OH Chemiluminescence Imaging Research Articles

Development Of An Endoscopic, Absorption Corrected And Calibrated OH Laser Induced Fluorescence Probe System for High Pressure Combustion Diagnostics

We present the design and preliminary results of an endoscopic OH laser-induced fluorescence (LIF) system that is capable of acquiring 1D-line measurements in high pressure combustion processes within large-scale test facilities. The system can be exploited to acquire line-of-sight OH-chemiluminescence images and absorption corrected absolute OH-species concentration from which additionally temperature distribution can be extracted in case of lean mixtures in chemical equilibrium. Different critical components, such as light guides for high power UV laser pulses and flexible UV image guides, were first tested and characterized in a laboratory setup. Optimization regarding pulse energy and beam quality led to the selection of an 1m long hollow core fiber for laser light delivery as a central element for the experimental setup despite high losses caused by bending the fiber. The image transfer is accomplished by a 7m long flexible UV image bundle, which is preferred to the use of a rigid borescope in harsh environments. We designed the endoscopic UV optics using readily available parts and achieved an approximated image resolution of 21.6 line pairs/mm. Additionally, post-processing methods, such as flat-field correction, can improve the image quality substantially and remove the pixilation effect of the fiber bundle structure nearly completely. To validate the complete system, probe-based qualitative OH-LIF measurements were performed in a well-known laminar flame of a matrix burner, which serves as a mock-up for anticipated applications in an industrial test environment with a technology readiness level (TRL) greater than 4. The final step consists of the integration of the miniaturized optics into a cooled probe system which protects the sensitive optics against the harsh environment of an aero-engine sector combustor. For this purpose, two probe systems were designed to include, on the one hand, the endoscope optics together with the fiber image bundle end tip and, on the other hand, the laser emitting and receiving system for performing an absorption measurement needed to quantify the recorded LIF signal.

Characteristics of turbulent flames in a confined and pressurised jet-in-hot-coflow combustor

Combustion in hot and low oxygen environments—such as those encountered in practical devices including inter-turbine burners and sequential gas turbines—is not yet fully understood at a fundamental level, particularly in terms of the effects of pressure. To meet this gap in understanding, a confined-and-pressurised jet-in-hot-coflow (CP-JHC) combustor has been developed to facilitate optical diagnostics of turbulent flames in hot and vitiated coflows for the studies of flame stabilisation, structure and soot formation at elevated pressures. The CP-JHC burner has been designed for steady operation at 10 bar with internal temperatures of up to 1975 K, with a water-cooled central jet issuing into a hot oxidant stream of combustion products from a non-premixed natural gas/H2 burner. This work describes the key features and operational capabilities of the CP-JHC burner and presents a selection of experimental results showing characteristics not previously available. Specifically, temperature measurements of the hot coflow are used to estimate the enthalpy deficit of the stream, revealing an increase in thermal efficiency with increasing heat input, and a decrease with increasing pressure. Chemiluminescence imaging of OH∗ and CH∗ is performed for turbulent jet flames to study the flame structure under various operating conditions, and true-colour imaging results are also included to highlight the change in soot formation under elevated pressures. The mean images indicate a change in stabilisation behaviour with changes in pressure and jet Reynolds number (Rejet), which is further investigated by a statistical analysis of the short-exposure CH∗ images. This analysis reveals that an increase in Rejet from 10,000 to 15,000 leads to an increase in the mean lift-off height (from the jet exit plane) from approximately 1.5 to 6 jet diameters at atmospheric pressure, while the flames at elevated pressures show significantly less variation and tend to stabilise at the jet exit for P > 3.5 bar(a). The experimental findings are complemented by numerical simulations of laminar opposed flow flames, providing additional insights into the fundamental chemical kinetics effects which influence these flames. In particular, a monotonic reduction in both the maximum and integrated OH∗ and CH∗ mass fractions is observed with increasing pressure. This reduction is particularly pronounced at lower pressures, with a reduction to 10% of the atmospheric-pressure value at 3 bar(a) for the integrated OH∗ mass fraction. Additionally, this behaviour is shown to be related to the combined effects of a shift in the formation pathways and the increased impact of collisional quenching.

