Convection at the indoor side of complex fenestration systems: the ASHWAT model revisited

This paper examines the three-temperature problem of convection at the indoor side of a complex fenestration system (CFS) based on recent theoretical developments, namely the extended Newton formulation and the dQdT technique. CFD solutions were obtained for natural convection at the indoor side of various CFS configurations. The dQdT technique was then implemented numerically to obtain the paired heat transfer coefficients of each configuration. The results were used to assess the approximate relations used in the ASHRAE Window ATachment (ASHWAT) tool. The comparison shows that while there is remarkable agreement between the ASHWAT estimates and dQdT results for roller blinds, discrepancies exist between results for venetian blinds. Furthermore, although use of a delta resistor network to model convection at the indoor side is valid and relatively accurate for roller blinds, the application of this model to CFSs with venetian blinds requires additional levels of approximation. Nevertheless, the heat transfer rates calculated based on the approximate resistor-network model are in close agreement with the CFD results.