Modeling Research On Predictions Of Vent Flows And Product Species Compartment Fires

The objective of this research is to develop practical techniques and engineering tools on predicting compartment fires regarding the fire-induced flows and fire product species.Vents in a compartment act as pathways for exchange of inside hot smoke and outside cool air. The hazards accompanied with vertical vents in a compartment, most common doors and windows, lie in supplying oxygen to fuel to keep burning and meanwhile spreading the fire and smoke to adjacent rooms. A mathematical study is made to compute the vertical vent flow behavior due to fire in a room. Two approaches were taken, first a model attempting to include the effect of fire entrainment and vent mixing; second was a model based on an ideal point source plume fire----both in the zone model concept. The equations sets include equations for entrainment, inflow, outflow, mixing, and a conservation equation regarding lower temperature under an adiabatic assumption. Limiting analytic results were found for the latter to give insight into the physics. A theoretical model based on an ideal point source fire plume is used to guide the form of the empirical correlation. The two vent configurations (doorway and window) are merged into one equation and a uniform formula for wall vent flow rate is developed based upon a theoretical derivation and mathematical fit to data. A thorough examination concerning the difference between the doorway and window flow modes is conducted. Both sill height and width of the windows pose key influence on the formula. The results were compared to available flow data and shown to be within 15% accuracy for a wide range of fire conditions. CFD computations were also explored by using FDS. Results from FDS and the zone model are compared with experimental data for a wide range of variables. The FDS model, as in any CFD model, attempts compute the complete physics, but lacks the full ability to resolve all scales of phenomena in current computational processes, thus giving rough results with up-to-40% error.Predicting and calculating the major species products in compartment fires is of great importance since these species pose great hazard on occupants'evacuation and fire protection. To better correlate the empirical data, an approach using the mixture fraction, or mass fraction of the fuel atoms, is introduced which can be used to predict the species yield for the unsteady-state fire situation. The derivation is based on a simplified two-zone modeled compartment fire scenario. As it is shown that the mixture fraction can be the only function of the species concentration, the available experimental data can re-correlated in the form of mixture fraction, instead of the equivalence ratio, employing the relationships between mixture fraction and the equivalence ratio. Thus the data can be used to compute species concentration for unsteady fire scenarios. Examples for compartment fire during the smoke filling period and smoke outflow period are derived and illustrated as applications of this method.