Experimental And Kinetic Modeling Study On Natural Gas Rich Combustion And Reformed Combustion

Natural gas rich combustion reformed combustion?RCRC?is a novel and promising technology which uses part cylinders?one or two?to generate hydrogen?H₂?and carbon monoxide?CO?via fuel-rich combustion and the exhaust gases of reformed cylinder reciculate to the intake manifold as a form of EGR to participate in combustion again.This thesis investigated the fundamental ignition and combustion characteristics of reformed gases and the best strategy when this technology is used in internal combustion engines by experimental and numerical methods to explore its fundamental combustion mechanism and the energy saving potential.The mechanism of emission behavior in the reformed cylinder and the non-reformed cylinders was studied by numerical method.Firstly,the ignition characteristics of reformed gases were investigated in a rapid compression machine?RCM?.It was found that the ratio of CO and H₂ was about 1.0¹.2and their mole fractions increased linearly as the reformed equivalence ratio increased.Hydrogen had prominent promotion in methane ignition which shortened the ignition delay times while carbon monoxide had little effect.Further sensitivity analysis revealed that the promotion on ignition delay time and flame propagation resulting from hydrogen addition was closely related to OH and H radicals.The reaction rate of H+O₂=OH+O was accelerated by hydrogen addition.The presence of H₂ and CO shortened the quenching distance of the flammable mixture and inceased the reaction rates,so the flammability limits were extended.The flame speeds of the reformed gases with different compositions were measured,and the correlation between flame speed and equivalence ratio was established at fuel-rich conditions.The combustion performance and emission characteristics were investigated in a single cylinder engine considering different application situations of RCRC combustion mode with the purpose of revealing the effects and mechanism of H₂/CO and engine parameters such as EGR rate,compression ratio on combustion and emission characteristics.The addition of H₂ and CO increased flame propagation speed and in-cylinder temperature,causing higher NO_x emissions and lower THC emissions.Hydrogen and carbon monoxide addition could increase knock intensity and the reason lied on their acceleration in flame propagation speeds and the end gas was much compressed with temperature and pressure increasing.By optimizing reformed equivalence ratio,EGR rate,spark strategy and compression ratio,with limited power losses in the reformed cylinder,the indicated thermal efficiency was increased by about 8¹2%in the non-reformed cylinders.In order to lower the power loss of reformed cylinder,a new reformed method,n-heptane/natural gas compression ignition,was tested.Compared to spark ignition reformed combustion,the indicated thermal efficiency was increased by more than 30%.To explore the mechanism of reformed gas combustion,firstly,the DREGP reduction method and sensitivity analysis based on temperature were used to reduce a detailed reaction mechanism.By modifying the exponent index of key reactions,the new reformed natural gas chemical kinetic model was developed which could precisely predict ignition delay times,laminar flame speed and species concentration of reformed natural gas oxidation.The three dimensional computation model was coupled with reformed gas chemical kinetic model to build a spark-ignition combustion model,to simulate the combustion process and analyze the reaction process of exhausted gases.In addition,the compression combustion of n-heptane/natural gas was numerical analyzed,which provided the explanations for the increase in thermal efficiency and extension in reformed equivalence ratio when the compression combustion was used as the form of reformed combustion.