Studies On Influences Of Internal Anisotropy Structure On Flame Spread Behavior Of Corrugated Cardboard

Building fire has become the most common fire scene in human daily life.Once it occurs,it will often cause huge economic losses and casualties,and may have a farreaching impact on the surrounding environment.Building fire scenarios,the most common is the combustion and fire spread of solid combustibles.It involves complex gas-solid two-phase heat and mass transfer problems(such as solid-phase pyrolysis,gas-phase transport and gas-phase combustion),which has attracted many scholars to study the typical solid fire spread behavior and combustion mechanism.These researches have been providing important theoretical support for the quality improvement of fire protection systems such as fire monitoring and fire detection in real life.Upward fire spread is one of the most dangerous scenarios.Once solid combustible is ignited,the flame will quickly spread upward along the solid vertical wall due to buoyancy.This situation is very common in warehouse facilities,and the huge fire load in warehouse will further enhance its fire complexity and risk.The most common fire loads in warehouse buildings is corrugated cartons that contain various commodity goods.At the beginning of the fire,the flame is usually concentrated on the outer surface of corrugated board which is a heterogeneous material made of lignocellulosic materials and has a hollow structure.The existing theoretical models and experimental studies mostly focus on the non carbonized homogeneous material PMMA.Therefore,it is necessary to carry out more in-depth study on the fire spread behavior of heterogeneous materials,such as corrugated board which is widely used in daily life.With the further development of the fire,the goods inside the corrugated box will also melt and pyrolysis under the heating action of the carton flame.The products in cartons are usually polymers(such as polystyrene plastic).Once they are involved in combustion,they will release extremely heated and toxic gases with complex components.At the same time,molten polymers may flow,resulting in the spread of fire through flowing molten materials,which will further improve the fire risk.In order to systematically study the combustion and fire spread behavior of typical heterogeneous combustibles,this paper conducts combustion experiments to study the effects of structural characteristics such as grain orientation and thickness on flame spread behaviors of corrugated cardboard.In addition,this paper also studies the multi fuel mixed combustion and fire spread behavior of corrugated carton(internally loaded with polystyrene plastic).The main work includes three aspects:(1)the effect of grain orientation and thickness on the vertical fire spread behavior over single-wall corrugated board;(2)the effect of grain orientation and thickness on the vertical fire spread behavior over double-wall corrugated board;(3)combustion characteristics and flame spread behavior of corrugated carton(filled with polystyrene plastic cup).The specific research contents are as follows:(1)The effects of grain orientation and thickness on the vertical fire spread behavior of single-wall corrugated board were studied.In the experiment,three kinds of single-wall corrugated boards with different thicknesses(set in two directions that along grain and across grain direction)were used to calculate the flame spread characteristic parameters such as pyrolysis front,flame front,mass loss rate and heat release rate on the surface of corrugated board.The results showed that when the corrugated board was in the direction along the grain,the flame combustion on the board surface is more intense,the fire spread rate is also greater,and the phenomenon of board cracking and delamination appears in the combustion area.Based on the mechanism analysis of mass energy conservation and heat and mass transfer,it was found that the internal hollow channel of along grain samples were parallel to the buoyancy direction,and the high-temperature pyrolysis gases in the combustion area of the paperboard was transported upward through the internal hollow gaps of the paperboard.Therefore,the unburned area on the upper part of the paperboard was preheated by both the flame on the surface of the paperboard and the high-temperature pyrolysis gases in the internal hollow gaps of the paperboard at the same time,which promoted the upward flame spread.The inner hollow gaps of across grain samples were perpendicular to the buoyancy direction,and the high-temperature pyrolysis gases cannot be transported upward through the inner hollow channel of the paperboard.The unburned area on the upper part of the paperboard was only preheated by the surface flame,so it shows a low fire spread rate.However,as the paperboard becomes thicker,the width of the hollow gaps in the cardboard increases,and the difference of combustion phenomenon between across grain samples and along grain samples decreased.Based on the experimental data and the theory of mass energy balance,a correlation model between pyrolysis height and heat release rate per unit length was established.The results revealed linear relationship between pyrolysis height and heat release rate per unit length.The fitting slopes were between 0.45 and 2.27.Samples of across grain and smaller thickness showed higher slope values while samples of along grain and larger the fitting slope showed lower slope values.(2)The effects of grain orientation and thickness on the vertical fire spread behavior of double-wall corrugated board were studied.Combustion experiments were carried out with BE double-wall corrugated board and BA double-wall corrugated board.It was found that BE double-wall corrugated board had cracking and delamination happened for two times over along the grain sample and one time over the across grain sample.Characteristic parameters flame spread behavior such as flame height,pyrolysis height,preheating zone length and mass loss rate reflected that the fire spread rate of the across grain sample of BE double corrugated board was lower than that of the along grain sample.Cracking and delamination only happed for one time both over the along grain sample and the cross grain sample of BA double-wall corrugated board.The comparison of flame height,pyrolysis height,preheating zone length and mass loss rate reflected that the fire spread rate of the across grain sample of BA double corrugated board was greater than that of the along grain sample.In addition,the flame spread rate of BA double-wall corrugated board(samples in two grain orientations)was greater than that of BE double corrugated board.Based on the relevant theoretical analysis of mass energy balance,it was found that the flame spread rate over doublewall corrugated cardboard was determined by the difference between the convective heating inside the hollow gaps in the outer cardboard and the convective heat dissipation inside the hollow channel in the inner cardboard.(3)The flammability and flame spread behavior over corrugated cartond commodity were studied.Class A plastic commodity(which is composed of corrugated carton and polystyrene plastic cup)was used for the combustion experiment.Through experiment observation and data measurement and analysis,the flame spread behavior of Class A plastic commodity is divided into five stages.The mass loss rate in these five stages increases first,then decreases,increases again and then decreases again.The final decrease slows down gradually.Based on the measurement results of temperature fields inside the carton and the simplified assumptions,the position of the pyrolysis front and then the combustion area inside the carton inside the carton is calculated.Based on the calculated burning area and the measured mass loss rate,the quantitative analysis of the B number(namely the mass transfer number)of mixed solid combustibles can be achieved.By comparing the combustion phenomena and characteristic parameters of each stage,it is determined that the third stage of the combustion of the corrugated carton(which includes the combustion of corrugated box and polystyrene cup)is more stable,and therefore,is more representative.At this stage,the experimentally-based calculated B number of mixed solid combustibles is 3.53,which is greater than the B number value of single corrugated board and polystyrene plastic proposed in previous studies,indicating that the fire risk of mixed materials is greater than that of single material.