Search

We present an exploration of two-dimensional flame thermometry and CH2O field measurement in a swirling flame within a two-stage radial stratified swirl combustor, operating under atmospheric pressure conditions. The research details the calibration and analysis of filtered Rayleigh scattering (FRS) signal intensity, employing a planar flame from a Hencken burner to establish a calibration curve for the FRS signal ratio and temperature. The 2-D temperature field results for the swirling flame are then showcased, all obtained using the calibrated FRS system. Parametric analysis focuses on the temperature fields of methane/air mixtures with equivalence ratios ranging from 0.65 to 1.05. The flame shape changes from V-shape to M-shape for a single-swirling flame with an equivalence ratio of 0.85, and that this change in shape is due to a combination of thermal diffusion effects and air-conducting effects. Furthermore, in the dual-swirling flame, the study identifies a transition from mergence to stratification during combustion in both the pilot and main stage flames under the same operating conditions. The article concludes by presenting and comparing the phase-averaged temperature field distributions and instantaneous CH2O distributions during the stratified phenomena of the dual-swirling flame. [ABSTRACT FROM AUTHOR]

The effects of hydrogen enrichment on flame and flow dynamics of a swirl-stabilised partially premixed methane-air flame are studied using large eddy simulation. The sub-grid reaction rate is modelled using unstrained premixed flamelets and a presumed joint probability density function approach. Two cases undergoing thermoacoustic oscillations at ambient conditions are studied. The addition of hydrogen modifies both thermoacoustic and fluid dynamical characteristics. The amplitude of the fundamental thermoacoustic mode increases with the addition of 20% hydrogen by volume. A second pressure mode associated with the chamber mode is also excited with the hydrogen addition. Intermittent single, double and triple helical instabilities are observed in the pure methane case, but are suppressed substantially with hydrogen addition. The results are analysed in detail to shed light on these observations. The feedback loop responsible for the thermoacoustic instability is driven by mixture fraction perturbations resulting from the unequal impedances of the fuel and air channels. It is shown that hydrogen addition increases the flame's sensitivity to these perturbations, resulting in an increase in amplitude. This higher amplitude thermoacoustic oscillation, along with a higher local heat release rate in the presence of hydrogen, is shown to considerably modify the flow structures, leading to a suppression of the helical instabilities. [ABSTRACT FROM AUTHOR]

The aim of this work is to investigate the effect of exhaust gas recirculation (EGR: water vapor and CO2), with and without O2 enrichment, on non-premixed turbulent flames stabilized on a swirl burner. The motivation includes CO2 capture applications using O2 and CO2, combustion of biogas that containing CO2 and the use of EGR or H2O in certain industrial applications to reduce pollutant emissions. Experiments were carried out on a coaxial swirl burner placed in a combustion chamber of 25 kW nominal power. The oxidant (air-O2, + H2O, + CO2) is introduced in the annular part though a swirler. The fuel (CH4) is fed though the central tube and injected radially at the exit section. The study focuses on laminar burning velocity, pollutant emissions, flame stability, and flow fields measurements with different fractions of O2, H2O and CO2 in the mixture. The fraction of diluents is varied from 0 to 20%, O2 concentration from 21 to 25% (in vol.) and the swirl number from 0.8 to 1.4. Different measurements are recorded: OH* chemiluminescence to locate the flame front, Stereo-PIV to analyze the flow field, pollutant emissions analysis (NOx and CO) and temperatures in the combustion chamber. Results show that dilution significantly influences flame characteristics. Dilution increases the lift-off height and reduces flame stability especially with high fractions (16–20%), whereas O2 enrichment decreases lift-off height and enhances flame stability. The increase dilution reduces NOx and increases CO emissions. Stereo-PIV measurements reveals the turbulent coherent structure of the swirling flow as well as the effect of dilution on the corresponding axial and tangential velocities. The effect of dilution on the underlying laminar burning velocity were determined by 1D calculation using COSILAB with GRI3.0 mechanism. [ABSTRACT FROM AUTHOR]

The acoustic characteristic of swirled and non-swirled turbulent inverse diffusion flame (IDF) for hydrogen-methane fuel is experimentally investigated with variation in Reynolds numbers (Re air) and equivalence ratios (Φ). For maintaining adequate working environment, noise characterization of hydrogen-methane fuel IDF is essential. Comparison of sound level indicates significant alteration in acoustic characteristics with change in Re air , Φ and swirl. The swirling IDF is noisier than the non-swirling IDF at the same Re air and Φ. The sound level increases by 9.9% for swirling and 10.2% for non-swirling IDFs with a surge in Re air. The variation of Φ (0.6 ≤ Φ ≤ 1.4) indicates that the noise of IDF with swirl is higher (an increase of 13.14%) than non-swirling IDF (an increase of 1.43%). Shearing of flow induced by swirl dominates sound generation. For swirling IDF, the rise in sound level is 3.67% when methane is blended with H 2 from 0% to 10%. • Effect of Swirl, momentum and hydrogen addition on acoustic of IDF is analysed. • Flow shear due to swirl increases noise levels. • Hydrogen addition enhances combustion dynamics and thus radiates large sounds. • Sound level of swirled IDF with hydrogen enhances by about 4%. [ABSTRACT FROM AUTHOR]

Abstract The mixing and combustion processes under different reference velocities in a gas turbine combustor were numerically investigated using the Flamelet Generated Manifold (FGM) model based on the Reynolds Averaged Navier–Stokes (RANS) method. The flow and combustion fields show strong self-similarity except on the slow auto-ignition in the mixing layer between fuel-rich product and fresh air upstream of the flame stabilization position. The time-scale analysis was carried out to understand the combustion modes inside the combustor. In general, the residence time of the fuel-mixture is much longer than both the chemical time scale and the mixing time scale. Thus, the combustion properties in each sub-zone were dominated by the mean flow structures. Furthermore, the combustion process exhibits a mixing-controlled feature in total. However, partially premixed combustion still appears on the flame base. Most of the fuel was found to be oxidized in the primary zone and the intermediate zone; however, the slow oxidization reactions also play a non-negligible role on the whole combustion process. Finally, a sketch map on the space of mixture fraction and combustion efficiency was proposed to understand the mixing and oxidization experiences of the fuel mixture.

