Increasing Flashback Resistance in Lean Premixed Swirl-Stabilized Hydrogen Combustion by Axial Air Injection

2015 ◽  
Vol 137 (7) ◽  
Author(s):  
Thoralf G. Reichel ◽  
Steffen Terhaar ◽  
Oliver Paschereit
Keyword(s):  
Flow Field ◽  
Gas Turbines ◽  
High Speed ◽  
Flame Temperature ◽  
Air Injection ◽  
Reacting Flow ◽  
Nox Emissions ◽  
Test Rig ◽  
Swirl Number ◽  
Lean Premixed

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a nonswirling jet on the central axis of the radial swirl generator which influences the vortex breakdown (VB) position. In the present work, changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by means of the qualitative light sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

Increasing Flashback Resistance in Lean Premixed Swirl-Stabilized Hydrogen Combustion by Axial Air Injection

2014 ◽  
Author(s):  
Thoralf G. Reichel ◽  
Steffen Terhaar ◽  
Oliver Paschereit
Keyword(s):  
Flow Field ◽  
Gas Turbines ◽  
High Speed ◽  
Flame Temperature ◽  
Air Injection ◽  
Reacting Flow ◽  
Nox Emissions ◽  
Test Rig ◽  
Swirl Number ◽  
Lean Premixed

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a non-swirling jet on the central axis of the radial swirl generator which influences the vortex breakdown position. In the present work changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by the means of the Qualitative Light Sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note, that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

Experimental and Numerical Investigations of Low-Swirl Multi-Nozzle Combustion in a Lean Premixed Combustor

2014 ◽  
Author(s):  
Weijie Liu ◽  
Bing Ge ◽  
Yinshen Tian ◽  
Yongwen Yuan ◽  
Shusheng Zang ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Equivalence Ratio ◽  
Flame Temperature ◽  
Main Reaction ◽  
Nox Emissions ◽  
Lean Premixed ◽  
Planar Laser ◽  
Reaction Zones ◽  
Promising Solution ◽  
Log Linear

This paper presents large-eddy simulations (LES) and laser diagnostic experiments of low-swirl lean premixed methane/air flames in a multi-nozzle combustor including five nozzles with the same structure. OH Planar Laser Induced Fluorescence (PLIF) is used to observe flame shapes and identify main reaction zones. NOx and CO emissions are also recorded during the experiment. The flows and flames are studied at different equivalence ratios ranging from 0.5 to 0.8, while the inlet velocity is fixed at 6.2 m/s. Results show that the neighboring swirling flows interact with each other, generating a highly turbulent mixing zone where intensive reactions take place. The flame is stabilized above the nozzle rim and its liftoff height decreases with increasing equivalence ratio. The center flow is confined and distorted by the neighboring flows, resulting in instabilities of the center flame. Mean OH radical images reveals that the center nozzle flame is extinguished when equivalence ratio is equals to 0.5, which is successfully predicted by LES. In addition, NOx emissions show log-linear dependency on the adiabatic flame temperature, while the CO emissions remain lower than 10 ppm. NOx emissions for multi-nozzle flame are less sensitive to the flame temperature than that for single nozzle. These results demonstrate that the low-swirl multi-nozzle concept is a promising solution to achieve stable combustion with ultra-low emissions in gas turbines.

Investigation of Lean Premixed Swirl-Stabilized Hydrogen Burner With Axial Air Injection Using OH-PLIF Imaging

2015 ◽  
Vol 137 (11) ◽  
Author(s):  
Thoralf G. Reichel ◽  
Katharina Goeckeler ◽  
Oliver Paschereit
Keyword(s):  
Flow Field ◽  
Flame Front ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Air Injection ◽  
Turbulent Flame Speed ◽  
Air Jet ◽  
Wide Range ◽  
Lean Premixed ◽  
Inlet Conditions

In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback (FB), is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on FB tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ=0.2−1.0), bulk air velocity u0 = 30–90 m/s, and burner preheat temperature ranging from 300 K to 700 K. The burner is mounted in an atmospheric combustion test rig, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against FB occurrence by applying the axial air injection strategy. Axial air injection constitutes a nonswirling air jet on the central axis of the radial swirl generator. While a high rate of axial air injection yields excellent FB resistance, reduced rates of air injection are utilized to trigger FB, which allowed to investigate the near FB flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection, alter the isothermal flow field as they cause a downstream shift of vortex breakdown and, thus, the axial flame front location. Such a shift is proven beneficial for FB resistance from the recorded FB limits. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front probability xF. By these means, it was revealed that for hydrogen xF is shifted downstream with increasing equivalence ratio due to the added momentum of the fuel flow, superseding any parallel augmentation in the turbulent flame speed. The parameter xF is identified to be governed by J, the momentum ratio between fuel and air flow, over a wide range of inlet conditions. These results contribute to the understanding of the sensitivity of FB to changes in the flow field, stemming from geometry changes or specific fuel properties.

