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To reduce fossil fuel consumption, blending sludge pyrolysis gas and ammonia (NH 3) for combustion enables the energy utilization of sludge pyrolysis gas while improving combustion characteristics of NH 3. This study employs chemiluminescence to investigate the structure of sludge pyrolysis gas/ammonia/air flame for the first time. A thorough investigation was conducted into the impacts of various swirl structure parameters on the stable combustion range and pollutant emissions. To further reduce pollutant emissions, staged combustion was used, and the effects of ammonia doping ratio, secondary air equivalence ratio, and secondary air inlet height on pollutant emissions were analyzed. The findings demonstrate that increasing the ammonia ratio narrows stable combustion equivalence ratio range. Additionally, at a thermal power of 5 kW, stable combustion was observed in the equivalence ratio range of 0.41–1.70, an improvement of 95.5%. Notably, staged combustion significantly reduced NO emissions, achieving concentrations as low as 32.7 ppm under stoichiometric conditions, compared to 1912.6 ppm in non-staged pure NH 3 combustion. This study provides valuable insights into the stability and emission characteristics of sludge pyrolysis gas/ammonia swirl burners for efficient and cleaner low-carbon fuels utilization. • First study on the stability and emissions of pyrolysis gas/NH 3 swirling flames. • Ammonia addition reduces OH* radiation intensity, particularly in lean combustion. • Increasing ammonia narrows the stable combustion equivalence ratio. • The optimal swirl parameters for stable combustion are proposed. • Staged combustion at 150 mm height reduces NO emissions to 32.7 ppm. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) has been gaining popularity as a carbon-free alternative fuel in recent years. OH∗ chemiluminescence is a promising method to study the structure of ammonia flames, but research on this aspect is still lacking. In this study, the OH∗ chemiluminescence distributions and structure characteristics in NH 3 /H 2 and NH 3 /cracked gas swirl flames are experimentally investigated. The OH∗ distributions of NH 3 /H 2 and NH 3 /cracked gas under different addition levels and equivalence ratios are compared. The results show that the increase of equivalence ratio and hydrogen doping level significantly increased the OH∗ chemiluminescence peak of NH 3 /air swirling flame. Especially in fuel-lean flames, the equivalence ratio effect is greater than the NH 3 dilution effect, while the two effects are comparable in equivalence and fuel-rich conditions. The stretching effect of NH 3 /cracked gas is stronger, and the flame width is narrower than that of NH 3 /H 2 flame, but the peak value of OH∗ radiation is higher. • Profiles of OH∗chemiluminescence in NH 3 /H 2 swirling flame are visualized. • The intensity of the OH∗ peak increases as φ and X NH3 increase. • The φ effect dominates the increase of OH∗ intensity during lean combustion. • Partially cracked NH 3 /air flame OH∗ chemiluminescence are investigated. [ABSTRACT FROM AUTHOR]

An experimental work was carried out to investigate the effects of swirl intensity on the flame structure and emission characteristics in a swirl-stabilized premixed oxy-fuel flame for application to oxy-fuel gas turbines. To analyze the flame structure, instantaneous photographs and OH radical chemiluminescence images were collected over various conditions. In addition, the gas emissions (NOx, CO, and O2) have also been measured and analyzed. The results show that the flame structure and the longitudinal distribution of CO concentration inside the combustor are strongly affected by the swirl intensity. At a higher swirl intensity, the flame shapes are short and flat; consequently, the CO dissipates rapidly downstream. In contrast, at a lower swirl intensity, flames are long-drawn and the CO remains longer in the downstream region of the combustor. Among the various swirl number, considered in this study, 0.8 is determined as an optimal value for ensuring flame stability and controlling emissions.

A reverse vortex flow gliding arc discharge in a fuel-rich premixed mixture was applied to a high swirl fuel-lean global combustion to accelerate fuel oxidation. Both the discharge and flame were generated in natural gas and air. To evaluate the role of the gliding arc in the process, a gas analysis of the exhaust gas was performed in the same operational conditions with and without plasma. The chemical measurements show that the plasma reduces carbon monoxide and unburned hydrocarbons contents with a low impact on the NOx level. Furthermore, the comparison of the relative decrease of the hydrocarbon emissions shows that the hydrocarbons have different sensitivities to the plasma application. [ABSTRACT FROM AUTHOR]

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO 2 and H 2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid. • The premixed formic acid-methane-air flames are stabilized for equivalence ratios of 0.5–0.8. • The addition of the formic acid up to mole fraction of 0.6 increases both the flashback and blowout limit. • Increasing the Reynolds number does not affect blow out limits. • The swirl number has a non-monotonous effect on the flashback and lean blowout limits. • The NO and CO emission of the lean formic acid-methane-air flames are low (<20 ppmvd). [ABSTRACT FROM AUTHOR]

• Performed single- and two-stage combustion of ammonia flames. • Decreased optimum primary equivalence ratio with increasing ammonia mixing ratio. • OH radical determines the optimum primary equivalence ratio. Rich-lean two-stage combustion concept was proposed for reducing emissions from ammonia-fueled gas turbine combustors. The primary combustion zone should be maintained at an optimum primary equivalence ratio for the simultaneous reduction of NO and unburned NH 3 ; however, the optimum primary equivalence ratio has been reported only for a limited number of fuel blends and ammonia mixing ratios. This study experimentally and numerically investigated the optimum primary equivalence ratio and studied its application in the rich-lean two-stage combustion of ammonia/methane/air and ammonia/hydrogen/air flames over a wide range of ammonia mixing ratios under atmospheric pressure. This study identifies factors determining the optimum primary equivalence ratio. The emissions were measured using a swirl combustor for single- and two-stage combustion. Unstretched freely propagating premixed flame simulations were performed using CHEMKIN-PRO for investigating the effects of chemical reactions on the optimum primary equivalence ratio. In both single- and two-stage combustion, the optimum primary equivalence ratios of non-premixed ammonia/methane/air and ammonia/hydrogen/air flames decreased linearly and exponentially with increasing ammonia mixing ratios, respectively. The rich-lean two-stage combustion eliminated unburned NH 3 and minimized NO over a wide range of ammonia mixing ratios for non-premixed ammonia/methane/air and ammonia/hydrogen/air flames. The numerical results confirm that OH radical plays an important role in determining the optimum primary equivalence ratio. [ABSTRACT FROM AUTHOR]

This study numerically investigates the combustion characteristics in a swirl burner for CO-H2 syngas by large eddy simulation. Firstly, experimental data are used to verify the applicability of the Boivin mechanism and the Sandia mechanism. The simulation results show that the Boivin mechanism predicts a more reasonable flame structure. Then, Syngas25, Syngas45, and Syngas100 (composed of 25%, 45%, and 100% H2 by volume, respectively) are set to numerically investigate the effect of hydrogen addition ratio on combustion characteristics by using the dynamic Smagorinsky model and the partially stirred reactor model. The detailed simulation data shows that the addition of hydrogen greatly influences the flame structure and flow field; with an increase in hydrogen addition, the flame length shortens, the temperature rises, and flashback caused by combustion induced vortex breakdown occurs. Furthermore, the precessing vortex core cannot exist stably in Syngas100, though it can be clearly observed in other cases.

