Propagation and quenching of premixed flames in a concentration-length-velocity diagram

Abstract

The flame propagation velocity (FPV) deviates from the laminar burning velocity (LBV) when a premixed flame is formulated in a combustion space having limited length-scales. In spite of many related studies, the relationship between the FPV and LBV has not been clarified because the FPV depends on the configuration or characteristic length-scale. In this study, variation of the FPV depending on the length-scale is experimentally evaluated using an improved, state-of-the-art annular stepwise diverging tube. Using a transient method, abundant experimental data are obtained for various fuel concentrations (e.g., equivalence ratio). Thus, a continuous physical surface could be established in a Concentration-Length-Velocity diagram (CLV-diagram). The CLV-diagrams for methane and propane flames are successfully described. Three projected planes of the CLV-diagram are related to conventional laminar flame theories, and a comparison of them reveals additional hidden mechanisms. These CLV-diagrams provide an overview of the effects of the length-scale on various characteristic behaviors of laminar premixed flames. The relationship between the critical FPVs (or LBVs) and the quenching distances is identified with the Lewis number. The flame stretch and Markstein length with respect to the FPV are also discussed.