SF-98-03-4 -- Guidelines for Modeling Projecting Fenestration Products

Heat transfer patterns in projecting fenestration products (greenhouse windows, skylights, etc.) are different from those in typical planar window products. The projecting surfaces often radiate to each other, thereby invalidating the commonly used assumption that fenestration product interior surfaces radiate to a uniform room air temperature. The convective portion of the surface heat transfer coefficient also is significantly different from the one used with planar geometries and is even more dependent on geometry and location. Projecting fenestration product profiles must, therefore, be modeled in their entirety. This paper presents the results of complete cross-sectional, variable film coefficient, two-dimensional heat transfer modeling of two greenhouse windows using the next generation of window-specific heat transfer modeling tools. The use of variable film coefficient models is shown to increase the accuracy with which simulation tools can compute U-factors. Simulated U-factors also are determined using conventional constant film coefficient algorithms. The results from both sets of simulations are compared with measured values.

Units: SI