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Splashing Morphology And Crown Dynamics Of Single Droplets Impinging Heated Liquid Films

Daniel Vasconcelos, André Silva, Jorge Barata

AEROG/LAETA - Aeronautics and Astronautics Research Center, Covilhã, Portugal


The phenomena of droplet impact can be encountered in modern applications, including heat exchangers, electronic cooling devices and internal combustion engines. The underlying mechanisms of two-phase flows, coupled with heat and mass transfer processes, such as evaporation, condensation and boiling, increase the complexity of physical systems. This requires a thorough investigation on interfacial dynamics associated with temperature gradients and mass transfer phenomena. However, existing studies in the literature mainly focus on droplets impacting onto liquid films under ambient conditions, neglecting thermal effects. Therefore, the main objective of this work is to evaluate the influence of the liquid film temperature on the droplet impact outcomes, namely splashing and crown dynamics. An experimental facility was designed and adapted to account for both ambient and non-isothermal conditions. The droplet is released from a hypodermic needle until impacting onto the liquid film. The impact surface is positioned above an aluminium block which acts as a heat source, heating the liquid film by conduction. Water, n-decane and n-heptane are the fluids considered for the experiments. Study cases include varying the droplet impact velocity and liquid film temperature, encompassing both spreading and splashing regimes for isothermal and non-isothermal conditions. Experimental results show that an increase in the liquid film temperature induces the transition from spreading to splashing regimes across all fluids. Qualitatively, this translates from smooth to irregular crowns due to the formation of cusps in the crown rim. These structures are the main source of secondary atomisation, which are promoted by increasing the liquid film temperature. Transitional regimes may display several irregularities, in which splashing is visualised at non-isothermal conditions, followed by the reduction/suppression of the splashing occurrence for higher liquid film temperatures. These require further attention in order to fully comprehend fluid and heat flow, such as Marangoni stresses and local evaporation.

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