Wide Range Of Equivalence Ratios Research Articles

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.

Experimental and Numerical Study of the Laminar Burning Velocity of Biogas–Ammonia–Air Premixed Flames

Biogas is a gas resulting from the digestion of biomass, which means transforming organic waste into energy. It is composed essentially of methane (CH4) and carbon dioxide (CO2) and can also contain ammonia (NH3) as an impurity. Biogas is generally used to generate electricity or produce heat in a cogeneration system. With the renewed interest in ammonia and the increasing development of biogas caused by the urge for an energetic transition, those two carbon-neutral fuels are being investigated as a mixture in this study through the laminar burning velocity (LBV). In this paper, the LBV of biogas ammonia air mixtures are investigated experimentally for the first time over a wide range of equivalence ratios and ammonia concentrations. The biogas studied was 60% CH4 and 40% CO2 in volume. The NH3 concentration in the fuel varied from 0 to 50% vol. while the equivalence ratio varied from 0.8 to 1.2. The experiments were conducted at constant pressure in a constant volume vessel at 300 K and 1 bar. Adding ammonia to biogas decreases the LBV while the Markstein length is not very sensitive to ammonia addition. The CEU-NH3-Mech-1.1 and Okafor mechanisms show good agreement with the experimental laminar burning velocity. The effect of radiative heat losses on the measurement is also investigated.

An experimental and kinetic modeling study of the impact of nitric oxide and hydrogen on iso-Octane combustion

The knowledge of the combined effects of exhaust recirculation and hydrogen enrichment on hydrocarbon fuels combustion is limited to the complexity of the ternary interaction. In this work, experimental and kinetic modeling techniques were combined to investigate the chemical effects of high-level NO, H2, or NO/H2 additions on hydrocarbon fuels. A series of jet-stirred reactor experiments were conducted on the oxidation of iso-Octane with NO/H2 additions at atmospheric pressure and over a wide range of equivalence ratios and temperatures. Experimental results reveal that high concentrations of NO improves iso-Octane oxidation in the temperature region of 800−1000 K and promotes its consumption more efficiently under the ultra-lean condition than stoichiometric and rich conditions. The addition of H2 slightly enhances iso-Octane oxidation by facilitating hydrogen abstraction reactions under the ultra-lean condition, but this effect is insignificant in the stoichiometric condition. The addition of combined H2 and NO demonstrates a more significant promoting effect on iso-Octane oxidation under both ultra-lean and stoichiometric conditions, indicating additive effect in the global reactivity. Existing kinetic models from the literature were incapable to capture these effects. We developed a detailed model which better accounts for the competing chain promotion/termination effects involved in NO sensitization on fuel oxidation, by modifying important reaction rate constants and introducing key nitrogen- containing C3 species. NO and combined NO/H2 additions both enhance the fuel initiation process, primarily through the NO+HO2=NO2+OH reaction, which promotes hydrogen abstraction pathways by converting HO2 to more active OH radical. The NO/H2 combination leads to intricate effects on intermediate species, driven by the competition between NO+HO2 and H+NO2 reactions for OH evolution, as well as the competition between H+O2 and H+NO2 for H consumption. This work contributes to the understanding of ternary hydrocarbon/NO/H2 interactions, providing insights into the design of improved fuel systems to optimize engine performance and mitigate emissions. Novelty and significance statementThis study investigates the effects of high-level NO, H2, or NO/H2 additions on iso-Octane oxidation through experiments and kinetic modeling. New JSR experiments were performed on the oxidation of iso-Octane with and without NO, H2, or NO/H2 additions at various equivalence ratios, 1 atm, and temperatures ranging from 600−1100 K. Experimental results reveal that high-level NO enhances iso-Octane oxidation under the ultra-lean condition in 800−1000 K. H2 has a moderate impact under the ultra-lean condition, but its effect is insignificant in the stoichiometric condition. Combined H2 and NO significantly enhance iso-Octane oxidation in ultra-lean and stoichiometric conditions. The developed detailed model outperforms existing ones in predicting these effects on fuel oxidation. This research contributes to understanding hydrocarbon/NO/H2 interactions and optimizing fuel systems.