Ignition Sensitivity Analysis for Energy-Assisted Compression-Ignition Operation on Jet Fuels with Varying Cetane Number

<div class="section abstract"><div class="htmlview paragraph">Local deposition of thermal energy can be used to assist the combustion process of low cetane number (CN) fuels in compression-ignition engines, here termed energy-assisted compression ignition (EACI). In the current work, a commercial ceramic glow plug, operated beyond its conventional operation range, was used as the ignition assistant (IA) and sensitivity of fuel jet ignition to operation parameters was studied for two fuels using EACI in an optical engine. A design-of-experiments (DoE) study was devised to determine which engine parameters influenced the energy-assisted pilot injection ignition process the most. The DoE was constructed with four parameters: injection pressure, injected mass, injection timing, and ignition assistant temperature. The fuels used were F24 (Jet-A with military additives) with a cetane number of 48 and a cetane number 35 fuel mixture consisting of 60% F24 and 40% of an alcohol-to-jet fuel (ATJ), blended on a volumetric basis. Simultaneous OH chemiluminescence and schlieren imaging were employed along with in-cylinder pressure measurements to assess ignition delay, jet development, and the combustion process. A Gaussian Kriging emulator was utilized to perform the sensitivity analysis. The results indicate that ignition of the fuel jet directed toward the ignition assistant is mainly driven by the ignition assistant temperature, with around 90% of the sensitivity for both fuels ascribed to ignition assistant temperature. However, for ignition assistant surface temperatures greater than ~1350 K ignition delay is approximately constant with increasing temperature. Ignition delay from the time the fuel reaches the ignition assistant to start of combustion, here termed hot surface ignition delay, was analyzed to gain a better understanding of the EACI ignition process. The hot surface ignition delay for ignition assistant temperatures <i>></i>1350 K ranged from approximately 200 to 500 <i>μ</i>s, independent of fuel cetane number. This indicates that ignition for ignition assistant surface temperatures <i>></i>1350 K is likely the result of high-temperature ignition chemistry.</div></div>

Laser diagnostic investigation of a confined premixed turbulent jet flame stabilized by recirculation

A preheated premixed CH4/air jet flame was operated in an optically accessible rectangular combustion chamber in order to study the flame stabilization and flame expansion mechanisms. Due to an off-center position of the jet, a pronounced lateral recirculation zone had formed in the combustion chamber which stabilized the lifted flame by mixing burned and unburned gases in the shear layer. Simultaneous single shot measurements of the flow field by particle image velocimetry (PIV), of OH by planar laser induced fluorescence (PLIF) and of heat release by OH chemiluminescence imaging from two directions were performed at a frame rate of 5 kHz. The image sequences provided a detailed insight into the interaction between the flow field and the flame, particularly into the effects of the vortices that developed in the shear layer between the jet and the recirculation zone. Special interest was devoted to the role of autoignition and turbulent flame propagation near the flame base.

Plasma Effects on Swirl Flames in a Scaled Dry Low Emission Burner

The effect of a rotating gliding arc (RGA) plasma discharge on the flame in a downscaled Siemens Energy dry low emission, SGT-750, burner was experimentally investigated under atmospheric combustion conditions. The central pilot section of the burner, named RPL (rich-pilot-lean), was redesigned with an integrated high-voltage electrode to generate an RGA. The exhaust gas was sampled and analyzed in terms of CO and emissions, and the CO emission data show that the RGA extends the lean blowout limit. High-speed OH chemiluminescence imaging was employed to understand the transient behavior of the flame in both conditions, with and without RGA, and also to study the process of flame restabilization by the assistance of the RGA. A flame kernel, initiated around the RGA channel, was observed to play an important role in the restabilizing process of the flame. Although the emission for the flame with RGA was found to be higher than that without RGA, it was still less than what previous data show for operating conditions with the RPL flame.

Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

Methane and hydrogen-enriched (25 vol% and 50 vol% H2-enriched CH4) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H2 concentrations. The effects of equivalence ratio (Φ), Re, and H2 enrichment on flame structure were examined employing OH-PLIF measurement. For CH4/air cases, the flame was stabilized with an M shape; while for H2-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H2 enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH4/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH4 flames with M shape, whereas the BLF occurs for all H2-enriched flames with П shape.

