A numerical study on extinction behaviour of laminar micro-diffusion flames

Abstract

We conducted a numerical study on the fluid dynamic, thermal and chemical structures of laminar methane–air micro flames established under quiescent atmospheric conditions. The micro flame is defined as a flame on the order of one millimetre or less established at the exit of a vertically-aligned straight tube. The numerical model consists of convective–diffusive heat and mass transport with a one-step, irreversible, exothermic reaction with selected kinetics constants validated for near-extinction analyses. Calculations conducted under the burner rim temperature 300 K and the adiabatic burner wall showed that there is the minimum burner diameter for the micro flame to exist. The Damköhler number (the ratio of the diffusive transport time to the chemical time) was used to explain why a flame with a height of less than a few hundred microns is not able to exist under the adiabatic burner wall condition. We also conducted scaling analysis to explain the difference in extinction characteristics caused by different burner wall conditions. This study also discussed the difference in governing mechanisms between micro flames and microgravity flames, both of which exhibit similar spherical flame shape.

Keywords:

• Micro flames
• Diffusion flame
• Extinction
• Scale effect
• Damköhler number

Acknowledgements

This study was supported in part by The Kurata Memorial Hitachi Science and Technology Foundation and in part by the Asahi Glass Foundation. We would like to acknowledge Mr Atsushi Kubota for his technical assistance in numerical calculations, Dr Kazu Kuwana for his valuable comments on numerical calculations and technical discussion on extinction behaviour of flames and Professor Paul Ronney for valuable suggestion about the minimum achievable flame power.