Windows and Daylighting Group

U2 - LBNL-1132E ER - TY - CONF T1 - Two-Dimension Conduction and CFD Simulations for Heat Transfer in Horizontal Window Frame Cavities T2 - 2005 ASHRAE Winter Meeting Y1 - 2005/02// A1 - Arlid Gustavsen A1 - Dariush K. Arasteh A1 - Christian Kohler A1 - Dragan C. Curcija AB - Accurately 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. JF - 2005 ASHRAE Winter Meeting CY - Orlando, FL VL - 111 U1 -Windows and Daylighting Group

U2 - LBNL-61250 ER - TY - CONF T1 - Two-Dimensional Computational Fluid Dynamics and Conduction Simulations of Heat Transfer in Window Frames with Internal Cavities - Part 1: Cavities Only T2 - ASHRAE Winter Meeting Y1 - 2005/02// A1 - Arlid Gustavsen A1 - Christian Kohler A1 - Dariush K. Arasteh A1 - Dragan C. Curcija AB - Accurately 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. JF - ASHRAE Winter Meeting CY - Orlando, FL U1 -Windows and Daylighting Group

U2 - LBNL-52509 ER - TY - JOUR T1 - Three-Dimensional Conjugate Computational Fluid Dynamics Simulations of Internal Window Frame Cavities Validated Using IR Thermography JF - ASHRAE Transactions Y1 - 2001/06// SP - 538 EP - 549 A1 - Arlid Gustavsen A1 - Brent T. Griffith A1 - Dariush K. Arasteh AB - This 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. CY - Cincinnati, Ohio VL - 107 IS - 2 U1 -Windows and Daylighting Group

U2 - LBNL-46825 ER -