A comparison of the blow-out behavior of turbulent premixed ammonia/hydrogen/nitrogen-air and methane–air flames

Ammonia has been identified as a promising energy carrier that produces zero carbon dioxide emissions when used as a fuel in gas turbines. Although the combustion properties of pure ammonia are poorly suited for firing of gas turbine combustors, blends of ammonia, hydrogen, and nitrogen can be optimized to exhibit premixed, unstretched laminar flame properties very similar to those of methane. There is limited data available on the turbulent combustion characteristics of such blends and important uncertainties exist related to their blow-out behavior. The present work reports experimental measurements of the blow-out limits in an axisymmetric unconfined bluff-body stabilized burner geometry of NH3/H2/N2-air flame, comprised of 40% NH3, 45% H2, and 15% N2 by volume in the “fuel” blend. Blow-out limits for the NH3/H2/N2-air flames are compared to those of methane–air flames. OH PLIF and OH chemiluminescence images of the flames just prior to blow-out are presented. Furthermore, two large-scale Direct Numerical Simulations (DNS) of temporally evolving turbulent premixed jet flames are performed to investigate differences in the turbulence-chemistry interaction and extinction behavior between the NH3/H2/N2-air and methane–air mixtures. The experiments reveal that the blow-out velocity of NH3/H2/N2-air flames is an order of magnitude higher than that of methane–air flames characterized by nearly identical unstretched laminar flame speed, thermal thickness and adiabatic flame temperature. Results from the DNS support the experimental observation and clearly illustrate that a methane–air mixture exhibits a stronger tendency towards extinction compared to the NH3/H2/N2-air blend for identical strain rates. Furthermore, the DNS results reveal that, even in the presence of intense sheared turbulence, fast hydrogen diffusion into the spatially distributed preheat layers of the fragmented and highly turbulent flame front plays a crucial role in the enhancement of the local heat release rate and, ultimately, in preventing the occurrence of extinction.

Apparatus to investigate liquid oxygen droplet combustion in hydrogen under microgravity conditions.

To investigate the fundamentals of liquid oxygen droplet combustion in hydrogen under microgravity conditions, a drop tower apparatus has been developed. In a cryogenic combustion chamber cooled with liquid nitrogen, single oxygen droplets with a diameter of 1 mm are suspended at the tip of a thin suspender. Ignition is accomplished after microgravity conditions are reached by a near-infrared laser, which generates a plasma spark positioned in the immediate vicinity of the oxygen droplet. The subsequent combustion is investigated with various optical diagnostics. Shadowgraph imaging is used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame are determined by OH chemiluminescence imaging. The oxygen/hydrogen combustion at two different pressure levels is reported. At a pressure of 0.1 MPa, irregular structures appear to form on the droplet surface during the combustion process, which lasts 137 ms before the droplet is completely consumed. The formation of these irregular structures is consistent with the condensation and freezing of water vapor in the cold-temperature region near the droplet surface. At a higher pressure of 4 MPa (reduced pressure pr,O2 =0.79), the water ice formation is significantly reduced due to the shorter burning time of 66 ms and the closer proximity of the flame to the droplet surface. The measured burning constant is k0.1 = 5.5 mm2/s at 0.1 MPa and k4 = 7.1 mm2/s at 4 MPa, respectively. This increase with increasing pressure is consistent with the smaller flame standoff distance.

Resolving flame thickness using high-speed chemiluminescence imaging of OH* and CH* in spherically expanding methane–air flames

