Low Swirl Burner Research Articles

Parameter variations and the effects of hydrogen: An experimental investigation in a lean premixed low swirl combustor

With a global focus on the reduction of fossil fuel consumption and harmful pollutant emissions, new technologies have been raised offering reduced emissions with the combustion of alternative and renewable fuels. Low swirl combustion and the addition of highly reactive fuels into the fuel stream are two methods that have been shown to meet these challenges. In the present study, the thermo-acoustic behavior of a lean premixed low swirl combustor is examined by the variation of several parameters: the equivalence ratio, bulk velocity, chamber pressure, and the addition of hydrogen into the fuel mixture. It is reported that the natural modes of the chamber employed shift upwards for both fuel mixtures examined when increasing the equivalence ratio. As additional heat is dumped into the chamber, the increase in acoustic energy is being pumped through these natural modes. An increase in the bulk velocity is found to have opposite effects on these dominant acoustic modes for the two mixtures investigated. The methane mixture shows negative shifts in frequency when increasing the bulk velocity, whereas the hydrogen-methane mixture displays upward-shifting frequencies. Elevating the chamber pressure results in an increase in the acoustic modes for both mixtures, although the trend is more consistently linear for the hydrogen-methane flames.

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.

Prediction of Flow Instabilities in an Atmospheric Low Swirl Burner Using URANS Models

Swirl-induced phenomena are used in gas turbine burners as a mechanism to stabilize the flame. The formation of coherent structures under turbulent swirling conditions plays a fundamental role in the stabilization and needs to be completely understood also in the absence of combustion. In this work, numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations for isothermal flow in an unconfined annular low swirl burner (50 kW) are reported. The standard k-ϵ and Reynolds stress models are used to run computational cases at a Reynolds number of 12,000 and two swirl numbers (S L = 0.57 and S H = 0.64). The numerical method is validated with the experiments reported by Legrand et al. [27]. Numerical results agree well with experiments for mean flow, temporal pressure measurements, and transient coherent structures. 2-D proper orthogonal decomposition (POD), 3-D iso-surfaces and advanced, vortex-related visualization methods are used to document the latter.

Simulation of nitrogen emissions in a premixed hydrogen flame stabilized on a low swirl burner

There is considerable interest in developing fuel-flexible, low emissions turbines for power generation. One approach is based on burning a variety of lean premixed fuels with relatively low flame temperatures. Such flames can be stabilized in a low swirl burner configuration, for example, using a variety of fuels such as pure hydrogen and hydrogen-seeded hydrocarbon mixtures. However, many hydrogen-rich fuels are thermodiffusively unstable and burn in cellular flame structures, which can have a significant impact on the local nitrogen chemistry. These cellular burning patterns are characterized by a local enhancement of fuel concentration and a corresponding increase in local flame temperature just downstream. In turn, these regions become sites for enhanced thermal NOx production. The structure of these cells, and their impact on the net emissions of a flame is influenced by the global flame stabilization mechanisms and by local turbulence properties. Here we investigate the role of thermodiffusive instabilities on NOx emissions in the context of a laboratory-scale low swirl burner fueled with a lean hydrogen–air mixture at atmospheric pressure. The simulations show how the cellular burning structures characteristic of lean premixed hydrogen combustion lead to local and global enhancements in the NOx emissions. We quantify the chemical pathways that lead to the formation of NO and N2O, and how they are enhanced within local regions of intense burning.

