Classification and lift-off height prediction of non-premixed MILD and autoignitive flames

Evans, Michael J.; Medwell, Paul R.; Wu, Hao; Stagni, Alessandro; Ihme, Matthias

doi:10.1016/j.proci.2016.06.013

Cited by 39 publications

(56 citation statements)

References 25 publications

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“…The Rayleigh-Raman diagnostic technique has been extended to one-dimensional data measurements, and extensive measurements of piloted jet flames have been performed to quantify scalar dissipation, length scales and flame-sheet orientation (Karpetis and Barlow, 2002;Barlow et al, 2005;Magnotti and Barlow, 2017), these have not been performed on flames in hot and diluted coflows. This is despite strong coupling between the burning state and heat release profiles of such flames to scalar dissipation rate (Oberlack et al, 2000;Özdemir and Peters, 2001;Ihme et al, 2012;Evans et al, 2016b;Ye et al, 2016Ye et al, , 2017.…”

Section: Measurements Of Temperature and Speciesmentioning

confidence: 97%

“…This conclusion has been further supported by direct numerical simulations (DNS) and transient flamelet modeling (Mastorakos, 2009). This technique has not, however, been performed under MILD combustion conditions in flames which appear visibly lifted or exhibit a "weak-to-strong transition" in a continuous flame-front (Medwell et al, 2008(Medwell et al, , 2016Medwell and Dally, 2012a;Evans et al, 2016b;Ye et al, 2017). The Rayleigh-Raman diagnostic technique has been extended to one-dimensional data measurements, and extensive measurements of piloted jet flames have been performed to quantify scalar dissipation, length scales and flame-sheet orientation (Karpetis and Barlow, 2002;Barlow et al, 2005;Magnotti and Barlow, 2017), these have not been performed on flames in hot and diluted coflows.…”

Section: Measurements Of Temperature and Speciesmentioning

confidence: 99%

“…Images of the OH radical may be interpreted as representing the lean side of the reaction-zone. Consequently, OH-PLIF can provide indications of flame stabilization mechanisms, such as the formation and growth of isolated ignition kernels (Gordon et al, 2008;Oldenhof et al, 2011), triple flame bases (Gordon et al, 2008) or a "weak-to-strong transition" in a continuous flame-front (Medwell et al, 2008(Medwell et al, , 2016Medwell and Dally, 2012a;Evans et al, 2016b;Ye et al, 2017), which may each be seen in Figure 5. The occurrence of these structures is largely dependent on the combination of fuel/oxidant composition and temperatures (Medwell et al, 2008(Medwell et al, , 2016Medwell and Dally, 2012a;Evans et al, 2016a,b), which dictates both the stoichiometric and most-reactive mixture fractions, and the underlying flow-field, which governs the local strain-field between the fuel and oxidant streams (Ye et al, 2016).…”

Section: Reaction-zone Imaging In Understanding Flame Stabilizationmentioning

confidence: 99%

“…Of these mechanisms, ignition kernel formation and autoignitive triple flames have been examined at significant length, and a full discussion is not provided here (Gordon et al, 2008;Mastorakos, 2009;Yoo et al, 2011;Arndt et al, 2012Arndt et al, , 2019Sidey and Mastorakos, 2015;Macfarlane et al, 2017Macfarlane et al, , 2018Macfarlane et al, , 2019Ramachandran et al, 2019). Observations from OH-PLIF (Medwell et al, 2008), and resulting numerical studies (Medwell et al, 2009b;Evans et al, 2016bEvans et al, , 2017a suggest that the least studied of these mechanisms-the "weak-to-strong transition"anchors as a weakly reacting diffusion flame close to the jet exit, in ≈ 0.2 (Evans et al, 2016a) under temperature and flowfield conditions almost identical to the laminar coflow stream. These weak diffusion flames allow for the diffusion of O 2 in the fuel stream (in a process which has been termed, "reactionzone weakening" Medwell et al, 2009b) and provide enhanced formation of precursors, such as CH 2 O (Medwell et al, 2009b;Medwell and Dally, 2012b), before a critical "transition point" where thermal run-away occurs (Medwell et al, 2008;Evans et al, 2016b).…”

Section: Reaction-zone Imaging In Understanding Flame Stabilizationmentioning

confidence: 99%

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Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

Evans

Medwell

2019

Front. Mech. Eng.

