A phenomenological model of near-field fire entrainment

A phenomenological model of air entrainment into combusting regions of axisymmetric fire plumes is developed and compared with some existing experimental entrainment data. The model considers engulfment of ambient air into a fire plume due to the action of the toroidal vortices that are formed periodically close to the source of an axisymmetric fire. It is shown that the diffusive entrainment rates of air towards a wrinkled flame sheet representing the turbulent diffusion flame severely underpredicts the entrainment rates by at least two orders of magnitude, suggesting that the turbulent large-scale engulfment is the primary mechanism of entrainment. By describing the entrainment process as periodic engulfment of ambient air around the toroidal vortex rings and the frequency of formation and passage of these vortex rings in combusting regions of fire plumes, it is shown that the modeled entrainment process gives good agreement with the experimentally determined fire plume entrainment or plume mass fluxes. This model accounts for the physical processes of unsteady fire plume dynamics, including the effects of pulsations for the first time. The model predicts the plume mass flux to grow linearly with height above the fire source and to be independent of the fire heat release in the visible flame region. The predicted scaling with respect to the source diameter appears to be in the range suggested by the experimental data. Overall, this new model provides a better foundation for the scaling of entrainment rates in the near-field of fire plumes. The methodology can be extended to predict entrainment rates in other periodic unsteady vortex dominated flows, such as pulsating buoyant non-reacting plumes. (C) 1998 Elsevier Science Ltd. All rights reserved.