Releases of pressurized liquefied gases: simulations of the Desert Tortoise test series with the CFD model FLACS

A methodology to handle releases of pressurized liquefied gases with the CFD model FLACS is presented and used to simulate the Desert Tortoise test series. Highly hazardous chemicals are mainly stored and transported as pressurized liquefied gases (e.g. chlorine, ammonia, propane…). Sudden releases of such pressurized liquefied gases result in a dense mixture of gas and aerosols which disperses into the atmosphere. A certain fraction of the liquid aerosols can deposit on the ground leading to the formation of a pool which gradually evaporates. In our methodology, the source term at the exit orifice where the flashing phenomenon occurs is estimated with a 1D expansion model. A homogeneous equilibrium model and a pool model have been implemented in the FLACS code. The homogeneous equilibrium model assumes that the liquid phase and the gas phase are in a dynamic and thermodynamic equilibrium. In the pool model the shallow water equations are solved and the evaporation rate is estimated from the conservation of enthalpy of the liquid material. The transient pool model is solved simultaneously with the 3D flow field. The flow field is modeled by the Reynolds Averaged Navier-Stokes equations which are closed by the standard k-epsilon turbulence model. Proper wind boundary conditions are applied by using the Monin-Obukhov similarity theory. This new methodology is used to simulate the Desert Tortoise test series which consisted in 4 pressurized liquefied ammonia spills. The temperature, concentration, size and shape of the ammonia clouds were determined in the experiments and are compared with the predictions of our 3D simulations.