Swirl Burner Research Articles

Effects of low temperature on near-nozzle breakup and droplet size distribution in airblast kerosene spray

Atomization of low-temperature fuel is encountered in extreme operating conditions of liquid propulsion systems such as cold start and high-altitude relight for aeroengines. Fuel temperature has a great impact on airblast spray characteristics by influencing fuel viscosity and thus the gas–liquid interaction, which raises the demand to clarify the temperature-dependent transition in near-nozzle breakup behavior and the corresponding droplet size distribution. A liquid-centered swirl coaxial injector is tested on the low-temperature swirl spray and combustion test rig at Zhejiang University, using 25 kHz high-speed digital off-axis holography. RP-3 aviation kerosene is atomized under ignition conditions at temperatures of 233, 253, and 301 K, fuel pressures of 0.03 and 0.69 MPa, and air pressure ranging from 0 to 4.0 kPa. Time-resolved near-nozzle dynamics suggest four types of elementary breakup processes: wavy-sheet breakup, pulsating breakup, membrane-type breakup, and nonaxisymmetric Rayleigh breakup. Each process alternately dominates the near field as fuel Reynolds number (Ref) and aerodynamic Weber number (Weg) decrease, corresponding to four primary breakup modes. A mode classification plot is summarized. Spray structures show an extended breakup length and reduced spray cone angle as fuel temperature (Tf) decreases. Increasing air pressure (Pg) promotes spray expansion at 0.03 MPa, but contracts spray cone at 0.69 MPa. Cross-sectional Sauter mean diameter (SMD) distribution indicates a solid-cone spray at 0.03 MPa and a hollow cone spray at 0.69 MPa. Lowering Tf will rise the SMD in the spray center at 0.03 MPa and transform the toroidal SMD distribution at 0.69 MPa into a solid one. Finally, a temperature-related SMD model is derived considering the exponential viscosity–temperature relationship, and a good fit with R2 > 0.95 is achieved. This research aims to deepen the understanding of the effects of low temperature on the transition of near-nozzle atomization characteristics for airblast sprays. Both spray visualization and SMD results provide reference for numerical simulations and near-nozzle spray modeling.

Ultralean Combustion Mechanism of Methane–Air Swirling Premixed Flame

ABSTRACT This study investigated a methane – air premixed flame in a partially tapered swirl burner. Experimentally, we succeeded in forming almost steady ultralean flames with an equivalence ratio as lean as 0.465. On the other hand, a numerical simulation of the flame with detailed chemistry revealed that the flame head existed in a large recirculation zone under ultralean conditions. In addition, high-value peaks of temperature, heat-release rate (HRR), and concentrations of OH, O, and H radicals were formed at the flame head, enabling ultralean combustion. Subsequently, to identify the formation mechanism of these peaks, adiabatic swirl burner flames were simulated with and without the effect of thermal-diffusive imbalance (a Lewis number effect). The results show that a Lewis number of less than unity caused a temperature increase in the flame located in the recirculation zone owing to the thermal-diffusive imbalance, which increased the peak heights of the HRR and the three radicals. The results also showed that even in a flame unaffected by thermal-diffusive imbalance, the backward convective transport could accumulate CO and OH at the flame head thus enhancing the chemical reactions at the head. (185 words)

CFD Analysis on the Effect of Ammonia Addition on Oxyfuel Coal Combustion in a Swirl Burner

CFD Analysis on the Effect of Ammonia Addition on Oxyfuel Coal Combustion in a Swirl Burner

Investigation on the effects of swirl-strength on flashback phenomenon of premixed CH[formula omitted]/H[formula omitted]/air flame in a bluff-body swirl burner

