Research On Spray And Combustion Characteristics Of Coal-based Fisch-Tropsch Diesel/Methanol Blended Fuel

Diesel engines still play an important role in the country’s social and economic development.Research on alternative fuels is an important means to achieve high-efficiency,clean combustion and low emissions of diesel engines.Based on the national conditions of rich coal and few petroleum in our country,F-T diesel and methanol formed by deep coal transformation can be used as good alternative fuels for diesel engines.F-T diesel has a high cetane number and contains no sulfur.Methanol is a low-carbon fuel with a high oxygen content.Considering the differences in viscosity,cetane number,oxygen content and other characteristics of F-T diesel and methanol fuel,the F-T diesel/methanol alternative fuel with excellent performance can be obtained by means of fuel design.Diesel engines using coal-based F-T diesel/methanol blended fuel can not only reduce the use of fossil fuels,but also effectively reduce diesel engine emissions.The application and basic research of coal-based F-T diesel and methanol is of great significance for ensuring energy security and protecting the ecological environment.According to the characteristics of the physical and chemical properties of coal-based F-T diesel/methanol blended fuel and diesel,by combining theory and experiment,the paper builds a fuel spray test bench,an engine combustion and emission test bench,and establishes a simulation model.The physical and chemical properties,spray characteristics,combustion characteristics and combustion products of F-T diesel/methanol blended fuels blended with 5%,10%and 15%methanol,and the chemical reaction pathway of combustion products with higher concentration of the blended fuel were discussed.The mechanism of methanol fuel on the secondary atomization process of F-T diesel/methanol blended fuel was proposed,and the characteristics of F-T diesel/methanol blended fuel combustion process were analyzed.The main research works of the paper are as follows:The measurement and analysis of the physical and chemical properties of F-T diesel/methanol blended fuels were carried out.Based on the liquid-liquid compatibility contact angle test,the miscibility of F-T diesel with methanol was evaluated.The physical and chemical characteristic parameters such as kinematic viscosity,density,lower calorific value,cetane number and surface tension of blended fuels with different methanol blending ratios were measured and analyzed.As the methanol blending ratio increases from 0 to 15%,the fuel density increases,the cetane number decreases,and the surface tension decreases,the kinematic viscosity is reduced,and the low calorific value is reduced.A visual spray test bench and a high-pressure jet spray model were established to study the effects of methanol blending ratio,fuel injection pressure and ambient back pressure on the spray characteristics of the blended fuel,and to discuss the primary atomization and secondary spray characteristics of the F-T diesel/methanol blended fuel.The results show that when the fuel injection pressure and the ambient back pressure remain unchanged,as the methanol blending ratio increases from 0 to 15%,the spray penetration distance decreases by about 2%,the spray cone angle increases by 5%,and the spray projected area increases by 23%.When the methanol blending ratio is constant and the ambient back pressure is 3MPa,as the injection pressure increases from 120MPa to 160MPa,the spray penetration distance increases by 13%,the spray cone angle increases by 4%,and the spray projected area increases by 33%;when the methanol blending ratio is certain,the fuel injection pressure is 160MPa,the ambient back pressure is increased from 3MPa to 5MPa,the spray penetration distance is reduced by 15%,the spray cone angle is increased by 5%,and the spray projected area is reduced by 16%.The spray simulation results showed that as the methanol blending ratio increased from 0 to 15%,the difference between fuel beam temperature and ambient temperature decreased,the high concentration region decreased,the concentration decreased more rapidly,and the front-end propulsion speed decreased by 1.6%to 5.0%and SMD decreased by 1.7%to 6.2%.With the addition of methanol,the viscosity of the fuel decreases,the surface tension decreases,and the droplets break up more easily.The speed of droplet increases and the Weber number increases,which changes the form of fragmentation,increases the probability of fragmentation,and makes it easier to form fine droplets.After F-T diesel is compression-ignited,it is easier to vaporize methanol,and these factors promote the secondary atomization blending process of blended