Non-premixed swirl combustion has been widely used in pieces of industrial combustion equipment such as industrial boilers, furnaces, and certain specific gas turbine combustors. In recent years, the combustion instability of non-premixed swirl flames has begun receiving attention, yet there is still a lack of related research in academia. Therefore, in this study, we conducted experimental research on a swirl stabilized gas flame model combustor and studied the heat release response characteristics of the swirl combustor through the flame transfer function. Firstly, the flame transfer function (FTF) was measured under different inlet velocities and equivalence ratios, and the experimental results showed that the FTF gain curve of the non-premixed swirl flame exhibited a significant "bimodal" shape, with the gain peaks located around 230 Hz and 330 Hz, respectively. Secondly, two oscillation modes of the flame near the two gain peaks were identified (the acoustic induced vortex mode Mv and the thermoacoustic oscillation mode Ma), which have not been reported in previous studies on swirl non-premixed flames. In addition, we comprehensively analyzed the flame pulsation characteristics under the two oscillation modes. Finally, the coupling degrees between velocity fluctuations, fuel pressure fluctuations, and heat release fluctuations were analyzed using the Rayleigh Index (RI), and it was found that in the acoustic-induced vortex mode, a complete feedback loop was not formed between the combustor and the fuel pipeline, which was the main reason for the significant difference in the pressure fluctuation amplitude near 230 Hz and 330 Hz. [ABSTRACT FROM AUTHOR]

The mixing and combustion processes under different reference velocities in a gas turbine combustor were numerically investigated using the Flamelet Generated Manifold (FGM) model based on the Reynolds Averaged Navier–Stokes (RANS) method. The flow and combustion fields show strong self-similarity except on the slow auto-ignition in the mixing layer between fuel-rich product and fresh air upstream of the flame stabilization position. The time-scale analysis was carried out to understand the combustion modes inside the combustor. In general, the residence time of the fuel-mixture is much longer than both the chemical time scale and the mixing time scale. Thus, the combustion properties in each sub-zone were dominated by the mean flow structures. Furthermore, the combustion process exhibits a mixing-controlled feature in total. However, partially premixed combustion still appears on the flame base. Most of the fuel was found to be oxidized in the primary zone and the intermediate zone; however, the slow oxidization reactions also play a non-negligible role on the whole combustion process. Finally, a sketch map on the space of mixture fraction and combustion efficiency was proposed to understand the mixing and oxidization experiences of the fuel mixture. [ABSTRACT FROM AUTHOR]

This paper reports results on the effect of oxygen enrichment on flow and turbulent flames in a coaxial swirl burner. The investigation was performed by S-PIV in CH4-air-O2 non-reacting and reacting flows. This work is the continuation of studies carried out previously by the authors on the same burner, with different new operating conditions. The burner consists of 2 concentric tubes, with a swirler located in the annular part through which the oxidant (air-O2) is supplied. The CH4 gas is delivered into the central tube and injected radially into the combustion chamber through small holes. The burner is placed in a parallelepiped combustion chamber with a volume of 120×50×50 cm3. The paper reports detailed results on mean velocity fields, profiles of the three velocity components (U,V,W) and velocity fluctuations (U',V',W') in the case of a 150 Nl/min oxidant flow rate, a swirl number Sn = 1.4 and an equivalence ratio Φ = 1. Three cases of O2 addition were investigated, 21%, 25%, and 30% (in vol.) corresponding to 9.4 kW, 11.7 kW, and 14.5 kW of combustion power, respectively. In-depth analysis of the field velocity differences between reactive and non-reactive cases was carried out and showed a significant difference in flow velocities between reacting and non-reacting flows. The profiles of the mean velocities, fluctuations and velocity decays differed, and higher values were observed in the reacting case. The enrichment of air with oxygen affected the velocity fields and their fluctuations in both distribution and values. The addition of O2 increased the flame temperature, leading to a greater radial expansion and a reduction in the size of the recirculation zone. The maximum axial velocity increased significantly as a function of the O2 rate, whereas the swirling velocity varied only slightly. The velocity fluctuation results also revealed a difference in profiles and values depending on O2 enrichments and locations along the flow. [ABSTRACT FROM AUTHOR]

This study analyzes six threshold approaches for measuring swirling flame properties including flame length, lift-off height, maximum width, area, and flame pulsating displacements in terms of flame center of gravity, length, and width under three operating conditions. Flame video images are captured with the use of a high-speed camera for this objective. The image processing of frames obtained from a high-speed video was fulfilled by utilizing the intermittency distribution approach in order to compare the flame properties quantitatively. The findings show that the Huang technique binarizes the experimental images of the swirling flames the best of the six threshold methods, whereas the Minimum approach produces a large error in flame morphology prediction. After the Huang algorithm, the Yen and Renyi's Entropy-based algorithms do well in forecasting flame morphology, respectively. When compared to the Huang algorithm, the Minimum technique reduces the flame length, maximum width, and area by approximately 34, 26, and 56% for fuel and airflow rates of 0.200 and 3 slpm, respectively. The two approaches of Otsu and Intermodes, on the other hand, yield almost identical flame characteristics. In addition, flame pulsating displacements in terms of center of gravity, length, and width exhibit linear dependency on the fuel flow rate (positive slope), and at a fixed fuel flow rate, pulsating displacement in terms of flame length is larger than pulsating displacement in terms of flame width. In addition, flame pulsating displacement in terms of center of gravity shows the least sensitivity to the fuel flow rate. [ABSTRACT FROM AUTHOR]

Earlier, an experimental and theoretical study of pulsating combustion in a pipe with a tangential supply of a mixture of propane-butane fuel with air was carried out. It was found that the results of calculating the frequencies of gas oscillations differ significantly from the experimental data. One of the reasons is the hypothetical nature of the axial distribution of the speed of sound in the combustion chamber. In this work, this distribution is found on the basis of calculated and experimental data on the gas temperature at the ends of the pipe.

This work investigates numerically a non-premixed swirling flame in a coaxial burner with a radial fuel injection under lean and rich conditions of mixtures. The swirler is placed in the annular part of the burner. The fuel is injected radially through eight holes symmetrically distributed on the periphery of the central tube. All simulations are carried out using the ANSYS-Fluent CFD code. The turbulence is captured using the Reynolds Averaged Navier-Stokes approach. Chemistry/turbulence interaction is resolved using the Eddy Dissipation Model. Simulations are performed with a global equivalence ratio ranging from 0.5 to 1.3. Model validation is achieved by comparing computed results to our experimental data of Stereoscopic Particle Image Velocimetry obtained in the case of the stoichiometric regime. Good agreement between numerical results and experimental measurements is assigned. The central recirculation zone and the swirling jet region due to the presence of the swirl are well predicted by the simulations. The effect of the global equivalence ratio on the profile of axial velocity, temperature distributions and pollutant emissions (CO and NOx) is numerically studied. From dynamic point of view, the equivalence ratio modifies the mean axial velocity of the swirling diffusion flame. The increase of equivalence ratio destabilizes the flame by increasing the liftoff height. NO production decreases by the increasing of the equivalence ratio. [ABSTRACT FROM AUTHOR]

