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

2008 ◽  
Vol 130 (3) ◽  
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.

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

2007 ◽  
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.

Characteristics of Combustion Processes in a Stagnation Point Reverse Flow Combustor

2006 ◽  
Author(s):  
M. K. Bobba ◽  
P. Gopalakrishnan ◽  
J. M. Seitzman ◽  
B. T. Zinn
Keyword(s):  
Stagnation Point ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Reverse Flow ◽  
Laser Sheet ◽  
Flame Structure ◽  
Stable Operation ◽  
Oh Radicals ◽  
Combustion Processes ◽  
Hot Products

The performance of dry, low NOx gas turbines, which employ lean premixed (or partially premixed) combustors, is often limited by static and dynamic combustor stability, power density limitations and expensive premixing hardware. To overcome these issues, a novel design, referred to as the Stagnation Point Reverse Flow (SPRF) combustor, has recently been developed. Various optical diagnostic techniques are employed here to elucidate the combustion processes in this novel combustor. These include simultaneous planar laser-induced fluorescence (PLIF) imaging of OH radicals and chemiluminescence imaging, and separate experiments with particle image velocimetry and elastic laser sheet scattering from liquid particles seeded into the fuel. The SPRF combustor achieves internal exhaust gas recirculation and efficient mixing, which eliminates local peaks in temperature. This results in low NOx emissions, limited by flame zone (prompt) production, for both premixed and non-premixed modes of operation. The flame is anchored in a region of reduced velocity and high turbulent intensities, which promotes mixing of hot products into the reactants, thus enabling stable operation of the combustor even at very lean equivalence ratios. Also, the flame structure and flow characteristics were found to remain invariant at high loadings, i.e., mass flow rates. Combustion in the non-premixed mode of operation is found to be similar to the premixed case, with the OH PLIF measurements indicating that nonpremixed flame burns at an equivalence ratio that is close to the overall combustor equivalence ratio. Similarities in emission levels between premixed and non-premixed modes are thus attributable to efficient fuel-air mixing in the nonpremixed mode, and entrainment of hot products into the reactant stream before burning occurs.

Numerical Investigation of a Stagnation Point Reverse Flow Combustor

2008 ◽  
Author(s):  
Yufeng Cui ◽  
Xuan Lu ◽  
Gang Xu ◽  
Jianli Chen ◽  
Chaoqun Nie ◽  
...  
Keyword(s):  
Stagnation Point ◽  
Equivalence Ratio ◽  
Reverse Flow ◽  
Flame Structure ◽  
Diagnostic Techniques ◽  
Combustion Efficiency ◽  
Numerical Results ◽  
Nox Emissions ◽  
Flameless Combustion ◽  
Flow Field Characteristics

Flameless combustion is characterized by ultra-low NOx emissions, high combustion efficiency and very stable flame, which is able to operate stably at very lean fuel-air mixtures without problems of combustion oscillation and flashback. Stagnation Point Reverse Flow (SPRF) combustors, as an important application of flameless combustion, have been experimentally studied by various optical diagnostic techniques. In this paper, Eddy Dissipation Concept (EDC) model with detailed chemical reaction mechanisms of natural gas GRI 2.11 is used to investigate the flame characteristics of a SPRF combustor operating at premixed mode with various mass flow rates and equivalence ratios of CH4 and air mixtures. The numerical results indicate that as the fuel and air mixture injection velocities increase, there are no distinct changes in the jet penetrations. However, flame temperatures and NOx emissions decrease, CO emissions increase, and OH is distributed in wider area and more evenly. For turbulent flow, intense reactions take place in the shear layer and the stagnation zone and they gradually shift to the combustor outlet as the jet velocities increase. As the equivalence ratios increase from 0.5 to 1, the NOx emissions always increase, although they are very low when equivalence ratio is below 0.7. However, the CO emissions decrease firstly, reaching the minimum value at equivalence ratio of 0.58, and then increase. The numerical results are compared with experimental data and it is verified that EDC model can capture the important flow field characteristics and flame structure and is appropriate for modeling SPRF combustor.

Precession Effects on the Relationship Between Time-Averaged and Instantaneous Swirl Flow and Flame Characteristics

2015 ◽  
Author(s):  
I. Chterev ◽  
G. Sundararajan ◽  
J. M. Seitzman ◽  
T. C. Lieuwen
Keyword(s):  
Stagnation Point ◽  
Large Scale ◽  
Reverse Flow ◽  
Axial Flow ◽  
Swirl Flow ◽  
Flame Stabilization ◽  
Coherent Motion ◽  
Stagnation Points ◽  
Topological Features ◽  
The Relationship

