S. R. Chakravarthy

23rdNational Conference onI. C. Engine and Combustion (NCICEC 2013) SVNIT, Surat, India

13-16, December 2013

New Insights into Mechanisms of Combustion Instability in Gas Turbine Type Combustors

S. R. Chakravarthy1), * 1)National Centre for Combustion Research and Development & Department of Aerospace Engineering

Indian Institute of Technology Madras, Chennai 600036 India

Abstract

Systematic experiments have been performed in combustors of identical dimensions containing a backward-facing step, a V-gutter bluff-body, or an annular swirler, to examine their combustion instability characteristics. The length of the combustor, the axial location of the step/bluff-body/swirler, fuel injection location or upstream of the flame holder, etc. have been varied. The air and (gaseous) fuel flow rates are widely varied to obtain acoustically induced blowout conditions as well as onset of transition from low amplitude broadband noise to high-amplitude discrete tones exhibited by combustion instability. Under most conditions, the onset of combustion instability is accompanied by shifts in dominant frequency from a constant Helmholtz number to constant Strouhal number representing a flow-acoustic lock-on. In the case of swirl combustor, this shift occurs between “mixed modes” of the combustor, i.e., neither purely acoustic nor purely hydrodynamic modes. Certain conditions indicate constructive and destructive interferences of this mechanism with equivalence ratio fluctuations due to fuel injection upstream of the flame stabilization zone. Upstream fuel injection at an intermediate location between non-premixed and premixed extremes in the swirl combustor clearly shows huge rise in amplitudes, suggesting flame structure fluctuations as a hitherto unacknowledged mechanism of heat release fluctuations driving combustion instability.

Keywords: Combustion instability,

1. Introduction Unsteady and unstable combustion has been a subject of investigation for long, owing to problems of noise generation and excitation of acoustic instabilities that leads to structural damages caused by enhanced vibrations or heat transfer in combustors of gas turbines, furnaces, etc. Candel [1] has reviewed the literature pertaining to different aspects of combustion dynamics and control. On the one hand, elementary processes of flame interactions with flow disturbances in the combustor are involved [2], and on the other, the role of heat release fluctuations in the combustion zone on the acoustic osci llations [3] is equally significant. One of the important driving mechanisms of combustion instability has been that due to vortex shedding, as highlighted by Schadow and Gutmark [4], who showed that the development of coherent flow structures and their breakdown into fine-scale turbulence can lead to periodic heat release. Many workers have investigated combustor geometries that include a predominant role for vortex shedding, such as dump combustors involving axisymmetric backward-facing step [5-9] and bluff-body flame-holders [10]. They have deduced the phase relationship between the vortex roll-up sequence and the heat release fluctuations under conditions of excitation of intense oscillations. Bloxsidge et al. [11] have utilized the observations

of heat release rate fluctuations in terms of the flame chemiluminescence reported in [10] to predict regimes of instability of combustor operation. Dowling [12] extended this to include saturation of the heat release rate fluctuations depending upon the instantaneous direction of the flow at the flame-holder, which leads to prediction of hysteresis and nonlinear behaviour including limit-cycle oscill-ations. Similarly, Dowling [13] developed a kinem-atic model of the flame oscillations with the flame anchor point fluctuating at the flame-holder depending upon whether the flame speed instantaneously exceeded the flow velocity. Lieuwen et al. [14] point out that the convective and chemical heat release time scales of the fuel-air mixture within the combustor may match the acoustic time scales for excitation of strong acoustic oscillations, and this could be amplified by equivalence ratio fluctuations [15] due to the response of the fuel feed line to the acoustic oscillations present in the combustion chamber. The vortex shedding at a location such as the dump plane in a dump combustor, for instance, would carry the fuel-air mixture in packets and modulate the heat release fluctuations. Hubbard and Dowling [16] have modelled the flame fluctuation in response to equivalence ratio fluctuations that are coupled to the acoustic oscillations in the combustor. Lieuwen [17] has reviewed in detail the flame-acoustic interaction in the context of modelling of combustion instability in premixed combustors. Lieuwen [18] has also reported experimental characterization of limit-cycle oscillations in a premixed dump combustor, including examples of transition from stable to

