액체 표면 위 단일 액적 충돌 현상에 대한 실험적 연구

Abstract

The phenomena of single droplet impact on various liquid surfaces were investigated experimentally in deep liquid pool under unburning and burning conditions. As the liquid surfaces, the water, ethanol, methanol, and n-Heptane were tested. The droplet diameter was controlled by the needle size and the impact velocity were changed by the free falling heights whose ranges were 0.01∼0.8 m and 0.5∼0.8 m under the unburning and burning conditions, respectively. In addition, for the different heat release rates under the burning conditions, various liquid pool diameters were prepared. For the unburning surfaces, four regimes were observed, which were the coalescence, single jet, splash (without or with secondary jet), and canopy. Among them, the splash regime with secondary jet and canopy regime appeared for the ethanol, methanol, and n-Heptane surfaces. Based on the present experimental observations, the regime map was constructed using Weber and Ohnesorge numbers, and the transitions between the regimes were compared with the previous correlations. The previous correlations for the transitions between the coalescence and single jet regimes, and between the single jet and splash (without or with secondary jet) showed better accuracy in the case of water surface than the others. Based on the previous and present experimental results, the new correlation for predicting the transitions of regimes were proposed. For the burning surfaces, the droplet diameter, impact velocity, and heat release rate were measured. The droplet diameter and impact velocity were decreased as compared with unburning surfaces. For the heat release rate measurements, the methanol was smallest while the n-Heptane was largest among the liquids tested. Through the visualizations, the single jet regime, splash regime with secondary jet, and canopy regime were observed. Under the larger droplet diameter and/or higher impact velocity conditions, the splash regime with secondary jet or canopy regime appeared, which were similar to the unburning cases. However, for the burning ethanol surface, the single jet regime, splash regime with secondary jet, and canopy regime were observed. In addition, for the burning methanol surface, the canopy regime did not appear, despite of the increases in droplet diameter and/or impact velocity. For the burning n-Heptane surface, the single jet and canopy regimes were observed.