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• A novelty micro-mixing multi-injector burner was constructed for burning high-hydrogen-content CH 4 /H 2 fuels. • Under all experimental conditions, the micro-mixing burner can achieve ultra-low NOx emissions (less than 8 ppm@15 %O 2). • The impact of H 2 content on the flame morphology, flame stability and emission characteristics were investigated. • Distinct mechanisms for flame stabilization between pure methane and high-hydrogen-content CH 4 /H 2 fuels were observed. Micro-mixing combustion technology premixes air and fuel in a microscale channel, enabling efficient mixing within a short distance can preventing flame flashback suitable for H 2 -enriched fuels. This approach effectively mitigates the formation of localized high-temperature zones in the flame and shortens the residence time of fuel in the high-temperature zone, thereby significantly minimizing NOx emissions. In this study, a self-designed micro-mixing multi-injector burner was constructed for burning high-hydrogen-content CH 4 /H 2 fuels. The H 2 / CH 4 volume ratios range from 0:10 to 8:2. The effects of fuel composition and equivalence ratio on flame structure, flame stability, and pollutant emission of the CH 4 /H 2 micro-mixing combustion were investigated experimentally. The results demonstrated that even at an adiabatic flame temperature of 2100 K, the micro-mixing burner can achieve low NOx emissions (less than 8 ppm@15 %O 2). Furthermore, compared to pure methane, highly hydrogen-enriched fuel can exhibit lower NO x emissions under stable combustion conditions. Based on the measurement of the distribution of OH radicals using Planar Laser-Induced Fluorescence (PLIF), and the captured OH* chemiluminescence signals, the impact of H 2 concentration, equivalence ratio and Reynolds number on the flame morphology were investigated. The measurements suggest that hydrogen plays a pivotal role in the formation of OH radicals. For fuels with higher H 2 concentration, the flame height and lift-off height are smaller, and the heat release area is more concentrated. The flame with higher H 2 concentration can be stabilized at a lower equivalence ratio. As the H 2 concentration decreases, the distance between flame jets shortens, leading to gradual coupling and eventual reunion of the jet flames. When the combustion state approaches the lean blowout (LBO) limit, the flame structure of high hydrogen concentration fuel exhibits notable disparities compared to that of pure methane fuel, indicating distinct mechanisms for flame stabilization between these two fuels. [ABSTRACT FROM AUTHOR]

• Wall confinement significantly affects flow macrostructures and flame topologies. • Unconfined flames are observed with much lower LBO limits than confined flames. • Unconfined and confined flames approaching LBO exhibit different flame dynamics. • Inhibited re-ignition processes result in higher LBO limits in confined flames. Dimethyl ether (DME) is a promising alternative fuel for gas turbines, considering its renewable nature and potential for large-scale synthesis from CO 2 feedstock. However, there are limited studies on the swirl combustion characteristics of DME, particularly regarding the stability performance. This work investigates the effects of wall confinement on flow, mixing, and lean blowout (LBO) characteristics of partially premixed DME/air swirl flames in a gas turbine model combustor. Flow and flame macrostructures are captured using particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements, respectively. The results show remarkable differences in flame topologies between unconfined and confined flames, especially the formation of inner recirculation zone and outer recirculation zone. The LBO limits are measured over various Reynolds numbers and flame dynamics approaching LBO are captured using 10 kHz OH* chemiluminescence imaging. Unconfined flames have unexpectively lower LBO limits (∼0.4) than the LBO limits of confined flames (over 0.5). Approaching LBO, unconfined flames display a quiet blowout mode featuring swirling spiral blow-off behaviors. In contrast, confined flames exhibit a violent blowout mode featuring large-scale quasi-instability behaviors, including periodic events of local extinction, flame lift-off, and re-ignition. Based on the numerical simulations and simultaneous OH- and CH 2 O-PLIF measurements, the large discrepancy in the measured LBO limits between the two configurations is found to result from different re-ignition processes related to the low-temperature ignition before the final blowout. These findings highlight the crucial roles of wall confinement in determining the stable flame topologies, LBO characteristics, and mechanisms driving the LBO physics for partially premixed DME/air swirl flames. [ABSTRACT FROM AUTHOR]

• High oxygen concentration and low co-flow velocity can make under-ventilated flames. • Blowout fuel velocity increases significantly with under-ventilated flames. • Effective velocity shows good relations with co-flow velocities and liftoff heights. • Mechanism of over-ventilated liftoff flames is related with the turbulent flame speed. Greenhouse gas emissions should be reduced to stop the advance of global warming. Oxy-fuel combustion is one of the methods to reduce CO 2 emissions. This paper aims to study the stabilization of CO 2 -diluted oxy-methane non-premixed jet flames and compare this with the results of methane-air flames. The lower the co-flow velocity and the higher the oxygen concentration in the co-flow, the higher the stability. If the flame stability is strong enough to withstand flame shape changes from the over-ventilated flame to the under-ventilated flame, the flammable range without blowout increases significantly under the under-ventilated flame. Otherwise, by decreasing the liftoff flame stability, the over-ventilated flame cannot change to the under-ventilated flame and blows out near the globally stoichiometric condition. The flame stability of a CO 2 -diluted oxy-fuel flame can differ from that of methane-air flames due to the properties of CO 2 and N 2. The effective velocity proposed by Guiberti et al. shows a good collapse of the dimensionless liftoff height among the O 2 /CO 2 co-flow, but the dimensionless liftoff height under the air co-flow is not on the same line with the O 2 /CO 2 co-flow. This study suggests a modified dimensionless liftoff height equation considering the fuel and co-flow mixture kinematic viscosity instead of the fuel kinematic viscosity and the ratio of the thermal diffusivity of the mixture. The thermal diffusivity ratio does not affect the result of the air co-flow. Additionally, the new coefficient β and turbulent Schmidt number are suggested because these numbers are factors depending on the burner geometry. The modified effective velocity using new coefficients and the modified dimensionless liftoff height show a good collapse for not only O 2 /CO 2 co-flows but also air co-flow. Also, the modified effective velocity shows the possibility of the critical liftoff velocity prediction. [ABSTRACT FROM AUTHOR]

• NO formation has been studied for NH3/DME binary fuel mixtures. • NO concentration profiles were determined with laser-induced fluorescence. • Maximal amounts of NO are formed in mixtures with around 50% NH 3 in the fuel. • Contemporary detailed reaction mechanisms are able to predict NO well. • Chemical interactions between C- and N-species are non-negligible for NO formation. Binary mixture of ammonia (NH 3) and dimethyl ether (DME) has been considered in literature as a potential fuel for practical use. Nitric oxide (NO) is a major product of combustion of NH 3 -containing fuels, and its formation routes have to be comprehensively studied. In this work, concentration profiles of NO were experimentally measured in laminar axisymmetric flames using planar laser-induced fluorescence. The molar percentage of NH 3 in the NH 3 /DME fuel mixture varied from 10% to 60%. Emission levels of NO have reached as much as around 1% for mixtures with around 50% NH 3. NO formation was analyzed with numerical simulations of 1D laminar flames and several detailed kinetic mechanisms. Modeling was performed in Chemkin with the steady-state burner-stabilized and free-propagating planar laminar flame reactor models. It was observed that the most recent versions of the contemporary NH 3 /DME models are able to reproduce the experiments, and their predictions agree with each other due to similarities in the NH 3 submechanisms. Kinetic analysis has revealed some disagreement was observed in terms of how much direct chemical coupling between carbon- and nitrogen-containing species affects NO formation. [ABSTRACT FROM AUTHOR]

