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This paper describes nitrogen oxide (NOx) measurements from a reacting jet in crossflow (RJICF). The NOx production of the RJICF is controlled by jet stoichiometry, crossflow temperature and compos...

A confluence of technology development, policy support, and industry investment trends is accelerating the pace of Hydrogen (H2) technology demonstrations, increasing the likelihood of power sector impacts. In preparation for a large scale power sector shift toward decarbonization for a low carbon future, several major power equipment manufacturers are developing gas turbines that can operate on a high H2 volume fuel. Many have H2 capable systems now that range from 5% to 100% H2. Units with 100% H2 capabilities are either using a diffusion burner or some version of a wet low emissions (WLE) burner. Most dry low emission/dry low NOx (DLE/DLN) technologies are currently limited to approximately 60% H2 or less. Therefore, research is currently underway to develop low NOx gas turbine combustion systems with improved Hydrogen capability. This paper provides an overview of the technical challenges of Hydrogen combustion and the probable technologies with which the manufacturers will respond.

This paper analyzes turbulence-chemistry interactions of an n-dodecane-air flame, focusing on the degree to which flame structure and fuel oxidation pathways change in turbulent flames relative to their corresponding laminar flames. This work is based on a lean (ϕ = 0.7) n-dodecane-air flame DNS database from Aspden et al. (2017). The relative roles of dominant reactions that release heat and produce/consume radicals are examined at various turbulence intensities and compared with stretched flame calculations from counterflow flames and perfectly stirred reactors. These results show that spatially integrated chemical pathways are relatively insensitive to turbulence intensity and mimic the behavior of stretched flames. In other words, the contribution of a given reaction to heat release or radical production, integrated over the entire flame, is insensitive to turbulence. Localized analysis conditioned on topological feature of the flame and on temperature is also performed. The former analysis reveals that larger alteration of pathways occurs in the positively-curved regions of the flame. Most significantly, it shows that the thermal structure of the flame is altered, as peak reaction rates and heat release in the low temperature (i.e., below 1200 K) region shift towards higher temperatures with increases in Karlovitz number. This result is particularly interesting given that prior work with lighter fuels (e.g., hydrogen) showed the opposite behavior. Various stretched, laminar flame calculations with altered transport effects were performed for reference. These calculations, using mixture-averaged and Le = 1 transport model, can capture similar shifts towards higher temperatures in laminar flames with increasing stretch. However, the stretch rate and transport values must be tuned to match these shifts in thermal structure differently, depending upon reactions, species, and Ka value. This effect is particularly prominent for low temperature species. Thus, these results show that flames do not simply shift to a Le = 1 thermal structure with high turbulence intensity, as previously suggested, but there is interplay between altered scalar diffusivity and stretch effects.

This study explores the effect of heat release on the growth of the shear layer vortical structures in a reacting jet in crossflow. Jets composed of mixtures of hydrogen, helium and nitrogen were used to independently vary the momentum flux ratio (J), jet to crossflow density ratio (S) and heat release. Velocity fields were obtained from 10 kHz high-speed stereoscopic particle image velocimetry (SPIV) and regions of elevated temperature/combustion products from simultaneous OH planar laser induced fluorescence (OH-PLIF). The shear layer vortices (SLV) originating from instabilities in the windward and leeward shear layers were identified using vortex identification indicator functions in order to track their spatial location and strength. The results show that the asymmetries in shear layer strength between the windward and leeward shear layers are dependent primarily on J, for both reacting and non-reacting flow-fields. The SLV growth rate dependencies on J and S is found to match trends noted by previous studies for non-reacting jets, where SLV growth rates increase with degree of global instability of the JICF. Heat release is also shown to suppress the SLV growth rates relative to non-reacting cases with the same jet parameters. Related to this point, the degree of lifting of the flame also has a significant impact on SLV growth. As flame lifting is directly related to autoignition times, this point shows strong coupling between kinetic rates and jet hydrodynamic stability.

This paper describes NOx measurements from reacting jets in crossflow (RJICF). This work is motivated by interest in axial staging of combustion as a means of reducing NOx emissions at high flame temperatures (>1900 K), where thermal NOx production rates are high. In this approach, the majority of the fuel is burned in a conventional lean-premixed flame, but additional fuel is injected from the combustor walls into the vitiated flow further downstream. The NOx emissions of RJICF are influenced by the secondary fuel jet stoichiometry, jet/crossflow mixing before combustion, as well as secondary combustion product mixing with the bulk product stream. In turn, jet/crossflow mixing is controlled by the hydrodynamic stability of the jet, as well as degree of flame lifting. A key challenge in understanding fundamental factors influencing NOx is decoupling the effect of bulk temperature rise due to the RJICF (∆T), JICF momentum flux ratio (J), and JICF stoichiometry (ϕJet), as they cannot be varied independently. As such, significant effort was made in developing a test matrix to differentiate their effects. Measurements reported here were obtained from rich premixed methane/air jets injected into a varying temperature (1650 K–1800 K) vitiated crossflow, for bulk temperature rises from 20 K–290 K, J values from 1.3–4.4, and ϕJet values from 1–9. These measurements show that NOx emissions monotonically increase with ∆T, as noted in prior studies, but the data reported here are able to differentiate the effects of ∆T and other parameters. In fact, for a given ΔT value, NOx values can vary by 2X depending upon other parameters. For example, the lifting of the flame (LO), which varies with ϕJet and J has significant effects on NOx emissions. These data suggest that the key fundamental JICF parameters influencing NOx emissions are ∆T, ϕJet, J, and LO.

This paper describes an analysis of the mechanisms of autoignition-controlled flame initiation and flame stabilization in a nonpremixed jet in crossflows, using simultaneous high-speed (10 kHz) tomographic particle image velocimetry, OH-PLIF and line-of-sight flame emissions. Measurements are conducted on a turbulent, transverse, reacting propane jet issued into a crossflow generated by combustion of natural gas at an equivalence ratio of 0.4 with the crossflow velocity of 10 m/s, the crossflow temperature of 1350 K and the jet momentum flux ratio of 41. While several prior studies have analyzed the lifted character of the flame in similar configurations, we show that several dynamic processes precede the leading edge of the lifted diffusion flame, including formation and evolution of “autoignition kernels”, “flame kernels” and “flame fragments”. “Autoignition kernels”, i.e., discrete compact reaction zones with the peak hydroxyl (OH) fluorescence intensity below that of the diffusion flame, initiate preferably at bulges along the jet periphery where the strain rates and the scalar dissipation rates are lower. The autoignition kernel grows in both size and the OH-fluorescence intensity as it convects downstream. An autoignition kernel transitions into a propagating flame kernel, which quickly gets distorted and elongated in the direction of the principal expansion strain rate to form a flame fragment. Neighboring flame fragments merge with each other and with the downstream diffusion flame via edge-flame propagation. Merging of upstream flame fragments with the downstream diffusion flame results in an upstream advancement of the diffusion-flame front. The diffusion flame front is intrinsically unsteady because of the rather random formation and evolution of autoignition kernels, flame kernels and flame fragments, presumably due to the stochastic velocity, the strain rate and mixture-fraction oscillations.

