Excitation of high frequency thermoacoustic oscillations by syngas in a non-premixed bluff body combustor

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Excitation of high frequency thermoacoustic oscillations by syngas in a non-premixed bluff body combustor Nikhil A. Baraiya*, S.R. Chakravarthy National Centre for Combustion Research and Development and Department of Aerospace Engineering, Indian Institute of Technology Madras, Chennai 600036, India

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abstract

Article history:

A laboratory-scale bluff body combustor is mapped for its stability and flame dynamics of

Received 25 January 2019

non-premixed flames with three fuels, namely, pure H2, H2eCH4 mixture, and H2eCO

Received in revised form

mixture, the last one representing syngas. Unsteady pressure measurements and high-

24 March 2019

speed OH* and CH*/CO2* chemiluminescence imaging are simultaneously performed. The

Accepted 11 April 2019

combustor behaviour with syngas is markedly different than the other two in exciting high

Available online 3 May 2019

frequency oscillations, typically at the third harmonic longitudinal acoustic mode of the duct at high air flow Reynolds numbers (Re). In contrast, the H2eCH4 excites only the

Keywords:

fundamental longitudinal mode, and pure H2 excites up to the first harmonic. The latter

Syngas combustion dynamics

two are observed to lock on to the shear layer mode of the bluff body wake, whereas the

High frequency acoustic oscillations

H2eCO case locks on to thrice the Strouhal number associated with the shear layer mode,

Shear layer vortices

commensurate with the excitation of the third harmonic natural acoustic mode. Time-

Acoustic time scales

averaged OH* and CO2* chemiluminescence images show large-scale structure for the H2eCH4 case compared to heat release rate zones aligned with the shear layer in the pure H2 and H2eCO cases. Cross-sectionally averaged chemiluminescence profiles exhibit a streamwise stagger in the peaks of OH* and CO2*, suggesting two heat release rate zones, that could excite the acoustic oscillations. The instantaneous profiles indicate a convective delay between the two heat release rate zones that is close to the third harmonic acoustic time scale. The sequential of H2 oxidation to OH followed by CO oxidation by OH to form CO2 are considered to be responsible for the high frequency excitation in the case with the H2eCO mixture when compared to the other two cases. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The use of alternative sources of thermo-chemical conversion has the potential to meet the growing demand for energy considering that fast depleting fossil fuel resources. The

requirement of sustainable future energy will be from biomass and also to a certain extent, inferior quality coal. Among the fuels derived from the listed sources, synthesis gas or syngas is prominent, which is used for powering fuelflexible gas turbines. Syngas primarily consists of hydrogen and carbon monoxide, besides carbon dioxide, nitrogen, and

* Corresponding author. E-mail address: [email protected] (N.A. Baraiya). https://doi.org/10.1016/j.ijhydene.2019.04.087 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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water, but might also contain methane in small quantities [1], even with significant presence of ash in the coal when gasification with relatively larger fraction of steam is more beneficial [2]. The chemical composition of syngas depends on the nature of coal used and the method of production. The combustion behaviour of these fuel mixtures are markedly different when compared to their individual fuel constituents due to differences in mixing, concentration, and kinetics that results in varying flame stabilization characteristics. Further, syngas, owing to its wide variation in composition as a result of its synthesis process and source provides the opportunity to explore fuel-flexibility in power sources, a topic of much importance [3]. Several investigators have focused on different aspects of combustion of syngas and related fuel mixtures, typically CO/ H2/CH4 combinations. Natarajan et al. [4] characterized the laminar flame speed of such fuel mixtures in the presence of diluents and affirmed higher flame speed with greater fraction of H2 in the mixtures. Ranga Dinesh et al. [5e9] have studied the variation in the flame structure with change in fuel composition i.e. H2/CO ratio for the case of non-premixed flame and different burner geometries. They emphasize on the role of proportion of syngas constituents on the flow-field as well as on the flame topology due to hydrogen diffusivity. Along-with the effect of syngas constituents, studies on the effect of diluents like CO2, H2O and N2 on flame properties of syngas mixtures have also been performed. It was observed that CO2 has stronger thermal, transport and chemistry influence in reduction of flame temperature and laminar burning velocity compared to H2O and N2 [10e14]. The fuel constituents were seen to influence the unsteady aspects of flow-field that has implications on the dynamic stability of a combustor. The role of hydrogen in particular is significant in dictating the fore-mentioned aspects of kinetics and transport, and as a result, on the steady state and dynamic behaviour, as illustrated in Ref. [15]. The work focusses on the ability of hydrogen enriched combustion in reducing emissions like NOX, due to the possibility of operation at leaner conditions. In the context of syngas, the effect of its composition on NOx production has been performed numerically for practical gas-turbine conditions by Refs. [16e18]. In addition to NOx, the effect of syngas variability on emissions viz. CO and CO2 has been reported in Refs. [19e21]. Similar to the effect of diluent on flame structure, literature concerning with the addition of CO2 and N2 on production of NOx in syngas combustion highlighted that dilution reduces the overall reaction rates and hence the maximum flame temperature [22,23]. Tuncer et al. [24] reported the correlation between NOx production and flash back. In this regard, the presence of hydrogen is seen to increase the propensity for flash-back due to increased flame speed [25e28]. In addition to premixed flames, performance of such fuel mixtures in a gas turbine combustor has also been evaluated in partially premixed and diffusion flame modes [29,30]. Lieuwen et al. [31] have considered CO/H2/CH4 fuel mixtures for several combustion characteristics such as turbulent flame speed, ignition delay, auto-ignition, blowout and flashback, and combustion instability. They note that the presence of methane increases the characteristic chemical time scales. This in-turn will dictate the emission, flame-topology and

