Effects Of N-butanol Addition On Soot Concentration And Morphological Behavior In Diesel Flame

Soot particles are ineluctable in the combustion process of diesel engine for fuel-rich regions always exist in the spray.These particles are the main source of PM2.5 in the atmosphere of the city,which cause great harm to human health.In order to reduce the emission of particles and ease the energy crisis,n-butanol is widely used as an alternative fuel.However,few researches were concentrated on the detailed soot formation processes of diesel/n-butanol mixture.Therefore,in this work,experimental and numerical investigations were carried out to explore the effects of n-butanol addition on the soot behaviors in diesel or diesel surrogate flames.The contents were as follows:?1?A reduced n-decane/?-methylnaphthalene/polycyclic aromatic hydrocarbon?PAH?kinetic model for the calculation of combustion and soot behavior of diesel and its surrogate fuel was developed,including 77 species and 287 reactions.The oxidation and pyrolysis reactions of fuel were obtained by pathway analysis from a detailed model,while the reactions of core species and PAHs were reduced by a directed relation graph with error propagation,computational singular perturbation method,and direct sensitivity analysis,sequentially.The model was validated by the mole fractions of main species and key PAH species in an ethylene premixed flame,ignition delay times of pure and mixed fuel in shock tubes,and the concentrations of major species in jet-stirred reactors.Finally,the new developed model was employed to calculate the combustion and soot processes in an optical diesel engine.The pressure in the cylinder,apparent heat release rate,and normalized soot fraction were obtained in this calculation.Both the fundamental modeling and engine modeling agree well with the data from the literature,indicating the newly developed model can be used to predict the combustion process of diesel fuel.?2?The soot volume fractions?SVF?and number densities in laminar coflow flames of n-heptane/n-butanol blends were experimentally and numerically investigated.A thermophoretic probe was used to sample particles at different height of the flames.SVF,soot number density,average primary diameter,and number of primary particles per aggregate were measured and calculated through transmission electron microscopy?TEM?images.CoFlame code was used to calculate the soot formation in the coflow flames with a newly developed n-heptane/n-butanol/polycyclic aromatic hydrocarbon?PAH?skeletal model and a fixed sectional soot method.The qualitative characteristics of SVF and number densities obtained from TEM images were well captured by the calculation.With the addition of n-butanol,the visible flame height,SVF and soot number density decrease overall.Inception by the dimerization of pyrene?A4?plays the most important role in the evolution of soot number densities,while hydrogen-abstraction-carbon-addition?HACA?process dominates the soot mass addition.Lower mole fractions of A4 and C₂H₂ are responsible for the suppression of soot mass fractions and number densities with the addition of n-butanol.?3?The soot morphological behaviors in laminar diesel/n-butanol coflow flames were experimentally investigated.The average primary diameter?d_p?,average radius of gyration of aggregate particles?R_g?,and average number of primary particles per aggregate?N_p?increase first and then decrease along the flame centerline.The fractal dimension?D_f?of the aggregates does not change significantly,while the prefactor increases first and then decreases because of particle collision and oxidation.With the addition of n-butanol in the fuel stream,the average d_p,R_g,N_p,and D_f are reduced overall,indicating that n-butanol can effectively reduce the size of the particles in the flame and produce more stretched chain-like particles.?4?The effects of CO₂ or H₂O vapor addition to the air stream on the soot mass fractions?SMF?and soot number densities were numerically investigated in a laminar coflow n-butanol/air diffusion flame.Results show that,the SVF,the SMF and number densities are reduced as CO₂ or H₂O vapor is added.Besides,the reactions and concentration distributions of species move downstream with the addition of CO₂.The decrease of soot number density is caused by the drop of temperature and mole fraction of pyrene?A4?that lead to a decrease of inception rate,while the decrease of SMF is caused by the drop of temperature,mole fraction of H and total surface area of particles that lead to a decrease of hydrogen-abstraction-carbon-addition?HACA?rate.?5?The soot behaviors in an n-butanol coflow flame at elevated pressure,a diesel engine-simulating condition,were numerically investigated.Results show that with the increase of ambient pressure,the flame narrows,while the SVF,soot number density and the rate of soot growth increase.Peak wing and centerline soot volume fractions are found to scale with P^4.3 and P^3.9 respectively.In addition,the number density of aggregate particles with smaller size decreases,while that with larger size increases with increasing ambient pressure.The effects of pressure on the flame structure and soot formation can be largely explained in terms of its direct impact on the mixture density.The enhanced air entrainment into the fuel stream with increasing pressure also plays an important role in affecting the flame structure and soot formation and oxidation.