Study Of Laminar Burning Velocity And Chemical Mechanism For Typical Small Molecular Combustible Gases

New combustion technologies for the purpose of high efficiency,low pollutant and greenhouse gas emission have been proposed and developed.Laminar premixed flame is the most basic and important topic in combustion engineering.The study of laminar premixed combustion characteristics of combustible gas can provide a theoretical basis for its industrial application.The laminar premixed mixture of low calorific gas and air,the oxygen enrichment combustion,the oxy-fuel combustion and the diluted combustion is a combination of fuel,oxygen and diluent.The purpose of this thesis is to study the charracteristics and chemical mechanism of laminar premixed combustion of typical small molecular combustible gases integratedly.The laminar flame velocities of CH₄-O₂-N₂-CO₂ mixture in different concentration ranges under ambient conditions were studied by numerical simulation.It was found that the correlation between laminar burning velocity of CH₄-O₂ and equivalent ratio showed a quadratic function;the correlation between normalized laminar burning velocity of CH₄-O₂-N₂ mixture and diluent fraction shows a first-order function.The correlation between normalized laminar burning velocity of CH₄-O₂-CO₂ mixture and diluent fraction shows a second-order function.The laminar burning velocity of CH₄-O₂-N₂-CO₂mixture can be calculated by mixing models of single diluent atmosphere.The Brokaw model based on mass fraction and the model based on adiabatic flame temperature have the highest accuracy among the eight models constructed and validated.The correlations and the mixing models have high accuracy by comparision of experimental results.A series of C₃H₈-O₂-CO₂ and C₃H₈-O₂-N₂ laminar combustion velocities were obtained by heat flux method and appropriate data processing methods.In light of these new experimental data,the performances of five detailed mechanism kinetic models were studied.All the five models predicted the tendancy of laminar burning velocity varies with equivalence ratio and diluent ratio,and UDEL showed the highest accuracy,especially in CO₂ diluent atmosphere.SANDIAGO predicted that the maximum laminar burning velocity of C₃H₈-O₂-N₂-CO₂ occurred in stoichiometric ratio,which was different from other mechanisms and experimental results.The laminar burning velocites predicted by POLIMI are much larger than other models and experimental results.At the same diluent ratio,the laminar combustion velocity in CO₂ is much smaller than that in N₂.Based on the analysis of numerical results,the correlation between N₂/CO₂ diluent ratio and normalized laminar combustion velocity is a second order polynomial.The first and second-order coefficients are related to the equivalence ratio by a second-order polynomial.The dilution effect on the laminar burning velocities of C₃H₈-O₂-CO₂ and C₃H₈-O₂-N₂ was interpreted by one-step global model based on large activation energy assumption and detailed mechanism.All global flame parameters?including adiabatic flame temperature,overall reaction order,overall activation energy,Lewis number,Arrhenius number etc.?vary with dilution ratio.Based on the one-step global model and sensitivity analysis,the transport effect have little effect on the laminar combustion velocity and the thermal effect is the dominant factor.With the increase of diluent ratio,all effects gradually decrease.The analysis based on detailed mechanism shows that the laminar burning velocities of all mixtures are related to radicals.There is a linear relationship between the laminar burning velocity and the maximum mole fraction of?H+OH+O?.The nonlinear correlation between laminar burning velocity and dilution ratio is interpreted from two aspects:for the single step global model,it is caused by the non-logarithmic correlation between the parameter?1-Ar/Ar₀?and dilution ratio;for the detailed mechanism model,it is the result of the decrease of free radicals including H,OH and O.The comparison with experimental data shows that the single-step global model overestimates the effect of dilution ratio,especially when the equivalence ratio is far away from stoichiometry.The fictious species were introduced to study the effects of CO₂ replacing N₂ on laminar burning velocity.The results show that the decrease of laminar burning velocity is mainly caused by thermal effect when CO₂ replaces N₂.Chemical effect is more sensitive to the change of dilution ratio and equivalence ratio than thermal effect.The transport effect is minimal and nigligible.The numerical simulation of propane oxidation in plug flow was studied and focus on the chemical effects of CO₂.Sensitivity and rate of production analyses shows that CO₂ nearly has no effect on the elementary reactions that initiate propane oxidation,but retarded some elementary reactions about intermediate oxidation and radical pool transformation.R99,CO+OH=CO₂+H is the most important elementary reaction about CO formation and consump-tion and R84,OH+H₂=H+H₂O is the most important elementary reaction about H₂ formation and consumption.These two elementary reactions play decisive role in the differences about premixed propane flame in N₂ and CO₂ atmosphere.By comparing the numerical results of four global mechanisms and one detailed mechanism,JL2 mechanism is proved to be the best mechanism for predicting propane oxidation at different temperatures,different equivalence ratios and different dilution gases,while WD1 mechanism is proven not suitable for propane diluted combustion,especially in fuel-rich conditions.