Experimental and numerical characterization of hydrogen combustion in a reverse-flow micro gas turbine combustor

This study conducts experimental and numerical investigations on a stagnation-point, reverse-flow combustor to examine the effect of its design features on the combustion behavior and emission characteristics of non-premixed, turbulent hydrogen/air flames. The study covers a wide range of equivalence ratios, from ultra-lean to stoichiometric, and thermal power inputs varying between 5 kW and 20 kW. The main motivation behind this design is to achieve passive recirculation of flue gas within the combustor to enhance dilution of the incoming charge, thus potentially leading to relatively uniform thermal fields, lower temperatures, and suppressed NOx emissions. This design concept is implemented in a micro gas turbine – a practical energy conversion device tapped to play a major role in decarbonizing CHP systems suitable for small-scale industries. Optical chemiluminescence measurements, in addition to thermocouple-based temperature and Quantum Cascade Laser (QCL) spectroscopy-based exhaust emissions measurements, were conducted for all the test cases. Spatial OH* signal intensity is used as heat release signature to identify distinct features of the test flames, allowing for useful information on the nature and dynamics of the combustion process within the combustor. The results show stable hydrogen/air flames for all the operating conditions; however, a more distributed combustion is observed at higher powers for all the given equivalence ratios. Relatively higher NOx emissions at φ >0.2 suggest the lower achieved mixing between burnt and fresh gases, particularly in the non-preheated case, prevent a thorough dilution of the fresh mixture. The latter is necessary to achieve low-temperature, uniform thermal fields – an essential for low thermal NOx emissions in gas turbine combustors. Further, CFD results show that preheating considerably changes flow field dynamics and recirculation pattern for better mixing; however, this also dramatically increases thermal NOx emission, with the lowest levels observed at φ = 0.2, while the peak occurs at φ = 0.4. Finally, a ‘brute-force’ sensitivity analysis shows that thermal NO and NNH pathways contribute the most to NO formation in hydrogen/air flames.

Roles of NH2 reactions in ammonia oxidation at intermediate temperatures: Experiments and chemical kinetic modeling

In recent years, a number of experimental and numerical studies on chemical kinetics of ammonia (NH3) have been performed and NH3 chemical kinetic models have been proposed. However, as NH3 oxidation at intermediate temperatures (below 1400 K) has been investigated by reactor experiments, discrepancies between measurements and model-prediction have been reported. In addition, NH3 oxidation in extremely fuel-rich conditions has not been well studied. This study aims to further explore the chemical kinetics of NH3 oxidation at intermediate temperatures using a micro flow reactor with a controlled temperature profile (MFR). NH3 and H2 in the oxidation of NH3/O2/Ar mixtures at atmospheric pressure were measured using a gas chromatograph and a quadrupole mass spectrometer connected to the MFR. The maximum wall temperatures were varied in the range of 1100–1400 K, and six equivalence ratios (ϕ = 0.5–4.0) were examined. An updated reaction model for NH3 was developed by placing special emphasis on NH2 reactions. The revised model satisfactorily reproduced the experimental and literature data. Reaction analysis revealed that the main channel of the NH2 + HO2 reaction is chain-terminating, as proposed by Klippenstein et al., which was critically important to reproduce the measured profiles of NH3 in the MFR. Furthermore, the contribution of NH2 + NH2 = N2H2 + H2 to H2 production showed large differences among the models because of large uncertainty of its rate constant. Models that used larger rate constants for this reaction overestimated H2 production. Novelty and significance statementThis study addresses unrevealed intermediate temperature reactions in NH3 oxidation where large discrepancies between experiments and model predictions as well as among models have been reported. Species profiles of NH3 and H2 in NH3 oxidation at intermediate temperatures (1100–1400 K) over a wide range of equivalence ratio (0.5–4.0) was obtained using a micro flow reactor with a controlled temperature profile. The model developed in the present study (adopted the rate constants of NH2 reactions in the latest literatures) satisfactorily reproduced the experimental results as well as literature data. Branching reactions of NH2 + + HO2 and NH2 + NH2 are found to be crucial for the NH3 oxidation and H2 production at intermediate temperatures, which would enhance further improvement of the ammonia chemical kinetic model and moreover, help control ammonia combustion.

