The potential consequences of an accidental release to the atmosphere of hydrogen fluoride in large quantities are severe, but risk-assessment studies are presently constrained by being unable to predict very reliably the released gas cloud behaviour. Due to a variety of complex thermody-namic and chemical interactions, a dispersing gas cloud may be heavier or lighter than air in different stages of its dispersion. If cloud buoyancy is significant then the reduced ground level concentrations due to the plume rise may be a major mitigation factor in any risk assessment. Atmospheric water vapour plays an important part in the development of cloud buoyancy and the borderline conditions lie around 10℃ and 15% humidity, typical of Northern European weather conditions. The only effective way of investigating these phenomena is with releases of hydrogen fluoride in the field under appropriate meteorological conditions. However, full-scale simulations are costly, time consuming and hazardous. A reduced-scale simulation greatly reduces these problems, but needs to be correctly scaled. The report considers these scaling requirements and shows that, within some constraints, they can be successfully achieved. As an example, a one tenth scale field experiment reduces the required released quantities from tonnes to kg. The required windspeeds reduce to about one half of the full scale, which is acceptable while placing some constraints on the lowest scalable windspeeds. It is more difficult to investigate stratified flows as a reduced-scale experiment atmospheric stratification appears less severe than its full-scale equivalent.
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