Study On The Seepage Characteristic And Immersion-Burning Behavior Of Liquid Fuel Leaking In Porous Media Sand Bed

The leakage accident of liquid fuel often occurs in the process of production,storage,transportation and usage.The leaked liquid fuel will penetrate into the porous media environment(soil,sand,etc.),and it is easy to cause a large scale of fire and explosion accidents under the influence of external heat source.In addition,the seepage of liquid fuel into porous media will also cause environmental pollution near the leakage region.At present,there are relatively few researches on seepage and infiltrationburning issues about porous media environment,such as soil and sand bed.Combined with the theoretical system of combustion science,heat and mass transfer of porous media,seepage mechanics,capillary mechanics,etc,the current work focuses on the seepage characteristics and infiltrationburning behavior of porous media soaked by liquid fuel through the theoretical analysis,small-scale experiment and numerical simulation.The seepage characteristics of typical liquid fuel leaked into porous media bed and the infiltration-burning behavior under different particle sizes,gradation configurations and infiltration depths were studied.The variation trend of characteristic physical parameters,such as burning rate,temperature distribution of porous media,plume temperature and flame height,etc.,were obtained,and the seepage mechanism and infiltrationburning mechanism of porous media soaked by liquid fuel were analyzed.The main works were summarized in the following four aspects:(1)The seepage characteristics of liquid fuel leaked into porous media sand beds were investigated.For the sand beds with smaller particle size and porosity,the seepage channels were narrower and more numerous,and the seepage velocity distribution was uniform.For the sand bed with larger particle size and porosity,there is an obvious main channel effect.It is revealed that the driving force of seepage in porous sand bed is mainly inertia force dominated by gravity effect,while the resistance term mainly includes the cohesion between fluid molecules and the adhesion between fluid and solid.Based on Darcy’s law and simple empirical correlation of structural parameters of microscopic porous media,the empirical correlation of apparent seepage velocity in porous media bed was obtained,which the porosity and effective particle size were used as the main influencing parameters.Under the condition with the same porosity,the apparent seepage velocity increases gradually with the increase of average particle size.Under the condition with the same average particle size,the apparent seepage velocity also increases with the increase of the porosity of porous media bed.In the range of particle size 2 mm≤d≤12 mm and50 %-75 % porosity,the relationship between apparent seepage velocity,porosity and particle size can be well characterized by the fitting of power law function.(2)The infiltration-burning characteristics of porous media bed soaked by liquid fuel under different particle sizes and two-component discontinuous gradation were studied.During the burning process of quartz sand bed infiltrated by ethanol liquid,the burning rate increases rapidly firstly and then decreases gradually.For the scenarios with different particle sizes,the burning rate and flame height decrease with the increase of particle size.The infiltration-burning rate of liquid fuel is influenced by the capillary effect,gradation configuration and the thermal conductivity of porous media.For the sand bed with fine particle,the capillary effect plays a major role,which promotes the fuel transport in porous media.For the porous media bed with good particle gradation,the spaces between large particles are well filled by fine particles,and there are more coordination numbers and better contact conditions between particles.The variation trend of burning rate is mainly influenced by the apparent thermal conductivity of porous media enhanced by good particle gradation.In the current experimental range(0.116 mm≤d≤4.50 mm),there is a positive correlation between the quasi-steady burning rate of porous quartz sand bed soaked by liquid fuel(ethanol)and the one-half power of the porosity of sand bed,and negatively correlated with the packing density of sand bed.(3)The infiltration-burning characteristic behavior of porous media bed soaked by liquid fuel and the variation law of fuel residual percentage under different infiltration depth were revealed.Due to the "heat insulation effect" of the dry-sand-layer on the upper part of sand bed during the middle-and-late burning stage,the burning rate of porous media bed infiltrated by liquid fuel in the quasi-steady burning stage decreases gradually with the increase of the porous media infiltration depth.Based on the temperature retardation phenomenon inside sand bed,the average moving velocity of fuel vapor zone was evaluated.The average moving velocity in porous media bed showed a decreasing trend with the increase of infiltration depth,and the maximum moving velocity of fuel vapor zone in current experimental range was 0.041 mm/s.The percentage of residual fuel increased with the increase of infiltration depth and particle size.For the scenarios with the large particles,deep infiltration depth,high viscosity and boiling point of combustible liquid,lower thermal conductivity of sand bed,infiltration-burning technology is generally not suitable for the decontamination of combustible liquid pollutants in soil,sand and other porous media environment.(4)The burning mechanism of porous media infiltrated by liquid fuel and the dominant mechanism of heat transfer were revealed,and the prediction model of burning rate of porous media infiltrated by liquid fuel was established.As the liquid level drops gradually,the dominant heat transfer mechanism of infiltration-burning process changes obviously.When the drop height of liquid level is small,the direct thermal feedback(convection and radiation)of flame on sand bed surface is the dominant mechanism of heat transfer,and the burning rate shows a rapid increase trend.When the liquid level drops obviously,the upper part of porous media bed forms a dry-sand-layer with a certain thickness,which hinders the direct effect of flame thermal feedback to the liquid fuel,and the heat conduction of dry-sand-layer becomes the dominant heat transfer mechanism.Based on the "three zones" characteristics of the temperature distribution inside sand bed during the burning process of the porous media bed soaked by liquid fuel,the mass and energy conservation analysis was established for the upper dry zone(fuel vapor and sand layer),the middle co-existing zone(fuel vapor,sand layer and liquid fuel),and the bottom wet zone(liquid fuel and sand layer)respectively.Combined with the concept of "long-time solution",a model is developed to predict the burning rate of liquid fuel impregnated porous media bed.Within the acceptable error range,the predicted results of the model agree reasonably with the measured results.The predicted errors are mainly attributed to the "back effect",the thermal conductivity of fuel pan sidewall,the unstable flame extinction characteristics,the random pore structure of porous media bed,the strong capillary effect of porous media bed with fine particles,and the introduction of "long-time solution",etc.In this work,a series of tests have been carried out on the seepage and infiltration burning of porous media bed soaked by typical liquid fuel.The seepage features and infiltration-burning characteristics of liquid fuel leaked into porous media bed under different particle sizes,particle gradations and infiltration depths were studied.The seepage characteristic behavior and main channel effect of porous media bed leaked by liquid fuel were revealed,and the influence mechanism of particle size,two-component discontinuous gradation and infiltration depth on the infiltration-burning characteristics of liquid fuel was analyzed.The variation law of characteristic parameters,such as burning rate,temperature distribution and heat transfer rate in porous media,temperature distribution of flame plume,burning duration,residual fuel percentage,etc.,of infiltration burning scenario were obtained.Based on the analysis of infiltration-burning mechanism and the dominant heat transfer mechanism of different burning stages,a prediction model of burning rate for the fire scenario of porous media bed soaked by liquid fuel was established.The conclusions of current work can provide some references for fire safety and burning-decontamination treatment for porous media environment,such as soil and sand bed.There are 95 figures,13 tables and 171 references in this dissertation.