Multi-nozzle Combustor Research Articles

Effects of Fuel Staging on the Hydrodynamic Stability of Multinozzle Swirl Flows

Abstract Hydrodynamic instability in lean premixed gas turbine combustors can cause coherent flow velocity oscillations. These can in turn drive heat release oscillations that when favorably coupled with combustor acoustic modes can result in combustion instability. The aim of this paper is to understand the impact of fuel staging on the characteristics of hydrodynamic modes in multinozzle combustors. We extend our recent numerical study on the hydrodynamic stability characteristics of a multinozzle combustor having three nozzles in a straight line with uniform fuel–air ratio in each nozzle, to the nonuniform fuel–air ratio case. As before, we construct the base flow model for this study by superposing contributions from individual nozzles, determined using a base flow model for a nominally axisymmetric single nozzle, at every point in the computational domain. The impact of fuel staging is captured by changing the burnt to unburnt gas density ratio parameter in the individual contribution from each nozzle. We investigate the characteristics of the most locally absolutely unstable mode for two cases. The first one is when the middle nozzle is made fuel rich when compared to the side nozzles and the second is when the side nozzles are made fuel rich relative to the middle nozzle. The impact of nonuniform fuel/air ratio on the local absolutely unstable temporal eigenvalues is seen to be small. However, significant changes in the spatial structure of the flow oscillations associated with the hydrodynamic eigenmodes are observed. In the first case, the flow oscillations with a different locally azimuthal nature on the middle nozzle when compared to the side nozzles emerge as the middle nozzle is made richer. In the second case, the oscillations on the two side nozzles are suppressed leaving the middle nozzle in a state that closely matches that of a single unconfined nozzle with the same nominal base flow velocity field. These types of internozzle variations in flow oscillation characteristics can explain the emergence of nonuniformity in heat release oscillation characteristics between individual nozzles in multinozzle combustors.

Data-driven Detection and Early Prediction of Thermoacoustic Instability in a Multi-nozzle Combustor

ABSTRACT Thermoacoustic instability (TAI) is a critical issue in modern lean-burn gas-turbine combustors, which is induced by a strong coupling between the resonant combustor acoustics and fluctuations in the heat release rate. This instability may lead to high-amplitude pressure waves that generate undesirable noise levels as well as fatigue stresses in mechanical structures of the combustor. The intense pressure fluctuations due to TAI may also cause large flow perturbations and possibly flow reversal that may lead to flame oscillations, flame liftoff, and even flame blow-out. Hence, there is a strong need for exercising control actions in a timely fashion to mitigate the TAI phenomena. Anomaly detection is an essential prerequisite to the design of a good controller and such a detector must be able to reliably predict a forthcoming TAI. To detect and predict the onset of a TAI from an ensemble of pressure time series, this paper investigates three data-driven methods: Fast Fourier transform (FFT), symbolic time series analysis (STSA), and hidden Markov modeling (HMM). The main focus of the paper is to make a comparative evaluation of these three anomaly detection methods for classification of the current regime of operation into stable and unstable categories as well as for real-time identification of precursors to impending instabilities with short-length time series of measured variables (e.g., pressure oscillations). The results, generated on experimental data from a multi-nozzle combustor apparatus, have been compared to evaluate the performance of FFT, STSA, and HMM methods for TAI analysis.