* Corresponding author. Fax: +91-44-22575025 E-mail address: [email protected]



unstable combustor operation in the form of supercritical and sub-critical bifurcations. Recently, Altay et al. [19] have reported experiments with fuel injection 280 and 930 mm upstream of a backward-facing step, typically at two air flow Reynolds numbers, 6500 and 8500. They observe no significant φ' fluctuations to occur at the step with fuel injection 930 mm upstream, but sufficient fluctuations with injection 280 mm upstream. They conclude that, even in the latter case, the φ' fluctuations play a secondary role, with the flame-vortex interaction being the primary mechanism of acoustic excitation. Combustion dynamics of swirl combustors has been of recent interest [20, 21]. The nature of the shear layers present in a swirl flow and their interaction are more complex than in other configurations. It is well known that a central toroidal recirculation zone (CTRZ) prevails in swirl flows, which lends itself to a vortex breakdown under strong swirl conditions, leading to a precessing vortex core (PVC) with an associated flame motion. In premixed or partially premixed systems, the vortex core precession could be amplified further in the presence of the flame [20]. In confined flows, a corner recirculation zone (CRZ) also prevails, which could interact with the CTRZ/PVC complex under oscillatory conditions. Such interactions could lead to momentary flame flashback. Under large expansion conditions, jet precession occurs along with associated flame motion, which is different from PVC. Interaction of combusting swirl flow shear layers with acoustic oscillations leads to different flame structure, depending upon the excitation of longitudinal versus transverse acoustic modes [21]. In the former case, typically at low frequencies, the effect is more global, leading to flame area variations. High frequency oscillations of the latter kind affect the flame locally and impart a limited effect on flame oscillation [22]. Huang and Yang [23] have also shown the effect of marginal increase in temperature or equivalence ratio on effecting a bifurcation between a stable and an unstable flame structure. The effect of externally imposed acoustic oscillations on swirl flames has been studied [24-27] with a view to measure the flame transfer/describing function (FTF/FDF) not only for input to linear stability analyses of combustion instability, but also to obtain insight into the flame dynamics under controlled oscillatory conditions. All the above works are on premixed combustion, performed in the context of stationary gas turbines used for power generation, which have stringent emission control norms, and therefore employ lean premixed pre-vaporized combustors, but they are also prone to intense instability problems. Very little attention is devoted to instabilities in non-premixed combustion systems such as those currently adopted in gas turbines for propulsion applications. Although the general view is that non-

premixed combustion is quite stable, it is not unconditionally so, however. Further, almost all the works above focus on investigations under conditions when the combustion oscillations are unstable. With a limited exception of Lieuwen [18] recently, there is no systematic work on a wide variation of geometric parameters of the combustor and flow conditions that span from a regime of low-intensity noise generation without appreciable acoustic feedback from the combustion chamber on the combustion process, to a regime of excitation of high-intensity discrete tones symptomatic of combustion instability. The Rayleigh criterion delineates regimes of unstable combustion from those of stable combustion, but it is only a necessary condition, which could be met by a variety and combination of physical mechanisms. These mechanisms would gradually vary in predominance and interplay with each other, leading to a transition from low-amplitude noise to high-amplitude instability conditions. A systematic variation would prompt investigation on the mechanisms that dictate the onset of instability. The data can also serve to identify precursors to instability that can be utilized in actively deploying certain passive control measures in practical combustion systems. The present paper discusses experimental results that have been systematically obtained at this laboratory on comparable size combustors with different flame-holding configurations, namely, backward-facing step, V-gutter bluff-body, and swirl flow. The details of the combustors are provided in [28-31]. Typically, these have 60 mm-a-side rectangular-shaped cross-sections. The air-flow Reynolds numbers vary in the range of 6000-80000 based on the step height, bluff-body width or swirler diameter, and inlet air velocity. The equivalence ratio is widely varied from extremely fuel-lean conditions to fuel-rich conditions. The fuel injection location is varied from the flame stabilization zone (nearly non-premixed) to progressively upstream locations, and in one case, completely premixed condition. Based on the above work, this paper presents a few new mechanisms of combustion instability or insights into interaction of previously identified mechanisms. As a starting point, we note that vortex shedding based combustion instability can be viewed as a flow-acoustic lock-on phenomenon. This is because of characteristic frequencies of intrinsic hydrodynamic instabilities that lead to vortex shedding, which interact with the duct-acoustic natural modes. Other mechanisms such as φ' fluctuations do not possess intrinsic characteristic frequencies and follow the acoustic modes without a lock-on. When the two mechanisms coexist, constructive and destructive interferences between them are observed. A third mechanism that comes about because of upstream fuel injection is that of flame structure fluctuations during a cycle of



oscillations, wherein the flame fluctuates from being nearly a premixed flame to that of a diffusion flame because of incomplete spatio-temporal fuel-air mixing also a consequence of φ' fluctuations. Finally, we show time-resolved PIV and high-speed chemiluminescence that illustrate the mechanism of vortex shearing at multiple time scales leading to bursts in pressure oscillations at near-unity equivalence ratios.

