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Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-2711-4687
2015 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 2, 025102- p.Article in journal (Refereed) Published
Abstract [en]

The concept of local isotropy in a chemically reacting turbulent wall-jet flow is addressed using direct numerical simulation (DNS) data. Different DNS databases with isothermal and exothermic reactions are examined. The chemical reaction and heat release effects on the turbulent velocity, passive scalar, and reactive species fields are studied using their probability density functions (PDFs) and higher order moments for velocities and scalar fields, as well as their gradients. With the aid of the anisotropy invariant maps for the Reynolds stress tensor, the heat release effects on the anisotropy level at different wall-normal locations are evaluated and found to be most accentuated in the near-wall region. It is observed that the small-scale anisotropies are persistent both in the near-wall region and inside the jet flame. Two exothermic cases with different Damkohler numbers are examined and the comparison revealed that the Damkohler number effects are most dominant in the near-wall region, where the wall cooling effects are influential. In addition, with the aid of PDFs conditioned on the mixture fraction, the significance of the reactive scalar characteristics in the reaction zone is illustrated. We argue that the combined effects of strong intermittency and strong persistency of anisotropy at the small scales in the entire domain can affect mixing and ultimately the combustion characteristics of the reacting flow.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015. Vol. 27, no 2, 025102- p.
Keyword [en]
Turbulent wall jet flow, reacting flow, combustion, local isotropy
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-160602DOI: 10.1063/1.4906370ISI: 000350551300028Scopus ID: 2-s2.0-84923814961OAI: oai:DiVA.org:kth-160602DiVA: diva2:790527
Funder
EU, FP7, Seventh Framework Programme, 339032
Note

QC 20150225

Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2017-12-04Bibliographically 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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Johansson, Arne V.

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