Comparative Study On The Effects Of Oxygen Enrichment And Oxygenated Fuel Addition On The Combustion Kinetics Of Laminar Premixed N-heptane/air Flames

Oxygen-enriched combustion(OEC)is a high efficiency combustion technology that occurs in the high O2/N2 environment through the adjustment of the combustion atmosphere.The fundamental research of Oxygen-enriched combustion has become popular in recent years,since it has been proven to significantly improve the combustion efficiency,reduce the fuel consumption,and also has a positive inhibitory effect on the production of many pollutants such as soot and carbon monoxide.Based on the Premixed code of CHEMKIN-PRO package,laminar premixed flame model has been solved to investigate the effects of oxygen-enriched atmosphere and oxygenated fuel atmosphere on the combustion characteristics of laminar premixed n-heptane/air flames.The flame structure date of different oxygen-enrichment ratio and methanol blending ratio were obtained by flame simulation.The effects of different oxygen enrichment rates on the variations of flame temperature,temperature gradient,heat release rate,the concentration of fuel and critical radicals,laminar flame velocity and reaction rate were analyzed.Based on the reaction flow analysis,the influence of oxygen-enriched air condition and oxygenated fuel addition on the oxidation path of n-heptane was studied.Variations of pollutants(including nitrogen oxides,formaldehyde and soot)concentration in the combustion process were studied,the formation chemistry of such pollutants has been explored.Combined with the thermal effect characteristics,the different effects of oxygen enriched atmosphere and oxygenated fuel addition on the combustion process has been conducted.The paper shows the following consequence:oxygen-enrichment atmosphere can significant enhance the thermal characteristic of n-heptane/air flames,the flame temperature,heat release rate has been largely promoted,the mole fraction of reactive radicals H,OH,O has been increased either.while methanol addition has no obvious effects on the thermal characteristic and the mole fraction of reactive radicals H,OH,O,while the concentration of HO2 has been significant promoted.Two Oxygen-enriched atmosphere has an positive role on the laminar flame speed of n-heptane/air flames.the oxidation pathway analyze shows that the enrichment of oxygen can accelerate the combustion process through the shortened oxidation pathway of some middle radicals in the reaction zone,while the addition of methanol does not participate in the early reaction of n-heptane oxidation pathway.Results of the contribution of consumption and production of such radicals shows that R1(H+O2<=>O+OH)has a key role on the combustion process,through it consumed most of the H radical and also the largest contribution reaction of O and OH radical.Besides,the results shows that the enrichment of oxygen has largely promoted the reaction rate of combustion process,but methanol addition has no obvious effects on reaction rate.The emission index of exhaust gas shows an parabolic relation with the increasing of oxygen concentration which has a maximum value at a fixed oxygen-enrichment ratio,while methanol addition has monotonous reduced the emission index of same exhaust gas,and it has an overall advantage compared to the oxygen enriched conditions.The comparative study shows that the oxygen-enriched flames has an absolute advantage than methanol addition in thermal characteristics such as flame temperature,heat release rate.The introduction of methanol into n-heptane combustion process will results in the common characteristics of the two fuels.From the point of view of the pollutants generation,methanol/n-heptane flame has more advantages.