Mechanism of flame stabilization in turbulent, lifted-jet flames

Abstract

Particle image velocimetry was used to study the velocity field in the stabilization region of lifted, turbulent CH4-jet flames over a range of Reynolds numbers from 7000 to 19,500. Measured velocities at the flame base are considerably below the turbulent flame speeds derived from previous studies and show a dependence on the Reynolds number. The average velocity at the stabilization point is nearly a factor of five below the premixed laminar burning velocity at the lowest Reynolds number and asymptotes to a value about 20% higher as the Reynolds number is increased. Planar images of OH show that the flame zone structure near the stabilization point is also highly dependent on the Reynolds number. Comparison of the present OH images with previous CH4 Raman imaging results shows that the flame thickness is determined by the width of the flammable region. At a low Reynolds number, the flame is stabilized near the jet exit where the flammable layer is thin, resulting in a thin flame zone. At an increased Reynolds number, the stabilization point is located farther downstream where the flammable region is wider, resulting in a correspondingly wider flame zone. It is proposed that the lower velocities observed at the flame base are related to thinning of the flame zone at low Reynolds, which results in greater curvature of the flame base. The increased flame curvature effectively defocuses the transport of heat and flame radicals to reactants upstream of the propagating flame front, resulting in reduced burning velocities. The implications of these results for mechanisms controlling turbulent flame stabilization, with an emphasis on the applicability of triple flame concepts to turbulent flows, are discussed.