Numerical Studies On The Combustion Mechanism Controlled By Mixture Reactivity And Concentration Stratification

Multi-fuel combustion mode,which has the ability to control the fuel reactivity and concentration distribution in-cylinder with high flexibility,has been widely concerned by worldwide researchers for its potential in high efficiency and clean combustion.In order to develop the applicable reduced mechanism of surrogates coupling with 3D-CFD,it is very important practical significance to search the suitable surrogates for gasoline and diesel to reproduce their combustion and emission characteristics.Moreover,it is important theoretical significance to study the effects of the cross-reactions among the multi-components on the reaction pathway and emission formation in the combustion.In this study,numerical and experimental investigations have been conducted to explore the fuel reactivity and concentration distribution in-cylinder and to develop more sophisticated reduced mechanisms of multi-fuel.Firstly,the effects of the different sizes of the reduced mechanisms from detailed mechanisms on the accuracy and efficiency of calculation are analyzed.N-heptane is chosen as the research object to investigate the influence of different size mechanisms on the accuracies of ignition delays,laminar burning velocities and mole fraction profiles.The main reasons are found to explain the errors between the reduced mechanism and the detailed one.Meanwhile,the calculation accuracy and efficiency of reduced mechanisms coupling with CFD are compared to establish the theoretical foundation for the construction of reduced mechanisms used in the combustion mode of fuel reactivity and concentration distributionSecondly,experiments were conducted to investigate the suitable gasoline surrogate among PRF（Primary Reference Fuels）,TRF（Toluene Reference Fuels）and TRFDIB（Toluene Reference Fuels with Diisobutylene）by comparing with the combustion and emission characteristics（including ignition delay,combustion duration,Soot,NOx,CO and UHC）at low,medium and high loads.TRFDIB was chosen as the ideal gasoline surrogate to develop a reduced chemical kinetic mechanism.The proposed mechanism contains 113 species and 561 reactions,the simulation results from KIVA-3V coupled with reduced mechanism show good agreements in predicting the combustion and emission characteristics,such as pressure in-cylinder,heat release rate,soot and NOx emissions.Thirdly,the reduced mechanism of gasoline surrogate was merged with a reduced mechanism of diesel surrogate to develop a multi-components surrogate for gasoline-diesel,which contained 145 species and 691 reactions including the sub-model of NOx and Soot.The multi-fuel mechanism could reproduce the combustion and emission characteristics of gasoline and diesel.Meanwhile,the differences between solo component and multi-components of surrogates on the detailed information of reaction pathway are compared.The effects of cross-reactions on the combustion process are studied.The results show that the multi-components surrogates can reproduce the combustion and emission characteristics of real gasoline and diesel more accurately;the cross-reactions between multi-components can improve the consumption of fuels,and it affects the reaction pathway and the PAH formation through C1^C4 small molecular and OH radicals etc.The reduced mechanism of gasoline-diesel was coupled with 3D-CFD to explore the effect of the injection strategy of diesel,the ratio of gasoline and EGR on the distribution of reactivity and concentration in-cylinder,the combustion process and emission formation.Results show that the injection strategy of diesel influenced a lot on the reactivity and concentration distribution of mixed gas.In the single injection strategy,injection timing is beneficial for the stratifications of reactivity and concentration.Early single injection strategy improve the reactivity of gas mixture.Diesel promote the oxidation reactions in low temperature and gasoline accelerate the reactions in high temperature;the mole fraction of H₂O₂ is high,less soot form in the homogeneous combustion.Mixture stratified in the late single injection strategy.Under this condition,the ratio of fuel throuth pyrolysis is high.The ratio of gasoline and EGR have important influence on the stratifications of reactivity and concentration.Through chemical mechanism analysis,abundant of H₂O₂and CH₂O are accumulated from the gasoline in high ratio.H₂O₂ and CH₂O oxidize immediately to raise the in-cylinder pressure and HRR.EGR reduce the conversion rate of reaction pathway HO₂?H₂O₂?OH to decrease the activity of mixture.EGR also lower the flux of CH₂O?HCO?CO?CO₂,which is beneficial to control the combustion process.