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emissions as described earlier. Analysis of the comprehensive literature thus suggests that, irrespective of the nature of flame and across a variety of flame holders, there exist significant differences in the nature of flame and associated phenomena, both in steady and in dynamic sense. These pose considerable challenges to the development of fuelflexible power stations with minimum hardware modifications. The present work is restricted to the role of fuelflexibility, particularly syngas and other hydrogen-enriched fuel(s) in effecting the acoustic characteristics of a nonpremixed bluff body combustor. Hence, subsequently, the rest of the literature survey presented here is focused upon this aspect. Combustion instability is a condition of intense flameacoustic interaction that leads to excitation of high amplitude levels of unsteady pressure in the combustor, which is oftentimes deleterious to the combustor performance and its structural integrity. Hence, this is of serious concern with the advent of non-conventional fuel mixtures such as syngas and other hydrogen enriched mixtures. Fergusson et al. [32] attempted to correlate the Wobbe indexdthe interchangeability parameter for changes in fuel compositiondwith the dynamic response of a lean premixed gas turbine combustor, and found it to be weak. This implies that a single interchangeability parameter cannot be adopted to identify the proneness of a combustor to instability when the fuel composition is varied. Lee et al. [33,34] investigated the mean flame structure of CO/H2 fuel mixtures and found an absence of a recirculation zone in this case as opposed to CH4 flames, which explained the low levels of acoustic excitation in the former case. In subsequent works, Park and Lee [35,36] have considered the full combination of CO/H2/CH4 and shown triangular stability diagrams of different compositions of this mixture in a swirl-stabilized partially premixed flame combustor, where the CO/H2 binary mixtures show very low amplitudes of acoustic excitation relative to other combinations. In effect, CO is considered to dampen pressure oscillations, so subsequent investigations by this research group is predominantly with H2/CH4 mixtures, with a few containing higher hydrocarbonsdethane or propanedadditionally. Choi and Lee [37] have compared the flame lengths based on proper orthogonal modes of OH* chemiluminescence images under combustion instability conditions with pure CH4 and with small quantities of H2 added. Recently, Yoon et al. [38] have considered higher contents of H2 in H2/CH4 mixtures to find to excitation of high harmonics of the combustor's acoustic modes, and report that the convective length scale is shorter under those conditions based on the flame length. Earlier, Figura et al. [39] have considered the “centroid of the flame” as a point source of heat release rate fluctuations based on mean CH* chemiluminescence images of H2/CH4 mixtures exciting combustion instability. This is a good indicator of the burner geometry as a lot of data for different fuel compositions and geometric/operating conditions collapse on a single curve. A similar approach with H2/hydrocarbon fuel mixtures has been adopted by Ghoniem and co-workers. They [40e42] have related the mean flame macro-structure and its changes with the combustion operating parameters such as the equivalence ratio and H2/CH4 ratio. Further, the length of mean

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recirculation zone is correlated with the chemical time scales based on the extinction strain rate of the flame corresponding to the fuel composition, equivalence ratio, inlet temperature, and operating pressure [43]. They also report in Ref. [15] that addition of hydrogen results in shifting the stability map of combustor, with no difference in the nature of instability per se. However, other literature have shown jumps in frequencies corresponding to acoustic mode shifts including high harmonics as operating conditions are varied, such as fuel composition, equivalence ratio and/or air-flow Reynolds number [35e38,44e47], with a variety of fuel combinations such as H2/CH4, H2/C3H8, and pure CH4. In general, higher H2 content in the fuel mixture excites higher harmonics, sometimes skipping intermediate harmonics as well, in order to satisfy the Rayleigh criterion at such high frequencies of excitation [45,48]. It can be seen from the above that there is scarce work reported in the literature on combustion instability characteristics of binary mixtures of H2/CO more directly representative of syngas, i.e., with hardly any CH4. Contrary to that reported in the past [35,36], such binary mixtures do excite high amplitude acoustic oscillations as shown in Ref. [49]. This leads us to extend the previous work with the objective of evaluating the dynamic stability characteristics of syngas with other hydrogen enriched fuel(s). Such a study, not attempted before can lead to answers, explaining the peculiarities of syngas combustion instability i.e., high frequency oscillations, that are not seen for other fuels including hydrogen enriched. This assumes significance, considering that only scarce literature exists describing the acoustic characteristics of pure hydrogen which in-turn leads to the ambiguity on the role of hydrogen in exciting higher frequency, as a single fuel and in combination with another fuel. As a result, the present work is devoted to identify and distinguish features of hydrogen enriched fuels from syngas to address the fore-mentioned issues. In this context, the role of fuel-air momentum ratio in exciting significantly different acoustic behaviour across the fuels is considered. This feature is seldom discussed as a governing parameter in the context of combustion instability. A bluff-body combustor is adopted for the purpose of investigating the thermo-acoustic behaviour with the above fuel mixtures. Since H2 is involved, holding a premixed flame is difficult for the range of velocities considered, hence the fuel is always injected at the rim of the bluff-body for the combustion to mostly occur in a non-premixed mode. Simultaneous unsteady pressure measurements and timeresolved chemiluminescence imaging has been performed to correlate the flame dynamics with the acoustic excitation. Taking advantage of the combustion of fuel mixture constituents considered in the present work, simultaneous time-resolved chemiluminescence imaging of OH* and CO2*/ CH* is performed, the choice of species in the latter imaging depending upon whether CO or CH4 is used in the fuel mixture. The simultaneous time-resolved chemiluminescence imaging of two species is reported here for the first time, and helps in resolving the presence of multiple heat release rate zones in the H2eCO case that is responsible for the excitation of higher harmonics when compared to the H2eCH4 case.

Experimental setup The bluff body type syngas combustor (Fig. 1(a)) used in the present work consists of a settling chamber of 280 mm diameter followed by a sudden contraction to a duct of 60
60 mm2 square cross-section. The air enters from the settling chamber to the primary combustor through an inlet duct consisting of a honeycomb section and wire meshes. The fuel line consists of a stainless steel pipe of 9 mm internal diameter and 12 mm outer diameter, 2000 mm in length providing sufficient mixing length for the fuel mixture. From the upstream end of the fuel line, carbon monoxide and hydrogen are supplied and allowed to mix; the downstream end terminates in a disc type bluff-body. The bluff body is 30 mm in diameter and 15 mm in width, having 16 equally spaced holes of 1.5 mm size along its circumference to inject the fuel to the combustor. The fuel line and bluff-body are axially movable, and are held along the centerline of the combustor by four vanes 0 angle to the inflow housed in the pipe of 25 mm diameter (Fig. 1 (b)), which provides sudden expansion of the air at 260 mm from the combustor inlet. The combustor consists of 60
160 mm quartz window for optical access with its upstream edge coinciding with the sudden expansion. The combustor is 1970 mm long. Four Alicat make mass flow controllers with the uncertainty of 0.8% of full scale and ±0.2% of the measured reading are used for controlling the mass flow rate of hydrogen line, carbon monoxide line, and main air line. The variables in the present study are the bluff body location lb relative to the sudden expansion (Fig. 1 (b)), air inlet Reynolds number Re (and global equivalence ratio 4), and the composition of the fuel mixture. Re corresponds to the average velocity of air evaluated at the dump plane, with length scale being the diameter of the bluff body. The bluff body location is fixed at lb ¼ 10 mm based on preliminary experiments where it is varied in the range of 0e30 mm in steps of 1 mm, for a fuel composition of H2:CO ¼ 75:25 by volume at Re ¼ 4600. In all the subsequent experiments, three fuel compositions are considered: (i) pure H2, (ii) H2:CO ¼ 75:25, and (iii) H2:CH4 ¼ 75:25 by volume. For these, the Re is swept in the range of 2200e8000 in steps of ~600. The dynamic pressure is measured with eight axially located piezoelectric pressure transducers (103B02 model, PCB make, sensitivity of 225 mV/kPa). The location of the transducers has been shown in Ref. [49]. The measurements are recorded at a sampling rate of 9 kHz over 3 s. In a separate set of experiments, the dynamic pressure measurement is performed simultaneously with timeresolved OH* and CO2*/CH* chemiluminescence imaging, the latter using two high-speed cameras (Phantom make, model V611) mounted on opposite sides of the combustor and focused on the same field of view. OH* and CO2*/CH* chemiluminescence imaging is used as flame marker in nonpremixed flame marker [50,51]. The cameras are used at their full frame resolution of 1280
800 pixels. For OH* chemiluminescence imaging, a 100 mm UV lens is used along with a Lambert intensifier and a filter of 310 ± 5 nm transmittivity. For CO2*/CH* chemiluminescence imaging, a 50 mm Nikor lens with a broadband bandpass filter of 430 ± 50 nm