This study numerically investigates the combustion characteristics in a swirl burner for CO-H2 syngas by large eddy simulation. Firstly, experimental data are used to verify the applicability of the Boivin mechanism and the Sandia mechanism. The simulation results show that the Boivin mechanism predicts a more reasonable flame structure. Then, Syngas25, Syngas45, and Syngas100 (composed of 25%, 45%, and 100% H2 by volume, respectively) are set to numerically investigate the effect of hydrogen addition ratio on combustion characteristics by using the dynamic Smagorinsky model and the partially stirred reactor model. The detailed simulation data shows that the addition of hydrogen greatly influences the flame structure and flow field; with an increase in hydrogen addition, the flame length shortens, the temperature rises, and flashback caused by combustion induced vortex breakdown occurs. Furthermore, the precessing vortex core cannot exist stably in Syngas100, though it can be clearly observed in other cases. [ABSTRACT FROM AUTHOR]

Ammonia, made up of 17.8 % hydrogen, has attracted a lot of attention in combustion community due to its zero carbon emission as a fuel in gas turbines. However, ammonia combustion still faces some challenges including the weak combustion and sharp NOx emissions which discourage its application. It was demonstrated that the combustion intensity of ammonia/air flame can be enhanced through adding active fuels like methane and hydrogen, while the NOx emission issue will emerge in the meantime. This study investigates regulation effect of methane and hydrogen on the emission characteristics of ammonia/air flame in a gas turbine combustor. The instantaneous OH profile and global emissions at the combustion chamber outlet are measured with Planar Laser Induced Fluorescence (PLIF) technique and the Fourier Transform Infrared (FTIR), respectively. The flames are also simulated by large eddy simulation to further reveal physical and chemical processes of the emissions formation. Results show that for NH 3 /air flames, the emissions behavior of the gas turbine combustor is similar to the calculated one-dimensional flames. Moreover, the NOx emissions and the unburned NH 3 can be simultaneously controlled to a proper value at the equivalence ratio (φ) of approximate 1.1. The variation of NO and NO 2 with φ for NH 3 /H 2 /air flames and NH 3 /CH 4 /air flames at blending ratio (Z f) of 0.1 are similar to the NH 3 /air flames, with the peak moving towards rich condition. This indicates that the NH 3 /air flame can be regulated through adding a small amount of active fuels without increasing the NOx emission level. However, when Z f = 0.3, we observe a clear large NOx emission and CO for NH 3 /CH 4 /air flames, indicating H 2 is a better choice on the emission control. The LES results show that NO and OH radicals exhibit a general positive correlation. And the temperature plays a secondary role in promoting NOx formation comparing with CH 4 /air flame. • The regulation effects of methane and hydrogen on NOx emission of ammonia/air flames in a swirl combustor are investigated. • The emission characteristics are verified similar between 1D simulation and the experimental results. • The flame structure detected by OH-PLIF is used to reveal NO emission qualitatively. • The relation of NO and OH is revealed by LES for the NH 3 /CH 4 /air flame. [ABSTRACT FROM AUTHOR]

• A fast responsive and non-invasive temperature measurement method for high-temperature flames was proposed and verified. • Dual-probe femtosecond time-resolved Coherent Anti-Stokes Raman Spectroscopy and a simplified computational model were used to extract the temperature. • The dynamic temperature of unsteady swirl flames was successfully monitored using this method. • The flame temperature can be obtained in real time during the measurement process. We propose a fast responsive and non-invasive optical temperature measurement method which can be used to monitor the temperature of air-fed flames in real time. This method is based on femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS). Two probe pulses with different wavelengths are employed in the CARS system to generate two CARS signals, and the intensity ratio of the two CARS signals has a definite numerical relationship with temperature and therefore can be used to rapidly measure the flame temperature. The temperature of methane/air swirl flames was monitored by the dual-probe fs-CARS thermometry with a sampling rate of 100 Hz. For the steady swirl flame, the measured average temperature in the inner recirculation zone is 1551 K with a standard deviation of 3.7%. While, at a higher methane/air equivalent ratio of 0.74, the flame cannot remain steady, and the temperature frequently jumps from around 1700 K to 2000 K, causing the vortex to expand and break. A significant advantage of the dual-probe fs-CARS thermometry is that the temperature of the flame can be obtained immediately during the measurement without any post-processing. Therefore, this method is a potential candidate for accurately monitoring the real-time temperature of turbulent combustion. [ABSTRACT FROM AUTHOR]

Operating lean-premixed gas turbine burners with fuels containing large amounts of hydrogen severely increases the risk of flame flashback. Enabling fuel-flexible burners, i.e. mitigating that risk for a wide range of hydrogen admixed to natural gas, for instance, is particularly challenging. An improved understanding of the responsible mechanisms and their impact on flashback limits is required to aid the development of such fuel-flexible and yet flashback-resistant burners. Only few detailed and systematic investigations on flame flashback have been conducted at elevated pressure and preheat temperature. In the current experimental study, boundary layer flashback of methane-hydrogen air flames has been investigated in an optically accessible swirl burner installed in a high-pressure test rig. High-speed imaging and laser diagnostics have been applied to characterize the flame propagation pathway and flame shapes. The current findings show that the flame propagation pathway, which determines the relevant flow-flame interaction for flashback, differs compared to previous swirl flame studies. The characteristics of flashback in the current configuration are found to be comparable to flashback in non-swirling boundary layers. Flashback limits have been measured at 2.5 bar and 200∘C preheat temperature to quantify the increase in flashback propensity as the amount of hydrogen is increased from 50% to 100% by volume. Counterflow premixed flame simulations have been performed to investigate the increase in flashback propensity with hydrogen addition. The results suggest that the investigated flames are close to being critically stretched when flashback occurs in the turbulent boundary layer, which allows deriving a Karlovitz number based criterion for an improved prediction of flashback limits. [ABSTRACT FROM AUTHOR]

