Low-swirl Injector Research Articles

Large Eddy Simulation and Acoustic Analysis of Combustion Instability in an Annular Combustor with Low-Swirl Flames.

Self-excited combustion instability in an annular combustor with low-swirl flames is studied with a combination of large eddy simulation (LES) and acoustic solvers. Acoustic analysis with a Helmholtz solver provides an estimate of frequencies and modal structures in the annular combustor. LES gives detailed modal dynamics for specific instability modes. Combustion instabilities in the annular combustor including longitudinal, spinning, and standing modes are successfully captured in a single LES. Numerical results show that the instability modes are not constant; they switch among these modes randomly and rapidly. The flow oscillates back and forth in phase with the largest pressure amplitude located near the outlet of the injectors for the longitudinal mode. The azimuthal instability oscillates in the 1A2L mode of the annular system. In the spinning mode, the pressure antinodes move forward while the modal structure keeps constant. For the standing mode, the locations of pressure antinodes are fixed in the annular combustor and the fluctuations at the pressure antinodes keep out of phase. The near-zero value of the mean spin ratio indicates that the dominant azimuthal mode is the standing mode. The azimuthal modes captured by LES are in good agreement with that predicted by Helmholtz solver in terms of frequency and modal structure. The maximum deviation of the predicted frequency is less than 5%. This adds values before the low-swirl injector is placed into the actual annular combustor.

Accurate weight coefficient estimation of multi-camera light field PIV through backpropagation neural network

Volumetric velocity measurement through multi-camera light field particle image velocimetry (LF-PIV) requires an accurate estimation of the weight coefficient (WC) of three-dimensional (3D) tracer particle distribution reconstruction. To achieve that, this study proposes a calibration method based on a backpropagation neural network (BP-NN) for the WC estimation of the multi-camera LF-PIV. The BP-NN model establishes a mapping relationship between the spatial voxels and pixels of the multi-cameras. The proposed method is compared with the direct ray tracing (DRT) method and it shows that the proposed method provides an accurate estimation of the WC. It also does not depend on the prior knowledge of angle separations of the multi-cameras as is required for the DRT method. The proposed method is initially evaluated by conducting synthetic tests of ring vortex field reconstruction and further verified by conducting experiments on a low-swirl injector (LSI) flow. Results show that the root mean square error of the ring vortex displacement field can be reduced from 0.71 voxels to 0.35 voxels by the proposed method. The relative errors of LSI flow axial and radial velocity components are smaller than 10%. Therefore, it demonstrates that the 3D flow velocity can be measured accurately by the multi-camera LF-PIV by incorporating the proposed BP-NN calibration method.

Heat transfer characteristics of a water heater operating on natural gas and cofiring with pulverized rumen contents

This study discusses the heat exchange optimization of a 450 kW water heater operating with natural gas (NG) and in cofiring mode burning solid pulverized rumen and NG. The rumen material is a byproduct from cattle slaughter processes, consisting of the waste contained in the cow stomachs and intestines. In NG mode, the gaseous fuel provides 100% of the heat input, and in cofiring mode 50% of the heat rate input is obtained from the rumen contents and NG balances the heat rate. The paper presents experimental analysis on a scaled prototype of 45 kW, and the results are complemented with CFD analysis of the aerothermochemistry of the combustion chamber and heat exchanger. The heterogeneous reactions are stabilized using a low swirl injector. The optimization seeks to use swirling flows inside the combustion chamber to improve convective heat transfer from the combustion products to the water-cooled surfaces, and the use of decaying swirl generators to enhance heat transfer at the second and third gas passes. The geometry variations attain effectiveness above 86%, with low emissions CO, THC and NOx, and to attain reaction stability of the rumen biowastes.

