Comparison of a spectral model for premixed turbulent flame propagation toDNS and experiments

A recently developed spectral model for premixed turbulent combustion in th e flamelet regime (based on the EDQNM turbulence theory) has been compared with both direct numerical simulations (DNS) and experimental data. The 128 (3) DNS is performed at a Reynolds number of 223 based on the integral leng th scale. Good agreement is observed for both single- and two-point quantit ies (i.e. ratio of the turbulent to laminar burning velocities, scalar auto correlation. dissipation and scalar-velocity cross correlation spectral for the two different values of u'/s(LO) considered. The model also predicts t he rapid transient behaviour of the flame at early times. An experimental s et-up is then described for generating a lean methane-ah flame and measurin g two- point spatial correlations along the midpoint of the flame brush (i. e. along the (C) over bar = 0.5 contour). The experimental measurements in the flamelet regime take the form of a discontinuous or 'telegraph' signal. The EDQNM model, in contrast, describes an 'ensemble' of flames, and thus is based solely on continuous variables. A theoretical relationship between the correlation obtained from the EDQNM model and the equivalent correlati on for a discontinuous (experimental) flame is derived. The relationship is used to enable a meaningful comparison between experimentally observed and model correlations. In general, the agreement is good for the three differ ent cases considered in this study, with most of the error occurring at the lowest Reynolds number (Re-L = 22). Furthermore, it is shown that consider ably more error would result if no attempt is made to convert the ensemble representation in the model to an equivalent single-flame or 'telegraph' si gnal.