Study On The Effects Of Hydrogen And Alcohol Fuels Blending On Soot Formation Of Diffusion Flames

Soot is the product of incomplete combustion of fuel.Soot formation and emissions not only lower the operating efficiency of the combustion device,but also negatively affect the atmospheric environment and human health.Therefore,it is of great practical significance to control the formation and emission of soot in the combustion process,and develop efficient and clean combustion technologies,which depends on a deep understanding and accurate simulation of the soot formation process.On the other hand,in order to reduce the soot formation and the dependence on traditional fossil fuels,clean and renewable fuels such as hydrogen and alcohol fuels have received widespread attention,and they are mostly blended with fossil fuels and used in engine combustion.However,the complex turbulent motion inside the engine cylinder is not conducive to the experimental measurement and simulation analysis of the soot formation process;this hinders our theoretical understanding on the soot formation mechanism of the blended combustion of hydrogen(or alcohol fuel)and hydrocarbon fuel,which cannot satisfy development requirements for the in-cylinder clean combustion technologies.For this reason,this thesis employed the laboratory fundamental flames,deeply explored the influencing mechanism of hydrogen and alcohol fuels blending on the soot formation of hydrocarbon fuels.The contents were as follows:Considering that the main combustion mode of energy power plants with high soot emissions is based on diffusion combustion,this thesis employed laminar counterflow flames as the foundation platform for soot formation study.This thesis firstly established light extinction and laser induced incandescence(LII)system for soot measurement,constructed a sectional soot dynamic model based on counterflow flame model(OPPDIF),considering the soot inception,coagulation,surface growth and oxidation process.This provides the basis for the experimental and numerical analysis of soot formation in this thesis.Then,the soot formation and evolution process of typical counterflow diffusion flames were investigated,which is the prerequisite for further analysis of the blending effects of hydrogen and alcohol fuel.Considering that ethylene has simple fuel molecular structure and a relatively accurate chemical kinetic model,this thesis employed ethylene as the baseline fuel,to systematically study the two different thermochemical structure(temperature field,intermediate species concentration field)and sooting zone structure of counterflow flame.Based on the results,this thesis discussed the effects of counterflow flame structure on their soot formation characteristics.Based on the analyses of the soot formation process of counterflow flames,this thesis then investigated the effects of hydrogen addition on soot formation of ethylene counterflow flames,and compared the results with the chemically-inert helium-doped case,to determine the chemical role of hydrogen in the soot formation process of ethylene flames.The results showed that compared with the helium-doped case,hydrogen-doped ethylene counterflow flame had lower soot volume fraction,indicating that hydrogen blending has a chemical inhibiting effect on soot formation of ethylene flames:hydrogen addition chemically inhibits the growth of soot precursors such as polycyclic aromatic hydrocarbons(PAH)and soot nucleation process.On the other hand,the effect of hydrogen blending on soot formation of methane flames was investigated,to analyze the relationship of hydrogen's chemical role with the baseline fuel structure.It was found that,the chemical role of hydrogen addition in the soot formation process of methane flame was dependent on the oxidant concentration(X_O).Subsequently,the influencing rules and mechanism of methanol blending on soot formation of ethylene counterflow flames were investigated,and it was found that methanol blending could significantly reduce soot formation.For example,a 20%methanol addition could reduce the peak soot volume fraction of the baseline ethylene flame by 68%.Kinetic analyses showed that most of the carbon atoms(?98%)in the methanol molecules were converted to CO,which helps to reduce soot formation.On the other hand,methanol blending showed a chemically inhibiting effect on the soot precursor formation and soot nucleation process of ethylene flame,which further increased the soot inhibiting effect of methanol blending.Finally,the effects of ethanol blending on the soot formation of ethylene counterflow flames were investigated.Compared with the soot inhibiting effects of methanol blending,ethanol blending showed a non-monotonic effect on soot formation of ethylene flames.Although ethanol has a smaller sooting tendency,a small amount of ethanol blending could promote the soot formation of ethylene flames.The phenomenon that soot formation of binary mixtures is enhanced compared to their individual fuel is called soot synergistic effects.Reaction pathway analyses showed that a large amount of CH₃ can be produced during ethanol decomposition process;this enhanced the chemical interaction between CH₃ and C2 species(i.e.CH₃+C2),promoting the formation of C₃H₃ and C4 species,eventually leading to an enhancement of PAH and soot formation.