This paper describes implementation of simultaneous, high speed (5 kHz) stereo PIV, OH and fuel-PLIF in a pressurized, liquid fueled, swirl stabilized flame. The experiments were performed to characterize the flow field, qualitative heat release and fuel spray distributions, and flame dynamics. Acquiring high speed OH-PLIF in pressurized, liquid fuel systems is difficult due to the strong overlap of the fuel's absorption and emission spectra with the OH fluorescence spectrum. To overcome difficulties associated with the overlap, the OH and fuel fluorescence signals were partially separated by using two cameras with differing spectral filters and data acquisition timing. Upon data reduction, regions containing fuel, OH and a mixture of fuel and OH are identified. Instantaneous and time-averaged results are discussed showing the flow field, flame position and dynamics, and spray distribution from the fuel signal for two multi-component liquid fuels, at two inlet temperatures and three pressures. These results are used to infer several important observations on coupled flow and flame physics. Specifically, the flame is “M-shaped” at higher preheat temperature and higher fuel/air ratio, as opposed to no visible reaction on the inside of the annular fuel/air jet at low temperature and fuel/air ratio conditions. While such fundamentally different flame topologies in gaseous, premixed flames are well known, these results show that there are also different families of flame shapes and heat release distributions in spray flames. In addition, the flame position with respect to the flow is different for the liquid-fueled flame than for gaseous, premixed flames—in premixed flames with this geometry, the flame lies in the low velocity shear layer separating the reactants and the recirculating products. In contrast, the flame location is controlled by the spray location in this spray flame, as opposed to the shear layer. For example, reactions are observed near the nozzle outlet in the core of the high velocity annular jet, something which would not be observed in the premixed flame configuration. Also of interest is the near invariance of the key flow features—such as jet core trajectory or shear layer locations—to the operating condition changes for this study, even as the spray penetration and distribution, and flame position change appreciably.

Axially staged combustors offer the potential for achieving low NO x emissions from gas turbines at the elevated temperatures required to achieve at least 65% combined cycle efficiencies. While there is evidence that premixing between the main burner products and secondary stream reduces NO x , it is important to characterize the specific requirements needed to achieve low emissions at gas turbine conditions. This study examines the sensitivity of NO x to finite-rate, large-scale entrainment of the main and secondary streams under the simplifying assumption of infinitely-fast small-scale mixing. Essentially, this allows us to isolate the effects of limited large-scale entrainment rates by using a homogeneous/uniform condition for the entrained and reacting gases. We use a reduced-order reactor network model to examine a generic staged-combustor whose inputs are physical time scales that embody the finite-rate entrainment characteristics. With simulations conducted over a large parameter space at typical operating conditions (25 atm, 650 K air), the results show that, even when small-scale mixing is infinitely fast and the reaction zone is uniform, entrainment significantly affects NO x emissions due to its influence on the equivalence ratio — and thus the temperature and time — at which the entrained mixture ignites. Fuel-air staging is shown to be vital to NO x reduction in most practical cases where entrainment times exceed roughly 1 ms and the secondary stream finishes entraining before the main burner products. A constrained NO x minimization indicates that using the leanest possible main burner and re-routing as much air as possible to the secondary stage is key to low NO x with finite-rate entrainment; for example, less than 10 ppm NO x can be achieved if the secondary stage contains 20% of the total mass flow.

© 2016 The Combustion Institute This paper considers the kinetic pathways of hydrogen oxidation in turbulent, premixed H2–air flames. It assesses the relative roles of different reaction steps in H2oxidation relative to laminar flames, and the degree to which turbulence–chemistry interactions alters the well understood oxidation pathway that exist in laminar flames. This is done by analyzing the turbulent, lean (ϕ = 0.4), H2–air flame DNS database from Aspden et al. [17]. The relative roles of dominant reaction steps in heat release and radical formation/consumption are analyzed at different Karlovitz numbers and compared with laminar stretched flame calculations from counterflow flames and perfectly stirred reactors. It is found that both the progress variable conditioned and spatially integrated contributions of the dominant reactions remain qualitatively similar between a highly turbulent and a laminar unstretched flame. Larger changes, up to a factor of about two, occur in the relative roles of reactions with secondary influences on heat release and radical production/consumption. These results suggest that the kinetic routes through which H2is oxidized remain essentially constant between laminar, unstretched flames and high Karlovitz number flames.

This paper describes measurements of nitrogen oxide (NOx) emissions from reacting jets in crossflow (RJICF). Primary factors that influence RJICF NOx emissions are: jet stoichiometry, mixing between jet and crossflow before combustion, and mixing of the remainder of the crossflow with the combustion products of the secondary combustion region. The aforementioned mixing is controlled by shear layer vortices and the counter-rotating vortex pair, as well as flame lifting. The coupled effects of bulk averaged temperature rise as a result of the RJICF (ΔT), jet stoichiometry (ϕJet), and momentum flux ratio (J) present a challenge in understanding critical factors controlling NOx production as it is difficult to vary them independently. Therefore, significant attention was paid to designing a test matrix that differentiated these effects. The data reported herein were obtained from the injection of premixed ethane/air or ethane/methane/air mixtures into a vitiated crossflow at one of two temperatures (1350K and 1410K). Varying the ethane/methane ratio allowed for systematic variation of flame lifting independent of ϕjet and J. The jets contained sufficient fuel to create an adiabatic bulk temperature rises from 75K–350K, with J values from 8–40, and ϕJet values from 0.8–8.0. The reported measurements confirm that NOx emissions increase monotonically with ΔT, as discussed in literature, but also indicates that the lifting of the flame significantly impacts NOx production. Lifting itself is a function of the variables described above and was quantified with chemiluminescence imaging. In fact, flame lifting is the dominant factor influencing NOx emissions, including ΔT.

This paper elucidates the behavior and dynamics of non-premixed flames responding to bulk fluctuations in flow velocity. It expands previous work on this problem by consistently incorporating finite Peclet number ( ) effects, and differentiating inflow boundary and dynamical effects on the flame dynamics. For analytical tractability, prior treatments of this problem generally prescribe the inflow boundary conditions into the domain. This paper shows, however, that prescribed inflow conditions, such as a step or constant local diffusive flux boundary condition, neglect axial diffusion effects in the region where their effects are most important; i.e., in the near-burner exit region where high transverse gradients and mass burning rates control the heat release dynamics. As the burning rate of non-premixed flames are controlled by mixture fraction gradients, the influence of axial diffusion substantively influences several burning rate characteristics of the flame. In addition, these effects cause the leading edge position of the flame front to oscillate, even for infinitely fast chemistry. Even in Pe ≫ 1 flames, axial diffusion introduces several fundamentally new features to the problem, resulting in exponentially decaying, dispersive flame wrinkle propagation with downstream distance. Also investigated here are asymptotic results for the heat release dynamics. It is shown that axial diffusion introduces a triple-zone asymptotic structure into the unsteady heat release characteristics, resulting in O(1) flame transfer function trends for St≪ 1 (to which an n–τ model is developed), O(1/St1/2) for intermediate Strouhal numbers, and O(1/St) for high Strouhal numbers. Finally, it is shown that the phase of the heat release response is approximately half that of a premixed flame with the same length, due to the concentration of unsteady heat release near the burner outlet, where transverse gradients are largest.

