Turbulent flow in an enclosed cavity or box is a model for many flows of practical interest: heating of a room; flow in a double glazing unit or ventilation, that are encountered in a number of situations of practical importance in our every day encounters. A three dimensional enclosure in the form of a rectangular enclosure contains a convectional heater built into one wall and having a window in the same wall. The heater is located below the window and the remain walls are insulated. The size of the window is varied while its position is fixed, and the heater position and size remain constant. The localized heating and cooling induces two boundary layers that collide in the region between the window and the heater. The timed averaged equations for continuity, momentum and energy, which are coupled to the turbulence equations, were solved using a finite difference technique. The vorticity vector potential formulation has been employed. A further use of difference false transient factors in different flow regions coupled with non-linear partial differential equations has been employed to hasten convergence of the numerical solution. The results were that the enclosure is stratified into three regions: a cold upper region, a hot region in the area between the heater and the window and a warm lower region. (C) 2003 Elsevier Ltd. All rights reserved.