Prediction of blowoff in a fully controllable low-swirl burner burning alternative fuels: Effects of burner geometry, swirl, and fuel composition

Although the lean-premixed low-swirl burner (LSB) has been successfully demonstrated in a variety of applications, the mechanisms of flame stabilization are not fully understood and practical approaches for predicting stability limits are lacking. Using a unique fully controllable LSB coupled with a stereoscopic particle image velocimetry system, flow field characteristics and flame stability were investigated over a wide range of inlet conditions (reactant flow rates, 400<Qtot<1200 SLPM; heat release rates, 15<HRRHHV<75kW), scales (38.1, 50.8 and 76.2mm LSB nozzles), swirl configurations (swirl angles, α of 0°, 35°, 37.5°, 40.4°, 44.4° and 47°), and fuel compositions (reference fuel of methane and 10 different alternative fuel mixtures). Two distinct stability modes were documented within the regime classically identified as low-swirl. The dominant ‘W-type’ flames were observed to be stabilized in the shear layer between the central core flow and annular flow of the LSB. The less common ‘R-type’ flames were characterized by a noticeable axial recirculation, even at low swirl numbers. Damköhler number calculations revealed that these two flame modes were separated by a transition region of roughly 0.3<Da<0.75, with blowoff in a W- and R-type mode generally occurring above and below this range. For hydrogen containing fuel mixtures, a third, attached-flame stabilization-mode was also observed at low burner exit velocities and/or high equivalence ratios. For the W-type regime, a semi-empirical correlation was developed based on the strength of the annular shear layer (Vdiff), which linearly related data for all available fuel mixtures and LSB sizes to the average exit velocity, Uave, such that (Vdiff/Uave)(SL/SL,CH4s)0.17=1.05±15% for all available data at blowoff. This result provides new insight into LSB flame stabilization and is a useful tool for LSB design applicable to a wide range of fuels, equivalence ratios, heat release rates, and scales.

Experimental Study on the Non-reacting Flowfield of a Low Swirl Burner

Lean premixed low swirl combustion is an effective way to control NOx emission. In the present work a low swirl burner fitting with vane swirler has been used to investigate the flowfield characteristics. Two swirlers (65° and 40° setting angle) have been adopted for velocity measurement by using PIV technique. The results show that the critical swirl number when recirculation zone appears scarcely changed with the swirler setting angle and central jet velocity, and the demarcation between high and low swirl flow can be set as 0.48 in this work. Compared to swirler II, a wider low speed zone and a more divergent flow emerge downstream swirler I. Dimensionless axial velocity profiles along central axis decline linearly and approximately come to zero at x/D=1.37 for swirler II and x/D=1.56 for swirler I. For two-dimensional turbulent kinetic energy profiles, two peaks downstream of the swirl mean higher burning velocity than central jet. And turbulent kinetic energy downstream swirler II is observed higher.

Atmospheric low swirl burner flow characterization with stereo PIV

The lean premixed prevaporized (LPP) burner concept is now used in most of the new generation gas turbines to reduce flame temperature and pollutants by operating near the lean blow-off limit. The common strategy to assure stable combustion is to resort to swirl stabilized flames in the burner. Nevertheless, the vortex breakdown phenomenon in reactive swirling flows is a very complex 3D mechanism, and its dynamics are not yet completely understood. Among the available measurement techniques to analyze such flows, stereo PIV (S-PIV) is now a reliable tool to quantify the instantaneous three velocity components in a plane (2D–3C). It is used in this paper to explore the reactive flow of a small scale, open to atmosphere, LPP burner (50 kW). The burner is designed to produce two distinct topologies (1) that of a conventional high swirl burner and (2) that of a low swirl burner. In addition, the burner produces a lifted flame that allows a good optical access to the whole recirculation zone in both topologies. The flow is studied over a wide range of swirl and Reynolds numbers at different equivalence ratios. Flow statistics are presented for 1,000 S-PIV snapshots at each configuration. In both reactive and cold nonreactive flow, stability diagrams define the domains of the low and high swirl topologies. Due to the relatively simple conception of the physical burner, this information can be easily used for the validation of CFD computations of the burner flow global structure. Near field pressure measurements reveal the presence of peaks in the power spectra, which suggests the presence of periodical coherent features for almost all configurations. Algorithms have been developed to identify and track large periodic traveling coherent structures from the statistically independent S-PIV realizations. Flow temporal evolution is reconstructed with a POD-based method, providing an additional tool for the understanding of flow topologies and numerical codes validation.