One of the main concerns regarding ammonia combustion is its tendency to yield high nitric oxide (NO) emissions. Burning ammonia under slightly rich conditions reduces the NO mole fraction to a low level, but the penalties are poor combustion efficiency and unburnt ammonia. As an alternative solution, this paper reports the experimental investigation of premixed swirl flames fueled with ammonia-hydrogen mixtures under very-lean to stoichiometric conditions. A gas analyzer was used to measure the NO mole fraction in the flame and post flame regions, and it was found that low NO emissions (as low as 100 ppm) in the exhaust were achieved under very lean conditions (ϕ ≈ 0.40). Low NO emission was also possible at higher equivalence ratios, e.g. ϕ = 0.65, for very large ammonia fuel fractions (X NH3 > 0.90). 1-D flame simulations were performed to elaborate on experimental findings and clarify the observations of the chemical kinetics. In addition, images of OH* chemiluminescence intensity were captured to identify the flame structure. It was found that, for some conditions, the OH* chemiluminescence intensity can be used as a proxy for the NO mole fraction. A monotonic relationship was discovered between OH* chemiluminescence intensities and NO mole fraction for a wide range of ammonia-hydrogen blends (0.40 < ϕ < 0.90 and 0.25 < X NH3 < 0.90), making it possible to use the low-cost OH* chemiluminescence technique to qualify NO emission of flames fueled with hydrogen-enriched ammonia blends. [ABSTRACT FROM AUTHOR]

Hydrogen is a promising carbon-free fuel for power generation in gas turbines. However, this raises some challenges associated with the storage and distribution of pure hydrogen. Storing hydrogen chemically in the form of ammonia is a safe and efficient alternative. However, ammonia as a fuel features a low chemical reactivity compared to hydrogen and natural gas and, as a consequence, stabilizing turbulent ammonia-air flames is challenging. Offsetting this low reactivity by enriching ammonia with some amount of hydrogen, which is much more reactive, is a promising strategy. In this study, the stability limits of technically-premixed ammonia-hydrogen-air flames are measured in a laboratory-scale swirl combustor for a wide range of ammonia fractions in the ammonia-hydrogen fuel blend. Results are compared to that obtained in the same combustor for reference methane-hydrogen-air mixtures. Data show that increasing the ammonia fraction in the fuel blend promotes lean blowout but reduces the flames' propensity to flashback. The latter effect is even more pronounced if the volume fraction of ammonia in the fuel blend exceeds 0.7. In that case, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blowout occurs instead, yielding a much wider range of stable equivalence ratios. This study also reports exhaust NO mole fractions, measured for large ranges of equivalence ratio and ammonia fraction in the fuel blend. Regardless of the ammonia fraction, data show that competitively low NO emissions occur for slightly rich equivalence ratios of φ ≥ 1.05, which is consistent with earlier studies. Stable flames and good NO performance are also found for very lean ammonia-hydrogen-air mixtures with φ ≤ 0.50, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen blends. • Ammonia-hydrogen-air flames feature wider stability limits than ammonia-air or hydrogen-air flames. • Nitrogen addition to ammonia-hydrogen fuel blends does not significantly alter stability limits. • Pure ammonia-air flames feature lower NO emissions than ammonia-hydrogen-air flames. • Good NO performance is found for "very" lean ammonia-hydrogen-air flames. [ABSTRACT FROM AUTHOR]

The present article investigates the interactions between the pilot and main flames in a novel stratified swirl burner using both experimental and numerical methods. Experiments are conducted in a test rig operating at atmospheric conditions. The system is equipped with the BASIS (Beihang Axial Swirler Independently-Stratified) burner fuelled with premixed methane-air mixtures. To illustrate the interactions between the pilot and main flames, three operating modes are studied, where the burner works with: (i) only the pilot flame, (ii) only the main flame, and (iii) the stratified flame (with both the pilot and main flames). We found that: (1) in the pilot flame mode, the flame changes from V-shape to M-shape when the main stage is switched from closed to supplying a pure air stream. Strong oscillations in the M-shape flame are found due to the dilution of the main air to the pilot methane flame. (2) In the main flame mode, the main flame is lifted off from the burner if the pilot stage is supplied with air. The temperature of the primary recirculation zone drops substantially and the unsteady heat release is intensified. (3) In the stratified flame mode, unique beating oscillations exhibiting dual closely-spaced frequencies in the pressure spectrum is found. This is observed within over narrow window of equivalence ratio combinations between the pilot and main stages. Detailed analysis of the experimental data shows that flame dynamics and thermoacoustic couplings at these two frequencies are similar to those of the unstable pilot flame and the attached main flame cases, respectively. Large Eddy Simulations (LESs) are carried out with OpenFOAM to understand the mechanisms of the time-averaged flame shapes in different operating modes. Finally, a simple acoustic analysis is proposed to understand the acoustic mode nature of the beating oscillations. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the flame-flow interaction during a fully-premixed swirl flame flashback from flame-frame-of-reference. To capture the flame front movement during upstream propagation, high-speed chemiluminescence imaging and simultaneous three-component PIV measurements are taken at 4 kHz. The upstream propagation of the flame occurs along a helical path around the center-body. For low-turbulence and high-swirl conditions (Re h = 4000, Swirl number ∼ 0.9), the lab-frame speed of the flame structure remains nearly constant during the period of investigation. Simultaneously, the leading side of the flame tongue retains its topology during propagation. The steady-state propagation behavior of the flame structure and stationarity of the flame topology allows us to make a frozen-flame-surface assumption. Applying space-time equivalence, the three-dimensional flame surface and flow field are reconstructed by shifting and stacking the time-series of the planar flame front profiles and the three-component planar velocity data. Further, the steady flow in the flame frame-of-reference provides a powerful means of investigating the flame-flow interaction. Quasi-pathlines are constructed in the unburnt and burnt regions of the flow field. The motion of the approach flow along a quasi-pathline is analyzed to understand the role of centrifugal and Coriolis forces. It is shown that the tug-of-war situation between Coriolis and centrifugal forces gets disrupted by the dilatation-driven blockage effect from the flame surface. It leads to a radial deflection of the approach flow, which results in reduction in the flame-normal approach flow speed, thereby assisting in the flame propagation. In the burnt gas, the Coriolis Effect bends the pathlines towards the center-body. We show - for the first time - that the azimuthal motion of the flame assists in the upstream propagation of the flame structure. Error assessment shows that the approximations made to construct the flame-surface and the flow-field retains the physics of flame-flow interactions. [ABSTRACT FROM AUTHOR]

