High-speed OH Research Articles

Influence of momentum ratio on heat release rate and noise of co/counter-swirl non-premixed diluted CO/H2 flames

Understanding co/counter-swirl or twin-swirl flames remains challenging due to the complex interaction of two swirling streams. In the present study, we investigate the heat release features of non-premixed co/counter-swirl syngas/air flames and their’ influence on combustor noise in a ∼20 kW combustor using simultaneous high-speed OH*-chemiluminescence (5 kHz) and microphone measurement (50 kHz) by varying the momentum ratio (M) from 0.4 to 0.95. Furthermore, the velocity field is examined using a low-speed two-dimensional particle image velocimetry (2D-PIV, frequency = 7 Hz). For all studied momentum ratios, the frequency spectra of noise measurements for both co/counter-swirl flames consistently exhibit a dominant frequency (∼285 Hz), close to the fundamental axial mode of the combustor. A further analysis using spectrograms, phase spaces, and recurrence plots reveals intermittent patterns in noise measurements, featuring periodic (P) and aperiodic regions (A). In periodic regions (P), noise synchronizes with the global fluctuation of the heat release rate as observed in chemiluminescence. Along with global fluctuation, the chemiluminescence also reveals a rotational component of heat release rate with distinct frequencies for co and counter-swirl configurations. This rotational motion possibly originated from a precessing vortex core (PVC) as indicated by the zig-zag arrangements of vortices in the inner shear layer. Furthermore, the impact of M on global fluctuation and rotational motion has been investigated using the frequency spectrum of OH*-intensity and the distribution of peaks in noise measurement. The global fluctuation is found to be suppressed when M increases while the rotational component becomes prominent at higher M. Therefore, the study elucidates the co-existence of global fluctuation and rotational motion and how these motions evolve with the varying momentum ratio (M), thus enhancing the understanding of combustion characteristics of the complex twin-swirl (co/counter-swirl) flames.

Influence of AM Generated Burner Surface Roughness on NOx Emissions and Operability of Hydrogen-Rich Fuels

ABSTRACT The additive manufacturing (AM) technique enables the fabrication of advanced burner components to enhance the hydrogen capability of the existing gas turbines (GTs) and reduce the carbon footprints of the power generation sector. This technique produces rough surfaces that may require post-processing to maintain the desired functionality of a burner, particularly for hydrogen fuel with unique thermo-physical properties. This study, therefore, compared the stability of three swirlers of variable surface roughness manufactured using AM and traditional machining methods, with one of the AM swirler post-processed by grit-blasting. The comparison included a conventional benchmark (100% CH4), low carbon (23%volCH4/77%volH2) and zero carbon (100% H2) fuels across a range of equivalence ratios. Additionally, the study quantified the flame topology and emissions performance of the fuel blends for each swirler using high-speed OH* chemiluminescence and exhaust gas emissions measurements, respectively. The experimental investigation concluded that the AM-generated surface roughness within the considered range does not detrimentally impact NOX emissions and the stability of the fuel mix. However, the flame location was observed to be influenced by surface roughness and shifted more toward the vertical centerline of the burner with increased roughness. From the practical perspective, the results showed that post-manufacturing surface finishing offers negligible performance advantages, indicating potential cost reductions. It is recommended that further studies should investigate the influence of increased surface roughness on burner performance, as well as numerical modeling techniques which could provide an insight into when AM surfaces are likely to be more influential.

Lifting of Transversely Forced Turbulent Nonpremixed Coaxial Jet Flames

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior.

Optical and thermodynamic investigation of jet-guided spark ignited hydrogen combustion

