A review of chemical diffusion: Criticism and limits of simplified methods for diffusion coefficient calculation

Giacomazzi, E.; Picchia, F.R.; Arcidiacono, N.

doi:10.1080/13647830701550370

Cited by 30 publications

(21 citation statements)

References 35 publications

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“…Preferential diffusion can lead to thermo-diffusive instability depending on the ratio between the thermal and mass diffusivity of reactants, i.e. the so-called Lewis number, Le i = α/D i [18]. When Le i < 1, as in the case of hydrogen, some thermo-diffusive instabilities [39, p. 357-365], [40] may grow and increase the flame surface and global burning rate [41, p. 56-59], [42,43], up to the formation of cellular flames when the flow is laminar [39, p. 349-365].…”

Section: Importance Of Preferential Diffusionmentioning

confidence: 99%

“…Since this is a zeroth-order model [35], it neglects some physics in the diffusion process (in particular, cross-diffusion, the Soret and Dufour effects); hence, adding up all species diffusion fluxes does not yield zero, and mass conservation is no longer automatically ensured. This problem requires a particular treatment of diffusive fluxes described in [18].…”

Section: Importance Of Preferential Diffusionmentioning

confidence: 99%

“…In this way, as discussed in Section 4.1.1, preferential diffusion of individual species is taken into account. A review of the calculation of molecular properties, especially focused on diffusion coefficients, can be found in [18].…”

Section: Molecular Propertiesmentioning

confidence: 99%

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Unsteady simulation of a CO/H₂/N₂/air turbulent non-premixed flame

Giacomazzi

Picchia

Arcidiacono

et al. 2008

Combustion Theory and Modelling

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The Sandia/ETH-Zurich CO/H 2 /N 2 non-premixed unconfined turbulent jet flame (named 'Flame A') is numerically simulated by solving the unsteady compressible reactive NavierStokes equations in a three-dimensional axisymmetric formulation, hence, in a formally twodimensional domain. The turbulent combustion closure model adopted is the Fractal Model, FM, developed as a subgrid scale model for Large Eddy Simulation. The fuel is injected from a straight circular tube and the corresponding Reynolds number is 16 700, while the air coflows. Since the thickness of the nozzle is 0.88 mm, and the injection velocity high, ∼104 m s −1 , capturing the stabilization mechanism of the actual flame requires high spatial resolution close to the injector. Results are first obtained on a coarse grid assuming a fast-chemistry approach for hydrogen oxidation and a single step mechanism for carbon monoxide oxidation. With this approach the flame is inevitably anchored. Then, to understand the actual flame stabilization a more complex chemical mechanism, including main radical species, is adopted. Since using this chemistry and the coarse grid of previous simulation the flame blows off numerically, attention is focused on understanding the actual flame stabilization mechanism by simulating a small spatial region close to the injection with a very fine grid. Then, analysing these results, an artificial anchoring mechanism is developed to be used in simulations of the whole flame with a coarse grid. Unsteady characteristics are shown and some averaged radial profiles for temperature and species are compared with experimental data.

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Section: Importance Of Preferential Diffusionmentioning

confidence: 99%

Section: Importance Of Preferential Diffusionmentioning

confidence: 99%

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Unsteady simulation of a CO/H₂/N₂/air turbulent non-premixed flame

Giacomazzi

Picchia

Arcidiacono

et al. 2008

Combustion Theory and Modelling

Self Cite

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“…It is common practice to prescribe the mass diffusion coefficients through a set of constant Lewis numbers, Le i [27][28][29][30]. For example, Hawkes and Chen [31] performed calculations of laminar strained premixed methane-air twin flames, showing that the mixture-averaged calculations were well reproduced by constant-Lewis number simulations over a wide range of strain rates.…”

