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Flow pattern and thermal comfort in office environment with active chilled beams.

In modern offices, the heat load per floor area has increased. With high cooling loads, the possibility of draft problems increases. The purpose of this article was to study the flow patterns and draft risk in an office environment where cooling and air distribution is implemented with active chilled beams. The study is based on experiments in a laboratory mock-up room in three load conditions: summer, winter, and midseason (spring/autumn). Thermal plumes from heat sources and warm or cold windows had a notable effect on the flow pattern and velocity distribution in the occupied zone. Areas with increased draft risk were found in locations where the supply jet turns down to the occupied zone. Draft risk can also be high at the floor level as a result of a circulating flow pattern in the room. This article concentrates on measurement and modeling results in a single-person office room. Comparisons are made with corresponding results in an open-plan office.

Introduction

In modern offices, the heat load per floor area has increased due to the higher density of workstations, an increased heat load from equipment, and a high solar load from large unshaded windows. With high cooling loads, the air distribution becomes more difficult and the possibility of draft problems increases. The flow pattern in the room becomes more unstable due to interactions between cool inlet jets and warm convection flows from heat sources, and large eddies appear into the room spaces (Muller et al. 2004). The outdoor condition can have a notable effect on the room airflows. Cool windows and outer wall surfaces can create downward flows, and heaters or warm surfaces upward flows, that affect the room flow pattern.

The purpose of this study was to examine the flow patterns and draft risk in two typical office spaces: a single-person office room and an open-plan office. Cooling and air distribution were implemented with active chilled beams. The study was based on experiments in a laboratory mock-up room in three different outdoor load conditions--summer, winter, and midseason (spring/autumn)--in the Scandinavian type of climate. This article concentrates on measurement and modeling results in a single-person office room. Comparisons are made with corresponding results in open-plan office. The results of the open-plan office case were reported in more detail by Koskela et al. (2010).

[FIGURE 1 OMITTED]

Methods

The experiments were carried out in the environmental chamber at the Finnish Institute of Occupational Health in Turku, Finland. The two test room layouts are shown in Figure 1. The test room was built to represent a typical building module of a modern flexible office building. The width of the module is defined by the distance between the construction beams of the building, typically 8.1 m. One wall of the test room had six windows of size 1.22 m x 1.47 m. Their surface temperature was controlled by blowing air into the chamber behind them. Convective heaters were placed under the windows and were only used in the winter test case. The division of the windows and installation of exposed chilled beams enables walls to be built in several locations. The single-person office room was separated with a wall from the right end of the module.

The single-person room had one workstation with a computer and display located at the center of a side wall. A simple plate was installed as a flow obstacle on the opposite wall to represent a typical location of a cupboard or a shelf. The room had one light fitting above the workstation at the height of 2.3 m. The open-plan office layout had eight work-stations arranged symmetrically in two groups.

The internal heat load to the room was produced with computers, displays, light fittings, and dummies representing persons. The purpose was to have approximately the same heat load per floor area in single-person room and open-plan office test cases in order to get comparable results. Therefore, somewhat higher load values were used in the single-person room case. The persons were simulated with painted cylinders (height, 1.1 m; diameter, 0.3 m) similar to those defined in DIN (1995), containing two light bulbs of total 80 W in the open-plan office and three light bulbs of total 120 W in the single-person room. Each workstation had a PC and display with power consumption adjusted to 90 W in the open-plan office and 120 W in the single-person room. The lamps had a power consumption of 120 W each.

Direct solar load was simulated by placing heater panels (size, 1.02 m x 0.55 m; height, 0.05 m) on the floor as shown in Figure 1. In the single-person room, the panels close to the windows were used in the summer conditions. The other panels farther off in the room were used in the spring/autumn conditions, when the elevation angle of the sun is lower. The heaters under the windows in the winter test case had dimensions of 1.2 m x 0.4 m x 0.1 m. They mainly produced a convective heat load due to the stainless steel covering on all vertical surfaces.

