|
...are compared; results from computational fluid dynamics (CFD) simulations with detailed radiation modeling are used as reference.
Four different frames were studied, two 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 5 mm (ISO 15099), and in some cases subdividing cavities with interconnections smaller than 7 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%). A maximum difference of 8.5% 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 incorporating these strategies may be more important for improving the accuracy of results than the use of CFD modeling for horizontal cavities.
INTRODUCTION
Window frames made of polyvinyl chloride (PVC) or aluminum contain air cavities that have a significant effect on the insulation capabilities of the frames. To understand total frame thermal conductance, it is important to accurately characterize the thermal impact of air cavities. The importance of these cavities is of particular concern in more highly insulating products.
This paper assesses the accuracy of the simplified frame cavity conduction/convection and radiation models presented in ISO 15099 and used in software (e.g., Blomberg [2000], Enermodal [2001], and Finlayson et al. [1998]) for rating and labeling window products. Temperatures and U-factors for several typical horizontal window frames that have internal cavities are compared; results from computational fluid dynamics (CFD) simulations with detailed radiation modeling are used as a reference. Unless otherwise noted, the term "CFD," as used below, includes detailed ray-tracing-based radiation modeling as well as direct modeling of convective heat transfer.
Four different window frames were examined, two made of polyvinyl chloride (PVC) and two made of aluminum. For each frame, six different simulations were performed, two with a CFD code and four with a building-component thermal-simulation tool that uses the finite element method (FEM). The FEM tool addresses convection using correlations from ISO 15099; it addresses radiation either with correlations from ISO 15099 or by using a detailed, view-factor-based radiation model. CFD calculations were performed 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.
Various sources prescribe rules for using building-component thermal simulation programs to simulate heat transfer in window frames (ASHRAE 1996; CEN 2003; ISO 2003). In a companion paper (Gustavsen et al. 2005), horizontal frame cavities were studied to assess the accuracy of current ISO 15099 procedures for modeling convection. In that analysis, the agreement between CFD modeling results and the results of the simplified models was moderate for heat transfer rates through frame cavities. The differences may be a result of the underlying ISO 15099 Nusselt number correlations being based on studies in which cavity height/length aspect ratios were smaller than 0.5 and greater than 5 (with linear interpolation assumed in between).
In addition, the results presented in Gustavsen et al. (2005) indicate that subdivision of complex cavities with small interconnections, as prescribed in the ISO 15099, is justified. However, the data in that study suggest that horizontal cavities with interconnections smaller than 7 mm should be subdivided, in contrast to the ISO 15099 rule, which sets the break point at 5 mm. In this paper, the focus is on how these different break points for subdividing the cavities influence the simulated U-factors and temperature distributions of real/realistic horizontal frames. When using the building component simulation code, the frame cavities were or were not subdivided as follows: in case 3, the cavities were not divided; in case 4, the cavities were divided when the interconnections were smaller than 5 mm; and in case 5, the cavities were divided when the interconnections were smaller than 7 mm. In addition, case 6, which was identical to case 4,was simulated using view factors instead of the ISO 15099 radiation model to calculate radiation.
The results presented in this paper are only for horizontal frame members because convection in vertical jambs requires three-dimensional CFD simulations that are beyond the scope of this project.
WINDOW FRAMES
The four simulated window frames that we studied are shown in Figures 1 and 2. Figure 1 shows, on the left, a PVC frame with complex cavities and, on the right, a relatively simple aluminum frame. This particular aluminum frame, which has a thermal break,...
NOTE: All illustrations and photos
have been removed from this article.
More articles from ASHRAE Transactions
Performance criteria for residential zero-energy windows., January 01, 2007
Re-inventing the wheel in HVAC & R technology: history does repeat!(he..., January 01, 2007
Carl von Linde--scientist, inventor, global player.(Biography), January 01, 2007
The heating system of the Roman baths., January 01, 2007
Best practices for data center thermal and energy management--review o..., January 01, 2007
Looking for additional articles?
Search our database of over 3 million articles.
Looking for more in-depth information on this industry?
Search our complete database of Industry & Market reports by text, subject, publication
name or publication date.
About Goliath
Whether you're looking for sales prospects, competitive information, company
analysis or best practices in managing your organization,
Goliath can help you meet your business needs.
Our extensive business information databases empower business
professionals with both the breadth and depth of credible,
authoritative information they need to support their business
goals. Whether it be strategic planning, sales prospecting,
company research or defining management best practices -
Goliath is your leading source for accurate information.
|
%20%20&position=1)