Section: Introductionmentioning

confidence: 99%

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

Burali

Lapointe

Bobbitt

et al. 2016

Combustion Theory and Modelling

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Accurate computation of molecular diffusion coefficients in chemically reacting flows can be an expensive procedure, and the use of constant non-unity Lewis numbers has been adopted often as a cheaper alternative. The goal of the current work is to explore the validity and the limitations of the constant non-unity Lewis number approach in the description of molecular mixing in laminar and turbulent flames. To carry out this analysis, three test cases have been selected, including a lean, highly unstable, premixed hydrogen/air flame, a lean turbulent premixed n-heptane/air flame, and a laminar ethylene/air coflow diffusion flame. For the hydrogen flame, both a laminar and a turbulent configuration have been considered. The three flames are characterised by Lewis numbers which are less than unity, greater than unity, and close to unity, respectively. For each flame, mixture-averaged transport simulations are carried out and used as reference data. The current analysis suggests that, for numerous combustion configurations, the constant non-unity Lewis number approximation leads to small errors when the set of Lewis numbers is chosen properly. For the selected test cases and our numerical framework, the reduction of computational cost is found to be minimal.

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“…This mechanism causes an increase in flame surface and amplifies the perturbations (Giacomazzi et al, 2007;Bastiaansy et al, 2007). Barenblatt et al (1962) reported linear analysis of thermal-diffusive instability of a plane flame front subject to long-wave perturbations, and derived the dispersion relation:…”

Section: Thermal Diffusive Instabilitymentioning

confidence: 99%

Flame structure and thermo-acoustic coupling for the low swirl burner for elevated pressure and syngas conditions

Emadi¹

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Reduction of the pollutant emissions is a challenge for the gas turbine industry. A solution to this problem is to employ the low swirl burner which can operate at lower equivalence ratios than a conventional swirl burner. However, flames in the lean regime of combustion are susceptible to flow perturbations and combustion instability. Combustion instability is the coupling between unsteady heat release and combustor acoustic modes where one amplifies the other in a feedback loop. The other method for significantly reducing NO x and CO 2 is increasing fuel reactivity, typically done through the addition of hydrogen. This helps to improve the flammability limit and also reduces the pollutants in products by decreasing thermal NO x and reducing CO 2 by displacing carbon. In this work, the flammability limits of a low swirl burner at various operating conditions, is studied and the effect of pressure, bulk velocity, burner shape and percent of hydrogen (added to the fuel) is investigated. Also, the flame structure for these test conditions is measured using OH planar laser induced fluorescence and assessed. Also, the OH PLIF data is used to calculate Rayleigh index maps and to construct averaged OH PLIF intensity fields at different acoustic excitation frequencies (45-155, and 195Hz). Based on the Rayleigh index maps, two different modes of coupling between the heat release and the pressure fluctuation were observed: the first mode, which occurs at 44Hz and 55Hz, shows coupling to the flame base (due to the bulk velocity) while the second mode shows coupling to the sides of the flame. In the first mode, the flame becomes wider and the flame base moves with the acoustic frequency. In the second mode, imposed pressure oscillations induce vortex shedding in the flame shear layer. These vortices distort the flame front and generate locally compact and sparse flame areas. The local flame structure resulting from these two distinct modes was markedly different.

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A review of chemical diffusion: Criticism and limits of simplified methods for diffusion coefficient calculation

Cited by 30 publications

References 35 publications

Unsteady simulation of a CO/H₂/N₂/air turbulent non-premixed flame

Unsteady simulation of a CO/H₂/N₂/air turbulent non-premixed flame

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

Flame structure and thermo-acoustic coupling for the low swirl burner for elevated pressure and syngas conditions

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A review of chemical diffusion: Criticism and limits of simplified methods for diffusion coefficient calculation

Cited by 30 publications

References 35 publications

Unsteady simulation of a CO/H2/N2/air turbulent non-premixed flame

Unsteady simulation of a CO/H2/N2/air turbulent non-premixed flame

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

Flame structure and thermo-acoustic coupling for the low swirl burner for elevated pressure and syngas conditions

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Unsteady simulation of a CO/H₂/N₂/air turbulent non-premixed flame

Unsteady simulation of a CO/H₂/N₂/air turbulent non-premixed flame