The exposed chilled beams were installed asymmetrically in the test rooms in the centerline of every second window. The distance from the ceiling was 0.15 m. The dimensions of the beams were 3.3 m x 0.41 m with a height of 0.18 m. The active length of the beams was 3.0 m, which were located symmetrically in relation to the centerline of the room.

The active chilled beam shown in Figure 2 was selected to represent a typical unit with exposed installation in the room. In the device, the outdoor air supply is combined with cooling of recirculated room air. Outdoor air supply is typically introduced through small nozzles inside of the beam. Outdoor air jets induce room air through a heat exchanger, where it is cooled, and the mixture (referred as inlet jets) is blown into the room through supply air openings. The flow rate of induced room air is typically three to five times the outdoor airflow rate. The heat exchanger consists of a cooling coil, which is cooled by water flow. In this case, the inlet jets are blown from inlet jet openings in the upper surface of the beam. The supply of inlet jets creates plane jets to both sides of the beam that normally attach to the ceiling, utilizing the Coanda effect. Figure 2 shows the operation principle of an active chilled beam.

[FIGURE 2 OMITTED]

The measurements of air velocity were carried out by using ultrasonic anemometers (Kaijo Denki WA-390, accuracy [+ or -]0.02 m/s), moved by an automated traversing system. The averaging time in each measurement point was 60 s. The measurement grid density was 0.1 m x 0.1 m. Additional measurements were done using Dantec 54N10 flow analyzer with hot sphere sensors. The flow pattern was visualized using smoke and video recorded during experiments. The cooling power of the chilled beams was determined by measuring the cooling water flow rate and the rise of water temperature in the heat exchanger.

[FIGURE 3 OMITTED]

In the single-person office room, distributions of air velocity and temperature were measured in horizontal and vertical planes shown in Figure 3. The horizontal planes were at the heights of 1.2 m and 1.7 m. The planes were 1.7 m wide and extended from the outer wall to the inner wall. A measurement height of 1.2 m was used with the automated traversing system instead of the standard height 1.1 m because of obstacles. In the open-plan office case, only horizontal planes at heights of 0.l m, 1.2 m, and 1.7 m were measured. The locations of the planes were selected to cover the four workstations in the central part of the room.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The computational fluid dynamics (CFD) simulations were carried out by using Ansys CFX software with an shear stress transport (S ST) turbulence model following the guidelines given by Nielsen et al. (2007). The grid was unstructured, with inflation layers on the room and heat source surfaces, and had 530,000 nodes in the single-person office room case and 1,350,000 nodes in the open-plan office case. The grid density was 1 cm in the supply opening, 5 cm in the supply jet area, and 10 cm in other parts of the room. No radiation model was used. The convective parts of the heat loads were given to the surfaces as heat fluxes. The radiation part was distributed to room surfaces based on their approximate view factors. The mean air speed was calculated from mean air velocity and turbulent kinetic energy using a correction formula reported by Koskela et al. (2001).

ISO Standard 7730 gives design criteria for maximum mean air speed in the office environment by defining three categories for different draft rate (DR) levels (ISO 2005):

* Category A (DR 10%): summer 0.12 m/s, winter 0.10 m/s;

* Category B (DR 20%): summer 0.19 m/s, winter 0.16 m/s;

* Category C (DR 30%): summer 0.24 m/s, winter 0.21 m/s.

These values are based on the assumption that the room temperature is in the lower end of the corresponding recommended temperature range and that the turbulence intensity is 40%.

[FIGURE 6 OMITTED]

Results

Three test cases were measured in the laboratory and simulated with CFD.

1. Summer case with 95-100 W/[m.sup.2] cooling load (warm windows and direct solar heat load).

2. Spring/autumn case with 45 W/[m.sup.2] cooling load (cold windows).

3. Winter case with 45 W/[m.sup.2] (cold windows and heaters under them).

In the summer case the heat load was 95 W/[m.sup.2] in the single-person office room case and 100 W/[m.sup.2] in the open-plan office case. The heat load levels in the test cases are presented in Table 1.

In the winter case, the heater below the window warmed up the window surface, lowering the calculated heat loss from the room to the window surface. In the single-person office room, some extra load was added with the floor panels in the spring/autumn case in order to achieve the heat load level 45 W/[m.sup.2].

