The impact of thermal loads on indoor air flow

Transcription

1 The impact of thermal loads on indoor air flow Risto Kosonen 1, Maija Virta 1 and Arsen Melikov 2 1 Halton Oy 2 International Center for Indoor Environment and Energy, Department of mechanical Engineering, Technical University of Denmark Corresponding risto.kosonen@halton.com SUMMARY The potential for draught discomfort and high air velocities in the occupied zone are often studied with only cooling design in mind. During the transition season, however, downward flows with high air velocities may occur in the occupied zone due to cold window surfaces. Airflow generated by supply air terminal dervices may further enhence the velocity in the occupied zone. Furthermore, convection flows caused by thermal loads may significantly affect the air flow conditions in the room as a whole and assist the occurance of high velocity near occupants. Analyses of results from full-scale measurements with chilled beams presented in this paper reveal that installations with possibilities of convection flow oposing the supplied flow should be avoided. Generally speaking, convection flows have less impact on air distribution in rooms with chilled beams installed in lengthwise direction than when installed crosswise in rooms. INTRODUCTION The local air velocity, temperatures of the room air and air jets, and fluctuations in air velocity are the key factors that determine the risk of a draught. In designing an indoor environment, it is important to evaluate the impact of the various solutions on the air flows in the room. It is particularly important to analyse air velocities in office buildings and on other business premises with relatively high requirements for cooling capacity. The maximum allowed speed in the occupied zone is specified in the present standards for the cooling and the heating seasons. No recommendations have been set for the transition season in the standards, which, in practice, means that designers are free to use the recommendations for either the winter or the summer season. Convection flows, for example caused by a cold window, may, however, be sufficiently strong during the transition season and may generate high velocity in the occupied zoned and cause draught discomfort. When chilled beams are used the air velocity in the occupied zone is affected by the window s surface temperature, the direction of the supplied air flow relative to the convection flow from the cold window, and the convection flow from operating radiator (if any) below the window. The effect of the convection flows on the air distribution in rooms is usually ignored and the assessment focuses only on the flow supplied from air terminal devices. This article addresses the effect of convection flows on the room air distribution. In particular the interaction of convection flows from heat sources with different strength and location and from warm window with the airflow supplied by chilled beams is studied and reported.

2 EFFECT OF CONVECTION FLOW ON JET DETACHMENT The effect of a convection flow created by a thermal load on the detachment point of the supply air jet as shown in Figure 1 was studied [1]. The measurements aimed at examining the variation occurring in the detachment point of the jet when the thermal load and the chilled beam's supply air flow rate were altered. The test room (4.0 x 2.8 x 2.8 m (H)) had a room-wide unidirectional beam and a 1.2-metre-wide band-shaped reheat coil (Fig. 1). In the measurements, the output capacities of the convector was set to 200 W (71 W/m), 300 W (107 W/m), 400 W (143 W/m), and 500 W (179 W/m), while the primary flow rate supplied from the chilled beam was 6, 8, and 10 l/s per m. Ld = 3,6 m 3 m 2 m 1 m 0 m Chilled beam Convector 4,0 m 1,2 m 2,8 m Figure 1. Test arrangement used to study the effect of convection flow on the detachment point of an opposing supply air jet of a room-wide unidirectional chilled beam. Figure 2 presents the distance of the detachment point of the supplied air jet as a function of the thermal load s capacity is shown. The supply air flow rate from the unidirectional chilled beam is a parameter. The results indicate that the convective flow corresponding to the cooling effect of a 150 W/m room device is sufficiently great to cause the detachment and jet droped in the occupied zone with all air flows studied. With the typical airflow rate of 8 l/s per m (2 l/s per m 2 ) for a unidirectional beam, the momentum flux caused by a thermal load of W/m is sufficient to release the jet before it reaches the opposite wall. In general, instalation of unidirectional chilled beams with supplied jets opposing convection flow from heat sources should be avoided.

3 3.5 3 Ld (m) (max 3,6 m) Linear (10 l/s,m) Linear (8 l/s,m) Linear (6 l/s,m) Pconv (W/m) Figure 2. The detachment point of the jet as a function of the convection load of a band-shaped heat source (W/m) and the supplied primary air flow (l/s/m) from a unidirectional chilled beam. SIGNIFICANCE OF THE POSITIONING OF THE AIR SUPPLY DEVICE The impact of the positioning of the air supply device on the flow distribution in the room during cooling (summer) and mid-season situations was studied in a test room (2.4 x 4.6 x 2.8 m (H)). A 300 mm wide chilled beam with a total length of 2,100 mm (1,900 mm effective length) was installed in both the longitudinal and latitudinal direction in the room as shown in Fig. 3. The air velocities in different points of the occupied zone were measured and compared. Figure 4 presents the air velocities measured in the cooling season (60 W/m 2 ), while Figure 5 for the transition season (25 W/m 2 ). In the case of 60 W/m 2, the heat loads were: a computer (100 W), a dummy (60 W), light fittings (144 W) and warm window (350 W). Heat loads in the transition case as before, except the window was not heated. CBD/C 2100/1900 CBD/C 2100/1900 Figure 3. Test room arrangements with a longitudinal and latitudinal active chilled beam.

