The Influence Law And Mechanism Of Material Inclined Angles And Concave Channels On Discontinuous Flame Spread

With the rapid development of urban construction in China,the shape of building fa(?)ade is increasingly diversified to meet the space,lighting and aesthetic requirements of building fa(?)ade,there are inclined,concave,closed and semi-closed structures.In addition,a large number of polymerizable combustible materials are widely applied to the field of external fa(?)ade insulation and decoration.Once a fire occurs,the fire develops and spreads rapidly in multiple ways,which could spread from the bottom of the building to the top and indoor in few minutes and form a large three-dimensional fire.At present,the existing research mainly concentrates on studying the continuous flame spread,ignoring the discontinuous distribution of combustibles,which is obviously different from the real building fire scenes.In reality,due to the existence of windows,corridors,isolation belts and various gap structures in the building external walls,combustibles are mostly non-continuous,which could lead to more dangerous fire scenarios.Therefore,this paper concentrates on the study of the influence law and mechanism of material inclined angles and concave vertical channels on discontinuous flame spread.Two typical polymers(Extruded Polystyrene and Polymethyl methacrylate)are selected as experimental materials in this paper.The scaling experiments of thermally-thick solid flame spread were carried out by changing the experimental parameters regularly,such as inclined angle,structure factor of concave channel,air gap height and window gap height,and the discontinuous flame spread characteristics are clarified,the coupling influence law and mechanism of multi-factor on discontinuous flame are revealed.Based on the previous flame spread models and the basic theories of heat and mass transfer,the physical models of discontinuous flame spread are also established.The main research results are shown below.(1)The influence of material inclined angles on discontinuous flame spread was studied by conducting the scaled flame spread experiment.The results show that when the inclination angle is less than or equal to 90°,the flame vortex phenomenon that accelerates the fire spread appears at the air gap.The average flame height,flame spread velocity and melting zone length all showed a decreasing trend with increasing inclined angle;the flame stagnation time at the air gap increases with the increase of inclined angle;With the increase of inclination angle,the high temperature region of the sample in the preheating zone gradually transfers from the lower surface of the sample to the upper surface,revealing the heat transfer mechanism in the preheating zone under different inclined angles.An empirical formula of discontinuous flame spread speed and inclined angle is proposed.Based on the previous flame spread model,a prediction model of discontinuous flame spread speed under different inclined angles is established.The experimental results are in good agreement with the prediction model.(2)The coupling effect of concave vertical channel and air gap on discontinuous flame spread was studied by conducting the scaled flame spread experiment.The results show that there are obvious differences between flame shape and flame height under different working conditions,and the development of flame height presents three stages.The average flame height increases first and then decreases with the increase of coverage rate,and reaches the maximum value when f = 0.64-0.76.There are three kinds of ’ jump spread ’ model in the pyrolysis front of discontinuous flame spread with different coverage.The flame spread rate increases first and then decreases with the decrease of coverage,and reaches the maximum when f =0.64-0.76.The flame spread speed increases first and then decreases with the increase of structural factor,and reaches the maximum when Π = 1.2.The prediction model of discontinuous flame spread speed was established,and the prediction results were consistent with the experimental results.(3)The coupling effect of concave vertical channel and window gap on discontinuous flame spread was studied by conducting the scaled flame spread experiment.Based on the basic theory of heat and mass transfer,the discontinuous flame spread model caused by window spacing is established,and the jump ignition model of discontinuous combustibles is proposed.The results show that the maximum temperature of fire overflow at the window interval increases with the increase of structural factor.When the structural factor remains unchanged,the average fire spread rate increases first and then decreases with the decrease of coverage rate.The flame spread rate with different coverage rates shows different changing trends with the increase of structural factors;The mass loss rate per unit area showed a linear decreasing trend with the increase of coverage rate,and an increasing trend with the increase of structural factor;The empirical formulas of flame height and pyrolysis height,heat flux and relative flame height were obtained.The research results of this paper can provide important reference for the improvement of fire protection standards,fire risk assessment and fire protection design of building facades.