Study On Radiation Ignition Behavior Of Adjacent Pool Fire On Liquid Fuel

In the process of fuel combustion,the flame presents a buoyancy driven flow.In this flow,the buoyancy caused by the increase of medium temperature or the decrease of density makes the fluid flow upward and induces the surrounding medium to flow to the plume.We call it the entrainment of flame to the surrounding air.In addition,the flame also emits thermal radiation to the surrounding,which may ignite nearby flammable substances.This kind of spreading fire caused by the radiation of adjacent fire sources often leads to more serious personnel and property losses.Therefore,it is necessary to study the ignition characteristics of liquid fuel under radiation.The experiment was carried out in a closed experimental chamber with a size of 2 m*6 m*4 m.The experimental system is mainly composed of the firing heat flux transmitting pool(hereinafter referred to as pool I)and the receiving heat flux to be ignited pool(hereinafter referred to as pool II)and data acquisition device.Among them,the former includes quartz glass pool with four diameters of 74,114,150 and 200 mm;The latter includes video acquisition system,temperature acquisition system,quality acquisition system and heat flux acquisition system.The main research work is as follows:The behavior and critical criterion of igniting liquid fuel by adjacent pool fire are studied.The ignition phenomenon of liquid fuel by radiant plate/adjacent pool fire is analyzed.The dual role of adjacent pool fire as radiation source and high-temperature heat source is explained.The ignition stage is divided into heating stage,vapor accumulation stage and instantaneous ignition stage.In the vapor accumulation stage,the temperature of n-heptane and ethanol slowly rises to about 40℃and 33℃.The critical ignition distance is discussed.The results show that with the increase of pool diameter,the critical ignition distance of both fuels increases.The critical ignition distance of ethanol is between 0.20 D-0.47 D,while that of n-heptane is between 0.95 D-1.40 D.The latter is much greater than the former,which is more dangerous;the radiant heat flux is analyzed,the mathematical relationship between radiant heat flux and pool distance is established,and the variation law of critical radiant heat flux with pool diameter is analyzed;It is found that the pool diameter has no obvious effect on the ignition time.The ignition time changes significantly with the pool distance.The pool distance increases,and the ignition time also increases.The regional radiant heat flux based on cylindrical radiation model is quantitatively characterized.Firstly,the point source radiation model is analyzed.It is found that the predicted value is much larger than the experimental value,and the average error is more than 100%.The error comes from the assumption of flame uniformity and isothermal in the point source model itself;the cylindrical model is analyzed.It is found that the calculated results of n-heptane fuel are in good agreement with the predicted values of the model.Among them,the calculated values are in good agreement with the experimental values when the radiant heat flux is in the range of 1.5-2.8 k W/m~2;when the radiant heat flux is greater than 2.8 k W/m~2,the calculated value and the experimental value tend to be discrete.The radiant heat flux received by the ethanol pool is in poor agreement with the experimental value,and the former is less than the latter as a whole.Based on the previous cylindrical model,the radiant heat flux received by the liquid level of the pool to be ignited is quantified,and the area is divided by MATLAB program,so as to obtain the average radiant heat flux received by the liquid level of the pool to be ignited,and then determine the equivalent radiation distance.It is found that there is a linear relationship between it and the pool distance.By integrating the radiant heat flux within the equivalent radiation distance,it is found that when D=150 and 200 mm,This ratio is up to more than 50%,which is significantly lower when D=74 and 114 mm than when the pool diameter is larger,and does not exceed 20%at the maximum.The heat transfer mechanism of liquid fuel ignited by adjacent pool fire was studied.Through the comparison between the experimental value and the predicted value of evaporation rate,it is found that the heat loss term and convection term can be ignored.The results show that the entrainment velocity of n-heptane is higher than that of ethanol under the same conditions.With the increase of height,the entrainment velocity of flame decreases gradually.The saturated vapor pressure of n-heptane and ethanol fuel when they are ignited is 12334 Pa and 11082 Pa respectively.Accordingly,the initial concentrations of the two fuels when they are ignited are 1.84%and 10.94%respectively.The vapor concentration at 5 mm above the liquid level is calculated by Fick’s second diffusion law.It is found that when the pool diameter is unchanged,the vapor concentration increases with the increase of the pool distance,and the variation trend of the vapor concentration at the ignition with the pool distance is not affected by the pool diameter.The combustion temperature limit of ethanol is 12-44℃.The fuel in oil pan II is not ignited at the beginning because the vapor concentration at a specific height above the liquid level increases with time,and the vapor concentration required for ignition is also different when the pool distance is different.It needs to be radiated for several times to reach this concentration before the fuel can be ignited.