Study On Dynamic Evolution Characteristics Of Flame Spread And Combustion Behaviors In Thin-layer Fuel With Different Thicknesses

In recent years,the demand for liquid fuel has been increasing all over the world.Fire accidents caused by liquid fuel leaks occur frequently.Once the liquid fuel leaks and is ignited by the ignition source,the flame spread and combustion phenomena of thin-layer fuel can occur.Due to the fluidity of liquid fuel,the development of flame spread and combustion area is dynamically developing,which may further burn oil storage tanks,tank trucks or combustible materials,and induce larger-scale fire accidents.Therefore,there is an urgent need for a thorough understanding of flame spread and combustion characteristics of thin-layer fuels.In this work,based on experimental research and theoretical analysis,dynamic evolution characteristics of flame spread and combustion behavior in thin-layer fuels with different thicknesses are studied.Specific work is as follows:(1)The diffusional combustion behavior of thin-layer fuel without boundary constraints was studied experimentally.Firstly,the relationship between flame height,fuel burning rate and burning time was studied.The transmittance of the fuel radiation and reflectance of substrate radiation were incorporated into the analysis of radiation heat,revealing the evolution mechanism of the output components of the radiation heat received by the fuel surface with the fuel thickness.The relationship between the position of the highest temperature rise on the substrate surface and the average combustion radius was explained.It was found that the temperature at certain positions may exceed the boiling point of the fuel,leading to a reversal of the direction of heat transfer between the fuel layer and the steel substrate.The convective heat transfer between the fuel layer and the substrate was quantitatively analyzed.Considering the heat loss,the evolution law of the heat feedback fraction with combustion time was revealed.It is found that with the increase of combustion diameter,the fraction of radiant heat feedback gradually increases,the fraction of convective heat feedback gradually decreases,and the main control mechanism of heat feedback will transition from heat convection to heat radiation.(2)The flame spread of thin-layer fuel with boundary was studied experimentally.Firstly,the relationships between the characteristic parameters of the subsurface flow,the fuel thickness and pool width were described.Based on the characteristic parameters of subsurface flow,the evolution law of the subsurface flow trajectory was revealed.Subsequently,a heat transfer model of the subsurface flow of flame spread with thin-layer fuel was established,revealing the evolution law of the ratio of the radiation heat from spreading flame to unburned fuel to total input heat with the change of fuel thickness.It was found that the ratio can reach 0.41 with a fuel thickness of 2.0 mm,indicating that radiation heat transfer is the key factor in maintaining the flame spread of thin-layer fuel.The relationship between the convective heat loss from the subsurface flow to the substrate and the fuel thickness was quantitatively analyzed,and the evolution mechanism of the convective heat transfer loss was revealed.Finally,the law of the external radiation of the spreading flame with the spreading time was expounded,and a dynamic multi-point radiation model was established to predict the external radiation of flame spread under different fuel thicknesses and pool widths.(3)The combustion behavior of thin-layer fuel with boundary was studied experimentally.The evolution laws of flame height,fuel mass loss rate and heat loss from fuel layer to substrate with combustion time were revealed,and the evolution mechanism of each output component of heat feedback in thin-layer fuel in the combustion enhancement stage was revealed.In the steady combustion stage,it was found that the main heat loss of the substrate tends to change from convection loss to radiation loss with increasing fuel thickness or burning diameter.In addition,the smaller fuel thickness or the larger combustion diameter leads to a larger percentage of the total heat loss in the total heat feedback.Considering the heat loss of the substrate,the heat feedback fractions of radiation,convection and conduction were calculated,and the variation law of the heat feedback fraction with the fuel thickness and combustion diameter was studied.The main control mechanism of the heat feedback during the combustion of the thin-layer fuel with boundary was revealed.