Swirling flame oscillation, with a local extinguishment-and-reignition phenomenon in advanced low-pollution lean premixed combustion technology, remains a challenge in understanding the underlying physics and predict in technical combustors. Here, a prediction method on swirling flame lean blowout (LBO) is proposed from flame image morphological features. In this method, flame features are first extracted by performing morphological algorithms on flame images. Then, the information of the time series of images is included. By designing the blowout state judgment criterion and the blowout state description method, the typical binary judgment is transformed into a numerical prediction. Finally, a random forest regression model is applied to build a predictive model for the swirling flame LBO. The results show that, with the data set from nine operating conditions, the model can achieve a determination coefficient of 0.9766 and a root mean square error of 3.78 on the 10% test set, which shows a strong generalization ability. This method exhibits potential for practical application in LBO control due to its simplicity and efficiency. [ABSTRACT FROM AUTHOR]

This work aims to implement an actuator integrating a swirl burner with electrodes to explore stable and low-emission combustion of ammonia/air. The cavity structure holds a plasma torch driven by low-power microsecond repetitively pulsed discharges (µRPDs) in the nozzle, which serves as a pretreatment for ammonia/air gas before it enters the combustion region. The plasma generates strong excited N2, OH, and atomic H* and O* signals, and fitting N2 spectra indicates a rotational temperature of ∼3000 K and vibrational temperature of ∼4500 K. The flame without plasma is easier to detach from the wall and operate as a cone-shaped structure in the confined quartz tube, while plasma can help attach the flame and extend the lean blow-off limit from ∼0.6 to 0.4∼0.5 in a Reynolds number range of ∼3000 to 7500. Then, optical diagnostics for OH radical and NH2* chemiluminescence are performed to enable analysis of intermediate chemistry. The NH2* signals are distributed at the edge of the OH profile in both attachment and detachment cases. Finally, flue gas analyzers are used to find an optimal lean equivalence ratio where plasma anchors a stable ammonia/air flame with a relatively low emission of approximately 200 ppm NO and zero NH3 and H2.

Non-premixed swirl combustion has been widely used in pieces of industrial combustion equipment such as industrial boilers, furnaces, and certain specific gas turbine combustors. In recent years, the combustion instability of non-premixed swirl flames has begun receiving attention, yet there is still a lack of related research in academia. Therefore, in this study, we conducted experimental research on a swirl stabilized gas flame model combustor and studied the heat release response characteristics of the swirl combustor through the flame transfer function. Firstly, the flame transfer function (FTF) was measured under different inlet velocities and equivalence ratios, and the experimental results showed that the FTF gain curve of the non-premixed swirl flame exhibited a significant “bimodal” shape, with the gain peaks located around 230 Hz and 330 Hz, respectively. Secondly, two oscillation modes of the flame near the two gain peaks were identified (the acoustic induced vortex mode Mv and the thermoacoustic oscillation mode Ma), which have not been reported in previous studies on swirl non-premixed flames. In addition, we comprehensively analyzed the flame pulsation characteristics under the two oscillation modes. Finally, the coupling degrees between velocity fluctuations, fuel pressure fluctuations, and heat release fluctuations were analyzed using the Rayleigh Index (RI), and it was found that in the acoustic-induced vortex mode, a complete feedback loop was not formed between the combustor and the fuel pipeline, which was the main reason for the significant difference in the pressure fluctuation amplitude near 230 Hz and 330 Hz.

Ammonia combustion has received intense research interest recently for its potential to reduce CO2 emission. This study aims to investigate the turbulent combustion characteristics in a bluff-body burner for CH4/NH3 mixtures with different ammonia blending ratios (15%, 30%, and 45% by mole fraction) through large eddy simulation and experiments. The simulations are conducted using openFOAM with a low Mach number solver and the partially stirred reactor combustion model with a detailed reaction mechanism. The flow field of one typical case is measured using the particle image velocimetry technique to verify the accuracy of the numerical results. The combustion characteristics are discussed. As the ammonia blending ratio increases, the flame height shortens, the flame color gradually changes from blue to orange, and the intermittent local quenching zone moves upstream, indicating that the combustion is becoming unstable. Meanwhile, the flow fields exhibit similar characteristics though the ammonia concentration varies greatly. The CO and NO emissions are also discussed. The CO emission decreases and the NO emission increases as the ammonia blending ratio increases. [ABSTRACT FROM AUTHOR]

Heavy fuel oil has an energy density content comparable to distillate fuels but a very high viscosity that necessitates extra heating before spray combustion inside a boiler. Heavy fuel oil is also characterized by high asphaltenes, carbon residues, trace metals, such as vanadium and nickel, fuel-bound nitrogen, and sulfur. Asphaltenes are heavy polycyclic aromatic compounds with embedded heteroatoms and significantly affect the physico-chemical properties of heavy fuels; this makes them very difficult to burn and leads to the formation of large cenospheres (lightweight, inert, hollow spheres), due to an inefficient burning process. The primary goal of this study is to experimentally investigate the influence of the asphaltene concentration on the combustion of HFO under a swirling flame, finally reducing gaseous and solid pollution. We tested HFO samples containing asphaltene concentrations of 4, 6, 8, 16, and 24 wt.%, prepared by blending the light oil of diesel and pure asphaltenes with HFO. This work provides quantitative information about the effect of different asphaltene contents on the fuel properties of viscosity, density, heating value, thermogravimetry with air and N2, and swirling flame combustion characteristics, including the analysis of gaseous and solid particle emissions. The results indicated that the high asphaltene content in the fuel was the critical factor for the high viscosity and incomplete combustion and also increased the density of the fuel sample. Reducing the asphaltene content in HFO improves its spray characteristics and combustion performance and reduces the solid emissions containing sulfur and metal elements. [ABSTRACT FROM AUTHOR]

Swirling flame oscillation, with a local extinguishment-and-reignition phenomenon in advanced low-pollution lean premixed combustion technology, remains a challenge in understanding the underlying physics and predict in technical combustors. Here, a prediction method on swirling flame lean blowout (LBO) is proposed from flame image morphological features. In this method, flame features are first extracted by performing morphological algorithms on flame images. Then, the information of the time series of images is included. By designing the blowout state judgment criterion and the blowout state description method, the typical binary judgment is transformed into a numerical prediction. Finally, a random forest regression model is applied to build a predictive model for the swirling flame LBO. The results show that, with the data set from nine operating conditions, the model can achieve a determination coefficient of 0.9766 and a root mean square error of 3.78 on the 10% test set, which shows a strong generalization ability. This method exhibits potential for practical application in LBO control due to its simplicity and efficiency.

