Swirl-stabilized Combustor Research Articles

Impact of precessing vortex core dynamics on the thermoacoustic instabilities in a swirl-stabilized combustor

Global instabilities in swirling flows can significantly alter the flame and flow dynamics of swirl-stabilized flames, such as those in modern gas turbine engines. In this study, we characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate. This study is performed using experimental data obtained from a model gas turbine combustor employing two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at five bar pressure. The flow split between the two streams is systematically varied to observe the impact of flow structure variation on the system dynamics at both non-reacting and reacting conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame and fuel flow behaviour, respectively. Spectral proper orthogonal decomposition and a complex network analysis are used to identify and characterize the dominant oscillation mechanisms driving the system. In the non-reacting data, a PVC is present in most cases and the amplitude of the oscillation increases with increasing flow through the centre nozzle. In the reacting data, three dominant modes are seen: two thermoacoustic modes and the PVC. Our results show that in the cases where the frequency of the PVC overlaps with either of the thermoacoustic modes, the thermoacoustic modes are suppressed. The complex network analysis coupled with a weakly nonlinear theoretical analysis suggests the mechanisms by which this coupling and suppression of the thermoacoustic mode occur.

Analysis of effusion cooling under realistic swirl reacting flow in gas turbine combustor

Cooling performance is experimentally and numerically investigated in an effusion-cooled, swirl-stabilized three-sector gas turbine combustor at realistic reacting flows. The temperature distribution on the perforated wall is measured by the infrared thermography and the overall cooling effectiveness is evaluated at the conjugate heat transfer condition. The effusion holes are inclined at 30°, 90° and 150° to explore the effect of cooling injection direction on the effusion cooling effectiveness and flame-cooling interaction. The pressure drop across the cooling plate and the equivalence ratio are varied to assess their influences on the effusion cooling behavior. Weak interaction between the main flow and cooling jets is observed for the forward cooling hole scheme, but the impingement region caused by the expanding main flow brushing on the liner wall suffers from the insufficient cooling protection. The orthogonal holes present relatively higher overall cooling effectiveness than the forward one because of the larger amount of air removing the heat loaded on the liner wall via convection flow inside the holes. In addition, the vertical cooling injections lift the main flow upwards and thus decrease the driven heat transfer temperature. It is most noteworthy that the backward injections present potentials to resist the impingement impact, its overall cooling effectiveness demonstrates to be the highest especially when the coolant amount is adequate enough.

Conjugate heat transfer characteristics of effusion cooling under realistic swirl flow in a three-sector gas turbine model combustor

• Convective heat transfer inside effusion holes contributes greatly to the heat removal even the film layer is damaged. • The full-coverage cooling film is developed and reachable after swirl flow-cooling jets intense interaction. • The swirl flow impinging location is not influenced by amount of main and cooling flows. · Increment of coolant. Conjugate heat transfer characteristics of the effusion cooling applied on the swirl-stabilized combustor liner is experimentally and numerically investigated in a three-nozzle gas turbine model combustor at non-reacting flow conditions. The interaction between the strong swirl main flow and multi-hole cooling jets inclined at 30° is stressed by varying the pressure drop across the nozzle ( Δ P m = 3–5%) and effusion plate ( Δ P e = 0–2%). The expanding swirl flow impinges on the liner wall and thus inhibits the injection of coolant via effusion holes locally, causing non-uniform cooling protection along both streamwise and lateral directions. The overall cooling effectiveness in the impingement region is much lower than other regions on the cooled plate. Despite of the detrimental effect of the main flow impingement on the formation of the effusion film, the cooling function doesn’t totally collapse due to the great capability of heat removal through convection heat transfer inside the cooling holes. Superposition effect induced full coverage of film on the wall is distinguished at further downstream. Enhancing the cooling air amount weakens the consequence of the mainstream impingement and lead the full coverage initial location to move upwards.

