Study On The Flow Dynamic Characteristics Of Thin Films Down Differently Shaped Fibers

Thin film fluid flowing down vertical fiber will show a very rich nonlinear dynamic phenomena due to the action of surface tension and gravity.The flow dynamic characteristics of a thin viscous film are associated with various industrial applications,especially,it has important industrial applications in optical fiber coating,new direct contact heat exchangers,and wet line towers.This topic is to study the flow dynamic characteristics of thin viscous films down differently shaped fibers(cylindrical fiber,square fiber,spiral fiber),a fiber coating experimental system is built and systematic experimental research is carried out,which reveals the flow regime transition mechanism and dynamic characteristics of liquid film flow.In terms of device platform,the fiber coating system was designed and built,which provided experimental platform for the study of the dynamic characteristics of a liquid film flowing down vertical fibers under different working conditions.The fiber coating experimental system adopted a split design,which was composed of four parts: liquid injection component,image acquisition component,liquid recovery component,and overall outer frame.It realized the image acquisition and recording of thin viscous films flowing down the vertical fiber under the action of gravity.The developed extraction and processing technology of liquid film boundary coordinates provided technical support for drawing the spatiotemporal evolution diagram and analyzing the dynamic characteristics of liquid film flow.In the experimental research,the flow behavior of a thin film falling on differently shaped fibers was studied by using the fiber coating experiment system.Firstly,the effect of fiber shape on the flow regime transition mechanism and dynamic characteristics of liquid film flow were studied.For square and spiral fibers,flow behaviors show three typical flow regimes as the cylindrical fiber,which indicates the isolated droplet regime,RayleighPlateau regime,and convective regime.However,the transition process of various fiber shapes is distinctively different.Unlike the cylindrical fiber,the flow regime transition process of square fiber is similar to that of cylindrical fiber,and the flow regime transition is carried out under a small flow rate;flow on a spiral fiber exhibits a wider range of flow rates in the Rayleigh–Plateau regime,which is helpful for the precise control of flow patterns in a relatively stable regime.We further quantitatively investigate three important characteristic parameters of flow dynamics of profiled fibers,i.e.,liquid bead velocity,thickness,and spacing.The spatiotemporal diagram of liquid film on differently shapes fibers under Rayleigh-plateau flow regime well describes the evolution law of liquid bead propagation velocity and spacing with time.Results reveal that a thin film on a spiral fiber has a higher liquid bead velocity,larger liquid bead thickness,and larger liquid bead spacing.Secondly,the influence of fiber diameter and liquid viscosity were studied.It is found that the critical flow rate of liquid film flow regime transition decreases with the increase of fiber diameter or liquid viscosity.For the three parameters of liquid film flow dynamic characteristics,the liquid bead velocity and liquid bead spacing decrease with the increase of fiber diameter or liquid viscosity,and the liquid bead thickness decreases with the increase of fiber diameter,while the viscosity has no significant effect on the liquid bead thickness.Finally,the non-axisymmetric coating of high surface tension liquid on differently shaped fibers was investigated.The results show that the liquid bead rotates downward around the fiber in a non-axisymmetric state,and the spiral fiber will accelerate the rotation speed.