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1
Krupa, R. J., T. F. Culbreth, B. W. Smith e J. D. Winefordner. "A Flashback-Resistant Burner for Combustion Diagnostics and Analytical Spectrometry". Applied Spectroscopy 40, n. 6 (agosto 1986): 729–33. http://dx.doi.org/10.1366/0003702864508232.
Testo completoAbstract (sommario):
A general utility burner for the production of laminar, homogenous diffusion flames, which is immune to flashbacks, is presented. Because the fuel and oxidant mix on the surface of the burner rather than within the spray chamber, the flames cannot flashback. A wide variety of gas mixtures has been investigated, including oxygen, nitrous oxide, and nitric oxide as the oxidants. Any combination of fuel and oxidant can be safely burned to produced a stable, laminar, and audibly quiet flame. Flame temperatures can be varied over a wide range either by changing the fuel-oxidant ratio or by diluting the flame gases with an inert gas. In this manner, the optimum flame temperature and composition can be achieved. These burners are of general use in analytical emission, fluorescence, and photoacoustic spectrometry, as well as combustion diagnostics.
2
Carlos E. Arrieta, Mario Luna-DelRisco, Arley Cardona, Jorge Sierra del Río, Alejandro Ruiz Sánchez, Lisandra Rocha-Meneses e Jhojan Stiven Zea Fernández. "Numerical Investigation of Operating Conditions that Lead to Flat Flames, Flashback, and Blowout in A Surface-Stabilized Combustion Burner". CFD Letters 15, n. 4 (16 febbraio 2023): 106–13. http://dx.doi.org/10.37934/cfdl.15.4.106113.
Testo completoAbstract (sommario):
Surface-stabilized combustion burners or surface-radiant burners use perforated ceramic plates, ceramic foams, or metal fibers to stabilize a premixed flame. These burners are the most straightforward alternative to have both, the benefits of the reactant preheating technique and a great amount of heat transferred by radiation from the burner to the load. However, in its design, one of the greatest difficulties is to predict the flame stability limits; especially under operating conditions that lead to flashbacks and blowouts. This work presents a computational methodology based on the finite volume method with a two-dimensional domain to predict the flame curvature towards the unburned and burned gas that occurs before flashback and blowout, respectively. In the methodology, continuity, momentum, energy, and chemical species equations are solved to obtain the increase in the surface area of the flame. It was observed that this value can be used as a criterion to predict whether an operating condition is stable. When comparing the numerical results with experimental results reported in the literature, good predictions of the operating conditions that lead to flashbacks and blowouts are observed
3
Eichler, Christian, Georg Baumgartner e Thomas Sattelmayer. "Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts". Mechanical Engineering 134, n. 12 (1 dicembre 2012): 52–53. http://dx.doi.org/10.1115/1.2012-dec-7.
Testo completoAbstract (sommario):
This article discusses the results of experimental investigation of turbulent boundary layer flashback limits for premixed hydrogen-air flames confined in ducts. A tube burner experiment was set-up to double-check the findings of the channel rig. Unconfined flashback experiments were carried out by stabilizing the flame on top of the pilot burner in free atmosphere. A confined flame configuration was achieved by simply fixing a ceramic ring with a diameter higher by 4 mm on top of the pilot burner. Flashback measurements with unconfined flame holding neatly reproduced literature values for fully premixed, atmospheric H2–air mixtures and turbulent flow. The results of unconfined and confined tube burner experiments were plotted. The results showed that the drastic decrease of wall flashback stability for confined flames was the very same for both, tube and channel.
4
Endres, Aaron, e Thomas Sattelmayer. "Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process". Fluids 4, n. 3 (1 agosto 2019): 146. http://dx.doi.org/10.3390/fluids4030146.
Testo completoAbstract (sommario):
Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. This was done by means of large eddy simulations with finite rate chemistry as well as detailed chemical kinetics and diffusion models at pressures between 0 . 5 and 3 . It was found that the flashback propensity increases with increasing pressure. The separation zone size and the turbulent flame speed at flashback conditions decrease with increasing pressure, which decreases flashback propensity. At the same time the quenching distance decreases with increasing pressure, which increases flashback propensity. It is not possible to predict the occurrence of boundary layer flashback based on the turbulent flame speed or the ratio of separation zone size to quenching distance alone. Instead the interaction of all effects has to be accounted for when modelling boundary layer flashback. It was further found that the pressure rise ahead of the flame cannot be approximated by one-dimensional analyses and that the assumptions of the boundary layer theory are not satisfied during confined boundary layer flashback.
5
KURDYUMOV, VADIM N., e AMABLE LIÑÁN. "STRUCTURE OF A FLAME FRONT PROPAGATING AGAINST THE FLOW NEAR A COLD WALL". International Journal of Bifurcation and Chaos 12, n. 11 (novembre 2002): 2547–55. http://dx.doi.org/10.1142/s0218127402006023.
Testo completoAbstract (sommario):
The flashback or propagation of premixed flames against the flow of a reacting mixture, along the low velocity region near a cold wall, is investigated numerically. The analysis, carried out using the constant density approximation for an Arrhenius overall reaction, accounts for the effects of the Lewis number of the limiting reactant. Flame front propagation and flashback are only possible for values of the near wall velocity gradient below a critical value. The flame propagation becomes chaotic for small values of the Lewis number.
6
Dias, David M., Pedro R. Resende e Alexandre M. Afonso. "A Review on Micro-Combustion Flame Dynamics and Micro-Propulsion Systems". Energies 17, n. 6 (10 marzo 2024): 1327. http://dx.doi.org/10.3390/en17061327.
Testo completoAbstract (sommario):
This work presents a state-of-the-art review of micro-combustion flame dynamics and micro propulsion systems. In the initial section, we focus in on the different challenges of micro-combustion, investigating the typical length and time scales involved in micro-combustion and some critical phenomena such as flammability limits and the quenching diameter.We present an extensive collection of studies on the principal types of micro-flame dynamics, including flashback, blow-off, steady versus non-steady flames, mild combustion, stable flames, flames with repetitive extinction, and ignition and pulsatory flame burst. In the final part of this review, we focus on micropropulsion systems, their performance metrics, conventional manufacturing methods, and the advancements in Micro-Electro-Mechanical Systems manufacturing.
7
Jiang, Xudong, Yihao Tang, Zhaohui Liu e Venkat Raman. "Computational Modeling of Boundary Layer Flashback in a Swirling Stratified Flame Using a LES-Based Non-Adiabatic Tabulated Chemistry Approach". Entropy 23, n. 5 (2 maggio 2021): 567. http://dx.doi.org/10.3390/e23050567.
Testo completoAbstract (sommario):
When operating under lean fuel–air conditions, flame flashback is an operational safety issue in stationary gas turbines. In particular, with the increased use of hydrogen, the propagation of the flame through the boundary layers into the mixing section becomes feasible. Typically, these mixing regions are not designed to hold a high-temperature flame and can lead to catastrophic failure of the gas turbine. Flame flashback along the boundary layers is a competition between chemical reactions in a turbulent flow, where fuel and air are incompletely mixed, and heat loss to the wall that promotes flame quenching. The focus of this work is to develop a comprehensive simulation approach to model boundary layer flashback, accounting for fuel–air stratification and wall heat loss. A large eddy simulation (LES) based framework is used, along with a tabulation-based combustion model. Different approaches to tabulation and the effect of wall heat loss are studied. An experimental flashback configuration is used to understand the predictive accuracy of the models. It is shown that diffusion-flame-based tabulation methods are better suited due to the flashback occurring in relatively low-strain and lean fuel–air mixtures. Further, the flashback is promoted by the formation of features such as flame tongues, which induce negative velocity separated boundary layer flow that promotes upstream flame motion. The wall heat loss alters the strength of these separated flows, which in turn affects the flashback propensity. Comparisons with experimental data for both non-reacting cases that quantify fuel–air mixing and reacting flashback cases are used to demonstrate predictive accuracy.
