Numerical Studies Of The Surrogate Model And Chemical Kinetic Mechanism Of LNG-diesel

Effected by the stringent emission regulation,the LNG-diesel dual fuel engines are drawing more and more attention.Recently,a lot of experimental and numerical studies about the LNG-diesel dual fuel engines have been done.However,the lack of the chemical mechanism of LNG-diesel mixtures constrains the development of the numerical studies of LNG-diesel dual fuel engines.LNG and diesel are both pretty complex mixtures,so it’s impossible to develop the chemical mechanism of LNG-diesel mixtures directly.Hence,the chemical mechanism of LNG-diesel mixtures was studied by developing the surrogate of LNG-diesel mixtures in this study.First,the methane/ethane/propane mixtures were considered as the surrogate of LNG,based on the contents of the components of real LNG.Then,the four representative species n-dodecane,iso-octane,cyclohexane and toluene were selected as the compositions of diesel surrogate,and the ratios of them were decided by coupling the H/C ratio,CN and low heat value of the mixtures with real diesel.Finally,the surrogates of LNG and diesel were combined and the surrogate of LNG-diesel consisted of methane,ethane,propane,n-dodecane,iso-octane,cyclohexane and toluene was obtained.The mechanism of the surrogate of LNG-diesel mixtures named WUT mechanism was developed by extracting the oxidation mechanism of propane from USCⅡmechanism with DRGEP method and adding it into the TJ mechanism.The sub-mechanisms of methane,ethane,propane,n-dodecane,iso-ocatne,cyclohexane and toluene were included in this mechanism.In addition,this mechanism was validated by the ignition delay time and the concentration of the critical intermediate species of every single composition.That is,this mechanism can predict the ignition delay time and the variation of the concentration of the critical intermediate species in the combustion of every single composition.The comprehensive combustion model for LNG-diesel dual fuel engines was constructed by coupling that mechanism with CFD in CONVERGE.In addition,the experimental data from Z6170 LNG-diesel dual fuel engine were used to validate this model.The results show that this comprehensive combustion model for LNG-diesel dual fuel engines can appropriately simulate the in-cylinder pressures and predict the NO_x emission,which means this model is reliable and it can be used to simulate the combustion of the surrogate of LNG-diesel mixtures.The comprehensive combustion model for LNG-diesel dual fuel engines was applied into the numerical studies about the effect of the diesel compositions on the engine operation and emission performance.By comparing the in-cylinder pressures and heat releasing ratios at different conditions,it was found that the effects of the change of the contents of diesel compositions on the engine operation performance were not obvious when the change is within 9%in mol.In addition,the parameter S,degree of sensitivity,is applied to study the effects of the change of the contents of compositions of diesel on the emissions of LNG-diesel dual fuel engines.The results show that in the condition of this study,the degree of sensitivity of compositions are different under different substitution rates of LNG,which means the effects of compositions on the emissions of LNG-diesel dual fuel engines would vary with substitution rates of LNG.In overall,it can be considerable that toluene is the most influential composition for the emissions of HC and CO in the dual fuel engines and n-dodecane is the most critical composition for the emission of NOx.In overall,during the numerical studies of LNG-diesel dual fuel engines,it is possible to ensure the precision of the prediction of operation performance of the model and improve the precision of the prediction of emission performance of the model at the same time,by change the contents of the compositions of diesel.