A detailed two-dimensional heat transfer analysis of complete fenestration systems was performed using computer models based on the Galerkin finite-element method. Laminar natural convection, coupled with radiative heat transfer in the cavity of the insulated glazing unit (IGU), was modelled simultaneously with conductive heat transfer in the solid parts (i.e., frame, glass panes, spacer, and sealants). Two prototype windows were developed for the purpose of this initial study - both wood, standard double-glazed windows. The overall dimensions of the prototype windows were 0.61m by 0.61m (24 in. by 24 in.) and 0.61m by 122m (24 in. by 48 in.). The two prototype systems have the same cavity width and gas fill (air), but they have different frame designs. Three different sets of boundary conditions were investigated, of which two incorporated fixed surface heat transfer coefficients on the indoor and outdoor window surfaces, and one incorporated variable coefficients. To validate the convective portion of the model, the glazing cavities from the two prototype models - subjected to a constant-temperature boundary condition on the vertical sides and zero heat flux on the horizontal sides - were modelled first and compared with other numerical and experimental results. In addition, a constant surface heat transfer coefficient was applied on the vertical boundaries of the interior glazing cavity, and the effects of radiation modelling inside the cavity were investigated. Component and overall U-factors for both prototype windows were then calculated and compared to the available experimental and conduction-only numerical results. Local heat fluxes and temperatures were plotted for the indoor and outdoor surfaces, and the effects of variable surface heat transfer coefficients were compared.

KEYWORDS: heat flow, heat transfer coefficient, windows, calculating, natural convection, double glazing, prototypes, surfaces, comparing, thermal conductivity, transmittance, temperature