Theoretical Analysis And Prediction Of Fire Dynamics For Pool Fires In Ceiling-vented Compartments

In ship engine rooms, nuclear stations, and undergound structures, compartments with the only opening located in the ceiling are very common. Due to the lack of vertical openings, fires in such compartments are different. Firstly, the environment in which fire occurs is unsteady because the smoke fills up the enclosure very soon after ignition. And also, the burning process would become more and more insufficent as the oxygen cannot be replenished in time. In this way, it is with great importance to carry out experimental and theoretical research on the vent flows in such ceiling vented compartment fires and also their impact patterns on the fire environment and further burning behaviors.The flow pattern and flow rates of the horizontal gas flows in the ceiling vented compartment fires. The vent flows are controlled mainly by the thermal environment and play important roles on the oxygen concentrations in the compartment in return. In ceiling vented compartment fires, the gas temperature will increase firstly and then rarely changes after a period of time. Following the ideal gas assumption, this suggests that the density difference across the vent would keep nearly constant and the pressure difference across the vent caused by thermal expansion would soon decrease. Classical horizontal vent flow models were employed to determine the transient flow pattern and to calculate the air inflow rates using the gas temperatures taken from the experiments. With very small horizontal opening, the vent flow was unidirectional at first and changed to bidirectional flow in relatively short time. While for larger ceiling vents, the vent flows were always bidirectional in the burning procedure. Air inflow rates were calculated because of its importance for supporting the burning. The air inflow rates rose fast initially and kept rather steady after the initial growing stages. Also, the inflow rates grew significantly with the increasing vent size.The temperature distribution assumption and prediction models for instantaneous temperature distributions. In very poor ventilated compartment fires,temperature appears to increase with height in the smoke layer and the frontier of the smoke layer descends to the floor very soon. Based on the observation, the linear temperature distribution assumption was proposed. The average temperature can be estimated from the energy conservation of the control volume, and the average temperature in the ceiling jet can be used as the highest temperature in the compartment. With the linear temperature assumption, the instantaneous temperature distribution can be predicted. When calculating the average temperature, the determination of detailed heat transfer process needs the calculation of heat transfer from walls and also the horizontal vent flows. Alternatively, a two-stage constant heat loss fraction model is developed to simplify the calculation of the average temperature.Before the frontier of the smoke layer reaches the bottom, the heat loss fraction is in the range of 0.6 and 0.9, and often a value of 0.7 would give sufficient accuracy. And after that, the heat loss fraction grows to a relatively high value, mostly above 0.95 indicating little heat would sustain in the gas phase.Prediction of combustion efficiency for pool fires using the concept of equivalence ratio. The concept of equivalence ratio was utilized to describe the ventilation conditions for the pool fires in ceiling vented compartments. In this way,the effects of descending oxygen concentrations in the bottom part of the comparment on the burning behavior of the pool fire can be revealed through this parameter. Also,the increasing of gas temperature in the compartment also works by influencing the entrainment process of the fire plume. Further, with the value of equivalence ratio of the flame, using the correlation established by Tewarson, the combustion efficiency of the pool fire can be determined accordingly. This method was proved to be reasonable using the data in compartment fires without any vent carried out by other researcher.Further, this model was applied to study some experiments. With the fuel mass loss rate histories taken from experiments, the heat release rates can be estimated using the prediction model of combustion efficiency. It can be found that the combustion efficiencies kept decreasing in the burning processes. The combustion efficiencies were smaller with larger fires. Increasing the vent size would significantly increase the combustion efficiency. However, the heat release rates of the same pool size changed little for varying opening sizes.Comprehensive prediction model for ceiling vented compartment fires.Based on the linear temperature distribution assumption, gas temperature distributions can be estimated. Thus, the flow pattern, the air inflow rates and smoke outflow rates can be determined. A comprehensive prediction model combining the calculation of horizontal vent flow and the prediction models for temperature distribution and the combustion efficiency was developed on the basis of the conservation of energy and oxygen in the compartment.