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Fig. 1 e Schematic of (a) Experimental setup and (b) bluff-body assembly (all dimensions are in mm). transmittivity is used. The dynamic pressure data acquisition and the high-speed imaging are performed at a sampling/ framing rate of 4000 samples/frames per second over 1 s. Synchronization and simultaneous triggering of cameras, intensifier, and pressure transducers are achieved with a BNC model 575 pulse/delay generator.

Results and discussion Effect of bluff body location on acoustic pressure The variation of sound pressure level excited with change in lb at a fixed inlet air Re is shown in Fig. 2. With an initial increase in lb, the amplitudes for lb < 6 mm are quite low and in the form of broadband noise, with the dominant frequency at the fundamental longitudinal mode of the combustor. The identification of the nature of mode being excited, is done by applying a technique adopted in Ref. [49], following measurements from the pressure transducers. The boundary conditions are open-open for the recorded frequency of oscillations during instability. Then, the pressure oscillations are seen to rise and reach peak amplitudes at lb ¼ 10 mm, and

subsequently decrease up to lb ¼ 14 mm. Correspondingly, the dominant frequency excited at high amplitudes for 6 mm < lb < 14 mm is the third harmonic of the longitudinal acoustic mode, which shifts to the second harmonic for lb > 14 mm, whereupon the pressure amplitudes remain low, implying that the combustor displays a stable behaviour. A similar trend has been reported earlier with change in bluffbody location in a similar combustor geometry [44,52]. As the bluff body position lb is increased, the flow velocity at its shoulder decreases for a given inlet velocity, which leads to a drop in the frequency of coherent structures associated with the bluff body, viz. shear layer. For an appropriate shoulder velocity range and other conditions like convective delay, this frequency gets close to the third harmonic frequency, and hence excitation of high amplitudes. A flowacoustic lock-on prevails, which causes a downward shift in the dominant frequency to the second harmonic with further increase in lb. A similar explanation has been provided by Yu et al. [53], where the dominant frequency of unsteady pressure was seen to change with inlet velocity. For the rest of paper, the combustor's acoustic characteristics at lb ¼ 10 mm, for different fuel compositions and varying Re have been investigated. The flow-acoustic lock-on noted above prompts the

Fig. 2 e Variation of (a) dominant frequency and (b) pressure amplitude with bluff body location at Re ¼ 4600, and 75% H2 e 25% CO.

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use of Re as the control variable of the flow, as is customary in the fluid mechanics literature on this aspect.

Effect of fuel compositions and Re on acoustic pressure The acoustics in the combustor is characterized to understand the effect of different fuel compositions on the combustor stability behaviour as a variation with Re. The results are shown in Fig. 3. Similar to Fig. 3, the amplitude spectrum of the spatially integrated OH* chemiluminescence is obtained for the listed fuel compositions. The integrated chemiluminescence is calculated by summing across all the pixels at a particular time instant. The results are shown in Fig. 4 for high Re (>4000). It is readily observed that the dominant frequency for both integrated OH* chemiluminescence and pressure are the same, thereby confirming that the instability is thermo-acoustic in nature. With increasing Re, the different fuel mixtures display drastic variations in the dominant frequency and amplitude trends. The least variation in the dominant frequency is shown by the H2eCH4 mixture, which shows a gradual linear increase from ~80 Hz to 160 Hzdaround the fundamental longitudinal mode frequency of the combustordin the low Re range up to ~4000, whereupon it remains constant at the fundamental longitudinal acoustic mode for the rest of the Re range. In the Re < 4000 range, the amplitudes are less, implying excitation of combustion noise, which accounts for the gradual linear rise in the dominant frequency. Thereafter, the corresponding amplitude levels show a sharp rise at Re ~4000, and continue to increase generally. On the contrary, both pure H2 and H2eCO fuels show relatively high amplitude excitation in the low Re range of 2000e4000, where the dominant frequency remains constant at 130 Hz, which is around the fundamental longitudinal acoustic mode frequency. The trends depart for the two cases thereafter, however. With pure H2, the amplitude levels are quite low up in the 4000 < Re < 5500 range, with the frequency continuing to remain the same. Thereafter, for Re > 5500, the dominant frequency shifts to 260 Hz, the first harmonic longitudinal mode frequency, and correspondingly, the amplitude shows a slight rise and fall in the subsequent Re range. On the other hand, with the H2eCO mixture, the amplitude significantly rises and falls in the 4000 < Re < 6000 range, and continues to be at low amplitude levels thereafter. The