8
Fooladgar, Ehsan, e C. K. Chan. "Large Eddy Simulation of a Swirl-Stabilized Pilot Combustor from Conventional to Flameless Mode". Journal of Combustion 2016 (2016): 1–16. http://dx.doi.org/10.1155/2016/8261560.
Testo completoAbstract (sommario):
This paper investigates flame and flow structure of a swirl-stabilized pilot combustor in conventional, high temperature, and flameless modes by means of a partially stirred reactor combustion model to provide a better insight into designing lean premixed combustion devices with preheating system. Finite rate chemistry combustion model with one step tuned mechanism and large eddy simulation is used to numerically simulate six cases in these modes. Results show that moving towards high temperature mode by increasing the preheating level, the combustor is prone to formation of thermalNOxwith higher risks of flashback. In addition, the flame becomes shorter and thinner with higher turbulent kinetic energies. On the other hand, towards the flameless mode, leaning the preheated mixture leads to almost thermalNOx-free combustion with lower risk of flashback and thicker and longer flames. Simulations also show qualitative agreements with available experiments, indicating that the current combustion model with one step tuned mechanisms is capable of capturing main features of the turbulent flame in a wide range of mixture temperature and equivalence ratios.
9
Chang, Liuyong, Boxuan Cui, Chenglin Zhang, Zheng Xu, Guangze Li e Longfei Chen. "Monitoring and Characterizing the Flame State of a Bluff-Body Stabilized Burner by Electrical Capacitance Tomography". Processes 11, n. 8 (10 agosto 2023): 2403. http://dx.doi.org/10.3390/pr11082403.
Testo completoAbstract (sommario):
Unstable combustion phenomena such as flame flashback, flame liftoff, extinction and blowout frequently take place during the operation of the bluff-body stabilized burner. Therefore, flame state monitoring is necessary for the safe operation of the bluff-body stabilized burner. In the present study, an electrical capacitance tomography (ECT) system was deployed to detect the permittivity distribution in the premixing channel and further characterize the flame states of stabilization, flashback, liftoff, extinction and blowout. A calderon-based reconstruction method was modified to reconstruct the permittivity distribution in the annular premixing channel. The detection results indicate that the permittivity in the premixing channel increases steeply when the flame flashback takes place and decreases obviously when the flame lifts off from the combustor rim. Based on the varied permittivity distribution at different flame states, a flame state index was proposed to characterize the flame state in quantification. The flame state index is 0, positive, in the range of −0.64–0, and lower than −0.64 when the flame is at the state of stable, flashback, liftoff and blowout, respectively. The flame state index at the flame state of extinction is the same as that at the flame state of liftoff. The extinction state and the blowout state can be distinguished by judging whether the flame flashback takes place before the flame is extinguished. These results reveal that the ECT system is capable of monitoring the flame state, and that the proposed flame state index can be used to characterize the flame state.
10
Huang, Kai, Louis Benteux, Wenhu Han e Damir M. Valiev. "Combined Impact of the Lewis Number and Thermal Expansion on Laminar Flame Flashback in Tubes". Fluids 9, n. 1 (19 gennaio 2024): 28. http://dx.doi.org/10.3390/fluids9010028.
Testo completoAbstract (sommario):
The understanding of the boundary layer flame flashback (BLF) has considerably improved in recent decades, driven by the increasing focus on clean energy and the need to address the operational issues associated with flashback. This study investigates the influence of the Lewis number (Le) on symmetric flame shapes under the critical conditions for a laminar boundary layer flashback in cylindrical tubes. It has been found that the transformation of the flame shape from a mushroom to a tulip happens in a tube of a given radius, as the thermal expansion coefficient and Le are modified. A smaller Lewis number results in a local increase in the burning rate at the flame tip, with the flame being able to propagate closer to the wall, which significantly increases the flashback propensity, in line with previous findings. In cases with a Lewis number smaller than unity, a higher thermal expansion results in a flame propagation happening closer to the wall, thus facing a weaker oncoming flow and, consequently, becoming more prone to flashback. For Le > 1, the effect of the increase in the thermal expansion coefficient on the flashback tendency is much less pronounced.
11
Fritz, J., M. Kro¨ner e T. Sattelmayer. "Flashback in a Swirl Burner With Cylindrical Premixing Zone". Journal of Engineering for Gas Turbines and Power 126, n. 2 (1 aprile 2004): 276–83. http://dx.doi.org/10.1115/1.1473155.
Testo completoAbstract (sommario):
Flame flashback from the combustion chamber into the mixing zone is one of the inherent problems of lean premixed combustion and essentially determines the reliability of low NOx burners. Generally, flashback can be initiated by one of the following four phenomena: flashback due to the conditions in the boundary layer, flashback due to turbulent flame propagation in the core flow, flashback induced by combustion instabilities and flashback caused by combustion induced vortex breakdown. In this study, flashback in a swirling tubular flow was investigated. In order to draw maximum benefit from the tests with respect to the application in gas turbines, the radial distribution of the axial and circumferential momentum in the tube was selected such that the typical character of a flow in mixing zones of premix burners without centerbody was obtained. A single burner test rig has been designed to provoke flashback with the preheating temperature, the equivalence ratio and the mean flow rate being the influencing parameters. The flame position within the mixing section is detected by a special optical flame sensor array, which allows the control of the experiment and furthermore the triggering of the measurement techniques. The burning velocity of the fuel has been varied by using natural gas or hydrogen. The characteristics of the flashback, the unsteady swirling flow during the flame propagation, the flame dynamics and the reaction zones have been investigated by applying high-speed video recordings, the laser Doppler anemometry and the laser induced fluorescence. The presented results show that a combustion induced vortex breakdown is the dominating mechanism of the observed flashback. This mechanism is very sensitive to the momentum distribution in the vortex core. By adding axial momentum around the mixing tube axis, the circumferential velocity gradient is reduced and flashback can be prevented.
12
Al-Tayyar, Mohammed A., Dhirgham Alkhafaji e Haroun A. K. Shahad. "An Investigation into Burner Configuration Effects on Premixed Flame Characteristics for LPG Diluted with CO<sub>2</sub>". Applied Mechanics and Materials 914 (15 maggio 2023): 53–66. http://dx.doi.org/10.4028/p-gu777j.
Testo completoAbstract (sommario):
Meeting stringent emission regulations, the demand for environmentally friendly fuels is increasing by the day. Alternative fuel must be burned alongside conventional fuel to increase the availability of such clean energy sources. The current experimental study investigates the characteristics of the premixed LPG flames with CO2 dilution in tube swirling and non-swirling burners. The study including testing the effects of equivalence ratios, φ, (0.8, 1, 1.2, & 1.4), CO2 dilution ratios (0%, 5%, 7.5%, & 10%), and aspect ratio of the non-swirling burner (2, 4, 6, 8, & 10). Two swirling burners with swirl number was tested, namely 0.78 & 0.48. The dilution of CO2 has been observed lengthens the flame, particularly at higher equivalence ratios and/or flow rates since there is more than one influence, they all agree on a similar influence on flame height. The flame shortens clearly when using a swirling burner. Besides, when increasing the swirl number, the flame height increases slightly. Also, the swirling burner divided the flame's inner core into segments equal to the number of swirl vanes, and a flower-shaped flame was generated at low flow rates. The burner’s aspect ratio affects flame height insignificantly. Flame stability limits increase for a higher equivalence ratio and it enhances due to CO2 addition. The LPG-CO2/air mixture has an improved reply to beat flame flashback. The addition of CO2 expands the flow rate of stable flame by about 40% and 25% for φ = 1 and 1.2 respectively. Utilizing a swirling burner improves flame stability greatly. The limit between flashback and blowout increased by about three times as a result of using a swirling burner.