Similarity phenomena of lean swirling flames at different bulk velocities with acoustic disturbances

In this study, flame responses to acoustic disturbances with different frequencies and amplitudes were experimentally investigated in a lean premixed swirl-stabilized combustor operating at different bulk velocities. The total heat release rate fluctuations and spatial CH* chemiluminescence distributions were captured using a photomultiplier tube and high-speed camera, respectively. The results indicate that the heat release rate exhibits a relatively drastic oscillation and high-order harmonics for low-frequency disturbances. When the bulk velocity and forcing frequency were doubled simultaneously, similar flame structures were observed in the CH* chemiluminescence distributions. As the bulk velocity increases, the gain of the Flame Describing Function (FDF) extends toward the higher frequencies, and the delay time of the flame response decreases. The similarity among FDFs at different bulk velocities was effectively captured by introducing a non-dimensional parameter, defined as the ratio of the flame response delay to the forcing time scale, to replace the dimensional forcing frequency. Furthermore, the availability of the newly defined non-dimensional parameter was verified for flames with different swirl numbers, as this played an important role in determining the flame structures and associated unsteady heat release rate.

Dynamic behavior and driving region of thermoacoustic combustion oscillations in a swirl-stabilized turbulent combustor

We attempt to identify the driving region of combustion oscillations by methodologies based on symbolic dynamics, synchronization, and complex networks. As the equivalence ratio is increased, the dynamic behavior of pressure fluctuations transitions from a stable combustion state to high-amplitude combustion oscillations via type-II intermittent combustion oscillations and the subsequent low-amplitude combustion oscillations. Rayleigh index-based transition network entropy is a useful indicator for determining the driving region of low-amplitude combustion oscillations. We clearly observe that a critical slowing down begins to appear in the combustion state close to the formation region of type-II intermittent combustion oscillations with temporally increasing equivalence ratio.

Effects of hydrogen addition on the forced response of H2/CH4 flames in a dual-nozzle swirl-stabilized combustor

The effects of hydrogen addition on the forced response of H2/CH4 flames are analyzed in a dual-nozzle swirl-stabilized combustor. The hydrogen volumetric content in the fuel is varied from 0% to 40%. Flame transfer function (FTF) is used to compare the forced response of the flames. The FTF gain featuring the local maximum and minimum values, which occurred commonly in the FTFs under all hydrogen contents, is determined by two different mechanisms: the change in the flame angle and the flame roll-up phenomenon. Among two mechanisms, the flame roll-up phenomenon has a more important role in determining the FTF characteristics. In addition, hydrogen addition attenuates the local maximum gains and decreases the FTF phase slope. The change in the flame roll-up behavior, which is induced by a short and compact flame distribution at high hydrogen contents, is the primary reason of these differences in the FTF.

Investigation on spray combustion characteristics of Boron-Loaded slurry fuel in a Swirl-Stabilized combustor

For many decades, liquid hydrocarbon fuels are being used to propel air-breathing engines. However, the energy content of liquid hydrocarbon fuels has reached close to its limitation. Any further improvement in energy value is quite difficult through chemical methods. Slurry fuels, which are colloidal suspensions of solid energetic particles in liquid fuel, became a potential alternative fuel. Among several solid additives, boron is considered the best choice because of its higher heating values. However, our knowledge about boron-loaded slurry fuel is very limited. Therefore, here in this study, we experimentally investigated the combustion characteristics of boron/Jet A-1 slurry fuel spray in a swirl-stabilized combustor. A particle loading of 10% by weight is considered here for the experiment. The feed boron, as well as the burnt boron samples, were characterized using standard material characterization techniques in order to understand the surface morphology, oxidation behavior, and active boron content. The combustion characteristics of slurry fuel were analyzed through spectroscopy and BO2* chemiluminescence imaging. Positive thermal contribution from boron combustion was quantified via temperature measurements at three different radial locations of the combustor exit. The spectroscopy and chemiluminescence signatures indicate that combustion of boron particles occurs downstream of the dump plane. The exit temperature of boron-laden slurry fuel measured at all three different radial locations is higher compared to neat Jet A-1. An increment of 19% in exit temperature was observed in the case of boron slurry fuel relative to neat Jet A-1. The X-ray diffractograms (XRD) and thermogravimetric analysis (TGA) show the complete oxidation of boron particles.

