Passive Filtration of Airborne Particles from Buildings Ventilated by Natural Convection: Design Procedures and a Case Study at the Buddhist Cave Temples at Yungang, China

Air exchange between interior spaces and the outdoor atmosphere can occur due to a variety of processes, including wind-driven flows and natural convectiondriven flows. As air is exchanged with the outdoors, airborne particles can be brought inside. Depending on the use of the indoor space, the presence of particles in indoor air could be a nuisance to the occupants or could be damaging to materials kept indoors. While one obvious solution to such problems is to install a mechanical air filtration system, that is not always practical. In particular, the character of some historical houses and some archaeological sites would be degraded by the presence of a mechanical air distribution system, and in some parts of the world the reliable electrical power supply needed for such a filtration system may not be available. In the present paper we consider principles for the design of passive filtration systems in which air motion through the filter material is induced by a natural convection flow rather than by a mechanical fan. A fluid mechanical model first is described for predicting the air flow through an interior space that acts as a thermal siphon. The effect of placing filter material in the path of such air flows is examined next. The indoor-outdoor air quality model of Nazaroff and Cass (1989a) is matched to the natural convection air exchange model, and calculations are performed to determine the relationship between the outdoor particle size distribution and indoor particle size distributions and particle deposition rates given a passive filtration system. Example calculations are worked for the case of a passive particle filtration system that could be installed to protect the interior of the Buddhist cave temples at Yungang, China. These are a collection of manmade cave temples dating from the 5th century AD, now situated in the middle of one of China's largest coal-mining regions with its accompanying air pollution problems.