Physics of steam explosion
Résumé
Steam explosions result from the flash of liquid water into steam. The leading parameters that drive those physical explosions are phase change associated to kinetics. As a consequence, whereas in chemical explosions the expansion ratio is about 8, it can reach 1700 for steam explosions. Steam explosions embrace very different phenomena such as Boiling Liquid Expanding Vapor Explosions (BLEVE) or Molten Material in contact With water (MMWW). BLEVE is an explosive vaporization that can happen if a pressure vessel containing water at a temperature above its atmospheric boiling point catastrophically fails. Fire, Pressure Relief Device (PRD) default,... are known to be possible causes of this catastrophic accident. Beyond overpressure effects, missiles and rockets can cause the most severe and distant damages. MMWW also relies on phase change but involves different circumstances. This latest phenomenon results from an almost instantaneous heat transfer from the molten material to the cold water causing explosive vapor generation. Although chemical reaction can play a role for some of the most reactive molten materials, it is widely accepted that its contribution is not required to explain the magnitude of the destructive steam explosion forces. MMWW is commonly divided into four successive steps: premixing of melt and coolant, triggering of the explosion, explosion propagation and finally work production. A steam film interface is formed when a hot material enters in contact with a coolant. The local film collapse is responsible for triggering the steam explosion through molten material fragmentation. Fragmentation offers a large heat exchange area to allow the instantaneous heat transfer to take place.