Development of the microscale Lagrangian particle dispersion model microspray for the simulation of two-phase releases

A new version of the Lagrangian dispersion model MicroSpray was developed to simulate the dispersion of two-phase aerosol clouds. The model extends the algorithms developed to take into account the effects of dense gas dynamics recently developed for the code, allowing the simulation of gas-aerosol jets forming from accidental release of toxic industrial chemicals (TIC) stored in liquid phase in a pressurized vessel or pipe. The mixture of contaminant liquid and vapor is considered as a single material and each particle tracked by the model simulates the contaminant liquid and vapor, water liquid and vapor and dry air taking into account all the possible phase transformations and the related effects on the dynamics of the plume. A system of differential equations is solved to follow at the same time the dynamics and the thermodynamics of the plume to evaluate the liquid and vapor mass fractions after the initial flashing, taking into account the presence of water initially in the mixture or entrained from the wet ambient air. The state of the contaminant at each time step is determined assuming homogeneous thermodynamic equilibrium. The equation of energy conservation is rewritten to include the latent heats of the liquid contaminant and atmospheric water. A sensitivity analysis showing the dependence of the model output on the time-step chosen and the influence of water vapor entrainment in the emission temperature drop has been performed. Further, to evaluate the MicroSpray ability to simulate the dispersion of two-phase releases of heavy gases, the model has been coupled with the diagnostic MicroSwift model, that provides the 3-D wind field in presence of obstacles and orography, and its performances compared in detail to a chlorine railway accident (Macdona). The simulations results, with and without the aerosol module, are presented and the differences are discussed.