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Modeling and Analysis of Turbulent Premixed Combustion

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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "B: Energy and Environment".

Deadline for manuscript submissions: closed (30 September 2021) | Viewed by 10735

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Prof. Dr. Nilanjan Chakraborty

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Guest Editor

School of Engineering, Newcastle University, Newcastle-upon-Tyne NE1 7RU, UK
Interests: computational fluid dynamics (CFD); turbulent combustion; fluid turbulence; heat transfer
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Prof. Dr. Markus Klein

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Guest Editor

Department of Aerospace Engineering, Institute for Applied Mathematics and Scientific Computing, University of the Bundeswehr Munich, 85577 Neubiberg, Germany
Interests: turbulent combustion; multiphase flow; reactive flow; aerodynamics; supersonic flows; gas explosions; computational fluid dynamics (CFD); numerical methods
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Dear Colleagues,
Modern combustion devices for power generation and propulsion purposes need to be simultaneously energy-efficient and environmentally friendly. This has increased the importance of premixed combustion, because thermal NOx formation can be controlled by homogeneously mixing fuel and oxidiser before the combustion process. In industrial applications, premixed combustion often takes place under turbulent conditions and in turbulent premixed flames; the underlying fluid turbulence is significantly affected by the thermal expansion induced by heat release from exothermic chemical reactions. This close coupling between fluid-dynamic and chemical processes poses a major challenge in the simulation and modelling of turbulent premixed combustion. These aspects become increasingly important in the presence of thermo-diffusive instability in the case of lean high hydrogen content (HHC) fuels, which are identified as being alternative fuels for power generation and propulsion. Moreover, premixed combustion in industrial applications often takes place under elevated pressures, where hydrodynamic instabilities (e.g. Darrieus–Landau instability) are more likely to occur due to a large scale separation between the integral length scale and flame thickness, and this increased scale separation for high-pressure premixed combustion also makes the sub-grid scale modelling, in the context of LES, a challenging task. These challenges are exacerbated further when hydrodynamic and thermo-diffusive instabilities interact with each other under elevated pressures. All of the aforementioned challenges make the analysis and modelling of turbulent premixed combustion a topic of significant intellectual and industrial interest. Therefore, we invite high quality original analytical, experimental and numerical contributions, and technical reviews for this Special Issue, which is expected to contribute to the analysis and modelling of turbulent premixed combustion—something which is urgently required in the interests of global challenges of energy economy and environmental safety.

Prof. Dr. Nilanjan Chakraborty
Prof. Dr. Markus Klein
Guest Editors

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Keywords

premixed combustion
turbulent premixed flames
direct numerical simulations
large eddy simulations
experimental diagnostics

Published Papers (6 papers)

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Research

28 pages, 17385 KiB

Open AccessFeature PaperArticle

Comparison of the Reactive Scalar Gradient Evolution between Homogeneous MILD Combustion and Premixed Turbulent Flames

by Hazem S.A.M. Awad, Khalil Abo-Amsha, Umair Ahmed and Nilanjan Chakraborty

Energies 2021, 14(22), 7677; https://doi.org/10.3390/en14227677 - 16 Nov 2021

Cited by 7 | Viewed by 1683

Abstract

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technique that can simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the differences in statistical behaviours of the surface density function (SDF = magnitude of the reaction progress variable gradient) between conventional premixed flames and exhaust gas recirculation (EGR) type homogeneous-mixture combustion under MILD conditions using direct numerical simulations (DNS) data. The mean values of the SDF in the MILD combustion cases were found to be significantly smaller than those in the corresponding premixed flame cases. Moreover, the mean behaviour of the SDF in response to the variations of turbulence intensity were compared between MILD and premixed flame cases, and the differences are explained in terms of the strain rates induced by fluid motion and the ones arising from flame displacement speed. It was found that the effects of dilatation rate were much weaker in the MILD combustion cases than in the premixed flame cases, and the reactive scalar gradient in MILD combustion cases preferentially aligns with the most compressive principal strain-rate eigendirection. By contrast, the reactive scalar gradient preferentially aligned with the most extensive principal strain-rate eigendirection within the flame in the premixed flame cases considered here, but the extent of this alignment weakened with increasing turbulence intensity. This gave rise to a predominantly positive mean value of normal strain rate in the premixed flames, whereas the mean normal strain rate remained negative, and its magnitude increased with increasing turbulence intensity in the MILD combustion cases. The mean value of the reaction component of displacement speed assumed non-negligible values in the MILD combustion cases for a broader range of reaction progress variable, compared with the conventional premixed flames. Moreover, the mean displacement speed increased from the unburned gas side to the burned gas side in the conventional premixed flames, whereas the mean displacement speed in MILD combustion cases decreased from the unburned gas side to the middle of the flame before increasing mildly towards the burned gas side. These differences in the mean displacement speed gave rise to significant differences in the mean behaviour of the normal strain rate induced by the flame propagation and effective strain rate, which explains the differences in the SDF evolution and its response to the variation of turbulence intensity between the conventional premixed flames and MILD combustion cases. The tangential fluid-dynamic strain rate assumed positive mean values, but it was overcome by negative mean values of curvature stretch rate to yield negative mean values of stretch rate for both the premixed flames and MILD combustion cases. This behaviour is explained in terms of the curvature dependence of displacement speed. These findings suggest that the curvature dependence of displacement speed and the scalar gradient alignment with local principal strain rate eigendirections need to be addressed for modelling EGR-type homogeneous-mixture MILD combustion. Full article

