Numerical And Experimental Study On The Soot Formation Mechanism Of Diesel Surrogate Fuel

Developing a detailed and accurate soot formation mechanism has very important theoretical significance and practical value for soot emission control of diesel engine.It is not realistic to develop a detailed kinetic mechanism for all components in real diesel fuel,and the use of surrogate fuels to characterize some important components of diesel fuel is an effective method at present.In this paper,the soot formation mechanism of diesel surrogate fuels was studied by using numerical simulation of CFD coupled chemical kinetic mechanism and experimental investigation.This work firstly built a numerical simulation platform KIVA-CHEMKIN,the 3D model was discretized using block-structured grid generation technology,the compression ratio of mesh model was compensated by using the squish-crevice volume method,and the model was verified the feasibility under actual engine conditions.A reduced n-heptane-n-butylbenzene-PAH mechanism(BRF-PAH)consisting of 111 species and 542 reactions was developed for diesel multi-component surrogate models using a semidecoupling methodology,and an evaluation system of most sensitive reactions was also presented.The n-butylbenzene sub-mechanism was simplified by DRGEP,sensitivity analysis and reaction pathway analysis.The critical kinetic parameters of the model were optimized based on the sensitivity coefficient diagrams of the ignition delay time.The results show that the optimized kinetic model is in good agreement with the experimental results in terms of ignition delay,laminar flame speed,species concentration and HCCI engine.In particular,the soot precursors of acetylene(C2H2),benzene(A1)and IND have shown good prediction.The DICI experiments of four different diesel surrogate fuels were carried out on a four-cylinder high-speed diesel engine.The combustion and emission characteristics of different fuels were compared,and the soot particle size distribution(PSD)was also analyzed.The results show that the addition of toluene or n-butylbenzene will make the ignition delay of blend fuels prolonged,while the effect of blending with the same proportion of n-butylbenzene on the ignition delay is weaken than that of toluene.The ignition delays of TRF20(20%toluene + 80%n-heptane)and BRF30(30%n-butylbenzene + 70%n-heptane)are close to diesel and similar on combustion and emission performances,so they can be regarded as diesel surrogate fuels.The PSD of BRF30 is different from TRF20,and close to diesel under the common engine conditions,especially on nuclear particles.In order to further elucidate the soot formation mechanism in different surrogate fuels,multi-dimensional numerical simulation of in-cylinder direct injection combustion and soot emission for different diesel surrogate fuels was performed using CFD code coupled with BRF-PAH mechanism.The results show that the prediction results are consistent with the test results.The addition of n-butylbenzene or toluene promoted the formation of PAHs and soot,while also improved the fuel-air mixing process to reduce soot formation.Therefore,the final soot formation depends on the competition results of the two processes above.From distributions of soot particle size and number density,BRF30 produces small particles more easily than TRF20,and the BRF30 can better perform the PSD of diesel.In conclusion,it is indicated that n-butylbenzene is more appropriate as the representative of aromatic compounds than toluene in diesel surrogate fuels.In addition,this mechanism can be easily extended to the more comprehensive diesel multi-component reduced mechanisms,which is of great significance for the accurate prediction of PAHs and soot emissions.