Characteristics of Lifted Triple Flames Stabilized in the near Field of a Partially Premixed Axisymmetric Jet

The characteristics of lifted laminar triple flames in the near field of the burner exit are experimentally and numerically investigated. The flames are established by introducing a nitrogen-diluted rich mixture of methane and air from an inner tube and a lean mixture from a concentric outer tube. The temperature and major species concentrations are measured using thermocouples and gas chromatography, respectively. The flame liftoff heights are determined from C2* chemiluminescence emission images recorded by an intensified CCD camera. A time-dependent, implicit numerical model that uses a detailed description of the chemistry and includes buoyancy effects is used to simulate the flame structures. The numerical results are validated through detailed comparisons with experimental measurements. Both the measured and simulated reaction zone topographies show that there is a similitude between the burner-stabilized and lifted flames. It is found that, unlike the highly nonlinear behavior of lifted flames in the far field of a jet, the liftoff height of near-field lifted flames exhibits a linear relationship with respect to the inner-flow velocity. Two kinds of oscillations, one induced by the instability in the inner flow and the other through buoyancy effects, have been observed. The former has a frequency of about 16 Hz and the latter of 4 Hz. After lifting, an unsteady triple flame moves downstream along the stoichiometric mixture fraction contour, and the axial velocity reaches a minimum value near the triple point. This velocity is close to the laminar burning velocity of unstretched stoichiometric methane-air flames. The hydrodynamic stretch increases with the increase in liftoff height and the curvature stretch decreases. There is a good correlation between the stretch rate, Lewis number, and flame propagation speed in both the rich and lean premixed zones of the lifted triple flame.