Extreme role of preferential diffusion in turbulent flame propagation

Thin reaction zones in constant-density turbulent flows at low Damköhler numbers: Theory and simulations

“…Accordingly, the CRDs are widely accepted as an appropriate framework for speculating what physical mechanisms govern the influence of turbulence on a reaction wave and under what conditions. In particular, "oversimplified" CRDs are discussed in almost every second archival publication on premixed turbulent combustion, even though important effects such as (i) preferential diffusion 2,4,[13][14][15] , (ii) variable density [16][17][18][19] , and (iii) complex chemistry [20][21][22] are well documented for flames but disregarded in the CRDs (certain recent combustion-regime diagrams 4,[23][24][25][26] allow for thermal expansion effects). It is also worth noting that a regime of premixed turbulent burning that was controlled by combustion chemistry was hypothesized by Shetinkov 27 as early as 60 years ago.…”

“…Indeed, the influence of turbulence on a reaction-wave surface and structure is an important ingredient of flame-turbulence interaction and a submodel of the former phenomenon is a basic building block for a typical model of premixed turbulent combustion [2][3][4]10,11,[24][25][26] . Even if effects due to preferential diffusion 2,4,[13][14][15] , variable density [16][17][18][19] , and complex chemistry [20][21][22] are well documented for turbulent burning but ignored in the present study, target-directed investigation of the influence of turbulence on a reaction-wave under the simplest conditions has its own value. Nevertheless, bearing in mind the above reservations, care should be taken when applying the results reported in the following to premixed turbulent combustion.…”

Transport equations for reaction rate in laminar and turbulent premixed flames characterized by non-unity Lewis number

“…Here, designates partial derivative with respect to time, is the flow velocity vector, and are the first-order partial derivatives of the rate with respect to and , respectively. Subsequently, application of a chain rule to , and yields (5) where , , and are the second-order derivatives with respect to and , and , and , respectively. Finally, using the continuity equation (6) we arrive at…”

“…This phenomenon is well documented in numerous experiments reviewed elsewhere [1][2][3], see also Refs. [4,5] as recent examples. The same phenomenon is also documented for lean turbulent flames of various H 2 /CO/CH 4 /O 2 /N 2 mixtures [6][7][8][9][10][11], but the magnitude of the effect is decreased when the mole fraction of hydrogen in the fuel blend is decreased by retaining the same equivalence ratio.…”

Surface-averaged quantities in turbulent reacting flows and relevant evolution equations

“…In spite of the paramount importance of the increase in the flame-surface area, the influence of turbulence on premixed combustion is not solely reduced to this physical mechanism. In particular, turbulent eddies can significantly change the local flame structure, thickness, and, hence, the local heatrelease rate, with these phenomena being of paramount importance in lean mixtures of light fuels (hydrogen or syngas) with the air [21][22][23][24][25][26][27]. Such effects are also often characterized using |∇c| and K. The former quantity directly yields the inverse of the local flame thickness and, according to the theory of laminar premixed flames subject to large-scale perturbations [17,28], difference between the local flame speed or burning rate and its unperturbed value is controlled by the local stretch rate.…”