13
Kurdyumov, V. N., E. Fernández e A. Liñán. "Flame flashback and propagation of premixed flames near a wall". Proceedings of the Combustion Institute 28, n. 2 (gennaio 2000): 1883–89. http://dx.doi.org/10.1016/s0082-0784(00)80592-5.
Testo completo
14
Kro¨ner, M., J. Fritz e T. Sattelmayer. "Flashback Limits for Combustion Induced Vortex Breakdown in a Swirl Burner". Journal of Engineering for Gas Turbines and Power 125, n. 3 (1 luglio 2003): 693–700. http://dx.doi.org/10.1115/1.1582498.
Testo completoAbstract (sommario):
Flame flashback from the combustion chamber into the mixing zone limits the reliability of swirl stabilized lean premixed combustion in gas turbines. In a former study, the combustion induced vortex breakdown (CIVB) has been identified as a prevailing flashback mechanism of swirl burners. The present study has been performed to determine the flashback limits of a swirl burner with cylindrical premixing tube without centerbody at atmospheric conditions. The flashback limits, herein defined as the upstream flame propagation through the entire mixing tube, have been detected by a special optical flame sensor with a high temporal resolution. In order to study the effect of the relevant parameters on the flashback limits, the burning velocity of the fuel has been varied using four different natural gas-hydrogen-mixtures with a volume fraction of up to 60% hydrogen. A simple approach for the calculation of the laminar flame speeds of these mixtures is proposed which is used in the next step to correlate the experimental results. In the study, the preheat temperature of the fuel mixture was varied from 100°C to 450°C in order to investigate influence of the burning velocity as well as the density ratio over the flame front. Moreover, the mass flow rate has been modified in a wide range as an additional parameter of technical importance. It was found that the quenching of the chemical reaction is the governing factor for the flashback limit. A Peclet number model was successfully applied to correlate the flashback limits as a function of the mixing tube diameter, the flow rate and the laminar burning velocity. Using this model, a quench factor can be determined for the burner, which is a criterion for the flashback resistance of the swirler and which allows to calculate the flashback limit for all operating conditions on the basis of a limited number of flashback tests.
15
Gruber, A., J. H. Chen, D. Valiev e C. K. Law. "Direct numerical simulation of premixed flame boundary layer flashback in turbulent channel flow". Journal of Fluid Mechanics 709 (29 agosto 2012): 516–42. http://dx.doi.org/10.1017/jfm.2012.345.
Testo completoAbstract (sommario):
AbstractDirect numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure.
16
Wierzba, I., e K. Kar. "Flame Flashback Within Turbulent Streams of Lean Homogeneous Fuel Mixtures and Air". Journal of Energy Resources Technology 114, n. 2 (1 giugno 1992): 142–45. http://dx.doi.org/10.1115/1.2905933.
Testo completoAbstract (sommario):
The flame flashback limit represents the minimum concentration of the fuel in the stream of homogeneous mixtures of fuel and air at which a flame can propagate upstream against the direction of the flow. Information about these limits is important in the assessment of the safety of operation in situations where a flowing homogeneous fuel-air mixture is present. The present work examines the flame flashback in turbulent homogeneous fuel-air streams with Reynolds number up to 16,000. The flashback limits were established for a number of common gaseous fuels, such as methane, propane, hydrogen, ethylene, and their binary mixtures. Taking into consideration the wide use of low heating value fuels, the effect of the presence of diluents (nitrogen and carbon dioxide) in the fuel on the flashback limit was also investigated. Correlations were proposed to calculate the flashback limits of fuel mixtures and their accuracy is discussed.
17
Al-Naffakh, Jameel T., Mohammed R. Al-Qassab, Barzan Tarish Neamah e Zaid Mohammed Hasan Al-Makhzoomi. "Experimental Investigation of Blending Acetylene with Iraqi LPG to Determine a Flame Stability Map". Journal of Petroleum Research and Studies 12, n. 1 (20 marzo 2022): 350–63. http://dx.doi.org/10.52716/jprs.v12i1.607.
Testo completoAbstract (sommario):
The difficult challenges facing the designers and engineers of combustion systems are the flame stability (flame stability map) represented by the limits of flashback and blow-off. In this study, acetylene gas was combined with Iraqi liquefied petroleum gas at rates (10% - 50%). The reason for choosing these two components is the low cost and ease of access to it. Where the flashback limits (critical velocity gradient) were obtained from (40-485) 1/sec, while the blow-off limits were (265-2510) 1/sec with a diameter of 25 cm for the burning nozzle diameter only for Iraqi LPG without mixing acetylene. While in the case of mixing 10% acetylene, the flashback limits (critical velocity gradient) were from (30-520) 1/sec and the blow-off limits (440-3985) 1/sec for the same diameter of the muzzle of 25 cm. Whereas, when mixing 30% of the acetylene, the flashback limits (critical velocity gradient) were from (55-575) 1/sec and the blow-off limits (570 - 4050) 1/sec. From the above three cases, noticed a relative expansion of the flame stability map for the flashback boundaries, while at the blow-off limits the amplitude was clear and large, which indicates the confidence in mixing acetylene with Iraqi LPG and obtaining a larger flame stability map. Thus, it stimulates its use in industrial fields and gas turbine power stations.
18
Scha¨fer, O., R. Koch e S. Wittig. "Flashback in Lean Prevaporized Premixed Combustion: Nonswirling Turbulent Pipe Flow Study". Journal of Engineering for Gas Turbines and Power 125, n. 3 (1 luglio 2003): 670–76. http://dx.doi.org/10.1115/1.1581897.
Testo completoAbstract (sommario):
A fundamental study has been performed on the upstream flame propagation of a turbulent kerosene flame, stabilized in a confined stagnation flow at atmospheric pressure. Besides temperature and equivalence ratio, mixture properties and fluid dynamic parameters have been varied. The flashback phenomenon is discussed in terms of critical mean velocities and additionally based on detailed LDV data at the outlet of the premixing duct. The largest critical velocities uc for flashback are found for the “perfectly” premixed case and equivalence ratios close to stoichiometric, which is in accordance with the theory on laminar flame propagation. In the case of a homogeneous mixture, flashback is determined by the velocity distribution at the outlet of the premixing section. In the undisturbed pipe flow the flame propagates through the wall boundary layer. The data for this case are compared with the theory of side-wall quenching in terms of a critical Peclet number and critical velocity gradients at the wall. Both are deduced from the experimental data. Reducing the velocity on the axis forces the flame to propagate through the center at a velocity predicted by correlations on turbulent flame velocity.
19
Zimmermann, Paul, Julian Bajrami e Friedrich Dinkelacker. "Validation of a Generic Non-Swirled Multi-Fuel Burner for the Measurement of Flame Stability Limits for Research of Advanced Sustainable Aviation Fuels". Energies 16, n. 22 (7 novembre 2023): 7480. http://dx.doi.org/10.3390/en16227480.
