Experimental and Numerical Evaluation of Toxic Pool Evaporation

To date, safety distances to toxic pool evaporation as measured by known models have been quoted in hundreds of meters, without a deeper study of the time variation of the evaporation rate. In order to evaluate this specific aspect, we designed an experimental study. This study included small-scale tests with a 0.1 m² evaporating pool, and medium-scale tests with 1 and 2 m² evaporating pools. For both small- and medium-scale tests, the experimental vertical velocity profile was built to reproduce an atmospheric profile after applying the Froude scaling procedure. The scope of this study focused on ammonia pool evaporation, with each test lasting long enough to highlight the time evolution of the evaporation rate. While many other parameters may have strongly influenced the evaporation rate, the influence of the most classical parameters was tested, including pool concentration, wind velocity, and ambient turbulence. During these tests, the metrology was designed to enable the measuring of evaporation rates with great precision, but other important components were also measured. This series of tests clearly showed a strong variation of the evaporation rate in the first 30 minutes after the release—the evaporation rate dropped to 20% of its initial value after this 30-min period. It is therefore obvious that such reactions should strongly influence the toxic consequences of the vapor atmospheric dispersion. The known influence of other parameters was also confirmed—typically, the higher the pool concentration and/or wind velocity, the higher the evaporation rate. The surrounding turbulence effect was also taken into consideration and was proven to have a lower influence on the evaporation rate. In light of these experiments, we present below a physical model named EVAP-Tox used to estimate the time variation of the evaporation rate of an ammonia solution.