Hydrogen as an alternative fuel for engines is gaining interest to decarbonize the heavy-duty sector. Hydrogen combustion in engines conventionally relies on the Otto cycle and either port fuel or direct injection. This combustion concept usually results in poor power density and non-ideal efficiencies due to the lean premixed operation and low compression ratio required to avoid preignition and knocking. In this study, hydrogen is directly injected at high pressures into the combustion chamber and spark-ignited with two different ignition systems (inductive and nanosecond repetitively pulsed discharge) at the periphery of the jet, with fuel conversion taking place predominantly in jet-guided (diffusion) mode while injection remains active.A rapid compression expansion machine (RCEM) is used to investigate the jet-guided hydrogen combustion thermodynamically and optically by combining high-speed Schlieren and OH* chemiluminescence imaging, and spark-induced breakdown spectroscopy (SIBS).SIBS reveals low air to fuel ratios at ignition time and location, ranging from 2.8 up to pure air depending on the condition and significant variation among repetitions. Both OH* chemiluminescence and Schlieren images illustrate how air entrainment pushes the flame-plasma kernel from the ignition location at the periphery of the jet towards the fuel-richer core, resulting in fast combustion of the already premixed fuel. Subsequently, the combustion process becomes dominated by the mixing dynamics. Under conditions of low in-cylinder pressures, the Heat Release Rate (HRR) closely follows the injected mass flow rate multiplied by the lower heating value of hydrogen, i.e., the combustion is mixing controlled. However, as the cylinder pressure increases, the mixing and combustion rates fall below the injected mass flow until lower densities are reached again.The delay between the start of injection and ignition primarily impacts the premixed peak, but the in-cylinder pressure and ignition source also play significant roles. This distinctive combustion process shows essential similarities to compression ignition Diesel combustion. The process holds the potential to achieve high efficiency, as hydrogen can be burned using process parameters typical of Diesel combustion without facing the constraints of knock or preignition and a complete absence of soot.

Characterizing lean blowout dynamics of DME/air premixed swirl flames in a gas turbine model combustor with and without confinement

This work investigates lean blowout (LBO) dynamics of dimethyl ether (DME)/air premixed swirl flames in a gas turbine model combustor with and without confinement. CH* chemiluminescence imaging and 2 kHz high-speed OH* chemiluminescence imaging are performed to capture the swirl flame macrostructures. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are used to visualize the flow fields and reaction zones, respectively. Compared to the unconfined flame, the addition of confinement results in the appearance of an outer recirculation zone (ORZ), variation in flame topology, enhanced overall heat release (HR) rate, increased turbulent kinetic energy (TKE), and extended LBO limits. Far from LBO, all flames are stabilized in the shear layer. Approaching LBO, the confinement enhances the flow vortex structures along the shear layers, which is associated with extinction at the flame base and shear layer. However, the unconfined flame is still stabilized in the shear layer due to the weaker flow vortex structures. Different extinction mechanisms during the LBO process are also discussed between the unconfined and confined flames. The unconfined flame suddenly leaves the inner shear layer (ISL) and recedes to the inner recirculation zone (IRZ) within about 100 ms before LBO. Abundant CH2O is transported into IRZ mainly by recirculation, which reduces the HR region area and give rise to LBO. For the confined flame, more and more cold reactants gather near the base plate and then are gradually brought into IRZ by ISL vortices, which makes the flame lift from the base plate and decrease the HR region area until LBO happens.

Advanced multiscale modal and frequency analysis of swirling spray flame near to lean blowout

This study provides an in-depth characterization of Jet-A1 swirled flames' dynamics using advanced decomposition techniques on high-speed OH* chemiluminescence images. Research conducted in a 300-kW combustor with global fuel-to-air ratios Φ = 0.36, 0.24, and 0.18 reveals dominant low-frequency components under ultra-lean conditions through statistical and wavelet analyses. Integrating Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD), and Multiscale Proper Orthogonal Decomposition (mPOD) enhances understanding by detailing the energy and frequency of flame modal structures. Observations show that as Φ decreases, flame size and intensity reduce, while fluctuations increase, indicating a transition towards unstable combustion dynamics. Furthermore, the frequency and intensity of burst phenomena increase, particularly in higher modes, indicating the system's approach to lean blowout (LBO). The application of nonlinear time series analysis, including autocorrelation (AFC) and phase space reconstruction, to mPOD eigenvalues, detects early warning signs of LBO. At lower Φ, the ACF reveals significant and frequent bursts, indicating high instability and intermittent behavior. Phase space reconstruction shows distorted orbits with a spiral-like structure, characteristic of substantial damping and irregular behavior near blowout conditions.

