The Effect Of Commodity Spacing On Flame Propagation Over Commodity Surface In High-rack Warehouses

Due to the high storage volume of combustibles in high-rack warehouses,in the event of a fire,the flame will quickly spread between the combustibles through the vertical or horizontal gaps of the racks,forming a large-scale three-dimensional fire,which will bring a great deal of economic loss.Therefore,the flame spread and fire evolution of high-rack warehouse fires has gradually become a research topic that researchers pay attention to.Previous studies have mainly focused on the temperature distribution of the smoke layer of high-rack warehouse fires and the extinguishing efficiency of automatic fire extinguishing systems.In a real high-rack warehouse fire scene,the process of flame spread between commodities is one of the most important characteristics that determine the direction of fire development.But the amount of research related to this is relatively small,and there are still many unsolved problems.This paper uses a combination of experimental research and theoretical analysis to analyze the influence mechanism of air gap on the flame spread on discrete solid surfaces by comparing changes in flame shape,flame height,mass loss rate,pyrolysis front position and flame spread rate,and applies this mechanism to the experimental study on the flame spread behavior in the high-rack warehouse fires.Major conclusions are as follows.Based on the experimental study on upward flame spread over discrete solid surface,it is found that the air gap has a dual effect on the development process of flame spread.As the length of the air gap length increases,the flame height,mass loss rate(burning rate),and flame spread rate over the solid surface all increase first and then decrease.The turning point of this trend occurs in the condition that the air gap length is between 6 and 8 cm.At the same time,with the increase of the air gap length,a certain ‘ignition delay time’ appears during the process of flame spread over the adjacent solid surface;when the air gap length is 18 cm(the fuel coverage rate is 30.8%),the flame cannot be dispersed.Then by analyzing the influence mechanism of the air gap factor on the heat flux distribution on the solid surface,the critical criteria for the occurrence of these two phenomena are given.Based on the experimental study on flame spread over commodity surface in a singlecolumn rack,two conditions are considered,namely flame spread with a ceiling and without a ceiling.It is found that the influence mechanism of air gap is suitable for experimental study on flame spread over commodity surface in a single-column rack.That is,as the fuel coverage rate decreases,the flame height over commodity surface,the mass loss rate of commodities and the flame spread rate all show a trend of first increasing and then decreasing.The turning point of the trend change occurs when the fuel coverage rate is between 40 and 50 %,and the critical fuel coverage rate where the flame cannot continue to spread is within the range of 10-20%.The ‘vertical channel’ between the rack and the fireproof backplane provides convenience for heat transfer and flame spread,so these two values are lower than the theoretical value.In addition,the thermal feedback of the ceiling increases the heat flux received by the commodity surface near the ceiling,and then speeds up the pyrolysis and combustion rate of the commodity surface,and causes its mass loss rate to be slightly higher and the ‘ignition delay time’ to be slightly lower than that of the corresponding conditions that are without a ceiling.Based on the experimental study on the flame spread over commodity surface between racks,it is found that during the process of flame spread over commodity surface in a doublecolumn rack,the vertical flame spread rate over commodity surface is much higher than the horizontal flame spread rate,and the overall flame spread rate shows a trend of first increasing and then decreasing with the decrease of fuel coverage.In the experimental study on flame spread between two adjacent racks,the overall flame spread rate decreases with the increase of the vertical aisle width,and the flame cannot spread between two adjacent racks when the vertical aisle width is 10 cm.As the width of the vertical aisle increases,the mass loss rate of commodities first increases and then decreases,and when the width of the vertical aisle is 6 cm,this value reaches its maximum.In addition,there are two main forms of flame spread between two adjacent racks.One is that the unburned commodity surface receives the heat flux from the flame,and when the critical heat flux for ignition is reached,the commodity surface instantaneously becomes ignited;the other is that burning commodities are disturbed by the airflow generated by the flame in the vertical aisle and fall,forming a ‘falling fire’ in the vertical aisle,and the unburned commodity in the lower layer of the rack are ignited by the ‘falling fire’.This paper enriches the research on the flame spread behavior of high-rack warehouse fires,and can provide a certain theoretical basis for the fire protection design of high-rack warehouses.This paper has 74 figures,7 tables,and 67 references.