Adiabatic Collapse of Gas Pockets

If an ideal gas is compressed adiabatically the final temperature reached is given by 

This method of initiation is particularly important because of the function of detonators and booster explosives as discussed under the topic of explosives train technology in Volume 2, Chapter 5. In the larger context of secondary explosives it is often associated with the premature or unwanted functioning of explosives subjected to mechanical and other forms of shock. 

explosive single crystal B, gas bubble S, shock front M, baffle  

The important observation in the above sequence is that the rapid collapse of the bubble causes initiation of explosion. However, from such sequences alone the exact mechanism responsible for the initiation is not clear. Various phenomena associated with the collapse of a bubble in a liquid have been studied jet formation [29-32], caused by involution of the bubble giving a high-speed jet which crosses the cavity in the direction of the initial shock production of a shock during the bubble rebound [29-32] and production of high temperature inside the collapsing bubble due to the adiabatic compression of the gas [22,33,34]. Experiments were designed to examine the influence of each phenomenon, and the conclusion was that the reaction was initiated by the rapid transfer of heat from the collapsed bubble to the crystal surface [28]. 

The bubble experiments described above show that silver and lead azide respond rapidly to the collapse of gas pockets as small as 50 fjm diam. Johannson and coworkers [35-37] pointed out that the temperature of the explosive surface will be lower than the temperature of the heated gas. However, the experiments described have been supported by calculations to show that sufficient heat reaches the explosive for initiation of fast reactions to take place [28]. For example, the energy in a 100-/xm air bubble compressed to its final volume is 300 J/m, and the required energy transfer to the crystal surface for initiation in 10 -10 sec is approximately 40 to 150 J/m. Bubbles much smaller than 50 /xm would probably not cause initiation because conditions during collapse would be increasingly isothermal, and the heat content would not be sufficient to exceed the hot spot requirements of temperature and size. 

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