Derivation of a theoretical model to explain the functioning of a window as a pre-heat ventilation device and its verification using physical models

‘Supply air’ windows are designed with an air gap between the inner panes of glass that is used as the incoming air path for room ventilation; air is pre-warmed within the window and thereby avoids the sensation of draughts. A series of tests, verified by model simulations, were carried out to determine those aspects of the window’s specification that govern the extent to which air is pre-warmed by the windows before entering rooms. The first was a laboratory experiment investigating the influence that the width of gap between the glass panes had on the pre-heating of the airflow, the results from which were simulated using an algebraic model based on fluid dynamics principles. The following experiments were carried out in test cells analysing the influence of the location of a low emissivity coating within alternative glazing assemblies, the relationship between pre-heat temperature and ambient temperature, and its variation with ventilation rate. The later stages were modelled by the use of the Computational Fluid Dynamics program FLOvent. The outputs of these experiments and simulations has enabled a clearer understanding of the physical processes at work, and the development of simulations that accurately predict the window’s performance as a pre-heat device.