Laminar Lifted Methane Jet Flames in Co-flow Air Diluted with Helium and Nitrogen

Abstract

Lifted flames in laminar jets of methane diluted with helium and nitrogen issuing into a coaxial co-flow air have been investigated experimentally. The chemiluminescence intensities of OH*, CH2O* radicals and the radius of curvature for tri-brachial flame were measured using an intensified charge coupled device (ICCD) camera, monochromator and digital video camera. The product of the OH* and CH2O* is used as a marker for heat release rate. Such methane jet flames could be lifted in buoyancy dominated and jet momentum dominated regimes despite the Schmidt number less than unity. The lifted flames were also stabilized due to buoyancy induced convection in buoyancy-dominated regime causing an increase of reactant mass flux to edge flame and thereby increasing the reaction rate and subsequently edge flame speed. It was confirmed that increased OH* and CH2O* concentration caused an increase of edge flame speed via enhanced chemical reaction in buoyancy-dominated regime. In jet momentum dominated regime lifted flames were observed even for nozzle exit velocities much higher than stoichiometric laminar flame speed. An increase in radius of curvature in addition to the increased OH* and CH2O* concentration stabilizes such lifted flames. Based on the stabilization mechanism, detailed discussion on the stabilization of such lifted flames is made.