The coupling of CFD simulations with detailed chemical kinetics presents great progress in predicting the complex behavior of reacting flows, but also requires validated input parameters in the form of experimental data. The spatial profile of a combustion wave represents one such parameter, which can be directly measured using chemiluminescence imaging of a spherically expanding flame. In this work, emission signals from electronically excited methylidyne (CH*) and hydroxyl (OH*) radicals near 434 nm and 315 nm, respectively, from spherically expanding methane–air flames at 1 atm and 298 K were recorded for equivalence ratios of 0.8, 1.0, and 1.2. Spatial profiles of normalized intensity were compared to predicted profiles from AramcoMech2.0. The effect of image resolution was investigated by repeating experiments for three levels of image pixel density. An Abel inversion was employed to extract intensity profiles of CH* and OH* at flame radii up to 6.5 cm. Measured flame thickness increased as flames grew in size, but this behavior diminished as image resolution increased. A linear stretch correlation was used to extrapolate measured thicknesses to an unstretched thickness for each experimental condition. Radical-based flame thicknesses and corresponding spatial profiles were found to be highly dependent on image resolution, and at high resolution, measured flame thickness appeared to approach the kinetically predicted radical-based thicknesses. This paper lays the foundation for future, comprehensive measurements of spherical, laminar flames that can resolve the flame zone details to a level of precision not typically seen in the literature, providing benchmark data for both kinetics model validation and CFD model inputs. As a result, the measurements thus far indicate that the measured flame zone thickness based on electronically excited species is much closer to the length scale typically predicted by kinetics models than what has been seen in most experiments to date.

Experimental characterization of flame/flow dynamics during transition between stable and thermo-acoustically unstable conditions in a gas turbine model combustor

This paper presents an experimental study of the flame/flow dynamics in a single-element, swirl-stabilized, lean premixed pre-vaporized gas turbine model combustor under spontaneously intermittent transition between stable and thermo-acoustically unstable conditions. Three regimes, corresponding to stable, transitional and thermo-acoustically unstable were considered. High-speed OH⁎ chemiluminescence (CL) imaging and planar particle image velocimetry (PIV) measurements revealed that the transition process was associated with periodic interactions between vortices and inner shear layers (ISLs), leading to large-scale back and forth propagation and reaction intensity fluctuations of the flame. Proper Orthogonal Decomposition (POD) analysis indicated that the transition featured the presence of precessing vortex core (PVC), which contributed to the helical motion of the flame. Additionally, it was also shown that the flame appeared in outer recirculation (ORZ) zone most frequently during stable state as compared to both transitional and unstable cases. Such phenomenon further suggested that the flame in the ORZ could only serve as a necessary but insufficient condition for the transition from stable to thermo-acoustically unstable states, depending on the geometry and thermal state of the combustor.

Experimental study on flame/flow dynamics in a multi-nozzle gas turbine model combustor under thermo-acoustically unstable condition with different swirler configurations

This paper presents an experimental study on the combustion stability characteristics and flame/flow dynamics in a multi-nozzle, lean premixed prevaporized (LPP), swirl-stabilized gas turbine model combustor with different swirler configurations in the presence of self-excited combustion instability. The flame structure was characterized using high-speed OH⁎ chemiluminescence imaging and the flow field across the centerline of three interacting flames was measured by high-speed planar Particle Image Velocimetry (PIV). Two sets of the swirler configurations were considered in this paper, featuring different combinations of swirl rotational directions. The first one consisted of three co-rotating swirlers, whereas the central swirler in the second configuration was replaced by a swirler with counter-rotation direction. These two configurations were termed as COS and CNS combustors respectively in the rest of the paper. It was found that these two combustors exhibited similar stable and unstable operating domains in terms of different equivalence ratios and inlet air velocities. At the same test condition, the amplitude of dominant instability of COS combustor was 130 dB, which was stronger than that of CNS combustor (120 dB). Phase-averaged PIV measurements showed that both COS and CNS combustor featured three recirculation zones downstream the swirlers, and high axial velocity was present after the merging of the adjacent flames. These two flow structures varied periodically, but in different manners for COS and CNS combustors. Phase-averaged OH⁎ chemiluminescence images indicated that the flame was primarily anchored in recirculation zones close to the swirler and most of the heat release was found to occur in flame interaction regions, where large-scale reaction intensity variations occurred. Furthermore, a greater phase delay between heat release rate and acoustic pressure was observed in CNS combustor, which contributed to a weaker instability comparing with that in the COS combustor.

