Study On Ignition And Chemical Kinetic Models Of Dimethyl Ether(DME) And DME Blended With Alkanes

Dimethyl ether(DME)has the merits of high oxygen content,no C-C bond,low auto-ignition temperature and high reactivity.Thus,it can be blended with diesel to reduce its soot emission and promote its combustion.The burning of fossil fuels leads to global warming,DME is a kind of bio-fuel because it can be extracted from biomass.Therefore,the pressurized oxy-fuel combustion combines with DME can realize the negative CO₂ emission.The chemical kinetic models of DME mixing with n-heptane(surrogate fuel of diesel)and pressurized oxy-fuel combustion of DME not only contribute to the development of related combustion equipments,but also help us have a deepen understanding of the combustion process.Ignition delay time(IDT)is important experimental date for evaluating kinetic models.The IDTs of DME/n-C₇H₁₆ mixtures in O₂/Ar atmosphere were measured using shock tube.Based on the LLNL n-heptane version 3.1 model and Zhao-DME model,the DME/n-heptane model is proposed,which can well predict the IDTs of DME/n-C₇H₁₆mixtures in O₂/Ar atmosphere at 0%to 100%DME proportion.The interaction between DME and n-C₇H₁₆ was found.When DME proportion increases from 0%to50%,the chemical reaction rate of the reaction CH₃OCH₃+CH₃=CH₃OCH₂+CH₄increases,resulting in the increase of H,CH₃ and HO₂ production,and the chemical reaction rates of OH formation promotion reactions.In addition,the production of C₂H₆increases with the increase of DME proportion,which leads to the increase of chemical reaction rates of OH formation promotion reactions.Therefore,the time corresponding to the OH peak decreases as the DME proportion increases and the IDT gets shorter.When the n-C₇H₁₆ proportion increases from 0%to 50%,the productions of H,C₂H₅,C₂H₄ and C₂H₃ increase,which lead to the increase of the chemical reaction rates of OH formation promotion reactions.Therefore,the time corresponding to the OH peak decreases as the DME proportion increases and the IDT gets shorter.The IDTs of DME/C₂H₆ mixtures in O₂/CO₂ atmosphere were measured by shock tube.Based on the OXYMECH 2.0 and DME sub-model in Aramco MECH 3.0,the OXY-Aramco model is constructed and validated by the experimental data measured in this work.The interaction between DME and C₂H₆ was found.The minimum IDTs of the DME/C₂H₆ fuel mixtures are less than those of pure DME and C₂H₆.When the DME proportion increases from 0%(DME00)to 50%(DME50),the importance of ignition promotion reaction CH₃OCH₃(+M)=CH₃+CH₃O(+M)is strengthened,which results in the acceleration of chemical reaction rates of OH formation reactions,and thus,the decrease of IDT.When ethane proportion increases from 0%to 50%,the chemical reaction rate of the reaction CH₃+HO₂=CH₃O+OH is accelerated,resulting in the acceleration of the chemical reaction rates of OH formation reactions.Thus,the IDT gets shorter.It is found that the minimum IDT of DME/C₂H₆ mixture changes from DME00 to DME20 when CO₂ replaces N₂ at P=10 atm and T=1300 K.The analysis shows that this phenomenon is caused by the third-body effect of CO₂,and the specific reactions are CH₃OCH₃(+M)=CH₃+CH₃O(+M)and H₂O₂(+M)=2OH(+M).The IDTs of DME/C₃H₈ mixtures in O₂/CO₂ atmosphere were measured by shock tube.The OXY-Aramco model is validated by the experimental data measured in this work.It is found that the interaction between DME and C₃H₈ is very weak in O₂/CO₂atmosphere.With the increase of DME proportion from 0%to 100%,the IDT decreases almost linearly.However,there is an obvious interaction between DME and C₃H₈ in O₂/N₂ atmosphere.This is because the third-body effect of CO₂ strengthens the third-body reactions C₃H₈(+M)=CH₃+C₂H₅(+M)and CH₃OCH₃(+M)=CH₃+CH₃O(+M),which promotes ignition,and weakens the interaction between DME and C₃H₈ in O₂/CO₂ atmosphere.The IDTs of DME in O₂/CO₂ atmosphere were measured by shock tube.A kinetic model for DME combustion at low and high temperature is proposed.The kinetic model was validated by the IDTs,laminar flame speeds and species profiles measured in O₂/N₂,O₂/Ar/CO₂,O₂/N₂/He/CO₂ and O₂/CO₂ atmospheres.The influence of CO₂ on DME ignition is very small in present temperature and pressure ranges.This is because the inhibition effect caused by the physical property of CO₂ counteracts the promotion effect caused by the chemical property of CO₂.The third-body effect of CO₂ plays dominant role in the chemical property of CO₂.With the increase of pressure,the promotion effect of third-body reactions CH₃OCH₃(+M)=CH₃+CH₃O(+M)and H₂O₂(+M)=2OH(+M)decrease,while the inhibition effect of third-body reaction2CH₃(+M)=C₂H₆(+M)increases.Therefore,the pressure has no obvious effect on the third body-effect of CO₂.