Improving the use of mixing factors for dilution ventilation design

In specifying dilution ventilation flow rate, a safety factor, K, is often used to provide a margin of safety and to compensate for uncertainties and health impact severity. In current practice, the selection of K is very subjective. Here the component of K accounting for imperfect mixing, Km, was studied to develop more effective and efficient design procedures. Air flow and contaminant distribution in a 10 m x 3 m x 7 m room with a single contaminant source on a 1-m high table were simulated for steady, isothermal conditions using computational fluid dynamics. A series of 10 simulations explored factorial combinations of air exchange rates (1, 2,4, 8, 16 ACH) and inlet types (a high wall jet and a ceiling diffuser). Nine additional simulations explored exhaust opening location effects and 13 other simulations investigated source location effects. Km was calculated at each of 25,600 grid locations within the room by linear regression of emission rate/flow rate (G/Q) on concentration (C). The linear relationship between C and G/Q at each of the points was nearly perfect (R2 > 0.97). For the simulations with varying dilution flow rate, Km ranged from 0.19 to 2.86 for the wall jet and from 0.94 to 4.34 for the ceiling diffuser. Holding G/Q at 100 ppm and varying source and exhaust location produced room average concentrations from 55.7 to 173 ppm. Unlike orthodox design approaches, this work suggests that air monitoring data often can be used to calculate dilution flow rate requirements. Also, dilution flow rate requirements may be reduced by enhancing room mixing with fans or altering air inlet configuration. However, mixing should not be increased if the altered room air currents could transport contaminant to an occupant's breathing zone or interfere with other control methods that depend on segregation of incoming air and contaminant.