This paper assesses the accuracy of the simplified frame cavity conduction/convection and radiation models presented in ISO 15099 and used in software for rating and labeling window products. Temperatures and U-factors for typical horizontal window frames with internal cavities are compared; results from Computational Fluid Dynamics (CFD) simulations with detailed radiation modeling are used as a reference.

Four different frames were studied. Two were made of polyvinyl chloride (PVC) and two of aluminum. For each frame, six different simulations were performed, two with a CFD code and four with a building-component thermal-simulation tool using the Finite Element Method (FEM). This FEM tool addresses convection using correlations from ISO 15099; it addressed radiation with either correlations from ISO 15099 or with a detailed, view-factor-based radiation model. Calculations were performed using the CFD code with and without fluid flow in the window frame cavities; the calculations without fluid flow were performed to verify that the CFD code and the building-component thermal-simulation tool produced consistent results. With the FEM-code, the practice of subdividing small frame cavities was examined, in some cases not subdividing, in some cases subdividing cavities with interconnections smaller than five millimeters (mm) (ISO 15099) and in some cases subdividing cavities with interconnections smaller than seven mm (a breakpoint that has been suggested in other studies). For the various frames, the calculated U-factors were found to be quite comparable (the maximum difference between the reference CFD simulation and the other simulations was found to be 13.2 percent). A maximum difference of 8.5 percent was found between the CFD simulation and the FEM simulation using ISO 15099 procedures. The ISO 15099 correlation works best for frames with high U-factors. For more efficient frames, the relative differences among various simulations are larger.

Temperature was also compared, at selected locations on the frames. Small differences was found in the results from model to model.

Finally, the effectiveness of the ISO cavity radiation algorithms was examined by comparing results from these algorithms to detailed radiation calculations (from both programs). Our results suggest that improvements in cavity heat transfer calculations can be obtained by using detailed radiation modeling (i.e. view-factor or ray-tracing models), and that incorporation of these strategies may be more important for improving the accuracy of results than the use of CFD modeling for horizontal cavities.

%B 2007 ASHRAE Winter Meeting %C Dallas, TX %8 01/2007 %G eng %1Windows and Daylighting Group

%2 LBNL-1132E %0 Conference Paper %B 2005 ASHRAE Winter Meeting %D 2005 %T Two-Dimension Conduction and CFD Simulations for Heat Transfer in Horizontal Window Frame Cavities %A Arlid Gustavsen %A Dariush K. Arasteh %A Christian Kohler %A Dragan C. Curcija %XAccurately analyzing heat transfer in window frames and glazings is important for developing and characterizing the performance of highly insulating window products. This paper uses computational fluid dynamics (CFD) modeling to assess the accuracy of the simplified frame cavity conduction/convection models presented in ISO 15099 and used in software for rating and labeling window products. Three representative complex cavity cross-section profiles with varying dimensions and aspect ratios are examined. The results presented support the ISO 15099 rule that complex cavities with small throats should be subdivided; however, our data suggest that cavities with throats smaller than 7 mm should be subdivided, in contrast to the ISO 15099 rule, which places the break point at 5 mm. The agreement between CFD modeling results and the results of the simplified models is moderate for the heat transfer rates through the cavities. The differences may be a result of the underlying ISO 15099 Nusselt number correlations being based on studies where cavity height/length aspect ratios were smaller than 0.5 and greater than 5 (with linear interpolation assumed in between). The results presented here are for horizontal frame members because convection in vertical jambs involves very different aspect ratios that require three-dimensional CFD simulations.

%B 2005 ASHRAE Winter Meeting %C Orlando, FL %V 111 %8 02/2005 %G eng %L LBNL-61250 %1Windows and Daylighting Group

%2 LBNL-61250 %0 Conference Paper %B ASHRAE Winter Meeting %D 2003 %T Two-Dimensional Computational Fluid Dynamics and Conduction Simulations of Heat Transfer in Window Frames with Internal Cavities - Part 1: Cavities Only %A Arlid Gustavsen %A Christian Kohler %A Dariush K. Arasteh %A Dragan C. Curcija %XAccurately analyzing heat transfer in window frame cavities is essential for developing and characterizing the performance of highly insulating window products. Window frame thermal performance strongly influences overall product thermal performance because framing materials generally perform much more poorly than glazing materials. This paper uses Computational Fluid Dynamics (CFD) modeling to assess the accuracy of the simplified frame cavity conduction/convection models presented in ISO 15099 and used in software for rating and labeling window products. (We do not address radiation heat-transfer effects.) We examine three representative complex cavity cross-section profiles with varying dimensions and aspect ratios. Our results support the ISO 15099 rule that complex cavities with small throats should be subdivided; however, our data suggest that cavities with throats smaller than seven millimeters (mm) should be subdivided, in contrast to the ISO 15099 rule, which places the break point at five mm. The agreement between CFD modeling results and the results of the simplified models is moderate. The differences in results may be a result of the underlying ISO correlations being based on studies where cavity height/length (H/L) aspect ratios were smaller than 0.5 and greater than five (with linear interpolation assumed in between). The results presented here are for horizontal frame members because convection in vertical jambs involves very different aspect ratios that require three-dimensional CFD simulations. Ongoing work focuses on quantifying the exact effect on window thermal performance indicators of using the ISO 15099 approximations in typical real window frames.

%B ASHRAE Winter Meeting %C Orlando, FL %8 02/2005 %G eng %L LBNL-52509 %1Windows and Daylighting Group

%2 LBNL-52509 %0 Journal Article %J ASHRAE Transactions %D 2000 %T Three-Dimensional Conjugate Computational Fluid Dynamics Simulations of Internal Window Frame Cavities Validated Using IR Thermography %A Arlid Gustavsen %A Brent T. Griffith %A Dariush K. Arasteh %XThis paper studies the effectiveness of one commercial computational fluid dynamics (CFD) program for simulating combined natural convection and heat transfer in three dimensions for air-filled cavities similar to those found in the extruded frame sections of windows. The accuracy of the conjugate CFD simulations is evaluated by comparing results for surface temperature on the warm side of the specimens to results from experiments that use infrared (IR) thermography to map surface temperatures during steady-state thermal tests between ambient thermal chambers set at 0 °C and 20 °C. Validations using surface temperatures have been used in previous studies of two-dimensional simulations of glazing cavities with generally good results. Using the techniques presented and a noncontact infrared scanning radiometer we obtained surface temperature maps with a resolution of 0.1 °C and 3 mm and an estimated uncertainty of +/-0.5 °C and +/-3mm. Simulation results are compared to temperature line and contour plots for the warm side of the specimen. Six different cases were studied, including a simple square section in a single vertical cavity and two four-sided frame cavities as well as more complex H- and U-shaped sections. The conjugate CFD simulations modeled the enclosed air cavities, the frame section walls, and the foam board surround panel. Boundary conditions at the indoor and outdoor air/solid interface were modeled using constant surface heat-transfer coefficients with fixed ambient-air temperatures. In general, there was good agreement between the simulations and experiments, although the accuracy of the simulations is not explicitly quantified. We conclude that such simulations are useful for future evaluations of natural convection heat transfer in frame cavities.

%B ASHRAE Transactions %C Cincinnati, Ohio %V 107 %P 538-549 %8 06/2001 %G eng %N 2 %L LBNL-46825 %1Windows and Daylighting Group

%2 LBNL-46825