Research On The Dynamic Behavior Of A Single Droplet Colliding With The Wall Surface

The phenomenon of droplet impinging on the solid surface is ubiquitous in daily life and industry.Controlling droplet-solid impacting behaviors is significant in researching droplet impinging dynamics and the mechanism of micro film adhesion and flow.The outcomes of a droplet impact on solid surfaces has important significance to industrial applications including petrochemical industry,fuel atomization,coating and so on.Supported by the "Zhejiang Natural Science Foundation Project"(LY15E06007)and the "Provincial new talent program"(2018R407064),the present work aims to focused on droplet-solid impacting behaviors.The dynamic behavior of the biomass oil droplets impinging on solid surface was experimentally investigated on a self-made experimental set-up.The evaporation characteristics of droplets on the heated surface was analyzed after stable adhesion.The main content and innovative point of the paper will be presented as follows.1)The dynamic behavior of a droplet impinging normally on a dry solid surface was experimentally investigated at the relatively high Weber number.The dynamic spreading factor was adopted to investigate droplet-solid impacting behaviors at room temperature.The effects of different parameters on the impinging results were analyzed,which were the typical surface structure(curvature),Weber number,and the physical properties of oil droplets,respectively.The corresponding theoretical analysis was conducted.The experimental results were compared with those of previous impacting flat researchers.The results show that the geometric structure has obvious influence on the spreading behavior of droplet.When the surface curvature is less,the dynamic behaviors of droplets will undergo the spread,retraction,oscillation,and stable attachment after impacting.Then the maximum spreading factor of the droplet is little influenced by the curvature,and gradually tends to that of the droplet impacting plane surface.While the impacting surface curvature is large,the maximum spreading diameter of liquid film will exceed that of solid surface,and some segments of liquid film rim even slide down directly.The experimental results of spreading are consistent with the numerical simulation results of Zhu et al.The results also show that the maximum spreading factor gradually increases with Weber number increasing,which shows an exponential distribution.A simple empirical correlation for the maximum spreading factor is obtained,which are agree well with the results of Clanet et al.2)The process of retraction and oscillation after a droplet impinging was experimentally investigated at the relatively high Weber number.Three kinds of surfaces were used.Liquid film retraction rate,retraction time,and its volume were examined in detail by the collision of droplet on typical surface with different Weber number and Oh number.The experimental results show that,the droplet has the largest retraction in the plane after maximum spreading,the retraction in cylindrical surface is the second,and the retraction in spherical surface is the smallest of the three structures.The results also show that the film retraction rate increased with the Weber number increasing,while the retraction time decreased with the he Weber number increasing.Moreover,the retraction volume of the liquid film increases clearly with the Weber number increasing with a small Oh number.While Weber number has little effect on the volume of retraction when Oh number is relatively large,and the behavior of retraction is obviously affected by viscosity.3)Research on the shape characteristics of stable liquid film after impacting at relatively higher Weber number was carried out.Three typical surface structures were used.The effects of surface curvature,impacting Weber number,and Oh number on the shape of stable adhesion liquid film were analyzed in detail.The mechanism of shape characteristics of liquid film(contact angle,diameter and height of liquid film)after stable adhesion was obtained.The results show that the contact angle of stable liquid film is totally different with structure.Under the same kind of structure,the surface curvature decreases,the contact angle increases gradually.And the contact angle shows largest in plate surface.In addition,the experimental result shows that the diameter of stable attachment oil film increases as Oh number increases,and the attachment film height decreases gradually.4)The dynamic behavior of oil droplets impinging on the heated surface coupled with the characteristics of stable adhesion film evaporation was studied.Dynamic behavior was examined by the impinging of oil droplets on plate surface with different Weber number and oil droplet properties and heated at different temperatures.The evaporation characteristics of liquid film on the heated surface were investigated after liquid film stable.The results show that the surface temperature has little effect on the spreading of oil droplets,while it shows obvious effect on the receding of liquid film.Besides,the liquid film receding speed increases gradually as surface temperature increases,which easily leads to film breakup in the period of shrinkage,even break into two split liquid films.Additionally,the evaporation patterns show different with different droplet properties.The "pinning effect" phenomenon of oil droplet evaporation gradually weakens with the ethanol concentration increasing,and disappears completely when the ethanol content increases to 95%(vol).Then,The evaporation pattern changes.The liquid film contact diameter and height decrease at the same time in this pattern.In addition,the evaporation rate increases rapidly with surface temperature increasing,and shows an exponential relationship.