MODELING INDOOR DISPERSION OF AEROSOLS OR VAPORS AND SUBSEQUENT VENTED FIRE OR EXPLOSION

Abstract

Considerable developmental work has gone into modeling dispersion of accidental outdoor releases; less has been applied to indoor releases. Indoor dispersion is characterized by the influence of a ventilation system and confining surfaces that facilitate aerosol rainout. Stratification can occur so that only part of a room contains flammable vapors, giving rise to so-called partial volume deflagrations. Indoor explosions are more complex to treat because there are two important regimes: before and after explosion vents, windows, or other panels open. We describe here a model, WELMIX, to calculate time-varying concentration changes within a room from time-varying inputs such as occur with pool evaporation. The model allows for gas sensors in the room connected to automatic controls of the fresh air/recycle ratio and ventilation rate. We illustrate here the influence of ventilation rate and fresh air/recycle ratio on concentrations. A mixing efficiency is applied that is useful when the flammable vapors would not fill the entire room such as when the spilled material has low volatility. We also describe two modules of the INVENT model to treat ignition of indoor vapor clouds. The INVENT model calculates the flame velocity, fireball growth with time, discharge rates, pressures, and temperatures inside and outside a room or dust bin. We concentrate here on the calculation of temperature and pressure before vents open.