(This article belongs to the Special Issue Modeling and Analysis of Turbulent Premixed Combustion)

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21 pages, 4806 KiB

Open AccessArticle

Analysis of Local Exergy Losses in Combustion Systems Using a Hybrid Filtered Eulerian Stochastic Field Coupled with Detailed Chemistry Tabulation: Cases of Flames D and E

by Senda Agrebi, Louis Dreßler, Hendrik Nicolai, Florian Ries, Kaushal Nishad and Amsini Sadiki

Energies 2021, 14(19), 6315; https://doi.org/10.3390/en14196315 - 03 Oct 2021

Cited by 5 | Viewed by 2052

Abstract

A second law analysis in combustion systems is performed along with an exergy loss study by quantifying the entropy generation sources using, for the first time, three different approaches: a classical-thermodynamics-based approach, a novel turbulence-based method and a look-up-table-based approach, respectively. The numerical computation is based on a hybrid filtered Eulerian stochastic field (ESF) method coupled with tabulated detailed chemistry according to a Famelet-Generated Manifold (FGM)-based combustion model. In this work, the capability of the three approaches to capture the effect of the Re number on local exergy losses is especially appraised. For this purpose, Sandia flames D and E are selected as application cases. First, the validation of the computed flow and scalar fields is achieved by comparison to available experimental data. For both flames, the flow field results for eight stochastic fields and the associated scalar fields show an excellent agreement. The ESF method reproduces all major features of the flames at a lower numerical cost. Next, the second law analysis carried out with the different approaches for the entropy generation computation provides comparable quantitative results. Using flame D as a reference, for which some results with the thermodynamic-based approach exist in the literature, it turns out that, among the sources of exergy loss, the heat transfer and the chemical reaction emerge notably as the main culprits for entropy production, causing 50% and 35% of it, respectively. This fact-finding increases in Sandia flame E, which features a high Re number compared to Sandia flame D. The computational cost is less once the entropy generation analysis is carried out by using the Large Eddy Simulation (LES) hybrid ESF/FGM approach together with the look-up-table-based or turbulence-based approach. Full article

(This article belongs to the Special Issue Modeling and Analysis of Turbulent Premixed Combustion)

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32 pages, 73152 KiB

Open AccessArticle

Comparison of Flame Propagation Statistics Based on Direct Numerical Simulation of Simple and Detailed Chemistry. Part 2: Influence of Choice of Reaction Progress Variable

by Felix B. Keil, Marvin Amzehnhoff, Umair Ahmed, Nilanjan Chakraborty and Markus Klein

Energies 2021, 14(18), 5695; https://doi.org/10.3390/en14185695 - 10 Sep 2021

Cited by 14 | Viewed by 1109

Abstract

Flame propagation statistics for turbulent, statistically planar premixed flames obtained from 3D Direct Numerical Simulations using both simple and detailed chemistry have been evaluated and compared to each other. To achieve this, a new database has been established encompassing five different conditions on the turbulent combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. The discussion includes interdependencies of displacement speed and its individual components as well as surface density function (i.e., magnitude of the reaction progress variable) with tangential strain rate and curvature. For the analysis of detailed chemistry Direct Numerical Simulation data, three different definitions of reaction progress variable, based on

{CH}_{4}, H_{2} O

and

O_{2}

mass fractions will be used. While the displacement speed statistics remain qualitatively and to a large extent quantitatively similar for simple chemistry and detailed chemistry, there are pronounced differences for its individual contributions which to a large extent depend on the definition of reaction progress variable as well as on the chosen isosurface level. It is concluded that, while detailed chemistry simulations provide more detailed information about the flame structure, the choice of the reaction progress variable definition and the choice of the resulting isosurface give rise to considerable uncertainty in the interpretation of displacement speed statistics, sometimes even showing opposing trends. Simple chemistry simulations are shown to provide (a) the global flame propagation statistics which are qualitatively similar to the corresponding results from detailed chemistry simulations, (b) remove the uncertainties with respect to the choice of reaction progress variable, and (c) are more straightforward to compare with theoretical analysis or model assumptions that are mostly based on simple chemistry assumptions. Full article

(This article belongs to the Special Issue Modeling and Analysis of Turbulent Premixed Combustion)