Prediction of NOx emissions and pathways in premixed ammonia-hydrogen-air combustion using CFD-CRN methodology

Ammonia-hydrogen blends have gained significance as they are carbon-free energy-dense fuels. However, NOx emissions have been a significant concern. In this study, the emissions from the premixed combustion of 70/30VOL.% NH3/H2 blend is studied using the computational fluid dynamics (CFD)- chemical reactor network (CRN) approach. The velocity, temperature, and species field are first obtained using CFD, based on which, a network consisting of perfectly stirred reactors (PSR) and plug flow reactor (PFR) is constructed. Three mechanisms have been implemented in the CRN to predict the NO and N2O emissions. It is shown that the trend of NOx is correctly predicted by the CRN over a wide range of equivalence ratios (φ) of 0.65–1.2 as compared to the authors’ recently published experimental data. It is demonstrated that a single CRN (based on the CFD for a specific φ) can be run to cover the range of φ = 0.65 to 1.2 b y scaling the temperature input to each reactor of the CRN. To contrast the NO pathways at different φ, quantitative reaction pathway diagrams (QRPD) are constructed, and dominant production and consumption pathways of NO for lean and rich combustion are established. The shifts in reaction pathways with φ are noted and found to be governed by OH, O, and H radicals. Next, the effect of stoichiometry on these radicals is established. Finally, the experimental trend of high NO close to stoichiometric combustion and high N2O in very lean combustion along with their respective pathways are explained.

Acceleration of premixed H[formula omitted]-Air-Steam flames when accounting for thermal radiation

Diluting hydrogen flames with steam can increase flame stability and reduce nitrogen oxide emissions. While the effects of dilution on the flame characteristics due to variations in chemistry or transport properties have been well investigated, this is not the case for those related to thermal radiation. Thermal radiation is indeed often neglected in free flame simulations. However, it can be critical in diluted flames in which participating species are present in the fresh gases. This study explores thermal radiation’s impact on H2-Air flames diluted with steam through 1D laminar flame computations. To this end, a reactive fluid solver is coupled with a semi-analytical thermal radiation code. Both the grey gas approximation, for a preliminary understanding, and the more accurate CK narrow-band model, for detailed simulations that account for spectral-dependent gas properties, are used. The combustion numerical setup is validated against literature results. Three main thermal radiation effects are highlighted: a preheating of the fresh gases, an increased laminar flame speed, and a decreased temperature in the burnt gases. It is found that domain length considerably affects this preheating and flame speed, which can then affects laminar burning velocity measurements. It is shown that thermal radiation and combustion can often be decoupled and that a preheated adiabatic flame can recover the main characteristics of the corresponding radiative flame. The increase of the laminar flame speed by thermal radiation is reported for a wide range of equivalence ratios, dilution levels, and pressures. The observed acceleration is especially significant for lean and very lean flames which can be up to 500% faster when considering thermal radiation.

Effect of the variation of oxygen concentration on the laminar burning velocities of hydrogen-enriched methane flames

The combustion properties of hydrogen-containing fuel mixtures and the effect of the variation of the oxygen content of the oxidizer are at the center of recent research interest. Laminar burning velocity measurements with varied oxygen content can help in the validation of reaction mechanisms for better simulations of combustion systems using exhaust gas recirculation (EGR) or oxygen-enriched atmosphere. Such measurements were carried out in hydrogen-enriched methane−air flames using the heat flux method with higher accuracy and a wider range of initial oxygen and hydrogen concentrations compared to the similar studies in the literature. The mole fractions of the hydrogen and oxygen contents of the initial fuel and oxidizer mixtures were varied between xH2 = 0 and 0.20, and xO2 = 0.14 and 0.23, respectively. The initial gas temperature and pressure were 298 K and 1 bar, respectively. It is demonstrated that the increase of combustion rate by the hydrogen enrichment can be compensated with the decrease of the oxygen content. This compensating effect was investigated in detail in a wide range of equivalence ratio (φ). The experimental data were simulated with 11 widely used methane combustion reaction mechanisms. The prediction accuracies of the mechanisms at lean and rich equivalence ratios were significantly different and the important reaction steps were identified using sensitivity analysis for three mechanisms. Mechanisms POLIMI-2014 and Caltech-2015 gave the best overall predictions.

