Experimental Study On The Evaporation And Ignition Characteristics Of Nano-fuel Pendent Droplets

Nano-fuel refers to the solid-liquid suspension consisted of nanoparticles and base fuel.The basic physical and chemical properties of fuel can be greatly changed with the enhanced surface energy of nanoparticles and the interaction between particles and fuel molecules.It has been proved that nano-fuel played a positive role in the field of engine combustion.In order to get deeply insight in the mechanism of heat and mass transfer between nanoparticles and fuel molecules during combustion.CNT,MoO₃ and CeO₂ nanoparticles with high thermal conductivity and catalytic activity were selected as fuel additives to obtain nano-fuel The effects of concentration,sizes and types for nanoparticles on oxidation and droplet evaporation and ignition process of nano-fuel were investigated.The main work of this paper are given as follows:（1）Preparation and stability analysis for nano-fuel.In this paper,CNT,MoO₃ and CeO₂nanoparticles with the size of 20 nm and 50 nm were selected as fuel additives,Cetyltrimethyl ammonium bromide（CTAB）served as the surfactant and the nano-fuels with the concentrations of 50 mg/L,100 mg/L and 150 mg/L were prepared by two-step method.The surface morphology of the nanoparticles were observed by using a thermal field emission scanning electron microscope（TFESEM）.As the images showed,CNT have a tubular structure,when MoO₃ have a hexahedral crystal structure and CeO₂ present a spherical shape,indicating that the nanoparticles selected in this paper were quite different in morphological characteristics and are representative.The sedimentation observation method and particle size distribution method were used to test the dispersion stability of the nano-fuel prepared in this paper.The results show that the size of the agglomeration in the nano-fuel samples are all below 600nm,indicating that the nano-fuels prepared in this paper have a little size agglomeration,which meets the requirements of dispersion stability for the follow-up tests.（2）Research for oxidation process and thermodynamic characteristics of nano-fuel.The non-isothermal oxidation process of diesel and nano-fuel in the temperature range of 30⁶00℃were investigated by using thermogravimetric analysis.The TG-DTG curve of fuel oxidation combined with the calculation of kinetic parameters were used to analyze the oxidation process of diesel and nano-fuel.The influence of the concentration,size and type of nanoparticle during the pyrolysis process were investigated.The nanoparticles enhance the thermal conductivity of fuel and the heat transfer area between sample and external heat source which improve the heat and mass transfer efficiency of nano-fuel.Compared with the pyrolysis curve of diesel,the pyrolysis curve for nano-fuel shifted to the low temperature region,and this trend was more obvious with the increase of particle concentration or the decrease of particle size.At the same mass concentration,CNT nanoparticles have the best effect on fuel oxidation process,followed by MoO₃ and CeO₂.（3）Study of evaporation characteristics for nano-fuel based on pendant drop method.A visual single droplet evaporation apparatus was built to study the evaporation characteristics of diesel and nano-fuel droplets by pendant drop method.The time sequence images of droplet evaporation process were recorded by a high-speed camera.Based on image processing technology,the image data processing program was compiled by matlab to obtain the droplet diameter during the evaporation process,and the droplet diameter was taken as the target parameter to analyze the evaporation characteristics of fuel droplet.Taking the temperature of cylinder burning temperature 600℃as the boundary,the effect of nanoparticles on the evaporation characteristics of fuel droplets ware investigated at evaporation ambient of low temperature（400℃）and high temperature（700℃）.It was revealed that at the evaporation temperature of 400℃,the evaporation of diesel and nano-fuel droplets underwent constant volume evaporation and stable evaporation,and the droplet evaporation basically conformed to the classical D² law.Owing to the enhanced surface tension of nano-fuel,the vaporization of surface droplet molecules were hindered,leading the evaporation rate of nano-fuel decreases,indicating that nanoparticles inhibit the evaporation of fuel droplets.The inhibition effect is enhanced with the increase of particle concentration or the decrease of particle size.At the same mass concentration,the surface tension of CNT nano-fuel droplets is the largest,so it has the lowest evaporation rate.At the evaporation temperature of 700℃,the evaporation of diesel and nano-fuel droplets underwent constant volume evaporation,fluctuating evaporation and stable evaporation,the micro-explosion phenomenon occurs and the volume changes abruptly until the end of evaporation,besides,the diameter change no longer conforms to the D² law.It is because that the nanoparticles became high-temperature heterogeneous nucleation sites inside the droplets at 700℃,which promote fuel evaporation around the particles,leading,the increases of evaporation rate for nano-fuel,indicating that nanoparticles promote the evaporation of fuel droplets.And,this promotion trend is enhanced with the increase of particle concentration or the decrease of particle size.Besides,various nanoparticles have different physicochemical properties,like density and thermal conductivity,which brought different evaporation rates for nano-fuel.CNT nanoparticles have the smallest density and the largest thermal conductivity,leading the effect of promoting the evaporation rate for fuel droplets by CNT is better than that of MoO₃ and CeO₂ nanoparticles.（4）Experimental study on high-temperature ignition characteristics of nano-fuel droplet.High-purity air was introduced into the heating chamber of the visual single droplet evaporation apparatus and the heating chamber was raised to 800℃to study the spontaneous ignition characteristics of nano-fuel droplets.It was revealed that nanoparticles became temperature heterogeneous nucleation sites to promote micro-explosion of the evaporated fuel around the liquid phase,thereby increasing the evaporation rate of the droplets to accelerate vaporization of the fuel and air mixing molecules to form a combustible gas mixture,therefore,the droplets ignition delay time of the nano-fuel droplets decreased.During the combustion of droplets,the larger specific surface area of the nanoparticles increases the combustion area of the droplets.On the other hand,the excellent radiation absorption capacity of nanoparticle increases the heat and mass transfer efficiency between the droplets and the external high-temperature heat source,leading the increase of nano-fuel droplets.As the particle concentration increased or the particle size decreased,the ignition delay time for nano-fuel decreased while the burning rate increased.The ignition delay time of CNT nano-fuel is the shortest and the burning rate is the largest,indicating that the ignition performance of CNT nano-fuel is better than that of MoO₃ and CeO₂nano-fuel.In this paper,the oxidation process of nano-fuel was studied,and the effect of nanoparticles on the evaporation characteristics for fuel droplets at different temperature and the Ignition characteristics of nano-fuels were also analyzed,which provides important basic data for optimizing atomization combustion of nano-fuel.