Study On The Mechanism Of Occurrence And Influence Factors Of Backdraft Phenomenon

With the acceleration of urbanization construction in China and the enhancement of building energy saving requirements,modern buildings pay more attention to their thermal insulation performance.Meanwhile,with the continuous improvement of living standards,indoor furniture is more diversified,its material contains a large number of combustible materials.When there is a fire in the room,the combustion of these combustible materials will produce a large amount of high temperature,toxic and harmful gases,among which there are various of combustible gases,such as incomplete combustion products and pyrolysis of fuel.As modern buildings have better thermal insulation properties,the heat released by the combustion cannot be lost to the external environment of the building as soon as possible,which leads the temperature inside the buildings rapidly rising.At the same time,due to the good sealing performance of modern buildings,the smoke produced by combustion and the combustible gaseous substances cannot be discharged as soon as possible,which leading the concentration of combustible gases in the buildings to rise continuously.With the development of combustion process in the building,the oxygen concentration is gradually drops below the flammable concentration limit,the flame on the fuel surface is extinguished,but the interior is filled with high temperature flammable gaseous material.When the ventilation condition of the building is improved(such as the fire rescue break down the building or the doors and windows are damaged by burning),the burning phenomenon will occur again.Although many scholars and research teams have conducted extensive research on the backdraft phenomenon during the fire development of liquid and gas,and have achieved fruitful results.However,most of the researches use liquid and gas as fuel and the ignition source is set at a specific location or change the ventilation condition when the fuel is still burning.It is rare to study only the free combustion of solid fuel and the disappearance of surface flame due to hypoxia,and backdraft phenomenon occur when change the ventilation condition.However,it is possible to have the above situation in the actual case of fire,and the phenomenon of backdraft often has a certain delay.At this time,it is very likely that the firefighters have already entered the building,once it happens,it will cause serious threat to the life of the personnel.Therefore,it has great practical significance to study the mechanism and influence factors of the backdraft of solid fuel.In this paper,we use the existing small size box of the laboratory to carry out experimental research on the backdraft phenomenon by solid fuel,and get the flame image of the development process of the solid fire,the space temperature,the gas composition and the concentration in the box,explore the mechanism and the influencing factors of the backdraft.The results show that after the windows of the box is closed,as the combustion progresses,the flame at surface of the solid fuel gradually disappears due to the lack of oxygen.When the window of the box is opened,the ventilation conditions are improved.When the mass of solid fuel is equal,the time required for the backdraft of the solid fuel is inversely proportional to the window area of the box,that is the larger the window area is,the shorter the time for the backdraft.When the opening area of the box is equal,the time required for the backdraft of solid fuel is inversely proportional to the surface area of solid fuel,the shorter the time required for the backdraft of solid fuel.Finally,this paper also uses CFD software to simulate the fire process in the case of the experiment,focusing on the flow process of the flue gas in each experimental stage and assisting the experiment.The results show that the simulation of the gas flow in the box can be consistent with the experimental phenomenon.