A key component of the thermoacoustic instability feedback mechanism is the excitation of hydrodynamic flow disturbances by narrowband acoustic fluctuations. In earlier work, we considered the response of time-averaged axisymmetric flames to helical disturbances. That study showed that although all helical modes cause local flame wrinkling, only the axisymmetric hydrodynamic mode, m = 0, contributes to the global heat release rate fluctuations. This paper extends this work to consider time-averaged flames that are non-axisymmetric. We show that distortions of the unforced flame shape changes its receptivity to helical disturbances, as it does not allow for the perfect destructive interference seen in axisymmetric flames. This results in a non-zero global flame response contribution from helical modes. Given that helical modes often have the largest amplitudes in jet flows, particularly those with swirl, these results show that the degree of non-axisymmetry of the flame has an important influence in determining which hydrodynamic modes, axisymmetric or helical, control the global heat release response of the flame. These points have been illustrated with example calculations that show how different control parameters and the degree of time-averaged non-axisymmetry influence the global flame response. An important implication of these results relates to scaling results from simplified geometries where a round jet is placed inside a round enclosure, to more realistic ones (such as where multiple jets are placed next to each other) where the flame shape will be distorted from being perfectly circular.

The Kemper County Project has demonstrated Transport Integrated Gasification (TRIG™) at a 2-on-1 Integrated Gasification Combined-Cycle (IGCC) facility located in Kemper County, Mississippi. Kemper is the largest IGCC project in the world, the first to use lignite as fuel, the first to capture and sell CO2, and the first to produce multiple byproducts from initial startup. The facility features two Siemens SGT6-5000F gas turbines, each capable of operating on a high-hydrogen syngas produced in the Transport Gasifiers from locally mined lignite. Using high-hydrogen syngas requires unique modifications to the combustion turbine design. Flame-diffusion combustors, rather than dry low-NOX designs, prevent flashback caused by the high hydrogen content of the syngas. Also, ports added to the turbine compressor casing allow air to be extracted from the compressor and used elsewhere in the plant, supplying up to one half of the air required by the gasifier. The Kemper facility has achieved the integrated operation of both gasifiers, including the production of electricity from syngas by both combustion turbines. Turbine operation on the high hydrogen syngas was smooth both during normal operations and during transitions, with efficiencies meeting or exceeding expectations. This paper describes the Kemper plant design, focusing on the combustion turbine design unique to Kemper. The paper also discusses turbine design challenges specific to Kemper, provides an overview of the robust control scheme used on both syngas and natural gas co-firing operations, and provides preliminary operational and performance results, including inspection findings.

This paper reports the results of a joint experimental and numerical study of the flow characteristics and flame structure of a hydrogen rich jet injected normal to a turbulent, vitiated crossflow of lean methane combustion products. Simultaneous high-speed stereoscopic PIV and OH PLIF measurements were obtained and analyzed alongside three-dimensional direct numerical simulations of inert and reacting JICF with detailed H 2 / CO chemistry. Both the experiment and the simulation reveal that, contrary to most previous studies of reacting JICF stabilized in low-to-moderate temperature air crossflow, the present conditions lead to a burner-attached flame that initiates uniformly around the burner edge. Significant asymmetry is observed, however, between the reaction zones located on the windward and leeward sides of the jet, due to the substantially different scalar dissipation rates. The windward reaction zone is much thinner in the near field, while also exhibiting significantly higher local and global heat release than the much broader reaction zone found on the leeward side of the jet. The unsteady dynamics of the windward shear layer, which largely control the important jet/crossflow mixing processes in that region, are explored in order to elucidate the important flow stability implications arising in the inert and reacting JICF. The paper concludes with an analysis of the ignition, flame characteristics, and global structure of the burner-attached flame. Chemical explosive mode analysis (CEMA) shows that the entire windward shear layer, and a large region on the leeward side of the jet, are highly explosive prior to ignition and are dominated by non-premixed flame structures after ignition. The predominantly mixing limited nature of the flow after ignition is examined by computing the Takeno flame index, which shows that ∼70% of the heat release occurs in non-premixed regions.

Fuel composition has significant influences on the turbulent flame speed of mixtures with strong stretch sensitivity. These fuels effects are associated with reactant thermal-diffusive properties and stretch sensitivities, causing local variations in the burning rate along the flame front. This study is motivated by leading point descriptions of the turbulent flame speed, which argue that S T is controlled by the flame characteristics at its positively curved leading edge. It has been argued that the leading edge of the flame approaches “critically stretched” values in thermo-diffusively unstable flames, implying that the appropriate laminar flame speed to parameterize the turbulent flame speed is the maximum flame speed across all potential values of flame stretch, S L,max , as opposed to its unstretched value, S L, 0 . This paper describes an experimental investigation of the characteristics of the flame leading point in high stretch sensitivity flames to assess this hypothesis more fully. Measurements of the flame curvature were obtained with a low swirl burner (LSB) for several H 2 /CO mixtures at velocities from 30–50 m/s. These data show that the leading point conditioned curvature statistics are a strong function of the turbulence intensity of the flow. Counter to our expectations, however, the measurements show relatively weak influences of fuel composition on the leading point curvature of the turbulent flame front. As such, these results do not seem consistent with prior arguments that the increased turbulent flame speeds seen with increasing hydrogen content are the result of increasing flame curvature/stretch rates, and therefore S L,max values, at the flame leading points. Additional analysis is needed to understand the physical mechanisms through which the turbulent flame speed is altered by differential diffusion effects.

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to a similarly excited premixed flame. Bulk axial or transverse excitation, in either case, leads to the excitation of wrinkles on the flame that propagate axially. Inclusion of axial diffusion in the non-premixed case, and burning velocity stretch sensitivity in the premixed case, cause wrinkle dissipation and dispersion. There are important differences in spatially integrated unsteady heat release dynamics between premixed and non-premixed flames. For general Strouhal numbers, mass burning rate fluctuations are the dominant contributor to non-premixed flame heat release fluctuations, while area fluctuations are the dominant contributor to premixed flame heat release fluctuations. Moreover, the heat release response of non-premixed flames rolls off much slower with frequency, O(St−1/2) compared to O(St−1) for premixed flames, and, hence, are more sensitive to flow perturbations than premixed flames at high Strouhal numbers. The asymptotic tendencies of the non-premixed flame, however, are largely controlled by the near burner exit region with high transverse gradients and, thus, are expected to be quite sensitive to burner exit details and finite chemistry effects.

Several studies have utilized leading points concepts to explain the sensitivity of turbulent burning rates to fuel/oxidizer composition, especially in negative Markstein length mixtures. Leading point theories suggest that the premixed turbulent flame speed is controlled by the flame front characteristics at the flame brush leading edge, or, in other words, by the flamelets that advance farthest into the unburned mixture (the so-called leading points). Furthermore, several authors have postulated that these leading edge flamelets have an inner structure similar to a critically perturbed laminar flame, i.e., the local stretch rate approaches the extinction value, near where the maximum possible laminar burning velocity is reached. In order to investigate these hypotheses for leading points burning rates, this paper analyzes the flame front structure at the leading edge of turbulent, lean ( ϕ = 0.31) premixed H 2 /Air flames, utilizing a database of direct numerical simulations (DNS) previously reported by Aspden et al. (2011). We calculate local flame front curvature, thickness, and burning velocity and compare these values to reference quantities obtained from stretched laminar flames computed numerically in different geometrical configurations (a counterflow twin flame, a tubular counterflow flame and an expanding cylindrical flame). These comparisons show that curvatures and burning velocities approach those of “critically” stretched laminar flames for the highest turbulent intensity case, but not for the lower turbulence intensity cases. In all cases, however, the structure of the flame front at the leading edge seems to closely mirror laminar flame calculations.