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There is a wealth of existing experimental data of flames collected using laser diagnostics. The primary objective of this review is to provide context and guidance in interpreting these laser diagnostic data. This educational piece is intended to benefit those new to laser diagnostics or with specialization in other facets of combustion science, such as computational modeling. This review focuses on laser-diagnostics in the context of the commonly used canonical jet-in-hot-coflow (JHC) burner, although the content is applicable to a wide variety of configurations including, but not restricted to, simple jet, bluff body, swirling and stratified flames. The JHC burner configuration has been used for fundamental studies of moderate or intense low oxygen dilution (MILD) combustion, autoignition and flame stabilization in hot environments. These environments emulate sequential combustion or exhaust gas recirculation. The JHC configuration has been applied in several burners for parametric studies of MILD combustion, flame reaction zone structure, behavior of fuels covering a significant range of chemical complexity, and the collection of data for numerical model validation. Studies of unconfined JHC burners using gaseous fuels have employed point-based Rayleigh-Raman or two-dimensional Rayleigh scattering measurements for the temperature field. While the former also provides simultaneous measurements of major species concentrations, the latter has often been used in conjunction with planar laser-induced fluorescence (PLIF) to simultaneously provide quantitative or qualitative measurements of radical and intermediary species. These established scattering-based thermography techniques are not, however, effective in droplet or particle laden flows, or in confined burners with significant background scattering. Techniques including coherent anti-Stokes Raman scattering (CARS) and non-linear excitation regime two-line atomic fluorescence (NTLAF) have, however, been successfully demonstrated in both sooting and spray flames. This review gives an overview of diagnostics techniques undertaken in canonical burners, with the intention of providing an introduction to laser-based measurements in combustion. The efficacy, applicability and accuracy of the experimental techniques are also discussed, with examples from studies of flames in JHC burners. Finally, current and future directions for studies of flames using the JHC configuration including spray flames and studies and elevated pressures are summarized.

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Section: Measurements Of Temperature and Speciesmentioning

confidence: 97%

Section: Measurements Of Temperature and Speciesmentioning

confidence: 99%

Section: Reaction-zone Imaging In Understanding Flame Stabilizationmentioning

confidence: 99%

Section: Reaction-zone Imaging In Understanding Flame Stabilizationmentioning

confidence: 99%

See 2 more Smart Citations

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

Evans

Medwell

2019

Front. Mech. Eng.

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“…It is worth mentioning that a recently presented new MILD combustion definition, based on an equivalent activation energy [69] and further observations [70], suggested that DJHC flames do not fully meet criteria for MILD combustion.…”

Section: The Test Case and Boundary Conditionsmentioning

confidence: 99%

Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Lewandowski

Ertesvåg

2018

Fuel

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Performance of the Eddy Dissipation Concept (EDC) in the regime of Moderate and Intense Low-oxygen Dilution (MILD) combustion is investigated. The special MILD features, where chemical and turbulence time scales are comparable (Damköhler number close to unity), have led several researchers to suggest modifications of EDC, mainly by changing model constants. EDC with standard and modified constants are compared, and the importance of each effect is outlined. Different fine-structure reactor models and their inflow/initial conditions are discussed and found to play a significant role. The reacting fraction of fine structures, which in virtually all other numerical studies is set to unity, is also discussed and found to be important. We observe better agreement with experiment when the reacting fraction is reduced below unity, which is also described by the original EDC. The results obtained with the variable reacting fraction are found to improve both the temperature distributions and the lift-off height predictions. The calculations are carried out with the use of open source software OpenFOAM. The main test case was the Delft Jet-in-Hot-Coflow burner emulating MILD regime at three different flow conditions (jet Reynolds numbers of 2500, 4100 and 8800).

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Influence of Boundary Conditions on the Flame Stabilization Mechanism and on Transient Auto-ignition in the DLR Jet-in-Hot-Coflow Burner

Arndt

Meier

2018

Flow Turbulence Combust

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Transient auto-ignition is a key factor for flame stabilization and flame initialization in several technical combustion systems such as internal combustion engines or gas turbine combustors. Reliable numerical simulations of auto-ignition stabilized flames are important for the development of new combustor systems. For detailed model validation, knowledge of the sensitivity of different system response quantities (SRQs) of interest to the boundary conditions in combination with the accuracy of boundary conditions is essential, especially with respect to uncertainty quantification of numerical simulations. In the current study, the flame stabilization and auto-ignition in the DLR Jet-in-Hot-Coflow burner was examined experimentally using high-speed OH* chemiluminescence. Here, methane was either injected continuously to study the flame stabilization mechanism of steady state lifted jet flames, or in a pulsed manner to study the formation of auto-ignition kernels, into the hot exhaust gas of a lean, premixed hydrogen/air flame. The flame stabilization height, and the location and time of initial auto-ignition kernels for a case with transient auto-ignition were evaluated with respect to several boundary conditions, such as coflow temperature as well as coflow-and jet-velocity. A relative sensitivity of the measured SRQs on the boundary conditions was introduced in order to quantitatively compare steady state flame to transient auto-ignition characteristics and to assess the quantitative influence of different boundary conditions. Comparison of the auto-ignition dynamics in the steady state and during transient fuel injection allowed assessing the role of auto-ignition in the flame stabilization mechanism for different boundary conditions; accompanying chemical kinetic calculations were used to quantify the influence of strain on auto-ignition and flame propagation for the current conditions, allowing further insight into the flame stabilization mechanism in Jet-in-Hot-Coflow flames.

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Classification and lift-off height prediction of non-premixed MILD and autoignitive flames

Cited by 39 publications

References 25 publications

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Influence of Boundary Conditions on the Flame Stabilization Mechanism and on Transient Auto-ignition in the DLR Jet-in-Hot-Coflow Burner

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