Effect of Internozzle Spacing on Lean Blow-Off of a Linear Multinozzle Combustor

This paper studies the effect of internozzle spacing on the lean blow-off of a linear combustor array. The combustor’s lean operability limit did not have a monotonic relationship with internozzle spacing; the configuration with the intermediate nozzle spacing was the most stable. Simultaneous multi-kHz repetition-rate OH planar laser–induced fluorescence (OH-PLIF) and stereoscopic particle image velocimetry (S-PIV) measurements were used to examine the fluid and flame kinematics during the transient blow-off process. Although the blow-off process of any individual nozzle—wherein local extinction led to mixing of cold fluid into the central recirculation zone (CRZ), leading to further extinction—was similar regardless of the internozzle spacing, the presence of adjacent flames influenced both the global and local kinematics. Cross-nozzle flame transport through convection of burning fluid was found to help stabilize the flames in the multinozzle configurations, but could not explain the improved stability of the intermediate spacing case. Furthermore, the intermediate spacing configuration was able to sustain attached flames with greater fluid dynamic strain rate than the other configurations, indicating that it was more resistant to local extinction, ultimately leading to total blow-off. The configuration with the intermediate nozzle spacing had the largest CRZ. A Damköhler number based on the CRZ axial length and bulk velocity at blow-off was relatively constant across the configurations, indicating that a large CRZ provides robustness against the blow-off process. These results demonstrate the complex interplay between fluid dynamics, geometry, and stability of combustion systems.

Flame Edge Dynamics and Interaction in a Multinozzle Can Combustor With Fuel Staging

The characterization and mitigation of thermoacoustic combustion instabilities in gas turbine engines are necessary to reduce pollutant emissions, premature wear, and component failure associated with unstable flames. Fuel staging, a technique in which the fuel flow to a multinozzle combustor is unevenly distributed between the nozzles, has been shown to mitigate the intensity of self-excited combustion instabilities in multiple nozzle combustors. In our previous work, we hypothesized that staging suppresses instability through a phase-cancelation effect in which the heat release rate from the staged nozzle oscillates out of phase with that of the other nozzles, leading to destructive interference that suppresses the instability. This previous theory, however, was based on chemiluminescence imaging, which is a line-of-sight integrated technique. In this work, we use high-speed laser-induced fluorescence to further investigate instability suppression in two staging configurations: center-nozzle and outer-nozzle staging. An edge-tracking algorithm is used to compute local flame edge displacement as a function of time, allowing instability-driven edge oscillation phase coherence and other instantaneous flame dynamics to be spectrally and spatially resolved. Analysis of flame edge oscillations shows the presence of convecting coherent fluctuations of the flame edge caused by periodic vortex shedding. When the system is unstable, these two flame edges oscillate together as a result of high-intensity longitudinal-mode acoustic oscillations in the combustor that drive periodic vortex shedding at each of the nozzle exits. In the stable cases, however, the phase between the oscillations of the center and outer flame edges is greater than 90 deg (∼114 deg), suggesting that the phase-cancelation hypothesis may be valid. This analysis allows a better understanding of the instantaneous flame dynamics behind flame edge oscillation phase offset and fuel staging-based instability suppression.

Large-eddy simulation of low-swirl multi-nozzle combustion with co- and counter-swirling arrays

Large-eddy simulation (LES) of CH4-air low-swirl flame was carried out in a multi-nozzle combustor with two burner configurations by using a premixed flamelet model. The multi-nozzle burner includes a co-swirling array where all five nozzles act in the same direction and a counter-swirling array where the center nozzle is in the opposite swirling direction to the outer nozzles. LES results are in good agreement with OH-planar laser-induced florescence data in terms of OH concentrations and combustion progress variables. Numerical results show that the flow of each nozzle is constant before merging. The neighboring flows interact with each other and generate a highvelocity zone with intensive turbulence. The kinetic energy in the interacting region for the co-swirling array is larger than that for the counter-swirling array. After neighboring flow combining, the flow develops into a unified swirling motion similar to a single swirling flow for the co-swirling array, whereas the flow maintains the individual swirling structures for the counter-swirling arrangement. However, the swirling array exerts minimal effect on multi-nozzle combustion in terms of the temperature distributions and combustion progress of premixed low-swirl multi-nozzle flames.