This article describes an analysis of the sound generated by swirling premixed flames when excited by helical disturbances, i.e., where the flow fluctuations have an azimuthal dependence of the form and denotes the helical mode number. These results illustrate the sensitivity of local and global sound generation characteristics to mode number, , swirl strength, and dimensionless frequency. An important result is the demonstration that specific mode numbers dominate particular aspects of the flame response. Specifically, particular helical modes dominate the magnitude of local flame wrinkling, , global heat release, , and sound generation, , and are generally not the same. For example, the helical mode that dominates the flame sound radiation, , is a function of flame compactness ratio, , while and are not. For compact, axisymmetric flames, , the axisymmetric, mode dominates the far-field sound radiation, both locally and in terms of global sound power. For and for , other helical modes dominate, generally...

Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well-defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially as a result of both the inherent dynamics of the vortex breakdown region and externally imposed oscillations. Motion of this flame stabilization point relative to the flow field may have an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This article describes an experimental investigation of flow forcing effects on ...

The chemical species composition of a vitiated oxidizer stream can significantly affect the combustion processes that occur in many propulsion and power generation systems. Experiments were performed to investigate the chemical kinetic effects of vitiation on ignition and flame propagation of hydrocarbon fuels using propane. Atmospheric-pressure flow reactor experiments were performed to investigate the effect of NOx on propane ignition delay time at varying O2 levels (14–21 mol%) and varying equivalence ratios (0.5–1.5) with reactor temperatures of 875 K and 917 K. Laminar flame speed measurements were obtained using a Bunsen burner facility to investigate the effect of CO2 dilution on flame propagation at an inlet temperature of 650 K. Experimental and modeling results show that small amounts of NO can significantly reduce the ignition delay time of propane in the low- and intermediate-temperature regimes. For example, 755 ppmv NOx in the vitiated stream reduced the ignition delay time of a stoichiometric propane/air mixture by 75% at 875 K. Chemical kinetic modeling shows that H-atom abstraction reaction of the fuel molecule by NO2 plays a critical role in promoting ignition in conjunction with reactions between NO and less reactive radicals such as HO2 and CH3O2 at low and intermediate temperatures. Experimental results show that the presence of 10 mol% CO2 in the vitiated air reduces the peak laminar flame speed by up to a factor of two. Chemical kinetic effects of CO2 contribute to the reduction in flame speed by suppressing the formation of OH radicals in addition to the lower flame temperature caused by dilution. Overall, the detailed chemical kinetic mechanism developed in the current work predicts the chemical kinetic effects of vitiated species, namely NOx and CO2, on propane combustion reasonably well. Moreover, the reaction kinetic scheme also predicts the negative temperature coefficient (NTC) behavior of propane during low-temperature oxidation.

Several studies have utilized “leading points” concepts to explain the augmentation of burning rates in turbulent flames by flow fluctuations. These ideas have been particularly utilized to explain the strong sensitivity of turbulent burning rates to fuel composition. Leading point concepts suggest that the burning velocity is controlled by the velocity of the points on the flame that propagate farthest out into the reactants – thus, they de-emphasize the classical idea that burning velocity enhancement is due to increases in flame surface area. Rather, within this interpretation, flame area creation is the effect, not the cause, of augmented turbulent burning velocities. However, the theory behind the implementation of leading point concepts in turbulent combustion modeling needs further development and the definition of “leading point” has not been fully clarified. For a certain class of steady shear flows, it is straightforward to demonstrate the leading point concept in an intuitive manner, but the problem becomes more complex when the leading points themselves evolve in time. In this paper, we use the G -equation to describe the flame dynamics and, utilizing results for Hamilton–Jacobi equations from the Aubry–Mather theory, demonstrate both the utility and limitations of leading points interpretations for front propagation, at least for deterministic problems. Specifically, we show how the large-time behavior of the solutions is controlled by discrete points on the flame under certain conditions and is, therefore, independent of the rest of the flow field details – a key hypothesis of leading points theories. However, it is possible to find other conditions where the large time behavior of the flame is not controlled by discrete points on the flame, but rather by the velocity field over its entire surface. Moreover, we also show that even in cases where the burning rate is controlled by discrete points, these points are not necessarily the most forward lying points in the flame front. Finally, we consider the case where the laminar flame speed is a function of flame front curvature and derive exact results for the sensitivity of the front speed to the Markstein length, l , for l > 0. These solutions explicitly illustrate how the reduction of front displacement speed for increasing l can be interpreted in terms of leading points dynamics in some cases.

This article analyzes the response of bluff-body stabilized flames to transverse acoustic waves. Data were obtained for bluff-body flames at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The key objective of the study is to compare measurements of the fluctuating velocity and flame wrinkling using the G-equation, e.g., to compare how the ensemble averaged unsteady flame wrinkling gain/phase predicted by solving the G-equation using the measured velocity field as inputs compares to the measured values. These results show good qualitative agreement between the comparisons and measurements, and quite good quantitative accuracy in many of the cases. These comparisons also...

This article describes measurements of the response of bluff-body stabilized flames subjected to transverse acoustic waves. It is the first of a two-article series. The objective of this work was to extend prior studies of this nature to much higher Reynolds numbers and more severe environments that more closely mimic conditions encountered in applications. To this end, experiments were performed at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The data show that the disturbance field and the flame front response amplitude exhibit a nonmonotonic spatial distribution with interference patterns. The phase of the flame front response at the forcing frequency varies nearl...

This paper describes measurements and correlations of turbulent consumption speeds, ST,GC, of hydrogen/carbon monoxide (H2/CO) fuel mixtures over a range of conditions. This work is set within the broader context of understanding the sensitivity of the turbulent flame speed to chemical kinetic and diffusive properties of the reactive mixture. Turbulent consumption speed data were obtained at mean flow velocities, turbulence intensities ( u rms ′ / S L , 0 ) , equivalence ratios and pressures ranging from 4–50 m/s, 5–45, 0.5–0.8, and 1–10 atm, respectively. Experiments were conducted where the mixture equivalence ratio, ϕ, was adjusted at each fuel composition to have nominally the same calculated un-stretched laminar flame speed, SL,0. Consistent with prior studies, these data show both fuel composition and pressure effects on the turbulent flame speed. For example, measured ST,GC values of the 90/10 H2/CO are about 3 times larger than CH4 blends having the same SL,0, turbulence intensity, and operating conditions. Similarly, the 5 atm data have ST,GC values that are consistently about 1.8 times larger than the 1 atm data, at identical conditions and fuel compositions. These data are correlated with a scaling law derived from quasi-steady leading points concepts using detailed kinetics calculations of highly stretched flames. For a given pressure, these scalings do an excellent job in scaling data obtained across the H2/CO fuel composition and equivalence ratio range. However, the pressure sensitivities are not captured by this scaling, which may be more fundamentally a reflection of the non-quasi-steady nature of the flame leading points. In support of this argument, we show that the spread in the data can largely be correlated with the ratio of a leading point chemical time scale to a flow time scale.

