Study On Optimized Mechanism And C₆H₆ Formation Of N-Heptane Combustion

Soot will be generated in the combustion process of automobile engine,which has two main harm:firstly,it will be discharged into the atmosphere with exhaust gas,forming harmful pollutants to the atmosphere;secondly,it may be attached to the engine,polluting the engine oil and affecting the normal operation of the engine.N-heptane is one of the common substitutes of diesel oil.It is the main reference fuel（PRF）and also a typical n-alkane.Its oxidation process has been widely studied and C₆H₆ is one of the precursor of soot.Based on the Chemkin software and mechanisms commonly used at present,this paper simulates the ignition delay time of n-heptane in shock tube and its concentration in plane flame,analyzes and compares the advantages and disadvantages of each mechanism,and then obtains the B-R（Blanquart-Revised）optimized mechanism by modifying the Blanquart mechanism.Based on the optimized mechanism,the correlation simulation is also done,and the results of sensitivity analysis and generation consumption contribution rate analysis of the optimized mechanism and Zhang mechanism are compared.Finally,through the optimized mechanism,the path of the formation of C₆H₆ by n-heptane combustion at different temperature and equivalence ratio,the concentration of C₆H₆ important precursor and the change of generation consumption reaction rate of its related important reactions are studied.The results show that the Blanquart mechanism lacks the key reaction in the low temperature stage,which makes the prediction deviation of ignition delay time larger in the low temperature stage;the Blanquart mechanism has less error in the prediction of C₆H₆ concentration than that of Zhang mechanism and LLNL mechanism;the addition of 14 key reactions in the low temperature stage of Sandiego mechanism greatly optimizes the ignition delay of Blanquart mechanism in the low temperature stage and these reduce the error of C₆H₆ concentration prediction results,and finally form a optimized mechanism including 192 species and 1052 reaction steps;the results of sensitivity analysis and contribution rate analysis of generation consumption of optimized mechanism and Zhang mechanism in C₆H₆ further show that the optimized mechanism can be used in other analysis related to C₆H₆;the formation path of C₆H₆ is mainly divided into three kinds,two are at different temperature,n-heptane is oxidized and decomposed to form C₆H₆.Another is that n-heptane is directly divided into ethylene and then converted into C₆H₆ at high temperature.In the path of high temperature stage,the increase of equivalence ratio will lead to the increase of C₆H₆conversion.Similarly,the increase of initial temperature will also lead to the increase of C₆H₆ conversion.C₃H₃ and A-C₃H₅ are the most important and direct precursors,and the contribution rate of R637（C₃H₃+A-C₃H₅<=>C₆H₆+H₂）to the formation of C₆H₆ is the highest;the difficulty degree of formation of C₆H₆ is different under different equivalence ratio,and the concentration of C₆H₆ under the condition of poor oxygen is significantly higher than that under the condition of mixed combustion of rich oxygen and chemical equivalent,so the combustion of engine should avoid the state of poor oxygen as much as possible.