Low Temperature Heat Release and ϕ-Sensitivity Characteristics of Iso-Octane/Air Mixtures

ABSTRACT Chemical energy release from high octane number fuels via low temperature heat release (LTHR) can help develop high-efficiency gasoline engines by promoting ultra-lean combustion in spark ignition engines and improving combustion control in gasoline compression ignition engines. A recently developed experimental technique that permits isolated LTHR investigations in motored engines was used to characterize the LTHR behavior of iso-octane/air mixtures ranging in strength from to at multiple inlet temperature conditions ( to °C). LTHR changes were studied by observing variations in exhaust CO emissions and exhaust temperature increase. Observed heat release results were explained using cylinder mixture pressure-temperature histories alongside supporting chemical kinetics modeling estimates of mixture reactivity in the form of chemical ignition delay (ID) time. The effects of fuel enrichment on iso-octane/air mixture reactivity were found to be non-uniform and dependent on mixtures’ thermal state trajectories in the LTHR ID peninsula. LTHR intensity measurements were used to discuss changes in mixture sensitivity at different engine inlet conditions. It was shown that by appropriately adjusting mixture thermal conditions via charge cooling from direct fuel injection and intake air heating, reactivity enhancements could be exploited maximally; and strong, positive, linear sensitivity of around 10 J per 0.1 increase in could be realized across a wide range of equivalence ratios from 0.05–1.2. It was also found that dominance of charge cooling effects at rich conditions resulted in negative and zero sensitivity regions.

Effect of radiation on laminar flame speed determination in spherically propagating NH3-air, NH3/CH4-air and NH3/H2-air flames at normal temperature and pressure

The use of spherically propagating flames is common for measuring the laminar flame speed in NH3-air, NH3/CH4-air and NH3/H2-air mixtures. However, the radiation-induced uncertainty in such mixtures has not been thoroughly investigated. Due to the low laminar flame speed of ammonia mixtures, it is anticipated that the radiation effect is considerable for such mixtures. This study aims to fill this gap by conducting numerical simulations using different chemical mechanisms and the adiabatic and optical thin radiation models to examine the effects of radiation on spherically propagating NH3-air, NH3/CH4-air and NH3/H2-air flames. The simulations are performed for mixtures at normal temperature and pressure (Tu=298 K and P = 1 atm) and wide range of equivalence ratios. The radiation-induced uncertainty in spherical flames is quantified and compared to planar flames. The importance of the radiation-induced flow and thermal effects in spherical flames is compared between different mixtures and a correlation is developed to determine the radiation-corrected flame speed for spherical NH3-air flames. Considering the radiation effect in NH3-air, it was found that using different mechanisms results in considerable discrepancies in laminar flame speed determination. Some mechanisms showed that the radiation-induced flame speed in spherical flames was underpredicted by more than two times compared to planar flames, and the radiation-induced uncertainty for lean and rich spherically propagating NH3-air flames exceeds 20%. However, the radiation-induced uncertainty at normal temperature and pressure in spherically propagating NH3/CH4-air and NH3/H2-air flames was less significant, not exceeding 11%. Finally, an updated correlation is proposed to determine the radiation-corrected flame speed for NH3-air flames that can be directly used in spherical flame experiments measuring the laminar flame speed.

Impact of Fe-doped H2/O2 flame equivalence ratio on the fate and temperature history of early particles