corresponding dominant frequency shifts to ~520 Hzdthe third harmonic frequency, skipping the first and second harmonicsdin the 4000 < Re < 6000 range. It further shifts to a much higher harmonic (~6th harmonic, skipping the 4th and 5th) at high Re > 6000. The latter is significant for the excitation of unusually high harmonics, but practically it is at very low amplitudes, so it is not considered for the present discussion. The excitation of the third harmonic at significant amplitude levels with the H2eCO mixture in the mid-Re (4000-6000) range in itself is the most significant observation in the present study. The preference for higher modes has been shown in Refs. [35,45], in the context of propane-hydrogen and methane-hydrogen flames, but observed for the first time in H2eCO flames without any hydrocarbon fuel in the reactant mixture. This is the focus of the rest of the present work. In discussing the above results, the linear increase in the dominant frequency at low Re range, 2000 < Re < 4000, is noticed, but at low amplitudes for the H2eCH4 case. A straight line fit to the dominant frequency data in this range passes close to Re ~0 (not shown), which indicates a hydrodynamic dominance at a constant Strouhal number of Stsl ¼ 1.68 (shown as a broken line). This is well within the range of the shear layer mode in the wake of the flow past circular discs [54,55]. For Re > 4000, the amplitudes drastically increase, and the dominant frequency is locked on to that of the fundamental longitudinal acoustic mode, in this case. However, the Stsl ¼ 1.68 line passes through the next frequency band at the first harmonic excited in the pure H2 case for Re > 6000. This implies that the mild rise and fall in the amplitude corresponding to this Re range in that case locks on to the same shear layer mode as exhibited at lower Re in the previous case of H2eCH4. Considering this, the excitation of the third and the sixth harmonic frequencies in the H2eCO case is found to fall in a band around a constant Strouhal number that is three times the shear layer mode, i.e., St ¼ 3 Stsl (also shown as a broken line), also passing through Re ~ 0 (not shown), in the Re > 4000 range. The multiple 3 here is commensurate with the multiples of 3 for the harmonics excited in this case. Evidently, since the St ¼ 3 Stsl line has a steeper slope than the Stsl ¼ 1.68 line, the third harmonic frequency band of high amplitude occurs in the mid-Re range with the H2eCO mixture when compared to pure H2. The second harmonic is not excited because the Rayleigh criterion is not satisfied for that mode shape with the given

Fig. 3 e Variation of (a) acoustic dominant frequency and (b) corresponding pressure amplitude with Re for different fuels.

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Fig. 4 e Amplitude spectrum of integrated OH* chemiluminescence at high Re for (a) H2eCH4 (b) H2 and (c) H2eCO fuels.

location of the bluff body flame holder. However, the question arises as to why the first harmonic registered in the pure H2 case is skipped in the case of H2eCO, and the third harmonic is directly excited, when Re is increased >4000. We seek answers to this question in terms of the flame structure, as observed from simultaneous time-resolved OH* and CO2* chemiluminescence images considered next. Note also another significant difference between the H2e CH4 case and the other two, namely, pure H2 and H2eCO cases, is the excitation of high amplitudes at low frequency in the latter two, whereas it is absent in the former. In the next subsection, the contrast in the extent of the heat release zones between the former and latter cases which explains the observed differences across the fuel compositions is illustrated from time-resolved images.

Time-resolved chemiluminescence images Fig. 5 shows sequence of time-resolved OH* chemiluminescence images for the three fuel mixtures considered, (a) H2eCH4 at Re ¼ 8000, (b) pure H2 at Re ¼ 6900, and (c) H2eCO at Re ¼ 4600. The choice of Re is based on the maximum amplitude attained in each case in the high frequency range, which is the focus of the present work. The flame patterns exhibited by the H2eCH4 flame (Fig. 5(a)) clearly shows significant variation in the chemiluminescence intensity pattern within a cycle of oscillations, explaining the high amplitudes excited in that case, seen in Fig. 3(b). Moreover, roll-up of large-scale vortical structures are observed, corresponding to the low frequency of fundamental natural acoustic mode excitation even at fairly high Re. These vortical structures are of the order of the bluff body size, and occupy the core of the combustor. Accordingly, the peak heat release rate locations are confined towards the combustor's centerline. On the contrary, the cases of pure H2 and H2eCO mixture do not show any large scale vortical structures to be exhibited by OH* chemiluminescence images (Fig. 5(b) and (c) respectively). In both the cases, the chemiluminescent intensity modulation is seen along the shear layer emerging along the edge of the bluff body. In the former case, peak heat release rate fluctuations are observed along the small-scale shear layer at the bluff-body edge. In the latter case, however, the chemiluminescent intensity pattern along the shear layer has a markedly serrated pattern, indicating a larger scale of

intensity modulation than with pure H2. This is commensurate with a higher amplitude of acoustic oscillations excited in the mid-Re range at the third harmonic natural acoustic mode with the H2eCO mixture than at the first harmonic with pure H2 in a higher Re range. Evidently, the small-scale structures in the intensity pattern would be more distinct in the low Re range in both cases of pure H2 and H2eCO, resulting in a larger modulation of the heat release rate and excitation of higher amplitudes of acoustic oscillations as observed in Fig. 3. The corresponding images, however, are not considered here in the interest of brevity. At low Re, at constant fuel flow rate in each case, firstly, the momentum ratio between the fuel jets from the bluff body rim and inlet air flow is high, so the fuel can diffuse upstream of the bluff body and react from there on besides further downstream, particularly with pure H2. Besides, at constant fuel flow rate, the equivalence ratio is closer to unity at lower Re. These factors result in larger heat release rate fluctuations, leading to higher amplitudes of acoustic excitation, in the lower Re range for both the pure H2 and H2eCO cases.

Time-averaged OH* chemiluminescence images The time-averaged OH* chemiluminescence images for the three fuel cases considered are shown in Fig. 6 for the same conditions as in Fig. 5 for the respective cases. These images provide an overall picture of the flame structure without regard to variations that could occur within a cycle of oscillations. The marked observation from these images is that the case of H2eCH4 is significantly different from the other two cases, in that the region just downstream of the bluff body along the centerline is predominantly occupied by OH* radicals in the former, which is not observed to the same extent in the other two. The following factors play an important role with regard to the above observation: (i) the H2eCH4 jet has a lower momentum ratio relative to the incoming air flow that pushes it closer to the core of the bluff body, which lets the fuel get entrained into the wake (Fig. 6(a)); (ii) the pure H2 jet has an even lower momentum ratio relative to the air flow, but it is much more and diffusive and reactive, hence the flame is held just outside the fuel injection holes along the bluff body rim, thereby extending the reaction and heat release zone along

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Fig. 5 e OH* chemiluminescence images at different phases and time-instants of an acoustic cycle for (a) H2eCH4 at Re ¼ 8000 (b) Pure H2 at Re ¼ 6900 and (c) H2eCO at Re ¼ 4600.

Fig. 6 e Time averaged OH* chemiluminescent (a) H2eCH4 at Re ¼ 8000 (b) Pure H2 at Re ¼ 6900 and (c) H2eCO at Re ¼ 4900.

the shear layer anchored there (Fig. 6(b)); (iii) the H2eCO mixture has a higher momentum ratio of all, and hence penetrates the air cross-flow and promoting the reactions radially outward from the bluff body rim, thus extending the reaction zone along the shear layer downstream (Fig. 6(c)). In fact, the last case shows higher intensity of chemiluminescence over the shoulder of the bluff body than then second case. Because of the above, as heat release occurs around the centerline downstream of the bluff body in the H2eCH4 case (Fig. 6(a)), it indicates the sense of the resulting baroclinic vorticity to aid the roll-up of the large-scale vortex structures observed the time-resolved images in Fig. 5(a). This is not so

with the other two fuels (pure H2 and H2eCO), and hence the wake vortex formation is countered by the baroclinic torque, leading to the reaction zones being confined to the shear layer in these two cases.

