44515-AC9
Evaporative Droplet-Particle Collision Dynamics
This project aims to examine the contact process of a liquid droplet on a hot particle or surface, which is relevant to many engineering applications such as fluid catalytic cracking (FCC). The long-term goal of this research is to provide useful information for the optimal design of the feed nozzle assembly for efficient liquid droplet atomization and evaporation in petrochemical reactor systems. A combination of the theoretical, computational, and experimental approaches are utilized in this research project, with the emphasis on the formulation of a multi-scale model which is based on the direct numerical simulation and sub-grid modeling. In the first year of the project, the multi-scale model has been established mainly for the impact of droplets on non-porous surfaces. The simulation results agree with the experimental results in a number of typical situations, including the normal/oblique collision of a droplet on a flat surface/spherical particle in the film-boiling/sub-cooled regime.
In the second year of the project, the study has been extended for the collision of a liquid droplet with a porous substrate. A new 3-D numerical model is developed for a droplet in collision with a porous surface under the film-boiling condition. Compared to the model originally developed for droplet collision with non-porous surfaces, the current model has the following features: (1) the momentum, heat and mass transport inside the porous substrate is modeled. The flow outside and inside the porous media is described by a unified set of equations for the entire domain, with the flow resistance by the porous material modeled by the solid drag term in the momentum equation. In this way, the interface condition at the surface of the porous substrate is incorporated natural in the equations, without the need to be explicitly specified as in the immersed boundary method employed in our previous models. (2) The modified vapor layer model considers the penetration of gas into the porous substrate, which leads to the decease of vapor pressure in the vapor layer. (3) The vapor mass transfer is modeled both inside and outside the porous substrate, with consideration of different transfer mechanisms in each domain.
Experimental studies have also been carried out for the collision on a porous alumina surface, which has a pore size of 76nm and porosity of 34%. Due to the small pore size, 300¢ªC is sufficient for the impact of the water droplet to be in the film-boiling regime. The collision process is recorded with a high-speed camera, and the droplet deformation and velocity are analyzed from the images. Numerical simulation using the newly-developed model is performed under the same condition as the experiment. The simulated droplet shape, spreading factor, and droplet height are in good agreement with the experimental results. It is found that the dynamic characteristics of the droplet and the heat transfer characteristics of the surface are similar to the impact of droplets on non-porous surfaces. However, the droplet has a longer residence time, and also appears to be less stable on the porous surface.
