Mixing of Dense or Light Gases with Turbulent Air- a Fast-Running Model for Lumped Parameter Codes

The release of gases heavier than air like propane, at ground level, or lighter than air like hydrogen, close to a ceiling, can both lead to fire and explosion hazards that must be carefully considered in safety analyses. Even if the simulation of accident scenarios in complex installations and long transients often appears feasible only using lumped parameter computer codes, the phenomenon of denser or lighter gas dispersion is not implicitly accounted by these kind of tools. In the aim to set up an ad hoc model to be used in the computer code ECART, fluid-dynamic simulations by the commercial FLUENT 6.0 CFD code are used. The reference geometry is related to cavities having variable depth (2 to 4 m) inside long tunnels, filled with a gas heavier or lighter than air (propane or hydrogen). Three different geometrical configurations with a cavity width of 3, 6 and 9 m are considered, imposing different horizontal air stream velocities, ranging from 1 to 5 m/s. A stably-stratified flow region is observed inside the cavity during gas shearing. In particular, it is found that the density gradient tends to inhibit turbulent mixing, thus reducing the dispersion rate. The obtained data are correlated in terms of main dimensionless groups by means of a least squares method. In particular, the Sherwood number is correlated as a function of Reynolds, a density ratio modified Froude numbers and in terms of the geometrical parameter obtained as a ratio between the depth of the air-dense gas interface and the length of the cavity. This correlation is implemented in the ECART code to add the possibility to simulate large installations during complex transients, lasting many hours, with reasonable computation time. An example of application to a typical case is presented.