ABSTRACT
To be in parallel with the development of theory, for practical reasons, fl ammability limits were measured in many laboratories: their knowledge for different mixtures was necessary to bring down the number of accidents in mines and in industry. Most measurements were made in vessels of various shapes. Hence, it was not a surprise that the obtained fl ammability limits were apparatus-dependent. Coward and Jones [8] gave a very good summary of empirical knowledge on this issue and also proposed a new standard apparatus for determining fl ammability limits. This was a vertical tube, 51 mm in diameter and 1.8 m long, closed at the upper end and open to the atmosphere at the bottom. If a mixture is ignited at the bottom of the tube and a fl ame can be formed, to propagate all the way to the top, the
CONTENTS
3.1 Flammability Limits: History and Mechanism of Flame Extinction ......................................................................................................................................................... 15 3.1.1 Introduction ......................................................................................................................................................... 15 3.1.2 Limit Flames Propagating Upward .................................................................................................................. 16 3.1.3 Flow Structure ..................................................................................................................................................... 16 3.1.4 Thermal Structure ............................................................................................................................................... 18 3.1.5 Flame Stretch ....................................................................................................................................................... 18 3.1.6 Extinction Mechanism of an Upward Propagating Flame ............................................................................ 20 3.1.7 Extinction Mechanism of a Downward Propagating Flame ......................................................................... 22
Acknowledgments ................................................................................................................................................................... 24 References ................................................................................................................................................................................. 24 3.2 Ignition by Electric Sparks and Its Mechanism of Flame Formation ..................................................................... 26
3.2.1 Introduction ......................................................................................................................................................... 26 3.2.2 Numerical Model Description .......................................................................................................................... 26
3.2.2.1 Numerical Method ............................................................................................................................... 26 3.2.2.2 Chemical Reaction Scheme ................................................................................................................. 27 3.2.2.3 Spark Ignition ....................................................................................................................................... 27
3.2.3 Calculated Results ............................................................................................................................................... 28 3.2.3.1 Spark Ignition Process ......................................................................................................................... 28 3.2.3.2 Behavior of Ion Molecules .................................................................................................................. 30 3.2.3.3 Effect of Spark Component on Ignition Process .............................................................................. 30 3.2.3.4 Effect of Equivalence Ratio on Minimum Ignition Energy ............................................................ 33
3.2.4 Conclusion ............................................................................................................................................................ 34 References .................................................................................................................................................................................. 34
mixture is said to be fl ammable. However, if the fl ame is extinguished while it is part of its way up the tube, the mixture is said to be nonfl ammable.