Combustion control using a lobed swirl injector and a plasma swirler

A low swirl injector incorporating a lobed swirler and a plasma swirler was developed for combustion control. The lobed swirler was used to replace the vane swirler to enhance fuel/air mixing and stabilize flame downstream of the injector. The plasma swirler installed at the exit of the injector was used to control the flame lift-off height, which is a critical parameter influencing the flame stability and thermal load. The plasma swirler is composed of three dielectric barrier discharge (DBD) actuators which were placed in a circular array around the axis to generate ionic wind in the circumferential direction. Experiments were carried out to investigate the aerodynamics and combustion characteristics of the low swirl injector. Non-reacting velocity distributions at the exit of the injector with different blockage ratios were captured with two-dimensional Particle Image Velocimetry. Reacting flow velocity profiles downstream of the lobed swirl injector were measured by two-dimensional Laser Doppler Anemometry. Based on the non-reacting flow field analysis, guidelines of the optimal design were deduced and a proper perforated plate was chosen to carry out further combustion experiments. Stable lifted flame was sustained by the lobed swirler with a vane angle of 30° and a blockage ratio of 75.5% which demonstrated that the flow field with a suspended central recirculation zone was suitable for stabilizing the lifted flame. It was found that the DBD actuation enhances the effectiveness of combustion control that can be used to adjust the position of the flame front.

Flame stability limits of premixed low-swirl combustion

The objective of this study is to gain a fundamental understanding of the flow-field and flame behaviors associated with a low-swirl burner. A vane-type low-swirl burner with different swirl numbers has been developed. The velocity field measurements are carried out with particle image velocimetry. The basic flame structures are characterized using OH radicals measured by planar laser-induced fluorescence. Three combustion regimes of low-swirl flames are identified depending on the operating conditions. For the same low-swirl injector under atmospheric conditions, attached flame is first observed when the incoming velocity is too low to generate vortex breakdown. Then, W-shaped flame is formed above the burner at moderate incoming velocity. Bowl-shaped flame structure is formed as the mixture velocity increases until it extinct. Local extinction and relight zones are observed in the low-swirl flame. Flow-field features and flame stability limits are obtained for the present burner.

Exploring Computational Methods for Predicting Pollutant Emissions and Stability Performance of Premixed Reactions Stabilized by a Low Swirl Injector

ABSTRACTThis article addresses the numerical modeling of NOx emissions and lean blowoff (LBO) limits of confined and pressurized turbulent premixed flames stabilized with a low swirl burner. The study also evaluates existing numerical methods that can be used to predict exhaust pollutant emissions and reaction instability close to the LBO limit. One of the strategies presented in the article consists of establishing a chemical reactor network (CRN), which is a simplified model of the fluid dynamics and energy balance of the system coupled with a detailed reaction mechanism. Since the computing turnaround time for a CRN model is several orders of magnitude less than the simplest computational fluid dynamics (CFD) case, the parameters controlling the pollutant formation and stability of the system can be quickly assessed over the complete flammable range. The results show the value of a simple reactor network as a design tool that can be used to optimize emissions and LBO limits of combustion systems. The parametric analysis examines the most important variables that control the formation of pollutant species (pressure, recirculation of gases, heat losses, geometry variables, air to fuel ratio, and fuel composition). Experiments were carried out at a pressure of 304 kPa using two fuel compositions: natural gas and natural gas enriched with up to 90% hydrogen (by volume). The results obtained with the CRN for NOx and LBO are in good agreement with those observed experimentally and show that, for ultra-low NOx burners, the lowest NOx emissions are concomitant to the onset of instabilities associated with LBO. In sharp contrast, the CFD-based simulations of NOx fail to accurately predict the effect of the fuel composition on the LBO limit. Further analysis of the CRN results show that, at lean conditions and pressures above three atmospheres, NOx is formed primarily through the N2O pathway, regardless of the fuel composition. The CRN model indicates that adding hydrogen to natural gas promotes the production of NOx through the Zeldovich, NNH, and N2O routes while reducing formation via the prompt route. Understanding the relative contributions of each route provides a starting point from which to propose modifications to the system to reduce NOx emissions.