This paper describes measurements and analysis of the unsteady characteristics of a turbulent reacting fuel jet in a vitiated cross-flow. This problem involves coupling between the hydrodynamic stability of the jet–wall system, combustion induced gas expansion and interactions of the autoigniting jet with the wake flow. Analysis of the unsteady jet motions shows that the reacting jet flaps in a sinuous manner with an amplitude that increases with downstream distance but is relatively independent of jet-to-cross-flow momentum flux ratio, J. This study particularly focuses on unsteady flame–wall interactions in these systems, showing that even though the time averaged flame trajectory may be well removed from wall regions, its instantaneous trajectory may be very near or even attached to the wall. Specifically, for lower J jets (J < ∼20), the flame intermittently attaches to the wall for some fraction of time that decreases with downstream distance and increasing J. From a practical point of view, intermittent flame–wall interactions cause an increase in the time averaged wall temperature, raising durability concerns. We suggest that this unsteady jet–wall attraction can be understood from analogies to the dynamics of co-flowing jet or wake pairs, as low J jets deflect into the cross-flow direction quickly. Side-by-side jets or wakes are “attracted” toward each other and exhibit global instabilities that are not present when they are isolated. In support of this argument, we show that data taken over the 1.7 < J < 7.2 range and a range of axial locations can be collapsed using the time averaged distance of the reacting jet from the wall.

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to that of a premixed flame. The space–time dynamics of the non-premixed flame sheet in the fast chemistry limit is described by the stoichiometric mixture fraction surface, extracted from the solution of the -equation. This procedure has some analogies to premixed flames, where the premixed flame sheet location is extracted from the G = 0 surface of the solution of the G-equation. A key difference between the premixed and non-premixed flame dynamics, however, is the fact that the non-premixed flame sheet dynamics are a function of the disturbance field everywhere, and not just at the reaction sheet, as in the premixed flame problem. A second key difference is that the non-premixed flame does not propagate and so flame wrinkles are convected downstream at the axial flow velocity, while wrinkles in premixed flames convect downstream at a vector sum of the flame speed and axial velocity. With the exception of the flame wrinkle propagation speed, however, we show that that the solutions for the space–time dynamics of the premixed and non-premixed reaction sheets in high velocity axial flows are quite similar. In contrast, there are important differences in their spatially integrated unsteady heat release dynamics. Premixed flame heat release fluctuations are dominated by area fluctuations, while non-premixed flames are dominated by mass burning rate fluctuations. At low Strouhal numbers, the resultant sensitivity of both flames to flow disturbances is the same, but the non-premixed flame response rolls off slower with frequency. Hence, this analysis suggests that non-premixed flames are more sensitive to flow perturbations than premixed flames at O(1) Strouhal numbers.

This paper describes numerical and theoretical analyses of the nonlinear dynamics of harmonically forced, stretch-sensitive premixed flames. A key objective of this work is to analyze the relative contributions of kinematic restoration and flame stretch upon the rate at which flame wrinkles, excited by harmonic forcing, are smoothed out. Kinematic restoration is an intrinsically nonlinear process with a two spatial-zone structure, whose amplitude dependence is fundamentally different near and far from the wrinkle excitation source. Flame stretch processes appear even in the small perturbation limit, and smooth out flame wrinkles in thermodiffusively stable mixtures. Which process dominates is a function of the perturbation amplitude, frequency, stretch sensitivity of the mixture, and spatial location. This paper presents computed results illustrating the solution characteristics, as well as key dimensionless parameters controlling the solution based upon a third order perturbation analysis.

This paper describes stereo-PIV, OH-PLIF and fuel-PLIE (planar laser induced emission) measurements in a pressurized, liquid fueled, swirl combustor. Data were obtained at globally fuel lean conditions, combustor pressures of 2–5 bar, and an inlet air temperature of 450 K. The experiments were performed to characterize the flowfield, heat release and fuel spray distribution. Several challenges are associated with OH-PLIF in pressurized, liquid fuel systems at sustained high repetition rates. For example, in addition to the significantly lower pulse energies of high repetition rate systems relative to low repetition rate ones, the ultraviolet laser used to excite OH also causes the fuel to emit, with the brighter liquid fuel signal overlapping the OH fluorescence spectrum. To overcome these challenges, two intensified high-speed cameras were used to maximize signal separation during data collection and perform signal subtraction in post-processing. The first camera used narrow band spectral filtering, and the intensifier was gated to miss much of the slower decaying fuel signal. As a result, it satisfactorily captures the OH fluorescence along with some of the stronger fuel fluorescence signal. The second camera detected primarily fuel emission with the intensifier gate delayed to capture the tail of the longer-lived fuel phosphorescence, and a long-pass spectral filter capturing all the fuel emission. This paper presents illustrative results showing the instantaneous flow field, flame position as indicated by OH-PLIF, and spray distribution from the fuel PLIE. Multiple flame topologies are observed — specifically, flames stabilized in the outer shear layer occur for all the cases studied, but inner shear layer stabilized flames are also seen in the higher pressure cases.

Blowoff sets important operational limits on a combustor system. While blowoff is intrinsically a system-dependent phenomenon, it is also dependent on the chemical and physical properties of the fuel. This paper describes an experimental study of the lean blowout limits of eight liquid fuels in a swirl-stabilized combustor, with data for both a pressure atomizer and an airblast atomizer. Three of the fuels were traditional jet fuels (an average Jet-A, JP-5, and JP-8) and the remaining five fuels spanned a range of physical and kinetic properties. These experiments were performed at a combustor pressure of 345 kPa and an air temperature of 450 K. In addition to some sensitivity of blowoff conditions to the thermal state of the combustor, results also clearly show sensitivities to fuel composition. Strong correlations were observed for pressure atomizer blowoff with fuel physical properties, particularly for boiling point temperature, indicating that fuels less easily atomized and vaporized are harder to blow off. These results are consistent with the idea that delaying atomization and/vaporization, and therefore reducing the level of premixing that drives the local fuel-air ratio towards the very lean global fuel/air ratio, is advantageous in order to promote regions of locally elevated flame temperatures. We suggest that this behavior occurs when the air preheat temperature is above the fuel flashpoint, as a similarly good correlation with boiling point temperature but with the opposite trend, has been previously reported in a study obtained for preheat temperatures below the fuel flashpoint. In contrast, the airblast results do not show strong correlations with fuel physical properties. Rather, the best correlation of the airblast atomizer results is with the percentage of iso-paraffins in the fuel. We speculate that this reflects a sensitivity to kinetic properties of the fuel, as the superior atomization characteristics of the airblast atomizer may de-emphasize the importance of physical properties.

This paper describes an analysis of the unsteady flow structures in a single nozzle and triple nozzle swirl combustor (with nozzle spacing of s/D=2.9). It was motivated by a prior study by our group which compared the time averaged and unsteady features for a different swirling nozzle, and found that the single and triple nozzle flow dynamics were quite similar upstream of the jet merging region. This work is motivated by the fact that realistic hardware, whether in can or annular combustion systems, almost always contains several nozzles. However, it is common to use test facilities with a single nozzle to study flame dynamics, a key component of the combustion instability problem [1]. Simultaneous OH Planar Laser Induced Fluorescence (OH PLIF) and Stereoscopic Particle Image Velocimetry (sPIV) techniques were performed at 5 kHz on a swirl methane-air flame. Two transverse forcing configurations were applied, so that the flame nominally lies in a pressure node/transverse velocity antinode, and vice versa. The time averaged flow fields of the single and triple nozzle configurations are compared, and several key differences are identified. Most prominently, there are non-negligible differences in the recirculation zone reverse flow velocity and flame spreading angle. However, the spatial variation of the disturbance magnitudes along the shear layers exhibit quite similar growth and decay trends, and the convection speeds along the shear layer were nearly identical. These results corroborate the findings of Aguilar et al. [2] and Szedlmayer et al. [3], which show that despite differences in time average quantities, comparable flow dynamics occur in single and multi-nozzle flames. These results imply that useful insights into the dynamics of multi-nozzle systems can be gleaned from appropriately designed single-nozzle hardware, with appropriate accounting for the differences in time averaged flow/flame characteristics.