Blending a low ratio of hydrogen into traditional hydrocarbon fuels can reduce the modification in structure of existing combustors. However, even a low hydrogen ratio can increase the propensity of flashback due to the extremely high reactivity of hydrogen. Swirl number is a key parameter of swirl combustors which determines the flow structure. To design reliable gas turbine combustors for hydrogen-blended fuels, the study in the effect of swirl strength on flashback characteristic and the underlying mechanism is necessary. In this study, the flashback characteristics of premixed 30% H2/70% CH4/air flame in a bluff-body swirl burner is investigated via both experiment and large-eddy simulations with the flamelet-generated-manifold method (FGM-LES). The flashback mode and flame-flow interaction are analyzed based on the LES results. It is found that the increase in swirl strength will increase the propensity of flashback and promote the flashback speed of premixed 30% H2/70% CH4/air flame in the bluff-body swirl burner. The turns from Mode II (small-scale flame bulges leading flashback) to Mode I (large-scale swirling flame tongue leading flashback) during flashback of hydrogen-blended flames due to the production of stronger reversed flow by swirling flow. It is also found that stronger swirl strength will result in a smoother flame front and narrower strain rate distribution range which suppresses the local hydrogen enrichment due to preferential diffusion of hydrogen, thus suppressing the flame propagating speed. However, this effect is not dominant in flashback process compared to the promotion of reversed flow by increased swirl strength.

Effect of Different Nozzle Arrangements on Combustion Flow Characteristics in a Swirl Coupling Multiple Direct Injection Combustor

ABSTRACT The influence of multiple direct injection nozzle arrangements with hexagonal, rhombus, and circular shapes on the combustion flow in gas turbines was numerically studied, and the combustion flow characteristics of swirl combustion combined with multiple direct injection combustion were analyzed, such as velocity and temperature distribution, vortex structure, NOX (nitrogen oxides) emission, flame stretch rate, etc. The result shows that for these different nozzle arrangements, the difference in velocity and temperature distribution is not obvious, but the difference in vortex structure is large. The hexagonal arrangement can form several counter-rotating vortex pairs (CVP) near the side wall, the rhombus arrangement can form several deflection vortex structures outside the center swirl, and the circular arrangement forms the least vortex structure. Furthermore, the circular arrangement is most effective in reducing the non-uniformity of the outlet temperature, OTDF is 0.00548, and it has a higher flame stretch rate, which means that it has a higher combustion intensity. However, the hexagon arrangement has the higher NOX emission, reaching 3.16 ppm.

STABILITY AND COMBUSTION/EMISSION CHARACTERISTICS OF STRATIFIED DUAL-SWIRL OXY-METHANE FLAMES: EFFECTS OF OXYGEN FRACTION

Abstract The stability, combustion, and emission features of stratified oxy-methane (CH4/O2/CO2) flames stabilized over a dual annular counter-rotating swirl (DACRS) burner, developed for gas turbine combustion applications, were investigated experimentally. The experiments were performed at fixed velocity ratio (Vr=Vp/VS=3.0) in both the primary and secondary streams at a constant primary stream velocity, Vp of 5 m/s and at fixed primary stream equivalence ratio, φP=0.9, and over ranges of oxygen fractions (OFP for the primary stream, OFS for the secondary stream) and secondary stream equivalence ratios. Measurements of flame macrostructure, temperature profiles, and exhaust emissions were recorded to characterize the flames and validate future numerical models. The testing findings revealed no flame flashback within the operational ranges of OFP and OFS and up to φs = 1.0. However, the near stoichiometric operation of the primary stream (φp = 0.9) at OFp =0.38, permitted the main secondary flame to tolerate exceptionally lean conditions (φs = 0.397 at OFs =0.34 and φs = 0.223 at OFs =0.39), raising the thresholds for flame blowout. Increasing OFP from 0.21 to 0.38 significantly reduced φS at blowout from 0.537 to 0.223, corresponding to a decrease in the combustor's global equivalence ratio (φg) at blowout from 0.554 to 0.254 at global oxygen fraction (OFg) from 0.38 to 0.39. Lower OFp values caused earlier flame lift-off, indicating the greater influence of OFp on flame macrostructures.

