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Flashback of premixed methane–air flames in the turbulent boundary layer of swirling flows is investigated experimentally. The premix section of the atmospheric model swirl combustor features an axial swirler with an attached center-body. Our previous work with this same configuration investigated the flame propagation during flashback using partic...
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... The swirling flow is widely utilized to enhance mixing and flame holding [12,13]. During the BLF in a swirl burner, the experiments [14][15][16][17] observed that a flame tongue, a convexshaped large-scale flame front, propagates upstream along the central bluff body. The flame tongue rotates along the bulk-flow direction for CH 4 /air and CH 4 /H 2 /air flames at 1 bar [15,16], whereas the flame front swirls against the bulk-flow direction for CH 4 /H 2 /air flames at 2.5 bar [17]. ...
... During the BLF in a swirl burner, the experiments [14][15][16][17] observed that a flame tongue, a convexshaped large-scale flame front, propagates upstream along the central bluff body. The flame tongue rotates along the bulk-flow direction for CH 4 /air and CH 4 /H 2 /air flames at 1 bar [15,16], whereas the flame front swirls against the bulk-flow direction for CH 4 /H 2 /air flames at 2.5 bar [17]. The mechanism for the switch of flame propagation modes is still unclear. ...
... The flame-tongue structure and the two propagation modes in the present LES-FSD agree well with experimental observations [15][16][17]. ...
We model the boundary-layer flashback (BLF) of CH 4 / H 2 /air swirling flames via large-eddy simulations with the flame-surface-density method (LES-FSD), in particular, at high pressures. A local displacement speed model tabulating the stretched flame speed is employed to account for the thermo-diffusive effects, flame surface curvature, and heat loss in LES-FSD. The LES-FSD well captures the propagation characteristics during the BLF of swirling flames. In the LES-FSD for lean CH 4 / H 2 /air flames at 2.5 bar, the critical equivalence ratio for flashback decreases with the increasing hydrogen volume fraction, consistent with the experiments. This is due to the improved modeling of effects of the flame stretch and heat loss on the local displacement speed. We also develop a simple model to predict the BLF limits of swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 14 datasets of CH 4 / H 2 /air swirling flame experiments, with the hydrogen volume fractions in fuel from 50% to 100%. The present model well estimates the flashback limits in various operating conditions.
... The swirling flow is widely utilized to enhance mixing and flame holding [12,13]. During the BLF in a swirl burner, the experiments [14][15][16][17] observed that a flame tongue, a convexshaped large-scale flame front, propagates upstream along the central bluff body. The flame tongue rotates along the bulk-flow direction for CH 4 /air and CH 4 /H 2 /air flames at 1 bar [15,16], whereas the flame front swirls against the bulk-flow direction for CH 4 /H 2 /air flames at 2.5 bar [17]. ...
... During the BLF in a swirl burner, the experiments [14][15][16][17] observed that a flame tongue, a convexshaped large-scale flame front, propagates upstream along the central bluff body. The flame tongue rotates along the bulk-flow direction for CH 4 /air and CH 4 /H 2 /air flames at 1 bar [15,16], whereas the flame front swirls against the bulk-flow direction for CH 4 /H 2 /air flames at 2.5 bar [17]. The mechanism for the switch of flame propagation modes is still unclear. ...
... The flame-tongue structure and the two propagation modes in the present LES-FSD agree well with experimental observations [15][16][17]. ...
We model the boundary-layer flashback (BLF) of CH$_4$/H$_2$/air swirling flames via large-eddy simulations with the flame-surface-density method (LES-FSD), in particular, at high pressures. A local displacement speed model tabulating the stretched flame speed is employed to account for the thermo-diffusive effects, flame surface curvature, and heat loss in LES-FSD. The LES-FSD well captures the propagation characteristics during the BLF of swirling flames. In the LES-FSD for lean CH$_4$/H$_2$/air flames at 2.5 bar, the critical equivalence ratio for flashback decreases with the increasing hydrogen volume fraction, consistent with the experiments. This is due to the improved modeling of effects of the flame stretch and heat loss on the local displacement speed. We also develop a simple model to predict the BLF limits of swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 14 datasets of CH$_4$/H$_2$/air swirling flame experiments, with the hydrogen volume fractions in fuel from 50% to 100%. The present model well estimates the flashback limits in various operating conditions.
... Despite its high relevance for various practical combustor designs, there are only a few studies on flashback in a bluff-body swirl burner. The Clemens group [13][14][15][16] experimentally identified two modes of flame propagation during flashback, i.e., (1) small-scale bulges propagating in the negative streamwise direction; and (2) large-scale flame tongues swirling with the bulk flow while leading flashback. More recently, a new flashback mode was discovered by Ebi et al. [17] , and they found that the upstream propagation of flame can be led by flame bulges. ...