• ISD combustion of hydrogen-enriched jet-fuel. • OH-CH 2 O-fuel-PLIF and NTLAF temperature imaging. • Effects of H 2 enrichment on flame propagation stability. • H 2 favors the suppression of local flame instability and temperature fluctuations. • H 2 inhibits CO and CO 2 emissions but promotes NO x emission. Hydrogen (H 2) and jet fuel co-firing is regarded as a promising transition strategy of decarbonization in aviation industry. Considering the simplicity and safety during highly hydrogen enrichment conditions, the inverse swirl diffusion (ISD) combustion holds research value. Given the current lack of understanding for the flame properties of H 2 -enriched pre-evaporative jet fuel ISD co-combustion, in this work, the simultaneous planar laser-induced fluorescence (PLIF) of hydroxyl (OH), formaldehyde (CH 2 O), and fuel, as well as the non-linear excitation regime two-line atomic fluorescence (NTLAF) thermography imaging, are used to study the effects of hydrogen enrichment on ISD flame intricate structure and temperature field. The results show the positive effects of H 2 enrichment. The ISD flames display a blue hue and the increase in doping hydrogen helps the flames stabilize under global-lean conditions with almost no CO emission and low CO 2 emission at the chamber exit. And the H 2 enrichment contributes to inhibiting the CH 2 O formation upstream, reducing the generation of isolated reaction islands and promoting the strength of the flame wings, favoring the suppression of flame instability. In contrast, when at low H 2 enrichment rate, the shift of the heat release zone and high-temperature zone, and the enlargement of the cold zone, are found in flame upstream. The analysis of the probability distribution functions (PDF) of temperatures confirms the effective effects of hydrogen on inhibiting the local flame fluctuations. Although the increase in the overall temperature level due to the hydrogen addition would lead to a promotion in NO x generation, the discussion on the structural analysis and flame propagation stability of ISD flames demonstrates the significance of H 2 enrichment for the achievement of leaner low-NO x combustion of ultra-H 2 -enriched jet fuel in the future. [ABSTRACT FROM AUTHOR]

• Detailed analysis of flame structures in cracked ammonia jet flames. • Insights into N 2 O formation in ammonia flames through NH and NO multiplication. • Impact of cracking ratio on chemical reactivity, flame stability, and NO emissions. • PLIF-derived statistics contribute to validating turbulence-chemistry interactions. The combustion of cracked ammonia is crucial for enhancing flame stability and reducing pollutant emissions in ammonia-based combustion systems. This study investigated the flame structure and pollutant formation in cracked ammonia jet flames (CAJFs) to improve our understanding of the turbulence-chemistry interactions in ammonia combustion. A simultaneous NH and NO planar laser-induced fluorescence (PLIF) technique was employed to analyze the flame structure of turbulent non-premixed CAJFs, emulated using ammonia-hydrogen–nitrogen mixtures. Experimental measurements were conducted across a range of pressures (1–5 bar) and cracking ratios (7 %-28 %). The results revealed the significant influence of the cracking ratio on the chemical reactivity and NH-layer characteristics of ammonia-hydrogen flames. NH effectively marked the heat release layer of CAJFs. Reducing the cracking ratio from 28 % to 7 % resulted in increased local extinction-induced NH-layer fragmentation and reduced reaction area. The reaction layer thickness exhibited low sensitivity to the cracking ratio and pressure, while the broadening of the NH layer displayed a slowdown pattern with increasing height, differing from premixed flames. The pollutant NO rapidly formed within the reaction zone, persisting in the outer hot products until reduced by turbulent mixing. The fragmented flame structure promoted decreased NO formation and potentially lower NO concentration. A novel approach of multiplying NH and NO was proposed to reflect the formation characteristics of another important pollutant N 2 O. The effect of turbulent disturbances on N 2 O formation needs to be considered in turbulent CAJFs because N 2 O formation is strongly suppressed when turbulent transport reduces local NO concentration. This research enhances understanding of turbulence-chemistry interactions in cracked ammonia combustion, benefiting ammonia flame stability and pollutant emission control. [ABSTRACT FROM AUTHOR]

• Effects of stretch, turbulence, and differential diffusion on H 2 -rich flames were assessed. • New method to determine the flame preheat zone thickness was proposed. • New evidence for flame preheat zone broadening accumulated. • Quantitatively measured flow field and flame structure with a set of optical diagnostics. • Empirical turbulent flame scaling law accounting for the synergetic effects was given. An experimental study on the structure and propagation of the stretched hydrogen-rich premixed turbulent flames in the thin-reaction-zone regime was conducted. A set of optical diagnostic techniques including the 2D/3D-particle image velocimetry, hot wire anemometer, intensified charge-coupled device (ICCD), and laser tomography visualization (LTV) methods were adopted to quantitatively measure the velocity field, turbulent intensity, and thickness of the stretched flame of the under various conditions. A new method to improve the measurements of the flame preheat zone thickness was developed by using the low-temperature available LTV method to identify the onset of the preheat zone and using the peak location of CH* to identify the reaction zone. The results showed that the flow field was in the inertial turbulent subrange and the turbulent flames obtained in the present counterflow configuration were categorized into the thin reaction zone regime in Borghi/Peters turbulent flame diagram. The experimental results showed that the flame structure of a turbulent H 2 -rich premixed flame was synergistically influenced by the hydrodynamic stretch, turbulence, and preferential diffusion. New evidence of the phenomenon accumulated that large-scale vortices wrinkled the surface of the flame while small-scale vortices penetrated into the flame preheat zone. Scaling laws for the turbulent flame propagation speed, apparent turbulent thermal conductivity, and were given with the consideration of the synergetic effect of hydrodynamic stretch, turbulence, and preferential diffusion. The scaling of the turbulent flame preheat zone thickness turned out to be sensitive, while those of the turbulent flame propagation speed and apparent turbulent thermal conductivity were insensitive, to the bulk flow strain rate. [ABSTRACT FROM AUTHOR]

• The transient auto-ignition of high-speed ethylene jet flame was studied. • A new high-temperature gas coaxial supersonic jet flame model burner. • The jet velocity affects the auto-ignition position but not the auto-ignition time. • As the jet velocity increased, the flame stabilization mechanism changed. The difficulty of ignition is one of the main problems in the combustion chamber of the air-breathing combined engine. Based on this, the transient auto-ignition in the turbulent non-premixed flames of ethylene in a high-speed jet in hot coflow was studied using a newly developed burner. High-speed hydroxyl (OH) planar laser-induced fluorescence (up to 10 kHz) and large eddy simulations (LES) were performed to investigate the auto-ignition process. The experimental conditions included injection velocities of 334–491 m/s, and the effect of jet velocity on the mixing process, ignition process, and flame stabilization mechanisms were investigated. The results showed that the numerical simulation results were consistent with the experimental data. The velocity could affect the auto-ignition position, whereas it had no effect on the auto-ignition time. The increase in fuel jet velocity is beneficial to the mixing of fuel and high-temperature gas and helps to increase the premixed combustion zone during the formation of fire kernels. Moreover, as the velocity increased, the flame status changed from a transitional flame to a lifted flame, the growth rates of the flame volume and total heat release increased, and the flame stabilization mechanism changed from flame propagation to auto-ignition. [ABSTRACT FROM AUTHOR]