Thermochemical analysis of premixed ammonia/biogas flames in a model gas turbine swirl combustion system

This study examined the premixed NH3/biogas combustion at near stoichiometric using an experimentally validated numerical method. Raising the NH3 wt.% in NH3/CH4 combustion at φ = 0.8 brought up the average reaction temperature (Tavg) due to heat retention. Intensified by CO2 addition, Tavg in NH3/biogas increased by a factor of 1.2 compared to NH3/CH4. At φ = 1.1, higher NH3 and CO2 wt.% reduced Tavg. The local Damköhler number (Da) was reduced marginally in the absence of CO2 as φ increased from 0.8 to 1.1. Conversely, local Da grew considerably in the presence of CO2 and was particularly sensitive to variations in the excess air ratio. Increased NH3 wt.% promoted NO emission, primarily via N + OH → NO + H and H + HCNO → CH2 + NO pathways. NH3/biogas produced more NO than NH3/CH4 from φ = 0.9 to 1.1, but as φ approached 1.1, NO is generally lowered. N2O is produced mainly by NH + NO → N2O + H. Fuel-lean operation generally results in a higher N2O than fuel-rich operation. The NH3/biogas combustion at φ = 0.8 is a potential clean fuel solution in lowering NO emissions, as compared to NH3/CH4 combustion.

Schlieren Image Velocimetry and Modal Decomposition Study of Preheated Isothermal Flow From a Generic Multi-Swirl Burner

Abstract This study presents an experimental investigation into the turbulent flow characteristics of an unconfined counter-rotating dual swirl burner under external acoustic excitation. Utilizing Schlieren image velocimetry (SIV), we capture the velocity field of the swirling jets. Mean velocity field analysis reveals the upstream propagation of the central recirculation zone within the burner passages. Through proper orthogonal decomposition (POD) analysis on instantaneous axial velocity fields, coherent structures are identified and the impact of different actuation methods on spatial modes is illustrated. Spatial modes of the unforced (natural) flow show the presence of a single and double helical precessing vortex core (PVC) modes at St = 0.53. Low-frequency acoustic actuation (St = 0.46) effectively suppresses the PVC mode, while high-frequency (St = 2) actuation stabilizes it. Broadband excitation of the flow field, however, induces the excitation of both single and double helical PVC modes.

Role of secondary hydrogen injection on flame stabilization of ammonia/air swirling flames

Ammonia is widely recognized as one of the most advanced hydrogen carriers and can be utilized as a fuel in gas turbines. Premixing hydrogen into the ammonia carbon free fuel system can substantially enhance stable limits, but it significantly promotes cross-reactions, leading to increased NO production. Recent findings related to other fuels suggest that the alternative approach for mitigating combustion instabilities involves introducing minimal pure hydrogen at the chamber inlet in a non-premixed mode. This may introduce a novel approach to stabilize ammonia/air swirl flames by extending the stability through a minimal hydrogen via secondary injection, with minimal impact on increasing nitrogen oxide emissions. In the present study, the flame stabilization mechanism and nitrogen oxide emission behaviors of swirl ammonia/air flames by a secondary hydrogen injection were compared with the premixed ammonia/ hydrogen/air flames. OH-/NO-PLIF and PIV techniques were applied to reveal the lean blow-off characteristics and the reacting flow features. The NOx emissions were measured by the FTIR gas analyzer. The large eddy simulation method with a developed dynamic thickened flame model was employed to further reveal the experimental findings. Experimental results show that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. The secondary hydrogen injection exhibits the stronger enhancement ability for the lean/rich blow-off limits, primarily due to the local diffusion hydrogen flame at the flame root providing more active radicals and higher temperature gases. The NO emission shows an increase with the addition of premixed hydrogen, while in the secondary hydrogen injection, NO emission deteriorates due to higher temperatures, with the NO emission increasing by less than 10% compared with the ammonia flame. In the large eddy simulation analyses, the physical effects of the secondary hydrogen injection enhance the flame stability by increasing the resistance of the flame root to extinction. The heat release rate and the mass fractions of NH and H near the flame root are significantly increased by 2% secondary hydrogen injection. The enhancement of the ammonia decomposition process is stronger with secondary hydrogen injection than with fully premixed hydrogen addition. For 2% secondary hydrogen injection case, the local hydrogen injection to form a non-premixed combustion mode can provide much higher capability to increase the local heat release rate compared to the fully premixed mode.Novelty and significance statement: Introducing small amount of hydrogen to ammonia flame can effectively improving the flame stabilization in a swirl combustor. In this study, it is found that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. Comparing the way of premixing hydrogen in the unburned mixture, a larger effect on blow-off limits extension was found when introducing a separate non-premixed hydrogen at the chamber inlet, which is mentioned as the secondary hydrogen injection. The lean blow-off limits can be extended from about 0.67 to 0.59 when only introducing 2% secondary hydrogen injection by heat value. The NO emission shows in the secondary hydrogen injection, NO emission deteriorates by less than 10% compared with the ammonia flame. A thickened flame model for non-premixed combustion was established and verified adequate with velocity field and flame structure. The mechanisms of enhancing flame stabilization by hydrogen introduction including physical and chemical effects for premixing and injection cases were revealed. Furthermore, the difference in the mechanisms of the two cases was emphasized.

