The coupled level set and volume of fluid method is applied to the numerical study on the successive impact of double droplets on a super-hydrophobic tube. The impact velocity varies from 0.25 to 2 m/s. These impact processes present spread, retract, rebound, breakup and splash. The out-of-phase impact takes place with the impact velocity from 0.25 to 1.25 m/s, while the in-phase impact takes place with the impact velocity from 1.44 to 2 m/s. With the impact velocity larger than 1.25 m/s, the liquid crown presents and deforms after the trailing droplet impact, then it would gather at the film edge, rebound or break up. When impact velocities range from 1.44 to 1.5 m/s, the finger liquid film presents before the liquid crown appearing. The finger head breaks with the impact velocity of 1.5 m/s during the leading droplet spreading. The zigzag liquid film becomes more obvious for larger velocities.

In this paper, the impact of successive double droplets on a super-hydrophobic tube surface is numerically studied using a three-dimensional model by the coupled level set and volume of fluid method. The effect of impact velocity on double droplets impact under different curvature ratios is studied. With the increase in impact velocity, two kinds of impact models (out-of-phase and in-phase impact) are obtained, and the coalescent liquid film presents rebound, breakup-rebound, and splash-rebound. With the increase in the curvature ratio, the spread of the liquid film along the circumferential direction is promoted, and the rim of leading liquid film up-warp can be observed earlier. The breakup of an extending liquid film is discussed in detail under the curvature ratio of 1, which is mainly influenced by the combined effects of local negative pressure, air flow motion, surface tension, and gravity of the gathering liquid. The air entrainment occurs for the impact velocity between 0.75 m/s (We = 15.4) and 1.25 m/s (We = 42.9), while no air entrainment occurs for the impact velocity exceeding 1.5 m/s (We = 61.7). The entrapped air bubbles would cause a center breakup of the liquid film or escape from the liquid film. The escaping direction is mainly determined by the pressure distribution in the liquid film.

In this paper, a solar desalination system with evacuated tube collectors (ETCs) and low-temperature multi-effect distillation (LT-MED) is developed. It solves the problem of shortage of fresh water, while it also avoids the consumption of non-renewable energy resource and environmental pollution. The system configurations designed consist of ETCs, heat storage tank, flash tank, multi-effect distillation, electrical heating and cooling, and so on. Mathematical and economic models are established based on mass and energy conservation, and the thermal and the economic performance of the solar desalination system is analyzed by utilizing MATLAB. In this paper, the fresh water cost reduces with both the increasing of the area of ETCs and collector outlet water temperature. Recommendations given in this paper could be helpful in future initiatives regarding the research and development of this promising solar desalination technology.