• Laminar burning speed of NH 3 /air flames over a wide range of conditions. • Buoyancy produced bean-shape flame structures at higher pressure. • Nonlinear decrease in laminar flame speed and thickness with pressure. • NH 3 /air flame was thicker than methane/air and hydrogen/air flames. • Equivalence ratio influenced O, H, and OH generation. • High pressure increased flame speed-inhibiting reactions. Ammonia (NH 3) is one solution for decarbonizing the economy. This study used high-speed schlieren imaging to investigate the NH 3 /air flame morphology and propagation characteristics (laminar burning velocity (LBV) and flame thickness) inside a constant volume combustion chamber (CVCC), at conditions relevant to different types of combustors used in power generation. Experiments were carried out for equivalence ratios (ϕ) from 0.7 to 1.3, initial pressures from 1 atm to 10 atm, and an initial temperature of 298 K using a variable gap ignition system. A novel passivation method that minimized the effect of NH 3 adsorption on stainless steel surfaces was used during the CVCC filling to reduce the uncertainty in the actual ϕ before ignition. Results showed that the low speed of NH 3 /air flames increased the effect of buoyancy on the flame shape, especially at high initial pressures or near the flammability limits when flames adopted a bean-shaped structure. A comparison of Rayleigh and Peclet numbers (indicators of buoyancy and diffusion phenomena in flames, respectively) supported these findings. Maximum LBV was measured at ϕ = 1.1 and decreased non-linearly with the initial pressure. Similarly, flame thickness decreased with pressure, and it was thinnest at ϕ = 1.1 for all the examined conditions. Numerical simulations using Cantera produced similar LBV values as the experiment. A sensitivity analysis indicated that reactions involving NH 2 and NO have an important role on LBV. Reactions leading to H, O, and OH radicals generation had the strongest effects on LBV enhancement, while LBV-inhibiting reactions had more accentuated effects at high pressures at which LBV is lower. [ABSTRACT FROM AUTHOR]

In this work, we present an investigation of the structure and formation of nitrogen oxides (NO x) in an axisymmetric laminar diffusion flame in which an ammonia–hydrogen fuel stream is surrounded by co-flowing air. Green ammonia, produced using renewable energy, is a promising carbon-free fuel for replacing hydrocarbon fuels over a diverse range of combustion applications. A key challenge for ammonia combustion is to understand the kinetics of nitrogen oxide formation for designing low-NO x combustion systems. In this experimental study, ammonia fuel is blended with hydrogen to increase fuel reactivity, with a fuel composition range of 15%–100% hydrogen. NO x emissions and fuel slip are measured by downstream sampling of NO, NO 2 , N 2 O and NH 3 with a Fourier Transform Infrared (FTIR) gas analyzer. Flame structure is visualized by planar laser-induced fluorescence (LIF) for NO, NH, and OH radicals, as well as OH* by filtered chemiluminescence. Species concentration profiles are compared to predictions from 2-dimensional axisymmetric numerical models of the laminar flame using recent kinetic mechanisms developed for ammonia combustion. No measurable ammonia slip was recorded for any fuel composition, in agreement with model predictions. Good agreement between predictions and measurements was obtained for exhaust nitrogen oxide concentrations and for in-flame radical profiles, although larger variation and deviations are observed for predictions of NO 2 and N 2 O than for NO. Numerical predictions for spatial distribution of flame radicals and flame liftoff height also showed good agreement with LIF and chemiluminescence measurements. For high fuel hydrogen content, the measured NH and NO profiles are shifted inward toward the fuel outlet, away from the peak temperature contour. Reaction path analysis is used to illustrate the important contributions to NO creation and destruction, including the differing role of key reaction pathways with varying fuel hydrogen content. • NH, OH and NO LIF confirms simulated structure for ammonia–hydrogen coflow flame. • NO emissions follow predictions closer than NO 2 and N 2 O for all kinetic mechanisms. • Ammonia slip below measurement limit for full range of NH 3 -H 2 fuel composition. • OH* chemiluminescence demonstrates accurate lift-off height for low-hydrogen fuel. • NH and NO profiles are shifted to the fuel side of flames with high fuel H 2 content. [ABSTRACT FROM AUTHOR]

It is shown that signals from both laser-induced incandescence (LII) of soot and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAHs) decrease with hydrogen addition (in volume fractions of 3%, 6% and 9% with respect to the fuel mixture) to ethylene–air inverse diffusion flames (IDFs). The structure of the IDF suppresses soot oxidation and the effect of hydrogen addition under these conditions has been studied. Experiments were performed using a frequency-doubled, pulsed dye laser to perform planar LIF of PAH, and a pulsed fibre laser to perform LII. In relative terms, the LII signal decreases more sharply than the PAH LIF signal. This would be consistent with the dependence of soot inception on PAH concentration as well as the suppression of soot growth via the reduced concentration of PAH and perhaps other precursors such as acetylene. Similar trends in relative signal decrease are observed at a range of heights above the burner, despite the measurement locations encompassing a wide range of absolute signal levels. As a comparison, the influence of adding methane to the IDF in the same volume fractions was also studied and found to suppress PAH LIF and LII signals but to a far lesser extent than in the case of hydrogen. [Display omitted] • LII using long-pulsed fibre laser was studied in an Inverse Diffusion Flame burner. • Peak LII/PAH signals were compared for various H 2 and CH 4 additions in C 2 H 4 –air IDF. • With increasing H 2 addition, reduction in LII signal was higher than the PAH signal. [ABSTRACT FROM AUTHOR]

• Turbulent planar natural gas flames are created in a concentric flow slot burner. • The flame stability is controlled by varying the mixture inhomogeneity. • The flame front curvature and flame surface density are extracted using OH-LIF images. • The mixing field was measured using Ryleigh scattering technique at the nozzle exit. • The effects of turbulence and mixture inhomogeneity on the flame are investigated. The present study is an extension of our earlier mixing field investigation in partially premixed flames. In the current work, the link between the mixing field and flame structure, and hence flame curvature and surface density are experimentally studied. The flame structure is captured using a high-speed OH-PLIF technique, whereas the crosswise mixing fields were previously presented in our earlier study. A concentric flow slot burner is employed for producing turbulent planar partially premixed natural gas flames at different levels of mixture inhomogeneity. The effects of the level mixture inhomogeneity, the equivalence ratio, the air to fuel stream velocity ratio, and Reynolds number "Re" on the flame characteristics are investigated. The link between the previously reported data on the mixing field and the flame structure are examined. The correlations between the mixing field and the flame structure were clearly observed in the current study. That is, as the mixture becomes highly inhomogeneous, in rich input jet conditions cases, altering Reynolds number and the mixing length led to a pronounced change in both flame structure and flame curvature while altering L/D influences the flame structure. On the other hand, the effects of the mixing length and Reynolds number on the flame structure and curvature in a more homogeneous mixture were not significant. The data showed that the flame surface density was inversely proportional to the level of the mixture inhomogeneity and directly proportional to the Reynolds number. The significant effect of the jet equivalence ratio on the level of mixture inhomogeneity leads to a significant change in the flame structure and flame curvature. In addition, if the mixture inhomogeneity is less pronounced the variation s of the mixing length and Reynolds number lead to minimal effects on the flame structure and flame curvature. [ABSTRACT FROM AUTHOR]