Combustion and exhaust emission characteristics of low swirl injector

The present experimental study aims to investigate the combustion and emission characteristics of the flow through a low swirl injector (LSI). An experimental study was carried out on the flame structure, the temperature distribution and the exhaust emission of low swirl pre-mixed combustion under the condition of different swirl number and different fuel composition. In order to qualitatively analyze the flame structure, the velocity distribution of the non-reacting flow through the LSI was measured using the particle image velocimetry (PIV) technique. Experimental results indicated that: (i) the LSI can generate a blue lift-off “W” type flame which consists of four clusters of flames connected together and holds up a long yellow pulsating flame, (ii) the blue flame structure converts the “W” type flame into the “broom” type flame and the distance between the front of the flame and the nozzle shortens with increasing swirl number, (iii) there exist high temperature region flanked by two peaks on the temperature profiles in the blue flame while uniform higher temperature in yellow pulsating flame, (iv) the NOx and CO emission level of the LSI mainly depends on the gas composition and thermal load.

Combined effect of spatial and temporal variations of equivalence ratio on combustion instability in a low-swirl combustor

In this experimental study, the combined effect of spatial and temporal variations of fuel–air mixture on self-excited combustion instabilities in a gas-turbine model combustor (∼60kW) with a low-swirl injector is reported. Detailed measurements were performed in 4 fuel split (to upstream/downstream injections) conditions while keeping the total equivalence ratio constant. The combustion stability was found to be very sensitive to the fuel split parameter which determined the local equivalence ratio distribution. The majority of the heat-release oscillations was generated in the flame-to-wall impingement region in a manner that satisfied the Rayleigh criterion. The driving force of the instability was considered the periodic interaction between the traveling vortex filled with fresh mixtures and the flame in the near-wall region as reported in the previous study on homogeneous mixture flame. However, the strength of the instability was sensitively modified by the change in the local equivalence ratio distribution. In the strongest oscillation case with inhomogeneous mixture, temporal variations of equivalence ratio exhibited a positive contribution to the thermoacoustic coupling. This suggested that temporal variations in equivalence ratio were enhancing the driving factor of the thermoacoustic instability in addition to the vortex-flame interaction mechanism.

Flashback and Turbulent Flame Speed Measurements in Hydrogen/Methane Flames Stabilized by a Low-Swirl Injector at Elevated Pressures and Temperatures

This paper reports flashback limits and turbulent flame local displacement speed measurements in flames stabilized by a low swirl injector operated at elevated pressures and inlet temperatures with hydrogen and methane blended fuels. The goal of this study is to understand the physics that relate turbulent flame speed to flashback events at conditions relevant to gas turbine engines. Testing was conducted in an optically accessible single nozzle combustor rig at pressures ranging from 1 to 8 atm, inlet temperatures from 290 to 600 K, and inlet bulk velocities between 20 and 60 m/s for natural gas and a 90%/10% (by volume) hydrogen/methane blend. The propensity of flashback is dependent upon the proximity of the lifted flame to the nozzle that is itself dependent upon pressure, inlet temperature, and bulk velocity. Flashback occurs when the leading edge of the flame in the core of the flow ingresses within the nozzle, even in cases when the flame is attached to the burner rim. In general the adiabatic flame temperature at flashback is proportional to the bulk velocity and inlet temperature and inversely proportional to the pressure. The unburned reactant velocity field approaching the flame was measured using a laser Doppler velocimeter with water seeding. Turbulent displacement flame speeds were found to be linearly proportional to the root mean square of the velocity fluctuations about the mean velocity. For identical inlet conditions, high-hydrogen flames had a turbulent flame local displacement speed roughly twice that of natural gas flames. Pressure, inlet temperature, and flame temperature had surprisingly little effect on the local displacement turbulent flame speed. However, the flow field is affected by changes in inlet conditions and is the link between turbulent flame speed, flame position, and flashback propensity.

