Explosive characteristics of premixed hydrogen-air subjected to various concentration gradients

Abstract

In this research, different concentrations of hydrogen-air were charged into separate compartments of a 4.7 m pipe with a circular cross-section. The high-speed laser schlieren technique and overpressure detection technique were employed to accurately acquire the kinetic parameters of the explosive flow field, i.e., the evolution of the flame morphology, the flame propagation velocity and the shock wave overpressure. Two concentration gradients of 15 %-17 % and 15 %-25 % were programmed for the experiment. Affected by R-T, D-L, and multidimensional shock wave perturbations, the flame morphology exhibits fingertip, planar, cellular, tulip, and twisted tulip structures. Prolonging the gas diffusion time, the mean flame speeds were observed to be 10 m/s and 20 m/s after 10 min of diffusion for 15 %-17 % and 15 %-25 %, respectively, smaller than 45 m/s and 150 m/s at uniform concentrations. Besides, with the prolongation of diffusion time, the flame rarely backflows, and the velocity is more homogeneous and stable. Furthermore, as the ignition position approaches the concentration gradient, the flame is “driven” by the reflected shock wave upstream coupled with the effect of the elevated gas concentration, which significantly enhances the propagation velocity and overpressure.