Study On Vertical Upward Flame Spread Characteristics And Heat Transfer Mechanism Of Hollow Structure Combustible

The vertical upward flame spread obviously speeds up the spread rate of building fire because the direction of induced air flow is consistent with the direction of flame spread.However,in the real building fire scenario,there are many hollow structures of combustible materials,such as various building pipes.Hollow structure of combustible in the fire will cause a strong chimney effect and induced airflow,which promote the spread of flame and greatly improves the risk of building fire.But at present,there is no research on vertical upward fire propagation of hollow structure materials.In this paper,corrugated paper tube is selected as the research object,and the characteristics of vertical upward fire spreading behavior of hollow structure combustible are systematically revealed by small-size experimental research method.Based on single-walled and double-walled building pipes,the influence mechanism of coupling effect of sample diameter D and sample thickness δ on flame propagation behavior of corrugated pipe was investigated under the two limited conditions.The main research contents and conclusions are as follows:(1)The variation law of flame spread rate with sample inner diameter and sample thickness is discussed.It is found that there exist two spreading conditions of uniform velocity and acceleration,and the critical acceleration conditions are divided.The average flame spread rate decreases with the increase of sample thickness,but it has little change under thermal thickness condition.The average flame spread rate increases monotonically with the increase of the inner diameter of the sample under single-sided restriction,but increases first and then decreases slightly under double-sided restrictions.The relationship between the dimensionless average flame spread rate and the dimensionless diameter and the relationship between the heat penetration thickness and the incident heat flux are established respectively.(2)The influence mechanism of sample inner diameter and sample thickness on mass loss rate was analyzed.The mass loss rate increases firstly and then decreases with the increase of sample thickness under the condition of single-sided restriction.However,when δ≥6 mm,the mass loss rate in this study conditions are almost unchanged with double-sided restrictions.The linear relationship between mass loss rate and sample inner diameter is established.In addition,convective heat transfer plays a dominant role throughout the combustion process.(3)The variation of flame length on the outer surface of the specimen was revealed.The flame length on the outer surface of the sample has a good power exponential relationship with the combustion time.The maximum flame length on the outer surface increases first and then decreases with the increase of the inner diameter of the sample.The power law relationship between the maximum flame length on the outer surface and the heat release rate per unit outer surface circumference is established.(4)The relationship between the jet flame height and sample thickness and sample inner diameter was studied.The average jet flame height changes little with the increase of sample thickness,but linearly increases with the increase of sample diameter,and the linear relation between the two is obtained.In addition,the linear relation between the dimensionless average jet flame height and the dimensionless group 1/5 power is also established.This study can not only supplement the content of hollow structure flame spread in fire science,but also further improve and expand the classical flame spread theory,which has certain academic value.In addition,the results of this study have important guiding significance for fire development prediction of high-rise buildings and fire prevention design of the internal and external walls of high-rise buildings.