Experimental performance and emissions of additively manufactured high-temperature combustion chambers for micro-gas turbines

To date, 59 countries, representing 54% of global greenhouse gas emissions have made pledges for net-zero emissions targets within this century. This will require cleaner and more efficient sources of energy which is driving research into small-scale engines and auxiliary power units for hybrid vehicles and stationary power generation. A suitable candidate for such applications is the micro gas turbine due to its high-power density, reliability and low emissions. Further development of such engines is required though due to their increasing parasitic energy losses relative to net power output as size decreases. Additive manufacturing offers the design freedom to not only increase efficiencies but to also reduce emissions when applied to the various components of micro gas turbines. This article reports the effects of several additive manufacturing (AM) enabled design features for micro gas turbine combustion chambers via experimental testing of full-scale parts. The main objective of the additively manufactured features is the reduction of exhaust emissions by improving the air-fuel mixture distribution and consequently ignition and combustion. Using additive manufacturing a novel conical radial swirl-stabilized tubular combustor with internal vane fuel injection was created as a baseline for the laboratory testing. Several other features, including augmented backside liner cooling surfaces, in-vane lattice structures for fuel mixing and upstream liner fuel injection rings were also generated to further the investigations into additively manufactured features and their effects on fuel mixing. Using multiple combinations of all these features, 10 geometries were generated and tested at a variety of operating conditions. Three inlet temperatures were tested (500°C, 600°C and 700°C) with varying fuel flow rates to investigate their operating limits at a constant inlet pressure of 4 bar absolute. Test results for the full range of equivalence ratios and operating conditions showed that the upstream liner fuel injection designs generated NOx, CO and THC emissions on par with the baseline but showed a reduction in the maximum and minimum operating ranges. This design, however, demonstrated the distinct advantage of being able to ignite at full air mass flow; this is not possible with the baseline designs and is also an added benefit to its main use which is the reduction of liner temperature. Overall, the test results underscore that designing combustion chambers for additive manufacturing can provide a myriad of benefits not only for micro gas turbines but also for other applications requiring high efficiency combustion chambers.

Dynamics of Thermoacoustic Oscillations in Swirl Stabilized Combustor without and with Porous Inert Media

Lean premixed (LPM) combustion processes are of increased interest to the gas turbine industry due to their reduction in harmful emissions. These processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms. To better study these instabilities, quantitative experimental resolution of heat release is necessary, but it presents a significant challenge. Most combustion systems are partially premixed and therefore will have spatially varying equivalence ratios, resulting in spatially variant heat release rates. For laminar premixed flames, optical diagnostics, such as OH chemiluminescence, are proportionally related to heat release. This is not true for turbulent and partially premixed flames, which are common in commercial combustors. Turbulent eddies effect the strain on flame sheets which alter light emission, such that there is no longer a proportional relationship. In this study, phased, averaged, and spatially varying heat release measurements are performed during a self-excited thermoacoustic instability without and with porous inert media (PIM). Previous studies have shown that PIM can passively mitigate thermoacoustic instabilities, and to the best of the authors’ knowledge, this is the first-time that heat release rates have been quantified for investigating the mechanisms responsible for mitigating instabilities using PIM. Heat release is determined from high-speed PIV and Abel inverted chemiluminescence emission. OH ∗ chemiluminescence is used with a correction factor, computed from a chemical kinetics solver, to calculate heat release. The results and discussion show that along with significant acoustic damping, PIM eliminates the direct path in which heat release regions can be influenced by incoming perturbations, through disruption of the higher energy containing flow structures and improved mixing.