Technical applicability of low-swirl fuel nozzle for liquid-fueled industrial gas turbine combustor

In this study, potentials of the liquid-fueled low-swirl burner technique for industrial gas-turbine combustor application are reported for the first time. A low-swirl fuel nozzle, which is a new implementation of the basic low-swirl burner design, is configured by the velocity measurement of methane–air open flames under atmospheric pressure and a low velocity (∼3m/s) condition. Flow properties, such as the axial stretch rate and virtual origins, are compared with the previously reported values with the axial vane type low-swirl injector, and it is confirmed that the flow field generated from the current implementation is of the typical low-swirl flow. Then, it is shown that the configuration successfully stabilize the lifted flame under much higher velocity (∼50m/s) condition with kerosene fuel injected by a typical pressure atomizer. Finally, the fuel nozzle is installed in a 290kW simple-cycle liquid-fueled gas turbine engine and is found to be operable over the entire operating range. The combustor inlet wall temperatures are shown to be within an acceptable range, even without the cooling air that was required for conventional combustors. This is an advantage of the lifted flame stabilized by the low-swirl technique. Although our focus is not on low emissions characteristics, NOx emissions is also found to be below maximum levels of current Japanese regulations (<84ppm@15% O2). In sum, the proposed fuel nozzle design shows promise for the application of liquid-fueled industrial gas turbine engines.

Self-induced unstable behaviors of CH4 and H2/CH4 flames in a model combustor with a low-swirl injector

Laboratory experiments and modeling analysis have been conducted to gain insight on the self-excited unstable flame behaviors of a model gas turbine combustor that utilizes a low-swirl injector (LSI). The combustor consists of a 5.7cm I.D. LSI firing into 18cm diameter by 32cm or 20.3cm long cylindrical enclosures at atmospheric condition. The experiments involved lean premixed turbulent CH4 (0.6<ϕ<0.7) and 0.9 H2/0.1 CH4 (0.3<ϕ<0.4) flames at bulk flow velocities of 12 and 18m/s. Acoustic spectral information was obtained from pressure transducers. A laser-based flame motion detection method was used to determine the locations and frequencies of flame oscillations. Phase-resolved 2D velocity statistics of the reacting flow fields was measured by particle image velocimetry (PIV). Self-excited unstable flame behaviors were found for the richer flames burning in the 32cm long chamber at 18m/s. The incited acoustic frequencies correspond to the first longitudinal mode of the combustor computed by the General Instability Model (GIM) program. Flame oscillations are found primarily in the region along the outer shear layer (OSL) of the LSI rim and the oscillation frequencies are consistent with the acoustic frequencies. Phase-resolved PIV show the OSL convects ring vortices shed from the rim. The trajectories of the vortex centers enable a linear instability analysis by GIM. For the CH4 flame, analysis shows that the roll up vortices from the dump plane are responsible for generating self-excited acoustically coupled flame instability. For the 0.9 H2/0.1 CH4 flame, the GIM analysis suggests a dominance of shedding vortices but is less conclusive due to flame attachment to the LSI rim resulting from the highly reactive H2 fuel and possible unsteady heat release contributions from bulk flow oscillations.

Measurement of Flame Frequency Response Functions Under Exhaust Gas Recirculation Conditions

Exhaust gas recirculation has been proposed as a potential strategy for reducing the cost and efficiency penalty associated with postcombustion carbon capture. However, this approach may cause as-yet unresolved effects on the combustion process, including additional potential for the occurrence of thermoacoustic instabilities. Flame dynamics, characterized by the flame transfer function, were measured in traditional swirl stabilized and low-swirl injector combustor configurations, subject to exhaust gas circulation simulated by N2 and CO2 dilution. The flame transfer functions exhibited behavior consistent with a low-pass filter and showed phase dominated by delay. Flame transfer function frequencies were nondimensionalized using Strouhal number to highlight the convective nature of this delay. Dilution was observed to influence the dynamics primarily through its role in changing the size of the flame, indicating that it plays a similar role in determining the dynamics as changes in the equivalence ratio. Notchlike features in the flame transfer function were shown to be related to interference behaviors associated with the convective nature of the flame response. Some similarities between the two stabilization configurations proved limiting and generalization of the physical behaviors will require additional investigation.

