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For Peer Review

Flow Pattern and Thermal Comfort in Office Environment

with Active Chilled Beams

Journal: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Manuscript ID: Draft

Publication: HVAC&R Research

Keywords: Commercial Buildings, Air Distribution Equipment, Research & Development

Review Copy Only. Not for distribution.

American Society of Heating, Refrigerating and Air-Conditioning Engineers

For Peer Review

Flow Pattern and Thermal Comfort in Office Environment with Active Chilled Beams Hannu Koskela Henna Häggblom Risto Kosonen, PhD Mika Ruponen, PhD

ABSTRACT

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 paper was to study the flow patterns and draft risk in 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 paper concentrates on

measurement and modeling results in a single-person office room. Comparisons are made with

corresponding results in open-plan office.

INTRODUCTION

In modern offices, the heat load per floor area has increased due to higher density of workstations,

increased heat load from equipment and 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 (Müller et al. 2004). The outdoor

condition can have a notable effect on the room air flows. Cool window and outer wall surfaces can create

downward flows and heaters or warm surfaces upward flows that affect the room flow pattern.

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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 Scandinavian type of climate.

This paper 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 have been

reported in more detail by Koskela et al. (2010).

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.

Figure 1. Dimensions and layout of the test rooms: open-plan office (left) and single-person

office room (right).

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The single-person room had one workstation with a computer and display located at the centre 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 workstations 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. 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 2 light bulbs of total 80 W in the open-plan office and 3

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 to 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 further 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 produced mainly convective heat load due to 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, height 0.18 m. The active length of the beams was 3.0 m and it was located symmetrically in relation to

the centerline of the room.

The active chilled beam model Halton CCE was selected to represent a typical unit with exposed

installation in the room. In the device, outdoor air supply is combined with cooling of re-circulated 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 3-5 times

the outdoor air flow rate. The heat exchanger consists of a cooling coil, which is cooled by water flow. In

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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. Operating principle of the active chilled beam.

The measurements of air velocity were carried out by using ultrasonic anemometers (Kaijo Denki WA-

390, accuracy ± 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.

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. Measurement height

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 0.1 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.

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Figure 3. Vertical and horizontal measurement planes in the single-person office room.

The CFD simulations were carried out by using Ansys CFX software with SST 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 the turbulence intensity is 40 %.

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RESULTS

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

1. Summer case with 95-100 W/m2 cooling load (warm windows and direct solar heat load)

2. Spring/autumn case with 45 W/m2 cooling load (cold windows)

3. Winter case with 45 W/m2 (cold windows and heaters under them)

In the summer case the heat load was 95 W/m2 in the single-person office room case and 100 W/m2 in

the open plan office case. The heat load levels in the test cases are presented in Table 1.

Table 1. Heat load levels in the test cases.

Single-person office room Open-plan office

Heat load type

Summer (W) Spring/

autumn

(W)

Winter (W) Summer (W) Spring/

autumn

(W)

Winter (W)

Persons (cylinders)

Computers and displays

Lights

Other internal loads

Heaters

Solar load (panels)

Windows (calculated)

120

120

120

170

500

120

120

120

200

100

-170

120

120

120

300

-170

640

720

360

740

910

640

720

360

-200

640

720

360

440

-640

Total load

Total load per floor area

1 030

95

490

45

490

45

3 370

100

1 520

45

1 520

45

In the winter case, the heater below the window warmed up the window surface, which lowered 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/m2.

CFD-modeling

The simplified model for the chilled beam was made of similar size as the actual device. The outlets

representing room air re-circulation openings were larger than in reality covering the whole vertical

surfaces of the beam. The inlet boundary conditions were determined based on the information obtained

from the manufacturer. The total inlet air flow 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 air flow was blown

with velocity 0.927 m/s to an angle of 26° 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 are reported in more detail by Koskela et al. (2010).

Single-person office room

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The flow patterns and air speed distributions in the single-person room are shown in Figs. 4-6. In the

summer and winter cases, the flow direction close to the windows is upwards. This upward flow creates a

circulation, which turns the inlet jet towards the corridor wall. The velocity maximum at 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 on opposite circulation compared to the

summer and winter cases. Supply jet turns towards the window boosting the downward flow of cool air to

the floor level.

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Figure 4. Measurement results in the summer test case with 95 W/m2 cooling load. Air flow

pattern from smoke experiments (top), air speed distributions in the vertical measurement

planes (middle) and air speed distributions at 1.2 m level (bottom left) and 1.7 m level (bottom

right).

Figure 5. Measurement results in the spring/autumn test case with 45 W/m2 cooling load. Air

flow pattern from smoke experiments (top), air speed distributions in the vertical measurement


right).

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Figure 6. Measurement results in the winter test case with 45 W/m2 cooling load. Air flow

pattern from smoke experiments (top), air speed distributions in the vertical measurement


right).