Comparative assessment of combustion characteristics of limonene, Jet A-1 and blends in a swirl-stabilized combustor under the influence of pre-heated swirling air

Biofuels are low carbon fuels and have emerged as an important source of energy for reducing pollutant emissions and greenhouse gas emissions in the aviation industry. In the present work, the spray combustion characteristics of limonene, Jet A-1, and limonene/Jet A-1 blends (10%, 30%, and 50% volumetric loading of limonene) have been experimentally investigated in a laboratory-scale swirl-stabilized combustor. To examine the effect of heated swirling air on the combustion of limonene, Jet A-1, and Jet A-1/limonene blends, the experiment has been conducted on non-preheat and two preheat conditions (swirling air temperature of 373 K and 473 K). Limonene is considered an emerging fuel alternative owing to its high calorific value and comparable physical properties with Jet A-1. The spray flame length of limonene is approximately 20% shorter than the flame of Jet A-1, while compared to the non-preheated case the flame length is reduced in preheat case (approximately 15% reduction in 373 K case and 25% reduction at 473 K). Compared to pure Jet A-1, the flames of limonene and limonene/Jet A-1 blends have given good results in terms of pollutant emissions and temperature profiles when limonene is added to Jet A-1. 50% limonene-loaded blends show a drop of 15% CO and neat limonene show a 35 % CO reduction. Additionally, 10%, 30%, and 50% limonene blends have shown better combustion characteristics compared to neat Jet A-1.

Control of Lean Blowout in a Swirl-Stabilized Dump Combustor at Different Levels of Premixing Based on Flame Colour

Control of Lean Blowout in a Swirl-Stabilized Dump Combustor at Different Levels of Premixing Based on Flame Colour

High Temperature Fuel Impacts on Combustion Characteristics of a Swirl-Stabilized Combustor

High Temperature Fuel Impacts on Combustion Characteristics of a Swirl-Stabilized Combustor

Design and Experimental Characterization of a Swirl-Stabilized Combustor for Low Calorific Value Gaseous Fuels

Abstract Low calorific value (LCV) gaseous fuels are generated as by-products in many commercial sectors, e.g., as mine gas or biogas. Their efficient exploitation can be a considerable source of primary energy. Typically, product gases from biomass are characterized by low lower heating values (LHVs) due to their high concentration of inert gases and steam. At the same time, their composition varies strongly based on the initial feedstock and may contain unwanted components in the form of tars and ammonia. These properties make the design of appropriate combustion systems very challenging and issues such as ignition, flame stability, emission control, and combustion efficiency must be accounted for. By employing a proprietary gas turbine burner at the TU Berlin, the combustion of an artificial LCV gas mixture at stoichiometric conditions has been successfully demonstrated for a broad range of steam content in the fuel. This work presents the stability maps and emissions measured with the swirl-stabilized burner at premixed conditions. It was shown that the flame location and shape primarily depend on the steam content of the LCV gas. The steam content in the fuel was increased until flame blow-out occurred at LHVs well below the target condition of 2.87 MJ/kg (2.7 MJ/mN3). The exhaust gas is analyzed in terms of the pollutants NOx and CO for different fuel compositions, moisture contents, and thermal powers. Finally, OH* measurements have been carried out in the flame. A simple reactor network simulation was used to confirm the feasibility of the experimental results.

Topology of turbulent premixed and stratified LPG/air flames

Premixed and stratified Liquefied Petroleum Gas (LPG)/air flames realized on a bluff body swirl-stabilized burner are studied in a confined environment. A laboratory-scale burner is designed in such a way that it can independently investigate the effects of mixture stratification, swirl number, swirl direction, and Reynolds number on a broad range of test conditions. A total of 80 different test cases are investigated, with four different swirl number combinations, two different swirl direction combinations, two velocity ratios, and five different mixture stratification ratios. The flame topology is investigated using the simultaneous acquisition of hydroxyl radical (OH*) and methylidyne radical (CH*) chemiluminescence and direct imaging. A series of premixed and stratified flame topologies are compared using mean Abel-deconvoluted chemiluminescence images and direct mean images. Premixed flames appear to be lifted V-flames influenced primarily by swirl number combinations rather than swirl direction or velocity ratio variations. For the same swirler combinations and velocity ratios, stratified flames are more compact than premixed flames, and higher stratified flames are similar in shape. A counter-rotating coaxial swirler arrangement is less likely to affect the flame structure than a change in swirl number. According to the study, the outer swirl angle determines the flame shape, whereas the inner swirl angle influences mixing, which aids in flame stabilization.

