Airflow patterns and tracer analyses

Airflows were not directly measured during the tracer experiments. Instead, they can be obtained from the transport model. Figure 3.1 displays the modeled airflows from Experiment 1. The overall airflow patterns in, out, and within the building are shown.

The airflow between the building and the surrounding environment, (i.e., infiltration and exfiltration) is dictated by the relative indoor to outdoor air temperatures, leakage characteristics of the envelope, and wind-induced pressures. The AHU recirculates

indoor air, and thus, does not actively play a role in ventilation (though it does in an indirect way as it alters the pressure distribution within the building). An important driving force is the stack effect. When the outdoor conditions are cooler than indoors, air enters through leaks in the 1st floor envelope, and exits from the 3rd floor. The staircase provides an easy pathway for this airflow to follow, and outside air will enter the interior rooms to a degree that depends on the positions of the staircase doors. When outdoor conditions are warmer than indoors, the stack flow is reversed, with air entering at the 3rd floor, and exiting at the 1st floor.

The airflow within the test unit is largely determined by the operation of the AHU, whenever it is on. The AHU contains only one ducted return pathway, located on the 1st floor. Because of this return, when the AHU is on, the 1st _{floor operates at a lower} pressure than the exterior and 2nd floor. Hence, air flows from the 2nd floor into the 1st floor. The airflow to and from the 3rd floor depends on the stack effect. Within the 1st floor, owing to the location of the return intake, air flows from Rooms 1.1 and 1.3 to Rooms 1.2 and 1.2a. Furthermore, the diffusers supplying airflow to Rooms 1.1 and 1.3 are horizontally oriented, and located in the doorway, which may facilitate the airflow to the adjacent rooms, 1.2a and 1.2b. On the 2nd floor, the researchers noted that air flows from Room 2.1 into the staircase, therefore drawing air from Room 2.2, and also from 2.3 into 2.2.

Based on the fan pressurization tests, it was evident that leakage between the interior and exterior for the first floor was relatively small, but that it was comparatively large for the second and third floors. The large leakage rates between the rooms on the first floor

are due to the presence of the air return registers, while the large leakage rates between the rooms on the second floor are due to large cracks in the floor and the walls.

Figures 3.2 – 3.5 show the propylene measurement results for Experiments 1, 4, and 13. Data are shown for this subset of experiments because they represent the greatest variability in experimental conditions and also are representative of the concentrations observed in the other experiments. Among these three experiments, there are 2 different release locations (i.e., the return intake in Experiment 1, and Room 1.3 in Experiments 4 and 13), and the AHU was both on and off (i.e., Experiments 1, 4: on; Experiment 13: off).

Figure 3.2. Time-dependent normalized tracer-gas concentrations for Experiment 1. Concentrations are normalized by theoretical peak concentration in the experiment; time is in reference to the time of the release; upper frame: first floor rooms; middle frame: second floor and third floor rooms; lower frame: staircases.

For Experiment 1, in which the release occurred at the return intake, the concentrations rise sharply in Rooms 1.2a, 1.2b, and all of the 2nd floor rooms, before

Relatively high concentrations also are observed in Stair 1, most likely owing to leakage from the AHU duct. The concentrations in Rooms 1.1 and 1.3 reach their peak values within a few minutes, but after the other rooms served by the AHU. An explanation is that a fraction of the supplied airflow to these rooms is diverted to the Rooms 1.2a and 1.2b. Therefore, less contaminant is transported to these rooms initially, and more time is required for the contaminant to reach the sensors, which were centrally located, in these rooms. The concentrations in Room 3.1, Stair 2, and Stair 3 increase toward their peak values more gradually, as these zones are not directly served by the AHU. The concentration profile is not smooth – that is, there are fluctuations, particularly for Rooms 1.2a, 1.2b, and Stair 1. These fluctuations indicate that well-mixed conditions are not instantaneously met within these zones. The fluctuations cease at ~ 5 minutes, which suggests that the mixing time for the respective zones is on this order.

Figure 3.3. Time-dependent normalized tracer-gas concentrations for Experiment 4. Concentrations are normalized by theoretical peak concentration in the experiment; time is in reference to the time of the release; upper frame: first floor rooms; middle frame: second floor and third floor rooms; lower frame: staircases.

In Experiment 4, the contaminant was released into Room 1.3. The highest concentrations are observed in the release room; in fact, the peak concentration in this room exceeds the peak concentration of all other zones by almost an order of magnitude.

1.2b. For the contaminant to be circulated to the other zones in the building, it must first be transported to the adjacent Room 1.2b, and then to Room 1.2a, before entering the intake to the AHU. Concentration fluctuations for Room 1.3 cease by ~ 8 minutes.

Figure 3.4. Time-dependent normalized tracer-gas concentrations for Experiment 13. Concentrations are normalized by theoretical peak concentration in the experiment; time is in reference to the time of the release; upper frame: first floor rooms; middle frame: second floor and third floor rooms; lower frame: staircases.

In Experiment 13, the AHU was off, and the contaminant was released in Room 1.3. The concentration profiles in this experiment are unique, mainly because the intrazonal transport is relatively slow, since all airflow occurred by nonmechanical means. The highest concentrations are observed in the release room, as expected. Since Room 1.3 communicates directly with Room 1.2b, this room experiences relatively high concentrations as well. Similarly, because Room 1.2b communicates with Room 1.2a (through an open pathway), the concentrations rise in Room 1.2a after several minutes. The contaminant eventually reaches Room 1.1, much later, and with a peak concentration an order of magnitude less than the peak concentration in the release room. In this experiment, the staircase doors were closed, inhibiting airflow from the main rooms to the stairwell. Because the stairwell is also the greatest pathway for vertical transport among floors, the concentrations in the 2nd and 3rd floors are significantly less than those on the first floor.

Figures representing the concentrations for the remaining experiments are included in the appendix (§3.A). The overall results do not differ significantly compared to the variability observed among Exps 1, 4, and 13. However there are some noticeable differences in the concentrations observed in the stairwell and on the 3rd floor. For example, if Experiment 2 is compared to Experiment 1, it is clear that the closed staircase doors inhibit contaminant transport to the staircase. Therefore, lower concentrations are observed in Experiment 2 in the stairwell. Because the stairwell is not actively served by the AHU, it is more greatly influenced by changing temperature and weather conditions and different interior door positions.