Investigation on Flowfield and Fuel/Air Premixing Uniformities of Low Swirl Injector for Lean Premixed Gas Turbines

2021 ◽  
Author(s):  
Fujun Sun ◽  
Jianqin Suo ◽  
Zhenxia Liu
Keyword(s):  
Penetration Depth ◽  
Residence Time ◽  
Gas Turbines ◽  
Structural Parameters ◽  
Combustible Mixture ◽  
Operating Conditions ◽  
Flame Stability ◽  
Nox Emissions ◽  
Auto Ignition ◽  
Lean Premixed

Abstract Based on the development trend of incorporating fuel holes into swirler-vanes and the advantages of wide operating conditions as well as low NOx emissions of LSI, this paper proposes an original lean premixed LSI with a convergent outlet. The influence of key structures on flowfields and fuel/air premixing uniformities of LSI is investigated by the combination of laser diagnostic experiments and numerical simulations. The flowfields of LSI shows that the main recirculation zone is detached from the convergent outlet and its axial dimensions are smaller than that of HSI, which can decrease the residence time of high-temperature gas to reduce NOx emissions. The fuel/air premixing characteristics show that the positions and diameters of fuel holes affect fuel/air premixing by changing the penetration depth of fuel. And when the penetration depth is moderate, it can give full play to the role of swirling air in enhancing premixing of fuel and air. In addition, the increase of the length of the premixing section can improve the uniformity of fuel/ar premixing, but it can also weaken the swirl intensity and increase the residence time of the combustible mixture within the LSI, which can affect flame stability and increase the risk of auto-ignition. Therefore, the design and selection of LSI structural parameters should comprehensively consider the requirements of fuel/air mixing uniformity, flame stability and avoiding the risk of auto-ignition. The results can provide the technical basis for LSI design and application in aero-derivative and land-based gas turbine combustors.

Study of a Rich/Lean Staged Combustion Concept for Hydrogen at Gas Turbine Relevant Conditions

2013 ◽  
Author(s):  
Felipe Bolaños ◽  
Dieter Winkler ◽  
Felipe Piringer ◽  
Timothy Griffin ◽  
Rolf Bombach ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Carbon Capture ◽  
Flame Temperature ◽  
Nox Emissions ◽  
Hydrogen Fuel ◽  
Auto Ignition ◽  
Wide Range ◽  
The Rich

The combustion of hydrogen-rich fuels (> 80 % vol. H2), relevant for gas turbine cycles with “pre-combustion” carbon capture, creates great challenges in the application of standard lean premix combustion technology. The significant higher flame speed and drastically reduced auto-ignition delay time of hydrogen compared to those of natural gas, which is normally burned in gas turbines, increase the risk of higher NOX emissions and material damage due to flashback. Combustion concepts for gas turbines operating on hydrogen fuel need to be adapted to assure safe and low-emission combustion. A rich/lean (R/L) combustion concept with integrated heat transfer that addresses the challenges of hydrogen combustion has been investigated. A sub-scale, staged burner with full optical access has been designed and tested at gas turbine relevant conditions (flame temperature of 1750 K, preheat temperature of 400 °C and a pressure of 8 bar). Results of the burner tests have confirmed the capability of the rich/lean staged concept to reduce the NOx emissions for undiluted hydrogen fuel. The NOx emissions were reduced from 165 ppm measured without staging (fuel pre-conversion) to 23 ppm for an R/L design having a fuel-rich hydrogen pre-conversion of 50 % at a constant power of 8.7 kW. In the realized R/L concept the products of the first rich stage, which is ignited by a Pt/Pd catalyst (under a laminar flow, Re ≈ 1900) are combusted in a diffusion-flame-like lean stage (turbulent flow Re ≈ 18500) without any flashback risk. The optical accessibility of the reactor has allowed insight into the combustion processes of both stages. Applying OH-LIF and OH*-chemiluminescence optical techniques, it was shown that mainly homogeneous reactions at rich conditions take place in the first stage, questioning the importance of a catalyst in the system, and opening a wide range of optimization possibilities. The promising results obtained in this study suggest that such a rich/lean staged burner with integrated heat transfer could help to develop a new generation of gas turbine burners for safe and clean combustion of H2-rich fuels.

