Study On Fire Spread Characteristics And Ceiling Temperature Distribution Of Cable Fire In Utility Tunnels

The utility tunnel is a type of modern and intensive urban infrastructure.The cable compartment has a high risk of fire because of the problems of short circuit of current,overload and overheating,and aging of insulation in cable.Once a fire breaks out somewhere,it could spread rapidly and ignite the surrounding cables,and the failure to control the fire in time would also cause the collapse of the utility tunnel and the interruption of urban power communication due to disasters,resulting in serious economic losses.At present,the burning characteristics of the cables,as well as the spreading features and temperature distribution law of the cable fire under confined conditions were not yet clear,and there is still a huge fire threat in the cable compartment of utility tunnels.Therefore,there is a current need for a systematic investigation of cable fires in utility tunnels.In this study,the cable combustion and temperature field distribution in the cable compartment of utility tunnel are analyzed from the theoretical model.Besides,combustion features,flame spread characteristics and the ceiling jet temperature distribution of the cable compartment are systematically studied by the small-scale experimental research method.The main works are described as follows:(1)In view of the problem that the current research methods of cable fire(continuous ignition by pool fire or using radiant heating)cannot reflect the combustion features by itself,this study research the YC rubber insulated copper core cables of different specifications for unassisted heating and combustion experiment.Based on the experimental phenomena,the fire behavior of cable combustion could be divided into five stages:ignition,single fire development,transition,dual fire development and dual fire recession.A calculation model of the combustion heat release rate of natural polymer compounds such as cable is proposed.After comparison and verification,it is found that the accuracy is high.And it quantitatively characterizes the severity of cable fires and lays a foundation for studying the relationship between heat release rate and fire characteristics.The research results show that the increase of the cable diameter and the number of copper cores would aggravate the cable burning degree,and the total burning time,oxygen consumption,heat release rate and mass loss rate of the cable also increase correspondingly,which indicate greater fire hazard.(2)According to the experiments,the flame characteristic parameters of the cable burning are obtained and the dimensionless heat release rate is introduced,and the dimensionless prediction formula of the maximum flame height of a single cable burning is proposed.Results show that the increase of heat release rate would augment the flame height,horizontal spreading speed and flame area.The maximum flame height,horizontal spreading speed and flame area are 8.83 cm,8.83 cm/s and18.21 cm~2,respectively.Once the vertical distances between multiple cables rises,the flame height and spreading speed both show increasing trend,and the phenomenon of flame stratification occurs.The horizontal flame spread of the upper cable is the slowest,followed by the middle layer,and the bottom layer is relatively slow.(3)The prediction model of ceiling temperature rise is proposed by combining experiments with relevant theories.The small-scale experiments of the near wall cable fire in airtight cable compartment are carried out and combined with the near wall fire theory of tunnel.Considering the influence of heat release rate,lateral distance near the wall and vertical height of the cable,a calculation model for the maximum ceiling temperature rise of near wall fire in closed cable compartment is proposed.It is found that the predicted value of this model is lower than that of some previous studies,which indicates that this model is more suitable for the fire scene where the cable burns without auxiliary heating in a closed environment.(4)The longitudinal attenuation trend of the maximum temperature rise of the ceiling is continuously explored through the fire experiments.Results show that the effects of heat release rate,vertical height and the lateral distance on the longitudinal distribution of the dimensionless maximum temperature rise could be negligible.In addition,it is found that the longitudinal distribution of the dimensionless maximum temperature rise has a double exponential decay trend with the dimensionless longitudinal distance.A prediction model for longitudinal attenuation of the maximum temperature rise of near wall fire in a closed power cabin is proposed.Comparing the prediction results of the model with the experimental value and predecessor model,a high degree of coincidence was obtained.It is more suitable for the fire research of the rectangular section utility tunnel in the closed environment.This study could provide important reference value for the formulation of fire prevention and control strategies in the cable compartment of utility tunnel.