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In this project, the collision dynamics of a liquid droplet with a solid surface is studied under various conditions. Such a collision process is relevant to a variety of industrial processes, including spray cooling, impingement of oil droplets on turbine engines, and droplet–particle collisions in fluid catalytic cracking (FCC) processes. A combination of experimental and numerical approaches is employed to investigate the problem, with an emphasis on the development of new numerical techniques and models. In previous two years, a multi-scale model based on the level-set method was developed to study the droplet dynamics on a high-temperature surface in the film-boiling and sub-cooled regimes. A model was also established to study the impact of a droplet on a hot porous surface, in which the transport inside the porous substrate was treated using averaged equations based on the macroscopic properties of the porous material.

During the extension period, further study has been carried out for droplet-porous substrate collision with the development of a new simulation technique that is able to account for the microscopic structures of the porous substrate. The new method is based on the lattice Boltzmann method (LBM), which is efficient in modeling multiphase flows and flows in complex geometries. Different from the previous level-set method, the LBM calculates the motion of the droplet surface using the pseudo-potential method, so that the gas and liquid phases are separated according to thermodynamics and therefore no interface capturing is needed. In order to resolve the small length scales inside the porous media, a novel adaptive mesh technique is developed for two-phase LBM simulation. Fine mesh resolution is applied near the droplet surface as well as inside the porous substrate to provide sufficient accuracy, while coarser mesh is applied in other regions to save computation time. The new LBM algorithm has been validated in both single and multiphase flow problems involving bubbles or droplets under various conditions.

In simulations involving porous material, a Monte Carlo deposition method is used to construct the porous substrate with spherical particles. The property of the porous substrate can be adjusted by varying the particle size, spacing and wettability.  A number of typical situations of droplet colliding with a porous substrate are simulated with the newly developed adaptive LBM scheme. The spreading and penetration of the droplet on the porous surface is analyzed. For non-wetting porous surfaces, the droplet is found to rebound from the surface, and the contact time can be estimated from the relation originally developed for impermeable surfaces. By coupling with the heat and mass transfer equations, the new model is also successfully applied to study the evaporation of the droplet during its collision with a hot porous surface.