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Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.ORCID iD: 0000-0002-3173-7502
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
INSA de Rouen.
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.ORCID iD: 0000-0002-2711-4687
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigate the properties of the subgrid-scale (SGS) stress tensor and SGS dissipation of kinetic energy and enstrophy,using the direct numerical simulation (DNS) data of a non-premixed reacting turbulent wall-jet flow, with and without heat release.The separation of scales, to obtain the SGS quantities, is achieved by application of a box filter.This study comprises an analysis on the topology of the resolved strain-rate andSGS stress tensors, through an assessment of their eigenvectors and their relative alignment. To find out the heat-release effects on the dynamics of the turbulent energy dissipation, SGS dissipation of kinetic energy andenstrophy are evaluated using length-scale, probability density functions (PDFs) and mean value analysis.

It is found that the mean SGS shear stress and turbulent kinetic energy are suppressed by the heat release, whilethe SGS anisotropy is substantially increased.Although, the topology of the resolved strain-rate tensor only marginally differs between the isothermal and exothermic cases in the near-wall wall region,substantial differences are observed in the shear layer in the jet area, where the compressibility effects are large andthe exothermic effects are found to promote compression states.The relative alignment between the SGS stress and resolved strain-rate tensors is also affected by the heat release.The mean SGS dissipation of kinetic energy is increased, while the SGS dissipation of enstrophy is decreased by the heat release.Interesting differences in the shape of the PDFs of the SGS dissipation are observed between the isothermal and exothermic cases, such as thechange in the intermittency of both SGS dissipation terms.

Keyword [en]
Subgrid-scale stress, turbulent wall-jet flow, combustion, heat release
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-160604OAI: oai:DiVA.org:kth-160604DiVA: diva2:790528
Note

QC 20160524

Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2016-05-24Bibliographically approved
In thesis
1. Numerical studies of turbulent flames in wall-jet flows
Open this publication in new window or tab >>Numerical studies of turbulent flames in wall-jet flows
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis deals with the fundamental aspects of turbulent mixing and non-premixed combustion in the wall-jet flow, which has a close resemblance to many industrial applications. Direct numerical simulations (DNS) of turbulent wall-jets with isothermal and exothermic reactions are performed. In the computational domain, fuel and oxidizer enter separately in a nonpremixed manner and the flow is compressible, fully turbulent and subsonic. The triple “turbulence-chemistry-wall” interactions in the wall-jet flow have been addressed first by focusing on turbulent flow effects on the isothermal reaction, and then, by concentrating on heat-release effects on both turbulence and flame characteristics in the exothermic reaction. In the former, the mixing characteristics of the flow, the key statistics for combustion and the near-wall effects in the absence of thermal effects are isolated and studied. In the latter, the main target was to identify the heat-release effects on the different mixing scales of turbulence. Key statistics such as the scalar dissipation rates, time scale ratios, two-point correlations, one and two-dimensional premultiplied spectra are used to illustrate the heat release induced modifications. Finer small mixing scales were observed in the isothermal simulations and larger vortical structures formed after adding significant amounts of heat-release. A deeper insight into the heat release effects on three-dimensional mixing and reaction characteristics of the turbulent wall-jet flow has been gained by digging in different scales of DNS datasets. In particular, attention has been paid to the anisotropy levels and intermittency of the flow by investigating the probability density functions, higher order moments of velocities and reacting scalars and anisotropy invariant maps for different reacting cases. To evaluate and isolate the Damkohler number effects on the reaction zone structure from those of the heat release a comparison between two DNS cases with different Damkohler numbers but a comparable temperature rise is performed. Furthermore, the wall effects on the flame and flow characteristics, for instance, the wall heat transfer; the near-wall combustion effects on the skin-friction, the isothermal wall cooling effects on the average burning rates and the possibility of formation of the premixed mode within the non-premixed flame are addressed. The DNS datasets are also used for a priori  analysis, focused on the heat release effects on the subgrid-scale (SGS) statistics. The findings regarding the turbulence small-scale characteristics, gained through the statistical analysis of the flow have many phenomenological parallels with those concerning the SGS statistics. Finally, a DNS of turbulent reacting wall-jet at a substantially higher Reynolds number is performed in order to extend the applicability range for the conclusions of the present study and figuring out the possible differences.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. x, 66 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:02
Keyword
Turbulence, combustion, direct numerical simulation, wall-jet, heat release effects, mixing scales, non-premixed flame, wall heat transfer
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-160609 (URN)978-91-7595-470-7 (ISBN)
Public defence
2015-03-12, F3, Lindstedsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20150225

Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2015-02-25Bibliographically approved

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Rasam, AminJohansson, Arne V.

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