in which the blind can both be raised at the bottom and lowered at the top have become popular. In the present study the effect of the hot air flow from a below window vent on the heat transfer rate from a recessed cold window covered by a top-down, bottom-up plane blind system has been numerically studied. The window, represented by a plane isothermal section recessed into the wall, is colder than the room air far from the window. It is assumed that the floor-mounted vent is located against the wall and has a uniform discharge velocity which is normal to the vent surface. The flow has been assumed to be steady. Situations involving both laminar and turbulent flow have been considered. The fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. This was dealt with using the Boussinesq approach. Radiant heat transfer effects have been neglected. The governing equations have been solved using the commercial CFD code ANSYS FLUENT©. The k-epsilon turbulence model with buoyancy force effects fully accounted for was used. Results have been obtained for a Prandtl number of 0.74, i.e., effectively the value for air. The effect of the dimensionless top and bottom openings on the window Nusselt number for a wide range of Reynolds and Rayleigh numbers has been studied.