Effects of inlet air holes on swirl flow characteristics and outlet temperature distribution in an axial swirl combustor

The air inlet holes on the swirl combustor are crucial for high-performance micro gas turbine (MGT), affecting internal airflow, fuel-air mixing, temperature distribution, and cooling. However, there are seldom studies that pay attention to the influence of the air inlet holes coupled with low volatility macromolecular fuels for fast and better fuel-air mixing and combustion processes in MGT. This study investigates the influence of primary and dilution hole positions on airflow, temperature, emissions, and combustion efficiency. Numerical simulations in ANSYS Fluent examine five inlet hole configurations in a two-stage axial swirl combustor. Simulation validation is conducted under three test conditions. The results showed that the cut-off effect of the primary hole affected the size of the swirl vortex core, and the backward movement of the primary hole resulted in a significant increase in the length of the recirculation zone and the coherence high-temperature zone, accelerated the droplet evaporation rate, and reduced CO and NO emissions. Backward movement of dilution hole alters flow field, affecting swirl vortex stability and reflow intensity. Case 3, with original dilution hole and backward extended primary hole, exhibits optimal outlet temperature distribution and combustion efficiency.

A novel low-NOx swirl burner with coal-ammonia co-firing: Effect of ammonia ratio in the dual channels on combustion and NOx formation

A novel low-NOx swirl burner with coal-ammonia co-firing: Effect of ammonia ratio in the dual channels on combustion and NOx formation

Application of an Improved Workflow for the Identification of Flame Dynamics to Swirl Stabilized Wet Combustion

Abstract The estimation of flame transfer functions (FTF) from time series data generated by large eddy simulation (LES) via system identification (SI) is an important element of thermoacoustic analysis. A continuous time series of adequate length is required to achieve low uncertainty, especially when dealing with turbulent noise. Limited scalability of LES codes implies that the wall-clock-time required for generating such time series may be excessive. The present paper tackles this challenge by exploring how the superposition of multiple simulations with the same excitation signal, but varying initial conditions, increases signal-to-noise ratio and leads to more robust identification. In addition, the established SI approach, which relies on broadband excitation, is compared to excitation with approximate Dirac and Heaviside signals, promising simpler pre- and postprocessing. Results demonstrate that the proposed workflow reduces significantly the wall-clock-time required for robust FTF identification. This reduction in wall-clock-time requires more parallel computational resources, but it does not significantly increase the overall computational cost while also enabling FTF estimation using Heaviside excitation. The proposed method is assessed on a partially premixed, steam enriched (‘WET’) swirl burner with significant turbulent noise levels. Steam enrichment is a combustion concept that reduces harmful emissions such as NOx and CO2 while increasing engine efficiency. However, the effect of steam on the flame response, needs to be better understood. To this end, a combustion model including an optimized global chemical mechanism for partially premixed wet methane combustion, is presented and validated against experimental data.

