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Figure 41. Flame<br />
distribution in a<br />
premixed gaseous<br />
mass. 12<br />
62<br />
as more energy is released. It is not the case, however, that the<br />
burning velocity will increase, the higher the proportion of<br />
fuel in the gaseous mass. It is rather that the burning velocity<br />
reaches its highest value when the stoichiometric mixture is<br />
roughly achieved and it then drops towards the upper fl ammability<br />
limit.<br />
Figure 40 shows the rate of laminar burning velocity for<br />
methane and propane respectively. The burning velocity will<br />
affect the build-up of pressure in a room. The nearer to the<br />
stoichiometric point, the faster the burning velocity.<br />
If we compare the heat released at the lower fl ammability<br />
limit and when the stoichiometric point is reached, more energy<br />
is released in the latter case, as there is a higher percentage<br />
of fuel combusted. This means that the burning velocity will<br />
be higher with the stoichiometric mixture, compared to at the<br />
lower fl ammability limit.<br />
Similar comparisons can be made between the stoichiometric<br />
point and the upper fl ammability limit. When we get<br />
into this upper section of the fl ammability range, it is the<br />
amount of oxygen which determines how much energy can be<br />
released.<br />
The richer in energy the gases are, the faster the burning<br />
velocity will be. The burning velocity (S u) is dependent on the<br />
energy released.<br />
Some plastics have a high energy value, which means that<br />
the smoke gases can contain a lot of potential energy if com-<br />
Ignition source<br />
Thin reaction zone<br />
Zone with unburnt gas<br />
Burning velocity<br />
Flame speed
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