• Revealing the coupled evolutions of primary and secondary vortex group and the cloud. • Secondary vortex group extends the distribution of the cloud. • Competitive mechanism between factors effecting flame propagation speed is revealed. To reveal the flow field-shock wave-flame coupling evolution law of LPG/DME blended gas explosion venting under different dimensionless vent ratio(K V), the numerical model for multi-blended gas explosion venting was established and research of blended gas explosion venting at K V = 0.02–0.21 was carried out based on experimental verification. The results show that, after the vent opens, the turbulence intensity increases, the velocity gradient of the flow field increases, the maximum explosion wind velocity(V max) of the outfield varies linearly with K V , and the flame of "mushroom shape" is elongated, and "break flame" occurs under the action of the relative motion of "primary and secondary vortex group". When K V ＞0.02, the flame propagation velocity increases locally because of the thermal-mass diffusion instability, and with increasing K V , the average flame propagation velocity decreases from 258.29 m/s to 226.58 m/s. In the process of explosion venting, a typical shock wave fluctuating pressure waveform curve is formed in the outfield. Under the combined action of the infield pressure venting effect and the outfield turbulence intensity, the maximum explosion overpressure in the outfield (P max) reaches maximum 423.97 kPa at K V = 0.07. Meanwhile, the larger the K V , the greater the shock wave velocity due to the lead role of the infield explosion venting shock wave and outfield flame compression shock wave on the shock wave propagation velocity. [ABSTRACT FROM AUTHOR]

• Chemical effect of H 2 on soot/PAH formation of jet fuel. • H 2 promotes the soot formation in co-flow diffusion flame. • PAH formation weakens as the PAH ring number increases. • Rapidly increased A1 and alkynes may lead to the promotion of soot formation. • Inhibiting effects on PAH-=>PAH affect the A4 formation. PAH and soot are harmful substances that can be produced in any type of combustion equipment including aircraft engines. The co-combustion of hydrogen and jet fuel has been applied in aero-engine combustors and large-scale hydrogen addition is a promising solution for reducing the consumption of fossil fuels in the aviation industry. However, it remains unclear how H 2 influences the soot and PAH formation characteristics of jet kerosene. In this study, to investigate the impact of H 2 on soot and PAH formation, planar laser-induced incandescence (PLII), planar laser-induced fluorescence of PAH (PAH-PLIF) and chemical kinetic simulation were conducted for the laminar diffusion flames of RP-3 jet kerosene and its surrogate S1 with different H 2 doping rates. It is found that the introduction of H 2 leads to the increased soot formation. However, the promotion effect of H 2 on the PAH formation weakens as the number of PAH rings increases, and the formation of A4 is significantly inhibited. But the rapidly increase of benzene and alkynes in the H 2 -doped kerosene flame may ultimately lead to the promotion of soot formation. Furthermore, the changes in direct synthesis reactions and PAH=>PAH- jointly affect the converse changes in A1 and A4 formation. These findings will contribute to the development of the soot model and soot/PAH-reduction strategy for the co-combustion of jet fuels and hydrogen. [ABSTRACT FROM AUTHOR]

• This work investigates the mixing field and OH distribution in turbulent planar partially premixed flames at the vicinity of the reaction zone. • Stoichiometric mixture fraction was observed near the reaction zone. • The level of mixture inhomogeneity was reduced by increasing the mixing length. • Increasing the mixing length leads to a shift from single to double reaction zone. • Lower flame front corrugation was observed at the highest stabilization point. Turbulent flames with compositionally inhomogeneous mixtures are commonly used in many combustion systems. In this work, turbulent planar jet flames issued from a concentric flow slot burner, CFSB, were used to study the impact of mixture inhomogeneity near the flame sheet. The CFSB burner can control the mixing inhomogeneity by changing the mixing length "L" between the concentric fuel and air slot nozzles. At various levels of mixture inhomogeneity, the mixing field, presented by mixture fraction, Z, distribution, and the flame structure was investigated via simultaneous Rayleigh and OH-PLIF imaging technique. Our previous study investigated the mixing field immediately downstream of the burner exit in non-reacting conditions. The PDFs of Z showed that the mixing field covered a wide range of mixture fractions, where the high flame stabilization occurred when a large portion of the PDF(Z) was located within the fuel flammability limits. This work showed that further downstream, the highly stabilized flames were also obtained when the range of fluctuations in the mixture fraction was close to the stoichiometric mixture fraction. Moreover, plotting the mixing field using the scatter plot within the mixing regime diagram for various flame conditions showed that the mixing field downstream of the burner exit consistently follows the mixing diagram classification. Moreover, a close inspection of the flame structure showed that the flame sheet varies from thick, and corrugated at a low and high levels of mixing to thin flame sheet but less corrugated and that at a particular mixing normalized length L/D = 7. The flame corrugation data showed that the flame stability occurs at a minimum corrugation factor. [ABSTRACT FROM AUTHOR]

• Quantitatively OH-PLIF measurements in laminar flames of n-heptane are achieved. • Absorption technique is adapted to the calibration of OH-PLIF results. • Effects of pressure on OH concentration in flames are analyzed. In the hydrocarbon fuel combustion, OH radicals play an important role, participating in most of the radical reactions and marking the location of the combustion product region. In this work, a two-dimensional quantitative measurement of OH radical concentration in n-heptane laminar diffusion flames was conducted across a pressure range from 1.0 bar to 4.0 bar. The primary objective was to investigate the impact of pressure on OH radical concentration and distribution in these flames. This investigation was carried out through the utilization of a combination of planar laser-induced fluorescence (PLIF) and calibration via absorption spectroscopy. Specifically, the Q1(8) band was selected for excitation, with an excitation wavelength of 283.55 nm employed for the measurements. As the pressure increases, the area occupied by the OH radical distribution region decreases, and the positions of the maximum concentration values on both sides of the flame axis shift closer to the flame axis itself and towards the burner exit. Besides, the integrated value and max value of OH concentration in the laminar diffusion flames decreases approximately linearly as the pressure increases. These experiments data provide reference for the application of optical techniques and the development of chemical reaction kinetics of n-heptane combustion. [ABSTRACT FROM AUTHOR]

• Measured laminar burning velocity for H2/CH4/air mixtures at elevated pressures. • Cellularity has been observed in all mixtures due to the DL and TD instabilities. • Strain-rate Markstein number varies with the pressure and hydrogen fractions. • A good agreement with predictions based on recently developed H2/CH4 mechanisms. Spherically expanding flame propagations have been employed to measure flame speeds for H 2 /CH 4 /air mixtures over a wide range of H 2 fractions (30 %, 50 %, 70 and 100 % hydrogen by volume), at initial temperatures of 303 K and 360 K, and pressures of 0.1, 0.5 and 1.0 MPa. The equivalence ratio (ϕ) was varied from 0.5 to 2.5 for pure hydrogen and from 0.8 to 1.2 for methane/hydrogen mixtures. Experimental laminar burning velocities and Markstein numbers for methane/hydrogen/air mixtures at high pressures, which are crucial for gas turbine applications, are very rare in the literature. Moreover, simulations using three recent chemical kinetic mechanisms (Konnov-2018 detailed reaction, Aramco-2.0-2016 and San Diego Methane detailed mechanism (version 20161214)) were compared against the experimentally derived laminar burning velocities. The maximum laminar burning velocity for 30 % and 50 % H 2 occurs at ϕ = 1.1. However, it shifts to ϕ = 1.2 for 70 % H 2 and to ϕ = 1.7 for a pure H 2 flame. The laminar burning velocities increased with hydrogen fraction and temperature, and decreased with pressure. Unexpected behaviour was recorded for pure H 2 flames at low temperature and ϕ = 1.5, 1.7 wherein u l did not decrease when the pressure increased from 0.1 to 0.5 MPa. Although, the measurement uncertainty is large at these conditions, the flame structure analysis showed a minimum decline in the mass fractions of the active species (H, O, and OH) with the rise in the initial pressure. Markstein length (L b) and Markstein number (Ma b and Ma sr) varied non-monotonically with hydrogen volume fraction, pressure and temperature. There was generally good agreement between simulations and experimentally derived laminar burning velocities, however, for experiments of rich-pure hydrogen at high initial pressures, the level of agreement decreased but remained within the limits of experimental uncertainty. [ABSTRACT FROM AUTHOR]

