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Heat release effects on mixing scales of non-premixed turbulent wall-jets: A direct numerical simulation study
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
2013 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 40, 65-80 p.Article in journal (Refereed) Published
Abstract [en]

The present study concerns the role of heat release effects on characteristics mixing scales of turbulence in reacting wall-jet flows. Direct numerical simulations of exothermic reacting turbulent wall-jets are performed and compared to the isothermal reacting case. An evaluation of the heat-release effects on the structure of turbulence is given by examining the mixture fraction surface characteristics, diagnosing vortices and exploring the dissipation rate of the fuel and passive scalar concentrations, and moreover by illustration of probability density functions of reacting species and scatter plots of the local temperature against the mixture fraction. Primarily, heat release effects delay the transition, enlarge the fluctuation intensities of density and pressure and also enhance the fluctuation level of the species concentrations. However, it has a damping effect on all velocity fluctuation intensities and the Reynolds shear stress. A key result is that the fine-scale structures of turbulence are damped, the surface wrinkling is diminished and the vortices become larger due to heat-release effects. Taking into account the varying density by using semi-local scaling improves the collapse of the turbulence statistics in the inner region, but does not eliminate heat release induced differences in the outer region. Examining the two-dimensional premultiplied spanwise spectra of the streamwise velocity fluctuations indicates a shifting in the positions of the outer peaks, associated with large energetic structures, toward the inner region.

Place, publisher, year, edition, pages
2013. Vol. 40, 65-80 p.
Keyword [en]
Turbulent wall-jet, Non-premixed combustion, Heat-release effects, Direct numerical simulation, Energy spectra
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-122335DOI: 10.1016/j.ijheatfluidflow.2012.12.005ISI: 000317326600006ScopusID: 2-s2.0-84875072425OAI: diva2:622537
Swedish Research Council, 621-2007

QC 20130522

Available from: 2013-05-22 Created: 2013-05-20 Last updated: 2015-02-25Bibliographically 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.
TRITA-MEK, ISSN 0348-467X ; 2015:02
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
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)

QC 20150225

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

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