This paper considers the kinetic pathways of hydrogen oxidation in turbulent, premixed H2-air flames. It assesses the relative roles of different reaction steps in H2 oxidation relative to laminar flames, and the degree to which turbulence-chemistry interactions alters the well understood oxidation pathway that exist in laminar flames. This is done by analysing turbulent, lean (φ =0.4), H2-Air flame DNS database from Aspden et al. (Proc. Combust. Inst. 35 (2015) 1321–1329). The relative roles of dominant reaction steps in heat release and radical formation/consumption are analysed at different Karlovitz numbers and compared with laminar stretched flame calculations from counterflow flames and perfectly stirred reactors. It is found that both the progress variable conditioned and spatially integrated contributions of the dominant reactions remain qualitatively similar between a highly turbulent and a laminar unstretched flame. Larger changes, up to a factor of about two, occur in the relative roles of reactions with secondary influences on heat release and radical production/consumption. These results suggest that the kinetic routes through which H2 is oxidized remain essentially constant between laminar, unstretched flames and high Karlovitz number flames.

Combustion instabilities are one of the most costly and technically challenging issues in lean, premixed combustion systems. While combustion instabilities, or thermacoustic oscillations more generally, have been noted in a variety of applications for several centuries, they are particularly problematic in lean, premixed combustion systems. Combustion instability is characterized by undesirably high acoustic and heat release rate oscillations inside a combustor chamber. In this chapter, we discuss the fundamentals of thermoacoustic feedback cycles, as well as the different coupling mechanisms by which combustor system acoustics create a feedback loop with flame heat release rate oscillations. Finally, an overview of combustion control strategies, particularly those employed in lean combustion systems, is discussed with references to future design and research directions.

This paper describes the response of a swirling premixed flame with constant burning velocity to non-axisymmetric harmonic excitation. This work extends prior studies of axisymmetric forcing, which have shown that wrinkles are excited on the flame that propagate downstream along the mean flame surface at a speed given by U o cos ψ , where U o is the mean flow velocity and ψ is the flame angle. The swirl component in the flow field introduces an azimuthal transport mechanism for disturbances on the flame. As such, the flame response at any given position is a superposition of flame wrinkles excited at earlier times, upstream axial locations, and different azimuthal positions. These swirl transport effects do not arise in problems where axisymmetric flames are subjected to axisymmetric excitation, but enter quite prominently in the presence of non-axisymmetries, such as when the flame is subjected to transverse excitation. The solution characteristics are strongly dependent upon the ratio of angular rotation rate to excitation frequency, denoted by σ = Ω / ω , which describes the fraction of azimuthal rotation a disturbance makes in one acoustic period. When σ ≪ 1 and σ ≫ 1, the axial wavelength of flame wrinkles scales with the convective wavelength, λ c , but becomes much longer for σ ∼ O (1). The spatial variation in phase of flame wrinkling is also strongly dependent upon σ . Regardless of swirl number, flame wrinkles propagate in helical spirals along the solution characteristics at a phase speed equal to the local tangential velocity. The axial phase characteristics of flame wrinkling at a fixed azimuthal location, as would be measured by laser sheet imaging, are much more complex. For σ σ , however, and becomes zero at σ = 1 for a compact flame. For σ > 1, the wrinkles actually have a positive roll-off character for the phase with axial downstream distance, indicating a flame wrinkle with a negative trace velocity, but whose actual propagation velocity is positive. Finally, these results show that while the flame response to transverse acoustic excitation is quite strong locally, its spatially integrated effect is much smaller for acoustically compact flames. This suggests that the dominant mechanism through which the flame responds globally to transverse excitation is the induced vortical and longitudinal acoustic fluctuations.

Improved understanding of turbulent flames characterized by negative consumption speed-based Markstein lengths is necessary to develop better models for turbulent lean combustion of high hydrogen content fuels. In this paper we investigate the topology and burning rates of turbulent, lean ( ϕ = 0.31), H 2 /air flames obtained from a recently published DNS database (Aspden et al., 2011). We calculate local flame front curvatures, strain rates, thicknesses, and burning velocities and compare these values to reference quantities obtained from stretched laminar flames computed numerically in three model geometrical configurations—a counterflow twin flame, a tubular counterflow flame and an expanding cylindrical flame. We compare and contrast the DNS with these model laminar flame calculations, and show both where they closely correlate with each other, as well as where they do not. These results in the latter case are shown to result from non-flamelet behaviors, unsteady effects, and curvature-strain correlations. These insights are derived from comparisons conditioned on different topological features, such as portions of the flame front with a spherical/cylindrical shape, the leading edge of the flame, and portions of the flame front with low mean curvature. We also show that reference time scales vary appreciably over the flame, and characterizing the relative values of fluid mechanic and kinetic time scales by a single value leads to erroneous conclusions. For example, there is a two order of magnitude decrease in chemical time scales at the leading edge of the front relative to its unstretched value. For this reason, the leading edge of the front quite closely tracks quasi-steady calculations, even in the lowest Damkohler number case, Da F ∼0.005.

This paper describes measurements of global turbulent consumption speeds, ST,GC, of hydrogen/carbon monoxide (H2/CO) mixtures. The turbulent flame properties of such mixtures are of fundamental interest because of their strong stretch sensitivity, and of practical interest since they are the primary constituents of syngas fuels. Data are reported at mean flow velocities and turbulence intensities of 4 1 u rms ′ / S L , 0 100 , respectively, for H2/CO blends ranging from 30% to 90% H2 by volume. Two sets of experiments are reported. In the first, fuel blends ranging from 30% to 90% H2 and mixture equivalence ratio, ϕ, were adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, SL,0. In the second set, equivalence ratios were varied at constant H2 levels. The data clearly corroborate results from other studies that show significant sensitivity of ST,GC to fuel composition. In particular, at a fixed u rms ′ and SL,0, values of ST,GC increase by a factor of almost 2 when H2 levels are increased from 30% (at ϕ = 0.61) to 90% (at ϕ = 0.48). Moreover, ST,GC in the 90% H2 case is three times larger than the ϕ = 0.9 CH4/air mixture with the same SL,0 value. An important conclusion from this work is these fuel effects are not simply a low turbulence intensity phenomenon – they clearly persist over the entire range of turbulence intensities used in the measurements. We also describe physics-based correlations of these data, using leading points concepts and detailed kinetic calculations of the stretch sensitivity of these mixtures. These results are used to develop an inequality for negative Markstein length flames that bounds the turbulent flame speed data and show that the data can be collapsed using the maximum stretched laminar flame speed, SL,max, rather than SL,0.

Swirling flows are widely used in industrial burners and gas turbine combustors for flame stabilization. In many cases, the flame is stabilized in the shear layer near the centerbody and/or abrupt expansion, where the high speed nozzle flow transitions into the larger combustor. Several prior studies have shown that the flame position becomes increasingly unsteady as it approaches blowoff, due to local extinction/re-attachment of the flame at one or both of these locations. This is apparently due to the local strain rate irregularly oscillating about the extinction strain rate values near the attachment point. In order to characterize these flame strain characteristics, PIV measurements were obtained of several hydrogen/methane mixtures in this spatial region. The fluid mechanic straining of the flame in this region is dominated by two gradients in velocity – that due to the strong shear near the centerbody and to the bulk flow deceleration as it expands from the smaller diameter nozzle into the combustor. These two velocity gradients cause positive and negative stretching of the flame sheet, respectively. The shearing velocity gradient is an order of magnitude larger than the flow deceleration term but, due to the fact that the flame is essentially parallel to the shear layer, actually has a secondary influence relative to the flow deceleration. As a result, the dominant flame straining term near the attachment point is apparently compressive – a somewhat counter-intuitive result, given that the flame is stabilized in the positively straining shear layer.

