| Most of the fuels in practical are blended liquids.Blended liquids is the fuels that contains two or more components.The blended fuels contain various components with different physicochemical properties.The interaction between the components will make the blends exhibit different characteristics during the combustion process.Therefore,this paper extensively studied the characteristics of the pool fire of the typical blended fuel based on the vapor-liquid phase equilibira of blends.The research in this paper mainly includes the following contents:(1)Introducing the vapor-liquid equilibrium theory into the research of evaporation and heat transfer of pool fires,the physicochemical properties of multi-component fuels are determined by microscopic molecular thermodynamics.Therefore,this paper describes the composition and physicochemical properties of the multicomponent liquid during the evaporation process in detail.The miscible blended fuels are divided into two types:azeotropic blends and near-ideal blends,and the basic physicochemical properties of the two liquids are described.(2)According to the phase equilibrium characteristics of the blends,two typical blended fuels were prepared:n-heptane/DMG blends and n-heptane/ethanol blends,and the pool fire experiments of two typical blended fuels were tested.The characteristic parameters of the two blended fuel pool fires were obtained,including burning rate,pool temperature,flame radiation characteristics,flame axial temperature.The results show that the burning process of the azeotropic fuels is similar to that of the single-component liquid.The physicochemical properties of the blended fuel during the combustion process are steady.The composition of the vapor from liquid pool is also constant,and the composition of the vapor is equal to the azeotropic proportion.Near-ideal liquids do not form the stable blends during the burning process,and the composition of the liquid and vapor phases is changing,and thus their physicochemical properties have also changed.Therefore,the azeotropic mechanism and the non－azeotropic mechanism have an important influence on the combustion process of the blended liquid pool fires.The T-X diagram of the blended solution describes the compositional changes of the two types of blends during the burning process.In addition,the burning modes of azeotropic blends in different proportions are described in detail.The typical combustion process of different proportions of azeotrope mixed liquids is discussed in detail.Three typical combustion modes are found,and the boundary values of the three regions are theoretically calculated.The liquid composition data of liquid pool in burning process based on cone calorimeter data is proposed.The liquid phase real-time component calculation method verifies the delineation of the three regions.(3)The combustion characteristics and yields of the actual oxygen-containing blended fuel were studied based on the cone calorimeter.Firstly,the combustion efficiency and combustion products of oxygen-containing blended fuels were studied.Basic data of combustion efficiency of blended fuel were obtained:carbon monoxide yield,CO/CO2 ratio,energy conversion rate and carbon conversion rate.The results show that the yield of CO cannot effectively characterize the incomplete combustion of liquid,and the energy conversion rate and carbon conversion rate data can effectively incomplete the combustion,which indicates that the incomplete combustion of the fuel is mainly product the soot.(4)The ignition characteristics of oxygen-containing blended fuel pool fires under external radiation conditions were studied by cone calorimeter.The results show that different types of oxygenated additives have different effects on the ignition time of blended fuel pool.The ignition time of the RP-3/DGM blends can be well fitted with the Janssens5 0.55 power exponential model,and the ignition time of the RP-3/ethanol blends cannot be fitted by the model.The boundary conditions derived by the Janssens ignition model are discussed and the applicability of the model is discussed.The result show that which is Janssens’ ignition time model is suitable for the blended fuel whose flash point is higher than the ambient temperature.(5)Based on the research of binary blended fuel burning process two typical pool fire burning processes were discovered,and the combustion characteristics of each process were described.The first is represented by aviation kerosene/DGM mixed fuel,and the combustion process can be divided into five stages.The second type is represented by RP-3 kerosene/ethanol mixed fuel,and its combustion process can be divided into two processes:process A and process B.Process A and Process B include three combustion stages,respectively.According to the analysis of the phase equilibrium temperature characteristics of the oil pool surface boiling temperature data and the blended fuels in different combustion stages,the two burning processes of RP-3 kerosene/ethanol blends are controlled by the combustion of ethanol and RP-3 kerosene respectively.The data calculated the burning rate of each component in the combustion process based on the HRR data is consistent with the conclusion of the combustion control mode.The combustion product data CO2/CO and carbon conversion data also have the same characteristics.When the volume fraction of ethanol is less than 40%,the initial boiling point of the RP-3 kerosene/ethanol blends is about 120 ℃.When the volume fraction of ethanol is greater than 40%,the initial boiling point of the RP-3 kerosene/ethanol blends is approximately equal to the boiling point of ethanol.The surface boiling point of RP-3 kerosene/ethanol blends in late burning stage is similar to the boiling point of pure RP-3 kerosene.(6)The study of burning characteristics of salt-ion blended fuel is performed.Several typical lithium ion electrolytes containing LiPF6 were used as researched blends,and the pool fire experiment was carried out.The vapor-liquid phase equilibrium theory is obtained,and the T-X diagram of the electrolyte solvent is obtained.Combined with the results of the electrolyte pool fire test,the combustion process of the electrolyte oil pool is mainly divided into three stages:(1)low-boiling component control stage,(II)intermediate transition phase,(Ⅲ)high-boiling component control stage.In low-boiling component control stage,a little LiPF6 salt is participates in combustion of pool fires.The lithium-ion salt changes the flame color of the initial flame through the flame reaction,but the radiation characteristics of the flame are not affected.The lithium-ion is mainly participating in pyrolysis reaction during the transition stage and high-boiling component control stage and a large amount of white fumes are released. |
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