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/m2. The workstation was moved from the normal location in

centre 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 centre of the side wall, the downfall of the supply jet occurs closer to the corridor wall.

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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 1.2 m level were somewhat smaller with the workstation in

the central position. However, the overall effect on the room air flow pattern was not large.

Figure 7. Effect of the workstation location on the air speed distributions at 1.7 m level (middle)

and 1.2 m level (bottom).

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/m2. In the standard configuration,

the plate turned one part the supply jet from the chilled beam towards the centre of the room instead of

continuing downwards along the side wall (Figure 8). This caused an interaction of the two inlet jets thus

widening the flow profile of the downward air flow. The maximum velocities close to the workstation were

somewhat smaller with the plate installed.

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Figure 8. Effect of the flow obstacle on the wall on the air speed distributions at 1.7 m level

(middle) and 1.2 m level (bottom).

CFD-modeling results

The flow patterns obtained from the CFD-simulations in the single-person room test cases are shown

in Figs. 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.

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Figure 9. Results of the CFD-simulation in the summer test case with 95 W/m2 cooling load. Air

flow pattern visualized with the 0.25 m/s iso-surface of air speed (top) and air speed

distributions at 1.2 m level from measurements (bottom left) and CFD-simulation (bottom

right).

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Figure 10. Results of the CFD-simulation in the winter test case with 45 W/m2 cooling load. Air

flow pattern visualized with the 0.25 m/s iso-surface of air speed (top) and air speed


right).

Figure 11. Results of the CFD-simulation in the spring/autumn test case with 45 W/m2 cooling

load. Air flow pattern visualized with the 0.25 m/s iso-surface of air speed (top) and air speed


right).

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Figure 12. Comparison of mean and maximum values of air speed in the occupied zone between

CFD-simulation and measurement results

Open-plan office

The main features of the open-plan office results are shown here, but they are 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 (on the left) together with the flow patterns

from the CFD simulation (on the 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 asymmetric layout of

the chilled beams compared to heat sources.

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Figure 13. Flow pattern in the summer case, experimental and CFD-results

Figure 14. Measured (top) and measured (bottom) air speed contours at 0.1 m (left) and 1.2 m

(right) levels in the summer case.

In the autumn/spring test case, the longitudinal circulation was weaker compared to the summer case

(Figure 15 on the 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 on the right). The heaters under the windows created upward

plumes as in the summer case. These plumes turned the inlet jets somewhat towards the corridor wall.

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Figure 15. Flow patterns in the spring/autumn case 45 W/m2 (left) and winter case 45 W/m2

(right) from smoke experiments.

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 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 %.

At 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).

At 0.1 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 air flows was the occurrence of large scale circulation in the room.

The cause of this circulation was different in 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

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circulation was mainly caused by the heat sources close to the window or the downdraft from cold window

surface.

Figure 16. Comparison of mean and maximum values of air speed between the open-plan office

and the single-person office room compared to ISO 7730 design criteria.

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

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the heater below the window. In the spring/autumn case the circulation turned the supply jet towards 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).

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

towards 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 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 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

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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 RANS-models to correctly predict the fluctuating room air flows.

Following general conclusions can be drawn based on this study:

1) Convection air flows 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

in this study. New methods of assessing the thermal conditions in rooms should be developed.

ACKNOWLEDGEMENTS

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

REFERENCES

DIN. 1995. DIN 4715-1; Chilled surfaces for rooms. Part 1. DIN, Germany.

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. International Organisation for Standardisation, Genève. 52 p.

Koskela, H., H. Häggblom, R., Kosonen, and M. Ruponen. 2010. Air distribution in office environment

with asymmetric workstation layout using chilled beams. Building and Environment 45 (2010) 1923-

1931.

Koskela, H., J. Heikkinen, R. Niemelä, and T. Hautalampi. 2001. Turbulence correction for thermal

comfort calculation. Building and Environment 2001; 36(2): 247-255.

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. Finnvac

ry, ISBN 978-952-998-3-6, Finland.

Müller. D., I. Gores, and R. Zielinski. 2004. Impact of the Thermal Load on the Room Airflow Pattern. In:

Roomvent 2004. Proceedings of 9th International Conference on Air Distribution in Rooms; 2004 Sept

5-8; Coimbra, Portugal

Nielsen, P. V., F. Allard, H. B. Awbi, L. Davidson, and A. Schälin. 2007. Computational fluid dynamics in

ventilation design. REHVA Guidebook no 10. REHVA.

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Zboril, V., A. Melikov, B. Yordanova, L. Bozkhov, and R. Kosonen. 2007. Airflow distribution in rooms

with active chilled beams. Proceedings of Roomvent 2007 10th international conference on air

distribution in rooms. Finvac ry. ISBN 978-952-99898-1-2, Finland.

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