Modeling the thermal performance of windows using a two-dimensional finite volume model

Abstract

The objective of this research is the numerical determination of overall heat transfer coefficients (U-Factor), solar heat gain factors (SHGF), and surface temperature profiles of windows. The program used takes account of the presence of the natural convection, the radiation between panes, and the thermal effects of the frame. Temperature profiles are useful for the calculation of thermal stresses in glazings and the prediction of condensation problems.

Results are compared with measurements and the numerical calculation of others. In benchmark solution comparisons errors, are less than 10^-2%. Comparisons with the numerically determined heat flux through a cavity bounded by two sheets of glass and spacer bars, i.e., a simple window, differed by less than 1%. In a comparison between simulated solar heat gain factors and solar calorimeter experimental results very good agreement is obtained for nine samples, with errors of 2.5% or less for seven samples, and errors of 12% and 5% for the other two.

From the simulation of more realistic windows, both geometrically and with respect to boundary conditions this work yielded the following observations: In modeling an entire wood frame window the error in U-factor is 0.38% while the maximum surface temperature difference is less than 0.5%; A quantitative comparison with a set of thermographic measurements for a flush mounted insulated glazing unit (IGU) yielded satisfactory results for clear double glazed units with the center glass temperature differing by about 1oC, and very good agreement for the triple glazed units and the double low-E unit with the center glass temperature differing by less than 0.25oC; And, the effects of heat transfer coefficients in flush and recessed mounted IGU's clearly reveal the importance of modeling a locally varying heat transfer coefficient.