Blowoff and Reattachment Dynamics of a Linear Multinozzle Combustor

This paper describes the coupled flow and flame dynamics during blowoff and reattachment events in a combustor consisting of a linear array of five interacting nozzles using 10 kHz repetition-rate OH planar laser-induced fluorescence and stereoscopic particle image velocimetry (S-PIV). Steady operating conditions were studied at which the three central flames randomly blew-off and subsequently reattached to the bluff-bodies. Transition of the flame from one nozzle was rapidly followed by transition of the other nozzles, indicating cross-nozzle coupling. Blow-off transitions were preferentially initiated in one of the off-center nozzles, with the transition of subsequent nozzles occurring in a random order. Similarly, the center nozzle tended to be the last nozzle to reattach. The blow-off process of any individual nozzle was similar to that for a single bluff-body stabilized flame, though with cross-flame interactions providing additional means of restabilizing a partially extinguished flame. Subsequent to blowoff of the first nozzle, the other nozzles underwent similar blow-off processes. Flame reattachment was initiated by entrainment of a burning pocket into a recirculation zone, followed by transport to the bluff-body; the other nozzles subsequently underwent similar reattachment processes. Several forms of cross-nozzle interaction that can promote or prevent transition are identified. Furthermore, the velocity measurements indicated that blowoff or reattachment of the first nozzle during a multinozzle transition causes significant changes to the flow fields of the other nozzles. It is proposed that a single-nozzle transition redistributes the flow to the other nozzles in a manner that promotes their transition.

The Effect of Fuel Staging on the Structure and Instability Characteristics of Swirl-Stabilized Flames in a Lean Premixed Multinozzle Can Combustor

Fuel staging is a commonly used strategy in the operation of gas turbine engines. In multinozzle combustor configurations, this is achieved by varying fuel flow rate to different nozzles. The effect of fuel staging on flame structure and self-excited instabilities is investigated in a research can combustor employing five swirl-stabilized, lean-premixed nozzles. At an operating condition where all nozzles are fueled equally and the combustor undergoes a self-excited instability, fuel staging successfully suppresses the instability: both when overall equivalence ratio is increased by staging as well as when overall equivalence ratio is kept constant while staging. Increased fuel staging changes the distribution of time-averaged heat release rate in the regions where adjacent flames interact and reduces the amplitudes of heat release rate fluctuations in those regions. Increased fuel staging also causes a breakup in the monotonic phase behavior that is characteristic of convective disturbances that travel along a flame. In particular, heat release rate fluctuations in the middle flame and flame–flame interaction region are out-of-phase with those in the outer flames, resulting in a cancelation of the global heat release rate oscillations. The Rayleigh integral distribution within the combustor shows that during a self-excited instability, the regions of highest heat release rate fluctuation are in phase-with the combustor pressure fluctuation. When staging fuel is introduced, these regions fluctuate out-of-phase with the pressure fluctuation, further illustrating that fuel staging suppresses instabilities through a phase cancelation mechanism.

Experimental Study on Instability Characteristics of Low-Swirl Flames in a Multinozzle Combustor With Different Swirling Arrays

This paper presents experimental study on self-excited combustion instability characteristics of premixed low-swirl flames in a multinozzle can combustor with counterswirl and coswirl arrays. Experiments were carried out over a wide range of inlet velocity from 4 m/s to 15.5 m/s and equivalence ratio from 0.5 to 0.85. Phase-locked OH planar laser-induced fluorescence was employed to measure flame shape and identify heat release rate. Four operation regions: stable combustion region, unstable combustion region, flashback region, and extinguish region are observed for both array burners. The amplitude of pressure fluctuation for counterswirl arrangement is less than the coswirl array, and the stable operating window of the counterswirl array is wider. In the unstable combustion region, the counterswirl flame triggers the 2L mode of the combustion system, while the coswirl flame incites three longitudinal modes with the highest amplitude near 3L. Rayleigh index distribution reveals neighboring flame interaction results in thermoacoustic coupling for multinozzle flames. Additionally, for the counterswirl array, thermoacoustic couplings also exit in the flame base region and shear region while, for the coswirl array, the instability driving zones also locate at the lip region and the tail of center flame which is totally different with counterswirl flame.

