Propagating characteristics of spherical flames in H2-N2O mixtures

Abstract

The laminar burning velocity and cellular instabilities of premixed flame in H2-N2O mixtures were studied experimentally at various equivalence ratios (φ) and initial pressures (p0). High-speed schlieren was used to record the morphology of the spherical flame. Based on the spherical flame method, the laminar burning velocity and Markstein length were derived. In addition, the Lewis number, flame thickness, thermal expansion and Zel’dovich number were evaluated numerically. The results show that both the laminar burning velocity and Markstein length increase with the equivalence ratio. The Lewis number (less than unity) increases with the equivalence ratio while the flame thickness exhibits an inverse evolution. Hence, the most unstable flames were found to be at φ= 0.4–0.6 due to the combined effects of the thermal-diffusion and hydrodynamic instabilities. At lower initial pressure, the flame front is smooth in spite of the diffusion-thermal instability. With the increase of initial pressure, both the diffusion-thermal instability and hydrodynamic instability dominate the cellular structure of the flame front. The flames undergo self-acceleration at φ= 0.4–0.6 and p0= 40 kPa, with the acceleration exponents close to 1.5 (the value of self-turbulization). Due to the local oscillation of velocity, the flame propagation speed decreases after the self-similar regime.