Experimental investigation of thermoacoustic coupling using blended hydrogen–methane fuels in a low swirl burner

In this study, experimental testing and analysis were performed to examine the combustion instability characteristics of hydrogen–methane blended fuels for a low-swirl lean premixed burner. The aim of this study is to determine the effect of hydrogen addition on combustion instability, and this is assessed by examining the flame response to a range of constant amplitude, single frequency chamber acoustic modes. Three different blends of hydrogen and methane (93% CH 4–7% H 2, 80% CH 4–20% H 2 and 70% CH 4–30% H 2 by volume) were employed as fuel at an equivalence ratio of 0.5, and with four different acoustic excitation frequencies (85, 125, 222 and 399 Hz). Planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) was employed to measure the OH concentration at different phases of acoustic excitation and a Rayleigh Index was then calculated to determine the degree of thermoacoustic coupling. It was found, as has been previously reported, that the combustion characteristics are very sensitive to the fraction of hydrogen in the fuel mixture. The flame shows significant increases in flame base coupling and flame compaction with increasing hydrogen concentration for all conditions. While this effect enhances the flame response at non-resonant frequencies, it induces only minimal compaction and appears to decreases the coupling intensity at the resonant frequency.

Cellular burning in lean premixed turbulent hydrogen-air flames: Coupling experimental and computational analysis at the laboratory scale

One strategy for reducing US dependence on petroleum is to develop new combustion technologies for burning the fuel-lean mixtures of hydrogen or hydrogen-rich syngas fuels obtained from the gasification of coal and biomass. Fuel-flexible combustion systems based on lean premixed combustion have the potential for dramatically reducing pollutant emissions in transportation systems, heat and stationary power generation. However, lean premixed flames are highly susceptible to fluid-dynamical combustion instabilities making robust and reliable systems difficult to design. Low swirl burners are emerging as an important technology for meeting design requirements in terms of both reliability and emissions for next generation combustion devices. In this paper, we present simulations of a lean, premixed hydrogen flame stabilized on a laboratory-scale low swirl burner. The simulations use detailed chemistry and transport without incorporating explicit models for turbulence or turbulence/chemistry interaction. Here we discuss the overall structure of the flame and compare with experimental data. We also use the simulation data to elucidate the characteristics of the turbulent flame interaction and how this impacts the analysis of experimental measurements.

Dynamic behavior of a freely-propagating turbulent premixed flame under global stretch rate oscillations

Dynamic features of a freely propagating turbulent premixed flame under global stretch rate oscillations were investigated by utilizing a jet-type low-swirl burner equipped with a high-speed valve on the swirl jet line. The bulk flow velocity, equivalence ratio and the nominal mean swirl number were 5 m/s, 0.80 and 1.23, respectively. Seven velocity forcing amplitudes, from 0.09 to 0.55, were examined with a single forcing frequency of 50 Hz. Three kinds of optical measurements, OH-PLIF, OH ∗ chemiluminescence and PIV, were conducted. All the data were measured or post-processed in a phase-locked manner to obtain phase-resolved information. The global transverse stretch rate showed in-phase oscillations centering around 60 (1/s). The oscillation amplitude of the stretch rate grew with the increment of the forcing amplitude. The turbulent flame structure in the core flow region varied largely in axial direction in response to the flowfield oscillations. The flame brush thickness and the flame surface area oscillated with a phase shift to the stretch rate oscillations. These two properties showed a maximum and minimum values in the increasing and decreasing stretch periods, respectively, for all the forcing amplitudes. Despite large variations in flame brush thickness at different phase angles, the normalized profiles collapse onto a consistent curve. This suggests that the self-similarity sustains in this dynamic flame. The global OH ∗ fluctuation response (i.e. response of global heat-release rate fluctuation) showed a linear dependency to the forcing velocity oscillation amplitudes. The flame surface area fluctuation response showed a linear tendency as well with a slope similar to that of the global OH ∗ fluctuation. This indicated that the flame surface area variations play a critical role in the global flame response.