... With this background, the objective of this work is twofold: (a) to investigate the impact of thermal boundary conditions on flashback of premixed CH 4 [13][14][15][16] (left) and the computational domain used in this work (right), which is superimposed with streamlines colored by the mean vorticity and time-averaged OH field. nisms responsible for flashback through the analysis of near-wall flame-flow interactions. ...
... The numerical simulations performed in this work refer to the experiments conducted in a bluff-body swirl burner developed by the Clemens group [13][14][15][16] , which consists of mixing tube, combustion chamber, and swirler, as shown in Fig. 1 . The combustion chamber has a diameter of 100 mm and length of 150 mm. ...
One of the major challenges of applying hydrogen (H2) enriched fuels in industrial combustion systems is the risk of flashback that can cause considerable damage on combustors and affect pollutant emissions. While flashback has been extensively investigated, comparatively less analysis is made to understand the impact of thermal boundary conditions on flashback of bluff-body swirling H2 enriched flames. The aim of this work is to fill a part of this gap through large-eddy simulations of premixed 95%H2/5%CH4-air flame flashback in a bluff-body swirl burner. The results show that flashback characteristics are very sensitive to the bluff-body thermal boundary condition. At Twall=350 K, flashback is led by a large-scale swirling flame tongue (Mode I) that can cause the deflection of streamlines ahead of the flame front and is responsible for flashback Mode I, while a small region of negative velocity induced by small-scale flame bulges cannot lead to a net upstream propagation of the flame front. However, when treating the bluff-body as an adiabatic wall, the flashback is led by multiple small-scale flame bulges, and the circumferential motion of the lowest flame tip is negligible (Mode II). These small-scale bulges can cause large reverse flow pockets and formation of a stagnation point ahead of the preheat zone that facilitates flashback, similar to non-swirling channel flashback. This is the main mechanism responsible for flashback Mode II. At Twall=500 K, flashback is led by different structures, switching between Mode I (upstream propagation of swirling flame tongue) to Mode II (upstream propagation of non-swirling flame bulges). Furthermore, it is found that as the boundary heat loss increases, the axial flashback speed decreases, while the azimuthal flashback speed increases due to the presence of Mode I. The results open up the possibility of extending the flashback limit of H2 enriched swirling flames by controlling the thermal condition of bluff body.
... In this context, hydrogen-enriched methane has been employed in gas turbines to reduce carbon emissions. It is noted that combustion of a CH 4 /H 2 /air mixture under stoichiometric or slight lean conditions could suffer from the danger of flashback [4][5][6][7][8][9] due to its high flame speed and at the same time would produce substantially more nitrogen oxides (NO ) than equivalent fossil fuels because of its high combustion temperature [10]. To Table 1 Summary of previous studies on premixed, hydrogen-enriched methane/air swirling flames since 2010, in which the equivalence ratio and operating pressure as well as hydrogen mole fraction H 2 in the methane/hydrogen blends are indicated. ...
... 0%-50% 30,000-100,000 1 1-5 Thermoacoustics NTNU [35] 0.65-1 0%-20% --1.5-3.3 Thermoacoustics PSU [43] 0.65-0.7 0%-40% 17,000 0.7 1 Thermoacoustics Imperial [12] 0.55-0.7 0%-40% 19,000 -1 Thermoacoustics UT/PSI [4][5][6][7][8][9] 0.4-0.9 0%-95% 3,000-40,000 0.7-0.9 ...
Hydrogen (H2) is regarded as a promising fuel to achieve decarbonization of power and propulsion systems. In this context, hydrogen enriched methane (CH4) combustion has attracted considerable attention in the development of low-emission gas turbines. To achieve low NOx emissions and avoid the dangers of flashback, combustion of CH4/H2/air mixtures under lean and/or ultra-lean operating conditions is of critical importance, while ultra-lean flames are prone to combustion instabilities and difficult to stabilize even in a bluff-body swirl burner. In this work, a series of confined lean premixed CH4/H2/air swirling flames with hydrogen enrichment (α H2) ranging from 0 to 80% is investigated under stable and ultra-lean conditions using simultaneous OH-PLIF and PIV measurements. The results suggest that decarbonization of combustion devices requires large volume fractions of H2 in the fuel mixture, e.g., 80% H2 to achieve half CO2 emission per heat of combustion. It is found that there is a flame topology transition when changing equivalence ratio and/or hydrogen enrichment. At a given α H2, the flames with 0 and 40% H2 always show “V” shapes, whereas an evolution from “M” to “V” shape can be observed for the 80% H2 flame when increasing the equivalence ratio. Moreover, at a given ϕ, the flame shape will shift towards “M” shape at α H2 = 80% from “V” shape at α H2 = 0,40%. Furthermore, H2-enriched flames would move to the inner recirculation zone (IRZ) and stabilize there when decreasing ϕ to ultra-lean conditions. Given that hydrogen enrichment can significantly enhance the resistance to flame strain and that under ultra-lean conditions, there is a strong diffusion of hydrogen from the swirling jet to the IRZ where the sufficient residence time and the increase in the local equivalence ratio contribute to the presence of flame pockets and flame stabilization in the IRZ.
