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44515-AC9
Evaporative Droplet-Particle Collision Dynamics
The phenomena of evaporative liquid droplets impacting onto a high-temperature solid object are relevant to many engineering problems, such as fluid catalytic cracking (FCC). Due to the complex nature of the hydrodynamic and thermodynamic properties involved, it is difficult to describe such process quantitatively. The purpose of this project is to examine various contact modes between droplets and particles through theory, computation, and experiments. The long-term goal of this research is to gain the ability to design the feed nozzle assembly for efficient liquid droplet atomization and evaporation operation in petrochemical reactor systems.
In the first year of this project, a 3-D numerical simulation model has been developed based on the multi-scale approach, which couples the micro-scale vapor-layer contact mechanics with the macro-scale flow fields inside the droplet as well as in the surrounding gas. The deformation of the droplet is modeled using the level-set method, which describes the motion of the gas-liquid interface. The heat and mass transfer equations are solved simultaneously with the momentum equation to account for the variation of the temperature field and the vapor generation. The model has been applied to study both the droplet-surface and droplet-particle collisions in the film-boiling regime. The simulation is able to successfully reproduce the dynamics of the droplet, such as the spreading, recoiling and rebounding process during the impact. The simulation results, including the momentum loss, spread factor, and residence time, are in good agreement with experimental data from the literature.
The model is then applied to investigate several typical problems, including oblique collision of a droplet on a flat surface, droplet collision with a moving particle, and droplet collision with a porous particle with mass transfer. For the oblique collisions, the simulation results indicate that the Weber number based on the normal impact velocity has the dominant effect. Higher Wen yields larger momentum loss as well as larger maximum spread factor of the droplet. For the droplet collision with a moving particle, the simulation results show that particle size and collision obliquity have significant effect on the collision dynamics. Small particle size and large obliquity will decrease the droplet-particle contact time, which leads to a lower heat loss of the particle. For particles whose size is comparable to the droplet, the contact time during the collision can be estimated by the first order vibration time. For small particles, the contact time is much smaller. The mass transport of the vapor during droplet collision with a porous particle is also studied. The simulation is able to predict the concentration of the vapor both inside the porous particle and in the surrounding gas phase.
