Ventilated Room
Often the ventilation of a room is ad-dressed in a macro scale level. Macro scale is the traditional level of description of air distribution in rooms as e.g. in standards where it is expressed that con-taminant removal effectiveness in a room with mixing ventilation should have the level of ε = 1.0. This means that the con-centration in any point of the occupied zone, coc, is equal to the concentration in the return opening cR. It also means that a person is exposed to the same contamina-tion from another source person (passive smoking, airborne cross infection) inde-pendent of the location in the room when the persons are not too close to each other (Figure 8.25A). When the occupants are close to each other the exposure can rise to a high level independent of the general contaminant level of the occupied zone. In other words, if the air distribution system is designed to make an efficient ventilation of the room there will still be a microenvi-ronment around people close to each other, which can’t be influenced by the general air distribution system (Figure 8.25B).
The microenvironment around two per-sons is visualized in Figure 8.26. The exhalation from a source person can be divided in two parts. One part of the exha-lation flows into the macroenvironment (the occupied zone), and the general air distribution system in the room dilutes and transports this part out of the room and creates a concentration distribution around the target person, coc. The other part of the air exhailed from the source person flows directly to the target person’s breathing zone, or to this person’s body thermal boundary layer. The target person is
ex-posed to a level, cexp, and this exposure therefore consists of an indirect exposure from the macroenvironment, coc, and a direct exposure from the source person’s exhalation. The concentration coc can be measured at the chest of the standing tar-get person. This concentration is also the concentration in the air inhaled by the target person if this person is not influ-enced by a direct exposure, because the inhalation normally originates from the thermal boundary layer (Brohus and Niel-sen 1996, Bjørn and NielNiel-sen 2002).
Figure 8.25. A) Fully mixed flow, same concentration everywhere. B) Illustration of the microenvironment with a local high exposure.
The concentrations cexp and coc are given in a dimensionless form where they are di-vided by the return flow concentration ce. coc/ce = 1.0 therefore corresponds to a fully mixed flow in the occupied zone, and cexp/ce = 1.0 corresponds likewise to a fully mixed exposure level.
A personal exposure index εexp (Brohus and Nielsen 1996, Mundt et al. 2004) can be defined as:
εexp = ce/cexp (8.19)
Figure 8.26 .A source manikin and a target manikin. The contaminant flow between the two manikins is visualized by smoke.
Measurements were performed in the mi-croenvironment of two breathing thermal manikins resambling standing persons. A ceiling radial diffuser is used for air distri-bution in a room. The air change rate is 5.0 h-1. The two breathing thermal manikins are surrounded by the natural convection boundary layer around their body. The layer is uninfluenced by downward draft from the diffuser above the manikins. The results of concentration measurements are shown in Figure 8.27. The concentration in the occupied zone, coc/ce, has a level of 1.25 to 1.50. The figure shows an in-creased effect of the exhalation flow in the microenvironment at reduced distance between the manikins. The personal expo-sure index εexp is 0.7 when the distance between the manikins is equal to or larger than 1.2 m, but is reduced to 0.5 (cexp/ce ~ 2) when the distance is 0.4 m. Exhalation through the nose seems to give the lowest level of exposure. Figure 8.27 shows the
avaraged exposure, but the peak values for exposure through both mouth and nose can be up to cexp/ce = 7.5 in shorter periodes (Nielsen et al. 2008).
The exposure will also be dependent on the persons activity level, difference in height and positions relative to each other.
Figure 8.27. Concentration in the occupied zone and exposure of the target manikin for two mani-kins of same height. (●) coc/ce, normalised con-centration in the occupied zone in case of exhala-tion through mouth. (▲) cexp/ce exposure in case of exhalation through nose. (■) cexp/ce exposure in case of exhalation through mouth.
The measurements are made in a room with mixing ventilation (ceiling-mounted radial diffuser).
8.6 Examples of mixing air distribution
Chilled beams
The impact of airflow interaction in a room with active chilled beams on the air distri-bution and occupants’ response was stud-ied. Experiments were performed in a full-scale test room, with size of 5.4 m x 4.2 m x 2.5 m (height). Three chilled beams (each 1.8 m long) were installed on the ceiling along the room (Figure 8.28). The primary airflow rate supplied by the chilled beams
was 1.5 L/s/m2. During the physical meas-urements two thermal manikins were used to simulate people in the room. The surface temperature of the manikins was controlled to be identical with the skin temperature of an average person in a state of thermal comfort. The manikins represented an aver-age female body size. They were dressed with clothing typical for an office type work, i.e. underwear, trousers, blouse, etc.