The three-dimensional turbulent swirling flame in an internally-staged combustor is numerically investigated. Four cases over a range of swirl intensities are explored by the Flamelet Generated Manifold model in this paper. Special attention is paid to analyzing the variation of the flow field, temperature, major species concentrations and emissions. The results clearly show the effects of swirl number on the size of the center recirculation zone, fuel distribution and combustion characteristics. When the third premixed stage swirl number increases from 0.6 to 1.2, the axial length of the center recirculation zone decreases by 3.7%, while the radial length increases by 6.9%. The characteristics of the flow field play an important role in the spatial distribution of the fuel, which further affects the temperature distribution in the combustor. The backflow effect is enhanced, resulting in a greater concentration of fuel at the outlet of the swirler. After the maximum temperature is reached at the exit position of the pilot stage, the temperature decreases compared to the peak temperature downstream as the proportion of premixed combustion mode increases. This creates a high concentration region of OH at the outlet of the pilot stage injector and the heat release region is squeezed upstream. At the same time, the volume of the high-temperature region downstream of the pilot stage is reduced. In addition, as the swirl number of the third premixed stage increases from 0.6 to 1.2, the emissions of NO and CO decrease by 28.7% and 75%, respectively. [ABSTRACT FROM AUTHOR]

Open and confined dimethyl ether/air swirling flames were investigated with an acoustically excited swirling burner. Simultaneous particle-image velocimetry and formaldehyde planar laser-induced fluorescence were measured using a high repetition rate burst-mode laser at 20 kHz. Time-averaged velocity field, vorticity and flame brush cloud were compared under different confinements and acoustic excitation conditions. The integrated fluorescence intensities are similar while the open flame has dramatically large fluorescence areas. Time-resolved velocity field and flame brush were measured to investigate the continuous evolution of swirling flame front and acoustically induced vortex. The flame brush was found to be much dispersed in the open flame due to the mixing effect from the surrounding air. Flame root and flame angle were extracted and analyzed statistically using the fluorescence images. Different stages of acoustically induced vortex were observed in both confined and open cases. The vortex trajectory along the outer shear layer determines that the flame tip can perceive the vortex instead of the flame base. The flame regions affected and unaffected by acoustically induced vortex indicate different perturbation mechanisms for the heat release. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study of a swirling flame generated through the interaction between a central fire and its surrounding fires is performed. A swirling flame can be configured by installing a secondary’s sources surrounding the central source, organized in an asymmetrical manner in order to ensure circumferential entrainment of the central flame by the supply puffs air. An analysis is performed to study this kind of flow. This analysis highlights the different zones that characterize the vertical propagation of a swirling flame; a first zone, close to the fire source, characterized by an acceleration of the flow and an increase of the temperature. A second zone marked by the passage of the temperature by a maximum while changing variation with a net decrease of the flow acceleration and a third zone where the thermal and dynamic fields change and gradually decrease. Moreover, this study shows an axisymmetric flow behavior with two different aspects of its global structure. A central region characterized by a block motion (solid core) where the flow is rotational, characterized by a concentration of vorticity, and surrounded by the rest of the space where the flow is irrotational. Moving vertically away from the active source, results show an attenuation of the axial vortex intensity which is accompanied by a disappearance of movement by block. The centerline evolution of the axial and azimuthal momentum flux enables also to highlight these different aspects of the global flow structure.

In this study, experimental measurements are performed on a swirl-stabilized combustor to study the nonlinear combustor dynamic characteristics under acoustic excitations. The imposed acoustic disturbances with different frequencies and amplitudes generated by a loudspeaker are applied to the combustion system. The acoustic pressure oscillations and heat release rate fluctuations are measured using acoustic pressure transducers and a photomultiplier tube equipped with a bandpass OH* filter, respectively. Various parametric studies are conducted to evaluate the effects of the equivalent ratio ϕ , amplitude Af, and frequency ff of the acoustic excitations on the combustor dynamics. The obtained results indicate that the combustor experiences frequency locking-in phenomenon accompanied by high amplitude acoustic oscillations when ff is close to the natural frequency fa of the combustor quarter-wave mode. Such physical phenomena are observed even for a low forcing wave amplitude. However, heat release rate fluctuations reach saturation at high amplitude excitations. Moreover, the combustor experiences super-harmonic resonances when ff is close to fa/2 or fa/3. Under such conditions, high amplitude excitations are required to stimulate the natural resonant mode, while the heat release rate fluctuations rarely reach saturation. Cross-correlation and cross-power spectral density analyses on the OH* chemiluminescence and acoustic pressure signals reveal that super-harmonic resonances are caused by "quasi-couplings" between the acoustic pressure and unsteady heat release. Further, the results demonstrate that the excited natural mode oscillations affect the acoustic wave characteristics at the inlet section, while they hardly influence the heat release rate fluctuations. The obtained results indicate that the equivalence ratio has a negligible effect on the combustor dynamics under forced conditions. Finally, discussions are provided to address the influences of the excited natural mode oscillations on the experimental measurements of the flame describing function (FDF). The achieved acoustic velocity disturbances at the injector outlet are suppressed by excited natural mode oscillations, which confine the velocity disturbance amplitude within a narrow range when ff matches fa. Hence, excited natural mode oscillations should be avoided in practice if we need to measure the FDF under a wide range of acoustic velocity amplitudes. The nonlinear characteristics of forced oscillations studied in this paper are crucial to the development of control strategy and FDF measurement. [ABSTRACT FROM AUTHOR]

Heavy fuel oil has an energy density content comparable to distillate fuels but a very high viscosity that necessitates extra heating before spray combustion inside a boiler. Heavy fuel oil is also characterized by high asphaltenes, carbon residues, trace metals, such as vanadium and nickel, fuel-bound nitrogen, and sulfur. Asphaltenes are heavy polycyclic aromatic compounds with embedded heteroatoms and significantly affect the physico-chemical properties of heavy fuels; this makes them very difficult to burn and leads to the formation of large cenospheres (lightweight, inert, hollow spheres), due to an inefficient burning process. The primary goal of this study is to experimentally investigate the influence of the asphaltene concentration on the combustion of HFO under a swirling flame, finally reducing gaseous and solid pollution. We tested HFO samples containing asphaltene concentrations of 4, 6, 8, 16, and 24 wt.%, prepared by blending the light oil of diesel and pure asphaltenes with HFO. This work provides quantitative information about the effect of different asphaltene contents on the fuel properties of viscosity, density, heating value, thermogravimetry with air and N2, and swirling flame combustion characteristics, including the analysis of gaseous and solid particle emissions. The results indicated that the high asphaltene content in the fuel was the critical factor for the high viscosity and incomplete combustion and also increased the density of the fuel sample. Reducing the asphaltene content in HFO improves its spray characteristics and combustion performance and reduces the solid emissions containing sulfur and metal elements.

