Transient behavior of turbulent scalar transport in premixed flames

Artikel i vetenskaplig tidskrift, 2011

Transition from gradient to countergradient scalar transport in a statistically planar, one-dimensional, developing, premixed turbulent flame is studied both theoretically and numerically. A simple criterion of the transition referred to is derived from the balance equation for the combustion progress variable, with the criterion highlighting an important role played by flame development. A balance equation for the difference in velocities is gradient during an early stage of flame development followed by transition to countergradient scalar transport at certain instant t (tr) . (2) The transition time is increased when turbulence length scale L is increased or when the laminar flame speed S (L) and/or the density ratio are decreased. (3) The transition time normalized using the turbulence time scale is increased by u'. Moreover, the numerical simulations have shown that the transition time is increased by u' if a ratio of u'/S (L) is not large. This dependence of t (tr) on u' is substantially affected by (i) the mean pressure gradient induced within the flame due to heat release and (ii) by the damping effect of combustion on the growth rate of mean flame brush thickness. The reasonable qualitative agreement between the computed trends and available experimental and DNS data, as well as the agreement between the computed trends and the present theoretical results, lends further support to the conditioned balance equation used in the present work.