The temperature and species concentration history experienced by the gas-borne nanoparticles during their evolution in the flame has a major impact on their size, morphology, composition, and crystallinity. In our recent work (Combust. Flame, 244 (2022) 112251), we have reported optical emission measurements of a Fe(CO)5-doped H2/O2/Ar fuel-lean (ɸ = 0.5) flame, revealing that the temperature of the early-formed nanoparticles exceeds the gas temperature by several hundred degrees, while the particle volume fraction increases sharply, followed by rapid disintegration in the reaction zone. This behavior, modeled by single particle Monte-Carlo simulations indicates involvement of heterogeneous reactive processes at the particle surface, such as particle reduction and oxidation, growth and etching. Within the refined approach of the current study, reactive and non-reactive collisions were treated consistently, assuming rapid thermalization between the impinging molecule and the particle, with subsequent random energy sampling to determine reactivity. In the present work, we test the limits and validity of the heterogeneous flame-particle interaction model by manipulating the oxidation–reduction and growth-etching balance by varying the equivalence ratio (0.25<ɸ<1.5). For the entire range of equivalence ratios studied in experiments and simulations, we find a deviation between the particle and gas phase temperatures with significantly higher particle temperature, which is continued until a full degree of iron oxidation within the particle (O/Fe ratio=3/2) is reached. Validating the simulations against the measurements of particle temperature and volume fraction over a wide range of equivalence ratios, emphasized the necessity to account for gas-phase Fe-atom concentration depletion. We incorporated nucleation theory to estimate initial cluster population, linking Fe-concentration variation in the gas phase and the stochastic particle evolution model. The surface reaction parameters in our current work were updated using density functional theory literature data, and validation of the model predictions against experimental data, across the entire range of equivalence ratios.

Measurement and Kinetics Prediction of Undiluted Methane–Oxygen Laminar Flame Speeds

This study presents new laminar flame speed () data for a wide range of equivalence ratios () at low pressure (0.5 atm) and multiple initial pressures (0.5–3 atm) at an equivalence ratio far from stoichiometry (). All experiments were conducted at room temperature using spherically expanding flames. At 0.5 atm, peaked slightly lean, reaching a maximum of 411 cm/s near . Far from stoichiometry, was approximately 72 and 34 cm/s at and 2.4, respectively. predictions were computed using several commonly cited chemical kinetics mechanisms for methane combustion, with performance approximately the same for all mechanisms. Every mechanism underpredicted at 0.5 atm for , with performance especially poor near stoichiometry ( underprediction near the peak). For , experiments were conducted for pressures from 0.5 to 3.0 atm, showing that varies approximately according to the power law . The mechanisms captured this pressure dependency, with GRI-Mech 3.0 performing especially well. Sensitivity analysis shows that many reaction adjustments are necessary to solve the underprediction near stoichiometry, with potentially a reaction of interest.

3-D Map Producing for Groundwater Level using Kriging Interpolation Method

The effect of the initial pressure upon the laminar flame speed, for a methane-air mixtures, has been detected paractically, for a wide range of equivalence ratio. In this work, a measurement system is designed in order to measure the laminar flame speed using a constant volume method with a thermocouples technique. The laminar burning velocity is measured, by using the density ratio method. The comparison of the present work results and the previous ones show good agreement between them. This indicates that the measurements and the calculations employed in the present work are successful and precise

Self-Acceleration and global pulsation of unstable laminar Hydrogen-Air flames

The self-acceleration and global pulsation of spherically expanding laminar hydrogen-air flames were studied in a spherical explosion vessel, over a wide range of equivalence ratios (0.4–2.0), initial temperatures (300–400 K), and initial pressures (0.1–0.7 MPa). Comprehensive quantitative data on the self-acceleration propagation of unstable laminar hydrogen-air flames were obtained. The results show that the self-similarity appears after the onset of instability, and the previous assertion of acceleration exponent α = 1.5 is not valid for hydrogen-air flames. The derived values of α for such flames vary from 1.125 and 1.39 in the present work. For the self-accelerating flame, the temporal evolution of the flame radius is described by ru=Atα, and temporal flame propagation speed is described by Sn=Aαtα-1. A modified theoretical expression of the constant A is proposed and validated against the present experimentally derived results. The acceleration phase after flame instability presents a global pulsating acceleration pattern. The frequency of global pulsation rises as the pressure or temperature of the flame increases. A pulsing flame speed equation can be used to reliably estimate flame speed after the onset of cellular instability. Self-acceleration observed in small laboratory explosions can serve as a predictive indicator for flame behaviour on a larger atmospheric scale.