This paper presents experimental results of the effects of CO2 and H2O dilution on CH4-air flames enriched by oxygen. The study focuses on pollutant emissions, flame stability through the determination of liftoff heights and flow fields by LDV measurements. Different parameters of the burner are studied such as the swirl number, the global equivalence ratio, and the fractions of O2, H2O, and CO2 in the mixture. The fraction of diluents varies from 0% to 20%, O2 enrichment from 21% to 30% (in vol.), and the swirl number from 0.8 to 1.4. The experiments are carried out on a coaxial swirled burner placed in a vertical combustion chamber. The flame is visualized by the chemiluminescence technique on OH* in order to locate the flame front. Measurements of combustion products such as NOx, CO, and CO2 are done using a HORIBA PG250 multi-gas analyzer. Flow fields are measured by LDV measurement for the longitudinal velocity component and its fluctuations. Results show that the CO2 and H2O dilution influences significantly the flame characteristics. With dilution the liftoff height increases but the flame remains stabilized. With O2 enrichment the liftoff height decreases and flame stability is enhanced. If we therefore want to dilute more, we should enrich with oxygen. The increase of dilution rate induces a decrease in NOx emission and exhaust gas temperature and an increase in CO emissions. LDV measurements showed the longitudinal velocity distribution of flow and its fluctuations in reactive and non-reactive conditions. Note that the presence of the flame induces an increase in the longitudinal velocity downstream of the flow due to the expansion of gases. [ABSTRACT FROM AUTHOR]

Summary: This paper is focusing on the numerical simulation of a swirling flame, resulting from the interaction of multiple fires, evolving in a free and unlimited environment. A typical system, formed by a central fire source surrounded by four heat sources, is used. Since the thermal characteristic of the surrounding sources is the main engine for the rotation of flame, a detailed study is performed by varying the heating flux of these sources. This study shows that an increase of the heating flux of surrounding sources has as a result an intensification of the penetrating air puffs through the openings between the surrounding four heat sources. These puffs tangentially drive the central flame, thus producing a marked improvement on the angular momentum. Moreover, this study shows that the flame height is strongly affected by the flame rotation. Moreover, two different aspects of the flame height evolution are observed from the flow visualization and the thermal and dynamic fields for the different cases studied. [ABSTRACT FROM AUTHOR]

The present paper compares the flow structure and flame dynamics during combustion of methane and syngas in a model gas-turbine swirl burner. The burner is based on a design by Turbomeca. The fuel is supplied through injection holes between the swirler blades to provide well-premixed combustion, or fed as a central jet from the swirler’s centerbody to increase flame stability via a pilot flame. The measurements of flow structure and flame front are performed by using the stereo particle image velocimetry and OH planar laser-induced fluorescence methods. The measurements are performed for the atmospheric pressure without preheating and for 2 atm with the air preheated up to 500 K. The flow Reynolds numbers for the non-reacting flows at these two conditions are 1.5 × 103 and 1.0 × 103, respectively. The flame dynamics are analyzed based on a high-speed OH* chemiluminescence imaging. It is found that the flame dynamics at elevated conditions are related with frequent events of flame lift-off and global extinction, followed by re-ignition. The analysis of flow structure via the proper orthogonal decomposition reveals the presence of two different types of coherent flow fluctuations, namely, longitudinal and transverse instability modes. The same procedure is applied to the chemiluminescence images for visualization of bulk movement of the flame front and similar spatial structures are observed. Thus, the longitudinal and transverse instability modes are found in all cases, but for the syngas at the elevated pressure and temperature the longitudinal mode is related to strong thermoacoustic fluctuations. Therefore, the present study demonstrates that a lean syngas flame can become unstable at elevated pressure and temperature conditions due to a greater flame propagation speed, which results in periodic events of flame flash-back, extinction and re-ignition. The reported data is also useful for the validation of numerical simulation codes for syngas flames.

This work investigates the dynamics of periodically-excited vortices in swirling cold flow and hot flame using high-speed particle image velocimetry, for three different Strouhal numbers, 0.68, 1.02, and 1.70. The periodic upstream perturbation induces coherent vortices in the shear layers both inside and outside of the flame surface. We find that the outer vortex rings (OVRs) play a dominant role in tuning the flame dynamics and heat release, whereas the inner vortex rings (IVRs) are related to the formation of the center recirculation zone (CRZ) and suffer from much stronger dissipation. The evolutions of core vorticity, circulation, trajectory, and convective velocity are quantitatively analyzed for the OVRs to understand two key mechanisms: vortex formation and vortex detachment. The results show that the growing and shedding of the OVRs are governed by the inlet shear layer as well as the variations in inlet velocity and acceleration. Furthermore, both the circulation increment during vortex formation and the vortex convection after detachment are qualitatively captured by existing vortex models, demonstrating the prospect of vortex dynamics in understanding general flame-vortex interaction problems. [ABSTRACT FROM AUTHOR]

Gain and phase are two key characteristics of flame transfer function (FTF) in evaluating unsteady flame response. Few studies have investigated the correlation between gain and phase of FTF compared to the extensive literature on the gain characteristics. In this study, we measured the gain and phase of acoustic-excited swirling flames with different flow rates, fuels, equivalence ratios, and burner structures on a single nozzle premixed swirl burner. We identified, for the first time, the consistent variation of FTF gain and derivative of phase over a wide range of acoustic frequencies, and revealed an essential linear relationship between the extremal frequencies of gain and phase derivative, demonstrating that the gain and phase of FTF are not independent. We further proposed an analytical decomposition of the periodically oscillated swirling flame in which FTF equals the combined complex vector of all the perturbing mechanisms. The synchronized variation characteristics of the gain and phase derivative for different flames and acoustic excitations can be explained using a two vector model based on vector decomposition. The distinct time lags and angular velocities of the perturbing vectors were determined by the different spatial positions of the flame tip and base. The local extrema of the FTF modulus and phase derivative are caused by the phase interference of the sub-vectors. Multiple complex vectors affecting heat release rate should be considered under high frequency. [ABSTRACT FROM AUTHOR]