Interaction of turblence and chemistry in a low-swirl burner

New combustion systems based on ultra-lean premixed combustion have the potential for dramatically reducing pollutant emissions in transportation systems, heat, and stationary power generation. However, lean premixed flames are highly susceptible to fluid-dynamical combustion instabilities, making robust and reliable systems difficult to design. Low-swirl burners are emerging as an important technology for meeting design requirements in terms of both reliability and emissions for next-generation combustion devices. In this paper, we present simlations of a laboratory-scale low-swirl burner using detailed chemistry and transport without incorporating explicit models for turbulence or turbulence/chemistry interaction. We consider two fuels, methane and hydrogen, each at two turbulent intensities. Here we examine some of the basic properties of the flow field and the flame structure. We focus on the differences in flame behavior for the two fuels, particularly on the hydrogen flame, which burns with a cellular structures.

Stability characteristics and flame structure of low swirl burner

Low swirl burner provides stable lifted flames for fundamental studies of flame structure and turbulence/chemistry interaction in well defined boundary conditions. In the present study the stability characteristics of the burner have been investigated with four tangential jets at the same stoichiometry as the main jet. Two different burner nozzles with 40 mm and 53.5 mm diameters have been used for the stability measurements. In addition, a combined two-dimensional Rayleigh/LIPF-OH technique has been applied for simultaneous measurements of temperature and OH-radical for reaction zone and flame front investigation. Three flames have been selected near extinction for detailed measurements. The data show that the relation between of the main jet velocity, U, and the velocity of the four tangential jets, u, is linear. For the present data set with the nozzles investigated the linear trend can lead to an almost constant ratio of UD/ u as 5.08 mm with D as the nozzle diameter of the burner. The flame structure varies from corrugated to highly wrinkle according to the turbulence level.

Combustion dynamics of a low-swirl combustor

The methodology for the measurement of dynamic combustor behavior has never been clearly established, due to the complexities associated with unsteady premixed flames and the difficulties in their measurement. The global and local distribution of Rayleigh index and the flame response functions are the main parameters normally employed to quantify and describe combustion dynamics. The Rayleigh index quantifies the thermoacoustic coupling, while the flame response function is a measure of the response of the system to outside disturbances. The primary objective of this work is to investigate the combustion dynamics of a commonly used low-swirl burner and to develop tools and methods for examining the dynamics of a combustion system. To this end, the effect of acoustic forcing at various frequencies on flame heat release behavior has been investigated. The current work uses OH–PLIF imaging of the flame region to produce phase-resolved measurements of flame behavior at each frequency. The response of the flame to the imposed acoustic field over the range of 22–400 Hz is then calculated from the processed images. This provides a starting point for an extension/extrapolation to practical acoustic ranges (∼5000 Hz). It was found that the thermoacoustic coupling was mainly evident in the shear mixing zone, producing a toroidal Rayleigh index distribution pattern. The phase shift of the flame fluctuation from the imposed acoustic wave seems to be very closely coupled to the vortices generated at the flame boundary due to shear mixing (Kelvin–Helmholtz instability), thus inducing the alternating toroidal structures. The peak value of the flame response function coincides with the peak absolute value of the Rayleigh index.

Effect of swirl on lean flame limits of pilot-stabilized open premixed turbulent flames

Increased combustion activity and the undesirable effects on our ecosystem resulting from the primary and secondary combustion emissions are major concerns with regard to maintaining a clean and green environment. This has driven combustion engineers to employ combustion methods or postcombustion techniques for achieving acceptable emission levels in burner systems. Among the methods developed, lean premixed combustion has gained importance as an effective measure for reducing combustion emissions. However, lean premixed burner flames are severely affected by reduced flame stability. It is known that burner stability can be improved by using swirling flow [1,2]. In recent years, a novel concept of lowswirl burners [3] has gained momentum as an effective combustion emission control strategy and for operating stable lean premixed flames and increased combustion efficiency. Chan [3] et al. developed a low-swirl burner operating with lean premixed flames stabilized at a certain distance above the burner exit, defying the long-held notion that lifted flames are inherently unstable. Several studies [3–6] have reported stability limits of lean premixed flames in different burner configurations. A study on unsteady flame dynamics [4] relating to a lean-premixed swirl-stabilized combustor revealed that the operating parameters, viz. inlet temperature and equivalence ratio, are the two major factors that

