One of the greatest difficulties in assessing the explosion hazard posed by an accidental spillage of hydrocarbons, such as natural gas, lies in the rational choice of a flame speed for incorporation into predictive models of combustion behaviour. The role of geometry and partial confinement in the generation of fast flames has been examined and a new approach to the problem highlights the dominant role of pockets of unburnt mixture behind the leading edge of the flame. Simple model problems indicate that rapid flame acceleration requires considerable confinement to channel and distort the flow field (at least for weak ignition in fuels of low chemical reactivity, such as methane). An analysis of published experimental data gives some support to this new approach.
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