ABSTRACT Air distribution was studied at numerous combinations of layout of heat sources and chilled beams arrangement in full scale mock up of a real office. The flow from the chilled beams had either low or high induction. Two thermal manikins were used to simulate blocking effect and heat generated by two occupants. Multichannel low velocity thermal anemometer was used during the measurements. Draught was identified lower than 20 % and the vertical temperature difference between 1.1 and 0.1 m level above the floor lower than 3 K for the studied cases. The draught risk and the temperature difference increased when the heat load in the room increased, but still remained relatively low. The draught risk was lower and the temperature difference higher when the diffusers were arranged to provide flow with a high induction than when they provided flow with low induction. Thermal flows generated by occupants and computers (stronger than from occupants) affected significantly the distribution of the flow supplied from the chilled beams. INTRODUCTION The use of chilled beams for ventilation of rooms and offices increases. However, air distribution and flow pattern in rooms with chilled beams when people are present have not been studied sufficiently. Effects of heat sources and their location in rooms ventilated by chilled beams have not been reported as well. Chilled beams combine air supply with cooling or heating. The main principle of the beam is induction of room air caused by the supply air. Inducted room air is sucked into the beam through the water-air heat exchanger where it is cooled or heated and subsequently mixed with the fresh air and supplied to the room from slots, as schematically shown in Figure 1. The air flows from the beam over the ceiling and falls down the walls. If there are no obstruction in the way air jet turns round and flows over the floor into the occupied zone. In case of parallel positioning of the beams the jets can meet in the middle, lose momentum and fall down to the occupied zone. In comfortable room environment an upward free convection flow exists around the human body. The airflow is slow and laminar with a thin boundary layer at the lower parts of the body and becomes faster and turbulent with a thick boundary layer at the height of the head. Body shape and posture, room air temperature, etc. define the mean velocity in the free convection flow which may be as high as 0.25 m/s and the thickness of the boundary layer may measure 0.2 m or more. Fairly strong thermal plume with velocity in the range of 0.2-0.3 m/s is established above the human body [1]. Computers, desk/table lights, etc. are heat sources that generate upward free convection flow as well. Figure 1 – Principle of chilled beam [4] Thus the upward flows, if combined, may substantially change the air distribution in a room and may cause local draught discomfort for the occupants. The objective of this study was to identify the effect of heat sources and their locations on the airflow distribution in rooms ventilated by chilled beams. The effect of the control of the supplied flow pattern was studied as well. Finally the effect of the chilled beam arraindement was studied, however it is not included in this paper. EXPERIMENT Measurements were performed in a room, with size of 5.4 x 4.2 x 2.5 m. Two thermal manikins, one with 17 body segments and one with 23 body segments, were used to simulate people in the room. The surface temperature of the segments was controlled individually by computer to be identical with the skin temperature of person in state of thermal comfort. The manikins represented an average female body size. They were dressed with clothing typical for office type work, i.e. underwear, trousers, blouse, etc. Surface temperature and heat flux from each body segment were measured AIR FLOW DISTRIBUTION IN ROOMS WITH CHILLED BEAMS Zbořil Viktor 1,2 , Bozhkov Lyuben 2 , Yordanova Boryana 2 , Melikov Arsen 2 , Kosonen Risto 3 1 Department of Environmental Engineering, Faculty of Mechanical Engineering, CTU in Prague, Prague 166 47 - Czech Republic 2 Internationl Centre for Indoor Environment and Energy, DTU, Lyngby 2800 - Copenhagen 3 Halton, Kausala 47400 - Finland
Transcript
ABSTRACTAir distribution was studied at numerous combinations of layout of heat sources and chilled beams arrangement in full scale mock up of a real office. The flow from the chilled beams had either low or high induction. Two thermal manikins were used to simulate blocking effect and heat generated by two occupants. Multichannel low velocity thermal anemometer was used during the measurements. Draught was identified lower than 20 % and the vertical temperature difference between 1.1 and 0.1 m level above the floor lower than 3 K for the studied cases. The draught risk and the temperature difference increased when the heat load in the room increased, but still remained relatively low. The draught risk was lower and the temperature difference higher when the diffusers were arranged to provide flow with a high induction than when they provided flow with low induction. Thermal flows generated by occupants and computers (stronger than from occupants) affected significantly the distribution of the flow supplied from the chilled beams.