Numerical Steady and Transient Evaluation of a Confined Swirl Stabilized Burner

Lean premixed combustion technology became state of the art in recent heavy-duty gas turbines and aeroengines. In combustion chambers operating under fuel-lean conditions, unsteady heat release can augment pressure amplitudes, resulting in component engine damages. In order to achieve deeper knowledge concerning combustion instabilities, it is necessary to analyze in detail combustion processes. The current study supports this by conducting a numerical investigation of combustion in a premixed swirl-stabilized methane burner with operating conditions taken from experimental data that were recently published. It is a follow-up of a previous paper from Farisco et al., 2019 where a different combustion configuration was studied. The commercial code ANSYS Fluent has been used with the aim to perform steady and transient calculations via Large Eddy Simulation (LES) of the current confined methane combustor. A validation of the numerical data has been performed against the available experiments. In this study, the numerical temperature profiles have been compared with the measurements. The heat release parameter has been experimentally and numerically estimated in order to point out the position of the main reaction zone. Several turbulence and combustion models have been investigated with the aim to come into accord with the experiments. The outcome showed that the combustion model Flamelet Generated Manifold (FGM) with the k-ω turbulence model was able to correctly simulate flame lift-off.

Numerical investigation of heat transfer effects on combustion in a generic gas turbine burner

PurposeThis study aims to methodologically investigate heat transfer effects on reacting flow inside a liquid-fueled, swirl-stabilized burner. Furthermore, particular attention is paid to turbulence modeling and the results of Reynolds-averaged Navier–Stokes and large eddy simulation approaches are compared in terms of velocity field and flame temperature.Design/methodology/approachSimulations consist liquid fuel distribution using Eulerian–Lagrangian approach. Flamelet-Generated Manifold combustion model, which is a mixture fraction-progress variable formulation, is used to obtain reacting flow field. Discrete ordinates method is also added for modeling radiation heat transfer effect inside the burner. As a parametric study, different thermal boundary conditions namely: adiabatic wall, constant temperature and heat transfer coefficient are applied. Because of the fact that the burner is designed for operating with different materials, the effects of burner material on heat transfer and combustion processes are investigated. Additionally, material temperatures have been calculated using 1 D method. Finally, soot particles, which are source of luminous radiation in gas turbine combustors, are calculated using Moss-Brookes model.FindingsThe results show that the flow behavior is obviously different in recirculation region for both turbulence modeling approach, and this difference causes change on flame temperature distribution, particularly in the outer recirculation zone and region close to swirler. In thermal assessment of the burner, it is predicted that material of the burner walls and the applied thermal boundary conditions have significant influence on flame temperature, wall temperature and flow field. The radiation heat transfer also makes a strong impact on combustion inside the burner; however, luminous radiation arising from soot particles is negligible for the current case.Originality/valueThese types of burners are widely used in research of gas turbine combustion, and it can be seen that the heat transfer effects are generally neglected or oversimplified in the literature. This parametric study provides a basic understanding and methodology of the heat transfer effects on combustion to the researchers.

Effect of Rotating Gliding Arc Plasma on Lean Blow-Off Limit and Flame Structure of Bluff Body and Swirl-Stabilized Premixed Flames

The effect of rotating gliding arc plasma on flame stabilization in a laboratory scale swirl-stabilized plasma-assisted combustor was experimentally investigated. The characteristics of a gliding arc plasma were studied using high-speed camera and simultaneous measurements of current and voltage waveforms. Effects of plasma on the extension of lean blow-off (LBO) limit and the swirl flames structures as well as OH radical distribution have been studied systematically. Results show that the discharge and combustion are coupled together effectively due to the dynamic processes of discharge. When the plasma is activated, the flame structures are drastically changed; oscillating flame and lifted flame convert to stable columnar flame tending to be attached to the plasma column. Besides, the plasma columns can promote OH formation and can produce much more energetic radicals due to the reactions between the discharge, methane and oxygen. The gliding arc plasma can stabilize the flame, provide an additional anchoring mechanism, and significantly extend the LBO limit. The plasma column can provide active radicals and continuous ignition to sustain the flame, and the thermal effect and kinetic effect may occupy the dominant role.