(a) Helmholtz number

(b) Amplitude

Figure 1. Unsteady pressure at different locations in a backward-facing step combustor with methane injection at the step. 2. Mechanisms of Combustion Instability

2.1 Flow-acoustic lock-on

Figure 1(a) shows the dominant frequencies observed in the pressure amplitude spectra of the sound excited during combustion for different lengths of the backward-facing step combustor, plotted in terms of the Helmholtz number. Figure 1(b) shows the corresponding amplitudes. The fuel is injected at the step in this case. We see that the frequency trend shifts from a nearly constant trend to a linearly increasing one at specific Reynolds number. Correspondingly, the amplitude trend shows a steep increase when the frequency shift occurs. This is an indication of flow-acoustic lock-on between the duct acoustic mode and vortex shedding. The linearly increasing frequency trend corresponds to a constant Strouhal number of 0.2 exhibited by the vortex shedding that occurs downstream of the step. The fuel-air mixing occurs in this vortex, followed by combustion and hence

heat release. Thus, the fluctuations in the heat release that excite the acoustic oscillations are modulated by the vortex shedding process, forcing a lock-on between the vortex shedding and the duct acoustics.

(a) Dominant frequency

(b) Amplitude

Figure 2. Unsteady pressure at different locations in a swirl combustor with LPG injection 120 mm upstream of the swirler exit plane.

Figure 2, likewise shows a similar trend for the case of swirl combustor, with the fuel (LPG) injected 120 mm upstream of the swirler exit plane. The amplitudes in this case are about 2 orders magnitude greater than in the previous case, and show an abrupt rise from nearly quiet levels when there is a shift in the trends of frequency with Reynolds number, again showing flow-acoustic lock-on. In this case, both the flow modes and acoustic modes are not clearly defined, and display “mixed modes”.

2.2 Competition between vortex shedding and φ '

As opposed to the above scenario, Fig. 3 shows a situation of constructive and destructive interference between the flow-acoustic lock-on mechanism and the φ' mechanism in terms of ups and downs in the amplitude when the frequency shift occurs. While the latter signifies flow-acoustic lock-on, the former implies the interference between the two mechanisms.

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The above data is for injection 190 mm upstream of the backward-facing step. The reason for the conclusion regarding the interference between the two mechanisms is the observation of high-speed chemiluminescence images of the flame that shows the vortex shedding pattern in accordance with the corresponding fluctuation in the fuel injection and acoustic velocity at the step, as shown in Fig. 4.

Figure 4. Vortex roll-up observed in the backward-facing step combustor due to fuel injection upstream of the step.

Figure 4 shows the flame rolling up along the shedding vortex during one half of the acoustic cycle and facing a fuel-lean part of the φ' pocket arriving at the step when it is about to unwrap out of the vortex as the latter sheds away, causing a local local

minimum in the pressure amplitude with Reynolds number as observed at one condition in Fig. 3.

3. Flame structure fluctuations

A third mechanism, which has not been advanced hitherto, is that of flame structure fluctuations. Figure 4 shows the maximum pressure amplitude registered for a fixed fuel flow rate for different combustor lengths, as a function of the upstream distance from the swirler exit plane where the fuel is injected.

Figure 4. Maximum amplitude as a function of upstream fuel injection location.

Here, it is noticed that, for the case of maximum amplitudes observed for the longest combustor, the amplitude attains a maximum for the partially premixed case of 50 mm upstream fuel injection from the swirler exit plane. This is investigated by means of phase-averaged OH-PLIF, shown in Fig. 5. With injection at the swirler exit (Xi = 0 mm, Fig. 5(a)), the flame is always multi-dimensional as in a jet flame structure. Fluctuations in the flame structure are apparent, with the flame being only partially present at 0° and 180°. For Xi = 50 mm (Fig. 5(b)), the flame exhibits a multi-dimensional structure similar to the above at some phases (e.g., 0° and 300°) and a distinct vortex roll-up at 60° due to the high amplitude of acoustic oscillations excited, similar to what is observed for Xi = 120 mm (Fig. 5(c)). At 120°, a nearly planar flame is observed right at the swirler exit plane (left edge of the image) with the post-flame OH zone, very similar to the case of 0° of Fig. 5(c). This flame appears weakened at 180°, and pushed out by a vortex roll-up (dark pattern) at 240° in Fig. 5(b), resuming the multidimensional structure for another half of the cycle. The case of Xi = 120 mm (Fig. 5(c)), on the other hand, exhibits the nearly planar flame mentioned above, followed by vortex roll-up (120°) and shedding (180°). It appears that the flame/flow processes contain more than one frequency of oscillations, so the phase-averaging does not clearly represent the whole sequence. For the present purposes, what is clear from Fig. 9, however, is that the flame for Xi = 50 mm, the intermediate upstream injection location (Fig. 5(b)), resembles that for Xi = 0 (Fig. 5(a)) in some parts of the cycle and resembles the flame for Xi = 120 mm (Fig. 5(c)) in other parts of the cycle. This indicates that there is a significant fluctuation in the flame

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Figure 3. Acoustic data: far-upstream injection.



structure itself from being similar to a diffusion flame to being similar to a premixed flame within a cycle of oscillations. Considering that this results in higher amplitudes of acoustic excitation than the rest, flame structure oscillations need to be considered as another mechanism of heat release rate fluctuations causing combustion instability, hitherto not discussed in the literature explicitly. In this scenario, it would not be possible to evaluate the heat release rate fluctuations from flame area calculations popularly resorted to or along the fluctuating stoichiometric surface as can be done with diffusion flames [32, 33]; the full flame structure needs to be determined.