(total clothing thermal insulation 0.7 Clo).
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 win-dows of size 2.0 m x 1.8 m and several smaller heating panels placed on the floor of total area 3.6 m2. The total heat load in the room was 50 W/m2 (windows 2x220 W, PC- 2x80W, thermal manikins – 2x75 W, floor heating panels – 280 W, lighting – 116 W). The room temperature was kept at 24.0 ± 0.1 ºC, supply air temperature was 20.0 ± 0.2 ºC and surface temperature of windows was 36.5 ºC. The amount of room air inducted into the beams was regulated by shutters placed in the beams. There were two positioning of the shutters’ settings, opened or closed, as shown in Figure 8.28.
The setting with closed shutters was named
“Induction 1” and the setting with opened shutters was named “Induction 2”.
Comprehensive measurements of mean velocity, turbulence intensity and air tem-perature field in the room were performed with multichannel low velocity anemome-ter. The mean velocity field and the tem-perature field in a plan across the chilled beams at the location of the thermal mani-kins are shown respectively in Figure 8.29 and Figure 8.30 (Zboril et al. 2007). The
impact of the interaction of the powerful thermal flow generated by warm window, computer and thermal manikin (left side of the figures) with the airflow supplied from the left side of the chilled beam is clearly seen: the thermal flow discharges the venti-lation flow to the opposite (right) side of the room. The manikin measurements iden-tify increased heat loss from the upper body segments of the manikin seated far from the windows (right side in the figure) due to the elevated velocity resulting from the airflow interaction.
Figure 8.28. Layout of chilled beams and workstations during the tests. The control of the air supplied from the chilled beams
(“Induction1” and “Induction 2”) is shown.
Figure 8.29. Mean velocity filed across the room at the location of the thermal manikins, velocities range from 0.05 m/s (blue) to
>0.3 m/s (red).
Figure 8.30. Air temperature filed across the room at the location of the thermal manikins, temperatures range from <22°C (grey) to
>26°C (red).
The same lay out of chilled beams and workplaces was employed to study the impact of the airflow interaction on occu-pants’ thermal comfort (Melikov et al.
2007, 2007a). Thirty subjects (15 males and 15 females) participated two at the time in three experiments all at primary supply flow rate of 1.5 L/s/m2. The heat load and the induction control during the three ex-periments was different: experiment 1 - 70 W/m2 and induction 2 on both sides of the chilled beams, experiment 2 – 70 W/m2, induction 2 on the left side and induction 1 on the right side of the chilled beams; ex-periment 3 – 30 W/m2 on both sides of the chilled beams. The heat load from the win-dows and the floor panels was increased respectively to 2x325 W and 520 W in order to obtain 70 W/m2. The 30 W/m2
were obtained with heat gain from the win-dow 2x200 W and people 2 x 75 W (in average).
Figure 8.31 compares the number of sub-jects who reported draught discomfort dur-ing the three experimental conditions. The largest number of draught complaints were reported at workstation 1 (WP1) during the experiment 1. In this case the strong ther-mal plume generated by the person, the warm window, the computer and the floor heating elements at the location of WP2 discharged the air supplied from the left side of the chilled beams toward the WP1.
As a result the person at WP1 was exposed to relatively high velocity which caused draught discomfort for 7 of the participants (more than 25% of the subjects participat-ing in the experiment). At the same time only 3 persons reported draught at WP2.
The use of the induction control (induction 1) on the right side of the chilled beam aimed to decrease the strength of the sup-plied flow and respectively the flow veloci-ty and thus to decrease the risk of draught at WP1. Indeed, as the results in Figure 8.31 show the number of dissatisfied sub-jects decreased almost to half the number, from 7 to 4 subjects. This action decreased also the draught complaints at WP2; only one person felt draught under these condi-tions. In the experiment 3 the decrease of the heat load on the left side of the room lead to weaker thermal plume generated only by the warm windows and the person.
The thermal plume did not have the strength to deflect the ventilation flow sup-plied from the chilled beam. Therefore the velocity at the WP1 decreased further the number of subjects complaining of draught.
At the same time the velocity at WP2 in-creased causing discomfort to four of the subjects participating in the experiment.
Figure 8.31. Number of subjects dissatisfied due to airflow interaction in a room with chilled beams.
No IC = “Induction 2”; with IC – “Induction 1”.
The example of airflow interaction and its impact on occupants’ comfort is typical in practice. In order to solve problems related to complaints from occupants knowledge on airflow pattern is important.
8.7 An experiment for transient