Influence of swirl intensity on combustion dynamics and emissions in an ammonia-enriched methane/air combustor

Ammonia (NH3) has been widely considered as a promising carbon-free energy and hydrogen carrier for various applications. The large-scale direct utilization of NH3 as fuel in gas turbine engines is currently attracting significant interest, with strong focuses on improving the efficiency and stability of the system and reducing the emissions of pollutants. The present study experimentally examined the impacts of swirl intensity on combustion stability and emissions in an NH3-enriched premixed swirl-stabilized CH4/air combustor under a wide range of equivalence ratios. Simultaneous high-speed OH* chemiluminescence and particle image velocimetry measurements suggested that increasing swirl intensity resulted in more compact flame shapes and expanded the recirculation zone, which promoted flame stability at higher NH3 ratios. However, under specified conditions, enhancing swirl intensity could increase the instability frequency and amplitude of pressure oscillations. The flame dynamics exhibited different behaviors depending on the swirl intensity. At high swirl intensity, the flames underwent high-frequency, small-amplitude periodic motion. At low swirl intensity, the flames oscillated axially with large amplitude and low frequency. For flow dynamics, the stability of the vortex at high swirl intensity contrasted with the periodic vortex shedding at low swirl intensity. Furthermore, the two-dimensional Rayleigh index indicated that the dominant positive thermoacoustic coupling regions were located near the flame shear layers and flame tail at low and high swirl intensities, respectively. Finally, the experimental results showed that swirl intensity affected pollutant emissions by influencing the temperature of combustion chamber and gas mixing efficiency. The pathway of fuel-type NOx was found to be dominant in the NOx emission of the NH3/CH4/air flames.

Influence of Variable Swirl on Emissions in a Non-Premixed Fuel-Flexible Burner at Elevated Ambient Conditions

Abstract As alternative fuels are designated for future energy applications, flexible combustor designs require considerable development to ensure stable operation with reduced NOx emissions. A nonpremixed variable swirl burner was used to experimentally appraise changes in NO production pathways, with CH4 NH3 and H2 flames, alongside intermediate fuel blends. Maintaining an equivalent thermal power and flame temperature between fuels, preheated reactants (500 K) were supplied to the burner, with parametric changes made to pressure (1–6 bara) and swirl number (0.8–2.0). NO production was characterized, alongside variations in flame structure and topology, with a correlation demonstrated for exhaust emissions. NO production was shown to be sensitive to combustor pressure, providing an expected increase for CH4 and H2 flames. Emission profiles from both NH3 and H2 flames are shown to be significantly augmented by a change in swirl number. As NH3 fractions were increased in the H2 blend, a decaying trend in NO emissions was observed with an increase in pressure, and as a function of mixture ratio. However, this behavior was markedly augmented by a change in swirl number and suggests that further reductions may be possible at increased pressure. At the low swirl/high pressure condition, the NH3/H2 blend outperformed pure H2, providing lower NO concentrations. Emissions data were normalized using the traditional dry/O2 correction, alongside mass scaled by thermal power, with a comparison provided. The corresponding differences in emission formation pathways were investigated, alongside high-speed OH* chemiluminescence to further elucidate findings.

Evaluation of high-pressure syngas ignition under high-CO2 dilution in shock tubes

Due to its importance as a candidate fuel for power cycles for future energy production, there have been active efforts to improve our understanding of the chemical kinetics of the Allam-Fetvedt cycle that utilizes syngas highly diluted with CO2. Shock-tube ignition delay times (IDT) of syngas-CO2 mixtures over a range of pressures, syngas blends, and equivalence ratios are now available in the literature, but their unique behavior deserves further study before the resulting data can be used to modify existing syngas kinetics models. In this study, an analysis of CO2-diluted syngas ignition behind reflected shock waves was conducted at the High-Pressure Shock Tube facility at Texas A&M University using an endwall imaging system. A syngas blend of 1:1 H2:CO at an equivalence ratio of 1 was studied at pressures around 12 atm for CO2 dilutions of 85, 88, and 94 % by volume. The combined results of the high-speed OH* imaging through the endwall and OH* time histories from sidewall windows indicate that the initial ignition event appears to be homogeneous and occurs first near the endwall, with no apparent premature ignition or localized flame kernels. An important finding of this investigation is that the existing syngas-CO2 IDT data in the literature are likely free from shock-tube ignition inhomogeneities. Therefore, they can be used to improve existing chemical kinetics mechanisms, which currently have trouble modeling the existing literature data over the range of temperatures of the shock-tube studies (∼1000 – 1400 K). Additionally, the lack of a measured post-combustion pressure rise makes it necessary to use a constant-pressure-enthalpy constraint when modeling the shock-tube IDT event.