Testo completoAbstract (sommario):
Future aviation concepts should be both CO2-neutral and without other emissions. One approach to reaching both targets is based on sustainably produced synthetic liquid fuels, which may allow very clean, lean premixed prevaporized (LPP) combustion. For that, fuels are needed with much longer ignition delay times and a lower flashback propensity than current jet fuels. We describe an experimental setup to investigate the flashback stability of liquid fuels in a multi-fuel burner. In this work, the measurement procedure and the determination of the experimentally obtained accuracy are in focus with regard to prevaporized and preheated iso-propanol/air flames in an equivalence ratio range of 0.85 to 1.05 involving three preheating levels (573, 673, and 773 K). As the determination of the accurate unburnt gas temperature just ahead of the flame is of strong importance for flashback but not directly possible, a model is implemented to determine it from the measurable quantities. Even with this indirect method, and also regarding the hysteresis of the experimental preheating temperature, it is found that the relevant quantities, namely, measured temperatures, mass flows, and values derived from them, can be determined with accuracy in the range below 1.7%.
20
Huang, Kai, Damir M. Valiev, Hongtao Zhong e Wenhu Han. "Numerical Study of the Influence of the Thermal Gas Expansion on the Boundary Layer Flame Flashback in Channels with Different Wall Thermal Conditions". Energies 16, n. 4 (13 febbraio 2023): 1844. http://dx.doi.org/10.3390/en16041844.
Testo completoAbstract (sommario):
In recent years, boundary layer flame flashback (BLF) has re-emerged as a technological and operational issue due to the more widespread use of alternative fuels as a part of a global effort to promote carbon neutrality. While much understanding has been achieved in experiments and simulations of BLF in the past decades, the theoretical modeling of BLF still largely relies on the progress made as early as the 1940s, when the critical gradient model (CGM) for the laminar flame flashback was proposed by Lewis and von Elbe. The CGM does not account for the modification of the upstream flow by the flame, which has been recently shown to play a role in BLF. The aim of the present work is to gain additional insight into the effects of thermal gas expansion and confinement on the flame-flow interaction in laminar BLF. Two-dimensional simulations of the confined laminar BLF in a channel are performed in this work. The parametric study focuses on the channel width, the thermal gas expansion coefficient, and the heat losses to the wall. This study evaluates the influence of these factors on the critical condition for the flame flashback. By varying the channel width, it is demonstrated that at the critical condition, the incoming flow in narrow channels is modified globally by the thermal gas expansion, while in wider channels, the flow modification by the flame tends to be more local. In narrow channels, a non-monotonic dependence of the critical-condition centerline velocity on the channel width has been identified. The variation of the heat loss to the wall confirms that the wall’s thermal conditions can significantly alter the flashback limit, with the flashback propensity being larger when the thermal resistance of the wall is high. To assess the general applicability of the CGM, the flame consumption speed and the flow velocity near the wall are quantified. The results confirm that the assumption of flame having no influence on the upstream flow, employed in the CGM, is not fulfilled under confinement for a realistic thermal gas expansion. This results in a general disagreement between the simulations and the CGM, which implies that the thermal expansion effects should be accounted for when considering the confined boundary layer flashback limits. It is shown that the critical velocity gradient increases with the gas expansion coefficient for the given channel width and wall thermal condition.
21
Schönborn, Alessandro, Parisa Sayad e Jens Klingmann. "Influence of precessing vortex core on flame flashback in swirling hydrogen flames". International Journal of Hydrogen Energy 39, n. 35 (dicembre 2014): 20233–41. http://dx.doi.org/10.1016/j.ijhydene.2014.10.005.
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Heeger, C., R. L. Gordon, M. J. Tummers, T. Sattelmayer e A. Dreizler. "Experimental analysis of flashback in lean premixed swirling flames: upstream flame propagation". Experiments in Fluids 49, n. 4 (9 maggio 2010): 853–63. http://dx.doi.org/10.1007/s00348-010-0886-0.
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ИДРИСОВ, Д. В., С. С. МАТВЕЕВ, Н. И. ГУРАКОВ, А. А. ЛИТАРОВА, О. В. КОЛОМЗАРОВ, А. С. САВЧЕНКОВА e А. Д. ПОПОВ. "NUMERICAL AND EXPERIMENTAL DETERMINATION OF THE FLAME FLASHBACK IN A METHANE-HYDROGEN FUEL USED IN COMBUSTORS OF GAS-TURBINE ENGINES AND PROPULSION FACILITIES". Физика горения и взрыва 60, n. 4 (31 luglio 2024): 103–11. http://dx.doi.org/10.15372/fgv2024.9445.
Testo completoAbstract (sommario):
Проведено расчетно-экспериментальное исследование границ проскока пламени при горении предварительно подготовленного метановодородного топлива в вихревом горелочном устройстве с закруткой потока, а также в модельной камере сгорания, являющейся прототипом используемых малоэмиссионных камер сгорания с предварительной подготовкой смеси. В результате проведенных исследований выработаны рекомендации, применение которых позволяет определить проскок пламени с погрешностью, не превышающей ±5 %. Эти результаты могут быть использованы для повышения точности определения границ проскока пламени при горении метановодородного топлива на этапе предварительного проектирования камер сгорания авиационных газотурбинных двигателей и энергетических установок. A comprehensive numerical and experimental study of the flame flashback boundaries in combustion of a premixed methane-hydrogen flame in a vortex burner with flow swirling and also in a model combustor, which is a prototype of low-emission combustors with fuel premixing is performed. Based on the results of the study, recommendations are given, which allow the flame flashback to be determined with an error smaller than ±5%. These results can be used to increase the accuracy of determining the flame flashback in combustion of the methane-hydrogen fuel at the stage of preliminary design of combustors for aviation gas-turbine engines and propulsion facilities.
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Ebi, Dominik, e Noel T. Clemens. "Experimental investigation of upstream flame propagation during boundary layer flashback of swirl flames". Combustion and Flame 168 (giugno 2016): 39–52. http://dx.doi.org/10.1016/j.combustflame.2016.03.027.
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Filomeno, Giovanni, Tommaso Capurso, Marco Torresi e Giuseppe Pascazio. "Numerical study of the lean premixed PRECCINSTA burner with hydrogen enrichment". E3S Web of Conferences 312 (2021): 11014. http://dx.doi.org/10.1051/e3sconf/202131211014.
Testo completoAbstract (sommario):
Hydrogen combustion is one of the most promising solution to achieve a global decarbonization in power production and transports. Pure hydrogen combustion is far from becoming a standard but, during the energy transition, hydrogen co-firing can be a feasible and economically attractive shortterm measure. The use of hydrogen blending gives rise to several issues related to flashback, NOx emissions and thermo-acoustic instabilities. To improve the understanding of the effect of hydrogen enrichment, herein a numerical analysis of lean premixed hydrogen enriched flames is performed by means of 3D unsteady CFD simulations. The numerical model has been assessed against experimental results for both cold and reacting flows in terms of velocity profile (average) and flame shape (mean OH* radical fields). The burner under investigation is the swirl stabilized PRECCINSTA studied at the Deutsches Zentrum für Luft-und Raumfahrt (DLR). The DLR’s researchers have shown the effect of hydrogen addition on the flame topology and combustion instabilities at various operating conditions in terms of thermal power, equivalence ratio and H2 volume fraction. Simulations are in good accordance with experimental data both in terms of velocity and temperature profiles. The numerical model provides a qualitative estimation of the flame shape.
