Investigation On Evolution Characteristics Of Leakage Fires Of Atmospheric Pressure Tanks Containing Typical Flammable Liquids

Petroleum is the primary energy source in the global energy mix and plays a vital role in sustainable economic development.Petroleum and its processed products,including flammable liquids such as n-heptane,ethyl alcohol and gasoline,are often stored in atmosoheric pressure tanks.However,fires due to accidental leakage of flammable liquids from tanks are a frequent occurrence,most commonly in tanker fires on the road,posing a serious threat to people and equipment.Once the flammable liquid leaks and burns,a pool fire will form and heat the tank.On the one hand,the pool fire will cause thermal radiation hazards to the surroundings.On the other hand,the fire impingement on a tank is likely to lead to the overpressure failure and rupture of the tank,causing explosive shock hazards to the surroundings.Therefore,pool fire and tank thermal response are affected with each other,resulting in an extremely complex fire evolution process.Thus,it is necessary to investigate the evolution characteristics of such fires by means of experimental and theoretical analysis.The small-scale experiments were conducted using two atmosoheric pressure tanks containing typical flammable liquids,including n-heptane and 92#gasoline.The measured parameters included pressure,temperature,and thermal radiation,mass burning rate and flame morphology.Based on the measured data and theoretical analysis,the major findings of this paper are as follows:Hazard characteristics of leakage fires of tanks were investigated.The arrival of the boiling point of the leaking liquid led to a sudden increase in fire hazard,and a numerical simulation approach was proposed to predict when the fire hazard increased suddendly.The fire was divided into four stages,including fire spread(stage Ⅰ),stable pool fire(stage Ⅱ),boiling pool fire(stage Ⅲ)and jet fire(stage Ⅳ).It is worth noting that liquid leakage occurred in stages Ⅰ、Ⅱ、Ⅲ,while gas leakage occurred in stage Ⅳ.After entering the the stage of boiling fire,the thermal radiation and pressure suddenly increased to the maximum.Thus,the fire hazard of stage Ⅲ was the largest,and it increased suddenly from stages Ⅱ to Ⅲ.The temperature of the leaking liquid reached the boiling point or the leaking liquid started to burn with boiling,indicating a sudden increase in fire hazard.The boiling temperatures of single-component liquid with fixed boiling point(n-heptane)and multi-component liquid with wide boiling range(92#gasoline)were 98.6℃ and 51.7℃,respectively.A CFD model of a tank with a leakage hole was established by ANSYS Fluent to predict the temperature of the leaking liquid,and the moment of a sudden increase in fire hazard was predicted.The maximum relative error between measurement and prediction was within 20%,indicating the CFD modeling approach prefermed well.Furthermore,the approach was expanded to establish a full-scale tank.CFD modeling approach was used to predict the moment of a sudden increase in fire hazard when the leakage fire occurred on a full-scale tank,providing the data support for the emergency decision of such fires.Burning characteristics of pool fire outside the tank were investigated,including the mass burning rate and thermal radiation.Before the fire hazard increased suddenly,the oil layer thickness increased more for smaller tray size,and both the leaking liquid and high-temperature tank wall could bring heat to the liquid pool,so the fire could be regarded as a thick-layer pool fire,under which condition the previous model of m"=m"∞(1-e-κβd)could be used to calculate the average mass burning rate.However,the oil layer thickness was consistently close to 0 for larger tray size,and the fire was only regarded as a thin-layer pool fire,under which condition the average mass burning rate was much smaller than the value calculated by the previous model.After the stage of boiling fire came with a sudden increase in fire hazard,the leakage flow failed to increase with the internal pressure.The leakage flow approached a maximum and tended to be constant as the filling level increased.Once the superheated liquid inside the tank was released to the outside,it vaporized and burned.The flame was no longer restricted by the tray and expanded outward,so the pool size was no longer equal to the actual tray size,under which condition the virtual pool modeling approach was proposed to predict the mass burning rate and thermal radiation.The maximum relative error between measurement and prediction was approximatedly 20%,validating the virtual pool modeling approach.The thermal response inside the tank was investigated,including pressure response and temperature response.During the stage of boiling pool fire,the liquid inside the tank was saturated and its temperature rising rate was almost constant.Thus,the saturated vapor pressure was solved using the Antoni equation and the real-time liquid temperature,under which condition the real-time pressure could be predicted.The peak pressure was positively correlated with the duration of the stage of boiling fire and the temperature rising rate of the liquid.The results showed that the peak pressure increased with decreasing leak hole,with increasing filling level,and with decreasing ignition time.The influence of tray size on the peak pressure was reflected in the presence or absence of burning with liquid spatter.As the tray size was larger,the liquid inside the tray almost burned out before the stage of boiling pool fire,and there was no buning with liquid spatter;thus,the peak pressure did not change with the tray size.As the tray size was smaller,liquid burned with liquid spatter in the stage of boiling pool fire.That is,the unburned liquid inside the tray was sputtered out,and the flame size increased sharply.As a result,the flame was deflected away from the tank and did not consistently heat the tank with high intensity,significantly reducing the temperature rising rate of the internal liquid.This resulted in a lower peak pressure,and thus the peak pressure decreased with decreasing tray size.The effect of fuel differences on the peak pressure varied with the filling level.As the filling level was lower,the peak pressure of nheptane tank was lower than that of gasoline tank.However,as the filling level was larger,the peak pressure of n-heptane tank was higher than that of gasoline tank.