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Numerical studies of turbulent flames in wall-jet flows
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
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 [en]
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: urn:nbn:se:kth:diva-160609ISBN: 978-91-7595-470-7 (print)OAI: oai:DiVA.org:kth-160609DiVA: diva2:790530
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
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 non-premixed turbulent wall-jets: A direct numerical simulation study
Open this publication in new window or tab >>Heat release effects on mixing scales of non-premixed turbulent wall-jets: A direct numerical simulation study
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.

Keyword
Turbulent wall-jet, Non-premixed combustion, Heat-release effects, Direct numerical simulation, Energy spectra
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-122335 (URN)10.1016/j.ijheatfluidflow.2012.12.005 (DOI)000317326600006 ()2-s2.0-84875072425 (Scopus ID)
Funder
Swedish Research Council, 621-2007
Note

QC 20130522

Available from: 2013-05-22 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved
3. Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets
Open this publication in new window or tab >>Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets
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
Keyword
Turbulent wall jet flow, reacting flow, combustion, local isotropy
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-160602 (URN)10.1063/1.4906370 (DOI)000350551300028 ()2-s2.0-84923814961 (Scopus ID)
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
4. DNS analysis of wall heat transfer and combustion regimes in a turbulent nonpremixed wall-jet flame
Open this publication in new window or tab >>DNS analysis of wall heat transfer and combustion regimes in a turbulent nonpremixed wall-jet flame
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Understanding the heat-release effects on the wall heat transfer in turbulent reacting flows, i.e. heat transfer with or without significant density variation, is essential for a wide variety of industrial flows, especially combustion problems. The present study focuses on the wall heat transfer and the near-wall reaction characteristics. The heat-release effects on the wall heat transfer and skin friction coefficients are investigated using three-dimensional direct numerical simulations of a turbulent reacting wall-jet flow with and without heat release. Reductions in the skin-friction coefficient are observed in the exothermic case, compared to the isothermal one, and the underlying mechanism is explained. The absolute wall heat flux also increases, while the corresponding Nusselt number decreases with increasing heat release. Furthermore, the wall effects on the near-wall average burning rate are assessed. It is found that the isothermal cold wall results in an appreciable decrease of the burning rate in the exothermic cases. We observed indications that the wall increases the chances for the development of the premixed mode and its occurrence is very fast in the wall normal direction.

National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-160605 (URN)
Note

QS 2015

Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2015-02-25Bibliographically approved
5. Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames
Open this publication in new window or tab >>Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames
(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
Subgrid-scale stress, turbulent wall-jet flow, combustion, heat release
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-160604 (URN)
Note

QC 20160524

Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2016-05-24Bibliographically approved
6. Reynolds Number Effects on Statistics and Structure of an Isothermal Reacting Turbulent Wall-Jet
Open this publication in new window or tab >>Reynolds Number Effects on Statistics and Structure of an Isothermal Reacting Turbulent Wall-Jet
2014 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 92, no 4, 931-945 p.Article in journal (Refereed) Published
Abstract [en]

Three-dimensional direct numerical simulation (DNS) is used to investigate the effects of changing the Reynolds number on dynamics of a reacting turbulent wall-jet. The flow is compressible and a single-step isothermal global reaction is considered. At the inlet, fuel and oxidizer enter the domain separately in a non-premixed manner. In this study, the bulk Reynolds number of the flow, in terms of the inlet quantities, varies from Re = 2000 to Re = 6000, which results in a comparable change in friction Reynolds numbers. The DNS database in Pouransari et al. (Phys. Fluids 23(085104), 2011) is used for the lower Reynolds number case and for the higher Reynolds number case, a new DNS is performed. One of the main objectives of this study is to compare the influences of changing the Reynolds number of the isothermal flow with the heat-release effects caused by the chemical reaction, that we studied earlier in Pouransari et al. (Int. J. Heat Fluid Flows 40, 65-80, 2013). While, both turbulent and flame structures become finer at the higher Reynolds number, the effect of decreasing the Reynolds number and adding the combustion heat release are compared with each other and found to be similar for some aspects of the flow, but are not always the same.

Keyword
Reynolds number effects, Turbulent, Combustion, Mixing scales, Wall-jet
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-147028 (URN)10.1007/s10494-014-9539-3 (DOI)000336310800006 ()2-s2.0-84901300211 (Scopus ID)
Note

QC 20140624

Available from: 2014-06-24 Created: 2014-06-23 Last updated: 2017-12-05Bibliographically approved

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