RELATIVE IMPORTANCE OF COHERENT STRUCTURES VS BACKGROUND TURBULENCE IN THE PROPAGATION OF A PREMIXED FLAME

For premixed flames in the ''flamelet'' regime, where turbulence lengt h scales are significantly greater than the flame thickness, it has be en asserted that the principal contribution to flame surface area is g enerated by vortical structures present in the reacting gas mixture. S everal direct numerical simulation (DNS) studies of premixed combustio n implicitly follow this assumption by only considering a flame intera cting with a single, well-defined vortical structure; however, the imp ortant issue of whether the majority of flame surface area is actually caused by vortical structures, as opposed to the featureless backgrou nd turbulence has not been satisfactorily addressed. We consider this question using direct numerical simulations (DNS). As shown by She, Ja ckson and Orszag (Nature 344:226, 1990), scrambling the phase of the v elocity held in Fourier space (or wave number space) eliminates cohere nt structures from the turbulent field. Consequently, DNS provides an opportunity to evaluate the importance of coherent structures by compa ring flames propagating through Navier-Stokes turbulence with those pa ssing through phase-scrambled (coherent-structure-free) turbulence. Th e idea of scrambling the phasing of the velocity components is extreme ly attractive since the scrambled velocity held is free of any coheren t vortical structures yet is nearly identical to the Navier-Stokes vel ocity in all other respects. The results show that the turbulent burni ng velocity is predominantly influenced by the featureless background turbulence over the range of parameters considered, although there is approximately a 4-7% increase in the flame speed that results from the presence of structures. The topologies of the flame surfaces from the scrambled and unscrambled turbulence are also very similar. (C) 1997 by The Combustion Institute.