(41ay) Liquid Rainout CFD Modeling, Testing and Empirical Equation

In liquid release dispersion modeling, it is important to correctly model the drag forces on the discharged liquid droplet, as it will affect the duration of the droplet travelling in the air, the amount of liquid that evaporates before touching ground, and the rainout locations. However, in liquid dispersion, each particle interacts with the carrier fluid individually and particles also interact with other particles, both of which changes as the liquid release disperses into the air, making it more difficult to model the drag effect with a simple drag coefficient.

In this study, a CFD model is used to model liquid dispersion, where individual droplets are modeled as Lagrangian Particles and the two-way coupling between the gas and particles, as well as the aerodynamic interactions between droplet particles, were both evaluated. It was shown that the CFD calculation predicted rainout distances and the droplet dispersion results agreed well with the full-scale testing results performed at BakerRisk’s testing facility. The CFD analysis requires modeling the transportation of individual particles, which is a very time consuming and resources intensive process for most quantitative risk analysis studies where calculating and using average particle velocity is sufficient for the purpose of most dispersion modeling scenarios.

Average particle velocities calculated from CFD modeling were analyzed to understand these gas-droplet and droplet-droplet interaction effects on the average droplet velocity and dispersion profile. A set of empirical equations that describe drag coefficients as a function of initial discharge diameter and pressure were derived, which was validated by a series of full-scale tests performed at higher discharge pressure and diameter, and for water, ethanal and diesel at BakerRisk’s test facility.

This study not only modeled liquid dispersion from a microscopic point of view through CFD analysis, but also analyzed key effects that determined liquid dispersion from a macroscopic point of view to develop empirical equations for the drag coefficient to provide a cost-effective way to calculate liquid dispersion.