Effects of fuel variation and inlet air temperature on combustion stability in a gas turbine model combustor

In this study, influences of fuel variation and inlet air temperature on the combustion stability characteristics in a gas turbine model combustor were experimentally investigated. Test fuels involved three selected single component hydrocarbons, including one linear alkane (n-decane), one branched alkane (iso-octane) and one cyclic alkane (methylcyclohexane (MCH)). RP-3 jet fuel was also selected as a technical reference. For all the fuels, experiments were conducted at a fixed equivalence ratio of 0.86 and varying inlet air temperatures from 383 to 483 K. Results showed that RP-3 and n-decane exhibited similar stability behaviors. At low inlet air temperatures, when the combustor was fueled with RP-3 and n-decane, the flame was stabilized and anchored in the combustor. The combustor then shifted to thermo-acoustically unstable state when the inlet air temperature exceeded a threshold value, which was associated by large-scale flame shape variations. On the other hand, noticeable differences can be observed for MCH and iso-octane flames, which featured thermo-acoustically unstable combustion throughout all the tested conditions, and unique mode-shift phenomenon was observed when the inlet air temperature was raised from 403 K to 423 K. Additional flame dynamics was visualized by OH⁎ chemiluminescence imaging. The underlying mechanisms that led to the differences in combustion stability and flame dynamics of the tested fuels were discussed with respect to their differences in physicochemical properties.

Application of image enhancement and mixture of Gaussian approach in combustion research

Chemiluminescence is one of the most commonly used optical diagnostic techniques in combustion research where a line-of-sight projected information is generated from spatial fields. The exactness and uniqueness of reconstruction along with ease of implementation gives Abel inversion an edge over the other existing single-view reconstruction techniques for efficient estimation of spatial field from line-of-sight projections. Though there exist many such algorithms, the primary focus of these has been to ensure tractable inversion through a systematic regularization by imposing a smoothness constraint on discrete data points. But these techniques do not have the provision to process the input image prior to deconvolution in order to prevent accumulation of noise infiltrated during data acquisition. Another major limitation of these algorithms is to adopt the changes in characteristics of the input data points while maintaining optimal storage and time complexity. To address these issues, we have proposed a new image processing technique using standard Abel inversion for the application in combustion research. It provides a suitable model to ensure regularized inversion by imposing a smoothness constraint on acquired raw data. The new algorithm has been implemented to yield the physically significant chemiluminescence emission from hydroxyl radicals in flames from line-of-sight integrated images. The effectiveness of this algorithm is highlighted using exemplary OH chemiluminescence images captured from a standard swirl stabilized research burner.

High-frequency transition characteristics of synthetic natural gas combustion in gas turbine

AbstractIn this study, the combustion instability and emission characteristics of flames of different H2/CH4compositions were investigated in a partially premixed model gas turbine combustor. A mode shift in the frequency of instability occurred under varying experimental conditions from the first to the seventh mode of longitudinal frequency in the combustor, and a parametric study was conducted to determine the reasons for this shift by using the length of the combustor, a factor that determines the mode frequency of longitudinal instability, as the main parameter. Furthermore, heat load and fuel composition (H2ratio) were considered as parameters to compare the phenomenon under different conditions. The GRI-3.0 CANTERA code, OH chemiluminescence and the Abel inversion process were applied to analyse the frequency mode shift. NOxemissions, which occurred through the thermal NOxmechanism, increased with increasing heat load and H2ratio. The instability frequency shifted from the first to the seventh mode as the H2ratio increased in the H2/CH4mixture. However, 100% H2as fuel did not cause combustion instability because it has a higher burning velocity and extinction stretch rate than CH4. Furthermore, the laminar flame speed influenced the frequency mode shift. These phenomena were confirmed by the flame shapes. The Abel inversion process was applied to obtain the cross section of the flames from averaged OH chemiluminescence images. Stable and unstable flames were identified from the radial profile of OH concentration. The combustor length was found to not influence frequency mode shift, whereas the H2ratio significantly influenced it as well as the flame shape. The results of this experimental study can help in the reliable operation of gas turbine systems in SNG plants.