26
Pappa, Alessio, e Ward De Paepe. "Humidification Towards Flashback Prevention in a Classical Micro Gas Turbine: Thermodynamic Performance Assessment". E3S Web of Conferences 414 (2023): 03010. http://dx.doi.org/10.1051/e3sconf/202341403010.
Testo completoAbstract (sommario):
Combustion air humidification has proven to be effective to stabilize hydrogen combustion and to avoid flashback apparition in a typical micro Gas Turbine (mGT). However, both the fuel alteration and combustion air dilution will impact the cycle performance. A complete characterization of this thermodynamic impact is essential to ensure that the mGTs become cleaner, and fully flexible to fit with the expectation of future small-scale decentralized power production. Therefore, the objective of this work is twofold: the determination of the necessary dilution for combustion stabilization, depending on the type of fuel, as well as the impact assessment on the cycle performance. In this framework, a hybrid model of the Turbec T100 mGT combustor, combining a 0D Chemical Reactor Network and 1D Laminar flame calculations, is used to first assess the flashback limits. The laminar flame speed is evaluated to predetermine the necessary minimal water dilution of the combustion air to avoid flashback for several CH4/H2 blends. Second, a thermodynamic analysis is performed to assess the impact of the flame stabilization measures on the cycle performance of the mGT using Aspen Plus. The 0D/1D simulation results show that the combustor of the Turbec T100 can operate with fuels containing up to 100% hydrogen. However, the thermodynamic analysis shows that the water dilution leads to a decreased electrical performance. Future work consists in the iterative coupling of both 0D/1D and the Aspen model to correctly predict the flashback limits, considering the altering operating conditions. To conclude, with this work, we provide a framework for future mGT operations with alternative fuels.
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Mohammad Nurizat Rahman, Norshakina Shahril e Suzana Yusup. "Hydrogen-Enriched Natural Gas Swirling Flame Characteristics: A Numerical Analysis". CFD Letters 14, n. 7 (17 luglio 2022): 100–112. http://dx.doi.org/10.37934/cfdl.14.7.100112.
Testo completoAbstract (sommario):
Increasing the amount of hydrogen (H2) in natural gas mixtures contributes to gas turbine (GT) decarbonisation initiatives. Hence, the swirling flame characteristics of natural gas mixtures with H2 are investigated in the current work using a numerical assessment of a single swirl burner, which is extensively employed in GT combustors. The baseline numerical and experimental cases pertained to natural gas compositions largely consisting of methane (CH4). The results show that the numerical model adequately describes the swirling component of the flame observed in the experiment. Altogether, the findings show that hydroxyl (OH) radical levels increase in H2-enriched CH4 flames, implying that greater OH pools are responsible for the change in flame structure caused by considerable H2 addition. The addition of 10 % H2 is predicted to raise the peak flame temperature by 4 % compared to the baseline CH4 flame. Therefore, adding 10 % H2 into a GT combustor without any flowrate tuning raises the risk of turbine material deterioration and increased thermal NOx emission. Due to the lower volumetric Lower Heating Value (LHV) of H2, which needs a higher volumetric fuel flow rate than burning natural gas/CH4 at the same thermal output, the addition of 2 % H2 is predicted to reduce the peak flame temperature by 4 % compared to the baseline CH4 flame. Hence, if 2 % H2 is fed into a GT combustor without any flowrate tuning, the required load may not be obtained. When compared to the baseline CH4 case, the addition of 5 % H2 is predicted to provide almost identical peak flame temperature, which can be postulated that the addition of 5 % H2 can produce roughly the same peak flame temperature as the pure CH4 flame because the Wobbe Index is comparable. Therefore, it reveals that incorporating 5 % H2 in the natural gas-fired GT combustor with nearly no modification is viable. More research, however, is required to fully capture the flame structure and strain for assessing transient-related phenomena such as flashback and blow off by raising the H2 proportion and utilising a higher precision turbulence model.
28
Kurdyumov, V., E. Fernández-Tarrazo, J. M. Truffaut, J. Quinard, A. Wangher e G. Searby. "Experimental and numerical study of premixed flame flashback". Proceedings of the Combustion Institute 31, n. 1 (gennaio 2007): 1275–82. http://dx.doi.org/10.1016/j.proci.2006.07.100.
Testo completo
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May, J. H., e T. M. Niemczyk. "An Argon-Sheathed Premixed Oxygen-Hydrogen Burner for Fundamental Studies". Applied Spectroscopy 42, n. 5 (luglio 1988): 788–91. http://dx.doi.org/10.1366/0003702884429120.
Testo completoAbstract (sommario):
A laminar flow premixed oxygen-hydrogen slot burner has been designed and constructed. The design allows operation over a wide variety of oxygen-hydrogen mixtures without flashback. One of the key design criteria was ease of machining, a feature that has resulted in considerable flexibility in the dimensions of the slots and the positioning of the sheath slots relative to the flame slot. The flame produced shows high stability and distinct regions which make it ideal for fundamental combustion diagnostic studies.
30
Shakir Mahmood, Abdulrahman, e Fouad Alwan Saleh. "FLAME STABILITY IN SWIRLING AND BLUFF-BODY BURNERS: A REVIEW". Jurnal Teknologi 85, n. 6 (17 settembre 2023): 1–15. http://dx.doi.org/10.11113/jurnalteknologi.v85.19588.
Testo completoAbstract (sommario):
Flame stability is one of the main challenges facing the different combustion applications. A lot of undesirable phenomena such as blow-off and flashback may occur according to the instability of the flame causing damage to the overall combustion system. Therefore, it is necessary to develop methods which help improve the combustion process and obtain wider flame stable regions. Among these methods, swirling flows and bluff-bodies are the most commonly used in flame stabilization. These may create central recirculation zones (CRZ’s) that, in turn, recirculate the heat and active chemical species to the root of flame, improving the reactants mixing and as a result, flame stability. Hence, the current article presents an overview of flame stability mechanisms and the operation map using swirling flows and bluff-bodies. The effect of the swirl number (S) and burner’s geometry, as well as the influence of the bluff-body shape, size and position on the flame stabilization mechanisms are discussed.
31
Gruber, A., A. R. Kerstein, D. Valiev, C. K. Law, H. Kolla e J. H. Chen. "Modeling of mean flame shape during premixed flame flashback in turbulent boundary layers". Proceedings of the Combustion Institute 35, n. 2 (2015): 1485–92. http://dx.doi.org/10.1016/j.proci.2014.06.073.
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Ranjan, Rakesh, Dominik F. Ebi e Noel T. Clemens. "Role of inertial forces in flame-flow interaction during premixed swirl flame flashback". Proceedings of the Combustion Institute 37, n. 4 (2019): 5155–62. http://dx.doi.org/10.1016/j.proci.2018.09.010.
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33
Zhao, Guoyan, Jianhui Du, Hongxi Yang, Tao Tang e Mingbo Sun. "Effects of injection on flame flashback in supersonic crossflow". Aerospace Science and Technology 120 (gennaio 2022): 107226. http://dx.doi.org/10.1016/j.ast.2021.107226.
Testo completo
34
Krupa, R. J., G. Zizak e J. D. Winefordner. "Shielded flashback-resistant diffusion flame burner for combustion diagnostics". Applied Optics 25, n. 20 (15 ottobre 1986): 3600. http://dx.doi.org/10.1364/ao.25.003600.
Testo completo
35
Nauert, A., P. Petersson, M. Linne e A. Dreizler. "Experimental analysis of flashback in lean premixed swirling flames: conditions close to flashback". Experiments in Fluids 43, n. 1 (5 giugno 2007): 89–100. http://dx.doi.org/10.1007/s00348-007-0327-x.