Abstract In an experimental study the effects of varied oxygen concentrations in the oxidizer gas on resulting flow fields, combustion products and general behavior of pulverized coal swirl flames under oxy-fuel conditions have been investigated. Experiments were carried out in a small scale down-fired cylindrical combustion chamber equipped with an annular swirl burner. Studied flames had a constant power output of 40 kW th and O 2 /CO 2 oxidizer gas mixtures with O 2 concentrations ranging from 23 to 33 vol%. Detailed two-dimensional flow field measurements are obtained from laser Doppler anemometry (LDA). Velocity profiles (Mean and RMS) have been obtained for all conditions investigated and serve as basis for identification of flow field characteristics. Velocity RMS values are provided as supplementary material. To complement flow field measurements, in-flame gas composition measurements were also conducted using a sampling probe combined with infrared gas absorption analysis via Fourier-transform infrared (FTIR) spectrometry. The results obtained show increased velocities, particularly along the main vortex for flames with increased oxygen contents, while lower velocities are found to occur inside the recirculation regions. The opposite occurs with lower O 2 concentrations, showing significantly reduced velocities in the main vortex, but stronger recirculation than the high oxygen counterparts. This effect is attributed to a modification of the swirl level introduced by the expansion of product gases. Measured NO and CO in-flame concentrations showed significant variations under different O 2 concentrations in the oxidizer. [ABSTRACT FROM AUTHOR]

The Lagrangian CMC method was implemented in the open source programme OpenFOAM and applied to turbulent nonpremixed bluff body and swirl flames. Lagrangian CMC is more efficient than Eulerian CMC with the number of Lagrangian flame groups much less than the number of computational cells for Eulerian CMC equations in general. It is based on the conditional flame structure depending on the residence time of the fuel of fixed Lagrangian identity from the nozzle. According to sensitivity study the injected fuel was divided into ten flame groups according to the injection sequence with the resulting conditional profiles between those by ISR and Eulerian CMC. Minor deviation from Eulerian CMC was attributed to the flame structure that is difficult to be characterised by the residence time only in elliptic recirculating flows of the bluff body and swirl flames. The Eulerian and Lagrangian CMC showed the same trend of deviation from measurements for conditional temperature, H2O, OH, CO and H2 mass fractions. The significant deviation of H2 was due to uncertainty in the reaction chemistry, as observed in the previous works based on other reaction mechanisms for methane and methanol. [ABSTRACT FROM AUTHOR]

This paper presents experimental investigations in an effusion cooled single sector gas turbine combustor under close-to-reality boundary conditions, i.e. elevated pressure and combustor inlet temperature, under varying staging conditions. Flow field, flame structure and gas-phase temperature measurements are performed using particle image velocimetry (PIV), planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) and coherent anti-Stokes Raman scattering (CARS), respectively. Additionally, isothermal mixing of the pilot and main stage is investigated using Acetone-PLIF. The influence of the pilot on the measured quantities can be identified up to 30 mm downstream of the burner head plate. These measurements are conducted within a novel test rig dedicated to the investigation of swirl-stabilized pressurized flames and effusion-cooling. The rig features full optical access for non-intrusive laser diagnostics from three sides and a modular effusion liner geometry. Important process parameters can be controlled independently in a wide range, providing a high versatility and reliability in terms of boundary conditions. Oxidizer and cooling air mass flows can be conditioned independently to 773 K and 973 K, respectively. Fuel staging can be gradually varied between 0% (fully premixed) and 100% (pilot only) at thermal loads up to 150 kW and a maximum pressure of 1,0 MPa. A movable block radial swirler allows for varying geometrical swirl numbers. [ABSTRACT FROM AUTHOR]

The recirculation zone and the swirl flame behavior can be influenced by the burner exit shape, and few studies have been made into this structure. Large eddy simulation was carried out on 16 cases to distinguish critical geometry factors. The time series of the heat release rate were decomposed using seasonal-trend decomposition procedure to exclude the effect of short physical time. Dynamic mode decomposition (DMD) was performed to separate flame structures. The frequency characteristics extracted from the DMD modes were compared with those from the flame transfer functions. Results show that the flame cases can be categorized into three types, all of which are controlled by a specific geometric parameter. Except one type of flame, they show nonstationary behavior by the Kwiatkowski–Phillips–Schmidt–Shin test. The frequency bands corresponding to the coherent structures are identified. The flame transfer function indicates that the flame can respond to external excitation in the frequency range 100–300 Hz. The DMD modes capture the detailed flame structures. The higher frequency bands can be interpolated as the streamwise vortices and shedding vortices. The DMD modes, which correspond to the bands of flame transfer functions, can be estimated as streamwise vortices at the edges.

This paper addresses the mechanisms through which transverse acoustic oscillations excite unsteady heat release. Forced and self-excited transverse acoustic instability studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that it is not the transverse disturbances themselves, but rather the induced axial acoustic disturbances, that control the bulk of the heat release response. This paper presents results from an experiment that controls the relative amplitudes of transverse and axial disturbances and measures the flow field and heat release response for an acoustically compact, swirling flame. 5 kHz, simultaneous sPIV and OH-PLIF measured the flow field and flame edge, and OH* chemiluminescence measured the relative heat release. Experiments performed with essentially the same transverse acoustic wave field, but with and without axial acoustics, show that significant heat release oscillations are only excited in the former case. The results show that the axial disturbances are the dominant cause of the heat release oscillations. These observations support the theory that the key role of the transverse motions is to act as the “clock” for the instability, setting the frequency of the oscillations while having a negligible direct effect on the actual heat release fluctuations. They also show that transverse instabilities can be damped by either actively canceling the induced axial acoustics in the nozzle (rather than the much larger energy transverse combustor disturbances), or by passively tuning the nozzle impedance to drive an axial acoustic velocity node at the nozzle outlet. [ABSTRACT FROM AUTHOR]

