Fluctuation Characteristics And Temperature Evolution For Droplet Impact On A Thin Liquid Film At Superhydrophilic Surfaces

Drop impact process widely exists in natural phenomena,daily life and so on.Due to the efficient heat and mass transfer performance,it has been extensively applied in industrial and agricultural fields such as spray cooling,pesticide spraying,desalination and aeronautical technologies.Deepen understanding of the droplet impact process is of great importance to the technological development and innovation in the relevant fields.Therefore,the hydraulic characteristics and temperature evolution of liquid droplet impinging on liquid films have been studied experimentally and numerically.In this paper,the superhydrophilic surfaces were prepared to sustain a thin liquid film on it and the spreading of droplet was accurately analyzed.A high-speed camera and an infrared imager were used to record the hydraulic characteristics and the temperature evolution during the impact process.Under lower We,a ring-shaped low temperature structure was formed on liquid film after impact on liquid films.The temperature shows increase first then gradually decrease trend from impact center to border.However,for drop impact on solid surfaces the droplet temperature distribution maintains a gradual increase trend from impact center to the boundary until the droplet temperature rises to the wall temperature.The effects of film thickness and We were comprehensively studied and the results on cold films were compared.The low temperature ring-shaped region appearing on the hot film at low We has evolved into a high temperature ring structure in cold films.With the increase of impact We the spreading of droplets gradually evolves to the formation of the coronal water spray or water splash.Moreover,the ring-shaped region has transformed into a hot center in temperature distribution.The droplet spreading distance decreases with the increase of the initial film thickness.The high temperature ring structure formed on the thin liquid film gradually moves closer to the center and finally merges together as the thickness of liquid film increases.The ring structure disappears when the liquid film is much thicker.Similar to the temperature distribution at higher We,a hot region was formed and maintained that distribution during the whole process.The critical We and initial film thickness ranges for generating the annular temperature distribution after impact was determined and the drop distribution after impact could be precisely controlled.In order to further explore the mechanism of ring-shaped temperature distribution after impact,a 2D / 3D impinging model was established to reveal the internal flow field of the liquid films and the distribution laws for droplet.CLSVOF method were used to study that process and the temperature distribution under different conditions.It is found that a vortex would formed inside the liquid films after impact.That leads to the droplet congested in front of the vortex region and results in the uneven heat transfer performance in the liquid films and the ring-shaped reversed temperature distribution was formed.With the increase of the We the hydraulic behavior after the impact changes from spreading to water splash.The vortex inside the liquid film has gradually weakened up to disappearing and the droplet tends to stay at the impact center,therefore the ring-shaped temperature distribution disappears.This is of great significance in intensifying and regulating the heat and mass transfer for the impact process.