4 Height, m 12 l/s,m 60 W/m2 center line 1,8 0,17 0,07 0,15 0,25 0,28 0,24 0,12 1,5 0,22 0,04 0,13 0,15 0,22 0,22 0,10 1,1 0,17 0,04 0,09 0,11 0,15 0,18 0,12 0,6 0,20 0,03 0,14 0,11 0,09 0,10 0,07 0,1 0,26 0,24 Distance from 0,18 0,11 0,12 0,14 0,10 0,05 0,26 0,28 back wall/window 0,16 0,12 0,11 0,13 0,10 0,1 0,4 1,8 1,5 1,2 0,9 0,6 12 l/s,m 12 l/s,m 12 l/s,m 60 W/m2 60 W/m2 60 W/m2 g d a 1,80 0,09 0,11 0,09 0,08 0,07 1,80 0,05 0,08 0,09 0,11 0,08 1,80 0,05 0,09 0,05 0,09 0,05 1,50 0,08 0,08 0,09 0,06 0,09 1,50 0,09 0,10 0,09 0,09 0,05 1,50 0,09 0,13 0,05 0,05 0,04 1,10 0,09 0,14 0,12 0,13 0,08 1,10 0,10 0,10 0,10 0,09 0,09 1,10 0,06 0,08 0,08 0,09 0,08 0,60 0,09 0,11 0,08 0,18 0,10 0,60 0,08 0,11 0,10 0,16 0,08 0,60 0,08 0,06 0,10 0,10 0,09 0,10 0,28 0,22 0,19 0,11 0,19 0,10 0,10 0,14 0,14 0,15 0,13 0,10 0,08 0,07 0,07 0,09 0,09 0,05 0,27 0,22 0,19 0,12 0,22 0,05 0,09 0,14 0,15 0,15 0,15 0,05 0,09 0,08 0,06 0,10 0,10 0,6 0,9 1,2 1,5 1,8 0,6 0,9 1,2 1,5 1,8 0,6 0,9 1,2 1,5 1,8 Figure 4. Air velocities (m/s) in the latitudinal (top cross-section) and longitudinal installation (bottom cross-sections: a = 0.6 m, d = 1.5 m, and g = 2.4 m from the window) measured with chilled beam (2100 mm length) at cooling capacity of 60 W/m 2. Cooling case (summer season). In the cooling situation (Fig. 4), the convection flow from heat sorces concentrated at the window wall (a warm window, human occupant, and computer) reverses the air flow from the latitudinal beam and the maximum velocities occur close to the corridor-side wall. In the longitudinal installation (Fig. 5), the effect of the convection flows is smaller, and the air velocities are lower, than in the longitudinal installation.

5 Height, m 12 l/s,m 25 W/m2, 14 window maximum values 1,8 0,28 0,21 0,11 0,10 0,14 0,08 0,12 1,5 0,23 0,10 0,08 0,12 0,15 0,08 0,08 1,1 0,21 0,15 0,09 0,09 0,19 0,09 0,10 0,6 0,20 0,17 0,16 0,15 0,15 0,09 0,07 0,1 0,15 0,21 0,21 0,27 0,32 0,28 0,34 0,05 0,10 0,19 0,23 0,29 0,31 0,25 0,35 0,1 0,4 1,8 1,5 1,2 0,9 0,6 Height, m 12 l/s,m 12 l/s,m 12 l/s,m 60 W/m2 60 W/m2 60 W/m2 g d a g d a 1,80 0,09 0,11 0,09 0,08 0,07 1,80 0,05 0,08 0,09 0,11 0,08 1,80 0,05 0,09 0,05 0,09 0,05 1,50 0,08 0,08 0,09 0,06 0,09 1,50 0,09 0,10 0,09 0,09 0,05 1,50 0,09 0,13 0,05 0,05 0,04 1,10 0,09 0,14 0,12 0,13 0,08 1,10 0,10 0,10 0,10 0,09 0,09 1,10 0,06 0,08 0,08 0,09 0,08 0,60 0,09 0,11 0,08 0,18 0,10 0,60 0,08 0,11 0,10 0,16 0,08 0,60 0,08 0,06 0,10 0,10 0,09 0,10 0,28 0,22 0,19 0,11 0,19 0,10 0,10 0,14 0,14 0,15 0,13 0,10 0,08 0,07 0,07 0,09 0,09 0,05 0,27 0,22 0,19 0,12 0,22 0,05 0,09 0,14 0,15 0,15 0,15 0,05 0,09 0,08 0,06 0,10 0,10 0,6 0,9 1,2 1,5 1,8 0,6 0,9 1,2 1,5 1,8 0,6 0,9 1,2 1,5 1,8 Distance from side wall Figure 5. Air velocities (m/s) in the latitudinal (top cross-section) and longitudinal (bottom cross-sections: a = 0.6 m, d = 1.5 m, and g = 2.4 m from the window) installation of a samesize beam with a cooling effect of 25 W/m 2 and a window surface temperature of 14 o C. During the transition season (Fig. 5), the supply air jet of a beam installed in the latitudinal direction emphasises the convection flow of a cold window, and high speeds ( m/s) are found at floor level (5 10 cm from the floor) at a distance of m from the window. The velocities are lower in a longitudinal installation than in a latitudinal one. However, rather high velocities occur at certain points, primarily in the centre of the room. Generally speaking, it should also be noted that a radiator installed below a window can eliminate the convection flow that increases the risk of a draught caused by a cold window. In practice, this means that, to minimise the draught risk, the radiator must be turned on during the transition season regardless of the cooling requirement. THE EFFECT OF THE LOCATION OF THE THERMAL LOAD ON ROOM AIR DISTRIBUTION The effect of the location of the thermal load on air velocities in the occupied zone was studied in a test room (6.5 x 4.0 x 2.8 m (H)) with an exposed active chilled beam. The chilled beam with a total length of 5 m was installed in longitudinal direction in the centre of the room. The beam s distance from the ceiling was 250 mm. The total cooling load was