A methodology is developed where the acoustic response of a swirl stabilized flame is obtained from a reduced set of simulations. Building upon previous analytical flame transfer functions, a parametrization of the flame response is first proposed, based on six independent physical parameters: a Strouhal number, the mean flame angle with respect to the main flow direction, the vortical structures convection speed, a swirl intensity parameter, a time delay between acoustic and vortical perturbations, as well as a phase shift between bulk and local velocity signals. It is then shown how these parameters can be deduced from steady and unsteady simulations. The methodology is applied to a laboratory scale premixed swirl stabilized flame exhibiting features representative of real aero-engines. In this matter, cold and reactive flow Large Eddy Simulations are first validated by comparing results with reference data from experiments. The high fidelity simulations are seen to be able to capture the flame structure and velocity profiles at different locations while forced flame dynamics for the frequency range of interest also match the experimental data. From the same analytical transfer function model, three methodologies of increasing complexity are presented for the determination of the model parameters, depending on the available data or computational resources. A first estimation of the flame acoustic response is obtained by evaluating parameters from a single stationary flame simulation in conjunction with analytical estimations for the acoustic-convective time delay. Flame dynamics and swirl related parameters can then be determined from a series of robust treatments on pulsed simulations data to improve the model accuracy. It is shown that good qualitative agreement for the flame transfer function can be obtained from a single non-forced simulation while quantitative agreement over the frequency range of interest can be obtained using additional reactive or non-reactive pulsed simulations at one single forcing frequency corresponding to a local gain minimum. The method also naturally handles different perturbation levels. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study of a swirling flame generated through the interaction between a central fire and its surrounding fires is performed. A swirling flame can be configured by installing a secondary's sources surrounding the central source, organized in an asymmetrical manner in order to ensure circumferential entrainment of the central flame by the supply puffs air. An analysis is performed to study this kind of flow. This analysis highlights the different zones that characterize the vertical propagation of a swirling flame; a first zone, close to the fire source, characterized by an acceleration of the flow and an increase of the temperature. A second zone marked by the passage of the temperature by a maximum while changing variation with a net decrease of the flow acceleration and a third zone where the thermal and dynamic fields change and gradually decrease. Moreover, this study shows an axisymmetric flow behavior with two different aspects of its global structure. A central region characterized by a block motion (solid core) where the flow is rotational, characterized by a concentration of vorticity, and surrounded by the rest of the space where the flow is irrotational. Moving vertically away from the active source, results show an attenuation of the axial vortex intensity which is accompanied by a disappearance of movement by block. The centerline evolution of the axial and azimuthal momentum flux enables also to highlight these different aspects of the global flow structure. [ABSTRACT FROM AUTHOR]

Effects of H 2 -enrichment on structures of CH 4 /air turbulent swirling premixed flames affected by high intensity turbulence in a gas turbine model combsutor are investigated by conducting direct numerical simulations. Two stoichiometric mixture conditions, of which volume ratio of CH 4 :H 2 = 50:50 and 80:20, are simulated by considering a reduced chemistry (25 species and 111 reactions). Results showed qualitatively different flame shapes and reaction zone characteristics between the cases. For the higher H 2 -ratio case, the flame is stabilized both in the inner and outer shear layers. For the lower H 2 -ratio case, the flame is stabilized only in the inner shear layer and extinction occurs in the outer shear layer. Comparison of the reaction zone characteristics with unstrained and strained laminar flames in phase space showed that H 2 mass fraction for the lower H 2 -ratio case and reaction rate profiles for both cases deviate from the corresponding laminar values. Analysis of fuel species conservation equation suggests that the turbulent transports are substantially influential to determine local and global flame structures. These findings would be useful for designing practical H 2 -enriched gas turbine combustor in the aspect of flame structures under high intensity turbulence. Image 1 • Effects of mixture conditions on CH 4 /H 2 /Air turbulent swirling premixed flames are investigated. • Qualitatively different global flame features between the mixture conditions are observed. • The role of H 2 in each mixture condition remains unchanged even under intense turbulence. • Reaction zone characteristics deviate from unstrained and strained laminar flame profiles. • Contributions by convection and diffusion are influential on the local and global flame structures. [ABSTRACT FROM AUTHOR]

The main objective of this study is to clarify effects of flame straining on flame structures and heat release rate (HRR) of swirling flames. This is achieved by analyzing results of direct numerical simulations (DNS) of hydrogen—air turbulent swirling premixed flames considering two swirl number and two equivalence ratio cases. Statistical characteristics of HRR are investigated by examining the mean HRR conditioned on a reaction progress variable and the total HRR in the computational domain. Conditional means of the HRR show that the magnitude of the HRR in reaction zones is smaller for higher swirl number cases than that for lower swirl number cases. A direct comparison between strained laminar and swirling flames shows the influence of the strain rate on the flame structure and the progress of elementary reactions. As strain rate increases in a laminar flame, the peak of the HRR by an exothermic reaction H2 + OH → H2O + H shifts toward the burnt side, implying active production of H in the burnt side. The HRR of the above reaction also shows an increasing tendency in a laminar flame under the strain rates greater than 106 s− 1. The strain–flame interaction with this tendency affects the HRR on highly strained flame surfaces of the swirling flames. It is also clarified that the local HRR intensity is dominated not only by strain rate but also by diffusion of H from the burnt to unburnt side. [ABSTRACT FROM AUTHOR]

This paper reports an experimental and numerical investigation of a methane-air diffusion flame stabilized over a swirler coaxial burner. The burner configuration consists of two tubes with a swirler placed in the annular part. The passage of the oxidant is ensured by the annular tube; however, the fuel is injected by the central jet through eight holes across the oxidizer flow. The experiments were conducted in a combustion chamber of 25 kW power and 48 × 48 × 100 cm3 dimensions. Numerical flow fields were compared with stereoscopic particle image velocimetry (stereo-PIV) fields for non-reacting and reacting cases. The turbulence was captured using the Reynolds averaged Navier-Stokes (RANS) approach, associated with the eddy dissipation combustion model (EDM) to resolve the turbulence/chemistry interaction. The simulations were performed using the Fluent CFD (Computational Fluid Dynamic) code. Comparison of the computed results and the experimental data showed that the RANS results were capable of predicting the swirling flow. The effect of the inlet velocity ratio on dynamic flow behavior, temperature distribution, species mass fraction and the pollutant emission were numerically studied. The results showed that the radial injection of fuel induces a partial premixing between reactants, which affects the flame behavior, in particular the flame stabilization. The increase in the velocity ratio (Rv) improves the turbulence and subsequently ameliorates the mixing. CO emissions caused by the temperature variation are also decreased due to the improvement of the inlet velocity ratio.

We report on a study of the impact of coherent helical vortex structures on the shape of the reaction zone and heat release in swirling methane/air flames in regimes with a vortex breakdown. Three kinds of atmospheric flames are considered, viz., fuel-lean and fuel-rich premixed flames and a partially premixed fuel-rich lifted flame. Based on the measurements of the velocity fields by a stereo PIV in combination with the OH PLIF and HCHO PLIF, the impact of the coherent flow structures on large-scale corrugations of the reaction zone is evaluated. Helical vortex structures, detected in both the non-reacting and reacting high-swirl flows by using proper orthogonal decomposition, are found to promote combustion both in the lean premixed and fuel-rich partially premixed flames. In the first case, based on the phase-averaged intensity of the HCHO×OH signal and the location of the helical vortex structure in the inner mixing layer, it is concluded that the vortex locally increases the heat release rate by enlarging the flame front and enhancing the mass exchange between the combustion products inside the recirculation zone and the fresh gases. The events of the local flame extinctions are detected in the instantaneous PLIF snapshots for the lean mixture, but they do not cause extinction of the entire flame or a blow-off. In case of the lifted flame, the outer helical vortex structure promotes combustion by locally intensifying the mass exchange between the fuel-rich jet with the surrounding air. [ABSTRACT FROM AUTHOR]