CFD modeling

The simplified model for the chilled beam was made of a similar size as the actual device. The outlets representing room air recirculation openings were larger than in reality, covering the all the vertical surfaces of the beam. The inlet boundary conditions were determined based on the information obtained from the manufacturer. The total inlet airflow rate was 125 L/s per unit. The dimensions of the inlet slots were 3.0 m x 0.025 m. This gives an inlet momentum flow rate of 0.139 N per unit. The airflow was blown with velocity 0.927 m/s to an angle of 26[degrees] compared to the vertical direction. The inlet jet temperature was calculated from the mean room temperature and the cooling power of the chilled beam. The modeling methods and results of the open-plan office case were reported in more detail by Koskela et al. (2010).

[FIGURE 7 OMITTED]

Single-person office room

The flow patterns and air speed distributions in the single-person room are shown in Figures 4-6. In the summer and winter cases, the flow direction close to the windows is upward. This upward flow creates a circulation, which turns the inlet jet toward the corridor wall. The velocity maximum at the 1.2-m level is therefore close to the corridor wall and does not cause draft to the workstation. In the spring/autumn case, the cool window surface causes a downward flow, which creates an opposite circulation compared to the summer and winter cases. The supply jet turns toward the window, boosting the downward flow of cool air to the floor level.

Effect of workstation location on the flow pattern

The effect of workstation location on the flow pattern was studied with a pair of experiments in summer conditions with a heat load of 65 W/[m.sup.2]. The workstation was moved from the normal location in the center of the side wall to a new location close to the window. The results are shown in Figure 7. When the workstation is in the center of the side wall, the downfall of the supply jet occurs closer to the corridor wall. This can be explained by the effect of the thermal plumes rising from the workstation heat sources to the supply jet. Also, the maximum air velocities at the 1.2-m level were somewhat smaller with the workstation in the central position. However, the overall effect on the room airflow pattern was not large.

[FIGURE 8 OMITTED]

Effect of a flow obstacle on the flow pattern

The effect of the horizontal plate on the wall representing a cupboard or a shelf was also studied with a pair of experiments in the summer conditions with a heat load of 65 W/[m.sup.2]. In the standard configuration, the plate turned one part of the supply jet from the chilled beam toward the center of the room instead of continuing downward along the side wall (Figure 8). This caused an interaction of the two inlet jets, thus widening the flow profile of the downward airflow. The maximum velocities close to the workstation were somewhat smaller with the plate installed.

CFD modeling results

The flow patterns obtained from the CFD simulations in the single-person room test cases are shown in Figures 9-11, with a comparison of air speed distributions in the measurement plane at the 1.2-m level. The mean and maximum values of air speed in the occupied zone from CFD simulations and measurements are compared in Figure 12. CFD simulations were able to correctly predict the main flow pattern in the single-person room in all three test cases. The predicted mean air speed values were also close to the measurement results in all cases. The maximum velocities, however, were notably higher in all cases.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Open-plan office

The main features of the open-plan office results are shown here, but they were presented in more detail by Koskela et al. (2010). The main flow patterns in the summer test case detected from the smoke experiments in the open-plan office are shown in Figure 13 (left) together with the flow patterns from the CFD simulation (right). Figure 14 shows the modeled distribution of air speed at two horizontal planes at 0.1-m and 1.1-m heights, with corresponding measurement results in the central part of the office module. A strong longitudinal circulation was formed in the room due to the asymmetric layout of the chilled beams compared to heat sources.

In the autumn/spring test case, the longitudinal circulation was weaker compared to the summer case (Figure 15, left). This is natural because of weaker buoyancy forces. Also, the downward plumes from the cool window were weak. In the winter case, the longitudinal circulation was also weaker compared to the summer case (Figure 15, right). The heaters under the windows created upward plumes, as in the summer case. These plumes turned the inlet jets somewhat toward the corridor wall.

