Flame acceleration in unconfined hydrogen/air deflagrations using infrared photography

• We investigated the influences of the flame acceleration on the blast wave of unconfined hydrogen/air deflagrations by using an infrared photography.
• Results demonstrate the self-acceleration due to flame front instability in the flame propagation definitely existed.
• The onset of self-acceleration in the early stage of the propagation depends on the intensity of diffusional-thermal instability and consequently the accelerations were mainly dominated by hydrodynamic instability.
• The flame front turbulence appeared and it accelerated in the flame propagation and thereby the blast wave intensity is dramatically increased.

Flame behavior and blast waves generated during unconfined hydrogen deflagrations were experimentally studied using infrared photography. Infrared photography enables expanding spherical flame behaviors to be measured and flame acceleration exponents to be evaluated. In the present experiments, hydrogen/air mixtures of various concentrations were filled in a plastic tent of thin vinyl sheet of 1 m3 and ignited by an electric spark. The onset of accelerative dynamics on the flame propagation was analyzed by the time histories of the flame radius and the stretched flame speed. The results demonstrated that the self-acceleration of the flame, which was caused by diffusional-thermal and hydrodynamic instabilities of the blast wave, was influenced by hydrogen deflagrations in unconfined areas. In particular, it was demonstrated that the overpressure rapidly increased with time. The burning velocity acceleration was greatly enhanced with spontaneous-turbulization.