Time-averaged streamwise variation of chemiluminescent intensity The line-of-sight integration implicit in the chemiluminescence imaging, is further exploited by integrating the chemiluminescent intensity in the vertical direction, i.e., perpendicular to the streamwise direction (left to right in Figs.

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5 and 6 as indicated by the white arrows at the top). This yields the cross-sectionally integrated intensity at any streamwise location, so that the streamwise variation of this integrated quantity can be examined. In this subsection, this streamwise intensity variation (SIV) for the time-averaged images is studied in detail and is shown in Fig. 6. In Fig. 7, two kinds of plots are presented for each case of fuel considered: (i) a superposition of the OH* intensity field in greyscale is superposed with contour plots of CH* or CO2* intensity in the case H2eCH4 or H2eCO mixtures respectively, and (ii) an overlay of the SIV plots of these intensities for the above two fuel mixtures (no such superposition/overlay is possible/required for pure H2). In a fuel containing H2 and CO, only CO2* radical is formed whereas CH* also is formed in the presence of hydrocarbon fuels [56,57]. Thus, the intensity captured by the camera with a filter of 400e500 nm wavelength range with peak around 440 nm is due to the blue continuum emissions from CO2* in the case of H2eCO flame and CH* predominantly in the case of H2eCH4. In Fig. 7(a), it can be seen that the intensity patterns of the OH* and CH* overlap quite well with each other spatially, with peak intensities in the region just downstream of the bluff body around the centerline, as already noted in Fig. 6(a) with OH*, but here with CH* as well. The corresponding SIV plots of the two intensities also show a good match with each other up to the major heat release region, i.e., downstream of the bluff body. Specifically, the peaks in the SIV plots of the two intensities overlap axially. This also implies that the production of CH* radical has the same rate of reaction as that of OH* in the present study. This fact has also been exploited in literature in study of hydrocarbon fuel based combustion instability, where CH* and OH* chemiluminescence has been used interchangeably as flame markers with identical dynamics [58]. The SIV plot of the OH* chemiluminescence in the case of pure H2 can be seen to peak just at the bluff body base, as can be directly compared with the greyscale intensity pattern shown above it (Fig. 7(b)). This, in turn, serves the purpose of comparison with the H2eCO case shown next in Fig. 7(c),

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where it can be seen that the OH* SIV plot is very similar to the case of pure H2, i.e., the hydrogen chemistry is reasonably replicated even in the presence of CO. Additionally, the contour plot of the CO2* chemiluminesence, superposed over the OH* greyscale intensity pattern at the top of Fig. 7(c), clearly shows the peak CO2* locations are shifted downstream of the bluff body and are near the walls of the combustor. Correspondingly, CO2* SIV plot shows a streamwise stagger in its peak relative to the OH* peak, and its variation is not as sharp as the latter. The above featuresdthe peak CO2* regions being near the walls, shifted downstream of the bluff body, with its SIV peak staggered relative to the OH* SIV peakdare markedly different than what is observed in the H2eCH4 case. It is evident that these distinct features of the H2eCO flame would be masked in the presence of CH4 as has always been considered in the literature thus far, and hitherto not unraveled. In order to understand the reason for the shift/stagger in the peaks between the two chemiluminescence intensities and how they might be related to the third harmonic mode acoustic excitation in the H2eCO case, the instantaneous evolution of its SIV plots is discussed next.

Time-resolved streamwise variation of chemiluminescent intensity in the H2eCO case Fig. 8 shows the temporal evolution of the instantaneous SIV plots of the two chemiluminescent intensities in the H2eCO case. The corresponding chemiluminescence images are shown beside for direct reference. There are a number of inferences that are possible from these plots. First, the wavy behaviour of the SIV plots and the peaks in the wavy pattern coincide with the vortex patterns shown from the chemiluminescence images in Fig. 5(c). This shows that the vortex roll-up in the shear layer leads to a corresponding spatial heat release rate variation. Second, the upstream peak in the OH* SIV plot is always coincident with the bluff body base, whereas the global peak in the CO2* SIV plot moves back and forth across the different instants, first

Fig. 7 e Time averaged OH* chemiluminescent (in gray) superposed by CH*/CO2* color line contour (on top) and streamwise intensity variation plots (at bottom) for (a) H2eCH4 at Re ¼ 8000 (b) Pure H2 at Re ¼ 6900 and (c) H2eCO at Re ¼ 4900. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 8 e Time resolved (a) OH* chemiluminescent, (b) CO2* chemiluminescent and (c) and overlaid SIV plots from OH* chemiluminescent (in orange) and CO2* chemiluminescent (in blue) images for H2eCO at Re ¼ 4900 in step of 0.5 ms (from top to bottom). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

occurring at the second local peak and then moving upstream to merge with the first local peak. In any case, there is always a stagger between the global peaks of OH* (always at the bluff body base) and CO2* SIV plots. The movement of the latter peak described above indicates a larger time scale taken for the CO2 production, during which time, the vortex pattern in which it occurs gets convected. Further, these vortices merge with each downstream, as expected in the shear layer, and shown by how the global peak convects downstream and shifts back to the one upstream, as noted above. The convective nature of the local peaks in the CO2* SIV plots is exploited to estimate the local streamwise velocity of the vortical structures in the shear layer originating from the shoulder of the bluff body. Tracking the peak that is global in the first frame shown in Fig. 8 till its subsequent decay yields a velocity of 13.17 m/s averaged over the first few frames that do not shown any vortex merger yet. This is very close to a theoretical estimate based on the adiabatic flame temperature of the 75% H2e25% CO mixture with air at the overall equivalence ratio of 0.74 corresponding to the conditions of Fig. 8, as

follows. The adiabatic flame temperature in this case is 2105 K, as obtained from chemical equilibrium calculations [59]. Based on this, at approximately constant pressure (which is atmospheric pressure) and for the average molecular weight of the products obtained from the above chemical equilibrium calculations, the velocity of the flow issuing from the shoulder of the bluff body is estimated as 15.45 m/s. The above validation of the flow velocity obtained from the convection of the CO2* SIV peak imposes confidence in the next step of the analysis, which is to examine the average spatial (streamwise) stagger between the instantaneous OH* and CO2* SIV global peaks obtained from successive images. The average stagger in the peaks from the images shown in Fig. 8 is 31.33 mm. For this distance to be covered at the convective velocity of 13.17 m/s seen above takes 2.38 ms. For the phenomenon to repeatedly occur at this time-scale would result in approximately a frequency of 420.4 Hz. This is close to the dominant acoustic frequency registered from the pressure transducer measurement during this run, which is 500.1 Hz.

