On the similitude between lifted and burner-stabilized triple flames: A numerical and experimental investigation

We have investigated lifted triple flames and addressed issues related to f lame stabilization. The stabilization of nonpremixed flames has been argued to result due to the existence of a premixing zone of sufficient reactivit y, which causes propagating premixed reaction zones to anchor a nonpremixed zone. We first validate our simulations with detailed measurements in more tractable methane-air burner-stabilized flames. Thereafter, we simulate li fted flames without significantly modifying the boundary conditions used fo r investigating the burner-stabilized flames. The similarities and differen ces between the structures of lifted and burner-stabilized flames are eluci dated, and the role of the laminar flame speed in the stabilization of lift ed triple flames is characterized. The reaction zone topography in the flam e is as follows. The flame consists of an outer lean premixed reaction zone , an inner rich premixed reaction zone, and a nonpremixed reaction zone whe re partially oxidized fuel and oxidizer (from the rich and lean premixed re action zones, respectively) mix in stoichiometric proportion and thereafter burn. The region with the highest temperatures lies between the inner prem ixed and the central nonpremixed reaction zone. The heat released in the re action zones is transported both upstream (by diffusion) and downstream to other portions of the flame. Measured and simulated species concentration p rofiles of reactant (O-2, CH4) consumption, intermediate (CO, H-2) formatio n followed by intermediate consumption and product (CO2, H2O) formation are presented. A lifted flame is simulated by conceptualizing a splitter wall of infinitesimal thickness. The flame liftoff increases the height of the i nner premixed reaction zone due to the modification of the upstream flow he ld. However, both the lifted and burner-stabilized flames exhibit remarkabl e similarity with respect to the shapes and separation distances regarding the three reaction zones. The heat-release distribution and the scalar prof iles are also virtually identical for the lifted and burner-stabilized flam es in mixture fraction space and attest to the similitude between the burne r-stabilized and lifted flames. In the lifted flame, the velocity held dive rges upstream of the flame, causing the velocity to reach a minimum value a t the triple point. The streamwise velocity at the triple point is approxim ate to0.45 m s(-1) tin accord with the propagation speed for stoichiometric methane-air flame), whereas the velocity upstream of the triple point equa ls 0.7 m s(-1), which is in excess of the unstretched flame propagation spe ed. This is in agreement with measurements reported by other investigators. Tn future work we will address the behavior of this velocity as the equiva lence ratio, the inlet Velocity profile, and inlet mixture fraction are cha nged. (C) 2001 by The Combustion Institute.