Lean Premixed Combustion of Carbon-Monoxide-Hydrogen-Methane Fuel Mixtures Using Porous Inert Media

2005 ◽  
Author(s):  
S. K. Alavandi ◽  
A. K. Agrawal
Keyword(s):  
Carbon Monoxide ◽  
Flame Temperature ◽  
Carbon Foam ◽  
Premixed Combustion ◽  
Nox Emissions ◽  
Lean Premixed Combustion ◽  
Lean Premixed ◽  
Porous Inert Media ◽  
Fuel Mixtures ◽  
Coated Carbon

Lean premixed combustion of carbon monoxide (CO), hydrogen (H2), and methane (CH4) fuel mixtures with air was investigated experimentally. Combustion at atmospheric pressure was stabilized within porous inert medium made of silicon-carbide coated carbon foam with 4 pores per centimeter. CH4 in the fuel was varied from 100% to 0% (by volume), with the remaining fuel containing equal amounts of CO and H2. Experiments at a fixed air flow rate were conducted by varying the adiabatic flame temperature and fuel composition. Profile of CO and NOx emissions in the axial and transverse directions were taken to identify the post-combustion zone and uniformity of combustion. At a given flame temperature, fuels with CO/H2 produced lower CO and NOx emissions compared to those for CH4. The temperature at the lean blow off limit was significantly lower (compared to CH4) if the fuel contained CO and H2, each greater than 35% by volume.

Impact of Cooling Air Injection on the Combustion Stability of a Premixed Swirl Burner Near Lean Blowout

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
A. Marosky ◽  
V. Seidel ◽  
T. Sattelmayer ◽  
F. Magni ◽  
W. Geng
Keyword(s):  
Flame Front ◽  
Equivalence Ratio ◽  
Flame Stabilization ◽  
Air Injection ◽  
Combustion Stability ◽  
Nox Emissions ◽  
Test Rig ◽  
Lean Blowout ◽  
Burner Exit ◽  
Cooling Air

In most dry, low-NOx combustor designs of stationary gas turbines, the front panel impingement cooling air is directly injected into the combustor primary zone. This air partially mixes with the swirling flow of premixed reactants from the burner and reduces the effective equivalence ratio in the flame. However, local unmixedness and the lean equivalence ratio are supposed to have a major impact on combustion performance. The overall goal of this investigation is to answer the question of whether the cooling air injection into the primary combustor zone has a beneficial effect on combustion stability and NOx emissions or not. The flame stabilization of a typical swirl burner with and without front panel cooling air injection is studied in detail under atmospheric conditions close to the lean blowout limit (LBO) in a full-scale, single-burner combustion test rig. Based on previous isothermal investigations, a typical injection configuration is implemented for the combustion tests. Isothermal results of experimental studies in a water test rig adopting high-speed planar laser-induced fluorescence (HSPLIF) reveal the spatial and temporal mixing characteristics for the experimental setup studied under atmospheric combustion. This paper focuses on the effects of cooling air injection on both flame dynamics and emissions in the reacting case. To reveal dependencies of cooling air injection on combustion stability and NOx emissions, the amount of injected cooling air is varied. OH*-chemiluminescence measurements are applied to characterize the impact of cooling air injection on the flame front. Emissions are collected for different cooling air concentrations, both global measurements at the chamber exit, and local measurements in the region of the flame front close to the burner exit. The effect of cooling air injection on pulsation level is investigated by evaluating the dynamic pressure in the combustor. The flame stabilization at the burner exit changes with an increasing degree of dilution with cooling air. Depending on the amount of cooling, only a specific share of the additional air participates in the combustion process.

Investigation of Lean Premixed Swirl-Stabilized Hydrogen Burner With Axial Air Injection Using OH-PLIF Imaging

2015 ◽  
Author(s):  
Thoralf G. Reichel ◽  
Katharina Goeckeler ◽  
Oliver Paschereit
Keyword(s):  
Flame Front ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Vortex Breakdown ◽  
Detection Algorithm ◽  
Air Injection ◽  
Swirl Generator ◽  
Air Jet ◽  
Lean Premixed ◽  
Inlet Conditions

In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback, is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on flashback tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ = 0.2–1.0), bulk air velocity u0 = 30–90m/s and burner preheat temperature ranging from 300K to 700K. The burner is mounted in the atmospheric combustion test rig at the HFI, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against flashback occurrence by applying the axial air injection strategy. Axial air injection constitutes a non-swirling air jet on the central axis of the radial swirl generator, thus, influencing the vortex breakdown position. High axial air injection yields excellent flashback resistance and is used to investigate the whole inlet parameter space. In order to trigger flashback, the amount of axially injected air is reduced, which allowed to investigate the near flashback flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection alter the isothermal flow field and cause a downstream shift of the axial flame front location. Such a shift is proven beneficial for flashback resistance. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front likelihood xF. The temperature and equivalence ratio dependence of the parameters xF is identified to be governed by the momentum ratio between fuel and air flow J. These results contribute to the understanding of the superior flashback limits of configurations applying high amounts of axial air injection over medium or none air injection.

