Assessment of the constant non-unity Lewis number assumption in chemically-reacting flows

Burali, Nicholas; Lapointe, Simon; Bobbitt, Brock; Blanquart, Guillaume; Xuan, Yuan Hu

doi:10.1080/13647830.2016.1164344

Cited by 52 publications

(32 citation statements)

References 51 publications

(78 reference statements)

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“…This is because the species Lewis numbers vary relatively little through the flame even when using mixture-averaged formulation. 31 Extension of these conclusions to other conditions, such as lean hydrogen/air flames, where thermo-diffusive instabilities occur, is unclear and should be the subject of future work.…”

Section: Discussionmentioning

confidence: 99%

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Vorticity isotropy in high Karlovitz number premixed flames

Bobbitt

Blanquart

2016

Physics of Fluids

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The isotropy of the smallest turbulent scales is investigated in premixed turbulent combustion by analyzing the vorticity vector in a series of high Karlovitz number premixed flame direct numerical simulations. It is found that increasing the Karlovitz number and the ratio of the integral length scale to the flame thickness both reduce the level of anisotropy. By analyzing the vorticity transport equation, it is determined that the vortex stretching term is primarily responsible for the development of any anisotropy. The local dynamics of the vortex stretching term and vorticity resemble that of homogeneous isotropic turbulence to a greater extent at higher Karlovitz numbers. This results in small scale isotropy at sufficiently high Karlovitz numbers and supports a fundamental similarity of the behavior of the smallest turbulent scales throughout the flame and in homogeneous isotropic turbulence. At lower Karlovitz numbers, the vortex stretching term and the vorticity alignment in the strain-rate tensor eigenframe are altered by the flame. The integral length scale has minimal impact on these local dynamics but promotes the effects of the flame to be equal in all directions. The resulting isotropy in vorticity does not reflect a fundamental similarity between the smallest turbulent scales in the flame and in homogeneous isotropic turbulence. Published by AIP Publishing. [http://dx

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Section: Discussionmentioning

confidence: 99%

“…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).…”

Section: B Governing Equationsmentioning

confidence: 99%

Vorticity isotropy in high Karlovitz number premixed flames

Bobbitt

Blanquart

2016

Physics of Fluids

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“…They reported that methane, n-heptane, iso-octane, and toluene flames have similar normalized turbulent flame speeds and fuel burning rates when neglecting differential diffusion, but flames using the nonunity Lewis number approximation underpredict the normalized flame speed when including differential diffusion due to reduced burning rates [3]. Building on these results, Burali et al [2] evaluated the relative accuracy of the nonunity Lewis number assumption relative to mixture-averaged diffusion for lean, unstable hydrogen/air flames; lean, turbulent n-heptane/air flames; and ethylene/air coflow diffusion flames. Their results demonstrated that the relative error associated with the nonunity Lewis number assumption could be minimized with careful selection of the Lewis number vector for a wide range of flames [2].…”

Section: Introductionmentioning

confidence: 94%

“…This is because calculating diffusion fluxes requires point-wise knowledge of the multicomponent diffusion coefficient matrix, which scales with the number of chemical species squared [1]. The unity Lewis number, non-unity Lewis number, and mixture-averaged diffusion assumptions have been used to reduce the costs associated with mass diffusion by approximating the full diffusion coefficient matrix as a constant scalar value, a constant vector, and a matrix diagonal, respectively [1][2][3][4]. In addition, several approaches, such as those used by Warnatz [5] and Coltrin et al [6], further reduce the system's complexity by approximating multicomponent diffusion processes in terms of equivalent Fickian processes.…”

Section: Introductionmentioning

confidence: 99%

“…Building on these results, Burali et al [2] evaluated the relative accuracy of the nonunity Lewis number assumption relative to mixture-averaged diffusion for lean, unstable hydrogen/air flames; lean, turbulent n-heptane/air flames; and ethylene/air coflow diffusion flames. Their results demonstrated that the relative error associated with the nonunity Lewis number assumption could be minimized with careful selection of the Lewis number vector for a wide range of flames [2]. Similar work by Schlup and Blanquart [4] examined the impact of multicomponent thermal diffusion on DNS of turbulent, premixed, high-Karlovitz hydrogen/air flames.…”

Section: Introductionmentioning

confidence: 99%

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A fast, low-memory, and stable algorithm for implementing multicomponent transport in direct numerical simulations

Fillo

Schlup

Beardsell

et al. 2020

Journal of Computational Physics

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Implementing multicomponent diffusion models in reacting-flow simulations is computationally expensive due to the challenges involved in calculating diffusion coefficients. Instead, mixture-averaged diffusion treatments are typically used to avoid these costs. However, to our knowledge, the accuracy and appropriateness of the mixture-averaged diffusion models has not been verified for three-dimensional turbulent premixed flames. In this study we propose a fast, efficient, low-memory algorithm and use that to evaluate the role of multicomponent mass diffusion in reacting-flow simulations. Direct numerical simulation of these flames is performed by implementing the Stefan-Maxwell equations in NGA. A semi-implicit algorithm decreases the computational expense of inverting the full multicomponent ordinary diffusion array while maintaining accuracy and fidelity. We demonstrate the algorithm to be stable, and its performance scales approximately with the number of species squared. We first verify the method by performing one-dimensional simulations of premixed hydrogen flames and compare with matching cases in Cantera. As an initial study of multicomponent diffusion, we simulate premixed, three-dimensional turbulent hydrogen flames, neglecting secondary Soret and Dufour effects. Simulation conditions are carefully selected to match previously published results and ensure valid comparison. Our results show that using the mixture-averaged diffusion assumption lead to a 15 % under-prediction of the normalized turbulent flame speed for premixed hydrogen air flames. This large difference in the turbulent flame speed raises questions on the appropriateness of using the mixture-averaged diffusion assumption for DNS of moderate to high Karlovitz number flames.

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Assessment of the constant non-unity Lewis number assumption in chemically-reacting flows

Cited by 52 publications

References 51 publications

Vorticity isotropy in high Karlovitz number premixed flames

Vorticity isotropy in high Karlovitz number premixed flames

A fast, low-memory, and stable algorithm for implementing multicomponent transport in direct numerical simulations

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