Effect of flame-flame interaction on scalar PDF in turbulent premixed flames

Direct numerical simulation (DNS) results are analysed to investigate the effect of flame-flame interaction on the scalar distribution in a large eddy simulation context. The DNS data consist of turbulent premixed planar flame, V-flame and swirl flame cases with different turbulence and equivalence ratio, and relatively large Damkohler number conditions are considered for the three DNS cases. The volumetric fraction of non-flamelet type reaction zones (Phi) over bar, which are caused by flame-flame interaction (FFI), is identified by the trained neural network. The quantification of scalar PDF mode is carried out by means of the bimodality coefficient. Various scalar PDFs are constructed with different sample volumes of the filter size. The PDFs with high bimodality coefficient samples show clear bimodal distributions, whereas low bimodality coefficient samples show unimodal or plateau distributions. The conditional PDF and conditional average of the bimodality coefficient conditioned based on (Phi) over bar clearly show negative correlation between these two quantities. These results suggest that the presence of FFI events leads to bimodality loss of the scalar field, even when the Damkohler number is large. Reaction zones with non-bimodal scalar distribution are also found to have very small scalar gradient. Thus, the reaction rate of such reaction zones could be underestimated in flamelet-type modelling. However, such deficit may be straightforwardly complemented additively, for example, by considering a zero-dimensional canonical reactor with the fraction (Phi) over bar. (C) 2021 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute.