Studies On Fire Development Inside Compartment And The Associated Ejected Facade Flame Behavior Under Different Ventilation And Two-floor Compartments Condition

As China's social and economic development and urbanization process accelerates,(super)high-rise buildings are increasing.There are about 200,000 high-rise buildings in mainland China,including more than 3,000 super high-rise buildings.The rapid development of high-rise buildings has brought about serious fire safety problems.Compartment fires and the associated facade fire are important factors influencing the spread of large-scale fires,and thus attract the attention of many research institutions and scholars.The current research focuses on the development law of compartment fire and the distribution of characteristic parameters(temperature,flame shape,etc.)of facade fire,and there is also many studies aimed at the effects of special boundary conditions on the development of compartment fires and the facade fire,such as different building structures(side wall,vertical and slopping wall)and plateau low pressure and low oxygen environment.However,the previous research was mainly based on a single compartment with one opening and the compartment was in a normal ventilation condition,which is different from the actual high-rise building large-scale fire spread(multi-layer simultaneous fire),and the high-rise buildings tend to be strongly affected by the environmental wind with their height increasing,in addition,research on the poor ventilation in the compartment is still lacking.In this paper,the characteristics of compartment fire evolution and the facade fire under different ventilation and two-floor compartment conditions are taken as the core,and the reduced-scale experiments and numerical simulation method are used for research.The reduced-scale(1:8)experimental model was used to carry out the fire experiment under different boundary conditions such as positive ambient wind,insufficient ventilation and two-floor compartment.At the same time,the numerical simulation of the same proportional size model was carried out to observe the parameters that can not be measured by experiments,such as the internal flow of the compartment,which are effectively complementary with the experimental data.In the analysis process,based on the classical models such as enclosure fire dynamics and facade fire entrainment,the influence law of different boundary conditions on the research content is revealed,the characterization model under the corresponding boundary conditions is developed by theoretical derivation,and the predicted values of the model are in good agreement with the experimental data.The research work in this paper mainly includes the following points:(1)The change of the state of fire development inside the compartment under ambient wind conditions is studied.It is found that the critical heat release rate of the compartment fire develops from the stratifed condition to the well-mixed condition decrease with increasing wind speed.A dimensionless relationship between the critical heat release rate and the ambient wind Froude number is proposed.In addition,the temperature difference between the upper and lower layers of the compartment is further analyzed to reflect the process of changing between different states of the compartment fire.The ventilation and mixing effect of ambient wind on the development of compartment fire was revealed,combined with the relationship between heat generation and heat loss inside the compartment,the prediction model of temperature difference between the upper and lower layers during the development of compartment fire is established.(2)The distribution law of the temperature of the facade fire under ambient wind conditions is studied.It is found that the vertical temperature under wind condition still conforms to the classical three-stage model.By introducing the dimensionless parameter to characterize the influence of the ambient wind on the entrainment of the fire plume,the vertical temperature distribution model under different wind speeds is constructed.Based on the Gaussian distribution function and the entrainment model of facade fire under ambient wind condition,the characteristic length scale of the effective thickness of the plume under ambient wind condition is proposed,and the theoretical model of the radial temperature distribution of the facade fire plume under the ambient wind condition is further established.(3)The evolution of compartment fire and the variation of facade fire flame height under the condition of insufficient compartment ventilation were studied.It is found that the compartment fire will turn from the traditional under-ventilated fire to the critical transition state under the condition of insufficient ventilation.Combined with the compartment fire dynamics theory,the critical fuel mass flow rate model of the compartment fire reaching the critical transition state is established.The law of flame height change of the critical transition state is revealed,by analyzing the difference between the physical mechanism of the traditional under-ventilated fire and the critical transition state,further gives the facade fire flame height prediction model under the critical transition state.(4)The fire evolution behavior of the two-floor compartments was studied.It was found that under the influence of the lower-floor compartment fire,the upper-floor compartment will present the three types of fire phenomenon,the flame inside the compartment extinguished,the flame first ejected outside and then extinguished,and the two flames of two-floor compartments merge.The effect of lower-floor compartment fire on the temperature and facade flame height evolution of upper-floor compartment fire was revealed.For the special three types of fire phenomenon of the two-floor compartments fires,the criterion of the corresponding critical state is proposed.