Study On Combustion And Fire Spread Behaviors Of Typical Wood Structure Buildings

Wooden structure building has been widely used in the world because of its green,environmental protection,energy saving and convenience.However,due to the flammability of wood and the high density of building fire load,wood structure buildings are prone to fire under the influence of external fire sources（such as electrical fire,lightning strike,etc.）.At the same time,due to the special material properties and structural types of wood structural members,fire often develops rapidly and is difficult to extinguish,resulting in heavy casualties and economic and property losses.Therefore,it is of great theoretical value and practical significance to study the fire behavior of timber buildings.Most of the existing wooden buildings have good historical and cultural heritage value.With the increase of service life and the change of use function,the fire risk increases synchronously.In this paper,the components of existing wooden buildings in different ages are selected to study their pyrolysis characteristics and combustion characteristics.At the same time,the combustion behavior of Chinese fir widely used in wooden buildings is studied.In summary,focusing on the pyrolysis characteristics,combustion characteristics of Chinese fir in different ages and the fire spread characteristics of timber structural components and single timber structural buildings,this paper adopts the method of combining experimental research,theoretical analysis and numerical simulation to study the fire characteristics of Chinese fir,timber structural components and single timber structural buildings under different working conditions.The specific research includes the following four aspects:（1）The pyrolysis behavior of Chinese fir under different natural aging time indoors and outdoors was studied,and the influence of natural aging time and heating rate on the pyrolysis characteristics of Chinese fir was revealed.Based on the model-free function method and the model fitting function method,the activation energy of each conversion rate and the reaction mechanism of each pyrolysis stage were clarified.The results showed that with the increase of heating rate,the pyrolysis curve shifted to the high temperature side,and the initial pyrolysis temperature and T5%decreased with the increase of natural aging time.The mass loss rate curve of Chinese fir with natural aging time of 70 years and 100 years shifted to the low temperature side by 15-20 K;the activation energy of indoor and outdoor Chinese fir with different natural aging time increases with the increase of conversion rate.The longer the natural aging time,the lower the activation energy of indoor and outdoor Chinese fir samples.The reaction mechanism of indoor and outdoor Chinese fir with the same natural aging time is the same.（2）The combustion characteristics of Chinese fir under different natural aging time and radiation intensity were studied.The key characteristic parameters such as ignition time,heat release rate,mass loss rate,total heat release and effective heat of combustion of Chinese fir were obtained and analyzed.The results show that the ignition time of Chinese fir is inversely proportional to the intensity of external radiation heat flux and proportional to the natural aging time.The heat release rate curve of Chinese fir with different natural aging time shows a consistent trend with the combustion stage,which increases slowly at first,and then decreases after reaching the peak value for a stable period of time,which is mainly related to the structural changes of Chinese fir after aging in the natural environment.The mass loss rate of Chinese fir showed a consistent trend,decreasing first and then increasing.The longer the natural aging time is,the smaller the total heat release difference of different external radiation heat flux intensity is.For Chinese fir of different natural aging time,the difference between O₂ content and CO₂ concentration in flue gas decreases with the increase of external radiation heat flux.（3）The characteristics of fire spread behavior on the surface of Chinese fir under the coupling of environmental wind speed and inclination angle were studied,and the characteristics of flame shape,flame scale,flame temperature and fire spread speed during fire spread were analyzed.The variation law of fire spread velocity of Chinese fir under the coupling effect of wind speed and inclination angle was revealed,and the empirical relationship between fire spread velocity and flame length was established.The influence mechanism of the coupling effect of wind speed and inclination angle on flame height and flame length and their oscillation amplitude was clarified.The results show that under the coupling effect of wind speed and inclination angle,there is a sudden critical wind speed of 1.0 m/s and a critical inclination angle of 30°on the surface fire spread of Chinese fir material.With the continuous increase of wind speed and inclination angle,the fire spread behavior will change from upper surface fire spread to lower surface fire spread.In the two cases of flame spread on the upper surface and the lower surface,the average flame height,the average flame length and the oscillation amplitude of the two show opposite changes with the change of wind speed and inclination angle.The flame spread velocity increases with the increase of inclination angle under different wind speed conditions.Under different inclination conditions,the flame spread velocity increases first and then decreases with the change of wind speed when the wind speed is 1.0 m/s.（4）The evolution law of fire behavior of typical wood structure single building was studied by means of experiment and numerical simulation.Firstly,through the fire experiments of different types of wood structure buildings,the variation law of temperature at different heights in the fire room of wood structure buildings is analyzed,and the mechanism of smoke convection heat transfer and flame radiation in the fire process of wood structure buildings of different building types is revealed.The influence of smoke convection heat transfer and flame radiation on the fire development process,fire spread behavior characteristics,indoor and outdoor temperature,heat release rate and flashover time of wood structure buildings is revealed,and the prediction model of flashover time is established.Based on the FDS software,a fire physical model of a typical official wood structure building was established.The influence of key factors such as external wind speed,fire source location and ventilation conditions on the fire spread characteristics,temperature of key parts and total heat release rate of wood structure buildings at different stages of fire development was revealed,and the fire risk characteristics of single wood structure buildings were clarified.There are 105 pictures,29tables and 116 references in this thesis.