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The door and window have significant effects on the displacement effectiveness of DVsystem (Lin et al., 2007). The displacement effectiveness forCO2 and VOCs are significantly reduced by the door openingdue to the change in airflow pattern. The added momentumfrom the incoming air causes pollutants to be displaced intothe breathing zone. The lateral displacement of the air dueto the window heat transmission may result in disruption ofthe displacement effect, which reduces the effectiveness ofDV systemin removing contaminants. The influence of furni-ture is related to the location and distribution. Furniture hasminimal impact on thermal comfort and IAQ when they donot obstruct the airflow from diffusers (Lau and Chen, 2007).Stratification boundary is the interface between the uppermixing zone and the lower zone with unidirectional flow. Theair of occupied zone will be sanitary only when the height ofthe stratification boundary is above the occupied zone. There-fore, the height of the stratification boundary is a very impor-tant parameter in DV system. Lee and Lam (2007) calculatedthe height by simulation and found that with a room heightof 2.4m and design room temperature of 25.5 C (defined at1.1mabove floor level), under the normal load to airflow ratioof 12,000Wm 3s and minimum supply temperature of 18 C,the height was 2.2 m, and was above normal breathing level.There are many factors that affect the height. The heightdecreased as the room load increased, but the height in-creased as the room load increased further when the roomload was greater than about 45Wm 2(Xing and Awbi,2002). The height increased as the flow rate increased fora given heat source at a fixed surface temperature. Fora fixed flow rate and a given heat source diameter, theheight decreased as the heat source temperature increased(Bouzinaoui et al., 2005). DV system is quite suitable forlarge size room due to its stratification characteristics.Future research could explore the possibility to apply DVsystem in ordinary rooms.
3.2.2. Personalized ventilation (PV)PV is a special DV system, and it really embodies the centralstatus of occupants. PV is able to provide occupants withimproved IAQ, thermal comfort and inpidual control of the micro-environment. Through adjusting the personalizedairflow rate, direction, temperature and turbulence intensity,occupants can achieve their preferred inpidual micro-environment, which is impossible in conventional air-conditioning systems.There are many types of PV system. For the desk-edge-mounted PV system, it was possible to obtain an air changeeffectiveness of approximately 1.5 when the supply flow ratewas 3.5–6.5 l s 1, which represented a 50% increase in effec-tive ventilation rate at the breathing zone (Faulkner et al.,2004). Niu et al. (2007) proposed a chair-based PV system(Fig. 6), and the position of the fresh air supply nozzle canbe adjusted by the seated user. The experimental resultsshowed that up to 80% of the inhaled air could be composedof fresh personalized air with a supply flow rate of less than3.0 l s 1. Perceived air quality was improved greatly by thechair-based PV system. Zhao and Guan (2007) investigatedthe dispersion characteristics of particles with aerodynamicdiameter of 0.5–10 mm in a room ventilated by a PV systemby computational fluid dynamics (CFD). The results showedthat PV was effective to remove particles smaller than2 mm, and that PV might not be the best ventilation modefor particles bigger than 7.5 mm due to resulting obviousparticle accumulation on the floor.Compared with total volume ventilation, PV is advanta-geous for many practical applications. Research is needed onexploring the potential of PV and ensuring its optimalperformance. Some of this research is outlined as follows(Melikov, 2004): Impact of airflow temperature and velocity on thermal com-fort and perceived air quality, and occupants’ compromisebetween thermal comfort and perceived air quality. Airborne transmission of infectious agents, and dispersionof large-droplet aerosols exhaled by occupants. Development of the supply air terminal devices (ATD) withhigh efficiency and of the technical solutions to installationof PV in practice. Development of PV systems with high performance for usein crowded spaces such as theaters and cinemas.