| It is a complex phenomenon that the droplets suspended on the fiber are oscillated or dropped off by the external air flow.The application of solid-liquid separation by applying air flow is not only reflected in gas-liquid separation filter and the gas diffusion layer of the fuel cell,but it is of special significance for the improvement of water storage technology in arid areas.Therefore,based on the image recognition method,this study analyzes the dynamic characteristics of clam-shell droplet suspended on the fiber under the influence of transverse air flow.In this paper,on the basis of the research in the gas-liquid field,the clam-shell droplet which easily to be affected by airflow is selected as the research object.The case of transverse air flow acting on a droplet is chosen based on the industrial context.An optical experimental system was designed and constructed in this work.Based on the image recognition method,the geometric characteristics of the droplet are obtained and the motion law of the droplet is summarized.Energy and force models of the droplet have been developed,and the dynamics of the droplet has been analyzed in depth by combining dimensionless quantities.The main findings of this paper are as follows:(1)An optical experimental system was designed and constructed to capture the motion state of the droplet under the influence of air flow.The experimental system ensures that the air flow to the droplet is laminar and pure in composition,and enables flow control at different flow velocity.At the same time,the system captures the motion state of the droplet in real time.For parameter data that could not be directly obtained during the experiment,calibration tests were performed on the experimental setup prior to the start of the experiment to obtain the mathematical relations.In order to deeply analyze the influence factors of external conditions on the droplet,different fiber diameters,droplet surface tension coefficients and air flow velocities were designed and controlled in this study.(2)The coordinates of the reference points of the droplet and fiber are obtained by contour recognition after binarization of the time sequence image of the droplet.Based on the reference points of the droplet profile,the geometric center position coordinates were obtained.And the droplet moving coordinate system was established to obtain the characteristic size of the drople,.(3)The kinetic model of the droplet was established.The 3-D model of droplet was reconstructed by numerical integration method,and the surface area and mass of droplet in each partition were obtained.Based on the motion characteristics of the droplet and the external influencing factors,the kinetic equation and energy equation of the droplet were constructed.The existing droplet stress model is introduced and its shortcomings are pointed out.Based on the energy change of droplet,a dynamic droplet stress model is proposed.Finally,the accuracy of the 3-D droplet model is verified by experimental method,and the error range is 0.8%-11.6%.(4)Based on the angular displacement and radial expansion of the droplet,the study found that the motion of the clam-shell droplet suspended on the fiber when subjected to the transverse air flow is mainly composed of the oscillation around the fiber axis and the vibration along its own radial direction.The coupling effect of the above two motions on the droplet is analyzed,and the motion stage of the droplet is divided into the swing-dominated and radial-vibrational-dominated ones according to the motion characteristics.Meanwhile,in this study,the fast Fourier transform of the droplet angular displacement and radial vibration was performed to analyze its frequency and influence factors.Finally,the relationship between the angular displacement and the radial vibration with the air flow velocity is analyzed by the Weber number of the droplet.It is found that the angular displacement of the droplet increases gradually with the increase of the airflow velocity,while the radial vibration of the droplet gradually increases.(5)Based on the kinetic model,it is found that the surface energy change of the droplet is the main factor affecting the energy change of the droplet,and then the surface deformation of the droplet is the main reason for the movement of the droplet.The external force of droplet is studied based on the dynamic force model of droplet.For the droplet with high surface tension,its internal force is mainly along the direction of air flow,while for the droplet with low surface tension,its force in the radial direction cannot be underestimated.Combined with the Weber number of the droplet,it is found that with the increase of the airflow velocity,the droplet is more susceptible to the larger drag force of the airflow,and the fluctuation amplitude of the drag force increases.(6)In the process of the droplet oscillating around the fiber,the surface tension of the droplet is represented by the damping force which hinders the droplet movement.The droplet surface can not completely buffer the effect of air flow when the air flow velocity is fast,and the droplet radial expansion becomes more intense.Finally,the variation of driving moment and angular displacement with surface tension shows that the influence of surface tension on average damping moment increases with the increase of surface tension coefficient and fiber diameter. |
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