The increase invelocity was due to the downward airflow motion caused bythe negative buoyancy force from the CC. For a cooling loadof 62Wm 2and CC temperature in range from 21 to 14  C,Loveday et al. (1998) measured maximum air velocity of0.11ms 1in the occupied zone. Behne (1995) measured theair velocity of 0.11–0.16ms 1for the same cooling load. Inboth studies, the velocities were far below the velocity thatcan cause draft. According to Behne (1995) and Fitzner(1996), there was no risk from draft caused by CC if the totalcooling load in a space was less than 100Wm 2(floor area).Downward convection below CC, downward convectionnear cold walls and upward thermal plumes from heat sour-ces have important effects on the height of the stratificationboundary, and the air quality in occupied zone in a roomwith CC/DV (Fig. 4). The downward convection below CCmay reduce the height of the stratification boundary in CC/DV systems. According to Loveday et al. (1998), the stratifica-tion boundary appeared at a height of about 2.0m regardlessof cooling load (from25 to 52Wm 2) in his experiments. How-ever, when CC system was on, the boundary layer wassuppressed to 1.5 m above the floor for a cooling load of62Wm 2. The downward motion from CC might suppressthe stratification boundary into the occupied zone, especiallyin room sections without heat sources. This causes unex-pected reverse effects on the air quality. Fitzner (1996)analyzed the influence of heat source and upward convectionon local air quality, and found that sitting person might haveremarkable improvement of the inhaled air quality due to thebuoyancy which drives the transport of fresh air from thelayer near the floor to the person’s nose (Fig. 4). Alamdari(1998) reported that, for a cooling load of 60Wm 2, upwardconvection was dominant in the vicinity of the occupantseven with the strong downward air motion from the coolingpanels. This study also showed that the downward convectionnear the sidewalls with low wall temperatures could cause a transport of pollutants from the upper mixed zone directlyinto the air supply layer with clean air.The occurrence of condensing water is not allowed abso-lutely on the surfaces of CC panels, so it is necessary todevelop accurate control technique to prevent condensation.In addition, the ratio of the cooling load removed by DV tothe cooling load removed by CC has significant effects onthermal comfort and IAQ, which deserves further study.3.2. Air supply modesProminent airflow distribution as well as outstanding ACsystems is necessary in order to achieve a comfortable andhealthy environment in occupied zone. Air supply mode isthe main influence factor of airflow distribution. Recentresearch is mainly focused on displacement ventilation (DV),personalized ventilation (PV) and under-floor air distribution(UFAD).3.2.1. Displacement ventilation (DV)DV is a new type of air supply mode, and it only improves theenvironment in occupied zone and not all the spaces. HenceDV not only saves much energy but keeps better IAQ. Indoorairflow pattern and distribution profiles of temperature andcontaminant concentration are shown in Fig. 5. Air tempera-ture is rather uniform at horizontal level except in the regionnear the air supply diffusers. Vertical temperature gradientsalways exist in the space with DV system and they are notlinear in all the space height. DV, compared with mixingventilation (MV), has higher ventilation efficiency and IAQdue to its piston flow effect and temperature stratification.However, DV may not provide better IAQ than MV if thecontaminant sources are not associated with heat sources,such as VOCs from buildingmaterials (Lin et al., 2005; Cheonget al., 2006). Additionally, DV system is not suitable forremoving ground level contaminants, or where the maincontaminants are emitted at the ground level, because thecontaminants would simply be displaced into the breathingzone. For gaseous contaminants, the displacement effect ofDV system is probably dependant on the molecular weightof contaminants. Since VOCs have greater mass, thedisplacement effect is significantly less than that for CO2.There are many factors that affect the design and theperformance of DV system. 室内空气空调系统英文文献及中文翻译(5):http://www.751com.cn/fanyi/lunwen_30304.html