Numerical Simulation of the Flow Field in a High Speed Multistage Compressor: Study of the Time Discretization Sensitivity

2015 ◽  
Author(s):  
Johannes Schreiber ◽  
Xavier Ottavy ◽  
Ghislaine Ngo Boum ◽  
Stéphane Aubert ◽  
Frédéric Sicot
Keyword(s):  
Flow Field ◽  
High Speed ◽  
State Of The Art ◽  
Axial Compressor ◽  
Time Discretization ◽  
Test Rig ◽  
Unsteady Rans ◽  
Multistage Compressor ◽  
Temporal Periodicity ◽  
Specific Contribution

The following numerical investigations are performed in the frame of a research project that aims at a better understanding of the flow unsteadiness that develops in a multistage high-speed axial compressor. First, the paper presents a new version of the 3.5 stages high-speed axial compressor CREATE (Compresseur de Recherche pour l’Etude des effets Aérodynamiques et TEchnologiques), which has been designed by Snecma and is based at the LMFA (Laboratory for Fluid Mechanics and Acoustics) on a 2MW test rig. This paper is based on numerical results obtained with 3D steady and unsteady RANS computations using the CREATE configuration. The unsteady RANS simulations are carried out over the whole spatial and temporal periodicity of the compressor. The main numerical setup has been fixed according to the state of the art. Second, the effect of three different time discretizations on the flow field in CREATE is discussed. The global performance of the compressor is not significantly affected. However the change in the time discretization impacts the structure of the flow at specific locations. The main focus of this study lies on the transport of flow structures and the analysis of their interactions. A double modal decomposition method, which highlights the specific contribution of the interactions on the overall flow field, is applied for the study of the highly complex and unsteady flow field. It allows identifying which interactions are more sensitive to the change in the time discretization.

A NOx Prediction Scheme for Lean-Premixed Gas Turbine Combustion Based on Detailed Chemical Kinetics

1996 ◽  
Vol 118 (4) ◽  
pp. 765-772 ◽  
Author(s):  
W. Polifke ◽  
K. Do¨bbeling ◽  
T. Sattelmayer ◽  
D. G. Nicol ◽  
P. C. Malte
Keyword(s):  
Flow Field ◽  
Gas Turbine ◽  
Concentration Field ◽  
Operating Conditions ◽  
Chemical Mechanism ◽  
Reacting Flow ◽  
Oxides Of Nitrogen ◽  
Detailed Chemical Kinetics ◽  
Nox Formation ◽  
Lean Premixed

The lean-premixed technique has proven very efficient in reducing the emissions of oxides of nitrogen (NOx) from gas turbine combustors. The numerical prediction of NOx levels in such combustors with multidimensional CFD codes has only met with limited success so far. This is to some extent due to the complexity of the NOx formation chemistry in lean-premixed combustion, i.e., all three known NOx formation routes (Zeldovich, nitrous, and prompt) can contribute significantly. Furthermore, NOx formation occurs almost exclusively in the flame zone, where radical concentrations significantly above equilibrium values are observed. A relatively large chemical mechanism is therefore required to predict radical concentrations and NOx formation rates under such conditions. These difficulties have prompted the development of a NOx postprocessing scheme, where rate and concentration information necessary to predict NOx formation is taken from one-dimensional combustion models with detailed chemistry and provided—via look-up tables—to the multidimensional CFD code. The look-up tables are prepared beforehand in accordance with the operating conditions and are based on CO concentrations, which are indicative of free radical chemistry. Once the reacting flow field has been computed with the main CFD code, the chemical source terms of the NO transport equation, i.e., local NO formation rates, are determined from the reacting flow field and the tabulated chemical data. Then the main code is turned on again to compute the NO concentration field. This NOx submodel has no adjustable parameters and converges very quickly. Good agreement with experiment has been observed and interesting conclusions concerning superequilibrium O-atom concentrations and fluctuations of temperature could be drawn.

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