6 85 W/m 2 when the primary (outdoor) supplied air flow rate was 2 l/s per m 2. In the measurement, the heat loads were: three computers (3x 123 W),three dummies (3x 120 W), light fittings integrated into the chilled beam (140 W), heat gain from floor panel (300 W) and warm window (350 W). The size of the window was 1.3 (W) x 1.9 (H) m. The area of the heated floor was 5.5 m². The velocity field was measured at three different thermal load distributions: 1) normal office room with thermal load on the window and side walls, 2) 50% on the window wall and 50% on the corridor wall, and 3) all load on the window wall (Fig. 6). Solar load Computers 50 % 50 % Solar load Dummies 100 % Dummies Solar load Figure 6. The location of thermal load and its impact on air velocities in the occupied zone. Top: normal office load. Middle: 50% on the window wall and 50% on the corridor wall. Bottom: 100% of the load on the window wall.

7 In all cases studied, the air velocities were relatively low (less than 0.22 m/s). The highest air velocities occurred in cases where the load had been altered from that of a normal office. The results shown in Fig. 6 by colored field of constant speed reveal that locating all heat sources close to the window wall generates air distribution with a maximum speed in the proximity of the corridor wall, while, when the heat sources were evenly distributed between the window and corridor walls, the maximum speed occurred in the middle of the room. Overall, the location of the thermal load did not significantly affect the operation of the chilled beam for the conditions of the performed measurements. DISCUSSION AND CONCLUSIONS Draught discomfort is one of the most often reported compaints in rooms. During design, the draught risk should be considered not only for the conditions which require maximum cooling (summer season) but also for the transition season between summer and winter, e.g. spring and authumn. Depending on the location of the air supply device and the convection flows generated in the room, the transition season may feature air velocities comparable or even greater than those of the maximum cooling sonditions. In general, the air velocities in rooms with latitudinal-installation of chilled beams are greater than than those in rooms with longitudinal-installation of the chilled beams. The area of occurrence of the maximum speed in the occupied zone depends on the streangth and the location of the heat sources, its momentum flux and on the supplied flow pattern with respect to the convection flow. Generally speaking, convection flows have less impact when the chilled beam is installed in the longitudinal direction than when it is installed in the latitudinal direction The results of this study identify a complex interaction of lows which takes in rooms ventilated with chilled beams. Simple analytic modelling of the jets and thermal flows is not sufficent to identify the airflow distribution in rooms. Therefore, it is possible to accurately analyse the various load situations only by full-scale measurements and/or CFD predictions. The design tools currently available enable studying the distribution of air supplied from chilled beams (and in general from other air supply terminal devices) however without taking account of the effect of the convection flows from heat sources. It should be noted that there are no standardised methods for presenting air velocities in rooms, and each manufacturer of air terminal devices has its way of presentation. ACKNOWLEDGEMENT The writers should like to thank TEKES (Technology Agency Finland) for funding this project. Special thanks go to Mika Komulainen (Lappeenranta University of Technology) for measuring the thermal sources and supply air jets and to Lyuben Bozhkov and Boryana Yordanova (International Centre for Indoor Environment and Energy, Technical University of Denmark) for the measurements related to the effect of the heat sourse location. REFERENCES 1. Komulainen, Mika. Warm convection flows caused by inner heat sources and their effect on supply air jet from the opposite direction. Master s thesis. Lappeenranta University of Technology, Department of Energy and Environmental Technology, 2006.

8 2. Boryana Yordanova and Lyuben Bozhkov, 2006, Active Chilled Beams: Airflow Interaction and Human Response, International Center for Indoor Environment and Energy, Department of mechanical Engineering, Technical University of Denmark, Master Theses/MEK-I-EP-06-04, p.97.