Measurements and Analysis of Alternating Flow Patterns in a Multinozzle Combustor

In some cases, a multinozzle combustor may exhibit flowfields in which individual nozzles hold two distinct flow or flame shapes in an alternating pattern. This study presents flowfield measurements for nonreacting and reacting flows in a rectangular combustor with two adjacent swirl-stabilizing nozzles at varying internozzle spacing. During tests of the wider nozzle spacings, with a reacting flow fueled by propane, there are differences between the flows of the two nozzles. Planar laser-induced fluorescence of the OH molecule (OH PLIF) shows that the flame remains anchored in the shear layer. Thus, the flame from one nozzle penetrates into the combustor, whereas the other flame is anchored close to, and almost parallel with, the dome wall. When the fuel is changed to methane, the asymmetry between the two nozzles is removed; therefore, the combustion properties, in addition to nozzle design, have an effect on the presence of this alternating flow pattern. A hypothesis based on turbulent opposed jets is presented to explain this change in the flowfields. When the nozzle design, flow, or combustion characteristics cause the shear layers of the adjacent nozzles to become sufficiently opposite in direction, the two flows can no longer mix. Instead, one shear layer goes underneath the other, which results in the differing flow features of the adjacent nozzles.

The Three-Dimensional Structure of Swirl-Stabilized Flames in a Lean Premixed Multinozzle Can Combustor

Flame structure can have a significant effect on a combustor's static stability (resistance to blowoff) and dynamic stability (combustion instability) and therefore is an important aspect of the combustion process that must be taken into account in the design of gas turbine combustors. While the relationship between flame structure and flame stability has been studied extensively in single-nozzle combustors, relatively few studies have been conducted in multinozzle combustor configurations typical of actual gas turbine combustion systems. In this paper, a chemiluminescence-based tomographic reconstruction technique is used to obtain three-dimensional images of the flame structure in a laboratory-scale five-nozzle can combustor. Analysis of the 3D images reveals features of the complex, three-dimensional structure of this multinozzle flame. Effects of interacting swirling flows, flame–flame interactions, and flame–wall interactions on the flame structure are also discussed.

Helical Flow Disturbances in a Multinozzle Combustor

This paper describes an experimental investigation of a transversely forced, swirl stabilized combustor. Its objective is to compare the unsteady flow structures in single and triple nozzle combustors and determine how well a single nozzle configuration emulates the characteristics of a multinozzle one. The experiment consists of a series of velocity field measurements captured on planes normal to the jet axis. As expected, there are differences between the single and triple nozzle flow fields, but the differences are not large in the regions upstream of the jet merging zone. Direct comparisons of the time-averaged flow fields reveal a higher degree of nonaxisymmetry for the flow fields of nozzles in a multinozzle configuration. Azimuthal decompositions of the velocity fields show that the transverse acoustic forcing has an important influence on the dynamics, but that the single and multinozzle configurations have similar forced response dynamics near the dump plane. Specifically, the axial dependence of the amplitude in the highest energy axisymmetric and helical flow structures is quite similar in the two configurations. Thus, upstream of the jet merging zone, the hydrodynamic influence of one swirling jet on the other is minimal. As such, that jet–jet interactions in this configuration do not have a significant influence on the unsteady flow structures.

Experimental investigations and large-eddy simulation of low-swirl combustion in a lean premixed multi-nozzle combustor

