Light Emission from Shock-front Compressed Air Layers in Tubes

Abstract

HESS¹ declared as far back as the turn of the century that the luminosity at the detonation of a free charge depends on ‘pneumatic heating’ of the surrounding air. Some decades later Lafitte² and Michel-Lévy and Muraour³ confirmed in extensive investigations that the light is mainly emitted by the surrounding air. When a cylindrical charge detonates in a tube the detonation gas acts as a piston with supersonic velocity. In front of this the gas atmosphere is collected in a compressed layer with a shock front⁴. Investigations by Bethe⁵, Fuchs, Kynch and Peierls⁶, Brinkley, Kirkwood and Richardson⁷, Hirschfelder and Magee⁸ and others are available for calculating the state of the layer. The compressing work is considerably greater than with a reversible adiabatic because the piston works from the very beginning against the terminal pressure. About half the work is consumed to provide the layer with kinetic energy and the remainder is converted into internal energy which gives the air a high temperature. If it is obstructed by an obstacle the kinetic energy is transformed into internal energy. For high explosives in air, the rise in temperature that is then obtained can be expected to increase the intensity of radiation by 10³–10⁴ times. This depends partly on the increase of blackbody radiation with temperature, but the main reason is that, according to Burkhardt⁹, the emission coefficient for air layers 1 cm in thickness varies very rapidly with the temperature below about 10,000° K.