Simultaneous PIV/OH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl flame

The diagnostic techniques for simultaneous velocity and relative OH distribution, simultaneous temperature and relative OH distribution, and three component velocity mapping are described. The data extracted from the measurements include statistical moments for inflow fluid dynamics, temperature, conditional velocities, and scalar flux. The work is a first step in the development of a detailed large eddy simulation (LES) validation database for a turbulent, premixed flame. The low-swirl burner used in this investigation has many of the necessary attributes for LES model validation, including a simplified interior geometry; it operates well into the thin reaction zone for turbulent premixed flames, and flame stabilization is based entirely on the flow field and not on hardware or pilot flames.

Large eddy simulation and experiments of stratified lean premixed methane/air turbulent flames

This paper presents a joint large eddy simulation and laser diagnostic investigation of premixed turbulent low swirl flames. A lean premixed methane/air mixture, of the equivalence ratio 0.60–0.66, is injected from a 50 mm diameter low swirl burner to a low speed co-flowing air at room temperature and pressure. The level-set G-equation is employed to simulate the inner layer flame front. Flamelet chemistry is used to determine the flame properties in the reactive zones. Mixing and heat transfer in the post-flame zone downstream are modeled using transport equations. In addition to large eddy simulation, simultaneous 2-D laser induced fluorescence of OH and 2-D particle image velocimetry are used to characterize the basic flame structure. Laser Doppler velocimetry is employed to further analyze the flow velocity along the central axis above the burner, and 2-D filtered Rayleigh scattering is used to measure the temperature field in the lower part of the flame. A bowl-shaped, highly wrinkled turbulent flame is stabilized at a position about one-half diameter above the burner. The flame consists of two distinct parts; around the burner axis, a premixed flame with uniform mixture fraction is stabilized in the low speed flow region induced by the inflow swirl; off the axis of the burner, a stratified lean premixed flame is found in the shear layer of the flow field. Flame holes (local extinction) owing to overly lean mixtures are observed in the off-axis lean stratified part of the flame. A unified level-set G-equation is developed to model the flame holes. The basic flow and flame structure from the model simulations are compared to the laser diagnostic measurements; the height of flame stabilization (lift-off height), the mean temperature profile, and the mean axial and radial velocity components together with rms velocity components are in fairly good agreement with measurement data.

CFD modeling of the in-cylinder flow in direct-injection Diesel engines

Three-dimensional flow calculations of the intake and compression stroke of a four-valve direct-injection Diesel engine have been carried out with different combustion chambers. A limited number of validation calculations of the compression stroke were first performed in order to explore the limits of CFD representation of the in-cylinder flow. The calculated flow field in three different combustion chambers was compared with laser Doppler velocimetry measurements; the comparison shows that the three-dimensional model is reasonably accurate for crank-angles around top dead center (TDC). In general, it performs better for low swirl combustion chambers while turbulence velocities are under-predicted when squish effects are important. In the main study, the flow characteristics inside the engine cylinder equipped with different piston configurations were compared. For this, complete calculations of the intake and compression strokes were performed under realistic operating conditions and the ensemble-averaged velocity and turbulence flow fields obtained in each combustion chamber analyzed in detail. The results confirmed that the piston geometry had little influence on the in-cylinder flow during the intake stroke and the first part of the compression stroke. However, the bowl shape plays a significant role near TDC and in the early stage of the expansion stroke by controlling both the ensemble-averaged mean and the turbulence velocity fields.

