Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.
There are many practical situations where a finite mass of a buoyant gas may become suddenly exposed to atmospheric air at a negligible pressure difference within vertical chambers or vessels that are either fully or partially open. The consideration of such situations when a fuel or toxic gas is released is of immense practical importance especially in relation to developing measures for protection against the potential resulting hazards of fire, explosion or toxic emissions. In such situations highly complex transient mixing processes are produced involving the formation and decay of flammable and explosive mixtures within the vessels and may extend with time to their immediate vicinity. Accordingly, the development of suitable predictive models may be effective in producing useful description of the phenomena and may assist in drafting measures for reducing the associated hazards.
In the present work, a fixed mass of a buoyant gaseous fuel is considered to be exposed suddenly with a negligible pressure difference to overlaying air at atmospheric conditions within vertical cylindrical enclosures that are either fully or partially open at the top to the atmosphere. The walls of the containing vessel can be at a different temperature from that of its gaseous contents. The paper describes features of a predictive axis-symmetrical model of the transient convective buoyant mixing viscous processes within the confines of the vertical cylindrical vessel and in its immediate vicinity. Results relating to the releases of a fixed mass of the highly buoyant methane representing natural gas into air are presented. These are then compared with those relating to those of a heavier than air fuel such as propane. The associated complex temporal changes of the spatial concentrations are described. Particular attention is given to the build up and decay of flammable regions within the open-top vessel as well as the release of the fuel gas into the outside atmosphere.