Effects of hydrogen on the thermo-acoustics coupling mechanisms of low-swirl injector flames in a model gas turbine combustor

The effects of hydrogen on the dynamic response of self-excited oscillations in a model low-swirl injector (LSI) gas turbine combustor have been studied by high-speed imaging of flame light emissions and OH∗ chemiluminescence on two thermoacoustically coupled unsteady flames burning CH4 (113kW) and a fuel blend of 0.9H2/0.1CH4 (74kW). The two flames have different shapes. The CH4 flame is lifted and bowl-shaped and the 0.9H2/0.1CH4 flame is attached and M-shaped. From phase-resolved PIV, the dominant unsteady flow structures of both flames are ring vortices shed from the LSI exit rim. The vortices of the CH4 flame burn intermittently and those in the 0.9H2/0.1CH4 flame burn consistently. Proper orthogonal decomposition analysis of OH∗ and calculation of Rayleigh indices show different flame structures are responsible for generating flame oscillations and acoustic coupling. CH4 flame oscillations are associated with flame folding near the standing ring vortex below the trailing edge of the lifted flame, intermittent burning of the shear layer, and irregular acoustic driving. The 0.9H2/0.1CH4 flame has regular and stronger forcing. A key contributor to the 0.9H2/0.1CH4 flame driving is merging of the central flat flame with the reacting rolled-up vortices, resulting in an instantaneous narrow band of localized high heat release density that is in phase with the pressure oscillation. Cyclic variations in the flow of the reactants also contribute to non-linear effects. High reactivity of H2 generates a situation where the geometry of the attached shear layer flame and central disk-shaped flame creates a very favorable situation for exciting higher-pressure oscillations at a lower total heat release than CH4 flames. To control combustion oscillations in a fuel-flexible LSI gas turbine combustor, the mitigation of flame attachment and/or formation of the outer shear layer would be necessary.

Development and Validation of a Thickened Flame Modeling Approach for Large Eddy Simulation of Premixed Combustion

The development of a dynamic thickened flame (TF) turbulence-chemistry interaction model is presented based on a novel approach to determine the subfilter flame wrinkling efficiency. The basic premise of the TF model is to artificially decrease the reaction rates and increase the species and thermal diffusivities by the same amount, which thickens the flame to a scale that can be resolved on the large eddy simulation (LES) grid while still recovering the laminar flame speed. The TF modeling approach adopted here uses local reaction rates and gradients of product species to thicken the flame to a scale large enough to be resolved by the LES grid. The thickening factor, which is a function of the local grid size and laminar flame thickness, is only applied in the flame region and is commonly referred to as dynamic thickening. Spatial filtering of the velocity field is used to determine the efficiency function by accounting for turbulent kinetic energy between the grid-scale and the thickened flame scale. The TF model was implemented into the commercial computational fluid dynamics code FLUENT. Validation in the approach is conducted by comparing model results to experimental data collected in a laboratory-scale burner. The burner is based on an enclosed scaled-down version of the low swirl injector developed at Lawrence Berkeley National Laboratory. A perfectly premixed lean methane-air flame was studied, as well as the cold-flow characteristics of the combustor. Planar laser induced fluorescence of the hydroxyl molecule was collected for the combusting condition, as well as the velocity field data using particle image velocimetry. Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.