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18 pages, 5692 KiB

Open AccessArticle

Comparison of Flame Propagation Statistics Extracted from Direct Numerical Simulation Based on Simple and Detailed Chemistry—Part 1: Fundamental Flame Turbulence Interaction

by Felix Benjamin Keil, Marvin Amzehnhoff, Umair Ahmed, Nilanjan Chakraborty and Markus Klein

Energies 2021, 14(17), 5548; https://doi.org/10.3390/en14175548 - 05 Sep 2021

Cited by 20 | Viewed by 1463

Abstract

In the present study, flame propagation statistics from turbulent statistically planar premixed flames obtained from simple and detailed chemistry, three-dimensional Direct Numerical Simulations, were evaluated and compared to each other. To this end, a new database was established encompassing five different conditions on the turbulent premixed combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. A detailed discussion of the advantages and limitations of both approaches is provided, including the difference in carbon footprint for establishing the database. It is shown that displacement speed statistics and their interrelation with curvature and tangential strain rate are in very good qualitative and reasonably good quantitative agreement between simple and detailed chemistry Direct Numerical Simulations. Hence, it is concluded that simple chemistry simulations should retain their importance for future combustion research, and the environmental impact of high-performance computing methods should be carefully chosen in relation to the goals to be achieved. Full article

(This article belongs to the Special Issue Modeling and Analysis of Turbulent Premixed Combustion)

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10 pages, 261 KiB

Open AccessArticle

Passive Front Propagation in Intense Turbulence: Early Transient and Late Statistically Stationary Stages of the Front Area Evolution

by Vladimir A. Sabelnikov and Andrei N. Lipatnikov

Energies 2021, 14(16), 5102; https://doi.org/10.3390/en14165102 - 19 Aug 2021

Cited by 1 | Viewed by 1119

Abstract

The influence of statistically stationary, homogeneous isotropic turbulence (i) on the mean area of a passive front propagating in a constant-density fluid and, hence, (ii) on the mean fluid consumption velocity

{\bar{u}}_{T}

is explored, particularly in the case of an asymptotically [...] Read more.

{\bar{u}}_{T}

is explored, particularly in the case of an asymptotically high turbulent Reynolds number, and an asymptotically high ratio of the Kolmogorov velocity to a constant speed

u_{0}

of the front. First, a short early transient stage is analyzed by assuming that the front remains close to a material surface that coincides with the front at the initial instant. Therefore, similarly to a material surface, the front area grows exponentially with time. This stage, whose duration is much less than an integral time scale of the turbulent flow, is argued to come to an end once the volume of fluid consumed by the front is equal to the volume embraced due to the turbulent dispersion of the front. The mean fluid consumption velocity averaged over this stage is shown to be proportional to the rms turbulent velocity

u^{'}

. Second, a late statistically stationary regime of the front evolution is studied. A new length scale characterizing the smallest wrinkles of the front surface is introduced. Since this length scale is smaller than the Kolmogorov length scale

η_{K}

under conditions of the present study, the front is hypothesized to be a bifractal with two different fractal dimensions for wrinkles larger and smaller than

η_{K}

. Finally, a simple scaling of

{\bar{u}}_{T} \propto u^{'}

is obtained for this late stage as well. Full article

(This article belongs to the Special Issue Modeling and Analysis of Turbulent Premixed Combustion)

17 pages, 2422 KiB

Open AccessFeature PaperArticle

LES Analysis of CO Emissions from a High Pressure Siemens Gas Turbine Prototype Combustor at Part Load

by Pascal Gruhlke, Christian Beck, Bertram Janus and Andreas M. Kempf

Energies 2020, 13(21), 5751; https://doi.org/10.3390/en13215751 - 03 Nov 2020

Cited by 1 | Viewed by 2208

Abstract

This work contributes to the understanding of mechanisms that lead to increased carbon monoxide (CO) concentrations in gas turbine combustion systems. Large-eddy simulations (LES) of a full scale high pressure prototype Siemens gas turbine combustor at three staged part load operating conditions are presented, demonstrating the ability to predict carbon monoxide pollutants from a complex technical system by investigating sources of incomplete CO oxidation. Analytically reduced chemistry is applied for the accurate pollutant prediction together with the dynamic thickened flame model. LES results show that carbon monoxide emissions at the probe location are predicted in good agreement with the available test data, indicating two operating points with moderate pollutant levels and one operating point with CO concentrations below 10 ppm. Large mixture inhomogeneities are identified in the combustion chamber for all operating points. The investigation of mixture formation indicates that fuel-rich mixtures mainly emerge from the pilot stage resulting in high equivalence ratio streaks that lead to large CO levels at the combustor outlet. Flame quenching due to flame-wall-interaction are found to be of no relevance for CO in the investigated combustion chamber. Post-processing with Lagrangian tracer particles shows that cold air—from effusion cooling or stages that are not being supplied with fuel—lead to significant flame quenching, as mixtures are shifted to leaner equivalence ratios and the oxidation of CO is inhibited. Full article

(This article belongs to the Special Issue Modeling and Analysis of Turbulent Premixed Combustion)

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