Effects of mean flow divergence on turbulent scalar flux and local flame structure in premixed turbulent combustion

Analysis of the balance equation for Favre-averaged progress variable variance developed previously are extended to open premixed flames in this paper. It is shown that both conditional mean dilatation and bulk-flow straining are linked to the turbulent scalar flux being countergradient (CGD) or gradient (GD) diffusion. The effect of bulk-flow straining is further investigated by using recent experimental data of joint particle image velocimetry/laser-induced OH fluorescence measurements in turbulent premixed Bunsen- and stagnation-type flames. A diverging mean flow field is found for these open flames with either CGD or GD transport, and the conditional mean dilatation always peaks near the burnt side of the local flame fronts. The coincidence of the conditional mean dilatation peaking at low. values of progress variable and the gradient-type turbulent scalar flux is expected only for confined premixed flames without lateral mean flow divergence or for simulated flames in direct numerical simulations (DNS) having no bulk-flow straining. A positive correlation has been observed between scalar diffusion and scalar dissipation in both DNS data and in laser measurements of open premixed flames. For these flames at high turbulent levels where gradient transport prevails, the conflicting behavior of local flame-front thinning or thickening is reconciled.