... This leads to turbulent flame speeds exceeding locally the convective flow velocities of the fresh gas. Especially at the boundary layers of the combustor walls [18,19] but for swirl flames also close to the central recirculation zones [20][21][22][23] flow velocities are locally small enough to facilitate flashback. In contrast to the higher temperatures, typical gas turbine operation pressures cause a decrease in laminar flame speeds [24,25]. ...
... This is most likely due to the local heating of the air-cooled prechamber and thus the increased likelihood of flashback due to hot walls [37]. It has been observed that small reversed flow flame pockets are formed at the entrance of the prechamber walls, as it has been reported, measured, and visualized by Ebi et al. [21][22][23] in several studies. Most likely, these low frequency reversed flow pockets are causing the fluctuation of the signal of the prechamber thermocouples. ...
Blending of natural gas with hydrogen is a viable pathway for the decarbonization of industrial gas turbines for combined heat and power applications. Very high blending ratios of hydrogen are needed to achieve significant CO2 emission reductions. However, burning high hydrogen contents in the gas turbine is challenging in terms of NOx emissions and the mitigation of flashback risks as well as suppressing thermoacoustic instabilities. This paper illustrates a design modification to improve the hydrogen capabilities of the Advanced Can Combustion (ACC) system and its ultra-low emission industrial swirl burner for the MGT6000 gas turbine that was originally designed for pure natural gas combustion. A flow conditioner is installed upstream of the swirler aiming to decrease the fuel amount close to the combustor walls and thereby increase the flashback resistance of the burner. High pressure (≈14bar) full power (≈4MWth) single can combustion tests and atmospheric burner tests are used for the assessment of the hydrogen capabilities for the original and the retrofitted burner. Different levels of hydrogen blending of up to 45 vol-% at high pressure and 93 vol-% at atmospheric conditions as well as different gas turbine relevant flame temperatures are assessed in terms of emissions, flame flashback and thermoacoustic stability. Low speed thermocouple measurements at the burner walls are identified as a good precursor for hydrogen induced flame flashback at the walls. The amplitude of the thermocouple fluctuation is observed to be similar for atmospheric and elevated pressure. Moreover, it is shown that the increase in NOX emissions associated to hydrogen blending can be transferred from atmospheric conditions to elevated pressure. The experimental dataset is used for the calibration of Computational Fluid Dynamics (CFD) calculations to allow for the assessment at different operating conditions and future modifications. The CFD is focused on the prediction of flashback resistance for different blends of hydrogen and natural gas at high pressure conditions.
... previous investigations have linked flashback to either CIVB, to the mechanisms seen in non-swirling boundary-layer flashback or to the density gradient across the flame front causing a rise in static pressure (Ebi et al., 2018). Given the common use of swirling flows for flame stabilisation in IGTs and that an increase in swirl has been shown to cause flashback (Heeger et al., 2010), it is important to understand and predict the effect of swirl on flashback. ...
... Ebi et al. (2020) showed that flashback limits, in terms of equivalence ratio, become smaller with increased proportions of hydrogen. The investigation also showed a different flashback mechanism for the swirling flow compared to that seen in the previous investigations (Ebi and Clemens, 2014, 2016Ebi et al., 2018). In this second mechanism, the flame propagated more similarly to flame propagation in non-swirling channel flows: the flame propagates locally against the oncoming flow (lead by the orange side in Figure 2.12) with small-scale bulges forming along the flame front, also facing this oncoming flow. ...
... The question remains how the swirling flow, and the generation of a radial pressure gradient or additional baroclinic torque, influence flashback. Swirling boundary-layer flashback has been linked to both combustion-induced vortex breakdown, where flashback is described as being caused by baroclinic torque generated by the radial pressure gradient (Kiesewetter et al., 2007), and the mechanisms seen in the con- The boundary-layer flashback of turbulent swirling flows has also been shown experimentally to be different to that of non-swirling flows (Ebi and Clemens, 2014, 2016Ebi et al., 2018). Rather than a series of bulges and cusps forming which propagate upstream (as seen for non-swirling upstream from a leading side of the flame tongue (highlighted in blue). ...