Understanding the ignition process and flame structure of conventional and oxygenated fuels under engine relevant conditions – An optical study

Renewably generated synthetic fuels such as poly-oxymethylene ethers (OME) have a significant potential to effectively break the soot-NOX trade-off in compression ignition engines by using exhaust gas recirculation (EGR) to maintain low nitrogen oxide (NOX) emissions while maintaining good efficiency and simultaneously contributing to circular carbon economy. However, owing to the fundamental differences in properties of OME when compared to fossil-based diesel fuels, it is critical to fully understand its ignition and combustion phenomenology to take advantage of this fuel to its utmost potential. In this context, this work outlines the results of a systematic experimental study performed in a heavy-duty, single-cylinder, optical engine probing the spatial and temporal progression of fuel decomposition and ignition behavior of OME when compared to n-dodecane, a diesel-fuel surrogate. Thermodynamic analysis and optical diagnostics techniques including simultaneous HCHO-PLIF and OH-PLIF complemented by high-speed OH* chemiluminescence were employed along with parametric sweeps of intake temperature and EGR dilution rates. OME does not exhibit any observable low temperature heat release irrespective of the ambient oxygen concentration. Differences in the observed diffusive flame structure such as longer flame lift-off length, less pronounced combustion recession, faster premixed burn at ignition (“volumetric” ignition), non-sooting behavior suggest that the inherent presence of fuel-bound oxygen in OME can skew the air-fuel ratio (AFR) distribution within the jet thereby reducing the reliance of combustion on mixing and air entrainment. This leads to rapid late-cycle oxidation leading to shorter combustion duration and favorable combustion phasing. Results also suggest that OME exhibits relatively weak negative temperature coefficient (NTC) behavior, however, the OME fuel-decomposition kinetic-pathways produce significant concentration of HCHO, which might be erroneously interpreted as a product of cool-flames.

Investigation of longitudinal self-excited combustion instability in a micromix hydrogen combustor

The increasing emphasis on low-carbon energy has heightened investigations into hydrogen combustion. This study focuses on the critical yet underexplored topic of thermoacoustic instabilities in micromix hydrogen combustors. We employ a newly designed micromix burner to explore the intricate relationships among acoustic modes, flame dynamics, and flow-vortex interactions. High-speed particle image velocimetry (PIV) and OH* chemiluminescence, supplemented by dynamic pressure measurements, are employed to capture dynamic behaviors and instability modes of the flame. Through thermoacoustic network analysis, we can identify two primary self-excited modes: a dominant low-frequency mode oscillating between 520 Hz and 565 Hz, and a secondary high-frequency mode surpassing 4400 Hz, originating from the hydrodynamic instability of the nozzle's jet flow. The low-frequency mode significantly influences the flame dynamics, generating a distinct longitudinal flame oscillation behavior. In particular, the inner premixed flame exhibits a periodic pattern of lifting and re-attachment phenomena. The outer diffusion flame, however, dynamically interacts with the outer shear layer (OSL) and the shed outer vortex rings (OVRs), which gives rise to notable deformations in the flame surface, manifesting as neck and swell structures propagating downstream with the OVRs. Furthermore, the dynamics of the OSL and OVRs are attributed to the fluctuations in the bulk flow velocity and local equivalence ratio at the nozzle exit, which are sustained by pressure oscillations induced by thermoacoustics. This presents important insights into the interplay between hydrodynamics and thermoacoustics in the formation of combustion instability in the micromix hydrogen combustor.

