A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

Abstract

Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterized by different density ratios σ are analyzed in order to study the influence of thermal expansion on flame surface area and turbulent burning rate. Obtained results show that, on the one hand, pressure gradient induced within the flame brush due to heat release in flamelets significantly accelerates unburned gas that deeply intrudes into combustion products in a form of an unburned mixture finger, thus, causing large-scale oscillations of the turbulent burning rate and flame brush thickness. Under conditions of the present simulations, contribution of this mechanism to creation of flame surface area is substantial and is increased by the density ratio, thus, implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal to one another. Apparent inconsistency between these results implies existence of another thermal expansion effect that reduces the influence of the density ratio on the flame surface area and burning rate. Investigation of the issue shows that the axial flow acceleration by the combustion-induced pressure gradient not only straightforwardly creates flame surface area by pushing a finger tip into products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to a high-speed (at σ = 7.53) axial 1 flow (a jet) of unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces a residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened due to a shorter residence time.

DNS Study of the Bending Effect Due to Smoothing Mechanism

“…The thermal expansion effects are not taken into account in the present simulations, because the physical mechanisms from Group (C) do not allow for them. We may also note that (i) the vast majority of approximations of experimental data on U T , e.g., see review papers [30,31], do not invoke the density ratio σ, thus, implying a weak influence of σ on U T or S T , (ii) recent target-directed experiments [32], as well as earlier measurements [33], did not reveal a substantial influence of σ on U T either, and (iii) recent DNS studies, e.g., Figures 10 and 11 in [34] or Figure 2a in [35], do not indicate such an influence.…”

Influence of Thermal Expansion on Potential and Rotational Components of Turbulent Velocity Field Within and Upstream of Premixed Flame Brush

“…Since the DNS data were discussed in details elsewhere [14,15] and were already used by various research groups [12,13,21,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50], let us restrict ourselves to a very brief summary of those compressible 3D simulations. They deal with statistically 1D and planar, equidiffusive, adiabatic flames modeled by unsteady continuity, Navier-Stokes, and energy equations, supplemented with the ideal gas state equation and a transport equation for the mass fraction * of a deficient reactant.…”

“…Recently, some of the present authors [12,13] analyzed the Nagoya DNS database [14,15] and showed that acceleration of the unburned gas by combustion-induced pressure gradient yielded unburned mixture fingers that deeply intruded into the products, thus, significantly increasing flame surface area, turbulent burning rate, and mean flame brush thickness. Appearance of such unburned mixture fingers was also documented in subsequent experiments [16,17].…”

Closure Relations for Fluxes of Flame Surface Density and Scalar Dissipation Rate in Turbulent Premixed Flames

“…In order to test Equation (9), we analyzed DNS data obtained earlier by Nishiki et al [15,16]. Because these data were used by different research groups in a number of papers [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36], we will restrict ourselves to a brief summary of these compressible simulations.…”