Study On The Soot Generation Mechanism Of Diesel/ethanol/biodiesel Mixed Fuel Combustio

The soot particles produced by incomplete combustion of diesel fuel are an important source of pollutants in the atmosphere,seriously endangering human health and ecological environment,and a realistic resistance to promote efficient and clean use of energy and achieve the goal of carbon neutrality.Compared with the high cost and long development period of internal combustion engine pollutant purification technology,exploring the ideal diesel alternative fuel is the fundamental measure to relieve the double pressure of resources and environment.Oxygenated fuels such as ethanol and biodiesel are increasingly favored by researchers due to their mature production technology,excellent physicochemical properties and wide range of raw material sources.However,the problems of high auto-ignition temperature,low calorific value and poor inter-solubility of ethanol with diesel,and poor low temperature,high viscosity and easy carbon accumulation of biodiesel have made the blending application of the three fuels to become an international research hotspot.At present,in the process of fuel blending application,the blending ratio for optimal performance of mixed fuels under the premise of mutual solubility needs to be solved,the influence law of each component of mixed fuels on the combustion process is still unclear,the competition/promotion mechanism of key radical reactions on soot generation is still inconclusive,and the essential reasons for the enhanced oxidation characteristics of soot particles by oxygenated fuels need to be deepened.The above-mentioned issues have been left unresolved for a long time,which restrict the further development of efficient combustion technology for diesel blended oxygenated fuels.Based on this,this paper investigates the effect of oxygenated fuels on the whole process of"fuel characteristics-combustion process-emission characteristics"of diesel fuel by combining numerical simulation and experiments,with the main technical objective of suppressing soot generation from diesel combustion:（1）The dissolution curve trends of DEB blends at different blending ratios and temperatures were comprehensively investigated,and the ternary phase diagrams of DEB blends with mutual solubility were constructed.The"lipophilic-hydrophilic"property of biodiesel can reduce the surface tension and enhance the interaction effect between adjacent molecules in different liquid phases,thus enhancing the mutual solubility of diesel and ethanol;temperature is a key factor affecting the solubility characteristics of DEB blends,and the mutual solubility region gradually expands with increasing temperature.At the same time,the influence of blending ratio on the low-temperature flow properties and ignition performance of DEB fuel blends was investigated.Under the constraints of kinematic viscosity,cold filtration point,cetane number,density and calorific value,temperature and dissolution characteristics,the optimum blending ratio of74.99%:5.79%:19.22%for DEB fuel blends in the temperature range of-10～40℃was determined.（2）A multi-level simplification strategy including the Directed Relation Graph（DRG）,the Directed Relation Graph with Error Propagation（DRGEP）,the Rate of Production（ROP）analysis method,and the sensitivity analysis method were used to simplify the detailed diesel/ethanol/biodiesel characterization fuel models for ethanol and biodiesel.The detailed mechanistic models of ethanol and biodiesel characterization fuels were simplified by multi-level simplification strategies such as DRG,Direct Relation Graph with Error Propagation（DRGEP）,Rate of Production（ROP）,and sensitivity analysis.The simplified mechanistic model was fused with the diesel characterization fuel mechanistic model to construct a new multi-component diesel/ethanol/biodiesel characterization fuel mechanistic model containing PAHs,including 266components and 1033 reactions.The accuracy of the model was verified using experimental values of molar component concentrations of combustion intermediates and end products,and the overall error was less than 5%,which better reproduced the fuel combustion process.（3）Based on a diesel/ethanol/biodiesel multi-component characterization fuel mechanism model,the effects of oxygenated fuels on diesel combustion thermodynamic parameters were investigated in depth,and key fundamental data such as flame temperature,temperature gradient,flame thickness,combustion heat release rate,and laminar flame propagation velocity were obtained.The low calorific properties of oxygenated fuels reduce the flame temperature,temperature gradient and combustion heat release rate of diesel fuel blends,while the differences in the initial cracking reaction path and combustion intermediate product reaction activity due to fuel specificity reduce the flame thickness of the blends and increase the laminar flame propagation speed.Meanwhile,in the process of exploring the fuel combustion reaction paths,it was found that free radical attack on the fuel for dehydrogenation reaction,C-H or C-C bond breaking of the fuel carbon molecule and dissociation of the fuel single molecule were the main reaction modes.The first benzene ring（A₁）is essential for the subsequent reaction of PAHs,and blending oxygenated fuels can reduce the A₁-A₄production content.（4）In order to further explore the inhibitory ability of oxygenated fuels on PAHs generation,the mechanisms of ethanol,biodiesel and biodiesel/ethanol on PAHs generation were investigated respectively,and the reaction rates of key radicals closely related to A₁,C₂H₂ and OH generation and consumption in PAHs generation,growth and consumption were obtained.The process of PAHs generation and oxidation reaction was depicted.Propargyl group closed ring formation reaction was the main pathway to form PAHs,small molecules such as C₂H₂ promoted the growth of benzene ring,and H,O and OH radicals accelerated the oxidation consumption of PAHs.Both ethanol and biodiesel can reduce the content of small molecules such as C₂-C₄ and make the concentration of reactive radicals such as OH increase significantly.Based on this,the idea that oxygenated fuels inhibit soot generation mainly by weakening the PAHs generation reaction and enhancing the oxidative PAHs consumption process is proposed.（5）Using the designed and developed fuel premix burner and soot particle collection device,soot particle samples of diesel(D₁₀₀),80%diesel/20%biodiesel(D_0.8B_0.2),10%biodiesel/80%diesel/10%ethanol(B_0.1D_0.8E_0.1)and 80%diesel/20%ethanol(D_0.8E_0.2)were successfully collected and analyzed in terms of oxidation The enhanced effect of oxygenated fuel on the oxidation activity of soot particles was analyzed in two different dimensions:oxidation characteristics and structural features.The oxygenated fuels lowered the conversion onset temperature Ts of soot particles under high temperature pyrolysis conditions,and the conversion onset temperature Ts=438℃and the average activation energy Ea=124 k J/mol were the lowest for D_0.8E_0.2 soot particles.The rise of oxygen content in soot particles enhanced the oxidation intensity of soot particles by the formation of CO/CO₂by oxygen atoms.A series of features such as the gradual decrease of the average diameter of soot particles with PAHs microcrystal size,the increase of microcrystal stripe length and curvature,and the obvious disorder feature of nanostructure are important manifestations of the enhanced oxidation reaction activity of soot particles by oxygenated fuels.