Abstract The development of a new standard for soot emissions proposed by ICAO-CAEP to reduce the environmental impact of civil aviation is moving increasingly research effort on the investigation of sooting flames. Formation and oxidation of the particulate matter are strongly affected by gas temperature, requiring an accurate prediction of the flow field from a numerical point of view. On the other hand, the temperature distribution within the combustor is modified by radiation, which depends on the soot concentration, leading to a very challenging coupled problem. In this work, a series of sensitivity analyses in RANS context are performed on soot, radiation and heat transfer modelling to assess their impact on the prediction of soot emission, gas temperature as well as wall heat fluxes distribution in the context of a high pressure sooting flame which is representative of a RQL combustor. These results are employed to set up a CHT (Conjugate Heat Transfer) simulation, using the multiphysics THERM3D procedure in a loosely-coupled manner where reactive CFD, radiation and heat conduction calculations are computed sequentially with a separate solver in a dedicated framework. These sensitivities can provide useful information for the numerical setup in high-fidelity simulations, as Scale Resolving Simulations. [ABSTRACT FROM AUTHOR]

Abstract The reliable prediction of the turbulent combustion processes in lean flames is nowadays of crucial importance in the design of gas turbine combustors. This work presents an assessment of the capabilities of Flamelet Generated Manifold (FGM) in the framework of Large-Eddy Simulation (LES), as implemented in the commercial CFD solver CONVERGE. One of the main characteristic of the code is the Adaptive Mesh Refinement (AMR) technique, namely the use of a dynamic mesh where elements size varies during the simulation. For validation purposes, the TECFLAM swirl burner, consisting of a strongly swirling, unconfined natural gas flame, has been chosen. Results highlight the advantages of AMR in describing turbulent flames, leading to a successful prediction of the main characteristics of the reacting flow field. [ABSTRACT FROM AUTHOR]

Abstract The accurate prediction of the turbulent combustion process in lean burn flames is of primary importance in the design of gas turbine low-emission combustors. In this framework, the correct account for high strain levels, combined with the heat loss of the flame, by numerical tools is of high technical relevance in order to improve the operational flexibility while reducing emissions. In fact, in high Reynolds lean combustion modelling, the quenching effects due to flame front distortion are expected to govern flame behaviour. The present work presents an assessment of the modelling strategies to introduce the stretch effects on the flame in Flamelet Generated Manifold (FGM) model, in both the framework of Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). At this purpose a premixed swirl burner experimentally studied at Cambridge University was chosen, consisting of a strongly swirling, confined natural gas flame. Results highlight that LES-FGM, coupled with an extended Turbulence Flame Closure model (TFC), succeeds in predicting the main characteristics of the flame at different operating conditions approaching blow-off, thus representing a valid tool to investigate lean burn flames in such context. [ABSTRACT FROM AUTHOR]

Large eddy simulations (LES) of combustion instabilities are often performed with simplified thermal wall boundary conditions, typically adiabatic walls. However, wall temperatures directly affect the gas temperatures and therefore the sound speed field. They also control the flame itself, its stabilization characteristics and its response to acoustic waves, changing the flame transfer functions (FTFs) of many combustion chambers. This paper presents an example of LES of turbulent flames fully coupled to a heat conduction solver providing the temperature in the combustor walls. LES results obtained with the fully coupled approach are compared to experimental data and to LES performed with adiabatic walls for a swirled turbulent methane/air burner installed at Engler-Bunte-Institute, Karlsruhe Institute of Technology and German Aerospace Center (DLR) in Stuttgart. Results show that the fully coupled approach provides reasonable wall temperature estimations and that heat conduction in the combustor walls strongly affects both the mean state and the unstable modes of the combustor. The unstable thermoacoustic mode observed experimentally at 750 Hz is captured accurately by the coupled simulation but not by the adiabatic one, suggesting that coupling LES with heat conduction solvers within combustor walls may be necessary in other configurations in order to capture flame dynamics. [ABSTRACT FROM AUTHOR]

Self-excited, high-frequency first transversal thermoacoustic instabilities in a cylindrical combustion chamber equipped with a premixed swirl-stabilized flame are investigated. Phase-locked image analysis of the phenomena shows the displacement of the flame and a higher burning rate in the region of elevated pressure. The impact of diffuser angle and fuel composition on the stability limits and the flame position is investigated. The Rayleigh-Index is computed for a three-dimensional domain based on analytical flame transfer functions for experimentally obtained data of OH*-chemiluminescence as measure for the spatial heat release. Two models from different sources are applied, which describe the interaction between flame and acoustic locally. The axial dependence of the amplitude of the transversal mode is computed by a numerical model, which takes the temperature distribution inside the combustion chamber into account. The comparison of the Rayleigh-Index of different operation points shows a correlation with the stability limits for some, but not for all investigated configurations. [ABSTRACT FROM AUTHOR]

An investigation into the response of non-premixed swirl-stabilized flames to acoustic excitation and jet in cross-flow (JICF) perturbations was carried out. The fluctuations of OH∗ chemiluminescence emission measured simultaneously with the pressure were used to determine flame describing function (FDF). It could be seen that the three specific conditions exhibited stronger heat release oscillations coupled with pressure fluctuations under acoustic forcing and JICF perturbations, which were expressed by the in-phase delay and positive flame response indices originated from Rayleigh criterion to conduct the stability analysis. Low acoustic excitation amplitude triggered more significant response in the flame heat release rate than high acoustic excitation amplitude at the particular forcing frequency, indicating that nonlinear saturation characteristic of the FDF is also emphasized in JICF. Poincaré map was adopted to analyze the lock-in amplitudes of the dynamics system and JICF effects on combustion instabilities under acoustic forcing frequencies nearby/far from the natural frequency. It was found that lock-in occurred more easily when the forcing frequency was greater than the natural frequency, showing that JICF did not affect the system dynamics due to their little impact on the vortex/flame interactions in this work studied. [ABSTRACT FROM AUTHOR]

Self-excited spinning mode azimuthal instability in an annular combustor with non-swirling flow is investigated using large eddy simulation (LES). Compressible Navier–Stokes equations are solved with a flamelet combustion model to describe the subgrid chemistry-turbulence interactions. Two flamelet models, with and without heat loss effects, are compared to elucidate the non-adiabatic wall effects on the thermoacoustic instability. The azimuthal modes are captured well by both models with only marginal differences in the computed frequencies and amplitudes. By comparing with the experimental measurements, the frequencies given by the LES are approximately 10% higher and the amplitudes are well predicted. Further analysis of the experimental and LES data shows a similar dominant anti-clockwise spinning mode, under which a good agreement is observed for the phase-averaged heat release rate fluctuations. Dynamic mode decomposition (DMD) is applied to shed more light on this spinning mode. The LES and experimental DMD modes reconstructed for their azimuthal mode frequencies agree very well for the heat release fluctuations. The DMD mode structure for the acoustic pressure from the LES shows a considerable non-zero profile at the combustor outlet, which could be essential for azimuthal modes to establish in this annular combustor. Finally, a low-order modelling study was conducted using an acoustic network combined with the flame transfer function extracted from LES. The results show that the dominant mode is associated with the plenum showing a first longitudinal and azimuthal mixed mode structure. By tuning the plenum length to match the effective volume, the predicted frequency becomes very close to the measured value. • A full annular combustor with 12 burners is simulated using compressible LES. • Self-excited thermoacoustic instability is reproduced well in the LES. • Low-order model is combined with LES to reveal physical insights for control. [ABSTRACT FROM AUTHOR]