Figure 5. Phase-averaged OH-PLIF images.

4. Burst pressure oscillations

Figure 6 shows typical burst pattern of pressure oscillations observed at an equivalence ratio of 0.898 in a premixed dump combustor. The inset shows the cycle for which high-speed PIV data is considered. The inset is enlarged and shown in Fig. 7 with instances of high-speed PIV labeled.

Figure 6. Burst pressure oscillations.

Figure 7. Inset in Fig. 6.

Figure 8(a) shows the sequence of velocity vectors and chemiluminescence images marked 1-8 in Fig. 7 corresponding to the growth in a cycle of a burst. The vector field shows the sequence of large-scale vortex roll-up (of the length-scale of the step height) downstream of the base of the step in the recirculation zone (marked by the line joining the loci of the vortex core). The corresponding chemiluminescence images show the flame rolling up along with this large-scale vortex. It is clear that this vortex mode of flow oscillation modulates the heat release to a high amplitude level, which in turn

excites the pressure oscillation seen growing during this time segment in Fig. 7. Thus the mechanism of the growth part for the cycle in the burst is the vortex combustion, as expected. However, the cause of the decay is interesting. As the acoustic amplitude rises, the duct mode gets amplified (due to its boundary conditions), whose time scales are shorter. Progressively, small-scale vortices begin to be shed from the step corner, even as the large-scale roll-up continues to occur at a larger time scale. As these small-scale vortices are shed at the acoustic time-scale (marked by the dashed line in Fig. 8(b)), they convect in the flow field around the large-scale vortex, and begin to shear at the latter’s edges. In general, the cascading of small-scale vortices shed at the shorter acoustic time scale rolling up around the large-scale step-mode vortex has been termed as “collective interaction” [4]. The other aspect of this is the shearing of the large-scale vortex by the small scales of sufficient strength (at high acoustic amplitude following the growth of the burst) that eventually weakens the large-scale vortex. It is well known that the flame is a low-pass filter. It smoothens the wiggles caused by the small-scales, but is modulated mainly by the large-scale vortex. When the latter is weakened, the flame can no longer oscillate in an organized manner, as can be clearly seen at late times in Fig. 8(b). The flame is the acoustic driver in this system. With no organized fluctuation, it cannot excite the acoustic pressure, leading to the latter’s decay seen marked 9-16 in Fig. 7. Thus, the large-scale vortex roll-up is progressively weakened by shearing action in this time interval (Fig. 8(b)).

Figure 8. Velocity and CH* intensity fields for the instants marked by +in Fig. 7.

661.62 ms(1)

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Conclusions

In this paper, a few different mechanisms of combustion instability that are not commonly discussed in the literature are presented. These are based on systematic experiments on nearly identical combustors containing a backward-facing step, V-gutter bluff-body or swirler conducted at this laboratory. Wide range of geometric and flow conditions have been varied. Four different mechanisms have been presented. The first is the flow-acoustic lock-on, wherein the dominant frequency of unsteady pressure oscillations shifts in its trend of constant Helmholtz number to constant Strouhal number, as the flow Reynolds number is varied. Accompanying this is a steep rise in the pressure amplitude. A similar behaviour is observed in the case of swirl combustor between modes that are constant in neither Helmholtz number nor Strouhal number. These are identified as “mixed modes” of the system. The second mechanism is the constructive and destructive interferences between the flow-acoustic lock-on due to vortex shedding and equivalence ratio fluctuations. High-speed chemiluminescence sequences are shown to demonstrate this mechanism. The third mechanism is that of flame structure fluctuations, shown in terms of phase-averaged OH-PLIF images, wherein the flame fluctuates from being a premixed flame to a diffusion flame during a cycle of acoustic oscillations, when the fuel is injected at an optimal location upstream of the swirler, for which maximum amplitude is observed when compared to at-the swirler or farther upstream fuel injection. The fourth is the mechanism of pressure burst oscillations due to interaction of different length scales of vortices shed at the step, one due to the preferred mode of vortex shedding and another at the acoustic frequency, the two vortices shearing each other to decay a burst in oscillations caused by heat release from a flame rolling up at the time scale of the large-scale vortex. These are new insights into the physical mechanisms that prevail during different facets of combustion instability.

Acknowledgement

The National Centre for Combustion Research and Development is supported by the Department of Science and Technology, Government of India.

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S. R. Chakravarthy