Swirling flows exhibit a variety of unsteady fluid mechanic features, including large scale vortical structures and precessing recirculation zones. This paper considers the specific influence of precession on the relationship between time-averaged and instantaneous flow and flame features. The objective of this study is to aid in developing insight into high fidelity computations or experimental results. In particular, we describe how certain topological features in the time-averaged flow, such as centerline axial jets, centerline stagnation points, and symmetry of the flow about the centerline are influenced by precession. Insight is built by presenting results from a simplified model of a two-zone flow, consisting of a precessing reverse flow region embedded in a positive axial flow. A particularly significant result of this work is in regards to aerodynamically stabilized flames, which rely on the low velocity interior stagnation points in the vortex breakdown region for flame stabilization. We show how precession causes systematic differences between the location of the stagnation point of the time-averaged velocity and the time-averaged position of the instantaneous stagnation point. Indeed, an important implication of this point is that a perfect prediction of the time-averaged flow field could still lead to a completely erroneous time-averaged flame position prediction. Finally, we discuss the influence of precession and coherent motion on convergence of estimated averaged quantities.

A Discussion of Turbulent Flame Structure in Premixed Charges

1985 ◽  
Author(s):  
John Abraham ◽  
Forman A. Williams ◽  
Frediano V. Bracco
Keyword(s):  
Turbulent Flame ◽  
Flame Structure

scholarly journals Flame stability limits of premixed low-swirl combustion

2018 ◽  
Vol 10 (9) ◽  
pp. 168781401879087 ◽  
Author(s):  
Yinli Xiao ◽  
Zhibo Cao ◽  
Changwu Wang
Keyword(s):  
Flow Field ◽  
Vortex Breakdown ◽  
Flame Structure ◽  
Field Measurements ◽  
Operating Conditions ◽  
Oh Radicals ◽  
Flame Stability ◽  
Atmospheric Conditions ◽  
Swirl Burner ◽  
Stability Limits

The objective of this study is to gain a fundamental understanding of the flow-field and flame behaviors associated with a low-swirl burner. A vane-type low-swirl burner with different swirl numbers has been developed. The velocity field measurements are carried out with particle image velocimetry. The basic flame structures are characterized using OH radicals measured by planar laser-induced fluorescence. Three combustion regimes of low-swirl flames are identified depending on the operating conditions. For the same low-swirl injector under atmospheric conditions, attached flame is first observed when the incoming velocity is too low to generate vortex breakdown. Then, W-shaped flame is formed above the burner at moderate incoming velocity. Bowl-shaped flame structure is formed as the mixture velocity increases until it extinct. Local extinction and relight zones are observed in the low-swirl flame. Flow-field features and flame stability limits are obtained for the present burner.

Investigation of Reverse Flow Slinger Combustor with Jet A-1 and Methanol

2021 ◽  
Author(s):  
Abhishek Dubey ◽  
Pooja Nema ◽  
Abhijit Kushari
Keyword(s):  
Fuel Injection ◽  
Reverse Flow ◽  
Lightweight Design ◽  
Injection Pressure ◽  
Combustion Efficiency ◽  
Flame Stabilization ◽  
Liquid Fuels ◽  
Flame Stability ◽  
Lean Blowout ◽  
Lean Fuel

Abstract This paper describes experimental investigation of a Reverse Flow Slinger (RFS) combustor that has been developed in order to attain high flame stability and low emissions in gas turbine engines. The combustor employs centrifugal fuel injection through a rotary atomizer and performs flame stabilization at the stagnation zone generated by reverse flow configuration. The design facilitates entrainment of hot product gases and internal preheating of the inlet air which enhances flame stability and permits stable lean operation for low NOx. Moreover, the use of rotary atomizer eliminates the need for high injection pressure resulting in a compact and lightweight design. Atmospheric pressure combustion was performed with liquid fuels, Jet A-1 and Methanol at ultra-lean fuel air ratios (FAR) with thermal intensity of 28 - 50 MW/m3atm. Combustor performance was evaluated by analyzing the lean blowout, emissions and combustion efficiency. Test results showed high flame stability of combustor and a very low lean blowout corresponding to global equivalence ratio of around 0.1 was obtained. Sustained and stable combustion at low heat release was attained and NOx emissions as low as of 0.4 g/Kg and 0.1 g/Kg were obtained with Jet A-1 and Methanol respectively. Combustion efficiency of 55% and 90% was obtained in operation with Jet A-1 and Methanol. Performance of the combustor was significantly better with Methanol in terms of emissions and efficiency.

CO and OH Imaging in Flames Using a Single Broadband Femtosecond Laser Pulse

2020 ◽  
Author(s):  
Pradeep Parajuli ◽  
Ayush Jain ◽  
Waruna D. Kulatilaka
Keyword(s):  
Laser Pulse ◽  
Reaction Zone ◽  
Turbulent Flame ◽  
Flame Structure ◽  
Chemical Species ◽  
Oxidation Reactions ◽  
Simultaneous Excitation ◽  
Spatiotemporal Information ◽  
Flame Burner ◽  
Fs Laser

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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