Cooling of Ventilated Room with Cooling Ceiling Beams

  
Project leader: Professor Mats Sandberg  and Professor Bahram Moshfegh
PhD student: Jan Fredriksson
Supported by: University of Gävle and KK-Foundation

KK-Fpundation logga
  

Introduction


Nowadays it is almost the norm for companies to equip offices with at least one computer terminal per person and it is not uncommon to see other heat generating equipment such as laser writers and telefaxes stacked on desks. To couple this kind of heat generating equipment and heat produced by people and lighting with limited ability to combat solar gains, you will reach some rather high cooling loads. Usually ventilation air is used for cooling in the Scandinavian countries. The current trend for meeting cooling and air quality requirements of the building centres on the integration of ventilation systems and cooling ceiling panels. The main purpose of the present study is to establish if the two systems work in harmony.
The cooling ceiling panel systems have shown to be good complements to more traditional systems like displacement ventilation and mixing ventilation. There can be several advantages with such cooling systems, e.g. energy savings, better thermal comfort and more silent operation. There are, though, some disadvantages too. The cooling system cannot by itself remove air pollutants, odours or latent heat, this has to be taken care of by the ventilation system. Therefore, it is important that the cooling system can co-operate with the ventilation system.
The objective of this research will be to explore the flow-rate and velocity characteristics created by chilled beams in rooms. The introductory work done so far has been focused to quantitatively explore the air flow pattern created by chilled beams.

Experimental set-up


The experiments have been performed at the Royal Institute of Technology, Department of Built Environment during the autumn of 1998. A (L x W x H, 4.2 m x 3.6 m x 2.5 m) test room was set-up to provide a realistic office environment. Figure 1 shows the two different layouts used during the experiments and table 1 shows the nondimensional magnitudes.
Cooling of ventilated room - experimental set-up
Figure 1
   
Table 1
LayoutB/LA/W
10.710.086
20.0740.83
For lighting the room was equipped with four fluorescent tube fittings with a heat load of approximately 80 watts each. To simulate common office environment the room was furnished with a PC-model with variable heat-load and a “manikin" with approximately the same area and heat load as a human. Electric heating foil was used to simulate solar irradiated windows. Thermo-couples were used to measure temperatures along the surface of a wall, at the floor and ceiling surfaces and of the air in the interior of the room. For the evaluation of the cooling effect the temperature of the inlet and outlet water were measured together with the water flow rate. Table 2 shows typical heat-balances from the experiments. The differences are caused by heat losses to the environment.
To document the flow pattern under the chilled beam a digital camera and slit illumination were used together with a smoke generator with the outlet above the chilled beam.
Table 2
CasePC-modelManikinLightHeat-foilChilled Beam
1533 W0320 W388 W973.1 W
2533 W0320 W0759.6 W
30100 W320 W0408.5 W

Temperature gradients


When the heat load increases the chilled beam is unable to remove the total heat and the temperature will rise. The temperature measurements show, see figure 2, that although the temperature increases, the temperature gradient remains almost unchanged.
Cooling of ventilated room - temperature gradients
Figure 2:  Temperature gradients

Smoke visualisation


Figure 3 shows the flow below the chilled beam. The flow is very unstable and oscillates from one side to the other. This oscillation is of very low frequency and also very irregular, although there seem to be a core in the middle of the flow which is relatively stable.
Cooling of ventilated room - visualisation
Figure 3:  Smoke visualisation

Further Research


The next step in this project is to measure the air flow-rate created by the chilled beam. As the results above indicates there is also a need to examine the oscillation flow. The slow drift might influence the thermal comfort causing a sensation of draught to the occupants. The change in flow direction will, by the occupants be interpreted as an intermittence of the velocity field. To evaluate this flow behaviour the idea is to use video recording, temperature measurements and hot wire anemometry.
Publicerad av: Liselotte Laurila Sidansvarig: Gunilla Mårtensson Sidan uppdaterades: 2013-10-17
Högskolan i Gävle
www.hig.se
Box 801 76 GÄVLE
026-64 85 00 (växel)