Investigation On N-pentanol/Diesel/Biodiesel Blended Fuel Spray Combustion And Combined Combustion Mechanism In Constant Volume Combustion Chamber

The development of clean and renewable alternative fuels for vehicles and the expansion of the types of vehicle energy will greatly help to reduce the dependence of the transportation industry on the oil energy,which would enhance the energy security of the country.From the aspect of combustion emissions,biofuels are usually types of oxygenated clean fuels,and could reduce emissions of carbon dioxide and other pollutants once used as the vehicle fuel.At the same time,biofuels come from the natural circulation system,and it will not increase the burden of greenhouse gases in the atmosphere after combustion.Therefore,it is of great practical significance for the research and development of alternative biofuels.In recent years,researchers have made a series of progress in the field of metabolic engineering and enzyme engineering,and the biotransformation rate of long-chain alcohols,such as pentanols,was gradually improved.Due to its excellent physicochemical property,pentanol is considered to be one of the most promising alternative biofuels.Biodiesel is regarded as another potential diesel alternative fuel,which has many advantages,such as rich resources,excellent compression ignition characteristics,and the molecular structure of large scale oxygen.Based on the theory of fuel design,this paper tries to integrate pentanol and biodiesel into existing petrochemical diesel fuel system,and takes the core part of diesel engine operation,fuel injection combustion process as research object.A series of experimental research and simulation work have been conducted to investigate the effect of fuel components on the spray,evaporation,ignition and combustion characteristics of blends.In the part of experimental study,this paper used two kinds of heating methods,including electric heating and premixed combustion heating,to construct the thermodynamic environment of the cylinder in the TDC of diesel engine on the basis of the combustion test platform.Coupling a variety of optical diagnostic methods and the corresponding image processing technology,the spray macroscopic characteristics(penetration length,spray cone angle and diffusion volume),evaporation characteristics(gas and liquid penetration length)and ignition and combustion characteristics(ignition time and position,flame lift length and high temperature soot radiation)have been investigated quantitatively.This paper selected diesel and biodiesel as the baseline fuels,and contrastive fuels were constructed by mixing with 20%and 40%proportion of n-pentanol.The influence of n-pentanol proportion on the spray,evaporation,ignition and combustion characteristics of the baseline fuels were investigated.For the spray characteristics,pure diesel has a longer spray tip penetration and a smaller cone angle compared to n-pentanol blends.While blending n-pentanol(up to 40%)does not affect the original spray penetration length for biodiesel.addition of n-pentanol could enlarge the spray angle and diffusion volume to a certain extent.However,a 1-D spray mixing model proposed by Musculus and Kattke has been employed to compare the equivalence fuel-air ratio distribution in different fuel sprays,and result illustrated blending n-pentanol could reduce the equivalence ratio distribution of diesel spray,but there was no clear distinction between the equivalence fuel-air ratio distribution of biodiesel and its n-pentanol blends.For the evaporation characteristics,on one hand,addition of n-pentanol would enlarge the gas diffusion volume of baseline fuels under high temperature environments.On the other hand,with the increasing blending ratio of pentanol,the high latent heat of vaporization and specific heat of pentanol take more important role to influence liquid length compared to the drop of fuel density resulted from the join of pentanol.It also warned that the risk of liquid impingement on the walls becomes higher when the pentanol blend ratio reaches 40%at low temperature combustion conditions.For the ignition and combustion characteristics,pure diesel has shorter ignition delay and shorter ignition distance comparing to n-pentanol blends,but a larger blend proportion of n-pentanol blend showed a greater sensitivity on the change of ambient oxygen content;Different from diesel,blending n-pentanol,an increase of n-pentanol ratio has a promoting effect on the ignition event of biodiesel spray under most conditions.The natural flame luminosity is found to be reduced significantly with an increasing n-pentanol ratio,meaning that addition of n-pentanol could reduce the soot level under all conditions.Especially for biodiesel,the effect of blending n-pentanol on reducing carbon smoke is obvious.In the part of simulation work,this paper firstly developed a combined combustion mechanism for combustion simulations of diesel/biodiesel/alcohol(up to n-pentanol).The new combined mechanism,which consists of 229 species and 902 elementary reactions,was constructed by integrating two separated reduced mechanisms based on reaction pathways analysis methodology.Then,a CFD simulation model focused on the fuel injection process in a constant volume bomb was constructed on CONVERGE platform.Coupling with the combined combustion mechanism,the CFD simulation model was used to simulate the ignition,combustion and pollutants formation characteristics of these blends.The results show that for the purpose of maintaining the law of heat release and combustion phase,long-chain alcohols have more advantages over short-chain alcohols to be the major alternative fuels for diesel engines.Moreover,addition of alcohols has a significant inhibitory effect on the soot emission in diesel spray combustion.But it should be noted that,the addition of n-pentranol will increase the content of NOx in diesel flame,especially in the case of 20%blending ratio.For biodiesel,the production of NOx in the flame decreased slightly with the increase of the blend ratio of n-pentranol.