Experimental Investigation Of The Dynamic Characteristics Of Droplet Impacting Different Finned Tube Surface

Heat exchanger is a necessary equipment in the industrial process, finned tube is core element that comprised of finned tube heat exchanger, its quality directly affect the heat exchanger performance. Comparing finned tube with smooth tube, with the same consumption of metal materials finned tube has more surface area, which brings the increase of heat transfer coefficient, and realizes the enhancement of heat transfer. This thesis studies the dynamic performance of the droplet and energy dissipation on the three kinds of finned tube surface in the process.When droplet impacts the solid surface, its spreading and shrinking process are widely used in industrial, such as combustion, spray cooling, pesticide antivirus spraying, ink-jet printing, fire fighting, etc. When the droplet impacts dry surface at lower speed, it will form a layer of liquid film. While at higher speed, it will splash. The behavior of impacting droplet is very complex; many detailed mechanisms are still not well explained.Using molecular self-assembly monolayer technology,18alkyl levels of super-hydrophobic, wing tip of super-hydrophobic/grooved hydrophilic and wing tip hydrophobic/grooved super-hydrophobic finned tube surfaces were prepared on copper finned tube surface. The contact angle and the contact angle hysteresis were obtained with the contact Angle measuring instrument, and the morphological characteristics were analyzed using the scanning electron microscopy (SEM). The results show that as pitch increases(0.55、0.66、0.91and1.27mm), the contact angle decreases, while the contact angle hysteresis increases, and the influence of the height is negligible.From the nature change of liquid, this paper investigates the dynamic characteristics of impacting, it found that:when droplets of water, ethanol with concentration of10%and30%contact S-2surface, the wavy behavior was observed, but for ethanol with concentration of50%and75%,there is no obvious wave occurred; During spreading, Dmax increases with pitch increasing, but decrease with the increase of concentration and viscous of liquid; In the second process of spreading it only counts for about70%-90%of the first, along with the increase of liquid concentration, the above ratio decreased.Considering of variation of the surface wettability in the process of spreading, for dry, droplet wetting and liquid film wetting state, the phenomenon of surface wave appears gradually apparent; For S-1dry surface,2～3regularly layers were present, for other situations this phenomenon can be observed, but not obvious. As pitch increases, for S-1dry surface, DMax get smaller at first than increases, and HMax increases at first than decreases; For S-2dry surface, for the column height of1.13mm, DMax get greater, while for the column height of1.2mm, it reverses; For S-3dry surface, the first three size DMax get greater, but for1.27mm pitch the droplets get into bar groove of the structure; droplet wetting S-3and liquid membrane wetting S-2surface, DMax get smaller; considering the whole dynamic process takes, for S-1dry surface and liquid film wetting S-2are less, as the practical case, for S-2dry surface, S-3dry surface and droplet wetting S-3it takes longer.