• Soot propensity of methane in a turbulent regime is significantly lower than in ethylene flames under similar conditions. • Soot concentration is dependent on the momentum flux ratio. • Methane bluff-body stabilized flames feature similar characteristics to ethylene flames. • Soot inception starts earlier in the recirculation zone of the methane flames. • An experimental dataset for methane flame is provided that can be used for model development. The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C 2 H 2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations. [ABSTRACT FROM AUTHOR]

• An optical injector is designed to investigate internal fuel distributions. • Heated RP-3 Fuel distributions are measured by using PLIF technique at elevated air pressures and temperatures. • The fuel displacements of liquid injections are controlled by centrifugal motion. • The fuel movements of gaseous injections are dominated by penetration. • Separated fuel distributions are found for near-critical and supercritical injections. The uniformity of fuel/air mixture in the mixing channel of the lean premixed and prevaporized injector is important for NOx emissions reduction in aircraft engines. In the present study, an optical annular mixing channel is designed to investigate the effects of heated RP-3 fuel on fuel/air mixing at liquid, gas, and supercritical injections. Planar laser-induced fluorescence is employed to obtain multiple cross-sectional fuel distributions along the axial direction of the mixing channel. The results show that for the liquid injections, the droplets tend to move closer to the outer wall due to the effect of the centrifugal force. Increasing air temperature in the range of 450 K to 670 K can promote liquid evaporation and suppress the centrifugal effect, and result in a 70 % increase in the fuel dispersion area. Compared with liquid injections, the gaseous injections are more evenly distributed and closer to the inner wall. And the displacement of the fuel is mainly determined by the fuel–air momentum ratio. Interestingly, the near-critical and supercritical fuel injections in the range of 638–700 K and 2.2–2.5 MPa show bimodal fuel distributions along the radial direction. The fuel distributed close to the outer wall is from the condensed part which moves under centrifugal effect, while the fuel distributed close to the inner wall is from the gaseous part which is mainly affected by injection penetration. [ABSTRACT FROM AUTHOR]

• Two-stage combustor: rich-catalytic first stage, swirl-stabilized second stage. • Overall lean combustion of syngas with near-zero NO x and low CO emission. • Flame topology different for co and counter-swirl syngas flames. • PLIF & PIV data used to explain differences in observed flame structure. • The recirculation zone behaves differently between co/counter-swirl when overall equivalence ratio changes. 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 (Ф first = 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 NO x concentrations are measured at the combustor exit. The CO emission ranges from 10 ppm to 70 ppm, whereas the NO x 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. [ABSTRACT FROM AUTHOR]

This work presents an experimental investigation of solid fuel combustion in a laminar flow reactor with high-speed laser diagnostics. The main focus lays on the development of image evaluation approaches for accurate determination of single-particle ignition based on optical measurement data. Homogeneous ignition of individual bituminous coal particles is visualized by simultaneous planar laser-induced fluorescence of OH radicals (OH-LIF) and diffuse backlight-illumination (DBI) techniques at 10 kHz. Two coal particle sizes of 90–125 μ m and 160–200 μ m are investigated in conventional air and oxy-fuel conditions with increasing oxygen concentrations. A comprehensive experimental data set is established, containing in total 1518 single-particle events and high-quality ground truth labels of ignition delay times. The previously proposed structure and signal (SAS) analysis is evaluated. Errors and uncertainties are evaluated in a sensitivity analysis with variation in the threshold selection, implying inherent restrictions of conventional image processing methods. An improved blob detection approach is applied, enabling feature detection at various scales and higher accuracy in predicting ignition delay times. Moreover, two deep learning architectures, namely residual network (ResNet) and feature pyramid network (FPN) are implemented. Both networks are capable of detecting ignition with significantly higher accuracy and precision than other algorithms in this study. Influences of training data and depth of networks on the prediction performance of a trained model are examined. The current study shows that the hierarchical feature extraction of the convolutions networks clearly facilitates data evaluation for high-speed optical measurements and could be transferred to other solid fuel experiments with similar boundary conditions. • Multi-parameter diagnostics allow for an extensive data set of particle ignition. • Ignition delay times in air and oxy-fuel atmospheres with increasing oxygen content. • The blob detection enables feature detection and accurate ignition detection. • RestNet and FPN reveal clearly higher accuracy and precision than other methods. • The performance of deep learning networks is systematically evaluated. [ABSTRACT FROM AUTHOR]

• The supercritical EHF jet momentum flux is affected by injection temperature and pressure. • EHF jet plume penetration depth increases with the momentum flux ratio increasing. • EHF jet plume presents a semi-circle shape and semi-Omega shape at different q. The mixing process and distribution characteristics of a supercritical endothermic hydrocarbon fuel (EHF) jet injected into a high-speed crossflow are investigated experimentally by the acetone Planar Laser-induced Fluorescence (PLIF) technique. The instantaneous and averaged distribution of supercritical fuel in the central plane and the cross-sectional planes with various Mach numbers, injection pressure, and temperature are measured and analyzed. A three-species surrogate is employed to calculate the flow parameters of the EHF jet. The boundary and penetration depth of the jet plume are identified by the probability density method. The mixing process of the supercritical fuel is analyzed using the fluorescence concentration distribution. Results show that the momentum flux of the EHF jet increases with the increase of injection temperature when the injection pressure is close to the critical pressure, while it decreases firstly and then increases as the injection pressure is larger than the critical pressure. The penetration depth is dominated by the jet-to-air momentum flux ratio (q), and the EHF jet plume boundary is enlarged with the increase of the q. In particular, the penetration depth formula is given in: y / d = 0.67 q 0.97 x / d 0.29 for supersonic crossflow and y / d =0.15 q 0.57 x / d 0.60 for subsonic crossflow, respectively. The EHF jet plume with lower q presents a semi-circle shape in the cross-sectional plane and mainly expands in the width direction. While it mainly expands in height direction downstream as a semi-Omega shape in the larger q condition. The results indicate that the increase in injection temperature and pressure can enlarge the EHF jet plume distribution and enhance the mixing effect. In comparison, increasing the injection pressure obtain more significant benefits, providing a reference for the fuel supply system and injector design. [ABSTRACT FROM AUTHOR]

