Ventilation Effect On Critical Fire Behaviors In Ceiling Vented Compartments

Compartment fire is one of the most classical issues in the fire research history. After decades of efforts, our knowledge to ordinary room fires has been significantly improved, and a plenty of simplification theories have been established to idealize the problem. However, many of these theories no longer apply for ceiling vented comportments such as ship chambers, underground structures and large atriums. Challenge goes together with chance in learning ceiling vented compartment fires.This research focuses on the ventilation effect on the critical fire behaviors in ceiling vented compartments. The critical fire behaviors, which means fire behaviors occurred in critical conditions, include two primarily behaviors:Self-extinction and Ghosting flame. Firstly, in this research, the buoyancy and pressure induced horizontal vent flow problem, which helps to determine the boundary conditions of the ceiling vented compartment fires, has been learned numerically. Then, based on theoretical analysis, experimental investigations and numerical simulations, characteristics, boundary conditions and controlling mechanisms of self-extinction and ghosting flame have been learned. And finally, prediction models of the typical fire parameters have been established using a dimensionless parameter derived from conservation equations. The principal conclusions obtained from our research under the specified conditions are as follows:The horizontal vent flow dominated by both buoyancy and pressure was learned numerically. Numerical simulations on purely buoyancy induced vent flow and buoyancy and pressure co-induced vent flow have been carried out respectively. The distribution of the flow field, the flow rate as well as the flow oscillation frequency were obtained. It was found that the buoyancy induced horizontal vent flow was mainly controlled by Rayleigh-Taylor instability, which presented alternant bi-directional flow with constant frequency. In each cycle, the fluids interface experienced linear deformation, nonlinear rise, shear dissipation and mixing separation in sequence. It could also be found that the pressure affects the vent flow significantly. When 0<△P/△Pcn< 1, the vent flow still presented bi-directional flow, but flow from higher pressure side to the other was clearly larger than the opposite. While △P/△Pcn> 1, the vent flow changed into uni-directional flow. The results proved that Epstein and Cooper’s vent flow calculation methods, which is concluded from salt-fresh water modelling, were also applicable for gaseous fluids.The self-extinction behavior of fire in ceiling vented compartments was investigated experimentally. Typical fire parameters, including mass loss rate of the pool fires, gas temperature and oxygen, were obtained from experiments in variation of ceiling vent size and fire size. Two fire extinction modes, which were ventilation limited self-extinction and burn-out extinction, were found in the ceiling vented compartments. The boundary of the two extinction mode was also determined. Based on the analysis of energy and species conservation equations, the transportation of oxygen within the compartment was found to be the key factor controlling self-extinction, while the energy transmission provided basis of the transportation. According to the adiabatic flame temperature theory, a model in predating self-extinction was obtained. It was found that the critical oxygen concentration for burning decreases slightly with environmental temperature.The behavior of ghosting flame in ceiling vented compartments was studied both experimentally and numerically. Experiments were carried out in two ceiling vented compartments, a cubic one and a narrow one. Numerical simulations were also accomplished according to the narrow compartment model. The effects of ventilation, fuel boiling, fire size and gas circulation on the behavior of ghosting flame were revealed. The results indicated that ventilation condition and fire size had significant influence on ghosting flame while fuel boiling, which though greatly increased fuel mass loss rate, was not directly relevant to ghosting flame. It is also important to find that only under large vent size circumstance can ghosting flame present. In other words. ghosting flame would never present for any fire size when the ceiling vent was small, which indicated that ghosting flame should be related to gas flow within the compartment. Using a flame stabilization theory, a generation mechanism of ghosting flame was explained. Since flame propagation rate, SL, has to be equal to the local flow rate, uL, when the flame is stabilized, it should be uL> SL when the flame was blown off. Thus, a model can be proposed by linking ghosting flame with local oxygen concentration and flow rate to predict the its occurrence.Non-dimensional parameters governing fires in ceiling vented compartment were obtained, and relationship between typical fire parameters and these dimensionless parameters were learned. By rearranging mass, energy and species conservation equations into dimensionless forms, two dimensionless parameters, Av5/4/Qt and hAstα/Qt. were derived. Av5/4/Qt characterizes the ventilation conditions of the ceiling vented compartments, while hAsTf/Qf is related to the heat loss from the compartment surface. By analyzing the results from ethanol pool fires, if can be found Qf/Av5/4 was a more impartment parameter in describing ceiling vented compartment fires. According the experimental results of Chapter 3 and 4. The relationship of Qf/Av5/4 with extinction time, gas temperature, plume temperature and oxygen concentration was figured. It could be concluded that the fire behaviors in ceiling vented compartments can be classified into four groups, includes:Ventilation limited self-extinction fire, transition fire, steady burning fire and free burning fire.