Studies On Mechanisms Of Fire Development Under Multi-factors And Smoke Control Method In High-rise Buildings

As the accelerating process of urbanization in the world,the urban population rises dramatically along with the expansion of cities.In recent years,many high-rise buildings have been built in the world.Disastrous high-rise building fires,which occurred frequently in recent decades,have drawn more and more public attentions to the fire safety in high-rise buildings.There are many vertical shafts in high-rise buildings,such as stairwells,elevator shafts,ventilating ducts and electric cable shafts.These shafts will become the potential paths of smoke spread during fires.Moreover,the statistics have shown that the fire-induced smoke is the most fatal threat in building fires,and about 85%of fatalities are caused by the hot and toxic smoke.Besides,buildings are usually in windy environments.The external wind may blow the flame to the leeward direction and affect the combustion inside the compartment.Therefore,the investigations on the characteristics of the fire behavior and smoke control method in the high-rise buildings are important in theory and engineering.According to the development of fire dynamics in high-rise buildings,the research topic is focused on the fire behaviors under multi-factors and positive pressure ventilation to prevent the smoke spreading in vertical channels.The researches are studied with the theoretical analysis,model scale experiments and computational fluid dynamics(CFD)simulations.The thesis is mainly composed of four parts.Firstly,the influence of stack effect on the dynamic equilibrium of fires was experimentally investigated.The results show that the position of the open window influences the location of the neutral plane,which determines the airflow velocity induced by the stack effect.Correlations are proposed for their relationship.Under stack effect,the flame tilt angles are quite close at the quasi-steady state regardless of the heat release rate of the fire and the position of the open window in a stairwell,which indicates that the inertial force of airflow induced by the stack effect equilibrates dynamically with the thermal buoyancy induced by fire plume.The velocity of airflow drawn into the fire compartment is directly related to the strength of stack effect,which is mainly influenced by the heat release rate of the fire.A new correlation for the flame tilt angle under the influence of stack effect is developed by taking the dynamic equilibrium into account.Secondly,the opposing wind force effect on combustion characteristics in compartment with simultaneous door and window opened were investigated experimentally.Results show that at low wind velocities the air flows into the compartment from lower part of the door while the hot gases and flame flow out from the window and upper part of the door.For the wind velocity being equal to the velocity at the transitional phase,the flame can no longer flow out of the window.At high wind velocities,the flames spill out from the door.An optimistic opening distance is determined for a theoretical model from the literature Chen’s theoretical model predicting the critical velocity for opposing wind force with downward flow.Moreover to predict the MLR,the heat feedback is taken into account by involving the compartment temperature in a modified model with wind velocity and the make-up air at the door.Thirdly,assisting wind force effects on the transition and extension of the flame projecting from a compartment with opposing openings were investigated experimentally.Results show that at low wind velocities,bidirectional flow can be seen at the door.The bidirectional flow will become unidirectional with the increasing wind velocity.Correlations for the hot gas velocity at the window are formulated based on the mass conservation of the flow in the compartment.The flame inside the compartment changes from the windward symmetrical rotation flame to the leeward deflection flame,as the fuel supply rate increases or the external wind velocity decreases.The flames eject from the compartment intermittently due to fluctuations of the gases ejected from the window.The model to predict flame projection probability is modified based on previous study without external wind.As the wind velocity increases,three states:namely a spill-like plume,horizontally ejected flame and no flame are identified outside the window.The Froude number can be used to determine the three states of the external flames.Then Correlations for the lengths and maximum temperature rises of the external flame are established.By validating coordinate values of trajectory databases for the spill-like plume,prediction correlations for the trajectory are developed with a notion of the physical process.Fourthly,the influence of positive pressure ventilation to prevent the smoke from entering the stairwell with different methods was investigated by CFD simulations.The results show that the minimum pressurization air supply to prevent the smoke from entering the stairwell is less in the group of 2 air supply outlets than those in the group of multiple air supply outlets.When the pressurization fan is located on the top of the stairwell,based on the mass and energy conservation,a simple method is developed to predict the airflow mass flow rates at the outlets.Further,the correlation of the average pressure in the stairwell on the fire floor and the pressurization air supply flow rate is developed.