Contribution of small scale turbulence to burning velocity of flamelets in the thin reaction zone regime

Abstract

There is a growing body of experimental evidence that passive surface character of the premixed flamelets may not be preserved beyond medium turbulence intensities, and their thermal structure deviate from that of a laminar flamelet. Further, the experimental measurements of flame surface characteristics indicate that the flame surface area is not the dominant factor in increasing the turbulent burning velocity under the conditions corresponding to the thin reaction zones regime. Approaches to estimating the turbulent burning rates based on the area increase of the premixed flame front surfaces may not be the right models and may require additional mechanisms for proper representation of the burning rate. This paper proposes a simple scheme to estimate the contribution of the flame front alteration by small scale turbulence on flamelet burning velocity. An expression was derived to estimate the contribution of flame front alteration as a consequence of the small scale turbulent eddies that may penetrate into the preheat layer of the premixed flame front. The derivation was based on the experimental evidence of flame front alteration by active eddies penetrating into the preheat layer and enhancing the transport. As a first approximation it was assumed that these active eddies have a characteristic size about the Taylor microscale. Further, the formalism that demonstrates that within the turbulence cascade the volume occupied by a certain size eddy and its characteristic velocity obey power-law relationships (i.e. structure functions) that are dictated by the intermittency of the turbulent field, was used. The predictions of the proposed expression were compared to the available experimental data.