Low-velocity detonation modes of grained pyroxylin powder

It is well known that low-velocity detonation excited by the explosion of a thin layer of a plastic explosive within charges of grained pyroxylin powder is propagated with a velocity that is practically constant along the charge. However, it varies depending on the power of the initiating pulse. The present paper is devoted to the elucidation of the mechanism of this unusual feature of detonation process. Experiments were carried out on charges of VTM grade grained single-channel powder with different initial density and were added by numerical modeling. It is shown that the property studied is the consequence of the relatively low intensity of the chemical transformation and the limited charge length (120 mm in the experiment and calculations). The reaction zone of the detonation wave has no time to form completely under these conditions, and the development process is interrupted at a stage when the wave characteristics change actively. The wave evolution was distinctly revealed on pressure profiles; however, the front trajectory, if excluding the initiation area, has an almost linear form. The wave velocity, close to constant, corresponds to this. To form a stationary wave with characteristics that are not dependent on the initiation conditions in a range corresponding to lowvelocity detonation mode, charges with much greater length are necessary. As regards the mechanism of the excitation of chemical transformation in the wave front, as numerical modeling showed, high-porosity charges operate by the gas-phase mechanism (the compression and heating in the high-speed gas flow in pores). In the case of compacted charges with a porosity of 0.2 and lower, heating and ignition of the powder occur by the solid-phase mechanism (because of dissipation at plastic deformations of the porous layer). Details of both mechanisms are considered.