Research On Droplet Evaporation Characteristics Of Diesel-Methanol Microemulsion Fuel

With the rapid increase of car ownership year by year,in China,and the increasing shortage of petroleum resources,the research and use of alternative fuels have attracted more and more attention.Methanol is an ideal alternative fuel containing oxygen because of its abundant sources and low emission of pollutants.The most convenient method of methanol blending combustion in diesel engine is microemulsion.The research object of this thesis is diesel-methanol microemulsified fuel.Good atomization state and evaporation characteristics of fuel are necessary conditions for energy saving and emission reduction of diesel engine.In this paper,the evaporation process of diesel-methanol microemulsion fuel droplet is studied by hanging drop single droplet evaporation test platform,the influencing factors of the evaporation process of droplets will be analyzed,and the mathematical model of droplet evaporation under atmospheric pressure and high temperature environment will be established.The main contents of this paper are as follows:First of all,oleic acid was selected as the main emulsifier and n-butanol as the co-emulsifier to prepare diesel-methanol microemulsified fuels with methanol volume fraction of5%,15%and 25%,respectively,which were denoised as D95M5,D85M15 and D75M25.After standing at room temperature for one week,there was no stratification phenomenon in the microemulsion fuel.Secondly,the evaporation test of diesel,D95M5,D85M15 and D75M25 microemulsified fuel droplets at different ambient temperatures under atmospheric pressure was carried out through the test device.The evaporation process of fuel droplets was filmed and recorded by a high-speed camera.Morphological method was used to process the droplet evaporation images.A batch image processing program was designed based on Matlab software to extract the droplet diameter data during the evaporation process.The influence of different ambient temperatures,methanol content and initial droplet diameter on the evaporation process was studied.The results show that the addition of methanol has slight influence on droplet evaporation,at the ambient temperature of 573K.During the whole evaporation process,the change rate of droplet diameter square with time is basically a constant,which conforms to the classical D~2 law.At673K,the evaporation characteristics of diesel-methanol microemulsion fuel change essentially.Because the boiling point of methanol is lower than that of diesel,methanol gasifies before diesel,and bubbles are formed on the surface and inside of the droplet.The bubbles on the surface of the droplet will quickly separate from the droplet and form jet phenomenon.Under certain circumstances,the bubbles in the inner part of the droplet will cause"micro explosion".In this case,the parent droplet will break and form many sub-droplet,which increases the evaporation surface area and accelerates the evaporation process of droplet.At this time,the change of droplet diameter no longer conforms to the classical D~2 law.With the same initial diameter,the evaporation rate of microemulsified fuel oil containing methanol is faster than that of diesel oil at the same ambient temperature,and it increases with the increase of methanol volume fraction and the evaporation life of the droplets is shortened at the same time.The influence of droplet initial diameter on droplet life is more obvious at 573K.Finally,based on the infinite heat and mass transfer model,the evaporation model of n-dodecane methanol two-component fuel droplet was established by replacing diesel oil with n-dodecane.The model was used to simulate the droplet evaporation process at 573K and 673K.The results were compared with the experimental results and the ideal results were obtained.