Air speed levels in the test cases

A comparison between the air speed levels in the occupied zone of the open-plan office and the single-person room is presented in Figure 16. The mean and maximum air speed results are compared with the target values of ISO Standard 7730 for summer and winter conditions. The standard does not give target values of air speed for spring or autumn conditions. Categories A, B, and C correspond to draft risk levels of 10%, 20%, and 30%, respectively.

At the 1.2-m height, the mean air speed values are at the same level (0.10-0.13 m/s) in both room types. Also the maximum air speed is at the same level in summer (0.27-0.28 m/s) and spring/autumn (0.20-0.21 m/s) cases. In the winter case, the maximum value in the single-person room is higher (0.26 m/s) than in the open-plan office (0.20 m/s).

[FIGURE 12 OMITTED]

At the 0. l-m height, the velocities in the open-plan office are higher in summer (0.24 m/s) and winter (0.27 m/s) conditions than the corresponding values in the single-person room (0.21 m/s and 0.18 m/s). In spring/autumn conditions, however, the velocities are higher in the single-person room (0.26 m/s) than in the open-plan office (0.23 m/s).

The main reason for the floor-level airflows was the occurrence of large-scale circulation in the room. The cause of this circulation was different in the open-plan office and single-person room cases. In the open-plan office, the main factor was the asymmetric layout of cooling units compared to heat sources and room geometry. This type of layout is typical in modern flexible offices. In the single-person room, the circulation was mainly caused by the heat sources close to the window or the downdraft from cold window surfaces.

Summary and discussion

The heat sources had a notable influence on the flow pattern, causing large-scale circulation and affecting the direction of inlet jets. Two main causes of draft risk were found:

1) downfall of inlet jets causing local maxima of air speed, especially at the head level, and

2) large-scale circulation causing high air speeds, especially at the floor level.

In the single-person room, the direction of the circulation in the room depended on the time of the year. In the summer case, it was caused by the upward plume from the warm window and, in the winter case, by the heater below the window. In the spring/autumn case, the circulation turned the supply jet toward the cool window. The combination of cool supply jet and downward flow from the widow caused relatively high velocities on the floor level. In the summer and winter cases, the highest velocities were found close to the corridor wall and away from the workstation. Findings are similar to those reported by Melikov et al. (2007) and Zboril et al. (2007).

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

[FIGURE 16 OMITTED]

In the open-plan office, the main circulation occurred in the longitudinal direction of the room. It was caused by the asymmetry of the heat loads and the cooling units. Also, a transverse circulation was created by the heat load of the windows or the heaters below the windows. It had the effect of turning the inlet jets toward the inner wall. The transverse circulation, however, was overridden by the stronger longitudinal circulation.

The collision and downfall of the inlet jets was another phenomenon causing high velocities in the open-plan office. The downfall occurs locally, and the position of draft risk areas can change in time and also due to changes in room heat sources.

The maximum air speed in the occupied zone was, in most cases, relatively high compared to the recommendations in Category C or above. The mean air speed was typically in Category B on the head level and in Category C on the floor level. No overall difference between the velocity levels in the two room types was found. The air speed in the open-plan office was highest in the summer case with a high cooling load. In the single-person room, the floor level velocities were high also in the spring/autumn case. It has to be noted, however, that the ISO Standard 7730 target values for air speed in Figure 16 assume that the temperature is in the lower end of the corresponding recommended temperature range, which may overestimate the draft risk.

The office rooms had only few flow obstacles, and the space under the tables was mainly open, which made the large-scale circulation flow along the floor possible. In real offices, there are usually more flow obstacles, which prevent the large-scale circulation and reduce the air speed. If the screens in the open-plan office block the flow totally under the tables, this type of circulation is not possible.

The location of the workstation had some effect on the air speed distribution. The plumes of the workstation heat sources seemed to change the location and air speed of the downfall of the inlet jet. The horizontal plate on the wall representing a cupboard or a shelf also had an effect on the flow pattern and air speed distribution in the occupied zone.

