Effects of thermal expansion on moderately intense turbulence in premixed flames

Sabelnikov, Vladimir; Lipatnikov, Andrei; Nikitin, N.; Pérez, Francisco E. Hernández; Im, Hong G.

doi:10.1063/5.0123211

Cited by 5 publications

(8 citation statements)

References 71 publications

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“…volume occupied by the flames) is significantly less than the mean flame brush volume, the injected kinetic energy can be statistically significant (Sabelnikov et al. 2022 a , b ) due to high local energy-injection rates controlled by the local dilatation. Moreover, the scalar is injected into the scalar field within such flames due to the local formation of combustion products.…”

Section: Resultsmentioning

confidence: 99%

“…2021 a , b ) and from the two present flames A and B (Sabelnikov et al. 2022 a , b ). The methods are discussed in detail elsewhere (Sabelnikov et al.…”

Section: Dns Attributesmentioning

confidence: 92%

“…2020, 2021; Sabelnikov et al. 2021 c , 2022 a , b ), only a brief summary of the DNS attributes is given below.…”

Section: Dns Attributesmentioning

confidence: 99%

“…Since the injected turbulent velocity field evolved along the -direction (Sabelnikov et al. 2022 b , figure 1), the flames interacted with turbulence whose characteristics, e.g. the r.m.s.…”

Section: Dns Attributesmentioning

confidence: 99%

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Backscatter of scalar variance in turbulent premixed flames

et al. 2023

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To explore the direction of inter-scale transfer of scalar variance between subgrid scale (SGS) and resolved scalar fields, direct numerical simulation data obtained earlier from two complex-chemistry lean hydrogen–air flames are analysed by applying Helmholtz–Hodge decomposition (HHD) to the simulated velocity fields. Computed results show backscatter of scalar (combustion progress variable $c$ ) variance, i.e. its transfer from SGS to resolved scales, even in a highly turbulent flame characterized by a unity-order Damköhler number and a ratio of Kolmogorov length scale to thermal laminar flame thickness as low as 0.05. Analysis of scalar fluxes associated with the solenoidal and potential velocity fields yielded by HHD shows that the documented backscatter stems primarily from the potential velocity perturbations generated due to dilatation in instantaneous local flames, with the backscatter being substantially promoted by a close alignment of the spatial gradient of mean scalar progress variable and the potential-velocity contribution to the local SGS scalar flux. The alignment is associated with the fact that combustion-induced thermal expansion increases local velocity in the direction of $\boldsymbol {\nabla } c$ . These results call for development of SGS models capable of predicting backscatter of scalar variance in turbulent flames in large eddy simulations.

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Section: Resultsmentioning

confidence: 99%

“…2021 a , b ) and from the two present flames A and B (Sabelnikov et al. 2022 a , b ). The methods are discussed in detail elsewhere (Sabelnikov et al.…”

Section: Dns Attributesmentioning

confidence: 92%

“…2020, 2021; Sabelnikov et al. 2021 c , 2022 a , b ), only a brief summary of the DNS attributes is given below.…”

Section: Dns Attributesmentioning

confidence: 99%

Section: Dns Attributesmentioning

confidence: 99%

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Backscatter of scalar variance in turbulent premixed flames

et al. 2023

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“…To test this hypothesis, the effects of dilatation and the residual solenoidal turbulence, which features zero divergence, are examined separately. The separate treatment of solenoidal and dilatational parts of turbulent flows has been applied in various studies on compressible flows (Wang et al 2018;Panickacheril John, Donzis & Sreenivasan 2021;Zheng et al 2021Zheng et al , 2022Sabelnikov et al 2023) using the Helmholtz decomposition (Helmholtz 1858). In this study, an approach similar to the Helmholtz decomposition is proposed to separate the solenoidal and dilatational contributions in the velocity gradient tensor in the context of premixed flames.…”

Section: Decomposition Of the Impact Of Dilatation And Solenoidal Tur...mentioning

confidence: 99%

Effects of dilatation and turbulence on tangential strain rates in premixed hydrogen and iso-octane flames

Chu,

Berger,

Gauding

et al. 2024

J. Fluid Mech.

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The tangential strain rate in premixed flames impacts significantly the flame surface area generation and thus the combustion process. Studies on incompressible isotropic turbulence have revealed that the mean tangential strain rate at material and iso-scalar surfaces is positive and exhibits a universal value when normalized by the Kolmogorov time. This is associated with the preferential alignment of the surface normal with the most compressive principal strain rate. The present study investigates such effects in premixed hydrogen and iso-octane flame kernels using direct numerical simulations. It is shown that the normalized mean tangential strain rate of the investigated flames has a very similar value compared with the incompressible flows. However, in the reaction zone, the flame surface normal aligns preferentially with the most extensive principal strain rate. Furthermore, this alignment depends on the reaction progress variable and the Lewis number, while the tangential strain rate remains independent of these parameters. Such counter-intuitive behaviour is systematically investigated by decomposing the effects of dilatation and residual solenoidal turbulence. It is found that the solenoidal turbulence influences significantly the tangential strain rate. A general effect of turbulence on the tangential strain rate is identified, which is consistent with incompressible flows and independent of the Lewis number and the reaction progress variable. This is a remarkable finding indicating that models of the tangential strain rate developed based on incompressible flows apply also to premixed flames with different Lewis numbers, and, for the modelling, only the solenoidal turbulence should be considered.

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Optimising subgrid-scale closures for spectral energy transfer in turbulent flows

Nabavi,

Kim

2024

J. Fluid Mech.

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Subgrid-scale (SGS) modelling is formulated using a local transport of spectral kinetic energy estimated by a wavelet multiresolution analysis. Using a spectrally and spatially local decomposition by wavelet, the unresolved inter-scale energy transfer and modelled SGS dissipation are evaluated to enforce explicitly and optimally their balance a priori over a range of large-eddy simulation (LES) filter widths. The formulation determines SGS model constants that optimally describe the spectral energy balance between the resolved and unresolved scales at a given cutoff scale. The formulation is tested for incompressible homogeneous isotropic turbulence (HIT). One-parameter Smagorinsky- and Vreman-type eddy-viscosity closures are optimised for their model constants. The algorithm discovers the theoretical prediction of Lilly (The representation of small-scale turbulence in numerical simulation experiments. In Proceedings of the IBM Scientific Computing Symposium on Environmental Sciences, pp. 195–210) at a filter cutoff scale in the inertial subrange, whereas the discovered constants deviate from the theoretical value at other cutoff scales so that the spectral optimum is achieved. The dynamic Smagorinsky model used a posteriori shows a suboptimal behaviour at filter scales larger than those in the inertial subrange. A two-parameter Clark-type closure model is optimised. The optimised constants provide evidence that the nonlinear gradient model of Clark et al. (J. Fluid Mech., vol. 91, issue 1, 1979, pp. 1–16) is prone to numerical instability due to its model form, and combining the pure gradient model with a dissipative model such as the classic Smagorinsky model enhances numerical stability but the standard mixed model is not optimal in terms of spectral energy transfer. A posteriori analysis shows that the optimised SGS models produce accurate LES results.

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Effects of thermal expansion on moderately intense turbulence in premixed flames

Cited by 5 publications

References 71 publications

Backscatter of scalar variance in turbulent premixed flames

Backscatter of scalar variance in turbulent premixed flames

Effects of dilatation and turbulence on tangential strain rates in premixed hydrogen and iso-octane flames

Optimising subgrid-scale closures for spectral energy transfer in turbulent flows

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