Laboratory investigation of an ultralow NO x premixed combustion concept for industrial boilers

A combustion concept to achieve ultralow emissions (NOx≤2 ppm and CO≤20 ppm) was tested on a 18 kW low-swirl burner (LSB). It is based on lean premixed combustion combusned with flue gas recirculation (FGR) and partially reformed natural gas (PRNG). Flame stability and emissions were assessed as a function of , FGR, and PRNG. The results show that PRNG improves flame stability and reduces CO, with no impact on NOx at =0.8. A one-dimensional flame simulation satisfactorily predicted prompt NOx under lean conditions with high FGR. Two catalysts were tested in a prototype steam reformer, and the results were used to estimate reactor volume and steam requirements in a practical system. An advanced Sud Chemie catalyst displayed good conversion efficiency at relatively low temperatures and high space velocities, which indicates that the reformer can be small and will track load changes. Tests conducted on the LSB with FGR and 0.05 PRNG show that boilers using a LSB with PRNG and high FGR andclose to stoichiometry can operate with low emissions and high efficiency.

Premixed turbulent flame structures in moderate and intense isotropic turbulence

Several 2-D imaging techniques including planar laser induced fluorescence for OH (OH-PLIF) have been used to investigate premixed turbulent flame structures under moderate to intense isotropic turbulence. Unconditioned velocity statistics were measured by laser Doppler anemometry. The experiments used a low-swirl burner that produces high intensity near-isotropic turbulence. The goal is to gain better insights into the flame structures at high turbulence and to test and verify the concept of the "distributed reaction zones" regime. Four methane/air flames (φ=0.7) have been studied with 0.5< u ′<2.2 m/s. A linear correlation for the flame speed, S f , is found: S f / S L =2.12 ( u ′/ S L )+1. Sets of 200 OH-PLIF images obtained for each flame clearly show that flame wrinkling is a random process. The probability of the flame having very small wrinkles is relatively low. This strongly suggests that the penetration of small intense eddies into the flame sheet to generate a "distributed reaction zone" is statistically an extremely rare event. The OH-PLIF images were processed to determine statistical properties of the mean flame curvatures and flame lengths for comparison with turbulence intensity and turbulent length scales. The results show that the increase in turbulent kinetic energy generates larger mean curvatures of the flame fronts, and a linear increase in the flame surface area ratio estimated from the mean flame length measurement.

The burning rate of premixed flames in moderate and intense turbulence

Burning rate measurements have been performed in lean (φ = 0.7) premixed methane/air flames stabilized on a low-swirl burner in moderate and intense turbulence, ( Re t = 460–1400). The purpose of this paper is to reconcile the burning rate determined by two widely used experimental methods: flow-velocity measurements and scalar measurements of the flame-surface density, Σ. The two sets of measurements, of conditional mass fluxes by Laser Doppler Velocimetry (LDV) and flame-surface density, were determined from the instantaneous flame-front positions obtained from OH Planar Laser Induced Fluorescence (OH-PLIF) images. It is important when making these comparisons to distinguish two measures of the turbulent burning velocity: the displacement, S D, and consumption speed, S C. Measurement of the consumption speed by the conditional flux method using laser Doppler velocimetry can account for the multidimensional affects of burner/flow geometry. In this burner geometry S C was found to be 30% of the cold-boundary flow velocity, S D. Both the turbulent displacement and consumption velocities were found to have linear dependencies on u′/ S L for values up to u′/ S L ∼7.3. The consumption speed is the more fundamental measure of the burning rate and is directly comparable to that derived from integration of flame-surface-density measurements. The similarity of the scalar and mass flux burning rates indicates that measurements of Σ, which are usually much simpler to perform than the flux measurements, can be used to test theoretical models and investigate the affect of upstream turbulence parameters on global flame-burning characteristics. Furthermore, comparison of the flux and scalar measurements has shown that even at flow/flame conditions above the Klimov-Williams criterion the affect of the turbulence on the local laminar burning rate, as expected from the Markstein number for this mixture, is small in the mean. Preliminary comparisons with models have been performed.