Transverse acoustic instabilities in premixed, swirl-stabilized flames are an important problem in low NOx combustors. Transverse excitation of swirling flames involves complex interactions between acoustic waves and fluid mechanic instabilities. This paper presents high-speed PIV characterization of the flow field characteristics of a swirling, annular jet under reacting and nonreacting conditions. These data show that the flame response to transverse acoustic excitation is a superposition of acoustic and vortical disturbances that fluctuate in both the longitudinal and transverse direction. In the nozzle near-field region, the disturbance field is a complex superposition of short wavelength and convecting vortical disturbances, as well as longer wavelength transverse and longitudinal acoustic disturbances. Very near the nozzle, distinct vortical structures are evident that are associated with the separating inner and outer annulus shear layers. Their relative phasing on the left and right side of the bu...

Concerns about green house gas emissions have encouraged interest in hydrocarbon combustion techniques that can accommodate carbon dioxide capture and sequestration. Oxy-fuel combustion, where the fuel is combusted in oxygen diluted with steam or CO 2 , is one promising approach for post-combustion carbon capture. In this paper we focus on CO 2 dilution effects and, in particular, on CO and O 2 emissions from these flames. The emissions issue must be considered from a different perspective than conventional power plants as the combustor effluents will be sequestered, and, thus, their interactions with the terrestrial atmosphere are not relevant. CO and O 2 are of interest for these systems as their presence in the exhaust stream represents wasted fuel and oxidizer. In addition, CO 2 pipeline specifications impose limitations on CO and O 2 levels, which also must then be controlled either through the combustion process or post gas cleanup. Equilibrium and kinetic modeling of CH 4 /O 2 /CO 2 combustion systems was performed in order to analyze CO 2 dilution effects upon CO and O 2 emissions level. Companion experiments were also performed in an atmospheric pressure, swirl stabilized combustor. These numerical and experimental results demonstrate the key tradeoffs associated with optimizing these systems, as well as the dependence of emissions on stoichiometry, pressure, CO 2 dilution and residence time.

This paper studies the heat-release oscillation response of premixed flames to oscillations in reactant stream fuel/air ratio. Prior analyses have studied this problem in the linear regime and have shown that heat release dynamics are controlled by the superposition of three processes: flame speed, heat of reaction, and flame surface area oscillations. Each contribution has somewhat different dynamics, leading to complex frequency and mean fuel/air ratio dependencies. The present work extends these analyses to include stretch and non quasi-steady effects on the linear flame dynamics, as well as analysis of nonlinearities in flame response characteristics. Because the flame response is controlled by a superposition of multiple processes, each with a highly nonlinear dependence upon fuel/air ratio, the results are quite rich and the key nonlinearity mechanism varies with mean fuel/air ratio, frequency, and amplitude of excitation. In the quasi-steady framework, two key mechanisms leading to heat-release sat...

The local turbulent flame speed of an attached flame is not only a function of the local flow and flame conditions, but also of upstream conditions – i.e., it is “non-local” or exhibits “memory”. Non-locality adds an additional degree of freedom to the classic problem of a freely propagating flame propagating normally to the time averaged flow. Non-locality occurs due to mean tangential flow along the flame brush, which causes flame wrinkles to translate downstream. As such, the wrinkling of the flame at any given point is not only a function of the local velocity disturbance, but also a superposition of flame surface perturbations from locations upstream at previous times. This causes the correlation length scale of turbulent flame wrinkles to differ from that of the underlying turbulent velocity fluctuations. The objective of this paper is to provide a physical description of the key flame kinematic processes that cause these non-local effects. Two approaches are adopted in this work. First, analytical solutions of the G-equation that explicitly describe the effect of non-locality in the low turbulence limit are developed. Second, numerical computations of the G-equation are performed that demonstrate the role of non-linearity in flame surface kinematics at higher turbulence intensities. Finally, these predictions are shown to be consistent with data from a turbulent Bunsen flame.

The linear response of 2D wedge-shaped premixed flames to harmonic velocity disturbances was studied, allowing for the influence of flame stretch manifested as variations in the local flame speed along the wrinkled flame front. Results obtained from analyzing the G-equation show that the flame response is mainly characterized by a Markstein number σ ˆ C , which measures the curvature effect of the wrinkles, and a Strouhal number, St f , defined as the angular frequency of the disturbance normalized by the time taken for the disturbance to propagate the flame length. Flame stretch is found to become important when the disturbance frequency satisfies σ ˆ C St f 2 ∼ O ( 1 ) , i.e. St f ∼ O ( σ ˆ C − 1 / 2 ) . Specifically, for disturbance frequencies below this order, stretch effects are small and the flame responds as an unstretched one. When the disturbance frequencies are of this order, the transfer function, defined as the ratio of the normalized fluctuation of the heat release rate to that of the velocity, is contributed mostly from fluctuations of the flame surface area, which is now affected by stretch. Finally, as the disturbance frequency increases to St f ∼ O ( σ ˆ C − 1 ) , i.e. σ ˆ C St f ∼ O ( 1 ) , the direct contribution from the stretch-affected flame speed fluctuation to the transfer function becomes comparable to that of the flame surface area. The present study phenomenologically explains the experimentally observed filtering effect in which the flame wrinkles developed at the flame base decay along the flame surface for large frequency disturbances as well as for thermal-diffusively stable and weakly unstable mixtures.

This paper overviews the dynamics of bluff body stabilized flames and describes the phenomenology of the blowoff process. The first section of the paper provides an overview of the fluid mechanics of the non-reacting and reacting bluff body wake flow. It highlights the key features of the flow (the boundary layer, separated shear layer, and wake), the flow instabilities that influence each of these features, and the influences of the flame on these instabilities. A key point from these studies is the large differences between the non-reacting wake (dominated by an absolutely unstable, sinuous instability associated with vortex shedding from the bluff body) and the reacting wake of high dilatation ratio flames. The latter are dominated by the lower intensity, convective instability of the shear layer. Very low dilatation ratio flames begin to approach the behavior of the non-reacting wake, as might be expected. Next, the paper presents a compilation of bluff body blowoff data from the literature and shows that the basic Damkohler correlations developed from prior studies are recovered, but without the need for semi-empirical fits or adjustable constants for chemical time estimation. The third section considers in detail the dynamics and phenomenology of near blowoff flames. It is shown that spatio/temporally localized extinction occurs sporadically on near blowoff flames, manifested as “holes” in the flame sheet that form and convect downstream. However, these extinction events are distinct from blowoff – in fact, under certain conditions the flame can persist indefinitely with certain levels of local extinction. The number of these events per unit time increase as blowoff is approached, eventually leading to large scale alteration of the wake. We hypothesize that simple Damkohler number correlations contain the essential physics describing the intial stage of blowoff; i.e., they are correlations for the conditions where local extinction on the flame begins, but do not fundamentally describe the ultimate blowoff condition itself. However, such correlations are reasonably successful in correlating blowoff limits because the ultimate blowoff event is related to the onset of this first stage. Key conclusions from this review are that blowoff occurs in multiple steps – local extinction along the flame sheet, large scale wake disruption, and a final blowoff whose ultimate “trigger” is associated with wake cooling and shrinking. A key challenge for future workers is understanding these latter processes that lead to ultimate blowoff of the flame.

