Structure of a high Karlovitz n-C7H16 premixed turbulent flame

Abstract

Results from a series of direct numerical simulations (DNS) of a high Karlovitz, slightly lean (φ = 0.9), n-C 7 H 16 /air premixed turbulent flame are presented. The flame is statistically flat and is subjected to an inflow of homogeneous isotropic turbulence. A 35-species and 217-reaction mechanism [Bisetti et al. Combust. Flame 159 (2012) 317-335] is used to represent the chemistry. Two simulations have been performed: one with unity Lewis number to asses the effects of turbulence on the flame structure in the absence of differential diffusion, and the other with non-unity Lewis numbers to analyze how turbulence affects differential diffusion. The Karlovitz numbers are 280 and 220 respectively. The first simulation reveals that the flame is strongly affected by turbulence as enhanced mixing largely thickens the preheat zone. However, the turbulent flame structure (i.e. the correlation between species and temperature) is similar to that of a one-dimensional flat flame, suggesting that turbulence has limited effet on the flame in temperature space, in the absence of differential diffusion. In the second simulation, the flame structure is affected by turbulence, as differential diffusion effects are weakened. It is suggested that this result is attributed to the fact that turbulence drives the effective species Lewis numbers towards unity through an increase in effective species and thermal diffusivities. Finally, the reaction zones of both the unity and the non-unity Lewis number turbulent flames remain thin, and are locally broken (only to some extent for the unity Lewis number flame, and more strongly for non-unity).

Turbulence–flame interactions in lean premixed dodecane flames

“…Little work has been done on direct numerical simulation of high molecular weight hydrocarbon fuels, particularly for high Karlovitz number premixed flames. In a sequence of papers, Savard et al [9,10] and Lapointe et al [11] discuss simulations of premixed heptane flames at high Karlovitz numbers with detailed chemistry and transport, building on previous work [12] based on the hydrogen simulations in [1]. The focus of these papers is on the impact of differential diffusion on flame response, even at high Karlovitz numbers; the authors note a broadening of the flame and a transition to composition versus temperature profiles characteristic of a unity Lewis number flame and a reduction in heat release and fuel consumption rates.…”

Three-dimensional direct numerical simulation of turbulent lean premixed methane combustion with detailed kinetics

“…Note that since the conditional means 365 are presented as a function of temperature, the shift to unity Lewis number behaviour is compounded by the thickening of the preheat zone presented in section 3.4. We argue that this shift in chemical composition is actually the beginning of the transition to distributed burning (see [15,16,9] and the references therein 370 for a detailed examination of the transition to distributed burning in supernova flames, lean premixed hydrogen, and larger hydrocarbons, respectively; other relevant studies include [21,22,10,11,23,24,25]). Distributed burning is the limiting behaviour when turbulent mixing dominates species and thermal diffusion; all fluid parcels are advected together, and the associated turbulent 375 diffusion dominates the molecular diffusion (and consequentially any changes in equivalence ratio) in this regime.…”

Vorticity isotropy in high Karlovitz number premixed flames

“…All simulations were performed in the work of Bobbitt et al 6 and were originally based on the previous work of Savard et al 11 and Lapointe et al 13 Only cases A, B, B Tab,1 , B 4 Tab,1 , C * , and D performed by Bobbitt et al 6 are considered here. All necessary information about the different simulations is provided in Table I.…”

“…11,30 Constant non-unity Lewis numbers were employed, 31 and the species Lewis numbers are the same as those listed in the work of Savard and Blanquart. 12 The chemical and transport models were compared against experimental data and numerical results using full transport (mixture-averaged formulation).…”

“…Investigating the smallest turbulent scales is also particularly important as these scales must be modeled in Large Eddy Simulations (LES) 7,8 and are known to alter the internal structure of the flame. [9][10][11][12][13] In premixed turbulent combustion, the coupling of the flame and turbulence has been observed to vary with the Karlovitz number. 2,9,[13][14][15][16][17][18] The Karlovitz number is defined as the ratio of the flame time scale (τ F ) to that of the smallest turbulent eddies (τ η ),…”