Investigation of the influence of the bluff-body temperature on a lean premixed DME/air flame approaching blowoff

In flames stabilized by a two-dimensional (2D) bluff body, the lean blowoff (LBO) limit can be extended by increasing the bluff-body temperature, which is verified experimentally in premixed dimethyl ether (DME)/air flames in this work. Two bluff-body temperatures of 573 K and 773 K are tested, respectively. High-speed OH* chemiluminescence (CL) imaging is applied to record the spatial distribution of the heat release. Particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) are performed simultaneously to explore the flow field and the combustion characteristics. Based on the results of OH*-CL, the heat release of the area near the bluff body is strengthened in the case of higher bluff-body temperature, and the enhanced chemical reactivity will further influence the downstream area. The turbulent flame speed obtained by OH-PLIF is increased in the entire area of interest. Due to the higher velocity gradient adjacent to the bluff body in the upstream region (y < 35 mm), the flame surface density (FSD), brush thickness, and flame wrinkling ratio are not sensitive to the bluff-body temperature. However, in the downstream region (y > 35 mm), a noticeable change can be found in these three parameters due to a higher turbulent flame speed and a much lower velocity gradient. Based on the PIV results, a limited effect on the velocity field by increasing the bluff-body temperature by 200 K. Furthermore, the probability density functions (PDF) of the vorticity and strain rate on the flame front are computed. The results indicate that the enhanced flame stability leads to a more pronounced overlapping between the flame front and the high-vorticity region, further increasing the values of these two parameters.

The Effect of Methane–Ammonia and Methane–Hydrogen Blends on Ignition and Light-Around in an Annular Combustor

Abstract The use of hydrogen and ammonia in gas turbines, either alone or blended with natural gas, poses various technical challenges for combustion systems, including ignition. Depending on the fuel composition, the laminar flame speed and the ratio of unburned to burned gas density (dilatation ratio) of hydrogen and ammonia flames can be well outside the range seen in natural gas flames. Previous studies in annular combustion chambers have provided evidence of the importance of these properties in determining the ignition dynamics including light-around times. So far, these studies have mostly considered hydrocarbon fuels, have been limited to only a few runs, and have not yet systematically investigated variations in the dilatation ratio and the flame speed but rather have considered them as a lumped parameter. To investigate these effects in more detail, experiments characterizing the light-around times were carried out on an atmospheric annular combustor in which the dilatation ratio and the laminar flame speed was independently varied. This was achieved by varying the equivalence ratio and employing a variety of different hydrocarbon fuels (ethylene, propane, and methane) and fuel blends of methane–ammonia and methane–hydrogen. Light-around times were evaluated from global chemiluminescence measurements obtained using an azimuthal array of photomultipliers placed round the combustor chamber as well as high speed imaging. To improve statistical certainty, more than 3000 ignition and light-around times were measured with 30 repetitions obtained for each operating condition. To provide some insight into the light-around dynamics in specific cases, 900 of the 3000 sets included high-speed OH* chemiluminescence images. Light-around times for premixed pure hydrocarbon flames showed a similar dependence on the laminar flame speed as reported in previous studies. For the range of ammonia fuel blends investigated, an increase in laminar flame speed leads to a predictable increase in the flame propagation speed, as in the case of hydrocarbon fuel. Furthermore, collapse of this dependence for all blends could be achieved when corrected for an effective Lewis number, noting that all Lewis numbers for these blends were above unity. However, for hydrogen fuel blends, a decrease in dilatation ratio was found to decrease the light-around time counter to existing experimental results on the ignition of hydrocarbon fuels for which we currently do not have an explanation.