Effects of exhaust tube vortex on the in-furnace phenomena in a swirl-stabilized pulverized coal flame

This paper presents an experimental investigation of the effects of the exhaust tube vortex (ETV) on pollutant emissions and carbon burnout performance. The novelty in this study is to improve understanding of combustion behavior with the ETV structure via detailed in-furnace measurements (temperature, gas-phase species concentration, carbon burnout, emissions, and etc.) in a 16-kWth coal-fired furnace. The experiment considered two different swirling flames (co- and counter-swirl) with four air staging positions. A hollow tubular structure such as the ETV structure has been created under the co-swirl condition. The values of NOx emissions are about 430 ppm, but the CO emission is lower and carbon burnout is approximately 2% higher than that of the counter-swirl unstaging condition at the furnace outlet. For the air staging conditions, the air staging position 3 (SL3) maintained the generated ETV structure stronger in the primary combustion zone. The SL3 condition produced the lowest CO emission at about 275 ppm, and a relatively low emission of NOx showed with the highest carbon burnout performance of approximately 94.5% or more. The SL3 condition is the most favorable condition for the synergistic effect of air staging technology on the ETV structure with a low-NOx swirl-stabilized burner.

Design, simulation, and validation of additively manufactured high-temperature combustion chambers for micro gas turbines

The demand for cleaner, more efficient, and durable sources of electricity is driving research into small-scale power generation. Micro gas turbines are especially suitable by virtue of their high power density and reliability, but a major drawback is their poor overall efficiency due to increasing parasitic energy losses relative to net power output as size decreases. Additive manufacturing offers design freedoms that could enable higher efficiency and lower emission combustors for micro gas turbine applications. A novel conical radial swirl-stabilized tubular combustor with internal vane fuel injection is designed and tested, and a validated reacting computational fluid dynamics model is used to design novel combustor features that can only be additively manufactured. Of the five different concepts tested, those benefitting from additively manufactured features outperform the traditional design in terms of peak temperature control and fuel–air mixing, translating to striking reductions in pollutant emissions, with up to 75% and 40% reductions in nitrogen oxides and carbon monoxides, respectively, while concepts incorporating upstream fuelling and a three-row lattice show a near 20% increase in mixture quality. As well as evaluating a number of novel and very promising additively manufactured combustor design features, this work provides guidance on the incorporation of additively manufactured features in combustors for any gas turbine application and demonstrates the clear benefits of additive manufacturing for low-emission combustor design.

Effects of hydrogen enhancement on mesoscale burner array flame stability under acoustic perturbations

Partial substitution of hydrocarbon fuel with hydrogen can effectively improve small-scale combustion system stability and performance, potentially opening the way for novel compact power generation and/or propulsion systems in the future. In this study, the effects of hydrogen enhancement between 0% and 40% hydrogen volumetric fractions in methane fuel were experimentally observed in a mesoscale burner array subjected to external acoustic perturbations. The mesoscale burner array utilizes an array of swirl-stabilized burner elements and their interactions with neighboring elements to improve the overall flame stability and simultaneously reduces the combustor length scale. OH∗ chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) were used to image various hydrogen-enriched flames at an equivalence ratio of 0.7, subjected to transverse acoustic perturbations at 320 Hz. Two acoustic modes were imposed by controlling the phase difference between two speakers perturbing the flow. OH∗ chemiluminescence images exhibited flame length scale reduction, leading to a denser flame array. Also, flame arrays with higher hydrogen enrichment were found to be more robust against transverse acoustic perturbations, demonstrated by reduced fluctuations in the global heat release rate. OH-PLIF images showed that flames with higher hydrogen enrichment initiated V- to M-shaped flame shape transition even under fuel lean conditions, thereby improving the combustion stability. OH-PLIF images were also used for flame stability analysis through spectral proper orthogonal decomposition (SPOD). The SPOD analysis showed hydrogen enrichment diminished flame fluctuation structures under fuel lean operation.