The CFD simulations were able to predict the main features of the flow pattern in the room. The predicted mean air speed values were close to the measurement results in all cases. The maximum air speed values, however, were notably higher than the measured values in all cases. This is a typical result and is caused by the inability of the steady-state Reynolds averaged Navier-Stokes (RANS) models to correctly predict the fluctuating room airflows.

The following general conclusions can be drawn based on this study.

1) Convection airflows have a notable effect on the thermal conditions in the room. The effects depend on the prevailing load conditions. Analysis of these effects requires full-scale experiments or simulation with CFD modeling techniques; analytical flow element models are not sufficient.

2) Midseason conditions must be analyzed also; extreme load conditions in summer or winter do not give the whole picture.

3) Current draft standards do not fully describe transient and asymmetric flow conditions such as those in this study. New methods of assessing the thermal conditions in rooms should be developed.

Acknowledgments

The financial support of National Technology Agency of Finland is greatly appreciated.

References

DIN. 1995. DIN Standard 4715-1, chilled surfaces for rooms. Part 1. Germany: DIN.

ISO. 2005. ISO Standard 7730, ergonomics of the thermal environment analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. Geneva, Switzerland: International Organisation for Standardisation.

Koskela, H., H. Haggblom, R., Kosonen, and M. Ruponen. 2010. Air distribution in office environment with asymmetric workstation layout using chilled beams. Building and Environment 45:1923-31.

Koskela, H., J. Heikkinen, R. Niemela, and T. Hautalampi. 2001. Turbulence correction for thermal comfort calculation. Building and Environment 36(2):247-55.

Melikov, A., B. Yordanova, L. Bozkhov, V. Zboril, and R. Kosonen. 2007. Human response to thermal environment in rooms with chilled beams. Proceedings of Clima 2007 Wellbeing Indoors. Helsinki, Finland, June 10-14.

Muller. D., I. Gores, and R. Zielinski. 2004. Impact of the thermal load on the room airflow pattern. Proceedings of 9th International Conference on Air Distribution in Rooms (Roomvent 2004), Coimbra, Portugal, September 5-8.

Nielsen, P.V, F. Allard, H.B. Awbi, L. Davidson, and A. Schalin. 2007. REHVA Guidebook No. 10, Computational fluid dynamics in ventilation design. Brussels: REHVA.

Zboril, V., A. Melikov, B. Yordanova, L. Bozkhov, and R. Kosonen. 2007. Airflow distribution in rooms with active chilled beams. Proceedings of 10th International Conference on Air Distribution in Rooms (Roomvent 2007), Helsinki, Finland, June 13-15.

Hannu Koskela, (1), * Henna Haggblom, (1) Risto Kosonen, (2) and Mika Ruponen (3)

(1) Institute of Occupational Health, Turku, Finland

(2) Halton Group, Finland

(3) Halton Indoors, Witham, UK

* Corresponding author e-mail: hannu.koskela@ttl.fi

Received November 8, 2010; accepted June 12, 2011

Hannu Koskela is Chief of Laboratory. Henna Haggblom is Researcher. Risto Kosonen, PhD, Member ASHRE, is Director of Technology Center. Mika Ruponen, PhD, is Indoor Environment Specialist.

DOI:10.1080/10789669.2011.603014
Table 1. Heat load levels in the test cases.

 Single-person office room

 Spring/
Heat load type Summer, W autumn, W Winter, W

Persons (cylinders) 120 120 120
Computers and displays 120 120 120
Lights 120 120 120
Other internal loads 200
Heaters 300
Solar load (panels) 170 100
Windows (calculated) 500 -170 -170
Total load 1030 490 490
Total load per floor area 95 45 45

 Open-plan office

 Spring/
Heat load type Summer, W autumn, W Winter, W

Persons (cylinders) 640 640 640
Computers and displays 720 720 720
Lights 360 360 360
Other internal loads
Heaters 440
Solar load (panels) 740
Windows (calculated) 910 -200 -640
Total load 3370 1520 1520
Total load per floor area 100 45 45
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Author:Koskela, Hannu; Haggblom, Henna; Kosonen, Risto; Ruponen, Mika
Publication:HVAC & R Research
Date:Aug 1, 2012
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