An explosion is an interaction between molecules which takes place at such a rate, and with such increase in volume and energy release, that it produces a major disturbance in adjacent matter.
The interaction can be chemical (e.g., Combustion) or physical (e.g., rapid phase transitions); single (i.e., vapor, liquid or solid explosions) or multiphase (e.g., mist or dust explosions); controlled (e.g., blasting) or uncontrolled (e.g., runaway reactions). Explosions can be unconfined (e.g., UVCE = unconfined vapor cloud explosion) or confined (in this case, their effect strongly depends on the extent of venting). Because of safety and hazard implications, the last category attracts a high level of attention relative to its frequency of occurrence. Most of these incidents are from combustion of a gaseous phase, which represents a suitable model for general description.
Gaseous explosions burn with a laminar, turbulent or detonative flame. The first two are deflagration waves in which the flame propagates to unburned reactants by diffusion; in a detonation the flame is constantly ignited by adiabatic shock compression. Depending on initial conditions, temperatures will range from 1,000–4,000°K, pressures from 1 to 1,000 bar, while flame speed across these three categories rises from 1 to 104 m.s–1
Explosion ignition only occurs if suitable reactants are premixed within their flammability/explosion concentration range in the presence of an ignition source. Heat released by the source will start and increase the rate of the exothermic reaction until autoignition occurs; the autoignition temperature is only a function of the reactivity of the reactants. A minimum ignition energy is required to ensure that the size and rate of growth of the flame will exceed critical limits, so that a stable flame will be established rather than extinguished by heat dissipation into unburned surrounding reactants.
The flame speed, Uf, of each gas mixture is approximately related to its burning velocity, Uo, by the expansion factor, E, so that
For a system of reactants, burning velocity is at a maximum at the stoichiometric mixture ratio, (see Combustion) it increases with temperature and reduces with increased pressure. E is related to the additional volume produced by conversion of reactants into products, which tends to be high for complex and high-density reactants. Especially in confined explosions, this volume increase behind the flame will accelerate flame speed.
The volume of burnt gases is a product of Uo and the flame area. Hence, flame speed is further enhanced by an increase of flame area from the minimal plane front, Ap, to a curved flame. Af. Also, initially spherical flames will slow down once partial confinement restricts further growth in all directions. However, boundary drag and shear increase flame area when flames travel through ducts, channels, pipes or even along walls or obstacles; confined or obstructed explosions therefore tend to accelerate faster than unconfined ones. In turn, shear and boundary drag also promote turbulence, expressed by a further factor, Ft, that leads to increased flame area and flame speed. Collectively,
When by the combined influence of these factors flame speed can rise above the local velocity of sound, it ceases to be controlled by mass and heat transfer processes and deflagration-to-detonation transition (DDT) takes place. The shock wave then initiates the fast reaction, and compression waves from the fast reaction maintain the shock front ahead. The critical factor is the strength of the weakest bond in the fuel molecule, which requires a minimum shock temperature for rupture and reaction initiation.
Detonation can also be initiated by shock impact from detonators, other explosives, or decompressions. When either the initiating shock or the DDT are too energetic an overdriven detonation may temporarily be established. Eventually, however, the detonation wave will settle down to its characteristic velocity.
The main damage from explosions is caused by the short-duration impact wave and shock pressures and the long-duration underpressures in the expansion wave. Light and heat are secondary factors.
Bowden, F. P. and Yoffe, Y. D. (1985) Initiation and Growth of Explosions in Liquids and Solids. Cambridge Science Classics.
Lewis, B. and von Elbe, G. (1987) Combustion, Flames, and Explosions of Gases. 3rd edn. Academic Press Inc. DOI: 10.1016/0016-2361(88)90335-3
Nettleton, M. A. (1987) Gaseous Detonations, Their Nature, Effects and Control. Chapman and Hall Ltd. DOI: 10.1016/0950-4230(88)80025-1