This thesis presents analysis and modelling of boundary-layer flashback processes for hydrogen rich gas-turbine combustion. The future use of industrial gas turbines will be dependent on lowering their carbon intensity, thus requiring flexible use of alternative fuels, such as those rich in hydrogen. Hydrogen has significantly different properties to traditional fuels, for example hydrogen shows an increased risk of flashback, where the flame propagates upstream from the combustion chamber into the premixing section of the gas turbine. Flashback is a significant safety concern which causes plant shutdowns and damage to equipment. The risk of flashback for hydrogen fuels, with their significantly higher flame speed, is particularly high in the case of boundary-layer flashback. Boundary-layer flashback has also been shown to be caused by an increase in swirl, which is particularly important for gas turbines where swirl is commonly used for flame stabilisation. To enable the use of hydrogen rich fuels in gas turbines it is therefore important to understand the physical mechanisms underlying boundary-layer flashback in swirling flows and to predict the effect of swirl on flashback speeds.
This thesis describes models of flashback in channels and annuli using a Froude number to describe the effect of swirl. The predictions of flashback speed, and physical mechanisms underlying them, are validated using both two-dimensional laminar simulations and three-dimensional turbulent simulations in planar channels and annuli. In non-swirling flows, boundary-layer flashback is dominated by flame propagation that is enhanced by volumetric expansion through the flame. In swirling flows, it is shown that the radial pressure gradient, resulting from centripetal acceleration, causes flow diversion around the flame and results in pressure-driven flashback. These two physical mechanisms are described by models using a momentum balance over the flame, and an additive model that combines a flame-propagation and a pressure-driven term. The trend in flashback speed with swirl is validated using the laminar simulations and experimental data from previous investigations. Finally, the laminar simulations are used to investigate and develop empirical models for the effect of bulk velocity, channel height and boundary-layer development on flashback speed.<br/
... They proposed the two concepts of a large-scale "flame tongue" and a small-scale "flame bulge" to describe swirl-flame propagation in a tube with a central body. The flame tongue denotes the large leading part of the propagating flame, where the negative axial velocity area around the flame front mainly concentrates in the radial direction and largely beyond the quenching distance [18]. The flame bulges feature a physical length scale that is an order-of-magnitude smaller than the flame tongue and are scattered on the trailing part of the swirl flame, where the induced reverse flow pocket is radially narrow, but reaches farther upstream compared to the flame tongue. ...
... However, the use of numerical simulations for modeling flashback is still limited by two aspects. First, flashback is a transient process with multi-physics interactions including turbulent near-wall flow [23], the interaction between reaction and near-wall quenching [12], and propagation in boundary layers [14,[16][17][18]. Second, the geometry of practical gas turbine combustors that employ swirling flows is complex, which introduces additional physical processes. ...
... The flashback occurs along the center body dominated by the presence of the flame tongue (introduced in Section 1). The flame front can be further divided into the leading side and the trailing side based on the flame motion and the lowest axial location (i.e., the flame base [18]). The effect of wall temperature on this flame structure is discussed later in Section 3.2.2. Figure 12 provides an overview of the velocity field and flame surface evolution from experiments and computations as the flame passes through the observation window (denoted by the vertical green line in the flame surface plots). ...
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.
... It has been recently demonstrated that the thermal expansion of burnt gas plays a role in boundary layer flashback of premixed turbulent flames [1] , enhancing the flame speed through the onset of the Darrieus-Landau (DL) instability, the associated flame corrugation and the insurgence of near-wall flow-reversal on the reactants' side of the flame. Recent experimental investigations confirmed the existence of these flow reversal regions in both swirling and non-swirling configurations [2][3][4][5] , with flame velocities approximately twice larger than the laminar flame speed. The three-dimensional simulations of Ref. [1] revealed that the flame in the boundary layer behaves in an almost laminar fashion, but its shape is modified by weak, highly anisotropic velocity fluctuations. ...
It has been previously demonstrated that thermal gas expansion might have a role in boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], inducing local flow-reversal in the boundary layer's low-velocity streaks on the reactants’ side of the flame and facilitating its upstream propagation. We perform a two-dimensional numerical investigation of the interaction between a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged streamwise velocity streaks with alternating regions of high and low velocities found in turbulent boundary layers in the vicinity of the walls. The parametric study focuses on the amplitude and wavelength of the periodic shear flow and on the gas expansion ratio (unburnt-to-burnt density ratio). With the increase of the amplitudes of the periodic shear flow and of the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of curved premixed flame velocity. The flame shape that is initially formed by the oncoming periodic shear appears to be metastable. At a later stage of the flame propagation, the flame shape transforms into the stationary one dominated by the Darrieus-Landau instability.
