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When a fire starts, the area around the ignition point starts to heat up. The atmosphere has no choice but to absorb heat. The heated air is more buoyant than the air surrounding it, so it begins to rise. The space vacated by the heated air is filled in by colder air. This cycle continues to the point that everything in the space begins to be heated. Thus, the fire grows.
This initial energy rising is known as a fire plumeThe fire plume follows some very basic laws of physics. Upon inception, it has a divergence of about 22 degrees around the central plane of the fire, from edge to edge.. It has energy, so it will be impacted in terms of velocity, according to the growthThe larger the fire grows, the more velocity in the plume. of the fire. The vertical movement of the plume will be impacted by any barrier either physical or environmentalIn an atrium, for example, the horizontal barrier may be airflow being injected into a space some distance above the floor. The airflow is colder than the fire plume, so it defeats the buoyant nature of the rising air. If the velocity of the plume is not great enough to penetrate the cold air layer, stratification will occur to some point. . The vertical movement will be deflected off of a horizontal surface, such as a ceiling, forming what is called the "ceiling jet." The ceiling jet will travel across the ceiling or horizontal barrier, again with a velocity that is dependent upon fire growth. The further from the fire's origin, the weaker the jet.
Understanding some basic plume dynamics is important to the placement of fire suppression and detection components. In the world of fire protection engineering, the performance of the fire detection and suppression system can be calculated if the variables in the development of the fire plume and ceiling jet are determined.
A word about plumes and jets...
© Hughes Associates,Inc. 2012