Application of conditioned structure functions to exploring influence of premixed combustion on two-point turbulence statistics

Artikel i vetenskaplig tidskrift, 2019

In order to investigate the influence of combustion-induced thermal expansion on turbulent flow within a premixed flame brush, a new method is introduced. The method consists in analyzing structure functions of the velocity field, which characterize velocity difference in two points A and B, with the structure functions being conditioned to various events; (i) unburned reactants in both points, (ii) combustion products in both points, (iii) intermediate states of the mixture in both points, (iv) the reactants in one point and the products in another point, (v) the reactants in one point and an intermediate state in another point, and (vi) the products in one point and an intermediate state in another point. Such structure functions and relevant probabilities are defined in the paper. Subsequently, the structure functions and the probabilities are extracted from Direct Numerical Simulation (DNS) data obtained from two statistically planar, 1D, fully-developed, weakly turbulent, premixed flames characterized by two significantly different (7.52 and 2.50) density ratios, with all other things being approximately equal. Obtained results indicate that (i) the conditioned structure functions differ significantly from the mean structure functions and (ii) the newly introduced approach could convey information important for understanding fundamentals of flame-turbulence interaction and finding issues that require further research. In particular, application of the approach to the aforementioned DNS data shows that the combustion-induced thermal expansion substantially affects small-scale two-point velocity statistics in the incoming constant-density turbulent flow of unburned reactants within a premixed flame brush.