Performance Investigation of Currently Available Reaction Mechanisms in the Estimation of NO Measurements: A Comparative Study

Ammonia (NH3) has been receiving the attention of researchers as an alternative promising green fuel to replace fossil sources for energy production. However, the high NOx emissions are one of the drawbacks and restrictions of using NH3 on a broad scale. The current study investigates NO production/consumption for a 70/30 (vol%) NH3/H2 mixture using kinetic reaction mechanism concepts to shed light on the essential reaction routes that promote/inhibit NO formation. Sixty-seven kinetic reaction mechanisms from the literature have been investigated and compared with recently reported measurements at a wide range of equivalence ratios (ϕ) (0.6–1.4), atmospheric pressure and temperature conditions. Both numerical simulations and experimental measurements used the same combustion reactor configuration (premixed stabilized stagnation flame). To highlight the best kinetic model for the predicting of the NO experimental measurements of NO, a symmetric mean absolute percentage error (SMAPE) has been determined as a preliminary estimation by comparing both numerical and experimental measurements. The results found that the kinetic reaction mechanism of Glarborg showed an accurate prediction with a minor error percentage of 2% at all lean and stoichiometric conditions. Meanwhile, the kinetic model of Wang accurately predicted the experimental data with 0% error at ϕ = 1.2 and underestimated the mole fraction of NO at 1.4 ϕ with an error of 10%. The sensitivity analysis and rate of production/consumption of NO mole fractions analysis have also been implemented to highlight the most important reactions that promote/inhibit NO formation. At lean and stoichiometric conditions, Glarborg kinetic model shows that the kinetic reactions of HNO + H ⇌ NO + H2, HNO + O ⇌ NO + OH, and NH + O ⇌ NO + H are the most important reaction routes with considerable effect on NO formation for 70/30 (vol%) NH3/H2 mixture. In contrast, the reactions of NH2 + NO ⇌ N2 + H2O, NH2 + NO ⇌ NNH + OH, NH + NO ⇌ N2O + H, and N + NO ⇌ N2 + O significantly consume NO to N2, NNH, and N2O. Further, Wang’s mechanism illustrated the dominant effect of each HNO + H ⇌ NO + H2, N + OH ⇌ NO + H, NH + O ⇌ NO + H in NO formation and NH + NO ⇌ N2O + H, NH2 + NO ⇌ NNH + OH, and NH2 + NO ⇌ N2 + H2O in the consumption of NO mole fractions.

Experimental and numerical comparison of currently available reaction mechanisms for laminar flame speed in 70/30 (%vol.) NH3/H2 flames

To achieve net zero carbon emissions, ammonia is gaining traction as a promising alternative fuel. However, the combustion characteristics of ammonia need further investigation. The current study aims to analyze the laminar flame speed, a fundamental physio-chemical property of any combustible mixture, through experimental measurements and kinetic reaction mechanism analysis. The laminar flame speed of 70/30 (%vol) NH3/H2 at atmospheric pressure and ambient temperature across a wide range of equivalence ratios (0.6–1.4) was studied experimentally and compared to the performance of 36 kinetic reaction mechanisms to appraise their performance concerning laminar flame speed prediction for the measured NH3/H2 mixture. The absolute percentage error (APE) formula has been adopted for preliminary estimation based on the experimental measurements of the present study and numerical data. The study found that Duynslaegher et al. 2012 model shows good performance speed across lean and stoichiometry conditions with an APE value between 0%-6%. The mechanism of Nakamura et al., 2017 and Gotama et al., 2022 demonstrates a good estimation of laminar flame speed under rich conditions. The sensitivity analysis revealed that the reactions H+O2=O+OH, NH2+NH2=N2H2+H2, and OH+H2=H+H2O are the most crucial reaction with considerable effect in promoting the laminar flame speed at all conditions, while the reactions of H+O2(+M)=HO2+M, NH2+H=NH+H2, and NH2+O=HNO+H play an essential role in the retardation of laminar flame speed at all conditions. The effect of the aforementioned reactions varies for the equivalence ratio, mainly due to changes in adiabatic flame temperature.

Application and comparison of multiple machine learning models for the prediction of the laminar burning velocity for CH3OH/H2/air mixtures

Recent investigations have indicated that co-firing CH3OH with H2 is a promising approach to develop a carbon-neutral energy system. However, accurate measurements of laminar burning velocities over a wide range of equivalence ratios, H2 mole fractions, pressures and temperatures are complicated and may not available. Hence, this research deeply investigates the application of several machine learning models in predicting the laminar burning velocities of CH3OH/H2 blended fuels. Results denoted that Random Forest Regressor is the most persuasive model based on a thorough comparison, as indicated by the correlation coefficient of 0.99707.

