Self-Acceleration and global pulsation of unstable laminar Hydrogen-Air flames

Abstract

The self-acceleration and global pulsation of spherically expanding laminar hydrogen-air flames were studied in a spherical explosion vessel, over a wide range of equivalence ratios (0.4–2.0), initial temperatures (300–400 K), and initial pressures (0.1–0.7 MPa). Comprehensive quantitative data on the self-acceleration propagation of unstable laminar hydrogen-air flames were obtained. The results show that the self-similarity appears after the onset of instability, and the previous assertion of acceleration exponent α = 1.5 is not valid for hydrogen-air flames. The derived values of α for such flames vary from 1.125 and 1.39 in the present work. For the self-accelerating flame, the temporal evolution of the flame radius is described by ru=Atα, and temporal flame propagation speed is described by Sn=Aαtα-1. A modified theoretical expression of the constant A is proposed and validated against the present experimentally derived results. The acceleration phase after flame instability presents a global pulsating acceleration pattern. The frequency of global pulsation rises as the pressure or temperature of the flame increases. A pulsing flame speed equation can be used to reliably estimate flame speed after the onset of cellular instability. Self-acceleration observed in small laboratory explosions can serve as a predictive indicator for flame behaviour on a larger atmospheric scale.