Shock waves and blast effects

Blast Waves

A Blast Wave is a general term for a rapidly propagating Overpressure or Shock Wave in the atmosphere with high pressure, high density and high particle velocity. Both types of Blast Waves have a positive phase followed by a negative (suction) phase. In the Blast Wave, the overpressure increases rapidly and decreases slowly below atmospheric pressure reaching the minimum and subsequently returning to atmospheric values. The Blast Wave can be the result of flame from propagation but in some scenarios, can be caused by a sudden release of non-flammable gas. The power of a Blast Wave relies on the pressure of the source, the duration of the explosion, and the distance from the source. The overpressure effect from the explosion is directly connected to the speed of flame propagation at the combustion front and each type of Blast Wave is specific for a flame propagation mode (Bjerketvedt, Bakke and Wingerden, 1992).

Types of Blast Waves

Overpressure Wave

The Overpressure Wave occurs if the combustion process within a gas explosion is relatively slow and consists of low-amplitude pressure waves. It is a rapidly propagating wave in the atmosphere causing a gradual increase in the gas-dynamic state: high density, pressure and particle velocity. The Overpressure Waves are associated with the Deflagration flame propagation mode (Bosch, 2005).

Shock Wave 

A Shock Wave occurs if the combustion process within a gas explosion is extremely rapid. It is characterised by a sudden increase in the gas dynamic state. The pressure increase is instantaneous forming a Shock Wave that is so rapid that the process is mostly adiabatic. The Shock Waves are typical for Detonation flame propagation mode (Bosch, 2005).

In some cases, the Blast Wave may occur during the sudden loss of containment of non-flammable compressed gas or pressurised liquefied gas and expansion. These events result only in overpressure effects and are referred to as Gas Blasts or BLEVE Blasts.

References

Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.

Bjerketvedt, D., Bakke, J.R., Wingerden, K.v. (1992). Gas explosion handbook. Bergen: CMR Gexcon.