Numerical investigation of particle diffusion in a clean room

Particle diffusion with gravitational sedimentation in a clean room with different ventilation modes has been investigated by computational fluid dynamics. A three-dimensional drift-flux model combined with the deposition boundary conditions for wall surfaces and considering the settling of particles under the effect of gravitational sedimentation was adopted to simulate particle diffusion. Since there is no clear criterion to judge when a particle should be treated as a passive contaminant this paper studies particle diffusion of a wide range of particle sizes compared with the diffusion of passive contaminant under the same conditions. Diffusion of particles with sizes of 0.3-20 mu m was simulated in two different clean rooms with two different air exchange rates and particle source locations after validating the numerical model. The results show that ventilation mode, particle source location and air exchange rate can influence particle distribution in a clean room. The product of nominal time of a ventilated room and the particle relaxation time may be a good indicator to decide if the particle could be treated as a passive contaminant. If the value of the indicator is big enough, the particles could not be treated as passive. The results can also be extended for other similarly ventilated rooms.