Testo completo
36
Mane, Deshmukh, Krishnamoorthy Arunagiri e Virendra Bhojwani. "Effect of mixture velocity for given equivalence ratio on flame development in Swiss roll combustor". Thermal Science, n. 00 (2019): 263. http://dx.doi.org/10.2298/tsci180604263m.
Testo completoAbstract (sommario):
Small-scale power generation using heat energy from hydrocarbon (HC) fuels is a proven technology. In this study, we analyzed 2D flame development in meso-scale Swiss roll combustor (SW). A mixture of 60% butane and 40 % propane was used (0.25-0.55 l/min). During all the analyses, equivalence ratio (1.1) was kept constant by adjusting air quantity against fuel quantity. The effect of increase in the mixture velocity on the development of flame shapes/patterns was monitored. We found different patterns of flame, e.g., Planar, Concave, conical, with the increase in mixture velocity. Increase in combustion chamber temperature was also noted. No Flashback was observed and blowout was observed with very high mixture velocity. Combustion chamber temperatures were found to be increasing with the increase in mixture velocity at the same equivalence ratio. Elongation of the flame was observed because of the increased flow velocity. Heat recirculation to the reactants enhances flame characteristics.
37
Hatem, F. A., A. S. Alsaegh, M. Al-Faham, A. Valera-Medina, C. T. Chong e S. M. Hassoni. "Enhancing flame flashback resistance against Combustion Induced Vortex Breakdown and Boundary Layer Flashback in swirl burners". Applied Energy 230 (novembre 2018): 946–59. http://dx.doi.org/10.1016/j.apenergy.2018.09.055.
Testo completo
38
Kiesewetter, F., M. Konle e T. Sattelmayer. "Analysis of Combustion Induced Vortex Breakdown Driven Flame Flashback in a Premix Burner With Cylindrical Mixing Zone". Journal of Engineering for Gas Turbines and Power 129, n. 4 (3 aprile 2007): 929–36. http://dx.doi.org/10.1115/1.2747259.
Testo completoAbstract (sommario):
In earlier experimental studies of the authors a previously unknown mechanism leading to flame flashback—combustion induced vortex breakdown (CIVB)—was discovered in premixed swirl burners. It exhibits the sudden formation of a recirculation bubble in vortical flows, which propagates upstream into the mixing zone after the equivalence ratio has exceeded a critical value. This bubble then stabilizes the chemical reaction and causes overheat with subsequent damage to the combustion system. Although it was shown earlier that the sudden change of the macroscopic character of the vortex flow leading to flashback can be qualitatively computed with three-dimensional as well as axisymmetric two-dimensional URANS-codes, the proper prediction of the flashback limits could not be achieved with this approach. For the first time, the paper shows quantitative predictions using a modified code with a combustion model, which covers the interaction of chemistry with vortex dynamics properly. Since the root cause for the macroscopic breakdown of the flow could not be explained on the basis of experiments or CFD results in the past, the vorticity transport equation is employed in the paper for the analysis of the source terms of the azimuthal component using the data delivered by the URANS-model. The analysis reveals that CIVB is initiated by the baroclinic torque in the flame and it is shown that CIVB is essentially a two-dimensional effect. As the most critical zone, the upstream part of the bubble was identified. The location and distribution of the heat release in this zone governs whether or not a flow field is prone to CIVB.
39
Mohammad Nurizat Rahman e Suzana Yusup. "Large Eddy Simulation of Hydrogen/Natural Gas/Air Premixed Swirling Flames and CIVB Flashback Risks". CFD Letters 15, n. 12 (30 ottobre 2023): 154–65. http://dx.doi.org/10.37934/cfdl.15.12.154165.
Testo completoAbstract (sommario):
The desire to develop gas turbines (GTs) that can utilise natural gas (NG) blended with hydrogen (H2) often encounters operability challenges related to combustion-induced vortex breakdown (CIVB) flashback issues. Hence, a detailed Large Eddy Simulation (LES) was employed in this study to examine the impact of H2-NG co-firing on central recirculation zones (CRZs), combustion properties, and the risk of CIVB flashback in a pilot-scale swirl burner facility. The LES model successfully replicated the swirling component of the flame observed in the experiment with reasonable accuracy. The findings revealed that the introduction of H2 into the burner increased the velocity and temperature of the burned gases. The higher reactivity of H2 resulted in faster burning rates and a shift in the reaction zone, indicating that NG-H2 firing burns more rapidly than pure NG firing. Additionally, H2 was found to enhance the velocity gradient, pushing the CRZ upstream. Changes in the location of the CRZ can disrupt density and velocity gradients, affecting the generation of vorticity by the baroclinic torque and potentially increasing negative axial velocity, thereby increasing the risk of CIVB flashback. Further research is necessary to comprehensively assess the CIVB flashback risk, particularly when the proportion of H2 exceeds 30 %.
40
Christodoulou, Loizos, Lipika Kabiraj, Aditya Saurabh e Nader Karimi. "Characterizing the signature of flame flashback precursor through recurrence analysis". Chaos: An Interdisciplinary Journal of Nonlinear Science 26, n. 1 (gennaio 2016): 013110. http://dx.doi.org/10.1063/1.4940154.
Testo completo
41
Chorpening, B. T., J. D. Thornton, E. D. Huckaby e K. J. Benson. "Combustion Oscillation Monitoring Using Flame Ionization in a Turbulent Premixed Combustor". Journal of Engineering for Gas Turbines and Power 129, n. 2 (30 agosto 2006): 352–57. http://dx.doi.org/10.1115/1.2431390.
Testo completoAbstract (sommario):
To achieve very low NOx emission levels, lean-premixed gas turbine combustors have been commercially implemented that operate near the fuel-lean flame extinction limit. Near the lean limit, however, flashback, lean blow off, and combustion dynamics have appeared as problems during operation. To help address these operational problems, a combustion control and diagnostics sensor (CCADS) for gas turbine combustors is being developed. CCADS uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. Previous development efforts have shown the capability of CCADS to monitor flashback and equivalence ratio. Recent work has focused on detecting and measuring combustion instabilities. A highly instrumented atmospheric combustor has been used to measure the pressure oscillations in the combustor, the OH emission, and the flame ion field at the premix injector outlet and along the walls of the combustor. This instrumentation allows examination of the downstream extent of the combustion field using both the OH emission and the corresponding electron and ion distribution near the walls of the combustor. In most cases, the strongest pressure oscillation dominates the frequency behavior of the OH emission and the flame ion signals. Using this highly instrumented combustor, tests were run over a matrix of equivalence ratios from 0.6 to 0.8, with an inlet reference velocity of 25m∕s(82ft∕s). The acoustics of the fuel system for the combustor were tuned using an active-passive technique with an adjustable quarter-wave resonator. Although several statistics were investigated for correlation with the dynamic pressure in the combustor, the best correlation was found with the standard deviation of the guard current. The data show a monotonic relationship between the standard deviation of the guard current (the current through the flame at the premix injector outlet) and the standard deviation of the chamber pressure. Therefore, the relationship between the standard deviation of the guard current and the standard deviation of the pressure is the most promising for monitoring the dynamic pressure of the combustor using the flame ionization signal. This addition to the capabilities of CCADS would allow for dynamic pressure monitoring on commercial gas turbines without a pressure transducer.