Bluff-body stabilized flames are susceptible to combustion instabilities due to interactions between acoustics, vortical disturbances, and the flame. In order to elucidate these flow-flame interactions during an instability, an experimental and computational investigation of the flame-sheet dynamics of a harmonically excited flame was performed. It is shown that the flame dynamics are controlled by three key processes: excitation of shear layer instabilities by the axially oscillating flow, anchoring of the flame at the bluff body, and the kinematic response of the flame to this forcing. The near-field flame features are controlled by flame anchoring and the far-field by kinematic restoration. In the near-field, the flame response grows with downstream distance due to flame anchoring, which prevents significant flame movement near the attachment point. Theory predicts that this results in linear flame response characteristics as a function of perturbation amplitude, and a monotonic growth in magnitude of the flame-sheet fluctuations near the stabilization point, consistent with the experimental data. Farther downstream, the flame response reaches a maximum and then decays due to the dissipation of the vortical disturbances and action of flame propagation normal to itself, which acts to smooth out the wrinkles generated by the harmonic flow forcing. This behavior is strongly non-linear, resulting in significant variation in far-field flame-sheet response with perturbation amplitude.

Lean premixed combustors are highly susceptible to combustion instabilities, caused by the coupling between heat release fluctuations and combustor acoustics. In order to predict the conditions under which these instabilities occur and their limit cycle amplitudes, understanding of the amplitude dependent response of the flame to acoustic excitation is required. This study presents an analysis of phase-locked OH PLIF images of acoustically excited swirl flames, to identify the key controlling physical processes and qualitatively discuss their characteristics. This analysis suggests that the flame dynamics are controlled by a superposition of the following processes: (1) annular jet fluctuations, (2) oscillatory turbulent flame brush development, (3) flame stabilization, and (4) fluid mechanical instabilities of the backward facing step, jet column, swirl, and shear layer. These results illustrate that the flame response is not controlled by any single physical process but, rather, by several simultaneously occurring processes which are potentially competing, and whose relative significance depends upon forcing frequency, amplitude of excitation, and flame stabilization dynamics.

Laminar flame speeds of lean H2/CO/CO2 (syngas) fuel mixtures have been measured for a range of H2 levels (20–90% of the fuel) at pressures and reactant preheat temperatures relevant to gas turbine combustors (15 atm and up to 600 K). A conical flame stabilized on a contoured nozzle is used for the flame speed measurement, which is based on the reaction zone area calculated from chemiluminescence imaging of the flame. An O2:He mixture (1:9 by volume) is used as the oxidizer in order to suppress the hydrodynamic and thermo-diffusive instabilities that become prominent at elevated pressure conditions for lean H2/CO fuel mixtures. All the measurements are compared with numerical predictions based on two leading kinetic mechanisms: a H2/CO mechanism from Davis et al. and a C1 mechanism from Li et al. The results generally agree with the findings of an earlier study at atmospheric pressure: (1) for low H2 content ( 60%) H2 content fuels, especially at very lean conditions. At elevated pressure, however, the effect is less pronounced than at atmospheric conditions. The exaggerated temperature dependence of the current models may be due to errors in the temperature dependence used for so-called “low temperature” reactions that become more important as the preheat temperature is increased. There is also evidence of slight radiative heat transfer effects on the laminar flame speed for lean syngas mixtures associated with CO2 addition to the fuel (up to 40%) at elevated pressure and preheat temperature.

This article addresses the impact of syngas fuel composition on combustor blowout, flashback, dynamic stability, and autoignition in premixed, steady flowing combustion systems. These are critical issues to be considered and balanced against emissions considerations in the development and operation of premixed combustors. Starting with blowout, the percentage of hydrogen in the fuel is suggested to be the most significant fuel parameter, which is more fundamentally related to the hydrogen flame's resistance to stretch induced extinction. Turning to flashback next, it is shown that multiple flashback mechanisms are present in swirling flows, and the key thermophysical properties of a syngas mixture that influence its flashback proclivity depend upon which flashback mechanism is considered. Flashback due to turbulent flame propagation in the core flow and the interaction of heat release with pulsations are less critical, whereas flame propagation in boundary layers and flashback due to the interaction of th...

Laminar flame speeds of lean H2/CO/CO2 (syngas) fuel mixtures have been measured over a range of fuel compositions (5–95% for H2 and CO and up to 40% for CO2 by volume), reactant preheat temperatures (up to 700 K), and pressures (1–5 atm). Two measurement approaches were employed: one using flame area images of a conical Bunsen flame and the other based on velocity profile measurements in a one-dimensional stagnation flame. The Bunsen flame approach, based on imaging measurements of the reaction zone area, is shown to be quite accurate for a wide range of H2/CO compositions. These data were compared to numerical flame speed predictions based on two established chemical mechanisms: GRI Mech 3.0 and the Davis H2/CO mechanism with detailed transport properties. For room temperature reactants, the Davis mechanism predicts the measured flame speeds for the H2/CO mixtures with and without CO2 dilution more accurately than the GRI mechanism, especially for high H2 content compositions. The stagnation flame measurements for medium levels of H2 at both 1 and 5 atm, however, show lower than predicted strain sensitivities, by almost a factor of two at lean conditions ( Φ = 0.6 – 0.8 ). At preheat temperatures comparable to those found in gas turbine combustors, the accuracy of the flame speed predictions worsens. For example in fuels with low levels of H2, both models underpredict the measurements. In contrast, for medium H2 content fuels, both measurement techniques show that the models tend to overpredict flame speed, with the discrepancy increasing as Φ decreases and temperature increases. In general, the Davis mechanism predictions are in good agreement with the measurements for medium and high H2 fuels for preheat temperatures up to 500 K but overpredict the measurements at higher temperatures.

An understanding of the amplitude dependence of the flame response to harmonic acoustic excitation is required in order to predict and/or correlate combustion instability amplitudes. This paper describes an experimental investigation of the mechanisms for nonlinearity of the heat release response to imposed acoustic oscillations using phase-locked, two-dimensional OH PLIF imaging. It focuses upon two representative conditions from a larger test set, corresponding to fundamentally different mechanisms of nonlinearity in flame response. The first mechanism is vortex roll-up at large disturbance amplitudes, similar to the observations of Balachandran et al. [R. Balachandran, B.O. Ayoola, C.F. Kaminski, A.P. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37–55.]. This roll-up causes the destruction rate of flame surface area by flame propagation to grow with disturbance amplitude, resulting in a flame surface area which does not grow proportionately with the disturbance amplitude. The second mechanism is unsteady flame liftoff, which occurs during the phase of the cycle of peak instantaneous axial velocity. This causes the flame attachment point to move off of the center body to a downstream location for part of the cycle. During this part of the cycle, the flame surface area decreases due to a merging of flame branches. Interestingly, while the flow field is highly three-dimensional and non-axisymmetric, the two key mechanisms identified here are essentially axisymmetric in nature. Furthermore, both mechanisms of nonlinearity are ultimately due to reduction in flame area by flame propagation normal to itself.

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical towards the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous PIV and CH-PLIF measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.