Experimental And Kinetic Studies On The Effect Of N-dodecane Addition On The Laminar Combustion Characteristics Of Methane

Natural gas has received extensive attention as a common alternative fuel for its clean burning,low prices and large reserves.Compared with the spark ignition engine,the compression ignition engine has a higher thermal efficiency.However,the natural gas is difficult to ignite in the compression ignition engine due to its high ignition point.The application of the natural gas in compression ignition engines is generally realized by injecting a small amount of pilot diesel into the cylinder to ignite natural gas.The premixed combustion of the natural gas/diesel/air mixture is one of important combustion stages of the natural gas/diesel dual-fuel engine.Therefore,to study the characteristics of the laminar premixed combustion of the mixture of nature gas/diesel can provide the support for the validation of the natural gas/diesel oxidation mechanism and the optimization of the combustion strategy of the dual-fuel engine.Since the natural gas and diesel are both complex mixtures,the methane/n-dodecane was selected as the alternative fuel of the dual-fuel in the present study.Hence,the characteristics for premixed laminar combustion of methane/n-dodecane mixture at different initial conditions and n-dodecane contents were investigated experimentally and numerically.In addition,the chemical kinetics analyses were performed to elucidate the related phenomena.The experiments and simulations were conducted in combustion chamber and CHEMKIN PRO via outwardly expending spherical flames and one dimension planar flames respectively.Results show that the laminar burning velocities of methane/n-dodecane mixtures increase first and then decrease with the increase of the equivalence ratio and decrease with the increase of initial pressure.Moreover,with the increase of n-dodecane content,laminar burning velocities and Markstein lengths of the methane-n-dodecane mixtures rapidly approach the values of n-dodecane,and then tend to become stable after the n-dodecane content reaches about 25%.Moreover,it can be seen that the prediction values of the laminar flame speed of methane/n-dodecane-air mixtures obtained by the chosen chemical kinetic mechanism model agree with the experimental results well.The results of chemical kinetics analyses show that there is a significant positive correlation between the laminar flame speeds and the concentration of free radicals O,H,etc.The sensitivity analyses indicate that R1(H + O2 = O + OH)and R31(CO +OH = CO2 + H)have the two highest positive sensitivity coefficient of laminar burning velocity.As the n-dodecane content increases,the production of main free radicals such as H and OH and the net reaction rates of elementary reactions of R1 and R31 sharply increased and tend to be gentle at the n-dodecane addition of 25%,which is consistent with the increasing trend of laminar burning velocity with the n-dodecane addition.In addition,the reaction pathway analyses show that the addition of n-dodecane can accelerate the combustion reactions of methane indicating that it can influence the methane reaction pathway to some extent.