Analysis of the information overlap between the PIV and [formula omitted] chemiluminescence signals in turbulent flames using a sparse sensing framework

The purpose of this study is to quantify the information overlap between the OH* chemiluminescence signal and the three-dimensional Particle Image Velocimetry (PIV) signal for a set of bluff-body stabilized, swirling and non-swirling turbulent methane/air flames issued from the Cambridge/Sandia Stratified Swirl Burner. The time-resolved velocity data was collected using high-speed stereoscopic PIV operated at 3 kHz. This sampling rate was sufficiently high to enable accurate tracking of the large-scale spatial and temporal features of the flow. To investigate flame-flow interactions, high-speed (8 kHz) OH* chemiluminescence imaging was applied on the same set of flames. The two datasets are first independently analyzed using multi-scale Proper Orthogonal Decomposition (mPOD) for the purpose of detecting the relevant flow and heat release rate structures. The resulting modes are qualitatively compared to assess if it is possible to identify the same flow structures from the two datasets. Finally, the information overlap is quantified by predicting the velocity field from the chemiluminescence signal using a sparse sensing framework. Sparse sensing is a mathematical technique that leverages the low-dimensional representation of a physical phenomenon to predict the entire state of the system using few measurements. The results show that the velocity and chemiluminescence mPOD modes display broadly similar features, from which is possible to retrieve the same flow instabilities in the corresponding frequency range. Moreover, the prediction of the velocity field obtained using chemiluminescence as input of the sparse sensing model achieves a good level of accuracy, pointing to an important degree of information overlap between the two signals.

The turbulent flame structure in a steam diluted H2/Air micromix flame

In order to safely organize hydrogen flame with low NOx emission in traditional gas turbines and industrial burners, an attractive combustion technology is the steam diluted micromix combustion technology. The effects of equivalence ratio (φ), steam dilution ratio (D) and nozzle diameter (d) on the turbulent flame structure in a steam diluted H2/air micromix flame were investigated experimentally with a 10 kHz high-speed OH∗ chemiluminescence imaging system to detect the instantaneous flame zone. With the increase of equivalence ratio (φ) or the decrease of steam dilution ratio (D), the overall OH∗ signal intensity and the area of OH∗ signal increase. The maximum OH∗ signal intensity along the axial direction (Z) is distributed far from the nozzle outlet with the increase of nozzle diameter (d). Two types of flame were found for time-averaged images, namely the anchored flame and the lifted flame. And the increase of hydrogen velocity could lead to the changing of flame from anchored to lifted due to lower uniformity of the fuel-air mixture at the nozzle outlet. The instantaneous flames for larger nozzle diameter appear as “M” or “V” shape, and the changing of flame shape indicates an unstable combustion. This was further discussed with the OH∗ spatial distribution uniformity (SDU) and its variation determined by the coefficient of variation (CV) parameter.

Optical study of knocking phenomenon in a spark-ignition engine by using high-speed OH* chemiluminescence imaging: A multiple ignition sites approach

Knock in a spark-ignition (SI) engine is a complicated combustion phenomenon that stimulates high-pressure oscillations inside the combustion chamber and restricts engine performance. This study presents a high-speed OH* chemiluminescence imaging technique to investigate the knock mechanism resulting from firing multiple spark plugs. The experiment was performed using a customized liner having three spark plugs installed at equal spacing, and to compare the results with conventional SI conditions, in which one spark plug was mounted at the center of the cylinder head. In addition, multiple pressure transducers were used at various locations to record the frequencies induced by the pressure oscillations inside the cylinder during knocking events. The results showed that firing a single central spark plug generated mild knock with late combustion phasing and lower power output. However, adding more spark plugs could advance the initiation of autoignition and produce higher knock intensity along with lower combustion duration for the same operating conditions. Additionally, a weak OH* chemiluminescence intensity oscillation was monitored before the autoignition of the unburned charge occurred. The crank angle location of peak OH* intensity and peak HRR showed a good linear curve fit with a positive slope. Furthermore, the larger amount of unburned mass fraction produced stronger pressure waves due to multiple autoignition sites, and the unburned mass fraction exhibited a good linear relationship with unburned temperature and end-gas area at the knock onset point. Moreover, the frequency spectrum recorded by the multiple pressure sensors illustrated that in the case of a single central spark plug only circumferential acoustic waves were formed with low power intensity. However, multiple ignition sites promoted both circumferential and radial pressure waves inside the combustion chamber because of multiple autoignitions occurring both near the center and cylinder wall.

