An experimental and numerical study on the structure and extinction of laminar premixed flames

The laminar premixed flames are studied to obtain the flame speed and flame thickness under the influence of stretch and preferential diffusion. A new concept of flame thickness which is defined by velocity profile of the flame structure is used. The extinction mechanism for the flames with different preferential diffusion effect is also studied and the extinction criterion is established for the adiabatic flames under stretch.;In this study, the laminar premixed flames are studied by both experimentation and numerical calculation. Experimentally, the counterflow configuration technique is applied to establish the adiabatic, planar twin flames. The velocity profiles for methane/air and propane/air flames with different equivalence ratios are measured by laser Doppler velocimetry to obtain the flame speed and flame thickness under different stretch and preferential diffusion conditions. The numerical calculations based on different chemical mechanisms for methane/air and propane/air flames are also carried out to obtain the flame speed, flame thickness and detailed flame structure in terms of different stretch rates and Le numbers for comparison with the experimental results.;The comparison between the new concept of flame thickness and the existing widely used flame thickness has shown that the new flame thickness defined by the velocity profile can be used as the definition of the flame thickness. Based on the experimentation and calculation results, the local flame speed and local flame thickness are used to analyze the extinction behavior of the flames. The local Karlovitz number is proposed based on the local flame speed and local flame thickness, which can truly represent the influence of the stretch on the flame. A new non-dimensional KT number is formed based on the local Karlovitz number and a temperature term, where the temperature term is the ratio between the temperature difference for the combustion and the flame temperature. The extinction will occur when the KT number is about unity.