Study of seawater droplet impacting and freezing coupling processes with combined phase-field and multi-relaxation-time lattice Boltzmann method

The phenomenon of seawater droplets impacting on low-temperature surface and freezing is ubiquitous in the field of marine engineering. An integrated phase field and multi-relaxation lattice Boltzmann method is employed in this paper to simulate the freezing behavior of seawater droplet impacting on low-temperature surface. The model can describe changes in the solid-liquid-gas three-phase interface, and obtain the concentration, temperature and velocity distribution inside the droplet. According to the simulation results, the droplet no longer retracts after impacting the surface due to the bottom solidification. A clear solid-liquid interface can be observed within the droplet. Above the solid-liquid interface, there is a high-concentration brine film. Below the salt water film, the frozen droplet exhibits a distinctly dendritic structure with plenty of high concentration brine pockets. Meanwhile, the effects of surface wettability, impacting velocity, surface temperature and initial salinity on dynamic factors and freezing efficiency are investigated. The results indicate that reducing surface wettability is beneficial for anti-icing, and the morphology of frozen droplet changes from central-pointy to central-concave with the increase of contact angle. When the time step is 25,000△t, the freezing efficiency at θ = 60° and 150° are 98.35 % and 84.48 %, respectively. Additionally, a drop in surface temperature leads to salt solution enrichment between dendrites. As the surface temperature drops from −10 °C to −20 °C, the max concentration inside the droplet increases from 0.108 kg/kg to 0.216 kg/kg. The current study serves to provide guidance for the development of anti-icing on the surface of marine structures and seawater freezing desalination by precisely revealing the coupling mechanism of droplet impact and seawater freezing.