The Influence Of Vertical Channel Structure On Flame Spread Of Building Facade

With the rapid development of urban construction,the number of buildings in China has increased significantly,with an annual increase of more than 2 billion square meters,and its growth rate has become the fastest in the world.There is no shortage of flammable materials in the materials used for building exterior walls,especially in recent years,the construction industry has promoted comprehensive energy conservation.XPS,EPS,PU,PMMA and other organic insulation materials have been widely used.Such materials without flame retardant treatment have a high risk of fire,causing frequent fire accidents on the building facade.High-rise buildings occupy a small area,improve land utilization,expand urban open space,help urban greening,and improve the sanitary environment;high-rise buildings move to high altitudes and make full use of limited land resources,which can effectively alleviate the problem of urban land tension,so the number and height of high-rise buildings has grown rapidly.Due to the negligence of management,high-rise building fires have occurred frequently in recent years.Therefore,the fire safety of external walls of buildings needs to be paid close attention to,and the flame spread behavior on the fa?ade needs to be further studied.There are many vertical channels on the external wall of modern buildings,such as the vertical channel formed by the concave external wall,the vertical channel between the curtain wall and the main structure.When a fire occurs in the facade insulation material,the flame spread will be significantly affected by the vertical passage.In order to study the flame spreading characteristics of the combustible material under the action of the vertical channel structure of the external wall,the research was carried out through experimental and theoretical research.The relevant experiment platform was established to carry out experimental research.The important characteristic parameters of the vertical channel include the structural factor(the ratio of the length of the side wall to the length of the back wall)and the front coverage.By changing the above characteristic dimensions,observing the flame spread characteristics,flame structure and temperature field,and based on the theoretical model of flame spread,the measured characteristic parameters described above are substituted into the previous model to establish a new relevant theoretical model.Finally,the experimental results are compared with the theoretically calculated results to verify the reliability of the theoretical model.The main research work includes:(1)The influence of closed vertical channel structural factors on the downward spread of XPS was studied.Characteristic parameters of the flame structure and flame spread rate of the countercurrent flame under the limitation of the side wall were obtained.By changing the length of the side wall,the induced airflow in the vertical channel and the temperature field on the surface of the curtain wall under different structural factors were obtained.The competitive mechanism between flame radiation and airflow cooling was found experimentally.Based on the heat transfer model,a theoretical model suitable for the downward flame spread in a closed vertical channel was established.The control mechanism of flame spread was analyzed from two aspects of heat transfer control and oxygen supply,and the importance of two methods of radiative heat transfer and convective heat transfer under the condition of heat transfer control was discussed.The image analysis method was used to obtain the flame height under different structural factors.The experiment found two factors affecting the flame height: oxygen supply and chimney effect.The airflow velocity and flow field structure under the chimney effect are analyzed,and a prediction model of induced airflow in the vertical channel during the flame spreading process is established.(2)The influence of the structural factors of the concave external wall on the upward flame spread of PMMA was studied.By monitoring the pyrolysis front,the linear relationship between the pyrolysis front and the speed of flame spread was obtained,and the relationship between the speed of flame spread and the structural factors was obtained.By introducing the entrainment coefficient,a related flame spreading model was established,the convection-controlled upward flame spread rate was predicted,and the existing radiation-controlled upward flame spreading model was optimized.Based on the theoretical basis between the heat release rate and the flame height,the relationship between the flame height and the pyrolysis height was calculated,and the mechanism of the effect of the concave vertical channel on the flame height and the flame spread rate was revealed.The reliability of the model is confirmed by comparing the relationship between the calculated value of the flame spread rate and the experimental value(3)Combined with the actual external wall structure,the XPS insulation board was used as the experimental material to study the upward flame spread characteristics under the coupling of external wall structural factors and curtain wall coverage.By changing the structural factors and the coverage,the temperature change inside the vertical channel and the flame height change were analyzed,and a fitting relationship between the pyrolysis length and the flame height was established.The relationship between the heated length,the structural factor,and the coverage rate was revealed,and an upward flame spreading model was established based on the heat transfer model.