This paper presents laser diagnostic experiments and large-eddy simulations (LES) of low-swirl lean premixed methane/air flames in a multi-nozzle combustor including five nozzles with the same structure. OH planar laser-induced fluorescence is used to observe flame shapes and identify main reaction zones. NOx and CO emissions are also recorded during the experiment. The flows and flames are studied at different equivalence ratios ranging from 0.5 to 0.8, while the bulk inlet velocity is fixed at 6.2 m/s. Results show that the neighboring swirling flows interact with each other, generating a highly turbulent interacting zone where intensive reactions take place. The flame is stabilized above the nozzle rim, and its liftoff height decreases with increasing equivalence ratio. The center flow is confined and distorted by the neighboring flows, resulting in instabilities of the center flame. Mean OH radical images reveal that the center nozzle flame is extinguished when equivalence ratio is equals to 0.5, which is successfully predicted by LES. In addition, NOx emissions show log-linear dependency on the adiabatic flame temperature, while the CO emissions remain lower than 10 ppm. NOx emissions for multi-nozzle flame are less sensitive to the flame temperature than that for single nozzle. These results demonstrate that the low-swirl multi-nozzle concept is a promising solution to achieve stable combustion with ultra-low emissions in gas turbines.

Experimental Study of a Multinozzle Combustor at Elevated Pressures

Emissions measurements and chemiluminescence imaging were performed on a novel multipoint lean direct injection combustor designed for low-NOx emissions and with the flexibility to operate at both high- and low-power settings. The combustor has five rows of fuel nozzles that can be staged in three independent circuits as required. The fuel nozzle stages are not identical; instead, they are optimized for different power settings. NOx, CO, and unburned hydrocarbons were measured for a variety of settings, including lean blowout, idle, intermediate power, and simulated full power. Effects of pressure, inlet temperature, equivalence ratio, and fuel flow distribution were studied. chemiluminescence was used to observe flame structure and interpret emissions trends. The local equivalence ratio, calculated with the fuel and air flow rates particular to each combustor stage, was a useful parameter for interpreting emissions. The combustor demonstrated the ability to operate at a wide range of conditions, from an overall equivalence ratio of 0.55 to lean blowout at 0.079. Low-NOx operation was achieved at high power by using all fuel stages and controlling fuel flow to obtain an even distribution among the nozzles and reduce peak temperatures while using staging to provide good stability at low power.

Development and Testing of a Low NOx Hydrogen Combustion System for Heavy-Duty Gas Turbines

Interest in hydrogen as a primary fuel stream in heavy-duty gas turbine engines has increased as precombustion carbon capture and sequestration (CCS) has become a viable option for integrated gasification combined cycle (IGCC) power plants. The U.S. Department of Energy has funded the Advanced IGCC/Hydrogen Gas Turbine Program since 2005 with an aggressive plant-level NOx target of 2 ppm at 15% O2 for an advanced gas turbine cycle. Approaching this NOx level with highly reactive hydrogen fuel at the conditions required is a formidable challenge that requires novel combustion technology. This study begins by measuring entitlement NOx emissions from perfectly premixed combustion of the high-hydrogen fuels of interest. A new premixing fuel injector for high-hydrogen fuels was designed to balance reliable flashback-free operation, reasonable pressure drop, and low emissions. The concept relies on small-scale jet-in-crossflow mixing that is a departure from traditional swirl-based premixing concepts. Single nozzle rig experiments were conducted at pressures of 10 atm and 17 atm, with air preheat temperatures of about 650 K. With nitrogen-diluted hydrogen fuel, characteristic of carbon-free syngas, stable operation without flashback was conducted up to flame temperatures of approximately 1850 K. In addition to the effects of pressure, the impacts of nitrogen dilution levels and amounts of minor constituents in the fuel—carbon monoxide, carbon dioxide, and methane—on flame holding in the premixer are presented. The new fuel injector concept has been incorporated into a full-scale, multinozzle combustor can with an energy conversion rate of more than 10 MW at F-class conditions. The full-can testing was conducted at full gas turbine conditions and various fuel compositions of hydrogen, natural gas, and nitrogen. This combustion system has accumulated over 100 h of fired testing at full load with hydrogen comprising over 90% of the reactants by volume. NOx emissions (ppm) have been measured in the single digits with hydrogen-nitrogen fuel at target gas turbine pressure and temperatures. Results of the testing show that small-scale fuel-air mixing can deliver a reliable, low-NOx solution to hydrogen combustion in advanced gas turbines.