Studies On Compartment-Facade Fires With A Wall-or Ceiling Vent Under Wind

In recent years,more and more high-rise buildings appear as the acceleration of China's urbanization process.The rapid development of high-rise buildings also brings fire risk and the building fire is usually caused by the fire in a single room in the building.When the air flowing into the room through the vent is insufficient to support the combustion inside the room,the fire enters the ventilation-control stage.The unburned fuel overflows from the vent of the room with high temperature smoke and burns on the facade(facade fire)forming the phenomenon of flame ejection.Flame ejection is the key factor to cause large-scale fire spread on building facade.On the other hand,the ceiling vent of the room usually undertakes the functions of natural smoke exhaust and lighting.The fire behavior,indoor flow field and fire evolution are different from those of the room fire with a wall vent,and the fire problem has also attracted the attention of scholars.However,previous studies on the flame ejection of a room fire with a wall vent and room fire with a ceiling vent are mainly concentrated on the conventional ventilation conditions,which are different from the ambient wind situation of building fire.The ambient wind usually has a significant impact on fire behavior and fire evolution,but the previous research is still relatively insufficient.In this paper,the characteristics of flame ejection of compartment fire with a wall vent and the fire evolution characteristics of compartment fire with a ceiling vent under the ambient wind are taken as the core.The characteristic parameters of facade flame under the boundary conditions of sidewall and horizontal eave and the fire evolution characteristics of compartment fire with a ceiling vent under the ambient wind were studied experimentally by using the reduced scale experimental platform and analyzed theoretically.At the same time,CFD numerical simulation was carried out to observe the flow field,oxygen concentration and other parameters in the experiment.Based on the facade flame entrainment model and the principle of fluid mechanics,the influence mechanism of different boundary conditions on the above-mentioned evolution behavior and characteristic parameters is revealed and the characterization model under the corresponding boundary conditions is proposed through theoretical derivation.The research work of this paper mainly includes the following aspects:(1)The facade flame height evolution ejected from a fire compartment under the coupling effect of ambient wind and sidewall was studied.The results show that facade flame height under the coupling effect increases with the sidewall separate distance decreases and decreases with the ambient wind speed increases.The air entrainment mechanism of facade flame was analyzed and the mechanism of the coupling effect of ambient wind and sidewalls was revealed.The coupling effect on facade flame height was theoretically characterized,and a prediction model of facade flame height under the coupling effect of ambient wind and sidewalls was established.(2)The extension behavior and mechanism of facade flame beneath the horizontal eave under the ambient wind ejected from a fire compartment was studied.It is revealed that the flame extension lengths in the direction parallel to-and perpendicular to the ambient wind beneath the horizontal eave decrease with the ambient wind speed increases and decreases with the horizontal eave height increases.And it is also found that the extension length in the direction parallel to the ambient wind is smaller than that in the direction perpendicular to the ambient wind.The effect mechanism of ambient wind on the facade flame extension behavior beneath the horizontal eave is analyzed assisted by CFD simulation,and the prediction model of flame extension length in the direction parallel to-and perpendicular to the ambient wind is established,respectively.(3)The temperature evolution and flame behavior characteristics of compartment fire with a ceiling vent under the ambient wind were studied.For Bernoulli flow through the vent(with a small vent size),the flame extinction phenomenon was found and the extinction heat release rate is independent of the ambient wind speed and fire source location,but increases with the vent size increases.For the oscillatory exchange flow through the vent(with a large vent size),the flame transition phenomenon under the ambient wind was found and the associated critical heat release rate decreases with the ambient wind speed increases and increases with the vent size increases,and is independent of the fire source location.The evolution law of temperature distribution and flame behavior is revealed.With the help of CFD simulation and based on the principle of fluid dynamics,the characteristics of indoor flow field are analyzed,and the influence mechanism of ambient wind and fire source location on temperature evolution and flame behavior is revealed.Based on the analysis of the air exchange rate through the vent,the Froude number is proposed and the characterization model of the critical heat release rate of flame transition is established.