Droplet Impact Dynamics On Non-isothermal Surfaces Based On Lattice Boltzmann Method

The droplet impacting the wall has excellent heat and mass transfer capability,which has a wide range of applications in daily life production and engineering.Study on droplet impacting problem is always a challenging task,because of the dynamic evolution and nonlinear nature of the solid-liquid phase interface and the three-phase contact line,complex interfacial motion,as well as the complexity of the flow and heat transfer process.Experimental studies are difficult to observe the fluid flow,heat transfer and complex boiling states inside the micro-nano structure.Therefore,the droplet impacting on the isothermal and non-isothermal wall is numerical studied based on the lattice Boltzmann method.The detailed research contents and main results are summarized as follows.First,the improved two-dimensional and three-dimensional vapor-liquid phase change lattice Boltzmann model is constructed based on the pseudopotential multiphase lattice Boltzmann method.Compared with the existing vapor-liquid phase change lattice Boltzmann model,the current thermal lattice Boltzmann model eliminates the calculation of some gradient terms by modifying the collision terms of the temperature evolution equation.Since the present approach adopts a simple linear equilibrium distribution function,it is possible to use the D2Q5 and D3Q7 lattice for the two-dimensional and three-dimensional cases considered here.Thus,the present model is simpler and more effective than the previous models.The present model was validated by droplet evaporation in open space,droplet evaporation on heated surfaces and nucleation boiling problem,and the numerical results show good agreement with the analytical results and the finite difference method.Our numerical results indicate that the present approach is reliable and efficient in dealing with the 3D liquid-vapor phase change.In addition,based on the scheme proposed by Zou and He,a pressure boundary condition for the D3Q19 lattice is proposed to model the multiphase flow in open systems.Then,a 3D multi-relaxation-time pseudopotential lattice Boltzmann model is adopted to study the double droplets impact on a wettability-patterned surface.The effects of several factors like the wettability difference,the Weber number and the droplet spacing on the droplet dynamic behavior are studied in detail.The numerical results show that the unbalanced Young’s force caused by the wetting difference will induce the droplets to rebound or migrate laterally toward the region with high-wettability.In addition,there exist three evolutionary phases for the double droplets impact on the wettability-patterned surface: the asymmetric spreading phase,asymmetric retracting phase,and wetting equilibrium phase.Further,when the Weber number is relatively smaller,a secondary spreading behavior could be observed which in turn causes an increase in the contact time.Finally,as far as the influence of the droplet spacing is concerned,we find that the flow patterns for different droplet spacings are largely different,and the presence of a air cavity could reduce the contact time.Finally,the droplet impact dynamics on Janus-textured heated substrates are numerically investigated with an improved thermal lattice Boltzmann method.A comprehensive parametric study is conducted by varying the wettability,the Jakob number,the Weber number and the surface topographies.With different control parameters,three distinct boiling regimes are observed,i.e.,contact boiling,transition boiling and film boiling(Leidenfrost phenomenon).To reveal the underlying physics of these regimes,the distributions of the unbalance Young’s force,thermophoretic force and vapor pressure difference in the system are theoretical analyzed.As for the self-propulsion behavior,we find that the droplet spreads directionally towards the denser side(area with more pillar arrays)for the contact boiling regime.However,when the droplet is under the Leidenfrost state,its bouncing dynamics depend on the combined effects of the Weber number and the wettability,and a decrease in wettability induces the droplet to migrate toward the sparser side(area with fewer pillar arrays).These physical insights enrich the fundamental understanding of the droplet bouncing dynamics on heated substrates and also provide guidelines for designing advanced surfaces to manipulate the droplet bouncing behavior.