The transient dynamics of the precessing vortex core (PVC), a globally unstable mode, in an intermittently shape-transitioning swirl flame is investigated by means of simultaneous OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (S-PIV) at a repetition rate of 10 kHz. A time-frequency analysis finds that the PVC is present in steadily lifted flames and at non-reacting conditions, whereas it is suppressed when the flame is steadily attached. While the flame shape spontaneously transitions between these two states, the onset of the growth/decay of the PVC amplitude occurs prior to the flame lift-off/reattachment. The transient formation of the PVC before flame lift-off is quantitatively elucidated by a transient linear stability analysis (TLSA), which well predicts the instability frequency and absolute growth rate. The TLSA reveals that the onset of the flow transition from convective to absolute instabilities, corresponding to the growth of the PVC, is attributed to continuous suppression of the convective perturbation and viscous diffusion, and afterwards the absolute instability is sustained only due to the weakened convection. The suppression of the convection and viscous diffusion terms is mainly due to the changes in the transient mean velocity and density fields within the inner shear layer, which are likely triggered in the first place by unsteady heat loss of the flame to the dump plane of the combustor. [ABSTRACT FROM AUTHOR]

We have performed a detailed analysis of the temperature field in a turbulent swirl flame operating with a self-excited thermo-acoustic instability. The temperature field was measured using 5 kHz chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The measurements are described in detail in the part 1 companion article. In this paper, part 2, a detailed analysis of the time-resolved temperature measurements and simultaneous pressure measurements is performed to provide insight into the dynamics and structure of the swirl-stabilized flame. This work is the first to capture the dynamics of the flame, flow, and coupled flow-flame processes using high-fidelity, spatially- and temporally-resolved thermometry in a flame of practical relevance. The time-averaged contour plot of the temperature field indicates that the flame is very flat and stabilizes approximately 10 mm downstream of the burner face. In this region, there are very significant temperature fluctuations indicating a very high level of unsteadiness. The temperature probability distribution functions (PDFs) are clearly bimodal in this region near the injector face. A Fourier analysis of the temperature time series revealed multiple coherent oscillatory modes. The strongest oscillation was found to be coherent and in-phase with an acoustic resonance at 314 Hz, as expected from the Rayleigh criteria for the unstable flame. An analysis of the phase-conditioned average temperature fields clearly shows an axial pumping of low-temperature reactants, which are consumed after a convective delay and result in a spike in the global heat-release rate. Continued analysis also revealed a 438 Hz oscillation that was found to correspond with the dynamics of convective transport by a helical precessing vortex core (PVC). The structure of the PVC, and its interaction with the flame, were studied based on the presence of this characteristic frequency in the power spectral densities computed throughout the flow. The precision and time-resolution of the CPP fsCARS measurements was also sufficient to enable computation of the integral time-scales as well as the PDFs of the temporal temperature gradients. A sample of state space trajectories were used to provide insight into the nature of coupling between the narrowband acoustic resonance and the broadband spectrum of turbulent flame processes. [ABSTRACT FROM AUTHOR]

In the present work, combustion instabilities of a modular combustor are investigated. The combustor operates with partially premixed, swirl-stabilized flames and can be operated in single- and different multiple-burner setups. The design parameters of the combustor prevent large-scale flame-flame interactions in the multiple-burner arrangements. The objective is to investigate how the interaction of the swirl jets affects the thermoacoustic stability of the combustor. Results of measurements of pressure oscillations and high-speed OH*-chemiluminescence imaging for the single-burner setup and two multiple-burner setups are discussed. Additionally, results of investigations with different flame characteristics are presented. These are achieved by varying the ratio of the mass flow rates through the swirlers of the double-concentric swirl nozzle. Several unstable modes with high pressure amplitudes are observed in the single-burner setup as well as in the multiple-burner setups. Numerical studies of the acoustic behavior of the combustor setups were per- formed that indicate that the different geometries show similar acoustic behaviors. The results lead to the conclusion that the interaction of the swirl jets in the multiple-burner setups affects the thermoacoustic response spectrum of the flame even in the absence of large-scale flame-flame interactions. Based on the findings in earlier studies, it is concluded that the differences in the flame response characteristics are induced by the reduction of the swirl intensity in the multiple-burner arrangements, which is caused by the exchange of momentum between the adjacent swirl jets. [ABSTRACT FROM AUTHOR]

The formation of inhalable fine particles (d p ≤ 10 µm) is an undesirable side effect of solid fuel combustion processes. These particles can accumulate in the human respiratory system and thus cause severe lung damage. Therefore, an understanding of the formation of these particles is of crucial importance to avoid or to reduce the amount of fine particulate matter (PM) released into the atmosphere. Until now, most of the studies have been carried out for laminar and early-stage solid fuel combustion under simplified conditions (e.g. flat flame burner with radial almost homogeneous conditions) and only a few studies have been carried out with more complex turbulent flame structures. As the majority of real applications on an industrial scale involves turbulent conditions, research in this area is highly important. For the investigation of particulate matter formation, a swirled methane-assisted pulverized solid fuel combustion test rig with full optical access which allows intrusive and non-intrusive measurements in different heights and radial positions was developed. In order to study the local influences of flame characteristics on the formation of ultrafine particulate matter, results from laser Doppler velocimetry (LDV), suction pyrometer, Fourier transform infrared (FTIR) spectroscopy, and scanning mobility particle sizer (SMPS) spectrometer measurements are presented and discussed. It is shown that the local differences - such as gas temperature, the velocity of the solid fuel particles, and concentrations species - have an influence on the formation of particulate matter. Especially in regions with high local velocities and shear zones, PM coagulates strongly due to the increased turbulence in this area. The data presented in this paper are an important foundation for further studies to validate CFD simulations using the measurements and to further develop existing PM formation models for turbulent cases. [ABSTRACT FROM AUTHOR]

