Published online by Cambridge University Press: 02 December 2009
Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure. The DNS was performed at a jet Reynolds number of 11,000 with over 940 million grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. A chemical flux analysis shows the occurrence of near-isothermal chemical chain branching preceding thermal runaway upstream of the stabilization point, indicative of hydrogen auto-ignition in the second limit. The Damköhler number and key intermediate-species behaviour near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. At the lifted-flame base, it is found that heat release occurs predominantly through ignition in which the gradients of reactants are opposed. Downstream of the flame base, both rich-premixed and non-premixed flames develop and coexist with auto-ignition. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base. In particular, the relative position of the flame base and the coherent flow structure induces a cyclic motion of the flame base in the transverse and axial directions about a mean lift-off height. This is confirmed by Lagrangian tracking of key scalars, heat release rate and velocity at the stabilization point.
Present address: School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea. Email: csyoo@unist.ac.kr
Movie 1. Logarithm of scalar dissipation rate. Red and yellow iso-surfaces represent χ values centered at 185 and 740 s-1, and white values greater than 2600 s-1. The maximum value in the movie is approximately 33,000 s-1.
Movie 2. Mass fractions of HO2 and OH. Red and yellow iso-surfaces represent YHO2 values centered at 3.4 × 10-6 and 1.8 × 10-4 respectively, and blue and purple YOH = 2.0 × 10-4 and 0.018 respectively. Maximum values of HO2 and OH mass fractions are 3.4 × 10-4 and 0.02, respectively.