Fast Deflagration in Open Spaces

In open volumes, flame acceleration caused by instability of the flame subjected to a compression wave is not that noticeable. Therefore, in large open volumes, significant flame acceleration has not been observed even for highly reactive mixtures. Thus, considerable flame acceleration has not been reported in experiments [5] on ethylene oxide + air mixtures in semi-spherical vessels of up to 5 m radius. It was proved in [1] that combustion velocity dependence on temperature and pressure also provides a weak effect on flame acceleration. 

Flame acceleration due to mixture temperature and pressure growth just assists the observed flame behavior in confined volumes, but in open volumes this assistance is insignificant. In addition to investigating the mixture temperature effect on flame acceleration, it should be noted that the sound speed growth in the fresh mixture, caused by its temperature increase, impedes shock wave generation and, therefore, reduces detonation probability. 

It has been noted in [1] that 2H2 + O2 mixture preheating has led to an increase in pre-detonation distance or eliminated the possibihty of detonation under conditions described in the experiments [1]. 

Blast wave generation dynamics and flame acceleration in H2 + air mixtures with a volume of 7.5-2,100 m - have been studied using semi-spherical clouds and spherical vessels [6]. Additional data on the pressure field generated by combustion of HAM hemispheres of 10 m radius in an initially stationary atmosphere can be found in [7]. 

However, confining surfaces increase the danger resulting in a hydrogen+air cloud explosion. It has been shown in [6] that when a H2 + air mixture is ignited in a U-shaped semi-open duct of 90 m volume, the combustion velocity can reach 80 m/ s and compression wave overpressure AP = 2 10 Pa. The turbulence artificially generated in U-shaped duct may cause transition of deflagration to detonation. 

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