20-03-2012, 04:26 PM
Detonation
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Introduction
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives,[1] as well as in reactive gases. The velocity of detonations in solid and liquid explosives is much higher than that in gaseous ones, which allows far clearer resolution of the wave system in the latter.
Gaseous detonations normally occur in confined systems but are occasionally observed in large vapor clouds. They are often associated with a gaseous mixture of fuel and oxidant of a composition, somewhat below conventional flammability limits. There is an extraordinary variety of fuels that may be present as gases, as droplet fogs and as dust suspensions. Other materials, such as acetylene, ozone and hydrogen peroxide are detonable in the absence of oxygen, fuller lists are given by both Stull[2] and Bretherick.[3] Oxidants include halogens, ozone, hydrogen peroxide and oxides of nitrogen and chlorine.
In terms of external damage, it is important to distinguish between detonations and deflagrations where the exothermic wave is subsonic and maximum pressures are at most a quarter[citation needed] of those generated by the former. Processes involved in the transition between deflagration and detonation are covered thoroughly by Nettleton
Theories
The simplest theory to predict the behavior of detonations in gases is known as Chapman-Jouguet (CJ) theory, 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 confines the chemistry and diffusive transport processes to an infinitely thin zone.
Applications
A pulsed detonation engine ground demonstrator operating at a frequency of 35 Hz (35 detonation waves per second). Fuel and oxidizer are supplied to the engine using a valving system that matches with the operating frequency.
The main cause of damage from explosive devices is due to a supersonic blast front (a powerful shock wave) in the surrounding area. Therefore, the detonation is primarily associated with explosives and the acceleration of various projectiles.
Engine knocking
Knocking (also called knock, detonation, spark knock, pinging or pinking) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.
The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.