Influence of the radial-lip concept design to achieve ultra-low soot emission reductions: An optical analysis

Nowadays, internal combustion engines must reduce pollutant emissions to achieve the upcoming strict regulations in the transport sector. In this context, new hardware designs have shown great potential to reduce pollutant emissions and comply with current and future regulations. This paper focuses on innovative bowl designs with radial lip protrusions, which are radially distributed inside the bowl. The potential of using these protrusions in order to reduce soot formation and improve soot oxidation has been demonstrated by different studies. For this reason, the present work addresses the impact of using two different radial-lip based geometries on soot formation and combustion behaviour in an optical single-cylinder compression ignition engine. High-speed OH* chemiluminescence imaging and 2-Colour pyrometry were applied simultaneously, in combination with a thermodynamic analysis of the in-cylinder pressure signal. The results show that more pronounced radial-lip geometry drive the flames closer to the bowl centre, where more oxygen is available to oxidize the soot, resulting in in-cylinder soot formation reduction. In addition, it has been observed that the improvement achieved by the more pronounced radial-lip geometry is more noticeable under high EGR conditions. This opens a path to reduce the soot-NOX trade-off in compression ignition engines.

Characterization of a high-pressure flame facility using high-speed chemiluminescence and OH LIF imaging

Characterization of a high-pressure flame facility using high-speed chemiluminescence and OH LIF imaging

Investigation of syngas combustion in a novel ultra-low emission 20-kW two-stage combustor

Two-stage rich-catalytic lean-swirl combustion is investigated in a 20-kW combustor using biomass-derived syngas as fuel. In the first stage, fuel-rich catalytic combustion of syngas (Фfirst = 4) is investigated using FeCr-alloy monolith and platinum (Pt) as a catalyst. The fuel undergoes partial conversion in the catalytic stage. The remaining unoxidized reactants from the first stage are burnt using additional air in the second stage. The flame is stabilized by two concentric swirling streams in the second stage, where the exhaust from the first stage flows through the inner stream, and the outer stream carries air. By changing the swirl direction of the inner stream, co or counter-swirl flame is established, whereas the overall equivalence ratio is changed by increasing the airflow rate. It is found that the flame topology is significantly different for the co-swirl and counter-swirl configurations. Transitions from a compact ‘M’ shaped flame to a shear-layer stabilized ‘V’-shaped flame is observed by changing the overall equivalence ratio. The combination of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) highlights the differences in the flow field and OH radical distribution, leading to the observed flame shape. The dynamic stability of the combustor is assessed using sound pressure level (decibel) obtained using a microphone and high-speed OH* chemiluminescence. Finally, the CO and NOx concentrations are measured at the combustor exit. The CO emission ranges from 10 ppm to 70 ppm, whereas the NOx emission is negligible in all conditions studied. Thus, the present study successfully demonstrates the feasibility of catalytic and twin-swirl combustion in achieving near-zero emissions while operating with biomass-derived syngas.

Lean blow-off of premixed swirl-stabilised flames with vapourised kerosene

Lean blow-off (LBO) conditions, duration of the extinction transient, and flame structure were investigated for swirl-stabilised, fully-premixed flames operated with methane and vapourised kerosene. High-speed (5 kHz) OH* chemiluminescence, OH-, and Fuel-PLIF imaging were used. Low-speed (10Hz) CH2O-PLIF imaging was also implemented to visualize the structure of CH2O. OH* chemiluminescence imaging of the LBO transient suggested that the primary reaction regions were confined within the central recirculation zone (CRZ). Sharp temporal increases in OH* signal were observed in the CRZ for both fuels, possibly due to flame re-ignition events during LBO transients. Both fuels showed a decrease in flame length and an increase in fragmentation in downstream regions. In the OH-PLIF images, pockets void of OH entered the CRZ from downstream. The CH2O- and Fuel-PLIF images suggest that these pockets contain either partially burned or fresh reactants. Furthermore, the CH2O- and Fuel-PLIF images indicated that CH2O and Fuel exist in the outer recirculation zone (ORZ) during the early stages of the LBO transient. The blow-off duration was calculated by integrating the OH* signal and for kerosene was found to be 1.5 times longer than for methane. In addition, the equivalence ratio at which kerosene blew-off was consistently lower than that of methane for a range of bulk flow velocities. These results indicate that fuel chemistry effects are important for the LBO condition and duration.