fuel injection.The in-cylinder combustion model of diesel engine was established to study the ignition and combustion characteristics of F-T diesel/methanol blended fuel.Under atmospheric pressure,as the methanol blending ratio increases from 0 to 15%,the ignition temperature of the blended fuel increases from 453K to 468K,and the ignition delay time at 474K increases from 0.6s to0.9s.When the load remains constant,with the increase of the methanol blending ratio,the in-cylinder temperature gradually decreases,the high equivalence ratio area increases,the area covered in the F-T diesel concentration field decreases,the concentration increases,and the ignition time delays.When the methanol blending ratio is constant,with the increase of load,the low temperature region in the cylinder expands,the high equivalent ratio region expands,the F-T diesel concentration increases,the methanol region expands,the ignition moment is advanced,and the maximum temperature in the cylinder increases.The change rules of the temperature field,concentration field,equivalence ratio and combustion characteristic parameters of each component in the cylinder were analyzed.The ignition process of F-T diesel/methanol blended fuel experienced three stages:compression ignition of F-T diesel,ignition of methanol and mutual promotion of combustion.The blended fuel compression ignition-ignition-facilitated ignition mode was established.Using n-tetradecane/isooctane as F-T diesel fuel to characterize the chemical reaction kinetic mechanism of combustion was constructed,and the chemical reaction pathway of F-T diesel/methanol blended fuel was discussed.A closed homogeneous reactor was used to study the combustion products of acrolein,benzene,1,3-butadiene and n-hexane of F-T diesel/methanol blended fuels with different methanol blending ratios,and the concentration of the combustion products in the methanol blending ratio,formation rate,sensitivity and chemical reaction pathways were analyzed.With the increase of the equivalence ratio of the blended fuel and air,the overall concentration peaks of acrolein,benzene,1,3-butadiene and n-hexane increase with the progress of the combustion time,and the peak time is earlier,the reaction time is shortened,and the positive reaction rate and reverse reaction rate peak increases.When the reaction conditions such as initial temperature,pressure and equivalence ratio remain unchanged,as the methanol blending ratio increases,the peak of acrolein appears later,the reaction time is prolonged,and the concentration peaks of benzene,1,3-butadiene and n-hexane decrease,the peak appears earlier,and the reaction time is shortened;the peaks of the forward and reverse reactions of acrolein,benzene,1,3-butadiene and n-hexane are basically unchanged,the peak of acrolein is delayed,and the peak of benzene,1,3-butadiene and n-hexane appeared earlier.After adding methanol,the three production paths of allyl alcohol become two paths for HCO formation:CH₃CHCHO=C2H₃CHO+H is the main formation reaction,and the absolute formation rate increases;in the reaction process of benzene production,2C₃H₃=C₆H₆ is the main reaction for the formation of benzene,adding methanol,the formation path of C₃H₃ changes,and the absolute formation rate decreases.The formation pathways of 1,3-butadiene changed from three to two,C₄H₇=1,3-C₄H₆+H was the main reaction for the formation of 1,3-butadiene,and the absolute formation rate decreased;the formation pathways of n-hexane changed from four to one,p C₂H₅+C₂H₅=n C₆H₁₄ is the main reaction to generate n-hexane,and the absolute formation rate first decreases and then increases.A 493 diesel engine performance test bench was built to measure the combustion characteristics and concentration distribution of over 30 combustion products in the cylinder of F-T diesel/methanol mixed fuels with different blending ratios.As the methanol blending ratio increases from 0 to 15%,the maximum cylinder pressure in the cylinder increases by 7.6%,the peak pressure rise rate increases by 12.8%,and the peak heat release rate increases by 33.8%under the rated working condition.The crank angle is delayed,the ignition delay period is extended,the combustion start point is moved backward,and the combustion duration is shortened.The addition of methanol can promote combustion.In the rated condition,as the methanol blending ratio increased from 0 to 15%,NO_x emissions decreased by 24.2%,soot emissions decreased by 46.6%,HC and CO increased.The emissions of olefins,alkanes,and aromatics decrease,while the concentration of aldehydes and ketones increases.Among them,the molar fractions of combustion products such as acrolein,1,3-butadiene,benzene,and n-hexane are relatively high.