Dynamics And Heat Transfer Characteristics In Droplet Impact On Flowing Liquid Film

Droplet impact is a very common phenomenon,which is widely used in aerospace,energy,chemical and other industrial technology fields.Due to the good heat and mass transfer ability in the process of droplet impingement,it has attracted many researchers’ attention.In fact,in the related application fields,the target surface of droplet impact is dry wall and static liquid film,which is less reflected as flowing liquid film.The interaction between droplets is inevitable during the impact process.So far,the research on the above contents is relatively less at home and abroad,and the analysis of local heat transfer and the influence of various parameters on the interface evolution characteristics and heat transfer performance are not clear.Comprehensive study has been done through experiments and three-dimensional numerical simulation.The evolution process and heat transfer characteristics of flowing film with single and double droplets are studied in detail.Firstly,the interface motion process of a single droplet impinging on a horizontal flowing liquid film is investigated experimentally.The effects of film flow and droplet impact velocity on the impact results are discussed.The variation of crown height,top radius and bottom radius are analyzed quantitatively.It shows that the asymmetric interface evolution phenomenon is caused by increasing the flow rate.Especially,the splash phenomenon occurs mainly in the upstream position at high flow rate,while the splash of the downstream liquid crown is restrained.With the increase of droplet velocity,the height of the upstream and downstream crowns increases in different degrees,and the height of the upstream liquid crowns is greater than that of the downstream.In addition,the critical impinging velocity of a single droplet impinging on the flowing liquid film is determined by a large number of experiments,and the linear expression of the critical splashing Weber number with respect to Reynolds number is obtained.Secondly,a three-dimensional numerical simulation method was used to study the impact of a single droplet on a horizontal flowing liquid film.The mathematical model of droplet splashing is established.The perturbation model in accordance with Gaussian random distribution is added to the momentum equation in the form of source term to realize the perturbation of the liquid region and clearly show the splash phenomenon at the top of the liquid crown.Revealed the distribution characteristics of gas-liquid two-phase and the non-uniform distribution of heat transfer.The effects of droplet impact velocity,film velocity,film thickness and three different kinds of fluids(water,ethanol and butanol)on the results were analyzed.Besides,the distributions of liquid crown height,wall temperature and convective heat transfer coefficient are discussed quantitatively,and the potential motion mechanism is explained reasonably.Finally,the process of successive and simultaneous impingement of two droplets on flowing liquid film is investigated.Emphasis is placed on the splash phenomenon and temperature field distribution as well as local heat transfer coefficient under different liquid film velocities when the impact momentum is large.It is found that the influence of film velocity on successive impact is greater than that of simultaneous impact.For the former,changing the film velocity will affect the impingement position of the second droplet,resulting in completely different interface evolution and complex distribution of wall temperature and convective heat transfer.While for the latter,the film velocity mainly affects the interface evolution and the move of the wall temperature and convective heat transfer coefficient curve along downstream direction.