Investigation of a Fuel-Flexible Diffusion Swirl Burner Fired with NH3 and Natural Gas Mixtures

Investigation of a Fuel-Flexible Diffusion Swirl Burner Fired with NH3 and Natural Gas Mixtures

Estimation of the Global Equivalence Ratio of a Swirl Combustor from a Small Number of Absorption Spectra Using Machine Learning.

A new optical diagnostic method that predicts the global fuel-air equivalence ratio of a swirl combustor using absorption spectra from only three optical paths is proposed here. Under normal operation, the global equivalence ratio and total flow rate determine the temperature and concentration fields of the combustor, which subsequently determine the absorption spectra of any combustion species. Therefore, spectra, as the fingerprint for a produced combustion field, were employed to predict the global equivalence ratio, one of the key operational parameters, in this study. Specifically, absorption spectra of water vapor at wavenumbers around 7444.36, 7185.6, and 6805.6 cm-1 measured at three different downstream locations of the combustor were used to predict the global equivalence ratio. As it is difficult to find analytical relationships between the spectra and produced combustion fields, a predictive model was a data-driven acquisition. The absorption spectra as an input were first feature-extracted through stacked convolutional autoencoders and then a dense neural network was used for regression prediction between the feature scores and the global equivalence ratio. The model could predict the equivalence ratio with an absolute error of ±0.025 with a probability of 96%, and a gradient-weighted regression activation mapping analysis revealed that the model leverages not only the peak intensities but also the variations in the shape of absorption lines for its predictions.

Research on the Characteristics of Oscillation Combustion Pulsation in Swirl Combustor

Research on the Characteristics of Oscillation Combustion Pulsation in Swirl Combustor

Linear analysis of a swirling jet with a realistic swirler model

The dynamics of an axisymmetrical swirling jet is studied via global linear stability and resolvent analyses. The modeled flow represents a combustor-like swirling jet, that is turbulent, compressible, non-parallel, and enclosed. In particular, the computational domain embeds a realistic axisymmetrical swirler model to resolve the mode conversion process. Swirl fluctuations are non-negligible on this configuration representative of a swirl burner, and match the analytical mode shapes of inertial waves of an inviscid uniform flow as obtained from global stability analysis. The stability map presents two eigenvalues driving a modal amplification. These eigenmodes couple a standing acoustic wave sustained in the mixing duct and the combustion chamber with the Kelvin-Helmholtz mechanism at the mixing duct exit and the acoustic-vorticity mode conversion process at the swirler, and act as a frequency selection criterion. Finally, the most amplified forcing from the resolvent analysis is similar to an unsteady heat source in the combustion chamber, and the identified optimal amplification mechanism is likely to be triggered in reacting flow with unsteady heat release rate.

Advancements in Sustainable Aviation Fuels: Impact of Nano-Additives and Ammonia-Based Strategies On Emissions

Abstract Our study investigates the impact of ammonia- and water-based nano-particulate additives on the combustion characteristics of Jet-A1 aviation fuel, using a 300-kW liquid swirl combustor. Experiments were conducted at two global equivalence ratios (Φ = 0.24 and Φ = 0.40), focusing on laminar flame speed (LFS) and flame properties through chemiluminescence imaging and modal analysis techniques. The primary objective was to understand how these nano-additives modulate flame dynamics and internal chemical reactions, alongside evaluating the environmental implications of combustion alterations. Results showed that integrating urea and water additives into the fuel matrix affected LFS, enhancing it at the lower equivalence ratio but having detrimental effects at the higher ratio. Modal analysis revealed a notable stabilizing influence on flame behavior, especially under leaner fuel conditions. The addition of water and urea influenced combustion chemistry and spray patterns, leading to more uniform sprays and more complete combustion. Chemiluminescence imaging demonstrated higher emission intensity of NH2* radicals compared to NH* radicals, varying with the global equivalence ratio. The data indicated a significant reduction in NOx emissions, particularly at lower equivalence ratios, accompanied by a slight increase in CO2 and CO emissions. This study highlights the potential of ammonia- and water-based nano-additives to enhance the combustion performance and environmental outcomes of Jet-A1 aviation fuel, with the trade-off of increased CO2 and CO emissions requiring further consideration.

