Turbulent lifted flames in a vitiated coflow investigated using joint PDF calculations

The joint velocity-turbulence frequency-composition PDF method is applied to a lifted turbulent jet flame with H2/N2 fuel issuing into a wide coflow of lean combustion products, which are at a temperature of 1045 K. Model calculations with detailed chemistry are performed using three existing mixing models (IEM, MC, and EMST) and two chemistry mechanisms (the Mueller and Li mechanisms). Numerically accurate results are obtained and compared with the experimental data. Recent experiments have shown that the stabilization height of this lifted flame is very sensitive to the coflow temperature, much more than to the inlet velocity profile or the initial temperature of the fuel. One percent (i.e., 10 K) change in the coflow temperature (which is well within the experimental uncertainty) can double the lift-off height. The joint PDF calculations capture this sensitivity very well and are in good agreement with the measurements for the velocity, mixture fraction, and species. The three mixing models give relatively similar results, implying that the cases studied here are mainly controlled by chemical kinetics. The Li mechanism results in earlier ignition than the Mueller mechanism and hence gives shorter lift-off heights over the whole test range. The joint PDF calculations generally give better agreement with the measurements than previous composition PDF calculations [A.R. Masri et al., Combust. Theory Modelling 8 (2004) 1-22]. A new parallel algorithm, involving domain partitioning of particles, has been implemented to facilitate these computations.