Propagation speed and stability of spherically expanding hydrogen/air flames: Experimental study and asymptotics

Abstract

Here, outwardly propagating spherical hydrogen/air flames are examined theoretically and experimentally with respect to flame propagation speed and the onset of instabilities which develop due to thermal expansion and non-equal diffusivities. Instabilities increase the surface area of the spherical flame, and hence the flame propagation speed. The theory applied here accounts for both hydrodynamic and diffusive-thermal effects, incorporating temperature dependent transport coefficients. Experiments are performed in a spherical combustion chamber over a wide range of equivalence ratios (0.6–2.0), initial temperatures (298–423 K), and initial pressures (1 atm to 15 bar). The evolution of the flame propagation speed as a function of flame radius is compared to predictions from theory showing excellent agreement. Also the wrinkling of hydrogen/air flames is examined under increased pressure and temperature for various equivalence ratios. Critical flame radii, defined as the point of transition to cellular flames, are extracted from high-speed Schlieren flame imaging. Overall, the critical radius is found to decrease with increasing pressure. The predictions yield the growth rate of small disturbances and the critical flame radius. Experimental flame radii, as expected, are underpredicted by the theoretical findings. Experimental data are provided in the form of an approximation formula.