Rapid compression machines (RCMs) have often been used to study auto-ignition phenomena and to validate chemical kinetic models at conditions relevant to engines. Many RCM designs were historically used for the latter purpose. First, the flat piston configuration was extensively used and contributed to valuable ignition delay data. Temperature heterogeneity in these devices was later addressed, and new designs were proposed. In particular, creviced piston configurations have been demonstrated to enhance the temperature homogeneity in the bulk volume of RCM chambers. Nevertheless, data from flat piston RCMs are still used in the community. Thus, additional understanding of experimental results from such devices is still useful, and it also helps to enhance the understanding of temperature stratification effects on auto-ignition phenomena. This paper addresses the interpretation of measured ignition delays from a flat piston RCM (without a circumferential crevice in the piston). First, we characterize local and temporal temperature evolution with a toluene planar laser induced fluorescence (PLIF) technique applied in oxygen-free gas under different test conditions. At a compression pressure ( P c ) of 11 bar and an adiabatic core temperature ( T c ) of 750 K, we find that the maximum measured temperature is very close to T c up to 30 ms after the compression stroke. In the second step, we qualitatively compare temperature fields with chemiluminescence images during the auto-ignition of n-hexane and an n-heptane/methyl-cyclohexane (MCH) blend for a fuel-air equivalence ratio of 0.4, P c values of 11 bar and 16 bar and T c values of 700–900 K. The chemiluminescence results of n-hexane show a fast sequential auto-ignition which strongly suggest that the combustion propagation regime is mainly a reaction fronts regime. In the n-heptane/MCH case, the tracking of ignition kernels demonstrates that the auto-ignition initiates in the cold vortex at T c values of 787 K and 834 K and in the adiabatically compressed region at T c values of 742 K and 900 K. In this study, flat piston RCM experiments demonstrate temperature heterogeneities with ranges up to 120 K and gradients of approximately 160 K/mm. This significantly influences the ignition delay measurement in the negative temperature coefficient (NTC) range. Through the example of n-heptane/MCH auto-ignition, it seems that despite temperature heterogeneities, the lower limit of the NTC range is well detected, whereas the higher limit is over-estimated. The determination of temperature thresholds in which ignition delay data may be confounded highly depends on the stratification level and on complex thermo-kinetic interactions. Outside these thresholds, when combustion is dominated by auto-ignition in regions at the adiabatic core temperature, the experimental data are easier to compare with simulation frameworks assuming a homogeneous hypothesis. In chemical kinetics-oriented studies, temperature heterogeneity should be considered when flat piston RCMs are used and when the piston crevice is not adequate. [ABSTRACT FROM AUTHOR]

• Fuel-tracer PLIF is used to quantify the equivalence ratio and temperature. • Misfire of PPC is due to synergistic effect of equivalent ratio and temperature. • In high direct injection pressure, the misfire is due to excessive premixing. • In late direct injection timing, the misfire is due to thermodynamic environment. • Misfire region most likely appears when the equivalence ratio is lower than 0.49. To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion. [ABSTRACT FROM AUTHOR]

• The flame and flow dynamics under air flow pulsations are resolved by performing validated LES and simultaneous OH-PLIF/PIV. • Temporal variations of stretch rate and heat release rate can be used as a criterion for flame extinction/reignition. • A novel decoupled simulation which integrates the plasma source terms and the CFD model is developed. • Thermal and kinetic effects of microsecond repetitively pulsed discharge on combustion are quantitatively interpreted. This paper intensively investigates the inlet pulsation-induced extinction of a premixed swirl flame (PSF) and its plasma-assisted stabilization using the microsecond repetitively pulsed (MRP) discharge. A well-designed low-frequency flow pulsation is applied to the air feedline for the mimicking of transient operations/disturbances in practical engines, which exhibits significant deterioration on the lean blowout (LBO) limit. The MRP discharge is utilized to improve the stability of perturbed flames. It extends the LBO limit at a proper time delay between the air flow pulsation and the discharge, with discharge power less than 1% of the combustion power. Further, the validated large-eddy simulations (LES) and the simultaneous OH planar laser-induced fluorescence/particle imaging velocimetry (OH-PLIF/PIV) measurements are performed to capture the unsteady evolution of flames approaching lean blowout. The nonlinear flame response to the flow pulsation quantitatively reveals the phase difference between the maximum local stretch rates (κ max) and volumetric heat release rates (q ̇ c). A combined effect of the excessive stretch and the reduction of heat release due to flow pulsation can be used to interpret the flame extinction. Finally, a novel LES-ZDPlasKin combined approach, which decouples the discharge and combustion processes, is dexterously developed to simulate plasma-assisted combustion behaviors. The reignition and stabilization due to the plasma effects are well reproduced in the predictive model, pronouncing the indispensable and synergistic thermal and kinetic effects of the MRP discharge on flame stabilization. [ABSTRACT FROM AUTHOR]

This paper describes numerical and experimental studies of gaseous fuel injection for CNG direct injection. To simulate the CNG direct injection, the injection sub-model was updated to include gaseous fuel injection methodology. The gaseous fuel injection methodology, which is similar to a liquid injection model, can be used in the KIVA-3V Release 2 code with some modifications. In addition, this model can be used to simulate gaseous fuel injection using a coarse mesh, which saves calculation time. The core region was defined as an inviscid region near the nozzle exit. The core length has an effect on the spray penetration where a longer core length results in longer spray penetration. The values of the turbulence kinetic energy, turbulence length scale, and turbulence kinetic energy dissipation rate were adjusted depending on the grid location since the RNG (re-normalization group) k – ε turbulence model is known to over-predict gas jet diffusion. Furthermore, a PLIF (planar laser induced fluorescence) method was used for the gaseous fuel injection experiments. Acetone was selected as a tracer and post-image processing was performed using MATLAB code. In this study, the simulation results of CNG injection were compared to experimental data. Through comparison of the spray tip penetration results to experimental measurements, the gaseous fuel injection model produced reliable results for gas fuel direct injection. [ABSTRACT FROM AUTHOR]

• The laminar flame speeds of NH 3 /O 2 /He/Ar mixtures at elevated pressures (up to 1.5 MPa) were firstly (to our knowledge) measured. • The measured data expanded the available database of ammonia kinetic model validation targets. • Physical and chemical effects of N 2 addition on the production of N 2 and NO x were discussed. • The main kinetic effect of N 2 addition on NO production: N 2 addition shifts the reaction N + NO<=>N 2 + O to the left-hand side. The present study aims to expand the available database of laminar burning velocities to a higher initial pressure for ammonia kinetic model validation and to investigate the ammonia flame chemistry especially for the effect of N 2 addition on N 2 and NO x production. In the present study, a high-pressure constant-volume combustion vessel with the pressure release tank was developed and validated. The variations of Lewis number with diluent (He/N 2 , He/Ar) ratio and the relationships of Lewis number and adiabatic flame temperature with initial pressures were investigated to prevent the flame instability effects and create safe experiments at high pressures. The laminar burning velocities of NH 3 /O 2 /He/Ar mixtures with Lewis number around 1.5 and the fixed adiabatic flame temperature of 2250 K at elevated pressures (up to 1.5 MPa) were measured and the overall reaction order of about 1.6 was determined, besides, the effects of pressure on laminar burning velocities and NO x formations were investigated. Moreover, to provide more useful validation data at high pressure, the laminar burning velocities of NH 3 /O 2 /He/Ar mixtures at different adiabatic flame temperatures and initial pressure of 1.0 MPa were presented. Additionally, the predictions of four kinetic models on the experimental data were compared and the discrepancies between the calculated and measured results were found. Since ammonia contains nitrogen element, the effects of N 2 addition on the laminar burning velocities and the production of N 2 and NO x in ammonia combustion were investigated and the physical and chemical effects of N 2 addition were separated. Results showed that the laminar burning velocities and NO x concentration for NH 3 /O 2 /N 2 flames decreased with the increase of N 2 dilution and a slight non-linear tendency was observed. Both physical and chemical effects inhibit the N 2 production in the NH 3 /O 2 /N 2 mixtures. The chemical effects play a dominant role in the inhibition of N 2 production at the N 2 dilution rates below 0.3. The main kinetic effect of N 2 addition on NO production is that the addition of N 2 shifts the reaction N + NO <=> N 2 + O to the left-hand side. [ABSTRACT FROM AUTHOR]