• Turbulent non-premixed swirling flames with oxygen enrichment. • Stereo-PIV measurements for different planes. • Velocity fields, turbulence intensity, swirl number and entrainment rates for reacting and non reacting flows. • Effects of swirl number and global equivalence ration. • Evolution of NOx emissions throughout gas entrainment rate measurements. This paper presents an investigation of the dynamic characteristics of non-premixed turbulent swirling flames using the stereo-PIV technique. To finely characterize the flow field, experiments are performed in the longitudinal and transverse planes in non-reacting and reacting conditions. The burner configuration studied in this work is a coaxial swirl stabilized burner used in the turbulent regime. The swirler is placed in an annular part supplying the oxidant flow and the central pipe delivers radially a fuel through eight holes symmetrically distributed on the periphery of the tube. The experiments are conducted using a 25 kW parallelepiped combustion chamber, 1 m high and 0.5 m wide, using circulating water as coolant. In the reacting case, the CH 4 -air flame is oxygen enriched from 21% to 30% in volume. Effects of the swirl number (0.8–1.4) and the global equivalence ratio (0.8–1) on the flows topology and structure are analyzed. Results of the present work concern the 3D mean velocity fields, the turbulence intensities, the swirl number measurements and the entrainment rate along the flow. SPIV results show the three-dimensional aspect of the flow including the recirculation zone and the annular zone with swirling jet effects. The comparison between reacting and non-reacting flows shows that the presence of the flame induces a larger radial flow expansion, higher mean velocities and higher turbulence intensities. The results show a bifurcation of the tangential velocity towards the jet central axis by crossing the flame front. It is also shown that the fluctuations of the tangential velocity are mainly responsible for the highest turbulence kinetic energy levels at the top of the central recirculation zone. The entrainment rate of the surrounding gases is measured by stereo-PIV as a function of the swirl number and equivalence ratio. The results explain one possible mechanism for the decrease of the NO x emissions when the global equivalence ratio increases via an increase of the entrainment rate at the flame base. [ABSTRACT FROM AUTHOR]

The light-round process in a transparent annular combustor which comprises 16 swirling injectors injecting lean premixed propane/air mixtures is experimentally investigated. This annular combustor is derived from the 'MICCA' combustor, pioneered by EM2C laboratory (Bourgouin et al., Combust. Flame 160(8), 1398, 2013).The annular chamber is formed by two transparent concentric quartz tubes, which provides optical access to high-speed imaging to diagnose the chemiluminescence of flame fronts. Two ignition modes (i.e. two extreme conditions in scheduling fuel delivery and igniter sparking), controlled by ignition procedure are investigated, thus the FFSL (Fuel First, Spark Later) and the SFFL (Spark First, Fuel Later) modes. These two ignition modes exhibit different patterns of injector-to-injector flame propagation during the light-round process. The light-around time and mean circumferential flame propagation speed are obtained for various conditions. The influence of thermal expansion, velocity fluctuation and convection to the light-round process is investigated and compared for two ignition modes. [ABSTRACT FROM AUTHOR]

Carbon particles (Soot) have been the one of primary pollutants inevitably with the combustion of fossil fuels. A better understanding on incipient soot was useful to build the models of soot formation and even control soot emissions. The present work focuses on the nanostructure and oxidation reactivity of nascent carbon particles (soot) formed from 2,5-dimethylfuran (DMF)/n-heptane non-swirling and swirling inverse diffusion flames (IDFs). The nascent soot samples were derived from three different fuels: 100% n-heptane, 50% n-heptane/50% DMF and 100% DMF. In addition, the effects of swirling combustion and collection time on characteristics of nascent soot were investigated in detail using the high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that nascent soot from pure n-heptane flames presented the film-like morphology and nanostructure of amorphous nature, while the nearly primary particles with more well-organized nanostructure were found in pure DMF soot. Swirling combustion increased the entire flame front areas and diameters due to the existence of tangential velocity at the outlet of burner. Moreover, swirling combustion could enhance the mixability of the oxidizer stream and fuel stream, leading to more young soot within the fuel stream being oxidized in the inception stage. However, the collection time on soot characteristics exhibited quite negligible impacts in comparison to the swirling effects. [ABSTRACT FROM AUTHOR]

• Effects of acoustic excitation on CH 4 /NH 3 /air swirling flame were studied. • The excited flame was visualized by the high-speed schlieren imaging system. • Effects of excitation frequency and intensity on CO and NOx emissions were studied. The blending characteristics of ammonia with conventional fuels have recently received much attention from researchers. This paper investigated the effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame on a laboratory-scale swirl burner. The high-speed schlieren imaging system was used to visualize the flame flow under acoustic excitation. The emission characteristics of CO and NOx at different excitation frequencies (70–270 Hz) and sound pressure levels (SPL, 117–128 dB) were determined. Results show that adding ammonia causes a significant increase in NOx emission and an increase in CO emission under fuel-rich conditions. The flame shows a layered V-shape with periodic development under weak low-frequency (70 Hz) acoustic excitation and an inverted cone with step distribution under high-frequency (270 Hz) acoustic excitation. Low-frequency acoustic excitation can reduce CO and NOx emissions under fuel-lean conditions. Especially for the excitation condition of 120 Hz (P rel = 0.3), NOx and CO emissions are reduced by 203 ppmvd and 170 ppmvd, respectively. However, when the excitation is stronger, the CO emission will rise sharply due to the destruction of the flame structure. Besides, the more intense acoustic excitation will cause a decrease in CO emission and an increase in NOx emission under fuel-rich conditions. [ABSTRACT FROM AUTHOR]

We model the boundary-layer flashback (BLF) of CH 4 / H 2 /air swirling flames via large-eddy simulations with the flame-surface-density method (LES-FSD), in particular, at high pressures. A local displacement speed model tabulating the stretched flame speed is employed to account for the thermo-diffusive effects, flame surface curvature, and heat loss in LES-FSD. The LES-FSD well captures the propagation characteristics during the BLF of swirling flames. In the LES-FSD for lean CH 4 / H 2 /air flames at 2.5 bar, the critical equivalence ratio for flashback decreases with the increasing hydrogen volume fraction, consistent with the experiments. This is due to the improved modeling of effects of the flame stretch and heat loss on the local displacement speed. We also develop a simple model to predict the BLF limits of swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 14 datasets of CH 4 / H 2 /air swirling flame experiments, with the hydrogen volume fractions in fuel from 50% to 100%. The present model well estimates the flashback limits in various operating conditions. [ABSTRACT FROM AUTHOR]