Topology transitions of swirl-stabilized flames in a cylindrical combustion chamber are analyzed for lean CH 4 /H 2 /air combustible mixtures. It is demonstrated that when the tip of a swirl flame is quenched at the combustor side wall, V to M flame topology transitions are triggered by flashback of the flame front along this wall. An analysis of the power spectral densities of the chemiluminescence luminosity reveals that these topological transitions are not consecutive to the appearance or disappearance of any axisymmetric or helical periodic flow structures interacting with the flame. It is shown that the appropriate space to analyze the stabilization regimes of these swirling flames corresponds to a diagram based on Karlovitz and Lewis numbers. Transitions from V to M flame shapes are found for a critical Karlovitz number below which the flame takes an M shape. The value of this critical Karlovitz number decreases when the Lewis number of the combustible mixture increases. The same type of law is found for M to V shape flame transitions, but this boundary does not coincide with the V to M shape transition boundary. In between these boundaries, the flame takes intermittently V or M shapes randomly. It is then shown that the critical Karlovitz number corresponding to the transition of V to M shape may be expressed by the product of a velocity gradient g calculated with the bulk injection velocity U and a characteristic scale of the outer recirculation zone, the laminar flame thickness α / S L , and the inverse of a vortex induced flame displacement speed S T θ . This displacement speed is related to the maximal azimuthal velocity of the swirling jet at the injector outlet and to the laminar burning velocity. Using this definition of the Karlovitz number, V to M flame topology transitions collapse roughly on the same curve for all flow conditions explored when the swirl number (0.3 ≤ S ≤ 0.68), injection bulk velocity (7 ≤ U ≤ 25 m · s − 1 ), equivalence ratio (0.5 ≤ ϕ ≤ 0.9), and hydrogen enrichment in the fuel blend ( 0 ≤ X H 2 fuel ≤ 0.37 ) are modified for rod stabilized and aerodynamically stabilized swirl flames. Results presented in this study might be used to ease predictions of the shape taken by swirl flames interacting with the chamber side wall when the flow operating conditions are modified. [ABSTRACT FROM AUTHOR]

The understanding of the liquid fuel spray and flow field characteristics inside a combustor is crucial for designing a fuel efficient and low emission device. Characterisation of the flow field of a model gas turbine liquid swirl burner is performed by using a 2-D particle imaging velocimetry(PIV) system. The flow field pattern of an axial flow burner with a fixed swirl intensity is compared under confined and unconfined conditions, i.e., with and without the combustor wall. The effect of temperature on the main swirling air flow is investigated under open and non-reacting conditions. The result shows that axial and radial velocities increase as a result of decreased flow density and increased flow volume. The flow field of the main swirling flow with liquid fuel spray injection is compared to non-spray swirling flow. Introduction of liquid fuel spray changes the swirl air flow field at the burner outlet, where the radial velocity components increase for both open and confined environment. Under reacting condition, the enclosure generates a corner recirculation zone that intensifies the strength of radial velocity. The reverse flow and corner recirculation zone assists in stabilizing the flame by preheating the reactants. The flow field data can be used as validation target for swirl combustion modelling. [ABSTRACT FROM AUTHOR]

The hydrodynamic instabilities in the flow field of a swirl-stabilized combustor are investigated theoretically. These instabilities give rise to large-scale flow structures that interact with the flame front causing unsteady heat release rate fluctuations. The streamwise growth of these coherent structures depends on the receptivity of the shear layer, which can be predicted numerically by means of linear stability analysis. This analysis is applied to the reacting flow field of a perfectly premixed swirl-stabilized combustor that is subjected to strong axial forcing mimicking thermoacoustic oscillations. The linear stability analysis reveals a clear correlation between the shear layer receptivity and the measured amplitude dependent flame transfer function. The stability analysis based on the natural flow predicts the distinctive frequency dependent flame response to low amplitude forcing. At these conditions, the flow reveals strong spatial amplification near the nozzle, causing the inlet perturbations to be significantly amplified before they reach the flame. At higher forcing amplitudes, the flow instabilities saturate, which manifests in a saturation of the flame response. The saturation of the shear layers predicted from the linear stability analysis is compared to phase-locked measurements of the forced flow field revealing good qualitative agreement. The analysis of the mean flow stability offers a powerful analytical tool to investigate the impact of shear flow instabilities on the flame describing function. [ABSTRACT FROM AUTHOR]

• The swirl flame structures of BFG fuel by gliding arc plasma with different equivalence ratios were investigated for the first time. • The experimental result demonstrates that there are six flame structures in the combustion process of blast furnace gas fuel with the decrease of equivalence ratio. • In all cases, the pressure pulsation of blast furnace gas fuel combustion is ± 10 Pa, and there is no obvious dominant frequency of heat release pulsation. • The gliding arc plays the role of anchoring the flame root, to stabilize the combustion of blast furnace gas fuel. As an essential by-product of the iron and steel enterprises, blast furnace gas fuel is used as an alternative fuel for gas turbine combustion to increase economic effect. However, the less combustible components and low heat value of BFG fuel make the combustion in combustor very challenging. In this paper, a gliding arc plasma was employed to assist the blast furnace gas /air combustion with different equivalence ratio. Flame chemiluminescence, the pressure and heat release rate pulsation were recorded simultaneously for showing the flame characteristics of blast furnace gas. The experimental result demonstrates that there are six flame structures in the combustion process of blast furnace gas fuel: A stable flame with short U-shaped (SS-Flame), a vertical oscillating flame (VO-Flame), a stable flame with long U-shaped (SL-Flame), a special gourd-shaped flame (SG-Flame), a lifted flame (L-Flame) and a continuous extinct flame (CE-Flame). According to characteristics of flame structures before flameout, all cases can be divided into three types of extinction modes: I: Direct extinction, II: Oscillation extinction, III: Lifted extinction. The equivalence ratio of extinction limits at the low volume flow rate of air is more likely to be extended by gliding arc plasma than those at the high volume flow rate of air. The pressure and heat release rate pulsation experiment result shows that the violent oscillation of the heat release can be one of the prediction indexes of the flameout of blast furnace gas fuel. In all cases, the pressure pulsation of blast furnace gas fuel combustion is ± 10 Pa, and there is no obvious dominant frequency of heat release pulsation. The amplitude of heat release rate decreases from 0.04 V to 0.01 V with gliding arc plasma addition at φ = 0.7 (E-1 case). Meanwhile, the flame structure of blast furnace gas fuel changes from VO-Flame to SS- Flame due to the gliding arc anchoring the flame root. [ABSTRACT FROM AUTHOR]

