Study On Flame Spread Rules And Smoke Flow Characteristics Of Cables In Long-Narrow Confined Space Under Multi-Factors

With the rapid development of urbanization in China,urban traffic tunnels,subways,utility tunnel,underground sidewalks and other long-narrow confined space are playing an increasingly important role in modern cities.Due to the particularity of long-narrow confined space structure and the closure of space,once the fire accident happens,it is easy to cause the serious accident of mass death and casualty,which puts forward a new challenge to the fire safety.There are a large number of cables in longnarrow confined space.In recent years,electrical fire accidents caused by cable faults are frequent.According to statistics,electrical fires account for 34%-38% of all fire accidents in China.At present,the fire protection specifications for cables in long-narrow confined space are not perfect,the fire risk assessment of cables in long-narrow confined space lacks experimental data and theoretical basis.The fire protection design of long-narrow confined space such as subways,tunnels,and utility tunnels need to be improved.Moreover,previous studies on cable fires in long-narrow confined space are not comprehensive enough,especially the influence mechanism of coupling of cable internal and external environmental factors on cable fire spread and smoke flow characteristics is still unclear,and the corresponding prediction model is lacking.Therefore,it is of great theoretical value and practical significance to study the fire propagation law and smoke flow characteristics of cables in long-narrow confined space under the effect of multi-factor coupling.This paper takes cable fire in long-narrow confined space as the research object,using a combination of experiment and numerical simulation,and mainly considers the influence of factors such as cable inclination,longitudinal wind speed and fire source power,which will affect the spread of cable fire and smoke in long-narrow confined space.The research on flow characteristics mainly includes the following aspects:(1)By carrying out cone calorimeter experiments,the influence rules of different cable core numbers,distance and external radiation intensity on cable combustion characteristics are studied.The linear fitting relationship between the core numbers and radiation intensity is good.Further,it is obtained that the cables with different core numbers are all thermal-thickness materials.Variation of ignition time,mass loss rate,heat release rate and other combustion characteristics with the number of cable cores,spacing and intensity of external radiation were analyzed.The experiment results show that the 3-core cable has the shortest ignition time and the highest peak heat release rate.The ignition time of 2-core cable is the longest,the mass loss rate is the largest,and the danger in the later stage of combustion is greater.When the cable spacing is 12 mm,the ignition time is the shortest,and when the cable spacing is 15 mm,the peak heat release rate is the highest,and the fire risk is the highest.(2)By carrying out the fire experiment of long-narrow confined space cable,the influence of the cable inclination angles and longitudinal wind velocities on the fire spread and smoke flow characteristics of long-narrow confined space cable is studied,and a prediction model of the flame spread rate considering the cable inclination angles is established,the experimental results are in good agreement with the prediction model.The results show that both the longitudinal wind velocities and the cable inclination angles will increase the flame inclination angle,lengthen the flame length,and promote the combustion of the cable.The flame spread rate is positively correlated with the cable inclination angles and the longitudinal wind velocities.When the longitudinal wind velocity reaches 0.65m/s,the flame spread rate accelerates.The highest temperature of the ceiling smoke is located near the fire source,and the longitudinal ventilation velocity will shift the highest temperature position of the ceiling.When the longitudinal wind velocity is less than 0.45m/s,the maximum ceiling temperature increases with the increase of wind velocity;when the longitudinal wind velocity is greater than 0.45m/s,the maximum ceiling temperature decreases with the increase of wind velocity.The critical wind velocity of the smoke back-layering length in the experimental study is0.65m/s.(3)The flame spread and smoke flow characteristics of the long-narrow confined space cable are studied by numerical simulation.The smoke flow,temperature distribution and CO concentration distribution under multi-factor coupling of different fire source power,cable inclination angles and longitudinal wind velocities are analyzed,the prediction model of longitudinal temperature rise of ceiling was established.The study found that the L-shaped long-narrow confined space is divided into two areas.The greater the inclination angle,the lower the temperature in area I,while the opposite is true in area II.With the increase of the vertical height from the fire source,the maximum vertical temperature decreases at a smaller inclination angle(0°,1°,2°),and at a larger inclination angle(4°),the temperature first decreases and then rises.The CO concentration is the lowest near the fire source,but the CO concentration far away from the fire source is higher,and the smoke is more dangerous.The angle of the cable has a significant effect on the CO concentration near the air inlet in Region I.When the cable angle increases to 4 °,the low concentration area of CO in region I increases to half of that in region.