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Fundamental studies of non-premixed combustion in turbulent wall jets using direct numerical simulation
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis deals with the fundamental aspects of turbulent mixingand non-premixed combustion in wall-jet flows. Direct numerical simulations(DNS) of compressible turbulent flows are performed in a wall-jet configura-tion, which has a close resemblance to many industrial combustion applica-tions. The triple ”turbulence-chemistry-wall” interactions are also present inthis flow set-up. These interactions have been addressed by first focusing onturbulent flow effects on the isothermal reaction, including the near-wall issues.Then, by adding heat-release to the simulations, it has been concentrated onheat-release effects on various phenomena that occur in the reacting turbulentwall-jet flow. In the computational domain, fuel and oxidizer enter separatelyin a non-premixed manner and the flow is fully turbulent and subsonic in allsimulations. In the first phase of this study, the case of a turbulent wall-jetincluding an isothermal reaction without heat release is addressed in order toisolate the near-wall effects and the mixing characteristics of the flow and thekey statistics for combustion are studied in the absence of thermal effects. Adeeper insight into three-dimensional mixing and reaction characteristics in aturbulent wall-jet has been gained through investigation of the probability den-sity functions, higher order moments of velocities and reacting scalars and thescalar dissipation rates of different species. In the second phase, DNS of turbu-lent reacting wall-jets including heat release is performed, where a single-stepglobal exothermic reaction with an Arrhenius-type reaction rate is considered.The main target was to identify the heat-release effects on different mixingscales of turbulent wall-jet flow. The scalar dissipation rates, time scale ratios,two-point correlations, one and two-dimensional premultiplied spectra are usedto illustrate the heat release induced modifications. It is observed that heatrelease effects delay the transition process in the chemically reacting cases andenlarge the fluctuation intensities of density and pressure, but have a dampingeffect on all velocity fluctuation intensities. Finer small mixing scales were ob-served in the isothermal simulations and larger vortical structures formed afteradding significant amounts of heat-release. Simulations with different Damk ̈h-  oler numbers, but comparable temperature-rise are performed and the expectedbehavior, a thinner flame with increasing Damk ̈hler number, is observed. Finally, some heat transfer related quantities are examined. The wall heat fluxand the corresponding Nusselt numbers are addressed. The near-wall reactioneffects on the skin friction coefficient are studied and further the reaction char-acteristics are investigated throughout the domain.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , viii, 44 p.
Series
Trita-MEK, ISSN 0348-467X ; 2011:09
Keyword [en]
Turbulence, non-premixed combustion, global reaction, direct numerical simulation, wall-jet, heat release, mixing scales, heat transfer
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-39028ISBN: 978-91-7501-089-2 (print)OAI: oai:DiVA.org:kth-39028DiVA: diva2:439193
Presentation
2011-09-28, Sal V2, Teknikringen 76, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish e‐Science Research Center
Note
QC 20110908Available from: 2011-09-08 Created: 2011-09-06 Last updated: 2012-05-24Bibliographically approved
List of papers
1. Direct numerical simulation of an isothermal reacting turbulent wall-jet
Open this publication in new window or tab >>Direct numerical simulation of an isothermal reacting turbulent wall-jet
2011 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 8, 085104- p.Article in journal (Refereed) Published
Abstract [en]

In the present investigation, Direct Numerical Simulation (DNS) is used to study a binary irreversible and isothermal reaction in a plane turbulent wall-jet. The flow is compressible and a single-step global reaction between an oxidizer and a fuel species is solved. The inlet based Reynolds, Schmidt, and Mach numbers of the wall-jet are Re = 2000, Sc = 0.72, and M = 0.5, respectively, and a constant coflow velocity is applied above the jet. At the inlet, fuel and oxidizer enter the domain separately in a non-premixed manner. The turbulent structures of the velocity field show the common streaky patterns near the wall, while a somewhat patchy or spotty pattern is observed for the scalars and the reaction rate fluctuations in the near-wall region. The reaction mainly occurs in the upper shear layer in thin highly convoluted reaction zones, but it also takes place close to the wall. Analysis of turbulence and reaction statistics confirms the observations in the instantaneous snapshots, regarding the intermittent character of the reaction rate near the wall. A detailed study of the probability density functions of the reacting scalars and comparison to that of the passive scalar throughout the domain reveals the significance of the reaction influence as well as the wall effects on the scalar distributions. The higher order moments of both the velocities and the scalar concentrations are analyzed and show a satisfactory agreement with experiments. The simulations show that the reaction can both enhance and reduce the dissipation of fuel scalar, since there are two competing effects; on the one hand, the reaction causes sharper scalar gradients and thus a higher dissipation rate, on the other hand, the reaction consumes the fuel scalar thereby reducing the scalar dissipation.

Keyword
boundary layer turbulence, chemically reactive flow, compressible flow, flow simulation, fluctuations, jets, Mach number, numerical analysis, pattern formation, shear turbulence, subsonic flow
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-39188 (URN)10.1063/1.3622774 (DOI)000294483500032 ()2-s2.0-80052338449 (Scopus ID)
Funder
Swedish Research Council, 621-2007-4232
Note

QC 20110908

Available from: 2011-09-08 Created: 2011-09-08 Last updated: 2017-12-08Bibliographically approved
2. Heat release effects on mixing scales of turbulent reacting wall-jets: a direct numerical simulation study
Open this publication in new window or tab >>Heat release effects on mixing scales of turbulent reacting wall-jets: a direct numerical simulation study
(English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-39191 (URN)
Note
QS 2011Available from: 2011-09-08 Created: 2011-09-08 Last updated: 2011-09-08Bibliographically approved
3. Numerical investigation of wall heat transfer and skin-friction coefficient in reacting turbulent wall-jets
Open this publication in new window or tab >>Numerical investigation of wall heat transfer and skin-friction coefficient in reacting turbulent wall-jets
(English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-39194 (URN)
Note
QS 2011Available from: 2011-09-08 Created: 2011-09-08 Last updated: 2011-09-08Bibliographically approved

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