1 Two-Dimensional Computational Fluid Dynamics and Conduction Simulations of Heat Transfer in Window Frames with Internal Cavities- Part 1: Cavities Only (original) (raw)
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Lawrence Berkeley National Laboratory, 2003
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 2 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.
Lawrence Berkeley National Laboratory, 2008
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 buildingcomponent 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.
Two-Dimension Conduction and CFD Simulations for Heat Transfer inHorizontal Window Frame Cavities
2005
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 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 withou...
Numeric Simulation of Heat Transfer Phenomena in Existing and Retrofitted Casement Windows
2020
Transparent elements of building enclosure components play a critical role in buildings' performance and integrity. Numeric simulation provides a tool to model and predict the thermal behavior of window constructions. This paper presents a two-dimensional numerical analysis of the thermal behavior of a casement window before and after implementing a vacuum glazing in the external pane. Two modelling approaches were considered, a conventional one (focusing predominantly on conductive heat transfer) and a more detailed one (involving coupled conduction and convection). The results of the study demonstrate the benefits of the aforementioned retrofit option in terms of reduced heat transfer, increase of surface temperature within the cavity during the cold season, and the reduction of surface condensation risk.
Journal of physics, 2015
Thermal performances of windows frames are established, in Europe, by the international standard UNI EN ISO 10077-2:2012. The standard introduces an equivalent thermal conductivity for air frame cavities thus simplifying the original combined heat transfer problem to a merely two-dimensional conductive one. The equivalence is referred to a rectangular cavity and is not able to fully recover the same radiative heat flux involved in the original problem. In view of that, the paper is focused on the radiative heat transfer taking place in the air cavities and aims to check if different equivalence criteria could lead to improved results. Thus, numerical tests involving an accurate description of radiative heat transfer in air cavities are compared to the simplified fully-conductive one provided by the standard. Results show that different criteria lead to quite different results. The optimal criterion turns out to depend on both geometrical and surface radiative parameters. It is also shown that, in any case, a proper radiative resistance but not the one suggested by the ISO 10077 should be adopted.
Journal of Building Physics, 2008
While window frames typically represent 20—30% of the overall window area, their impact on the total window heat transfer rates may be much larger. This effect is even greater in low-conductance (highly insulating) windows that incorporate very low-conductance glazing. Developing low-conductance window frames requires accurate simulation tools for product research and development. Based on a literature review and an evaluation of current methods of modeling heat transfer through window frames, we conclude that current procedures specified in ISO standards are not sufficiently adequate for accurately evaluating heat transfer through the low-conductance frames. We conclude that the near-term priorities for improving the modeling of heat transfer through low-conductance frames are: 1. Add 2D view-factor radiation to standard modeling and examine the current practice of averaging surface emissivity based on area weighting and the process of making an equivalent rectangular frame cavity....
Journal of Building Performance Simulation, 2016
Windows are a key factor for designing energy-efficient buildings, particularly the frame area that can produce high thermal bridging. This paper deals with accurately estimating heat transfer through window frames under fluctuating film coefficients. The one-dimensional frame conductance model traditionally used by building simulation programs is analysed and an alternative model is proposed, which takes into account the non-planar morphology of studied frames. This model shows a positive agreement with the results obtained from a two-dimensional heat-transfer simulation program, demonstrating that the thermal performance of high-conductance non-planar frames strongly depends on the ratio between the boundary surface area and the projected frame area. According to the results, the traditional conductance model seems to be suitable for all frames with a thermal transmittance lower than 5 W/m 2 K; however, frames with a U-factor higher than 6.2 W/m 2 K need an alternative conductance model that better reflects the 2D nature of frame sections.
Implications of Model Complexity for the Simulated Thermal Behavior of a Casement Window
IOP Conference Series: Earth and Environmental Science, 2019
Windows have been a subject of interest for research in building industry due to their multifaceted and significant implications for indoor environmental quality and energy use. In this context, the present contribution explores the retrofit opportunity toward retrofit of casement windows via application of vacuum glazing. The first scenario included conductive and convective heat transfer processes. The second, more detailed scenario took, in addition, the effects of (long-wave) radiation phenomena into account. The main objective of the present contribution is thus to contrast the results obtained from primarily conductive and convective computations with those that involve a detailed coupled conductive, convective, and radiation simulation. The study benefits from a CFD (Computational Fluid Dynamics) model to evaluate the airflow patterns (temperature and velocity) within the window’s interstitial space by assuming isothermal boundary conditions. Based on simulation results, the ...
In this paper, two ways to improve the PVC window frame thermal transmittance without frame geometrical dimension and material variations are presented. The first variation considered relies on inserting polyurethane foam into the air gaps. The second variation counts on low-emissivity coating on PVC surfaces in the frame air gaps. To investigate these modifications two-dimensional CFD simulations of PVC window frames were used which then were validated by measurements performed in a calorimetric chamber. A hot box methodology was implemented for the measurements. The experimental work was focused on verification of simulation results of modeled frame thermal transmittances. A calorimetric chamber was used, consisting of a metering box, simulating indoor conditions (warm side), and a climate box, simulating outdoor conditions (cold side). It was concluded that the air gap filling with polyurethane foam in window frames can reduce window frame thermal transmittance by about 27% while covering PVC surfaces with low-emissivity coating can reduce window frame thermal transmittance by about 28%. These variations do not change the frame geometry or its total thickness. To answer the question what would be the expected impact of proposed frame variations on annual energy demand of different types of buildings additional simulations of two buildings with a Design-Builder with EnergyPlus engine software were performed for a set of different climatic conditions.
Key elements of and material performance targets for highly insulating window frames
Energy and Buildings, 2011
The thermal performance of windows is important for energy efficient buildings. Windows typically account for about 30-50 percent of the transmission losses though the building envelope, even if their area fraction of the envelope is far less. The reason for this can be found by comparing the thermal transmittance (U-factor) of windows to the U-factor of their opaque counterparts (wall, roof and floor constructions). In well insulated buildings the U-factor of walls, roofs and floors can be between 0.1 and 0.2 W/(m 2 K). The best windows have U-factors of about 0.7-1.0. It is therefore obvious that the U-factor of windows needs to be reduced, even though looking at the whole energy balance for windows (i.e., solar gains minus transmission losses) makes the picture more complex.