INTRODUCTIONThe use of chilled beams for ventilation of rooms and offices increases. However, air distribution and flow pattern in rooms with chilled beams when people are present have not been studied sufficiently. Effects of heat sources and their location in rooms ventilated by chilled beams have not been reported as well.
Chilled beams combine air supply with cooling or heating. The main principle of the beam is induction of room air caused by the supply air. Inducted room air is sucked into the beam through the water-air heat exchanger where it is cooled or heated and subsequently mixed with the fresh air and supplied to the room from slots, as schematically shown in Figure 1. The air flows from the beam over the ceiling and falls down the walls. If there are no obstruction in the way air jet turns round and flows over the floor into the occupied zone. In case of parallel positioning of the beams the jets can meet in the middle, lose momentum and fall down to the occupied zone.
In comfortable room environment an upward free convection flow exists around the human body. The airflow is slow and laminar with a thin boundary layer at the lower parts of the body and becomes
faster and turbulent with a thick boundary layer at the height of the head. Body shape and posture, room air temperature, etc. define the mean velocity in the free convection flow which may be as high as 0.25 m/s and the thickness of the boundary layer may measure 0.2 m or more. Fairly strong thermal plume with velocity in the range of 0.2-0.3 m/s is established above the human body [1]. Computers, desk/table lights, etc. are heat sources that generate upward free convection flow as well.
Figure 1 – Principle of chilled beam [4]
Thus the upward flows, if combined, may substantially change the air distribution in a room and may cause local draught discomfort for the occupants. The objective of this study was to identify the effect of heat sources and their locations on the airflow distribution in rooms ventilated by chilled beams. The effect of the control of the supplied flow pattern was studied as well. Finally the effect of the chilled beam arraindement was studied, however it is not included in this paper.
EXPERIMENTMeasurements were performed in a room, with size of 5.4 x 4.2 x 2.5 m. Two thermal manikins, one with 17 body segments and one with 23 body segments, were used to simulate people in the room. The surface temperature of the segments was controlled individually by computer to be identical with the skin temperature of person in state of thermal comfort. The manikins represented an average female body size. They were dressed with clothing typical for office type work, i.e. underwear, trousers, blouse, etc. Surface temperature and heat flux from each body segment were measured
1Department of Environmental Engineering, Faculty of Mechanical Engineering, CTU in Prague, Prague 166 47 - Czech Republic
2Internationl Centre for Indoor Environment and Energy, DTU, Lyngby 2800 - Copenhagen3Halton, Kausala 47400 - Finland
and recoded on a computer. The manikins sat behind desks simulating seated person performing office work. Two computers and two table lights were used as additional heat sources. Solar radiation was simulated by two heating panels (attached at one of the walls) for simulation of windows of size 2.0 x 1.8 m and several smaller heating panels placed on the floor. Four chilled beams were installed on the ceiling as shown in Figure 2. Shutters, placed in the chilled beams were used to modify the pattern of the supplied outdoor air and thus to achieve two level of induction of room air.
Figure 2 – Arrangement of the chilled beams
Two different layouts of the heat sources were measured in the full-scale experiment (Figure 3). Case 1 was symmetrical layout where the two manikins sat next to the walls, one near the windows and the other one on the opposite side. Manikins’ pelvises were situated in the middle of the length of the room and 0.9 meters far from the wall. In Case 2 the manikins sat face to face next to the wall with simulated windows. The distance between the manikins and the wall was 0.9 meters. Tables met each other in the middle of the length of the room.
Figure 3 – Layout and measuring grid in Case 1 (left) and Case 2 (right).
Eight low velocity thermal anemometers with omni-directional transducers were used to measure mean velocity (in fact mean speed), turbulence intensity and temperature. The velocity sensor was spherical with
a diameter of 2 mm and the temperature sensor was shielded against radiation. The measuring period at each point was 3 min.
Twenty-three resistance thermometers were used for measuring inlet and outlet water temperatures, inlet and outlet air temperatures in every beam, surfaces temperatures of the walls and air temperature in the room. All sensors were connected to a computer and temperatures were recorded every 6 seconds by a data logging system. The measurements were performed under steady state conditions.
The amount of room air inducted into the beams was regulated by shutters placed in the beams. There were two positioning of the settings, opened or closed, as shown in Figure 1. The setting with closed shutters was named “Induction 1” and the setting with opened shutters was named “Induction 2”.
