| A computational research is conducted to investigate the effects of hydrogen addition to methane,ethylene and methane addition to hydrogen-diluted ethylene laminar diffusion flames on soot formation characteristics by applying the open source flame code Co Flame,respectively.And the inhibition mechanism of hydrogen addition on soot formation was analyzed in the diffusion flames.Numerical calculations were carried out using a detailed C1-C2 gas phase reaction mechanism and a soot model consisting of two pyrene molecules colliding into a dimmer as nucleation,surface growth and oxidation follow the hydrogen-abstraction-C2H2-addition mechanism(HACA).In this paper,the flame temperature,soot volume fraction,mole fraction of main species and the distribution of important parameters describing soot growth process were obtained numerically in hydrocarbon fuel diffusion flames,respectively.Then,the chemical,dilution and thermal effects of hydrogen addition on soot formation were further analyzed by comparing the results of hydrogen and helium addition.Finally,the chemical reaction paths of soot precursor formation in CH4 and C2H4 diffusion flames were analyzed in detail,which can provide a theoretical basis for reducing the soot emission.Adding different proportion of H2to CH4 diffusion flame,the numerical results show the flame temperature has little change,so the thermal effect of hydrogen is not the major factor affecting soot formation.However,hydrogen can effectively reduce the mole fraction of soot precursors C2H2,A1 and A4,thereby slowing down nucleation rate,surface growth rate,condensation rate and number density,and inhibit the soot formation ultimately.Heliun is significantly better than hydrogen in suppressing soot formation,showing that the dilution effect of hydrogen inhibiting the soot formation is stronger than its chemical promotion.It is mainly attributed to higher mole fraction of H atom provided by the main chemical reaction R151 promotes the formation of A1 and soot nucleation rate by increasing the reaction rate of R389 which dominates the formation of A1in CH4 diffusion flames.Adding different proportion of H2to C2H4 diffusion flame.The numerical results show that the chemical effect of hydrogen addition does not change the distribution of temperature in the C2H4 flames,the maximum temperature is about 2100K.In addition,by comparing the results of hydrogen and helium addition,hydrogen is significantly better than helium in suppressing soot formation.The chemical effect of H2 in C2H4diffusion flames causes the lower nucleation rate and surface growth rate together.The mole fraction of A4 and soot nucleation rate decrease with H2 addition.The smaller soot surface growth rate with H2 addition is attributed to the lower mole fraction of H radical and C2H2in the bottom of ethyene diffusion flames.the chemical effect of H2 in C2H4diffusion flames also promotes the formation of A1 Simultaneously,it is attributed to reaction R394,R397 and R403,especially for the R397 affecting the formation of A1in CH4 diffusion flames.With the introduction of H2into C2H4 fuel,it can promote the mole fraction of H atom and the formation of A1 through the main chemical reaction R151 ultimately.Adding methane to a hydrogen-diluted ethylene diffusion flame can further suppress soot formation effectively.Adding CH4 consume more H atoms effectively by the reverse reaction of R4 CH4+H?CH3+H2,which is detrimental to the growth of A1,A2,A3 and A4.And the condensation rate of PAHs and the surface growth rate of HACA were further inhibited.The higher the proportion of methane,the more obvious the inhibition effect is. |
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