Turbulent Flame Structure and Dynamics: Combustion Regimes

Abstract Carbon monoxide (CO) and hydroxyl (OH) are the two key intermediate species formed during chemical reactions inside gas turbine combustors. Spatiotemporal information and a detailed understanding of CO formation in the reaction zone are important during the combustion processes as a major part of the heat release is obtained from the oxidation of CO to CO2. Turbulent flame structures and reaction zone in different flame conditions can also be visualized through the spatial distribution profiles of OH. In the current study, both these species are excited simultaneously using a single ultrashort, broad spectral bandwidth of approximately 100-femtosecond (fs) duration laser pulse at λ = 283.8 nm. Subsequent fluorescence signals are separated through spectral filters of appropriate bandwidth and imaged using two cameras. This present study was performed in a McKenna flat-flame burner with ethylene/air as a pilot flame and non-premixed turbulent ethylene jet at the center. The partial spectral overlap of CO–X (4,0) and OH A–X (1,0) transitions are utilized for simultaneous excitation, thereby characterize the overall flame structure (via OH) and regions of oxidation reactions (via CO) in a range of flame conditions. Besides, CO and OH profiles follow the trends obtained from model predictions for a range of equivalence ratios in ethylene/air flames stabilized over the Hencken calibration burner. These results are used for obtaining quantitative calibrations of CO and OH signals. Overall, the present study extends the applicability of a single, broadband fs laser pulse for simultaneous imaging of multiple chemical species in flame.

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

Flow Turbulence and Combustion ◽

10.1007/s10494-020-00228-9 ◽

2020 ◽

Author(s):

Thorsten Zirwes ◽

Feichi Zhang ◽

Peter Habisreuther ◽

Maximilian Hansinger ◽

Henning Bockhorn ◽

...

Keyword(s):

Turbulent Flame ◽

Detailed Chemistry ◽

Partially Premixed Combustion ◽

Processing Step ◽

Different Types ◽

Partially Premixed ◽

Definition Of ◽

And Diffusion ◽

Combustion Regimes ◽

Term Analysis

Abstract 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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Validation of a Turbulent Flame Speed Model across Combustion Regimes

Combustion Science and Technology ◽

10.1080/00102200903341587 ◽

2010 ◽

Vol 182 (3) ◽

pp. 284-308 ◽

Cited By ~ 44

Author(s):

H. Kolla ◽

J. W. Rogerson ◽

N. Swaminathan

Keyword(s):

Turbulent Flame ◽

Flame Speed ◽

Turbulent Flame Speed ◽

Combustion Regimes

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Direct Numerical Simulations of the Impact of High Turbulence Intensities and Volume Viscosity on Premixed Methane Flames

Journal of Combustion ◽

10.1155/2011/746719 ◽

2011 ◽

Vol 2011 ◽

pp. 1-12 ◽

Cited By ~ 4

Author(s):

Gordon Fru ◽

Gábor Janiga ◽

Dominique Thévenin

Keyword(s):

Numerical Simulations ◽

Heat Release ◽

Turbulent Flame ◽

Flame Structure ◽

Turbulent Flames ◽

Direct Numerical Simulations ◽

Turbulent Reynolds Number ◽

Volume Viscosity ◽

Flame Structures ◽

The Impact

Parametric direct numerical simulations (DNS) of turbulent premixed flames burning methane in the thin reaction zone regime have been performed relying on complex physicochemical models and taking into account volume viscosity (κ). The combined effect of increasing turbulence intensities (u′) andκon the resulting flame structure is investigated. The turbulent flame structure is marred with numerous perforations and edge flame structures appearing within the burnt gas mixture at various locations, shapes and sizes. Stepping upu′from 3 to 12 m/s leads to an increase in the scaled integrated heat release rate from 2 to 16. This illustrates the interest of combustion in a highly turbulent medium in order to obtain high volumetric heat release rates in compact burners. Flame thickening is observed to be predominant at high turbulent Reynolds number. Via ensemble averaging, it is shown that both laminar and turbulent flame structures are not modified byκ. These findings are in opposition to previous observations for flames burning hydrogen, where significant modifications induced byκwere found for both the local and global properties of turbulent flames. Therefore, to save computational resources, we suggest that the volume viscosity transport term be ignored for turbulent combustion DNS at low Mach numbers when burning hydrocarbon fuels.