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The above analysis implies that there are two distinct heat release zones, one associated with OH production and another with CO2 production (as signaled by the OH* and CO2* chemiluminescence SIV peaks above), that are spatially staggered with respect to each other, and similarly in time. The second heat release kicks in 2.38 ms after the first heat release for every vortex shed along the shear layer at the bluff body shoulder, which occurs at a frequency close to the third harmonic longitudinal natural acoustic mode frequency of the combustor. Hence, the third harmonic is directly excited in the H2eCO case as opposed to the first harmonic in the pure H2 case, and the shear layer mode of St ¼ 3 Stsl locks on to this acoustic mode, seen in Fig. 3(a). The fact that second heat release is delayed from the first heat release suggests that the corresponding key reaction steps are sequential. That is, the production of CO2 from CO should utilize the availability of OH, or in other words, CO is oxidized by OH to form CO2. Indeed, this is the key step in the CO oxidation leading to CO2 formation [60e63]. Thus, the two heat release zones referred to above relate, first to the oxidation steps of H2 leading to the formation of OH, as seen in the fixed peak location of the OH* chemiluminescence at the bluff body shoulder, and second to the oxidation of CO by OH to produce CO2, as seen by the CO2* peak staggered downstream from the former. These sequential reactions, first of all, result in the stagger. The other point to note is that the OH formation step above has a shorter reaction time scale, resulting in its occurrence always at the bluff body shoulder; whereas, the CO2 production by oxidation of CO by OH has a longer reaction time scale, as can be seen by the convection of the local CO2* SIV peak and the spatial variation in its global SIV peak in association with the vortex merger seen above. In the present work, the stagger resulting from sequential oxidation of CO2 by OH* radicals supplied by hydrogen combustion is further altered due to the different transport properties of the syngas fuel constituents. The net effect is the presence of the two heat release rate zones. It should be noted that the transport process are significantly influenced by Re. The cumulative result of all the considerations is there is a certain Re range, where the stagger between the peak OH* and peak CO2* corresponds to a time scale based on flow velocity is around the acoustic time-scale and the flame stabilizes in a region where the hydrodynamic time scale is in proximity to the acoustic time scale. The latter modulates the heat release rate fluctuations of the key chemical reaction steps involving OH production and CO oxidation by OH, resulting in the peculiar nature of high frequency excitation seldom observed for hydrogen-hydrocarbon and pure hydrogen combustion.

Conclusion The combustion dynamics of a laboratory scale bluff-body combustor is examined for three different fuel/fuel mixtures, pure H2, H2eCH4 and H2eCO. Non-premixed mode of combustion is considered since the presence of H2 causes flame flashback past the bluff body in the velocity regime tested in the present work. The combustion dynamics of the H2eCO fuel mixture without the presence of CH4 or other hydrocarbon fuels has not been investigated hitherto in

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much detail. This fuel mixture has been considered to exhibit low amplitudes of self-excited acoustic oscillations relative to the other fuels/fuel mixtures involving CH4 or other hydrocarbons and H2. In the present study as well, the amplitudes of the pure H2 and H2eCO cases are quite lower than the H2e CH4 case, but they are not insignificant. More importantly, high frequency oscillations up to the 3rd and even the 6th harmonic longitudinal natural acoustic modes of the combustor duct are excited, particularly with the H2eCO mixture, which is the aspect of investigation in the present work. Abrupt jumps in the dominant frequency of the acoustic oscillations observed for the H2 and H2eCO cases as compared to the H2eCH4 case. The latter shows a linear increase in the frequency at low Reynolds numberdwhich is at a constant Strouhal number of 1.68 that corresponds to the shear layer mode of flow past a circular disc bluff body, and subsequently remains constant at the fundamental natural longitudinal duct acoustic mode frequency of the combustor. In comparison to this, the abrupt frequency shift in the pure H2 case is between the fundamental and first harmonic longitudinal duct acoustic modes, indicating a flow-acoustic lock-on between the same shear layer mode (Stsl ¼ 1.68) and the above two acoustic modes. On the other hand, the H2eCO case avoids the fundamental, and first and second harmonic frequencies, and directly locks-on between the 3rd and 6th harmonic modes of the duct with a shear layer frequency at St ¼ 3 Stsl. Time-averaged chemiluminescence images show a largescale vortex of the order of the bluff body width suggesting the predominance of the wake mode in the H2eCH4 case, whereas the heat release is confined to the shear layer in the other two cases. The H2eCH4 mixture's jet momentum ratio with the incoming air stream is sufficiently low as to deflect it towards the centerline, where the peak heat release occurs then. Whereas, the pure H2 jet, despite having a low momentum ratio, is more diffusive and reactive so that it almost instantaneously reacts along the shear layer. On the other hand, the H2eCO mixture has sufficient momentum ratio to penetrate the air flow and react mainly along the shear layer. The cross-sectionally averaged chemiluminescent intensity of OH* and CH*/CO2* have coincident peaks along the streamwise direction in a time-averaged sense in the H2eCH4 case, but they latter is staggered downstream of the former in the H2eCO case, by contrast. This suggests a double kicked oscillator model of two zones heat release rate fluctuations exciting the acoustic oscillations in quick succession to result in the 3rd harmonic frequency recorded. Indeed, the time scale of convection across the stagger between these two peaks averaged over several instantaneous measurements matches the observed acoustic time scale. The above results suggest that the key heat release rate steps responsible for the acoustic excitation should be sequential, i.e., the oxidation of H2 to form OH is necessarily followed by the oxidation of CO by the OH to form CO2. Indeed, the reaction time scales of the former oxidation are quite smaller than that of the latter. This results in the presence of two heat release rate zones in syngas, whose stagger results in the excitation of high frequency oscillations.