[Display omitted] • The flow and spray fields in a realistic staged HTR combustor were investigated. • Strong coupling between the pilot and main swirling flows has been identified. • A precessing vortex core was identified and analyzed using proper orthogonal decomposition. • Spray distribution and its correlation with the flow dynamics were illustrated. The present work investigated the flow and spray fields in a realistic concentric staged high-temperature-rise (HTR) combustor in isothermal conditions using high-speed (10 kHz) particle image velocimetry (PIV) and 10 Hz kerosene planar laser-induced fluorescence (kerosene-PLIF). The two stages of the combustor, including a pilot stage and a main stage, were operated both individually and jointly to investigate their corresponding flow fields and the interactions between the flow from the two stages. From the time-averaged flow field results, strong coupling between the pilot and main swirling flows has been identified, leading to the formation of a primary recirculation zone (PRZ) and a lip recirculation zone (LRZ). From the instantaneous flow fields, the presence of a precessing vortex core (PVC) was observed when the two stages were flowing jointly, and the dominant PVC unsteady motions located in the inner shear layer of the pilot swirling was identified using snapshot proper orthogonal decomposition (snapshot POD) with dominant frequencies at 1400 and 1515.4 Hz. Comparing the flow field with the spray field indicates that the unsteady flow motion, in addition to the mean bulk flow, plays a key role in the understanding of the spray distribution. Noticeably, it is shown in present study that the dominant unsteady flow motions properly captured from the POD analyses show good agreement with some primary features of the spray distribution, which can be valuable to unraveling the complex flow-spray coupling in the HTR combustor operated under realistic conditions. [ABSTRACT FROM AUTHOR]

• The spherically expanding flames of TRF/air/CO 2 mixtures at elevated temperatures and pressures were measured. • The laminar burning velocity variation with respect to the temperature, pressure and dilution ratio of CO 2 was investigated. • The impact of CO 2 addition on the markstein lengths and the flame instabilities were discussed. • The individual effect of CO 2 addition was decoupled and numerically analyzed respectively. Spherically expanding flames of TRF/air mixtures diluted with different levels of CO 2 content at elevated temperatures and pressures were experimentally studied in this work to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO 2 in EGR gases on the laminar flame characteristics of gasoline fuel. The experiments have been performed using a constant volume combustion bomb with schlieren method. The laminar burning velocities (LBV) and markstein lengths of the TRF flame with different dilution level of CO 2 (0–20%) at the elevated temperature of 373 K, 443 K and pressures of 1 bar, 3 bar and 5 bar have been studied. Numerical decoupling simulations were then applied to investigate the individual dilution, thermal and chemical effects of CO 2 , respectively. The CO 2 addition has a suppressing effect on the LBV of the TRF fuel at all the tested conditions. The LBV of the TRF mixture diluted with 20% CO 2 reduced to about 60% of the non-diluted flame at 373 K and atmospheric pressure. The LBV were promoted by the elevation of initial temperature, while inhibited by the promotion of initial pressure. The increasing of initial temperature extended both the lean and rich limits, while the elevation of initial pressure would facilitate the combustion on the leaner flame. The Markstein length had a slight increase, indicating a small increase of flame instability, with the increase of the of CO 2 dilution ratio for the rich flames and decreased for the lean flames. The crossover equivalence ratio, φ*, was witnessed to shift slightly towards the leaner side, while the critical equivalence ratio, φ c , moved to the richer side. Among the three factors, the dilution effect played the dominant role and the chemical effect came up the next. The thermal effect took up the least portion, yet non-negligible. In addition, the chemical effect showed a more sensitive respond to the temperature variation. [ABSTRACT FROM AUTHOR]

• MILD combustion reduces NOx and CO emissions by 43–70% and 63–81% respectively. • The hot coflow causes local self-ignition upstream, which leads to the combustion zone moving upstream. • BGG MILD combustion can be established by hot coflow with φ > 1, which is the difference between BGG and CH 4 in terms of establishing MILD combustion. Biomass gasification is a promising clean technology. However, biomass gasified gas (BGG) suffers from problems related to actual utilization. BGG contains approximately 65% dilution components, that cause ignition difficulty, combustion instability, and high pollutant emissions. Within this context, Moderate or intense low-oxygen dilution (MILD) combustion is a clean and efficient combustion mode under dilution conditions, which is naturally suitable for BGG; however, the mechanism of establishing BGG MILD combustion remains unclear. Here, we investigated the MILD combustion of BGG with a hot coflow generated by a non-premixed CH 4 -air flat flame. Chemiluminescence and OH planar laser-induced fluorescence (PLIF) imaging were performed to characterize flame structures. Further, NOx and CO were detected in flue gas. Results show that CO emission can be reduced by 43–70% and NOx emission by 63–81% under MILD combustion. Decreasing the equivalence ratio (φ) of the hot coflow can reduce NOx emissions and increase CO emissions. The BGG MILD combustion can also reduce NOx from the hot coflow, which supports the hot coflow to enter industrial or civil burner applications. Introduction of the hot coflow causes local self-ignition; therefore, the combustion zone of the BGG flame moves upstream. This effect becomes more significant with an increase in φ. Further, BGG MILD combustion can be established by a coflow with a φ of 1.1; O 2 in the hot coflow was no longer necessary to establish the MILD combustion. The limitations of BGG MILD combustion applications can be reduced considerably, and the mixed combustion of natural gas and BGG based on this jet in a hot coflow configuration can easily establish MILD combustion. [ABSTRACT FROM AUTHOR]

[Display omitted] • The well-designed rapidly-mixed swirl burner anchors compact ammonia/air flames. • Gliding arc discharges significantly extend the lean blow-off margin by repetitive ignition. • The flame structure and intermediate plasma chemistry are optically inspected. • Extremely lean ammonia flames assisted by plasma achieve NO x emissions below 100 ppm. Aiming at the potential development of ammonia as a carbon-free renewable fuel, this work investigates the flammability and NO x emission of swirling ammonia/air flames and utilizes plasma to enhance ammonia combustion. First, the well-designed rapidly-mixed swirl burner can anchor compact ammonia/air flames under a wide range of flow conditions, with the current maximum heat release of approximately 4.7 kW. The flame is progressively detached from the quartz confinement tube and goes blow-off when the equivalence ratio drops below approximately 0.7–0.8. Then, to alleviate the problem of low flammability, gliding arc discharges driven by a 12.5 kHz alternating current (AC) power supply are facilitated to extend the lean blow-off margin to approximately 0.3–0.4. The localized flame kernels surrounding the discharge column are detected by high-speed photography. The planar laser-induced fluorescence (PLIF) imaging of OH radicals, optical emission spectroscopy, and NH 2 * chemiluminescence measurement are performed to interpret the intermediate chemistry. Finally, the NO x emission of the swirling ammonia/air flame is measured by a flue gas analyzer. Results show that although the AC-powered gliding arc exhibits weak global effects on the NO x reduction of burner-stabilized ammonia/air flames prevailing at higher equivalence ratios (e.g. , φ > 0.75), leaner flames stabilized by discharges can achieve NO x emissions below 100 ppm due to the thermal DeNO x mechanism. [ABSTRACT FROM AUTHOR]

