DIRECT NUMERICAL-SIMULATION OF H-2 O-2 N-2 FLAMES WITH COMPLEX CHEMISTRY IN 2-DIMENSIONAL TURBULENT FLOWS

Premixed H-2/O-2/N-2 flames propagating in two-dimensional turbulence have been studied using direct numerical simulations (DNS: simulations in which all fluid and thermochemical scales are fully resolved). Sim ulations include realistic chemical kinetics and molecular transport o ver a range of equivalence ratios Phi (Phi = 0.35, 0.5, 0.7; 1.0, 1.3) . The validity of the flamelet assumption for premixed turbulent flame s is checked by comparing DNS data and results obtained for steady str ained premixed flames with the same chemistry (flamelet 'library'). Th is comparison shows that flamelet libraries overestimate the influence of stretch on flame structure. Results are also compared with earlier zero-chemistry (flame sheet) and one-step chemistry simulations. Cons istent with the simpler models, the turbulent flame with realistic che mistry aligns preferentially with extensive strain rates in the tangen t plane and flame curvature probability density functions are close to symmetric with near-zero means. For very lean flames it is also found that the local flame structure correlates with curvature as predicted by DNS based on simple chemistry. However, for richer flames, by cont rast to simple-chemistry results with non-unity Lewis numbers (ratio o f thermal to species diffusivity), local flame structure does not corr elate with curvature but rather with tangential strain rate. Turbulent straining results in substantial thinning of the flame relative to th e steady unstrained laminar case. Heat-release and H2O2 contours remai n thin and connected ('flamelet-like') while species including H-atom and OH are more diffuse. Peak OH concentration occurs well behind the peak heat-release zone when the flame temperature is high (of the orde r of 2800 K). For cooler and leaner flames (about 1600 K and for an eq uivalence ratio below 0.5) the OH radical is concentrated near the rea ction zone and the maximum OH level provides an estimate of the local flamelet speed as assumed by Pecker et al. (1990).