(60f) A Method for Determining the Flammable Limits of Gases in a Spherical Vessel

The flammable limits of gases are very important to prevent fires or explosions in industrial and domestic applications. When a flammable or combustible gas is mixed with an oxidizer, such as air, in the proper concentration in the presence of a source of ignition, rapid combustion or an explosion will occur. The flammable limits are the boundary concentrations of flammable gas that will support the propagation of flame through the volume of the mixture after ignition. Flammable limits are also defined for both upward and downward flame propagation and are, for most chemical species, found at different concentrations.

The current method for determining the flammable limits for a gas in a closed spherical vessel is based on a specification of the maximum pressure increase during the combustion, usually from 5 to 10% of the initial ambient pressure. This approach is completely arbitrary and is not fundamentally based. For most hydrocarbons this pressure boundary and hence the flammable limit is easy to determine experimentally since an abrupt pressure drop occurs at the flammable limit as the fuel concentration in air is adjusted. However, for some species, particularly hydrogen mixed with air, the drop in maximum combustion pressure is not very abrupt and the fuel concentration can range several percentage points depending on the arbitrary criterion used for the flammable limit.

This paper will discuss a new approach for determining the flammable limits for a gas in a spherical vessel. The approach is based on the maximum second derivative of pressure rise. The second derivative is indicative of an acceleration of the combustion rate and is, hence, fundamentally based. Furthermore, we have identified a new approach to determine the downward propagating flammable limits based on the combustion time, that is, the time that the gas actually burns in the vessel.