Ignition of a sequential combustor: Evidence of flame propagation in the autoignitable mixture

Ignition of the second stage in a lab-scale sequential combustor is investigated experimentally. A fuel mixing section between jet-in-cross-flow injection and the second stage chamber allows the fuel and vitiated, hot cross-flow to partially mix upstream of the main heat release zone. The focus of the present work is on the transient ignition process leading to a stable flame in the second stage. High-speed OH-PLIF as well as OH chemiluminescence imaging is applied to obtain complementary planar and line-of-sight integrated information on the ignition. We find experimental evidence for the co-existence of two regimes dominating the chamber ignition, i.e. autoignition and flame propagation. As the mass flow of the dilution air injected downstream of the first stage is increased (i.e. mixing temperatures in the fuel mixing section are decreased), we transition from an autoignition to a flame propagation dominated regime. Hysteresis in the ignition behavior is observed indicating that the first stage in a sequential combustor may be operated at leaner conditions than required for ignition of the second stage. The time traces of integral heat release obtained simultaneously with a photomultiplier tube show distinct features depending on the dominating regime, which is important for high-pressure testing with limited optical access.

Local supersonic and subsonic combustion mode transition in a supersonic jet flame

An experimental and computational study has been carried out for a supersonic jet flame by using OH chemiluminescence imaging, shadowgraph visualization, temperature measurement by TDLAS, pressure measurement by transducers, and large eddy simulation (LES) together with a skeletal reaction mechanism involving 13 species and 41 steps. Agreements have been found between experimental data and LES results, which are subsequently used to analyze the flow, mixing, combustion, and heat release processes involved. A systematic method is adopted to qualitatively as well as quantitatively investigate different combustion modes and their contributions to heat release in the combustor. Influences of airstream temperature and pressure on combustion mode and heat release are also discussed by comparing four different cases. Results show that the heat is released from a combination of supersonic combustion mode and subsonic combustion mode even when the main flow is at supersonic speed. Local mode transition occurs as the jet flame propagates and interacts with shocks that enhance mixing because of baroclinic effects and induce subsonic combustion due to deceleration effects. It is also observed that subsonic combustion releases more than 50% of heat at the base of the jet flame because of recirculation zones behind the strut. Supersonic combustion mode gradually becomes prominent in the turbulent far field with small values of heat release rate. The overall dominant combustion mode is dependent on not only inflow conditions but also combustion intensity.

Effect of after injections on late cycle soot oxidation in a small-bore diesel engine

After injection, for which a short fuel injection follows the main injection, is proved to be effective in reducing soot emissions in diesel engines. The present study aims to better understand this in-cylinder soot reduction mechanism of after injection with a particular emphasis on its efficacy in a small-bore diesel engine in which more significant flame-wall interactions could cause different behaviour compared with extensively studied heavy-duty diesel engines in the literature. With the main injection only case as a baseline, two after-injection cases of close-coupled and long-dwell timings have been investigated in a single-cylinder common-rail optical engine. Various optical/laser-based imaging diagnostics have been performed including line-of-sight integrated chemiluminescence imaging of OH*, planar laser-induced florescence of hydroxyl (OH-PLIF), planar laser-induced incandescence of soot (PLII) and transmission electron microscope (TEM) imaging of thermophoretically sampled in-flame soot particles to visualise the spatial and temporal evolution of high temperature reaction and soot as well as particle structure. The results indicate that both after-injection strategies introduce a secondary heat release that leads to additional bulk gas temperature rise and thereby promoting the late-cycle soot oxidation. However, the efficacy is found to be dependent upon the after-injection timing. The OH-PLIF and PLII images show that the close-coupled after-injection induces additional high-temperature reaction and soot formation due to high ambient gas temperature. The new reaction occurs near the jet–wall impingement point, which is well separated from the main combustion zone as the main fuel jet travelled along the bowl wall due to significant jet–wall interactions. Although the main and after-injection soot are decoupled, soot morphology analysis based on TEM images suggests that the close-coupled after-injection does enhance the oxidation of main combustion-generated soot particles through the breakdown of large soot aggregates at elevated temperatures and with increased OH radicals. In comparison, both OH-PLIF and PLII images show no visual evidences of decoupled after-injection combustion for the long-dwell case due to insufficient temperature as the additional reaction occurs later in the expansion stroke. However, the increased OH radicals and breakdown of soot aggregates in the main combustion region are evident in OH-PLIF and TEM images. Overall, the close-coupled after-injection adds more soot but induces a higher degree of oxidation for the main combustion-generated soot particles. By contrast, the promotion of soot oxidation is less significant for the long-dwell after-injection but it produces no extra soot.

