Industry support continues for leading-edge research on flashing liquid jets and two-phase droplet dispersion.
The third phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion was recently completed, and a fourth phase has recently been kicked off. The aim of the project is to increase the understanding of the behaviour of liquid jets, and to improve the prediction of droplet behaviour (Figure 1). The JIP includes theory and software development performed by DNV Software, complemented by a programme of experimental work.
Introduction
Many accidents involve flashing two-phase releases of hazardous chemicals into the atmosphere. Rainout results in reduced concentrations in the remaining cloud, but may also lead to extended cloud duration because of re-evaporation of the rained-out liquid. For accurate hazard assessment, one must predict both the amount of rainout and re-evaporation of the pool.
Phase I of the JIP included a review (Reference i) on flashing liquid jets and two-phase dispersion. Phase II involved the application of Phase I recommendations for scaled water experiments (Ref. ii) and model improvement (Ref. iii). Cardiff University performed scaled twophase experiments for water for conditions above boiling point.
The experiments measured velocity and droplet size distributions close to the orifice after expansion. A criterion was derived for the transition between nonflashing and fully flashing jets, and droplet size correlations were proposed in the regimes of mechanical break-up, transition to flashing and fully flashing. Recommendations were made for the droplet size distribution and the critical size of the droplet below which no rainout occurs.
Figure 2. PDA droplet size measurement
for butane jet experiment (INERIS).
Third phase of JIP
Phase II was limited to scaled experiments for water with initial droplet-size data measured at a single superheat value. Furthermore, the modelling assumed a single averaged droplet size with rainout at one point. The recently completed Phase III – sponsored by DNV Software, Gaz de France, RIVM, TOTAL and Statoil -
Hydro – addressed some of these limitations.
It also included scaled experiments for cyclohexane, butane, propane and gasoline, to ensure that the derived droplet size correlations are also valid for other chemicals than water. Furthermore, largescale butane experiments were carried out by INERIS (France) to ensure that for more realistic scenarios the derived droplet size correlations are accurate (Figure 2).
| Current Phast 6.531 SMD correlation |
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| New Phase III JIP SMD correlation |
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| Figure 3. Validation of SMD droplet size correlation against Phase III Cardiff experiments. |
Model improvement and validation was carried out by DNV Software, including validation of release rate and initial droplet size against experiments. An improved droplet size correlation was formulated and implemented in the Phast discharge model. This was compared with other published droplet size and rainout correlations, in conjunction with validation against an extensive set of experiments. It was shown that the improved droplet size correlation agrees much better with experimental data than the previous Phast correlation (Figure 3).
To improve the rainout predictions further, the Phast dispersion model was extended to allow simultaneous modelling of a range of droplet sizes and distributed rainout (Figure 1). It also included improvements to pool and dispersion modelling after rainout, and validation for dispersion from LNG and LPG pools.
Fourth phase of JIP
Phase IV of the JIP – sponsored by DNV Software, ExxonMobil, HSE, RIVM, StatoilHydro and TOTAL – was initiated in September this year.
This work includes rainout experiments for octane, performed by the UK Health and Safety Laboratories (HSL),
with measurement of release rate, initial droplet size distribution, cloud concentrations and distributed rainout. Experiments will be carried out for a range of release pressures, hole sizes and weather conditions. The main purpose of the work is to validate the new distributed rainout model developed as part of Phase III, and to make refinements where deemed necessary.
DNV Software intends to make the new Phase III droplet size correlation (Sauter Mean Diameter) available in Phast 6.6, and the Phase IV distributed rainout model available in a subsequent version of Phast.