Stability and combustion characteristics of dual annular counter-rotating swirl oxy-methane flames: Effects of equivalence and velocity ratios

An experimental study was carried out to investigate flow/flame interactions, stability, combustion, and emissions properties of CH4/O2/CO2 stratified flames stabilized on a dual annular counter-rotating swirl (DACRS) burner for gas turbine combustion applications. The effects of two targeted parameters, namely equivalence ratios (for primary - ϕp, secondary - ϕs, and global - ϕg streams) and velocity ratio (Vr = Vp/Vs: primary to secondary stream inlet velocity ratio), are studied at fixed volumetric oxygen fraction (OF) in the oxidizer mixture (O2 + CO2) of both primary and secondary streams of 34 % at fixed Vp of 5 m/s. The experimental results indicated that no flame flashback was recorded in the domain of any operational ϕp up to stoichiometric operation of the secondary stream (ϕs = 1.0). At near stoichiometric operation of the primary stream, the main secondary flame can persist in extremely lean conditions (ϕs = 0.45 @ Vr = 3), thereby extending the thresholds of flame blowout. Moreover, raising ϕp from 0.4 to 1.0 results in significant reduction of ϕs at blowout from 0.595 to 0.456, corresponding to reducing the combustor ϕg at blowout from 0.577 to 0.499. At lower ϕp, the oxy-methane flame tends to lift and extinguish earlier.

CO formation characteristics in a centrally staged swirl combustor

Based on a validated numerical simulation methodology, this study focused on a centrally staged swirl combustor to investigate the combustion flow field inside the chamber under typical operating conditions. The CO formation characteristics of each region in the combustor were analyzed. In terms of elementary reactions, this study analyzed crucial reaction pathways contributing to CO formation in distinct regions utilizing the eddy dissipation concept model. Results reveal the occurrence of CO formation in the flame front and its rapid oxidation to form CO2 in the post-flame zone. CO2 partially dissociates into CO at high temperatures. Furthermore, air film quenching substantially impacts CO formation and emission, and air film greatly affects CO oxidation more than its formation. Local cooling and dilution effects from the air film contribute to the freezing of CO oxidation reactions. Therefore, a substantial quantity of CO remains unreacted, leading to increased CO emissions at the combustion chamber outlet.

Nox Emissions Assessment of a Multi Jet Burner Operated with Premixed High Hydrogen Natural Gas Blends

Abstract Decarbonization of gas turbine combustion creates a pressing demand for new technical solutions for the combustion process. While switching to hydrogen fuels may solve the problem of carbon emissions and associated pollutants it can also lead to stability issues for swirl-stabilized combustors due to its increased reactivity. However, with jet flame burner systems, the required flashback safety can be achieved with high axial flow velocities even for premixed combustion of 100% hydrogen fuel. The development of such an engineering solution, however, requires significant effort to reach the maturity of today's swirl burners. This study examines the capacity of a premixed multi-tube jet burner to manage the chemical reactivity change over a range of volumetric blends from pure natural gas to pure hydrogen fuel. NOx emissions are measured and analyzed for atmospheric tests. The changes in emissions originate not only from altered combustion chemistry but also from changes in flame shape and turbulence intensity. To get a deeper understanding of the NOx formation process, a low-order model is designed and compared to the experimental data of technically and perfectly premixed combustion tests. Parameter variations of the low-order model are conducted to assess the influences on the NOx emission production of the multi jet burner. The information on the combustion process required for the model is obtained computationally and experimentally. Therefore, flame images are recorded and analyzed.