Differential diffusion effects, distributed burning, and local extinctions in high Karlovitz premixed flames

Abstract

a b s t r a c tDirect numerical simulations of premixed n-heptane/air flames at different Karlovitz numbers are performed using detailed chemistry. Differential diffusion effects are systematically isolated by performing simulations with both non-unity and unity Lewis numbers. Different unburnt temperatures and turbulence intensities are used and their effects on the flame structure and chemical source terms are investigated. As the unburnt gases are preheated, the viscosity ratio across the flame is reduced and the Karlovitz number at the reaction zone is increased. The increase in turbulence intensity suppresses differential diffusion effects on the flame structure (i.e. species dependence on temperature). However, differential diffusion effects on the chemical source terms are still noticeable even at the highest Karlovitz number simulated. Simulations with differential diffusion effects exhibit lower mean fuel consumption and heat release rates than their unity Lewis number counterparts. However, the difference is reduced as the reaction zone Karlovitz number is increased. Transition to distributed burning is characterized by a broadening of the reaction zone resulting from enhanced turbulent mixing. Local extinctions in the burning rate are observed only in non-unity Lewis number simulations and their probability decreases at high Karlovitz numbers. These results highlight the importance of using the reaction zone Karlovitz number to investigate the effect of turbulence on the chemical source terms and to compare flames at different unburnt temperatures.

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.…”

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.…”

“…9,10,12,15 In order to isolate the effects of the flame on the turbulence and to reach high Karlovitz numbers, the DNSs analyzed here were statistically planar and used turbulence forcing to prevent the decay of turbulence. Similar methods have been used in the past to investigate both the dynamics of the flame 12,13,[45][46][47] and small scale turbulence. 6,15,16,22 Further analysis and justification of the current configuration were performed by Bobbitt et al 6 The evolution of enstrophy through the flame was in agreement with the unforced, shear-driven, high Karlovitz number slot Bunsen flames of Sankaran et al 43 In the simulations under consideration, linear forcing, adapted from the work of Carroll and Blanquart, 27 was employed by appending the following term to the momentum equation:…”

“…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 ),…”

A flame particle tracking analysis of turbulence–chemistry interaction in hydrogen–air premixed flames

“…The local flame geometries were also investigated, and the effects of strain rate and curvature on different regions across the flame were analyzed in detail [7,8]. Large scale DNS at higher Reynolds and Karlovitz numbers considering detailed chemistry have also been conducted [9][10][11][12][13], with an emphasis on the fractal characteristics.…”