Detonation

Theories

The simplest theory to predict the behaviour of detonations in gases is known as the Chapman–Jouguet (CJ) condition, developed around the turn of the 20th century. This theory, described by a relatively simple set of algebraic equations, models the detonation as a propagating shock wave accompanied by exothermic heat release. Such a theory describes the chemistry and diffusive transport processes as occurring abruptly as the shock passes.

A more complex theory was advanced during World War II independently by Zel'dovich, von Neumann, and Döring. This theory, now known as ZND theory, admits finite-rate chemical reactions and thus describes a detonation as an infinitesimally thin shock wave, followed by a zone of exothermic chemical reaction. With a reference frame of a stationary shock, the following flow is subsonic, so that an acoustic reaction zone follows immediately behind the lead front, the Chapman–Jouguet condition.

There is also some evidence that the reaction zone is semi-metallic in some explosives.

Both theories describe one-dimensional and steady wavefronts. However, in the 1960s, experiments revealed that gas-phase detonations were most often characterized by unsteady, three-dimensional structures, which can only, in an averaged sense, be predicted by one-dimensional steady theories. Indeed, such waves are quenched as their structure is destroyed. The Wood-Kirkwood detonation theory can correct some of these limitations.

Experimental studies have revealed some of the conditions needed for the propagation of such fronts. In confinement, the range of composition of mixes of fuel and oxidant and self-decomposing substances with inerts are slightly below the flammability limits and, for spherically expanding fronts, well below them. The influence of increasing the concentration of diluent on expanding individual detonation cells has been elegantly demonstrated. Similarly, their size grows as the initial pressure falls. Since cell widths must be matched with minimum dimension of containment, any wave overdriven by the initiator will be quenched.

Mathematical modeling has steadily advanced to predicting the complex flow fields behind shocks inducing reactions.{{cite journal |last1=Sharpe |first1=G. J. |last2=Quirk |first2=J. J. |title=Nonlinear cellular dynamics of the idealized detonation model: Regular cells |journal=Combustion Theory and Modelling |volume=12 |issue=1 |pages=1–21 |year=2008 |doi=10.1080/13647830701335749

Applications

When used in explosive devices, the main cause of damage from a detonation is the supersonic blast front (a powerful shock wave) in the surrounding area. This is a significant distinction from deflagrations where the exothermic wave is subsonic and maximum pressures for non-metal specks of dust are approximately 7–10 times atmospheric pressure. Therefore, detonation is a feature for destructive purposes while deflagration is favored for the acceleration of firearms' projectiles. However, detonation waves may also be used for less destructive purposes, including deposition of coatings to a surface or cleaning of equipment (e.g. slag removal) and even explosively welding together metals that would otherwise fail to fuse. Pulse detonation engines use the detonation wave for aerospace propulsion. The first flight of an aircraft powered by a pulse detonation engine took place at the Mojave Air & Space Port on January 31, 2008.

In engines and firearms

Unintentional detonation when deflagration is desired is a problem in some devices. In Otto cycle, or gasoline engines it is called engine knocking or pinging, and it causes a loss of power. It can also cause excessive heating, and harsh mechanical shock that can result in eventual engine failure. In firearms, it may cause catastrophic and potentially lethal failure.

Pulse detonation engines are a form of pulsed jet engine that has been experimented with on several occasions as this offers the potential for good fuel efficiency.

References

References

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