Experimental Study Of Multi-droplet Impact On Solid Wall And Liquid Film

Droplet impact is common in energy,chemical,aerospace and other industrial applications.Because of its good heat and mass transfer ability,it has attracted many scholars' attention.In the actual process,the droplet impact is more reflected in multi-droplet impact than single droplet impact.The main characteristic of multi-droplet impact different from single droplet impact is the interaction between adjacent droplets or the interaction between different phenomena produced by a single droplet.Due to the lack of knowledge of this kind of interaction,the prediction of the heat and mass transfer capacity in the actual process by using the single droplet correlation often has a big deviation.In this paper,high-speed camera technology is used to systematically and deeply study the flow problems in the process of multi-droplet impact on solid surface and liquid film.Firstly,the interface evolution process of synchronous and asynchronous multi-droplet impact on solid surface is studied,and the effects of vertical and horizontal distance of droplets and impact velocity on the height of central liquid sheet and the spread factor of combined droplets are discussed.The results show that with the increase of the vertical distance between droplets,the viscous dissipation increases,the height of central liquid sheet decreases,and the spreading factor first decreases and then increases.With the increase of droplet horizontal distance,the spreading factor increases and the height of central liquid sheet decreases,but with the increase of impact velocity,the effect of droplet horizontal distance decreases gradually.Secondly,the wave forming and moving process of droplet film interaction are studied.The wave diameter,dynamic wavelength and propagation speed of capillary wave are discussed,and the corresponding prediction method is given.The results show that the increase of Weber number has a certain inhibitory effect on the wave propagation,but the larger the Weber number is,the less obvious the effect is.Especially for highly viscous fluids,the wave propagation is very insensitive to the Weber number.Based on the theory of motion discontinuity,a prediction method of capillary wave diameter and dynamic wavelength is established.Then,the interaction process between the low speed impact droplet and the static droplet on the plate is studied.The relations between the maximum spreading factor,the minimum height,the duration of instability,the period of oscillation,and the variation of height with Weber number and eccentricity distance are discussed.The results show that with the increase of the eccentricity distance,the thickness of the edge of the retracted side decreases obviously.The height of combined droplets is more representative of liquid oscillation than the diameter.The interaction can be divided into four stages: impact stage,unsteady damping stage,steady damping stage and equilibrium stage.Finally,the evolution process of multi-drop impinging on the liquid film is studied.The effects of vertical distance,horizontal distance,impingement velocity and film thickness on the height of the liquid crown,the diameter of the bottom of the liquid crown and the height of central liquid sheet are discussed.The results show that with the increase of the vertical distance,the height of the central liquid sheet decreases,and the height of the liquid crown first decreases and then increases.The diameter of the bottom of the liquid crown decreases with the increase of the vertical distance of the liquid drop,no obvious change with the increase of horizontal distance.With the increase of the horizontal distance between droplets,the height of central liquid sheet decreases,and the height of the liquid crown first increases and then decreases.The increase of impact velocity has a positive effect on the change of above characteristic parameters.With the increase of the liquid film thickness,the height of the liquid crown increases first and then decreases,the diameter of the bottom of the liquid crown has no obvious change,and the jet height of central liquid sheet increases gradually.