Interestingly, there was significant fluid separation in all three initial scenarios, with the denser pollutant sinking to the ground to form a thick concentrated blanket. This layer was important to consider because any disruptions to it would form a cloud of highly concentrated and hazardous pollutant.
The importance of this was seen in Scenario 1, where the purifier being placed parallel to the direction of ventilation did not reduce the velocity of the air coming out of the purifier, contrary to that in Scenario 2 where opposing velocities minimised the final velocity out of the purifier. This higher comparative velocity in Scenario 1 led to the formation of a recirculation zone at the back wall, significantly disrupting the pollutant blanket. Comparing the results (at the occupant breathing zone) in all three scenarios, Scenario 1 showed more occupants breathing in hazardous regions of fluid. The recirculation zone also caused the nearby occupant to feel uncomfortably cold, and this is important as studies have highlighted that excessive cooling aggravates symptoms of the Sick Building Syndrome.
Stagnant zones were also evident in all three scenarios. The differing results are put down to increased purifier velocity in Scenario 1, where the Coanda effect was more prominent, with air travelling along the walls and reaching further, dispersing more of these stagnant zones.
These results, along with a few others, highlighted the significant effects of purifier placement. Thus, it was decided that a new purifier efficiency metric would be designed, considering the wider-system performance instead of just filter strength. The final formula developed compared the overall pollutant removed by the purifier – accounting for filter strength – against the level of pollution in the breathing zone, as reducing pollution in this region is paramount in ensuring safety of the occupants in the room.
Pulling the necessary results from each scenario, a purifier efficiency rating was calculated. Interestingly, Scenario 2 had the highest efficiency rating, followed by Scenario 4. (Reminder: The purifier in Scenario 4 has the improved filter and is placed in a different location to the one in Scenario 2.) Even though Scenario 2 had a higher room concentration and a poorer performing filter, the concentration in the breathing zone was very similar to that of Scenario 4. This example highlights the need to consider not only filtration efficiency, but also the placement of the purifier.