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Acknowledgements The National Centre for Combustion R & D was supported by the Science and Engineering Research Board. This work was also partially supported by the UK-India Education and Research Initiative.

references

[1] Lieuwen T, Yang V, Yetter R. Synthesis gas combustion: fundamentals and applications. 2009. [2] Vijay Kumar K, Bharath M, Raghavan V, Prasad BVSSS, Chakravarthy SR, Sundararajan T. Gasification of high-ash Indian coal in bubbling fluidized bed using air and steam e an experimental study. Appl Therm Eng 2017;116:372e81. [3] Richards GA, McMillian MM, Gemmen RS, Rogers WA, Cully SR. Issues for low-emission, fuel-flexible power systems. Prog Energy Combust Sci 2001 Jan 1;27(2):141e69. [4] Natarajan J, Lieuwen T, Seitzman J. Laminar flame speeds of H2/CO mixtures: effect of CO2dilution, preheat temperature, and pressure. Combust Flame 2007;151:104e19. [5] Ranga Dinesh KKJ, Jiang X, Kirkpatrick MP, Malalasekera W. Combustion characteristics of H2/N2 and H2/CO syngas nonpremixed flames. Int J Hydrogen Energy Nov 2012;37e21:16186e200. [6] Ranga Dinesh KKJ, Jiang X, van Oijen JA, Bastiaans RJM, de Goey LPH. Hydrogen-enriched nonpremixed jet flames: effects of preferential diffusion. Int J Hydrogen Energy 15 April 2013;38e11:4848e63. [7] Ranga Dinesh KKJ, Luo KH, Kirkpatrick MP, Malalasekera W. Burning syngas in a high swirl burner: effects of fuel composition. Int J Hydrogen Energy 17 July 2013;38e21:9028e42. [8] Ranga Dinesh KKJ, Jiang X, van Oijen JA. Hydrogen-enriched non-premixed jet flames: compositional structures with near-wall effects. Int J Hydrogen Energy 12 April 2013;38e12:5150e64. [9] Ranga Dinesh KKJ, Jiang X, van Oijen JA. Hydrogen-enriched non-premixed jet flames: analysis of the flame surface, flame normal, flame index and Wobbe index. Int J Hydrogen Energy 12 April 2014;39e12:6753e63. [10] Shang R, Zhang Y, Zhu M, Zhang Z, Zhang D, Li G. Laminar flame speed of CO2 and N2 diluted H2/CO/air flames. Int J Hydrogen Energy 2016;41(33):15056e67. [11] Xie Y, Wang J, Xu N, Yu S, Huang Z. Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures. Int J Hydrogen Energy 2014;39(7):3450e8. [12] Li Hong-Meng, Li Guo-Xiu, Sun Zuo-Yu, Zhou Zi-Hang, Li Yuan, Yuan Ye. Investigation on dilution effect on laminar burning velocity of syngas premixed flames. Energy 2016;112:146e52. [13] Santner Jeffrey, Dryer Frederick L, Ju Yiguang. The effects of water dilution on hydrogen, syngas, and ethylene flames at elevated pressure. Proc Combust Inst 2013;34(1):719e26. [14] Xu Huanhuan, Liu Fengshan, Sun Shaozeng, Zhao Yijun, Meng Shun, Tang Wenbo. Effects of H2O and CO2 diluted oxidizer on the structure and shape of laminar coflow syngas diffusion flames. Combust Flame 2017;177:67e78. [15] Taamallah S, Vogiatzaki K, Alzahrani FM, Mokheimer EM, Habib MA, Ghoniem AF. Fuel flexibility, stability and emissions in premixed hydrogen-rich gas turbine combustion: technology, fundamentals, and numerical simulations. Appl Energy 2015 Sep 15;154:1020e47.

[16] Ahmed SF, Santner J, Dryer FL, Padak B, Farouk TI. Computational study of NO x formation at conditions relevant to gas turbine operation, Part 2: NO x in high hydrogen content fuel combustion at elevated pressure. Energy Fuels 2016;30(9):7691e703. [17] Santner J, Ahmed SF, Farouk TI, Dryer FL. Computational study of NOx formation at conditions relevant to gas turbine operation: Part 1. Energy Fuels 2016;30(8):6745e55. [18] Asgari N, Ahmed SF, Farouk TI, Padak B. NOx formation in post-flame gases from syngas/air combustion at atmospheric pressure. Int J Hydrogen Energy 2017;42(38):24569e79. [19] Safer K, Tabet F, Safer M. A numerical investigation of structure and NO emissions of turbulent syngas diffusion flame in counter-flow configuration. Int J Hydrogen Energy 2016;41:3208e21. [20] Sahu AB, Ravikrishna RV. A detailed numerical study of NOx kinetics in low calorific value H2/CO syngas flames. Int J Hydrogen Energy 2014;39(30):17358e70. [21] Yang K, Shih H. NO formation of opposed-jet syngas diffusion flames: strain rate and dilution effects. Int J Hydrogen Energy 2017:1e15. [22] Park J, Bae DS, Cha MS, Yun JH, Keel SI, Cho HC, Kim TK, Ha JS. Flame characteristics in H2/CO synthetic gas diffusion flames diluted with CO2: effects of radiative heat loss and mixture composition. Int J Hydrogen Energy 2008;33(23):7256e64. [23] park J, Kim JS, Chung JO, Yun JH, Keel SI. Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames. Int J Hydrogen Energy 2009;34(20):8756e62. [24] Tuncer O, Acharya S, Uhm “Dynamics JH. NOx and flashback characteristics of confined premixed hydrogen-enriched methane flames. Int J Hydrogen Energy January 2009;34e1:496e506. [25] Garcia-Armingo T. J. Ballester “Operational issues in premixed combustion of hydrogen-enriched and syngas fuels. Int J Hydrogen Energy 12 January 2015;40e2:1229e43. [26] Dam B, Love N, Choudhuri A. Flashback propensity of syngas fuels. Fuel February 2011;90e2:618e25. [27] Reichel TG, Paschereit CO. Interaction mechanisms of fuel momentum with flashback limits in lean-premixed combustion of hydrogen. Int J Hydrogen Energy 16 February 2017;42e7:4518e29. [28] Schonborn A, Sayad P, Klingmann J. Influence of precessing vortex core on flame flashback in swirling hydrogen flames. Int J Hydrogen Energy 2014;39:20233e41. [29] Lee MC, Park S, Kim U, Kim S, Yoon J, Joo S, et al. Effect of hydrogen content on the gas turbine combustion performance of synthetic natural gas. Vol. 1B Combust. Fuels Emiss. ASME 2013. V01BT04A067 [30] Park S, Kim U, Lee M, Kim S, Cha D. The effects and characteristics of hydrogen in SNG on gas turbine combustion using a diffusion type combustor. Int J Hydrogen Energy 2013;38:12847e55. [31] Lieuwen T, McDonell V, Petersen E, Santavicca D. Fuel flexibility influences on premixed combustor blowout, flashback, autoignition, and stability. J Eng Gas Turbines Power 2008;130:011506. [32] Ferguson D, Richard GA, Straub D. Fuel interchangeability for lean premixed combustion in gas turbine engines. Proc ASME Turbo Expo 2008;3:1e9. [33] Lee MC, Seo S Bin, Chung JH, Kim SM, Joo YJ, Ahn DH. Gas turbine combustion performance test of hydrogen and carbon monoxide synthetic gas. Fuel 2010;89:1485e91. [34] Lee MC, Seo S Bin, Yoon J, Kim M, Yoon Y. Experimental study on the effect of N 2, CO 2, and steam dilution on the