42
Eichler, Christian, e Thomas Sattelmayer. "Experiments on Flame Flashback in a Quasi-2D Turbulent Wall Boundary Layer for Premixed Methane-Hydrogen-Air Mixtures". Journal of Engineering for Gas Turbines and Power 133, n. 1 (14 settembre 2010). http://dx.doi.org/10.1115/1.4001985.
Testo completoAbstract (sommario):
Premixed combustion of hydrogen-rich mixtures involves the risk of flame flashback through wall boundary layers. For laminar flow conditions, the flashback mechanism is well understood and is usually correlated by a critical velocity gradient at the wall. Turbulent transport inside the boundary layer considerably increases the flashback propensity. Only tube burner setups were investigated in the past, and thus turbulent flashback limits were only derived for a fully developed Blasius wall friction profile. For turbulent flows, details of the flame propagation in proximity to the wall remain unclear. This paper presents results from a new experimental combustion rig, apt for detailed optical investigations of flame flashbacks in a turbulent wall boundary layer developing on a flat plate and being subject to an adjustable pressure gradient. Turbulent flashback limits are derived from the observed flame position inside the measurement section. The fuels investigated cover mixtures of methane, hydrogen, and air at various mixing ratios. The associated wall friction distributions are determined by Reynolds-averaged Navier-Stokes (RANS) computations of the flow inside the measurement section with fully resolved boundary layers. Consequently, the interaction between flame back pressure and incoming flow is not taken into account explicitly, in accordance with the evaluation procedure used for tube burner experiments. The results are compared with literature values, and the critical gradient concept is reviewed in light of the new data.
43
Prieur, Kevin, Guillaume Vignat, Daniel Durox, Thierry Schuller e Sébastien Candel. "Flame and Spray Dynamics During the Light-Round Process in an Annular System Equipped With Multiple Swirl Spray Injectors". Journal of Engineering for Gas Turbines and Power 141, n. 6 (9 gennaio 2019). http://dx.doi.org/10.1115/1.4042024.
Testo completoAbstract (sommario):
A successful ignition in an annular multi-injector combustor follows a sequence of steps. The first injector is ignited; two arch-shaped flame branches nearly perpendicular to the combustor backplane form; they propagate, igniting each injection unit; they merge. In this paper, characterization of the propagation phase is performed in an annular combustor with spray flames fed with liquid n-hepane. The velocity and the direction of the arch-like flame branch are investigated. Near the backplane, the flame is moving in a purely azimuthal direction. Higher up in the chamber, it is also moving in the axial direction due to the volumetric expansion of the burnt gases. Time-resolved particle image velocimetry (PIV) measurements are used to investigate the evaporating fuel droplets dynamics. A new result is that, during the light-round, the incoming flame front pushes the fuel droplets in the azimuthal direction well before its leading point. This leads to a decrease in the local droplet concentration and local mixture composition over not yet lit injectors. For the first time, the behavior of an individual injector ignited by the passing flame front is examined. The swirling flame structure formed by each injection unit evolves in time. From the ignition of an individual injector to the stabilization of its flame in its final shape, approximately 50 ms elapse. After the passage of the traveling flame, the newly ignited flame flashbacks into the injector during a few milliseconds, for example, 5 ms for the conditions that are tested. This could be detrimental to the service life of the unit. Then, the flame exits from the injection unit, and its external branch detaches under the action of cooled burnt gases in the outer recirculation zone (ORZ).
44
Hoferichter, Vera, e Thomas Sattelmayer. "Boundary Layer Flashback in Premixed Hydrogen–Air Flames With Acoustic Excitation". Journal of Engineering for Gas Turbines and Power 140, n. 5 (21 novembre 2017). http://dx.doi.org/10.1115/1.4038128.
Testo completoAbstract (sommario):
Lean premixed combustion is prevailing in gas turbines to minimize nitrogen oxide emissions. However, this technology bears the risk of flame flashback and thermoacoustic instabilities. Thermoacoustic instabilities induce velocity oscillations at the burner exit which, in turn, can trigger flame flashback. This article presents an experimental study at ambient conditions on the effect of longitudinal acoustic excitation on flashback in the boundary layer of a channel burner. The acoustic excitation simulates the effect of thermoacoustic instabilities. Flashback limits are determined for different excitation frequencies characterizing intermediate frequency dynamics in typical gas turbine combustors (100–350 Hz). The excitation amplitude is varied from 0% to 36% of the burner bulk flow velocity. For increasing excitation amplitude, the risk of flame flashback increases. This effect is strongest at low frequencies. For increasing excitation frequency, the influence of the velocity oscillations decreases as the flame has less time to follow the changes in bulk flow velocity. Two different flashback regimes can be distinguished based on excitation amplitude. For low excitation amplitudes, flashback conditions are reached if the minimum flow velocity in the excitation cycle falls below the flashback limit of unexcited unconfined flames. For higher excitation amplitudes, where the flame starts to periodically enter the burner duct, flashback is initiated if the maximum flow velocity in the excitation cycle is lower than the flashback limit of confined flames. Consequently, flashback limits of confined flames should also be considered in the design of gas turbine burners as a worst case scenario.
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Eichler, Christian, Georg Baumgartner e Thomas Sattelmayer. "Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts". Journal of Engineering for Gas Turbines and Power 134, n. 1 (4 novembre 2011). http://dx.doi.org/10.1115/1.4004149.
Testo completoAbstract (sommario):
The design of flashback-resistant premixed burners for hydrogen-rich fuels is strongly dependent on reliable turbulent boundary layer flashback limits, since this process can be the dominant failure type for mixtures with high burning velocities. So far, the flashback data published in literature is based on tube burner experiments with unconfined flames. However, this flame configuration may not be representative for the most critical design case, which is a flame being already present inside the duct geometry. In order to shed light on this potential misconception, boundary layer flashback limits have been measured for unconfined and confined flames in fully premixed hydrogen-air mixtures at atmospheric conditions. Two duct geometries were considered, a tube burner and a quasi-2D turbulent channel flow. Furthermore, two confined flame holding configurations were realized, a small backward-facing step inside the duct and a ceramic tile at high temperature, which was mounted flush with the duct wall. While the measured flashback limits for unconfined tube burner flames compare well with literature results, a confinement of the stable flame leads to a shift of the flashback limits towards higher critical velocity gradients, which are in good agreement between the tube burner and the quasi-2D channel setup. The underestimation of flashback propensity resulting from unconfined tube burner experiments emerges from the physical situation at the burner rim. Heat loss from the flame to the wall results in a quenching gap, which causes a radial leakage flow of fresh gases. This flow in turn tends to increase the quenching distance, since it constitutes an additional convective heat loss. On the one hand, the quenching gap reduces the local adverse pressure gradient on the boundary layer. On the other hand, the flame base is pushed outward, which deters the flame from entering the boundary layer region inside the duct. The flashback limits of confined flames stabilized at backward-facing steps followed this interpretation, and experiments with a flush ceramic flame holder constituted the upper limit of flashback propensity. It is concluded that the distribution of the flame backpressure and the flame position itself are key parameters for the determination of meaningful turbulent boundary layer flashback limits. For a conservative design path, the present results obtained from confined flames should be considered instead of unconfined tube burner values.
46
Hoferichter, Vera, Christoph Hirsch e Thomas Sattelmayer. "Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory". Journal of Engineering for Gas Turbines and Power 139, n. 2 (13 settembre 2016). http://dx.doi.org/10.1115/1.4034237.
