Investigation of the Thermal Performance of a Well Insulated Window and its Influence on the Heating and Ventilation Systems

  
Project leader: Professor Bahram Moshfegh
PhD student: Ulf Larsson
Supported by: University of Gävle and KK-Foundation

KK-Fpundation logga

Background


Windows are crucial for people's experience of the indoor climate, especially in the Nordic Countries with cold climate and short days during the winter. The current trend to reduce the heat losses by building components (in order to reduce the energy consumption) has resulted in many design modifications of windows in order to improve the thermal performance. Modifications have provided both positive and negative effects on the thermal climate and the perception of the windows. For example, well-insulated windows provide a higher surface temperature of the pane, a lower cold radiation and a weak downdraught, but on the other hand they reduce the penetration of daylight and separate people from outdoor surroundings.

The aim

The objective of this project is divided into two tasks. First the effect of the thermal performance of the window and the size of the window's niche on the downdraught from the window will be investigated. In the second task, the interaction between the thermal performance of the window, heating and ventilation systems on the indoor climate will be analysed. Both experimental and numerical methods will be used for the analysis. The measurements will be carried out in an well-insulated room in the laboratory hall at the Department of Built Environment, Royal Institute of Technology, Gävle, Sweden. The simulation of the temperature and velocity distribution in the room is performed by solving the governing equations for the conservation of mass, momentum, energy and the radiative heat exchange between the surfaces by the finite element method. The measurements will be used for validation of the model for the numerical calculations.

Presented results


The first part of the project was to investigate the thermal performance of a triple glazed window with emissive layers and the inert gas krypton inside the closed spaces between the glasses. The intention of that investigation was to analyse, both numerically and experimentally, the simultaneously acting mechanisms of conduction, convection and radiation heat transfer through the window construction. The measurements are performed for thermal boundary conditions which are relevant for Nordic Countries for winter time. Furthermore, a mathematical model, which can be used for heat transfer analysis through the window construction have been developed. The flow and temperature characteristics through the window construction are also obtained by solving the governing equations for the conservation of mass, momentum, energy and the radiative heat exchange between the surfaces numerically by the finite method. The results of this part of investigation are presented in the following paper, "Thermal Analysis of Super Insulated Windows", are publicated in the Energy and Building, Vol. 29, pp. 121-128, 1999.  

Based on numerical calculations within the enclosure in a window construction a new correlation for the Nusselt number in a rectangular enclosure has been obtained. The numerical calculations have been performed for a physical model with two vertical panes with constant surface temperatures. The correlation for the Nusselt number is a function of the aspect ratio and the Rayleigh number. The correlation covers aspect ratios and Rayleigh numbers that are of interest for window constructions. The numerical results have been validated based on numerical studies and measurements from other investigators. The results from this work are presented in the following paper, "Numerical convection within a Rectangular Enclosure in a Window Construction", which where presented at the CIB World Building Congress 1998.  

The effect of thermal performance of the window is investigated on the downdraught for different supply air velocity and different sizes of the niche of the window. Temperatures and air velocities are measured in the area below the window and in the occupied zone close to the floor. The results of this project are presented in the following paper, “ Experimental Investigation of Downdraught from Well-Insulated Windows", which is submitted for publication in Building and Environment, The International Journal of Building Sciences and its Application. Also a conference paper from this part of the project with the title, “Effect of Window Bay on the Downdraught from a Well-Insulated Window" and is published in the Proceedings of the RoomVent 2000, 9-12 July 2000, Reading, UK. At the end of 1999 are The Licentiate Thesis “Thermal Analysis of Well-Insulated Windows". was presented in 1999.

Future work


In the future the influence of window on the indoor climate will be investigated numerically. The analysis will include the effect of the size of the window's niche, thermal performance of the window, heating and ventilation systems on the downdraught from the window and indoor climate. A three-dimensional model will be used for numerical simulation of the flow and heat transfer in the room. The numerical model will include the governing equations for the conservation of mass, momentum, energy and the equation for the radiative heat exchange between the surfaces in the room. The commercial CFD codes Fluent and Phoenics will be used for the simulation. The results for this part of the research are intended to be published both in an International journal and also a conference paper presumably in CMEM 2001, 4-6 June, Alicante, Spain.

Publications

  • U. Larsson, B. Moshfegh and M. Sandberg, "Thermal Analysis of Super Insulated Windows", Energy and Buildings, Vol. 29, pp. 121-129, 1999.
  • U. Larsson, B. Moshfegh and M. Sandberg, "Natural convection within a Rectangular Enclosure in a Window Construction", Proceedings of CIB World Building Congress, 1998.
  • U. Larsson and B. Moshfegh, "Experimental Investigation of Downdraught from Well-Insulted Windows", submitted for publication in Building and Environment, The International Journal of Building Sciences and its Application, 2000.
  • U. Larsson and B. Moshfegh "Effect of Window Bay on the Downdraught from a Well-Insulated Window, Proceedings of RoomVent 2000 Conference, 9-12 July 2000, Reading, UK.
  • U. Larsson “Thermal Analysis of Well-Insulated Windows (Numerical and Experimental Investigations)" Licentiate Thesis, ISBN 91-7170-495-7, 1999.
Publicerad av: Liselotte Laurila Sidansvarig: Gunilla Mårtensson Sidan uppdaterades: 2013-10-17
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