Chemical kinetic effects in nonpremixed flames of H₂/CO₂ fuel

Finite rate chemical kinetic effects are investigated in diffusion flames of H₂/CO₂ fuel. The CO₂ diluent is not totally inert since at high temperature some of it will react to give CO. Calculations of laminar flames and measurements in turbulent flames are presented. The calculations are performed, using a detailed chemical kinetic mechanism with 13 species and 34 chemical reactions. The structure of laminar counterflow diffusion flames with a range of stretch rates are calculated. Using the joint Raman-Rayleigh-LIF technique, simultaneous space- and time-resolved measurements of temperature and the Raman scattering from CO, CO₂, H₂, H₂O, O₂, and N₂, as well as laser-induced fluorescence (LIF) from OH have been made in turbulent nonpremixed flames of H₂/CO₂. As these flames are pilot-stabilized they blow off, at high enough jet velocities, in a region down stream of the pilot where turbulent mixing rates are high and the finite rate chemistry effects are significant. It is found that for laminar flames with low stretch rates, the peak calculated temperatures exceed the equilibrium temperature and the peak temperatures measured in turbulent flames. This is mainly due to differential diffusion effects that are substantial in the laminar flame calculations and cause hydrogen enrichment of lean mixtures. Such effects are not as significant in turbulent flames. The hydrogen radical plays a major role at extinction, where the competition is very intense between the formation-destruction processes within the reaction zone and diffusive processes towards nonreactive regions. In laminar flames, the radicals OH, H, and O are in superequilibrium concentrations regardless of the stretch rates. Measurements in turbulent flames show that the hydroxyl radical is in superequilibrium concentrations as well. The turbulent's flame approach to global blowoff is monomodal. Local extinction in turbulent flames increases very sharply only in the flame's final approach to blowoff when the jet velocities are greater than ∼90% of the blowoff velocity.