Two-phase jet releases and droplet dispersion: scaled and large-scale experiments, droplet-size correlation development and model validation

Many accidents involve two-phase releases of hazardous chemicals into the atmosphere. Rainout results in reduced concentrations in the remaining cloud, but can also lead to extended cloud duration because of re-evaporation of the rained-out liquid. For accurate hazard assessment one must accurately predict both the amount of rainout and re-evaporation of the pool.

This presentation provides an overview of the results of a third phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The aim of the project is to increase the understanding of the behaviour of sub-cooled (non-flashing) or superheated (flashing) liquid jets, and to improve the prediction of droplet atomisation, droplet dispersion and rainout. Phase II of the JIP was limited to scaled experiments for water with initial droplet-size data measured at a single value of the superheat only. Furthermore the modelling simplistically assumed one single averaged droplet size (Sauter Mean Diameter, SMD) with rainout at a single point only. As a result Phase III was started to account for these issues. It was sponsored by DNV Software, Gaz de France, RIVM (Dutch Government), TOTAL, Norske Hydro and Statoil (currently merged into StatoilHydro).

The tri-linear modelling approach proposed in Phase II is endorsed in Phase III via additional scaled water experiments at Cardiff University including measurements of flow rate and initial droplet size across the full relevant range of superheats. Phase III also included scaled experiments for cyclohexane, butane, propane and gasoline at the gas turbine research centre (Cardiff University), to ensure that the derived droplet size correlations are also valid for other chemicals than water. Furthermore large-scale butane experiments were carried out by INERIS (France) to ensure that for more realistic scenarios the derived droplet size correlations are accurate.

Model validation and model improvements were carried out by DNV Software, including validation of release rate and initial droplet size against the above scaled and large-scale experiments. New refined correlations for droplet size correlation and SMD were formulated and implemented into the Phast discharge model. It was compared against a range of other droplet size and rainout correlations published in the literature, in conjunction with validation against an extensive set of experiments. It was shown that the new droplet size correlation agrees much better against experimental data than the existing Phast correlation.

To further improve the rainout prediction, the Phast dispersion model (UDM) was also extended to allow simultaneous modelling of a range of droplet sizes and distributed rainout (rather than rainout at one point). It also included further improvements to pool and dispersion modelling after rainout, and validation for dispersion from LNG and LPG pools.