Laboratory Investigations of Low-Swirl Injectors Operating With Syngases

The low-swirl injector (LSI) is a lean premixed combustion technology that has the potential for adaptation to fuel-flexible gas turbines operating on a variety of fuels. The objective of this study is to gain a fundamental understanding of the effect of syngas on the LSI flame behavior, the emissions, and the flowfield characteristics for adaptation to the combustion turbines in integrated gasification combined cycle clean coal power plants. The experiments were conducted in two facilities. Open atmospheric laboratory flames generated by a full size (6.35 cm) LSI were used to investigate the lean blow-off limits, emissions, and the flowfield characteristics. Verification of syngas operation at elevated temperatures and pressures were performed with a reduced scale (2.54 cm) LSI in a small pressurized combustion channel. The results show that the basic LSI design is amenable to burning syngases with up to 60% H2. Syngases with high H2 concentration have lower lean blow-off limits. From particle image velocimetry measurements, the flowfield similarity behavior and the turbulent flame speeds of syngases flames are consistent with those observed in hydrocarbon and pure or diluted hydrogen flames. The NOx emissions from syngas flames show log-linear dependency on the adiabatic flame temperature and are comparable to those reported for the gaseous fuels reported previously. Successful firing of the reduced-scale LSI at 450 K&lt;T&lt;505 K and 8 atm verified the operability of this concept at gas turbine conditions.

Laboratory investigations of a low-swirl injector with H 2 and CH 4 at gas turbine conditions

Laboratory experiments were conducted at gas turbine and atmospheric conditions (0.101 < P 0 < 0.810 MPa, 298 < T 0 < 580 K, and 18 < U 0 < 60 m/s) to characterize the overall behaviors and emissions of the turbulent premixed flames produced by a low-swirl injector (LSI) for gas turbines. The objective was to investigate the effects of hydrogen on the combustion processes for the adaptation to gas turbines in an IGCC power plant. The experiments at high pressures and temperatures showed that the LSI can operate with 100% H 2 at up to ϕ = 0.5 and has a slightly higher flashback tolerance than an idealized high-swirl design. With increasing H 2 fuel concentration, the lifted LSI flame begins to shift closer to the nozzle exit and eventually attaches to the nozzle rim and assumes a different shape at 100% H 2. The STP experiments show the same phenomena. The analysis of velocity data from PIV shows that the stabilization mechanism of the LSI remains unchanged up to 60% H 2. The change in the flame position with increasing H 2 concentration is attributed to the increase in the turbulent flame speed. The NO x emissions show a log linear dependency on the adiabatic flame temperature and the concentrations are similar to those obtained previously in a LSI prototype developed for natural gas. These results show that the LSI exhibits the same overall behaviors at STP and at gas turbine conditions. Such insight will be useful for scaling the LSI to operate at IGCC conditions.

Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Adaptation to Fuel-Flexible Turbines

The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultralow emissions (&lt;5 ppm NOx and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling, or costly materials and catalysts. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural-gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO2. Using particle image velocimetry, the nonreacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data, an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed has been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enables the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.

Fuel effects on a low-swirl injector for lean premixed gas turbines

Laboratory experiments have been conducted to investigate the fuel effects on the turbulent premixed flames produced by a gas turbine low-swirl injector (LSI). The lean-blow off limits and flame emissions for seven diluted and undiluted hydrocarbon and hydrogen fuels show that the LSI is capable of supporting stable flames that emit <5 ppm NO x (@ 15% O 2). Analysis of the velocity statistics shows that the non-reacting and reacting flowfields of the LSI exhibit similarity features. The turbulent flame speeds, S T, for the hydrocarbon fuels are consistent with those of methane/air flames and correlate linearly with turbulence intensity. The similarity feature and linear S T correlation provide further support of an analytical model that explains why the LSI flame position does not change with flow velocity. The results also show that the LSI does not need to undergo significant alteration to operate with the hydrocarbon fuels but needs further studies for adaptation to burn diluted H 2 fuels.