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Flame Structure and Stabilization Mechanisms in a Stagnation-Point Reverse-Flow Combustor

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.2836614 ◽

2008 ◽

Vol 130 (3) ◽

Cited By ~ 24

Author(s):

Mohan K. Bobba ◽

Priya Gopalakrishnan ◽

Karthik Periagaram ◽

Jerry M. Seitzman

Keyword(s):

Stagnation Point ◽

Reverse Flow ◽

Turbulent Flame ◽

Flame Structure ◽

Reaction Rates ◽

Diagnostic Techniques ◽

Flame Stabilization ◽

Oh Radicals ◽

Spontaneous Raman Scattering ◽

High Turbulence

A novel combustor design, referred to as a stagnation-point reverse-flow (SPRF) combustor, was recently developed to overcome the stability issues encountered with most lean premixed combustion systems. The SPRF combustor is able to operate stably at very lean fuel-air mixtures with low NOx emissions. The reverse flow configuration causes the flow to stagnate and hot products to reverse and leave the combustor. The highly turbulent stagnation zone and internal recirculation of hot product gases facilitates robust flame stabilization in the SPRF combustor at very lean conditions over a range of loadings. Various optical diagnostic techniques are employed to investigate the flame characteristics of a SPRF combustor operating with premixed natural gas and air at atmospheric pressure. These include simultaneous planar laser-induced fluorescence imaging of OH radicals and chemiluminescence imaging, and spontaneous Raman scattering. The results indicate that the combustor has two stabilization regions, with the primary region downstream of the injector where there are low average velocities and high turbulence levels where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product recirculation levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on the chemical time scales and the flame structure is quantified using simple reactor models. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zone regime throughout the combustor. The flame tends to become more flameletlike, however, for increasing distance from the injector.

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Flame Structure and Stabilization Mechanisms in a Stagnation Point Reverse Flow Combustor

Volume 2: Turbo Expo 2007 ◽

10.1115/gt2007-28231 ◽

2007 ◽

Cited By ~ 11

Author(s):

Mohan K. Bobba ◽

Priya Gopalakrishnan ◽

Karthik Periagaram ◽

Jerry M. Seitzman

Keyword(s):

Stagnation Point ◽

Reverse Flow ◽

Turbulent Flame ◽

Flame Structure ◽

Reaction Rates ◽

Diagnostic Techniques ◽

Flame Stabilization ◽

Oh Radicals ◽

Spontaneous Raman Scattering ◽

High Turbulence

A novel combustor design, referred to as a Stagnation Point Reverse Flow (SPRF) combustor, was recently developed to overcome the stability issues encountered with most lean premixed combustion systems. The SPRF combustor is able to operate stably at very lean fuel-air mixtures with low NOx emissions. The reverse flow configuration causes the flow to stagnate and hot products to reverse and leave the combustor. The highly turbulent stagnation zone and internal recirculation of hot product gases facilitates robust flame stabilization in the SPRF combustor at very lean conditions over a range of loadings. Various optical diagnostic techniques are employed to investigate the flame characteristics of a SPRF combustor operating with premixed natural gas and air at atmospheric pressure. These include simultaneous Planar Laser-Induced Fluorescence (PLIF) imaging of OH radicals, chemiluminescence imaging, Spontaneous Raman Scattering. The results indicate that the combustor has two stabilization regions, with the primary region downstream of the injector where there are low average velocities and high turbulence levels where most of the heat release occurs. High turbulence level in the shear layers lead to increased product recirculation levels, elevating the reaction rates and thereby, the combustor stability. The effect of product entrainment on the chemical timescales and the flame structure is quantified using simple reactor models. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor. The flame tends to become more flamelet like, however, for increasing distance from the injector.

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