Testo completoAbstract (sommario):
Premixed combustion is a common technology applied in modern gas turbine combustors to minimize nitrogen oxide emissions. However, early mixing of fuel and oxidizer opens up the possibility of flame flashback into the premixing section upstream of the combustion chamber. Especially, for highly reactive fuels, boundary layer flashback (BLF) is a serious challenge. For high preheating and burner surface temperatures, boundary layer flashback limits for burner stabilized flames converge to those of so-called confined flames, where the flame is stabilized inside the burner duct. Hence, the prediction of confined flashback limits is a highly technically relevant task. In this study, a predictive model for flashback limits of confined flames is developed for premixed hydrogen–air mixtures. As shown in earlier studies, confined flashback is initiated by boundary layer separation upstream of the flame tip. Hence, the flashback limit can be predicted identifying the minimum pressure rise upstream of a confined flame causing boundary layer separation. For this purpose, the criterion of Stratford is chosen which was originally developed for boundary layer separation in mere aerodynamic phenomena. It is shown in this paper that it can also be applied to near-wall combustion processes if the pressure rise upstream of the flame tip is modeled correctly. In order to determine the pressure rise, an expression for the turbulent burning velocity is derived including the effects of flame stretch and turbulence. A comparison of the predicted flashback limits and experimental data shows high prediction accuracy and wide applicability of the developed model.
47
Beerer, David, Vincent McDonell, Peter Therkelsen e Robert K. Cheng. "Flashback and Turbulent Flame Speed Measurements in Hydrogen/Methane Flames Stabilized by a Low-Swirl Injector at Elevated Pressures and Temperatures". Journal of Engineering for Gas Turbines and Power 136, n. 3 (5 novembre 2013). http://dx.doi.org/10.1115/1.4025636.
Testo completoAbstract (sommario):
This paper reports flashback limits and turbulent flame local displacement speed measurements in flames stabilized by a low swirl injector operated at elevated pressures and inlet temperatures with hydrogen and methane blended fuels. The goal of this study is to understand the physics that relate turbulent flame speed to flashback events at conditions relevant to gas turbine engines. Testing was conducted in an optically accessible single nozzle combustor rig at pressures ranging from 1 to 8 atm, inlet temperatures from 290 to 600 K, and inlet bulk velocities between 20 and 60 m/s for natural gas and a 90%/10% (by volume) hydrogen/methane blend. The propensity of flashback is dependent upon the proximity of the lifted flame to the nozzle that is itself dependent upon pressure, inlet temperature, and bulk velocity. Flashback occurs when the leading edge of the flame in the core of the flow ingresses within the nozzle, even in cases when the flame is attached to the burner rim. In general the adiabatic flame temperature at flashback is proportional to the bulk velocity and inlet temperature and inversely proportional to the pressure. The unburned reactant velocity field approaching the flame was measured using a laser Doppler velocimeter with water seeding. Turbulent displacement flame speeds were found to be linearly proportional to the root mean square of the velocity fluctuations about the mean velocity. For identical inlet conditions, high-hydrogen flames had a turbulent flame local displacement speed roughly twice that of natural gas flames. Pressure, inlet temperature, and flame temperature had surprisingly little effect on the local displacement turbulent flame speed. However, the flow field is affected by changes in inlet conditions and is the link between turbulent flame speed, flame position, and flashback propensity.
48
Duan, Zhixuan, Brendan Shaffer, Vincent McDonell, Georg Baumgartner e Thomas Sattelmayer. "Influence of Burner Material, Tip Temperature, and Geometrical Flame Configuration on Flashback Propensity of H2-Air Jet Flames". Journal of Engineering for Gas Turbines and Power 136, n. 2 (28 ottobre 2013). http://dx.doi.org/10.1115/1.4025359.
Testo completoAbstract (sommario):
Flashback is a key operability issue for low emission premixed combustion systems operated on high hydrogen content fuels. Previous work investigated fuel composition impacts on flashback propensity and found that burner tip temperature was important in correlating flashback data in premixed jet flames. An enclosure around the jet flame was found to enhance the flame–burner rim interaction. The present study further addresses these issues using a jet burner with various geometric configurations and interchangeable materials. Systematic studies addressing the quantitative influence of various parameters such as tip temperature, burner material, enclosure size, and burner diameter on flashback propensity were carried out. A comprehensive overview of the flashback limits for all conditions tested in the current study as well as those published previously is given. The collective results indicate that the burner materials, tip temperature, and flame confinement play significant roles for flashback propensity and thus help explain previous scatter in flashback data. Furthermore, the present work indicates that the upstream flame propagation during flashback is affected by the burner material. The material with lower thermal conductivity yields larger flashback propensity but slower flame regression inside the tube. These observations can be potentially exploited to minimize the negative impacts of flashback in practical applications.
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Novoselov, Alex, Dominik Ebi e Nicolas Noiray. "Accurate Prediction of Confined Turbulent Boundary Layer Flashback Through a Critically Strained Flame Model". Journal of Engineering for Gas Turbines and Power, 31 agosto 2022. http://dx.doi.org/10.1115/1.4055413.
Testo completoAbstract (sommario):
Abstract A novel boundary layer flashback model is developed based on previous measurements that showed flashback limits may be related to strained premixed flame extinction. According to the model, flashback occurs at the equivalence ratio where the strained extinction limit flame speed matches the mean axial flow velocity one thermal distance from the wall. The model is validated by comparison with experimental measurements of flashback of confined non-swirling turbulent hydrogen-air flames. This comparison shows that the proposed model is capable of predicting confined turbulent boundary layer flashback across a large range of wall velocity gradients and preheat temperatures. The model is extended to methane-hydrogen-air flames in a swirling configuration using information about a single flashback event and shows good agreement with experimental measurements as a function of both hydrogen mole fraction in the fuel and pressure. In addition, inclusion of a mean non-reacting velocity field computed via Large Eddy Simulation allows for a significant increase in the accuracy of the model when applied to swirling flows. Ultimately, this model provides a new pathway for the design of flashback resistant gas turbines, even with the addition of fuels like hydrogen.
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Jaeschke, Alexander, Bernhard Ćosić, Dominik Wassmer e Christian Oliver Paschereit. "Experimental Investigation Of A Multi Tube Burner For Premixed Hydrogen And Natural Gas Low Emission Combustion". Journal of Engineering for Gas Turbines and Power, 8 settembre 2023, 1–14. http://dx.doi.org/10.1115/1.4063378.
Testo completoAbstract (sommario):
Abstract Hydrogen as an essential part of future decarbonisation of the energy industry makes it a crucial necessity to replace conventional, natural gas based concepts in gas turbine combustion. This paper presents an experimental study of a multi tube jet flame burner. The study is carried out with natural gas and pure hydrogen fuel at gas turbine relevant conditions at atmospheric pressure. To identify key differences between hydrogen-air and natural gas-air flames on the overall robustness and flame flashback behaviour, air bulk velocity (80-120 m/s), adiabatic flame temperature (1235-2089 K) and air inlet temperature (623-673 K) are varied over a wide range, covering a range of Reynolds numbers of 10.000 to 20.000. Depending on flame temperature, two different flame shapes are observed for natural gas-air flames. The shape of the hydrogen-air flame changes less over the range of flame temperatures tested, but is generally more compact. The process of fuel-air mixing is further investigated by concentration distribution measurements in a water tunnel setup. Therefore, planar laser-induced fluorescence is utilized for visualization. The measured concentration distributions confirm the overall good mixing quality but also give an explanation on the observed flashback behaviour of the different burner designs at reacting tests. The findings of the study are composed in a flash back correlation combining the observed flashback drivers for the burner configurations investigated.