Assessment of Finite Rate Chemistry Large Eddy Simulation Combustion Models

Large Eddy Simulations (LES) of a swirl-stabilized natural gas-air flame in a laboratory gas turbine combustor is performed using six different LES combustion models to provide a head-to-head comparative study. More specifically, six finite rate chemistry models, including the thickened flame model, the partially stirred reactor model, the approximate deconvolution model and the stochastic fields model have been studied. The LES predictions are compared against experimental data including velocity, temperature and major species concentrations measured using Particle Image Velocimetry (PIV), OH Planar Laser-Induced Fluorescence (OH-PLIF), OH chemiluminescence imaging and one-dimensional laser Raman scattering. Based on previous results a skeletal methane-air reaction mechanism based on the well-known Smooke and Giovangigli mechanism was used in this work. Two computational grids of about 7 and 56 million cells, respectively, are used to quantify the influence of grid resolution. The overall flow and flame structures appear similar for all LES combustion models studied and agree well with experimental still and video images. Takeno flame index and chemical explosives mode analysis suggest that the flame is premixed and resides within the thin reaction zone. The LES results show good agreement with the experimental data for the axial velocity, temperature and major species, but differences due to the choice of LES combustion model are observed and discussed. Furthermore, the intrinsic flame structure and the flame dynamics are similarly predicted by all LES combustion models examined. Within this range of models, there is no strong case for deciding which model performs the best.

Investigation into the combustion instability of synthetic natural gases using high speed flame images and their proper orthogonal decomposition

In this study, the characteristics of the combustion instability of synthetic natural gases that contained small amount of H2 were investigated to determine the allowable content of H2. Because H2 burns faster and hotter than natural gas, which is the primary fuel of gas turbines, it can cause severe damage to fuel nozzles and contribute to combustion instability. Careful inspection of the combustion instability phenomena in synthetic natural gases containing 0% (as reference), 1%, 3%, and 5% H2 was conducted by measuring pressure fluctuation and high-speed intensified OH chemiluminescence images of oscillating flames and their proper orthogonal decomposition. When the H2 content was changed from 0% to 5%, two representative indicators of combustion instability, pressure fluctuation and heat release oscillation, responded sensitively and nonlinearly. The results of pressure fluctuation and heat release oscillation both showed that synthetic natural gas containing 1% H2 did not change the instability characteristics as much as synthetic natural gases containing 3% H2 and higher did. A box plot showing statistically meaningful values, such as the mean and median, disclosed characteristics regarding the amount of heat release from flame evolution and decay, which could be obtained from neither frequency nor time series plots of pressure fluctuation and heat release oscillation. In particular, quantitative analysis using Pearson correlation coefficients for the time coefficients of proper orthogonal modes and the fast Fourier transform of pressure fluctuation validated a close relationship in synthetic natural gases at 0% and 1%. Based on these results, it was concluded that in synthetic natural gas, 1% H2 is a permissible limit because it did not change the characteristics of combustion instability when natural gas that contained no H2 was substituted into synthetic natural gases in which all H2 could not be eliminated for economic reasons.

Influence of self-sustained jet oscillation on a confined turbulent flame near lean blow-out

Premixed methane–air turbulent flame is generated in a single-nozzle jet-stabilized combustor designed based on the FLOX® concept11FLOX® is a registered trademark of WS Wärmeprozesstechnik GmbH, Renningen, Germany. Confinement-induced, self-sustained jet oscillation is observed. Its influence on combustion stability near lean blow-out (LBO) is investigated using simultaneous particle imaging velocimetry (PIV), planar laser-induced fluorescence of OH radicals (OH PLIF), and OH chemiluminescence imaging at 5-kHz repetition rate. Via proper orthogonal decomposition (POD) of the velocity field and extended POD of the scalar fields, pronounced variations in the flame shape are observed during a cycle of jet oscillation. In extreme cases, flame is partially blown out in the combustor due to jet impingement on the wall during the first half of its oscillation cycle. In the subsequent half cycle following jet detachment, flame is restabilized after robust flashback and re-light. Statistical analysis shows that such pattern is by far the most prevalent mechanism for blow-out and restabilization to take place at the operating condition. Additionally, these events are found with much higher probability during slow-paced jet oscillations.