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Article Excerpt
INTRODUCTION
Occupants near windows often experience thermal discomfort. During the winter, window interior temperatures usually fall below indoor air temperature, causing thermal discomfort due to radiant temperature asymmetry, low operative temperature, and downdraughts. High-performance windows provide solutions to these problems and improve glazing interior surface temperatures in winter. The main HVAC system, which is usually variable air volume, provides ventilation, cooling, and often some heating as well. In cold climates, perimeter heating is traditionally used in office buildings during cold periods as a secondary heating system near fenestration in order to improve thermal comfort near facades and to avoid oversized HVAC systems. However, as this paper will show, by utilizing a high-performance envelope, the main HVAC system can supply the necessary heating without a secondary heating system while ensuring good comfort conditions.
Perimeter heating systems can be convective or radiative. Electric baseboard heaters are most common; they are usually placed under the windows and exchange heat with the air by convection and with the person, glass, and room surfaces by radiation. The warm air rises and increases window interior surface temperatures, thus reducing thermal discomfort. Radiant ceiling/floor panels are sometimes used instead. These electrically-heated panels exchange heat by radiation with the person and the surfaces of the windows, walls, floor, and furniture. Another variation is electrically-heated windows. In this case, the window surface itself is heated with a current provided on a film deposited on the interior glass between the glass panes. Therefore, the glass surface is directly heated, and thermal discomfort due to radiation exchange can be eliminated.
The first part of this paper investigates and compares the performance of three different perimeter heating systems and presents air velocity and temperature gradients near a glazed facade with different perimeter heating variations (baseboard heaters, radiant ceiling panels, and heated windows) in a controlled test chamber. Although air turbulence affects occupant thermal comfort, this impact was not considered in the present study. The temperature field was monitored using a grid of thermocouples, while air speed near the facade was measured using a particle image velocimetry (PIV) system mounted on a traverse (Karava et al. 2007). The PIV technique was applied for the first time for measurements near a full-scale facade. Previous work by Hosni and Jones (2002) considered application of PIV in a large-scale room but focused on the evaluation of air velocity field around a standing person. It is the first time that this type of configuration is used for measurements near full-scale facades. The effect of the presence of a roller shade on the window was also studied. Following the experimental measurements, a three-dimensional numerical CFD model was developed in order to validate the software and be able to generalize temperature and air velocity modeling near facades.
In commercial buildings, the air-conditioning system near the perimeter provides heating to the zones close to cold facades, with outlets often positioned under the windows or nearby ceiling diffusers. It is quite common in Canada to have both a perimeter heating system and an air-based HVAC system; however, the presence of two systems increases capital and operational costs. Larsson and Moshfegha (2002) investigated the effect of thermal performance, window bay, and displacement ventilation on the downdraught and showed that well-insulated windows may result in reduced air velocity and turbulence intensity. In another study, Rueegg et al. (2001) measured draughts and thermal comfort near a facade with different window sizes and insulation characteristics under variable outdoor temperatures. They found that the window frame is a more critical parameter than the glazing itself and that highly insulated window frames will ensure acceptable draughts. In the presence of solar radiation, glass temperatures may be still low, depending on the type and properties of glazing. In the winter, shading devices can act as a radiant barrier, absorbing solar radiation and decreasing the radiation heat exchange between the cold window surface and the occupant. Heat transfer through windows with shading devices has been extensively studied experimentally (Collins et al. 2001; Duarte et al. 2001; Collins & Harrison 2004) and numerically (Collins et al. 2000; Collins 2004).
The second part of the paper explores the possibility of employing a high-performance envelope to eliminate the need for perimeter heating and utilize the main HVAC system to provide both ventilation and all air-conditioning required. Measurements in the test chamber and near a glazed facade in a real office space were, therefore, conducted without any perimeter heating to examine thermal and airflow conditions in the space under winter conditions. The experiments also considered the impact of solar radiation and shading attachments on thermal comfort (Bessoudo 2008). The experimental results were used to validate a numerical simulation model, which was developed based on a finite difference thermal network approach. Furthermore, a CFD model was utilized to investigate the conditions required to eliminate secondary perimeter heating--namely glazing properties, diffuser location, and supply velocity.
STUDY OF THREE PERIMETER HEATING SYSTEMS--PARTICLE IMAGE VELOCIMETRY AND THERMAL MEASUREMENTS
Experimental Facilities
The first part of the study consists of experimental measurements of air velocity and temperature gradients near a glazed facade with different perimeter heating configurations. The experiments were performed in the Hydro-Quebec research laboratories (Laboratoire des Technologies de l'Energie) in Shawinigan, Quebec. An environmental test chamber with a 4.2 m glazed facade that separates a cold room from the test space (3.65 m deep X 2.3 m high) was used for the measurements (Figure 1). The temperature in the cold room was controlled (using a cooling system) and could be decreased to -18[degrees]C to imitate cold (winter) outdoor conditions.
[FIGURE 1 OMITTED]
The test space is equipped with three controllable perimeter heating devices: (1) a 690 W baseboard heater under the windows, (2) electrically-heated windows (108-216 W/[m.sup.2] power), and (3) a radiant ceiling panel right above the windows. Only one heating source was used during each measurement in order to study them separately and compare their performance. The ventilation system was turned off so that air movement due to natural convection only would be measured during the experiments. The window properties are shown in Table 1. The windows were also furnished with interior retractable standard commercial roller shades, which were used to study the impact of a shading layer on airflow and temperature patterns. The experimental equipment consisted of the following:
Table 1. Window Properties of the Test Facade Window thickness, m 0.025 Power for heating, W/[m.sup.2] 108-216 Visible transmittance, % 73 U-value, W/[m.sup.2]*K 1.873 SHGC 0.67
* A PIV system for air velocity field measurements
* A two-dimensional automated traverse system for moving the PIV system (controlled by a computer)
* A grid of thermocouples (constant and movable) for air and surface temperature measurements
* An infrared camera for surface temperature measurements
* A data acquisition system (connected to the computer) for automatic air velocity and temperature data collection and processing
The experiments included air velocity and temperature field measurements near the facade for the following cases:
* Baseboard heater on
* Baseboard heater on and windows covered with roller shade
* Heated windows ON
* Radiant ceiling panel ON
* No heating (cold windows)
The following sections present the experimental method and results for each case.
Air Velocity Measurements Using...
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