The heat load calculated for this type of office, orientated to the south and with heat sources described above, was 50 W/m2 for both, Case 1 and Case 2. Additionally, in Case 2, heat load of 80 W/m2 was simulated with heating panels placed on the floor to simulate floor area exposed and heated by the sun.
Figure 4 – Measuring grid above the sources.
Combinations of these parameters (Cases, Inductions and Heat loads) defined several simulated conditions. In each of simulated conditions measurements were performed in two areas. One of them was the occupied zone defined in ASHRAE Standards [2], where sensors were placed 0.1, 0.6, 1.1 and 1.7 meters above the floor at measuring locations as shown in Figure 3. The second area was above the heat sources as shown in Figure 4 in a plan above the manikin’s head and monitor.
The room temperature was kept at 24 °C. In this paper are presented only cases when the flow rate of the supplied outdoor air was 1.5 l/s1.m2, that is 122 m3/h1 and the ventilation rate was 2.15 ACH. Humidity was not a subject of research and was measured for control only.
RESULTSFigures 5 – 6 show fields of mean velocity and draught rate (5, 6) in occupied zone. To show conditions near the occupant, only one cross-section from the occupied zone measurement close to the manikin is presented. This cross-section is marked in the Figure 3 with thicker line. Velocity field and draught rate field above the heat sources are shown in Figure 7. Detailed measurement above the heat sources gives an insight to what happens to the jets from the beams and to the plumes from the sources. For 50 W/m2 in all situations draught rate in occupied zone is lower than 20 % and temperature difference between 0.1 m level and 1.1 m level is lower than 3 K, which means that the requirements for thermal comfort category B and C as defined in the European Guidelines EN 1752 are fulfilled.
Lower draught rate in the occupied zone, compared to Induction 1, is visible with Induction 2 (shutters in the beam were closed). This is due to lower velocities in the room. Difference between flow patterns and field of mean velocity above the manikins is shown in Figure 7. In this case lower inlet air temperature was needed in order to reach required room temperature, leading to greater temperature difference between the inlet air and room air.
Table 1 – Draught rate, temperature differences between inlet temperature and inside temperature, temperature difference between temperature in 0.1 m and 1.1 m above the floor.
As far as flow patterns are concerned results presented in Figure 7 show that plumes from computers are stronger than plumes from people. In some case it is visible that plumes together affected jets from the beams and turned them down untimely.
The high heat load in the room (80 W/m2), caused by high radiation from the floor, if shading was not used, caused higher draught rate in occupied zone (Figure 5e, Figure 6e). The difference between the inlet temperature and room temperature was almost 7 K.
Table.1 shows temperature difference between the lowest inlet air temperature and the highest temperature in the room, the highest draught rate in the occupied zone and the highest temperature difference between 0.1 m (ankle) and 1.1 m (head) above the floor [3] for every situation.
CONCLUSIONSDraught was lower than 20 % and the vertical temperature difference between 1.1 m and 0.1 m level above the floor lower than 3 K for the studied cases. The draught risk and the temperature difference increased when the heat load in the room increased, but still remained relatively low.
The control of the supplied flow pattern had an impact of both velocity distribution and the draught rating in the rooms. Draught risk was lower and the temperature difference higher when the diffusers were arranged to provide flow with a low induction than when they provided flow with high induction.
Thermal flows generated by occupants and computers (stronger than from occupants) affected the significantly the flow supplied from the chilled beams.
ACKNOLEDGEMENTSThis research is supported by Halton Oy, Finland.
REFERENCES[1] Hyldgaard, C.E., “Thermal Plumes above a person”, Proc. of Roomvent’98, Stockholm, Sweden, June 1998, vol. 1, pp.407-413.
[2] “Thermal Environmental Conditions for Human Occupancy“, ASHRAE Standard, USA, Atlanta , 1992.
[3] “Ventilation for Buildings – Design Criteria for the Indoor Environment“, CR 1752, CEN Report, Brussels, 1998.
[5] EN ISO 7730. (1995). International Standard ISO/DIS/7730: Moderate Thermal Environments-Determination of the PMV and PPD Indices and Specification of the Conditions for Thermal Comfort, Geneva: International Standard Organization for Standardization.
[6] ASHRAE. (1992). ANSI/ASHRAE Standard 55-1992: Thermal environmental conditions for human occupancy. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
[7] CEN Report 1752. (1998). CEN Report: Ventilation for buildings. Design criteria for the indoor environment, CEN/TC 156/WG 6 (main reference).
