| The nanoscale solid particles with strong catalytic performance or strong heat conductivity added into the fuel can depress the formation of emissions such as CO,HC and PM with good catalytic and oxidation capacity.Moreover,the heat transfer performance of base fuel is increased due to the strong heat conductivity of nanoparticles,and hence the fuel droplet can absorb heat faster,which is conducive to the formation of combustible mixture.In order to better understand the effect of nanoscale sloid particles as fuel additives on the evaporation performance of the base fuel,droplet evaporation visualization test and the numerical simulation of internal liquid field in the droplet evaporation process was combined to study the evaporation and distortion behavior of the sessile nano-fuel droplet on heated horizontal substrate,and the influence of the adding concentration,diameter and type of nanoparticle on the researched parameters was also investigated.Nanoparticles used in this paper were CNT、CeO2 and Co3O4,each type of particle is studied with two size,20 nm and 50 nm,and three adding concentration,50mg/L,100mg/L and 150mg/L.Single component liquid tetradecane(C14)was chosen as the base fuel instead of diesel and CTAB as surface active agent.(1)The basic physical parameters of nano-fuel like density,dynamic viscosity and gas-liquid surface tension were measured by experimental method.The density of base fuel was increased due to the addition of nanoparticles,but its influence on the experiment results can be neglected because of the small addition quantity.Compared with base fuel,the dynamic viscosity of nano-fuel was increased remarkably,and the effect was even more evident with the increase in concentration and decrease in nanoparticle diameter.The increase extent of three types of nanoparticles on fuel dynamic viscosity is CNT>CeO2>Co3O4.Pendant-drop method was adopted to measure the gas-liquid surface tension of the nano-fuel,and the results turned out that the gas-liquid surface tension of the base fuel was slightly reduced due to the addition of nanoparticles,and the reducing effect grew with the increase in nanoparticle concentration and decrease in nanoparticle diameter.CNT nanoparticle with diameter of 20 nm and adding concentration of 150mg/L has the most obvious reduction effect,the value of which was reduced by 5.10% comparedwith the base fuel.(2)The contact angle measuring instrument was adopted to measure the evaporation process of nano-fuel droplet on a 180℃ cast aluminum plate.The initial contact angle and diameter of nano-fuel on plate is about 4.10 mm and 31.0°.The variation of effective evaporation time,the dimensionless contact diameter,height,mass and mass changing rate changed with evaporation time is analyzed.(3)In the evaporation process of sessile droplet on heated horizontal substrate,four stages successively appear which are spreading stage,constant contact line evaporation stage,constant contact angle evaporation stage and mixing evaporation stage.The constant contact line evaporation stage is prolonged with increase in adding concentration and decrease in nanoparticle diameter due to the ‘self-pinning’ effect.In constant contact line evaporation stage at the ‘liquid-solid-gas’ three-phase line,which is dominated by evaporation,the internal thermal convection is weakened and heat transfer is increased due to increase in nano-fuel viscosity and heat transfer coefficient,which is not conducive for the droplet center liquid to flow towards the ‘liquid-solid-gas’ three-phase line and the establishment of liquid heat accumulation area in the edge.As a result,the decline rate of droplet mass is lower than that of base fuel droplet.In constant contact angle evaporation stage and hybrid evaporation stage,the temperature gradient is small in the droplet interior,so there is virtually little thermal convection and evaporation occurs on the entire droplet surface.The decline rate of droplet mass is increased significantly owing to the increase effect of nanoparticle on the heat conduction of base fuel.Therefore,the mean evaporation rate of nano-fuel droplet in the effective evaporation process is higher that of base fuel droplet,the effective evaporation time is reduced.The effects mentioned above are all strengthened with increase in adding concentration and decrease in nanoparticle diameter,effects of nanoparticle species are CNT>CeO2>Co3O4.(4)CFD flow field simulation software is adopted to simulate and analyze the evolution process of the internal liquid flow field and temperature field of C14 pure fuel and 20 nm C50 nano-fuel in the constant contact line stage.In the initial stage of evaporation,influenced by buoyancy and Marangoni effect,a convective vortex was generated on both sides of the inner axis of the droplet,which goes from the center of thedroplet to the bottom and then to the edge,where the heat transfer is dominated by heat convection.Compared with the base fuel,the inner liquid flow rate of the nano-fuel in droplet evaporation process is reduced due to the increase in the density of nano-fuel while the internal temperature rises rapidly owing to the increase in heat transfer rate.With the advance of evaporation time,the temperature gradient inside the droplet is decreased gradually and the heat convection intensity is weakened slowly.In later stage of evaporation when the contact angle is close to the critical contact angle,the heat convection inside the droplet has almost disappeared,the temperature of the droplet becomes uniform and approaches that of the plate and heat transfer is dominated by heat conduction.The evaporation process of single sessile nano-fuel droplet was studied by experiment and simulation method and the influence mechanism of the effect of nanoparticle on the evaporation rate and morphological change in different droplet evaporation stages was analyzed from microscopics in this paper,laying a theoretical foundation for the better understanding of the effect of nanoparticles on heat absorption and evaporation of fuel. |
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