Revisiting effective Lewis number of combustible mixtures

Lewis number (Le) is of importance in the field of premixed combustion. In this study, the effective Lewis numbers of combustible mixtures containing a single fuel (CH4/air, C3H8/air, H2/air, C2H4/O2/N2), two fuels (CH4/C2H4/air, H2/NH3/air, H2/CH4/N2O) and two oxidizers (CH4/N2O/O2) over a wide range of equivalence ratios were evaluated via various methodologies. The Markstein length was obtained from previous studies, or extracted experimentally from the outwardly propagating flames. Firstly, the mass diffusivity and Lewis number of a reactant in mixture were evaluated using different models. Secondly, the effective Lewis number (Leeff) was calculated based on various theories, which was compared with the experimental ones. The results indicate that depending on the components, the heat-release, volumetric and diffusion models can be optimal for the depiction of Leeff. For the studied mixtures, the measured Le based on λ(x)=x (Bechtold & Matalon, Combust. Flame, 2001, 127(1–2): 1906–1913) was found to be more universal, where λ is the non-dimensional thermal conductivity. Additionally, the experimental Lewis numbers derived from different models can be very close, and the key parameters were compared and analyzed.

Experimental study of H2/air turbulent expanding flames over wide equivalence ratios: Effects of molecular transport

The turbulent expanding flames of hydrogen/air mixtures are investigated over a wide range of equivalence ratios (ϕ = 0.4–5.0) using the fan-stirred combustion chamber. The effects of molecular transport on turbulent flame propagation are interpreted over a wide Lewis number range, including turbulent flame morphology, turbulent burning velocity, and its general correlation. The wrinkled degree of turbulent flame surface becomes strengthened at fuel-lean conditions than that of fuel-rich conditions at the same normalized turbulence intensity (u’/SL), and the flame morphology becomes more wrinkled with the flame propagating outwardly. The equivalence ratio of peak turbulent burning velocity is shifted towards fuel-lean side compared to that of laminar burning velocity. At fuel-lean conditions (ϕ = 0.6–1.0), the normalized turbulent burning velocity (ST/SL) decreases sharply with the equivalence ratio, while at fuel-rich conditions it first decreases slowly (ϕ = 1.0–2.0) and then nearly unchanged (ϕ = 2.0–5.0). The general correlations of turbulent burning velocity under different Lewis numbers can be obtained in different u’/SL ranges, and the effects of molecular transport on turbulent burning velocity are strengthened at the intense turbulent field. All of these indicate that the molecular transport has significant effects on hydrogen turbulent flames.

Uncertainty quantification of the premixed combustion characteristics of NH3/H2/N2 fuel blends

Green ammonia is a candidate fuel to decarbonise shipping and other industries. However, ammonia features a lower reactivity compared to conventional fuels and is therefore difficult to burn. To resolve this issue, thermo-catalytic cracking of ammonia using waste heat is often employed to produce NH3/H2/N2 blends as fuel. However, on-site operational variations in this process can become sources of uncertainty in the fuel composition, causing randomness of the flame's physicochemical properties and challenging flame stability. In the present work, a surrogate model is built using the polynomial chaos expansion (PCE) method to investigate the impact of fuel composition variability on combustion characteristics at different operating conditions. Impacts of 1.5% deviation in the fuel composition on the flame properties for different initial pressures (Pi) and unburnt fuel temperatures (Tu) are investigated for a wide range of equivalence ratios covering lean and rich mixtures. The uncertainty effects defined by the coefficient of variation (COV) fluctuate for equivalence ratios greater than 1.1, while no fluctuation is observed in COV for near stoichiometric combustion conditions. It is shown that H2 variation in the fuel blend has the strongest effect (over 80%) on the uncertainty of all investigated physicochemical properties of the flame. The least affected property is the adiabatic flame temperature with variations of about 2.5% in richer fuel conditions. The results further show that preheating of the reactants can significantly reduce the COV of laminar flame speed. The consequences of these uncertainties upon different combustion technologies are then discussed and it is argued that moderate and intense low oxygen dilution (MILD) and colourless distributed combustion (CDC) technology may remain resilient.