Experimental And Numerical Investigation On Evaporation Characteristics Of Single Nano-fuel Droplet

Nano-fuel is a kind of fuel-based nanofluid which is prepared by mixing a certain proportion of nanoscale solid particles with high energy density into traditional fuel by an appropriate method.Nanoparticles changes the liquid structure of base fuel,thus affecting the basic physical properties.In the meanwhile,most kind of nanoparticles possess favorable catalytic oxidation ability,which contributes to promote combustion and reduce the generation of pollutants.In order to further explore the mechanism of nanoparticles facilitating heat transfer and improving combustion,much attention was paid to the evaporation characteristics of single nano-fuel droplet in this paper and the following work was carried out:（1）Effects of crucial parameters in the preparation of nano-fuel by two-step method on the its stability was explored,and the physical performances of nano-fuel were determined by experiment or theoretical calculation.Due to the favorable thermal conductivity and catalytic ability,Ce O₂ with the particle size of 20 nm and 50 nm was served as the nanoparticles.The N-tetradecane（C14）was selected as the base fluid,and the Cetyltrimethyl Ammonium Bromide（CTAB）was worked as surfactant to reduce the agglomeration.The effects of surfactant/nanoparticle mass ratio,ultrasonic oscillation time and water bath temperature during the preparation on the nano-fuel’s stability were studied by combining the single factor method and response surface method.For nano-fuel sample containing 20 nm Ce O₂ nanoparticles with concentration of 50 mg/L,the optimum dispersion results can be achieved at the condition of water bath temperature being 39.5℃,mass ratio being 0.96 and ultrasonic vibration for 40 min.And the nano-fuel sample can be stable for about 7 days.After standing for 5 days,most of nanoparticle clusters’size was detected around about 300 nm,which indicates the fine stability and dispersibility of nano-fuels.The nano-fuels have little higher density than that of C14.The density was improved with increasing concentration of nanoparticles,but it was irrelevant to the nanoparticle size according to the formula.The dynamic viscosity,surface tension and thermal conductivity of nano-fuels also increased since there are interaction forces between nanoparticles or between nanoparticles and liquid molecules.What’s more,these parameters were increased with the increase of nanoparticle concentration and decrease of nanoparticle size.（2）The evaporation experiment of nano-fuel was conducted on a single pendant droplet evaporation bench.A special device was established for single nano-fuel droplet evaporation to study the evaporation behavior with an ambient temperature ranging from 373 K to 773 K.The relationship between evaporation characteristics of nano-fuels and particles’concentration or size was emphatically expounded.A high-speed camera was employed to collect the images of droplet profiles during the evaporation.The equivalent droplet diameter was obtained by extracting the pixel points through image processing program,and then the evaporation rates and mass transfer of the nano-fuel droplets can be calculated.The results showed that the whole evaporation process was divided into the instantaneous heating stage and stable evaporation stage.The effects of nanoparticles on evaporation characteristics were closely related to the ambient temperature.At the low temperature of 373 K,nanoparticles inhibited the evaporation,resulting in lower evaporation rates than that of C14.However,nanoparticles promoted the evaporation process when the ambient temperature rose to 573 K,and all nano-fuels droplets evaporated faster than C14droplet.Furthermore,the impact of nanoparticles on the evaporation was intensified with the growth of nanoparticles’concentration.Specifically,the evaporation suppression at 373 K and the evaporation promotion at 573 K and 773 K were enhanced with the increase of nanoparticles’concentration.In addition,the small-size nanoparticles are more susceptible to external forces and lead to strong micro-convection.Compared with large-size nanoparticles,small-size nanoparticles were more favorable for evaporation.（3）Numerical simulation for single nano-fuel droplet evaporation in high temperature environment was conducted based on ANSYS FLUENT software.And the evolution of temperature field and concentration field in the computational domain during the evaporation were analyzed.The simulation results showed that the temperature field and concentration field were distributed in a gradient and the boundary layer of temperature and concentration were formed.Nano-fuel droplets absorbed heat from the external environment,and the temperature of the nano-fuel droplets kept rising until reaching the evaporation equilibrium temperature.In general,the evaporation equilibrium temperature of the droplets was discrepant at different temperatures and it increased with raising the ambient temperature.The evaporation equilibrium temperature of nano-fuels droplets was higher than that of C14 at the temperature of 573 K.In addition,it increased with the increment of nanoparticle concentration and reduction of the nanoparticle size.In the beginning of the evaporation,the vapor volume fraction of nano-fuel was relatively low and it increased slowly with time.As the evaporation process continued,the evaporation rate of nano-fuel droplets increased.The liquid nano-fuel was constantly evaporating to the fuel vapor,and the vapor volume fraction was increased.The change rate of the vapor volume fraction over time was faster at higher ambient temperature.The vapor volume fraction was increased during the same evaporation period when elevated nanoparticles concentration and minished nanoparticles size.In this paper,experimental research and numerical simulation were conducted to investigate the evaporation characteristics of single nano-fuel droplets.The effect of Ce O₂ nanoparticles on the evaporation characteristics of base fuel droplets at different ambient temperatures were studied in detail.Besides,the evolution of the temperature field and concentration field of nano-fuel droplets during evaporation were emphatically discussed.The results could provide the basic data and theoretical foundation for the futural application of nano-fuel.