Transverse fuel jet in turbulent cross-flow: influence of fuel composition on near field flame stabilization

摘要

Compressible three-dimensional direct numerical simulations (DNS) of transverse fuel jet in turbulent boundary layer cross-flow of air (JICF) are performed to gain fundamental insight into the influence of fuel composition on the near field flame stabilization characteristics. A transverse jet of synthesis gas fuel, diluted by 30% nitrogen by volume, at 420 K is injected from a circular nozzle of 1 mm diameter with a bulk velocity of 250 m/s into a turbulent cross-flow of air at 750 K and free stream velocity of 55 m/s, representing nominal conditions of the premixer section of stationary gas-turbines. The aim is to develop an understanding of those mechanisms that are critical to flame stabilization in this configuration and aid the design of gas turbine burners that are fuel-flexible and intrinsically flashback-safe. The DNS incorporates detailed chemical kinetics and mixture-averaged transport properties. A chemical mechanism involving 12 species and 29 reaction steps is considered and three fuel fuel compositions with varying ratios of H_2:CO mole fractions are investigated: 1:0 (pure H_2), 1:0.06 (H_2 rich) and 1:1 (H_2 lean). The laminar flame speeds for the three mixtures at nominal conditions (stoichiometric mixture, unburnt temperature of 420 K) are in the ratio 1:0.93:0.49 respectively. In spite of the large difference in the laminar propagation speeds the flame stabilizes on average at roughly the same location in all three cases: 1 mm downstream of the nozzle center in the stream wise direction and 4 mm downstream in the wall-normal direction. This location corresponds to a region of low velocity magnitudes and near stoichiometric mixtures. The dilatational influence of the flame on the turbulent flow field is likely the same in the three cases since the density ratios for the three mixtures are roughly equal to 4.5. Significant differential diffusion effects are observed with the elemental mixture fraction of hydrogen being much larger than carbon in the vicinity of the flame anchoring region. This suggests that the influence of adding CO in the fuel stream is negated somewhat by differential diffusion which continually provides an excess of H_2 to the flame compared to CO, relative to the fuel stream.