• Three ZrO 2 -based ceramics coatings were prepared to investigate their effects on flame stability. • Blending Y 2 O 3 , MgO, or Al 2 O 3 was beneficial to decrease quenching distances. • Wall thermal and chemical effects were qualitatively evaluated. • Reasonable allocation of material components was an effective strategy to regulate combustion. From the perspective of the thermal/chemical performance management of a slot burner, three kinds of blended coatings with the low thermal conductivity and certain chemical activities, including 30wt.%Y 2 O 3 /ZrO 2 , 30wt.%MgO/ZrO 2 and 30wt.%Al 2 O 3 /ZrO 2 , were prepared on stainless steel walls plated by atmospheric plasma spraying. The effects of coating material, wall temperature, and equivalence ratio on fuel-rich methane flame characteristics were investigated experimentally. The OH planar laser-induced fluorescence (OH-PLIF) technique was used to obtain OH intensity and flame anchoring location. The physical and chemical properties of different coatings before and after spraying were analyzed by a series of material and surface characterization techniques. Results show that compared with pure ZrO 2 coating, the quenching distances of three ZrO 2 -based blended coatings with Y 2 O 3 , MgO, and Al 2 O 3 decrease to some extent, resulting in the minimum quenching distance of Al 2 O 3 /ZrO 2 coating, and the maximum one for Y 2 O 3 /ZrO 2 coating. Meanwhile, the blended coating walls result in the higher flame anchoring location than that of pure ZrO 2 in most cases, in which the coating of Y 2 O 3 /ZrO 2 displays the highest location. The blended coatings have higher thermal conductivity, which is beneficial to balance part of the heat loss through the axial heat recirculation of wall structures, and thus expanding stability limits at low temperatures. In addition, partial lattice solid solutions are formed in mixture systems after the ultra-high-temperature thermal spraying, leading to more surface lattice oxygen and stronger chemical promotion. In summary, reasonable allocation of ZrO 2 -based ceramic components as coating wall is an effective alternative strategy to control flame stabilization in small-scale combustion devices. [ABSTRACT FROM AUTHOR]

• Comparisons of flame propagation using SI and different pre-chambers were carried out. • Pre-chamber ignition fueled with hydrogen was adopted to further improve the lean flammability limit. • The pressure oscillation was clearly observed in experiments of prechamber ignition fueled with hydrogen. The combustion characteristics of CH4-air mixture under the lean-fuel conditions ignited by different methods including SI, unfueled pre-chamber ignition and fueled pre-chamber ignition were investigated experimentally in a constant volume combustion chamber (CVCC) through high-speed schlieren photography. And a self-designed fueled pre-chamber turbulent jet ignition (TJI) combustion system with a single orifice configuration and small volume was employed. Results show that TJI could enhance combustion including the rapid increases of flame tip velocity and flame area growth rate, which also was manifested by increasing peak pressure and shortening time to peak. The pre-chamber ignition fueled with hydrogen can greatly extend the lean flammability limit in the main chamber and reduce the sensitivity of flame development with the excess air coefficient. However, the oscillations in flame propagation and pressure evolution due to the flame-shock interaction were obvious under the pre-chamber fueled with hydrogen. Overall, in the present work, the turbulent flame development in the main chamber depends on the flame consumption velocity in pre-chamber, orifice size of pre-chamber, and the mixture concentration in main chamber. These results are of great help to achieve the highly-efficient combustion and to shorten the combustion duration in practical SI engines. [ABSTRACT FROM AUTHOR]

• A two-phase fluid dynamic model for the simulation of entrained flow gasification is created. • A free jet approach from literature is empirically adapted to the two-phase free jet. • The model is validated with experimental data from an entrained flow gasifier. • The flame structure is analyzed using OH*- chemiluminescence. • Droplet size and velocity is measured using phase Doppler anemometry (PDA). We present a steady-state two-phase fluid dynamic model based on a classical free jet approach for the simulation of the main reaction zone in an atmospheric entrained flow gasifier. Radial Gaußian profiles of the mixing ratio and velocity for single-phase free jets taken from literature are adapted to the two-phase free jet. Exchange of momentum and mass between the jet and the surrounding is described by a parameter derived from atomization experiments under ambient conditions. With the free jet equations, a pattern of gas phase velocity and temperature is calculated. Droplets are introduced at the nozzle, accelerated, heated up and evaporated. Initial droplet size fractions are measured under ambient conditions. Sub-process models for fuel decomposition, oxidation and the water gas shift reaction are included in the model. The interaction of the sub-process models and the free jet equations are considered via balance equations for momentum, mass and enthalpy, solved in each control volume. The two-phase free jet model (2Ph-FJM) calculates the local composition, velocity and temperature of the gas phase as well as velocity, temperature and evaporation of the fuel droplet fractions. Simulation results are approved for a set of experimental data (i.e. droplet velocity, droplet size distribution and flame structure via OH*-chemiluminescence imaging) from the bench-scale atmospheric entrained flow gasifier REGA. [ABSTRACT FROM AUTHOR]

• The NOx and CO formation characteristics in thin reaction zone have been investigated. • The small-scale turbulence is quantified in preheat zones of 1-D premixed flames. • An efficient emission modelling approach has been formulated and demonstrated. • The fuel/air unmixedness is important for the quantitative NOx prediction. It is practically important to accurately predict NOx and CO emissions in lean premixed flames for the development of fuel-efficient and low-emission combustion systems. In this study, the efficient modelling with pre-tabulated formation characteristics of NOx and CO is systemically investigated. Their formation characteristics for turbulent flames in the flamelet and thin reaction zone regimes are first quantified with one-dimensional adiabatic premixed flames through the incorporation of turbulence induced diffusion. Results show that the formation of NOx and CO can be characterized by two different stages, i.e., a rapid increase of both in flame front and the linear growth for NOx and exponential decay for CO in post-flame zone. An efficient NOx and CO modelling approach is formulated and demonstrated in a full-scale methane gas turbine combustor, in which species NOx and CO are transported and solved, with the source terms being modelled by pre-tabulated formation characteristics from one-dimensional premixed flames with detailed chemical kinetics. Realizable k-epsilon model and finite rate/eddy dissipation model in conjunction with a two-step global mechanism are employed to primarily predict the flow and flame characteristics. Results show that the predicted NOx and CO agree with experimental measurements over a wide range of equivalence ratios. The discrepancy in NOx emission can be accounted by fuel/air unmixedness. For CO, the rapid increase due to the incomplete CO oxidation resulting from its increasing characteristic oxidation time near lean blow-out (LBO) is correctly captured. It is further shown that the predicted CO emission is sensitive to combustion/kinetics model parameters and the accurate prediction of flame shape and dynamics is crucial to capture the trend of CO formation near LBO. [ABSTRACT FROM AUTHOR]