| The process of a drop impact on surfaces is common in industrial equipment. Due to its high heat and mass transfer capabilities, it is also widely applied in the fields of energy, power, chemical engineering, metallurgy, aerospace and so on. Thus, an in-depth understanding of this process is crucial for the key technology in the relevant industrial equipment. Besides, as a typical free surface flow problem, the drop impact on surfaces has always been the focus in the academic community. Comprehensive study has been done for the drop impact on solid surfaces, while research focused on the drop impact on wetted surfaces, especially inclined and curved wetted surfaces is less concerned. In this work, a systematic and deep study was carried out for a single drop impacting on wetted surfaces using the experimental observation and numerical simulation.A two-dimensional incompressible laminar flow model was established to simulate the drop impact on a liquid film using the coupled level set and volume of fluid method, which was verified by comparing simulations with numerous experimental results. The formation mechanism of jets in the neck region during the initial stage after impact was revealed and it was found that the jets arise from the local large pressure difference in the drop-film contact region. Influences of gas properties on the liquid sheet morphology and drop coalescence were analyzed. It was pointed out that both gas density and viscosity can affect the liquid sheet and coalescence evolution, and the influencing mechanism was explored. The effect of the impact process on heat transfer performance between the solid surface and the pre-existing film was investigated. The heat flux distribution and effects of impact velocity, the film thickness and the drop diameter on the average heat flux were discussed as well.The drop impact on wetted inclined surfaces was observed using a high speed camera, and outcomes after impact were analyzed. It was found that using the normal critical Weber number to signify the splashing occurrence could eliminate the influence of the impact angle. The front and back spreading factors were discussed, and it was found that the front spreading factor increases with the decrement in the impact angle, while the back one appears the opposite trend. Also the decreasing of the impact angle can weaken the Ohnesorge number effect on the back spreading factor. It was revealed that both the drop front and back initial spreading velocity grow with the impact angle. Some three-dimensional simulations were performed focused on the drop deformation process on inclined wetted surfaces, and the pressure accumulation effect in the film was shown. Dynamic processes from a single liquid drop impact on wetted cylindrical and spherical surfaces were experimentally and numerically investigated, and the effect of the target-drop curvature ratio on the outcomes after impact was analyzed. Characteristic parameters of drop rebound, spreading and the liquid sheet on wetted cylinders were discussed. A predictive model for the spreading factor and an empirical correlation for the liquid sheet width in the cylinder generatrix direction were provided. Also the drop rebound critical numbers with respect to the curvature ratio were analyzed. It was found that the upper limit of the rebound critical Weber number increases with the decrement in the curvature ratio, while it keeps constant for the curvature ratio of less than0.5. According to energy conservation, drop rebound was analyzed theoretically as well, and a predictable model for the lower limit of the rebound critical Weber number was obtained. Based on experimental data, a linear increasing model during drop spreading on wetted spherical surfaces was proposed, and the spherical bending effect on the splashing critical Weber number was identified. When the curvature ratio is less than0.224, the splashing critical Weber number does not change with the curvature ratio and the spherical bending effect vanishes when splashing occurs. |
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