Atmospheric dispersion of ammonia: an ammonia fog model

A simplification to the two-phase ammonia vapor-droplet fog problem has been implemented to study the dispersion of a spill of 40 tons of ammonia. We have circumvented the necessity of adding the partial differential equations for mass, momentum, and energy for the ammonia in the liquid phase by certain assumptions. It is assumed that the ammonia fog behaves as an ideal gas including the droplets. A temperature-dependent molecular weight was introduced to simulate the transition from a vapor-droplet cloud to a pure vapor cloud of ammonia. Likewise, the vaporization of ammonia was spread out over a temperature range. Mass, momentum, energy, and total ammonia is conserved rigorously. The observed features of the ammonia spill simulation have pointed out phenomena that could not be predicted in simpler calculations. Perhaps the most obvious feature is the cloud bifurcation due to the strength of the gravity current relative to the ambient wind. The gravity spreading of the denser ammonia fog significantly perturbs the unidirectional windfield in the vicinity of the spill, setting up complex eddy patterns in the cloud which are enhanced by ground heating and warm dry air entrainment. The lower concentrations appear to lift off by a buoyancy-induced flow. The ammoniamore » cloud, rather than being cigar shaped as assumed in simpler models, ranges from pancake shaped to pear shaped, depending upon the ambient windfield. The fact that the ammonia cloud remains cold, very low, and wide is in qualitative agreement with some of the large-scale ammonia spill accidents. 14 figures.« less