Analysis of Markers for Combustion Mode and Heat Release in MILD Combustion Using DNS Data

Doan, Nguyen Anh Khoa; Swaminathan, Nedunchezhian

doi:10.1080/00102202.2019.1610746

Cited by 20 publications

(16 citation statements)

References 44 publications

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“…Another approach uses the chemical explosive mode analysis (CEMA) to differentiate between premixed propagation and diffusion flame characteristics. For example, compared the flame index to the chemical modes in a turbulent hydrogen flame and Doan and Swaminathan (2019) compared the regime prediction of the flame index with the chemical mode in MILD combustion and concluded that the CEMA based approach identified large parts of the flame as premixed whereas the flame index identified a non-premixed regime. Nordin-Bates et al (2017) applied a similar concept to a scram jet flame.…”

Section: Introductionmentioning

confidence: 99%

Identification of Flame Regimes in Partially Premixed Combustion from a Quasi-DNS Dataset

Zirwes

Zhang

Habisreuther

et al. 2020

Flow Turbulence Combust

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Identifying combustion regimes in terms of premixed and non-premixed characteristics is an important task for understanding combustion phenomena and the structure of flames. A quasi-DNS database of the compositionally inhomogeneous partially premixed Sydney/Sandia flame in configuration FJ-5GP-Lr75-57 is used to directly compare different types of flame regime markers from literature. In the simulation of the flame, detailed chemistry and diffusion models are utilized and no turbulence and combustion models are used as the flame front and flow are fully resolved near the nozzle. This allows evaluating the regime markers as a post-processing step without modeling assumptions and directly comparing regime markers based on gradient alignment, drift term analysis and gradient free regime identification. The goal is not to find the correct regime marker, which might be impossible due to the different set of assumptions of every marker and the generally vague definition of the partially premixed regime itself, but to compare their behavior when applied to a resolved turbulent flame with partially premixed characteristics.

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Section: Introductionmentioning

confidence: 99%

Identification of Flame Regimes in Partially Premixed Combustion from a Quasi-DNS Dataset

Zirwes

Zhang

Habisreuther

et al. 2020

Flow Turbulence Combust

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“…They concluded that the appearance of a distributed combustion is due to the interaction of thin reaction zones [19][20][21][22][23], which were also reported based on OH-PLIF visualisations [4,10,14,15]. These analyses have been further extended by Doan, Swaminathan and co-workers for stratified-mixture MILD combustion [24][25][26] and provided important insights into the flame structure [24,25], reaction-zone topology [24,25] and markers of combustion mode [26]. A review of the physical insights gained from DNS can be found in [27].…”

Section: Introductionmentioning

confidence: 63%

“…Several Reynolds averaged Navier-Stokes (RANS) and large eddy simulations (LES) investigations of MILD combustion using conventional flamelet and eddy dissipation approaches [8,9,13,16,17] have also been conducted which reported good agreement with experimental data in terms of mean velocity and temperature fields, but discrepancies have been reported in terms of peak temperature and minor species (e.g., CO and OH) concentrations [8,12]. The advancements in high-performance computing have enabled direct numerical simulations (DNS) of MILD combustion [18][19][20][21][22][23][24][25][26][27]. Van Oijen [18] carried out DNS of autoigniting mixing layers of CH 4 /H 2 fuel-blends and provided important insights to the roles of non-unity Lewis number and O 2 dilution on autoignition delay under MILD combustion conditions.…”

Section: Introductionmentioning

confidence: 98%

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

et al. 2021

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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.

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“…The image shown on the left is from the experimental studies of Scherrer et al (2016) and the one on the right is from the DNS case NP3. The numerical Schlieren, obtained as explained by Doan and Swaminathan (2019a), should be compared qualitatively to region marked as "Combustion" in the experimental image and the later image also shows shock waves. The similarities between these two images are quite interesting and offer support to the above deduction.…”

Section: Relevance To Supersonic Combustionmentioning

confidence: 98%

“…The OH-PLIF (planar laser-induced fluorescence) commonly used for the combustion diagnostics will pick the signals, coming from mixtures with low heat release, corresponding to the right peak and is likely to miss the signals from regions with large heat release rate since Y OH is almost the same as the background value, Y c OH . Thus, one needs extra care for studying MILD combustion using PLIF techniques (Doan and Swaminathan, 2019a).…”

Section: Mild Combustion Inceptionmentioning

confidence: 99%

Physical Insights on MILD Combustion From DNS

Swaminathan

2019

Front. Mech. Eng.

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MILD combustion is gaining interest in recent times because it is attractive for green combustion technology. However, its fundamental aspects are not well-understood. Recent progresses made on this topic using direct numerical simulation data are presented and discussed in a broader perspective. It is shown that a revised theory involving at least two chemical timescales is required to describe the inception of this combustion not only showing both autoignition and flame characteristics but also a strong interaction between these two phenomena. The reaction zones have complex morphological and topological features and the most probable shape is pancake-like structure implying micro-volume combustion under MILD conditions unlike the sheet-combustion in conventional cases. Relevance of the MILD (micro-volume) combustion to supersonic combustion is explored theoretically and qualitative support is shown and discussed using experimental and numerical Schlieren images.

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Analysis of Markers for Combustion Mode and Heat Release in MILD Combustion Using DNS Data

Cited by 20 publications

References 44 publications

Identification of Flame Regimes in Partially Premixed Combustion from a Quasi-DNS Dataset

Identification of Flame Regimes in Partially Premixed Combustion from a Quasi-DNS Dataset

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

Physical Insights on MILD Combustion From DNS

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