... This flashback mode is different from swirl flame boundary layer flashbacks at higher swirl numbers, where previous studies found that the flame is swirling in the same direction as the bulk flow for swirl numbers of about 1.0 and 0.9, respectively[14,15]. In such a high-swirl boundary layer flashback mode, the relevant flow-flame interaction occurs on the leeward side of larger flame tongues[16]. In contrast, in the current swirl burner configuration, the flame structure and alignment between approach flow and flame propagation direction shows a lot of similarity with generic, turbulent boundary layer flashbacks (BLFs) in non-swirling channel or pipe flows[2].At time instant t 2 , the portion of the flame leading the flashback is on the back side of the center body. ...
Operating stationary gas turbines on hydrogen-rich fuels offers
a pathway to significantly reduce greenhouse gas emissions
in the power generation sector. A key challenge in the design of
lean-premixed burners, which are flexible in terms of the amount
of hydrogen in the fuel across a wide range and still adhere to the
required emissions levels, is to prevent flame flashback. However,
systematic investigations on flashback at gas turbine relevant
conditions to support combustor development are sparse. The
current work addresses the need for an improved understanding
with an experimental study on boundary layer flashback in a
generic swirl burner up to 7.5 bar and 300°C preheat temperature.
Methane-hydrogen-air flames with 50 to 85% hydrogen by
volume were investigated. High-speed imaging was applied to
reveal the flame propagation pathway during flashback events.
Flashback limits are reported in terms of the equivalence ratio
for a given pressure, preheat temperature, bulk flow velocity
and hydrogen content. The wall temperature of the center body
along which the flame propagated during flashback events has
been controlled by an oil heating/cooling system. This way, the
effect any of the control parameters, e.g. pressure, had on the
flashback limit was de-coupled from the otherwise inherently associated change in heat load on the wall and thus change in wall
temperature. The results show that the preheat temperature has a
weaker effect on the flashback propensity than expected. Increasing
the pressure from atmospheric conditions to 2.5 bar strongly increases the flashback risk, but hardly affects the flashback limit
beyond 2.5 bar.
... See Refs. [9,10] for a recent excellent experimental characterization of flashback in swirling flows. ...
... In the past, practical difficulties in performing accurate experimental measurements in the near-wall region of reactive flows have represented a considerable challenge and only recently improved laser-based diagnostic techniques have enabled the acquisition of high-quality empirical data on flame-wall interactions [18] and near-wall flame propagation [19]. Recent experimental and numerical investigations of swirling and non-swirling reactive flows [10,[19][20][21] have revealed the presence of flameinduced flow reversals in the viscous layer (y + 20) immediately upstream of the flame surface. These flow reversal "pockets" are consistently associated with regions of the flame front that are convex towards the reactants. ...
Direct numerical simulations are performed to investigate the transient upstream flame propagation (flashback) through homogeneous and fuel-stratified hydrogen-air mixtures transported in fully developed turbulent channel flows. Results indicate that, for both cases, the flame maintains steady propagation against the bulk flow direction, and the global flame shape and the local flame characteristics are both affected by the occurrence of fuel stratification. Globally, the mean flame shape undergoes an abrupt change when the approaching reactants transition from an homogeneous to a stratified mixing configuration. A V-shaped flame surface, whose leading-edge is located in the near-wall region, characterizes the nonstratified, homogeneous mixture case, while a U-shaped flame surface, whose leading edge propagates upstream at the channel centerline, distinguishes the case with fuel stratification (fuel-lean in the near-wall region and fuel-rich away from the wall). The characteristic thickness, wrinkling, and displacement speed of the turbulent flame brush are subject to considerable changes across the channel due to the dependence of the turbulence and mixture properties on the distance from the channel walls. More specifically, the flame transitions from a moderately wrinkled, thin-flamelet combustion regime in the homogeneous mixture case to a strongly wrinkled flame brush more representative of a thickened-flame combustion regime in the near-wall region of the fuel-stratified case. The combustion regime may be related to the Karlovitz number, and it is shown that a nominal channel-flow Karlovitz number, Kainch, based on the wall-normal variation of canonical turbulence (tη=(ν/ε)1/2) and chemistry (tl=δl/Sl) timescales in fully developed channel flow, compares well with an effective Karlovitz number, Kaflch, extracted from the present DNS datasets using conditionally sampled values of tη and tl in the immediate vicinity of the flame (0.1<C<0.3).