• The macrostructure and blow-off characteristics of swirling flame with different confinement rates were studied. • The increase of CR in the confined combustor is helpful to enhance the resistance to stretch-induced extinction. • The ISL vortexes accelerate the flame blow-off near the lean limit. • The downstream ORZ is helpful to stabilize the flame near blow-off with CR = 3. • The transition process from a stable to an unstable flame is extended by increasing the CR of the confined combustor. Blow-off of swirl flames as well as its mechanism are fundamental issues and attract continuous attention due to its wide application in modern combustors. This study experimentally investigated the blow-off and flame macrostructure transition behavior of unconfined and confined premixed CH 4 /air flames stabilized on a swirl and bluff-body burner. PIV and OH-PLIF techniques were used to measure the flow field and instantaneous OH profile, respectively. It was observed that the combustor confinement could significantly broaden the blow-off limits, which was further widened with the increase of confinement ratio (CR) of the confined combustor. The influence of CR on flame stabilization was revealed by analyzing flame structures and flow fields. For the unconfined combustor, when the flames are far away and close to the blow-off, the shear layer and inner recirculation zone (IRZ) play the critical role in stabilizing the flame front, respectively. For the confined combustor, when the flame was far from blow-off, both the shear layer and the outer recirculation zone (ORZ) contributed to the stabilization. With the decrease of equivalence ratio, the stabilization effect of ORZ started to decrease and gradually weakened. When near lean blow-off limit, for CR = 2 (Φ = 0.48), the flame was mainly stabilized by the IRZ, while for CR = 3 (Φ = 0.46), the flame stabilization depended on the upstream IRZ/inner shear layer (ISL) and the downstream IRZ/ISL and ORZ/outer shear layer (OSL). Furthermore, the ISL vortexes would further accelerate the blow-off near the lean limit for confined and unconfined flames. The transition from stable to unstable was extended by increasing the CR in the confined combustor. [ABSTRACT FROM AUTHOR]

A scaled model of a gas turbine (GT) burner with coaxially mounted swirlers has been used to study the effects of fuel staging on the behavior of lean premixed methane air flames. Lean flames are known to be susceptible to instabilities that can lead to unsteady operation, flame extinction, and thermo-acoustic oscillations. High speed (10 kHz) laser and optical diagnostic techniques have been used to investigate the fuel staging effect on the mechanisms involved in such instabilities. Methane air flames at atmospheric pressure have been investigated at a constant thermal power of 58 kW. The global equivalence ratio was kept constant, while the fuel staging was varied. The bulk flow velocity at the exit plane was kept constant at 20 m/s. Simultaneous high speed OH PLIF, OH* CL, and acoustic measurements were performed at kHz repetition rate to characterize the flames and determine the operability limits of the combustor. The characterization measurements reveal significant changes in flame shape for various staging ratios as well as onset of self-excited thermo-acoustics in flames with more than 55% fuel injection in the outer swirler. The phase resolved analysis of the OH* CL revealed pulsation in the heat release due to acoustics in flames with higher percentage of fuel in the outer swirler. Comparison of the pressure oscillation in the combustion chamber with the heat release yielded a clear picture regarding the feedback mechanism that sustains the self-excited thermo-acoustic pulsations. The variation of local equivalence ratio of the mixture seems to be the driving force that initiates the onset of acoustics pulsations. [ABSTRACT FROM PUBLISHER]

This paper utilises large eddy simulation (LES) to study swirling reacting flows by comparison with experimental observations. The purpose is to provide further insights in engineering designs, as well as to improve modelling. A reduced-scale swirl burner has been developed for the experiments. Comparison of particle image velocimetry (PIV) measurements with LES results using finite rate chemistry shows that LES captures all the salient features of an unconfined flame including velocity and temperature distributions. However, when the flame is confined within a cylindrical combustor, the simulated flame shape is initially not consistent with experimental observation. Investigations show that the discrepancy is caused by the often practised assumption of adiabatic wall temperature. With the use of an assumed wall temperature distribution guided by laboratory observation, results of LES are consistent with experiments. Although the latter LES approach requires more computational resources, the improvement is found to be justified. [ABSTRACT FROM PUBLISHER]

The structure and dynamics of swirled, strongly pulsed, turbulent jet diffusion flames were examined experimentally in a co-flow swirl combustor. The dynamics of the large-scale flame structures, including variations in flame dimensions, the degree of turbulent flame puff interaction, and the turbulent flame puff celerity were determined from high-speed imaging of the luminous flame. All of the tests presented here were conducted with a fixed fuel injection velocity at a Reynolds number of 5000. The flame dimensions were generally found to be more impacted by swirl for the cases of longer injection time and faster co-flow flow rate. Flames with swirl exhibited a flame length up to 34% shorter compared to nonswirled flames. Both the turbulent flame puff separation and the flame puff celerity generally decreased when swirl was imposed. The decreased flame length, flame puff separation, and flame puff celerity are consistent with a greater momentum exchange between the flame and the surrounding co-flow, resulting from an increased rate of air entrainment due to swirl. Three scaling relations were developed to account for the impact of the injection time, the volumetric fuel-to-air flow rate ratio, and the jet-on fraction on the visible flame length. [ABSTRACT FROM PUBLISHER]

Abstract: The nonlinear heat release response of a premixed swirl flame to velocity perturbations is investigated using Large Eddy Simulation. The nonlinear heat release response is required for the prediction of thermo-acoustic limit cycle pressure amplitudes and is represented here by the Flame Describing Function (FDF). As test case a premixed atmospheric swirl flame, for which experimental data on the FDF is available, was used. First a steady reacting LES solution was obtained and compared to experimental data. The simulation was then excited by superimposing a mono-frequency harmonic wave on the inlet boundary condition. Both the frequency and amplitude of the acoustic wave were varied to obtain the FDF. The calculated FDF was in good agreement with experimental data. At a frequency of 115Hz, the flame was found to saturate for larger excitation amplitudes. A detailed analysis of the LES results revealed that the mechanism causing the saturation was a nonlinear evolution of the area of the flame surface with increasing perturbation amplitudes. Furthermore it was shown that for the present swirl flame, the heat release and flame surface fluctuations were linearly dependent on the tangential component of velocity fluctuations upstream of the flame, while they increased nonlinearly with the axial component of velocity fluctuations. [Copyright &y& Elsevier]