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 5 5 9 8 e1 5 6 0 9

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

combustion performance of H2 and CO synthetic gas in an industrial gas turbine. Fuel 2012;102:431e8. Park J, Lee MC. Combustion instability characteristics of H2/CO/CH4 syngases and synthetic natural gases in a partially-premixed gas turbine combustor: Part Idfrequency and mode analysis. Int J Hydrogen Energy 2016;41:7484e93. Park J, Lee MC. Combustion instability characteristics of H2/ CO/CH4 syngases and synthetic natural gases in a partiallypremixed gas turbine combustor: Part II - time lag analysis. Int J Hydrogen Energy 2016;41:1304e12. Choi O, Lee MC. Investigation into the combustion instability of synthetic natural gases using high speed flame images and their proper orthogonal decomposition. Int J Hydrogen Energy 2016;41:20731e43. Yoon J, Joo S, Kim J, Lee MC, Lee JG, Yoon Y. Effects of convection time on the high harmonic combustion instability in a partially premixed combustor. Proc Combust Inst 2015;36:3753e61. Figura L, Lee JG, Quay BD, Santavicca DA. sea, and air DABTATE 2007: P for L. The effects of fuel composition on flame Structure and combustion dynamics in a lean premixed combustor. In: ASME Turbo Expo 2007: Power for Land, Sea, and Air; 2007. p. 181e7. Taamallah S, Labry ZA, Shanbhogue SJ, Ghoniem AF. Thermo-acoustic instabilities in lean premixed swirlstabilized combustion and their link to acoustically coupled and decoupled flame macrostructures. Proc Combust Inst 2015;35:3273e82. Taamallah S, LaBry ZA, Shanbhogue SJ, Habib MAM, Ghoniem AF. Correspondence between “stable” flame macrostructure and thermo-acoustic instability in premixed swirl-stabilized turbulent combustion. J Eng Gas Turbines Power 2015;137:071505. Shanbhogue SJ, Sanusi YS, Taamallah S, Habib MA, Mokheimer EMA, Ghoniem AF. Flame macrostructures, combustion instability and extinction strain scaling in swirlstabilized premixed CH4/H2combustion. Combust Flame 2016;163:494e507. Michaels D, Shanbhogue SJ, Ghoniem AF. The impact of reactants composition and temperature on the flow structure in a wake stabilized laminar lean premixed CH4/ H2/air flames; mechanism and scaling. Combust Flame 2017;176:151e61. Balachandran R, Chakravarthy SR, Sujith RI. Characterization of an acoustically self-excited combustor for spray evaporation. J Propuls Power 2008;24:1382e9. Altay HM, Speth RL, Hudgins DE, Ghoniem AF. Flame-vortex interaction driven combustion dynamics in a backwardfacing step combustor. Combust Flame 2009;156:1111e25. Hong S, Shanbhogue SJ, Speth RL, Ghoniem AF. On the phase between pressure and heat release fluctuations for propane/ hydrogen flames and its role in mode transitions. Combust Flame 2013;160:2827e42.

15609

[47] Chakravarthy SR, Sivakumar R, Shreenivasan OJ. Vortexacoustic lock-on in bluff-body and backward-facing step combustors. Sadhana - Acad Proc Eng Sci 2007;32:145e54. [48] Lieuwen TC. Unsteady combustor physics. vol. 9781107015. 2009. [49] Baraiya NA, Chakravarthy SR. Effect of syngas composition on high frequency combustion instability in a non-premixed turbulent combustor. Int J Hydrogen Energy 2019;44e12:6299e312. [50] Kothnur PS, Tsurikov MS, Clemens NT, Donbar JM, Carter Campbell D. Planar imaging of CH, OH, and velocity in turbulent non-premixed jet flames. Proc Combust Inst 2002;29(2):1921e7. [51] Kelman JB, Masri AR. Reaction zone structure and scalar dissipation rates in turbulent diffusion flames. Combust Sci Technol 1998;133(4e6):17e55. [52] Baraiya NA, Nagarajan B, Chakravarthy SR. Experimental investigation of combustion dynamics in a turbulent syngas combustor. Proceedings ASME Turbo Expo 2017. https:// doi.org/10.1115/GT2017-64849. Paper No. GT2017-64849, pp. V04BT04A051; 10 pages; ISBN: 978-0-7918-5085-5.  A, Daily JW. Low-frequency pressure [53] Yu KH, Trouve oscillations in a model ramjet combustor. J Fluid Mech 1991;232:47e72. [54] Berger E, Scholz D, Schumm M. Coherent vortex structures in thewake of a sphere and a circular disk at rest and under forced vibrations. J Fluids Struct 1990;4:231e57. [55] Zhong W, Liu M, Wu G, Yang J, Zhang X. Extraction and recognization of large-scale structures in the turbulent near wake of a circular disc. Fluid Dyn. Res 2014;46:025507. [56] Ning D, Fan A, Yao H. Effects of fuel composition and strain rate on NO emission of premixed counter-flow H2/CO/air flames. Int J Hydrogen Energy 2017;42:10466e74. [57] Shih HY, Hsu JR. Computed NO x emission characteristics of opposed-jet syngas diffusion flames. Combust Flame 2012;159:1851e63. [58] Ghoniem AF, Annaswamy A, Park S, Sobhani ZC. Stability and emissions control using air injection and H2 addition in premixed combustion. Proc Combust Inst 2005;30 II:1765e73. [59] NASA-CEA (https://cearun.grc.nasa.gov). [60] Lewis B, Von Elbe G. Combustion, flames and explosions of gases. Elsevier; 2012. romne s A, Metcalfe WK, Heufer KA, Donohoe N, Das AK, [61] Ke Sung CJ, et al. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combust Flame 2013;160:995e1011. [62] Rahnama P, Paykani A, Reitz RD. A numerical study of the effects of using hydrogen, reformer gas and nitrogen on combustion, emissions and load limits of a heavy duty natural gas/diesel RCCI engine. Appl Energy 2017;193:182e98. [63] Joshi AV, Wang H. Master equation modeling of wide range temperature and pressure dependence of CO þ OH / products. Int J Chem Kinet 2006;38:57e73.