Numerical study of a new ventilation tower system for fresh air supply in an air conditioned room.

In an air-conditioned office building, the ventilation air is normally mixed with the return air from the room in the air-handling unit. Therefore, the value of the air exchange efficiency defined by age of air is usually about 1.0, which is close to the perfect mixing case. If the fresh air and air-conditioning air are supplied separately, it is possible to increase the value of the air exchange efficiency at the breathing zone if the former is supplied directly to the breathing zone. In this paper, the results of the CFD investigations for the ventilation tower system are described. In this system, fresh air is directly supplied to the breathing zone without mixing it with the return air from the room. This is achieved by supplying fresh air through a vertical duct rising from the floor and distributing fresh air to breathing zone height. With this system, the indoor air quality at the breathing zone will be improved and the requirement of the fresh air will be reduced in comparison with usual air-distribution systems. The CFD simulation for isothermal conditions was carried out to compare the value of the air exchange efficiency between a conventional air-conditioning system and the ventilation tower system. The results from CFD simulation show that the age of air at the breathing point is about 37 minutes for the conventional air-conditioning system and about 20 minutes for the ventilation tower system when the nominal time constant for the room was about 31 minutes. This result shows that the IAQ at the breathing zone provided by the ventilation tower system is better than the conventional systems.