The response of a lean premixed flame subjected to acoustic perturbations is a complex phenomenon that depends highly on the type of flame and the operating conditions. Swirl introduces additional complexities due to the azimuthal component of the flow. In this work, a bluff body stabilised burner is studied under non-swirling and highly swirling conditions by placing a removable axial swirl upstream of the burner. The influence of swirl is assessed in terms of the flame describing function which is the ratio of heat release rate fluctuations response to incoming velocity oscillations and the spatial flame dynamics at high forcing amplitudes. The effect of flame interaction with the wall on the flame response is also explored by considering an enclosure with a larger diameter. It is found that swirl can affect the non-linear characteristics of the flame at medium frequencies (Strouhal numbers around unity) by altering the flame roll-up mechanisms. This is related to the variation of the local swirl number in space and time. For Strouhal numbers that are considerably lower than unity, the effect of swirl is small due to the high convective wavelengths. The size of the enclosure can also change the flame response characteristics, specifically for large forcing frequencies. With a small enclosure, where the flame interacts with the wall, the flame break-up is more significant and the vortex formation is interrupted. This does not happen when the enclosure is enlarged and it can affect the non-linear behaviour of the flame. • Swirl can alter the flame roll-up mechanisms affecting at Strouhal numbers around 1. • Swirl has a small effect on the Flame Describing Function at low Strouhal numbers. • A small enclosure affects the flame response by interrupting the vortex formation. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to monitor the dynamic cross-sectional behavior of swirling flames using the tunable diode laser absorption spectroscopy (TDLAS) tomographic system. The newly developed online and highly spatially resolved imaging system based on TDLAS tomography was employed to monitor and reveal directly the reaction process in the swirling flame by reconstructing the 2-D distributions of temperature and H2O concentration over a cross section of the flame. The system was demonstrated to be capable of capturing the temperature distribution accurately and inferring the thermal expansion over the cross section of interest in the swirling flame generated by a model swirl injector operating in partially premixed combustion mode. As the equivalence ratio was decreased, thermal oscillations extracted from the real-time 2-D reconstructions were used to infer the instability of the swirling flame. Furthermore, the developed system was applied to capture dynamically the process of blowout of the swirling flame, illustrating that the system can provide firsthand and reliable visual data to help prevent the flame from lean blowout (LBO). This paper reports the first experimental observation of the dynamic cross-sectional behavior of the swirling flame, enabled by the high spatial and temporal resolutions provided by the TDLAS tomographic imaging system. The developed system can help better understand the LBO mechanism so as to improve the performance of low-emission gas turbine combustors. [ABSTRACT FROM AUTHOR]

Investigation results on unsteady flow dynamics in a gaseous jet flame with strong swirl, vortex breakdown, and precession of a vortex core obtained by panoramic optical methods are presented, as well as the results of theoretical analysis of the fastest growing modes of hydrodynamic instability. Characteristics of the most unstable self-oscillating mode in the initial region of the turbulent strongly swirling propane-air jet burning in the atmospheric air in the form of a lifted flame are determined. Analysis of data by principal component analysis and linear stability analysis revealed that evolution of the dominant self-oscillating mode corresponds to quasi-solid rotation with constant angular velocity of the spatial coherent structure consisting of a jet spiral vortex core and two spiral secondary vortices. [ABSTRACT FROM AUTHOR]

Effects of oxygen enrichment and CO2 dilution on the characteristics of non-premixed methane-air turbulent flames in a coaxial swirl burner are investigated in this article. The work primarily focuses on pollutant emissions (NOx and CO), flame structure, and stability investigations. The experiments are conducted using a 1-m high, 0.5-m wide, 25-kW parallelepiped combustion chamber, cooled by outside water. The burner configuration consists of two concentric tubes in which a swirler is placed inside the annular part for air or air-O2-CO2 supply that can allow for the rotation of the oxidant. Fuel is injected radially from the central tube. OH* chemiluminescence measurements are performed to describe the structure and stability of the flame providing information on the flame lengths and lift-off heights. The measurements are conducted with the oxygen concentrations ranging from 21% to 35%, CO2 concentrations ranging from 0 to 20%, swirl numbers ranging from 0.8 to 1.4, and global equivalence ratios ranging from 0.8 to 1. The lift-off heights, the fluctuations of the flame base, and the flame lengths are determined as a function of these parameters. The results show that oxygen enrichment stabilizes better the flame, promotes the formation of NOX and CO2, and decreases the formation of CO. The dilution by CO2 changes significantly the flame shape and its behavior. The flame becomes longer, less intense, and unstable. CO2 dilution reduces considerably the flame temperature which obviously reduces the NOx formation, but it is observed that CO2 and CO concentrations in the flue gases are increased. [ABSTRACT FROM AUTHOR]

Revamp of air powered industrial boilers to oxy-combustion operation raises several issues. In this study, the stabilization of CO 2 -diluted premixed swirling CH 4 /O 2 flames is compared to operation with CH 4 /air flames by a set of experiments in a generic labscale combustor equipped with an axial-plus-tangential swirler. The investigated flames are stabilized aerodynamically within the swirling flow without help of any solid anchoring device. The structure of the turbulent swirling flames is examined by recording their OH ∗ chemiluminescence. Laser induced OH fluorescence measurements are carried out to delineate the location of the flame front and burnt gases and infer the shape taken by the flame. Particle imaging velocimetry measurements reveal the corresponding velocity field. The temperature is also recorded with thermocouples in the internal and the external recirculation zones of the flow and inside the combustion chamber walls. These diagnostics reveal similarities between the topology of CH 4 /air and CH 4 /O 2 /CO 2 flames near their stabilization point. For a fixed swirl number, it is found that N 2 - and CO 2 -diluted CH 4 /O 2 flames at the same equivalence ratio feature very similar shapes provided the adiabatic flame temperature and the ratio of bulk velocity to laminar burning velocity are kept the same although the absolute value of bulk velocity and the general velocity level inside the combustor are different. This result was found for injection Reynolds numbers varying from Re = 8500–28 000 and for swirl numbers ranging from S 0 = 0.5 to 1.2. The operability range of well stabilized CO 2 -diluted flames is however reduced. It is also found that the temperature of the burnt gases in the outer recirculation zone differs between the CO 2 - and N 2 diluted combustible mixtures sharing the same adiabatic temperature, but this does not affect the stabilization of the flames near the burner outlet, the temperature in the burnt gases and inside the combustor sidewalls. The flame leading edge is stabilized on average off-axis due to the structure of the W axial velocity profile produced by the axial-plus-tangential swirler at the burner outlet. This study indicates that CO 2 -diluted CH 4 /O 2 premixed swirling flames can be stabilized with similar shapes as CH 4 /air flames without design modification provided the suggested similarity is obeyed. [ABSTRACT FROM AUTHOR]