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Amidst the increasing deployment of renewable electricity, the demand for energy storage rises. To address medium to long-term storage needs, utilizing excess electricity for hydrogen production emerges as one of the most promising solutions. In the context of a modern energy grid characterized by intermittent power sources and challenges in constructing large power plants in densely populated areas, the concept of a Decentralised Energy Systems (DES) network appears practical. Consequently, there is an urgent need for research into efficient micro Gas Turbine (mGT) units fuelled by hydrogen. Most notably, it is important to assess the impact of several key combustor design parameters to minimise the NO x emissions. As a result, the study provides optimal values for both the momentum (3–4) and the primary equivalence ratio's (0.3–0.4) in order to optimise the hydrogen injection depth and minimise the high temperature zones near the combustor head wall, which will curtail NO x emissions. • Rising demand for energy storage with renewable electricity growth. • Low NO x hydrogen mGT in a Distributed Energy Systems (DES) framework. • Research on design and optimisation of micromix combustor. • Optimise design of nozzles to minimise hydrogen injection depth and wall temperature. • Optimal values of momentum (3–4) and equivalence ratio (0.4) decrease typical high NO x causes. [ABSTRACT FROM AUTHOR]

Internal combustion engines (ICEs) fuelled with hydrogen can play a major role in the short-term future transportation sector since they abate all criteria pollutants at engine-out reducing tailpipe CO2 emissions to near-zero levels. However, optimizing hydrogen ICEs is a challenging task that can be addressed through the development of a robust simulation tool capable to predict the H2 combustion process. In this study, a previously developed two-zone combustion model has been updated considering different laminar flame speed computations, both based on a detailed chemistry scheme: a polynomial correlation function and a tabulated approach. The predictive capabilities of the combustion model have been validated against experimental data coming from a 0.5L PFI single-cylinder engine under several operating conditions. The tabulated approach for laminar flame speed definition proved to be the best solution, leading to a combustion duration average error lower than 3 deg over a dataset containing more than 45 different operating conditions. • A predictive combustion model for hydrogen combustion was developed. • The air-hydrogen laminar flame speed calculated using detailed chemical kinetics. • Two approaches tested for implementing the laminar flame speed calculations. • Tabulated laminar flame speed provides high predictive capabilities. [ABSTRACT FROM AUTHOR]

Chemiluminescence of OH∗ is commonly employed as an optical indicator for the combustion process. It is widely recognized that the generation of OH∗ in hydrogen flames is primarily governed by the reaction H + O + M=>OH∗+M. In this study, the time histories of OH∗ chemiluminescence from the A→X band around 308 nm were recorded behind reflected shock waves for stoichiometric and lean H 2 /O 2 mixtures highly diluted in argon over the temperature range of 1130–3150 K and at the pressure of approximately 0.9 atm. Based on the time histories of OH∗ chemiluminescence, the best-fit reaction rate coefficient for the reaction H + O + M=>OH∗+M was determined as k = 3.17 × 1027exp(-121968 cal mol−1/RT) + 2.25 × 1017exp(-4246 cal mol−1/RT) cm6 mol−2 s−1 over the temperature range studied. Furthermore, by using the current rate coefficient in mechanism predictions, the predicted time histories and relative peak intensities of OH∗ chemiluminescence are in good agreement with experimental results reported in literature. • The time histories of OH∗ chemiluminescence up to 3150K are recorded. • The rate coefficient for H + O + M=>OH∗+M is determined. • The OH∗ sub-mechanism with the fitted rate coefficient has been verified. [ABSTRACT FROM AUTHOR]

Compared with pure NH 3 fuel, the lower NO emission conditions of NH 3 –H 2 mixtures under the oxygen-enriched combustion mode have been identified quantitatively. Furthermore, the effects of the H 2 addition on the NO productions of premixed oxygen-enriched NH 3 –H 2 flames are studied numerically, while the major reactions responsible for the variation of total NO are identified. Results show that the H 2 addition is eligible to reduce the NO emissions of the oxygen-enriched NH 3 combustion if the laminar burning velocity is fixed to a larger value (>23 cm/s). The quantitative equations are obtained to determine the optimum oxygen-enriched conditions of the NH 3 –H 2 mixture, aiming to achieve lower NO emission and similar or improved combustion stability compared to pure NH 3 fuel by restricting the minimum and maximum O 2 percentages for the NH 3 –H 2 mixtures. Based on the rate of production (ROP) analysis, for the oxygen-enriched NH 3 –H 2 combustion, R144 (HNO + H<=>H 2 +NO) is consistently the most significant NO production reaction, while R85 (NH + NO<=>N 2 O + H) and R91 (N + NO<=>O + N 2) play significant roles in the NO consumption. When the laminar burning velocity (S L) is kept to a lower value, the increased total NO of the oxygen-enriched NH 3 flames with the H 2 addition is ascribed to more efficient suppressions on the NO consumption reactions of R85, R91 and R77 (NH 2 +NO<=>NNH + OH). In contrast, thanks to the extra impact of the higher O 2 percentage on the H/O/OH radical pool at larger S L , the NO production reactions begin to suffer stronger suppressions of the H 2 addition, resulting in the reduced total NO of NH 3 –H 2 mixtures. Furthermore, R144 is identified as the key reaction influencing the variation trend of total NO at both lower and higher S L values. • NO emissions of oxygen-enriched NH 3 –H 2 combustion are investigated. • Lower NO emission conditions of NH 3 –H 2 mixture than NH 3 are identified. • H 2 addition leads to opposite change of NO emission at small and large S L. • Effects of H 2 addition on NO formations are analyzed at different S L. • R144 plays a decisive role in determining the opposite change of NO emission. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional, high fidelity LES of a fully premixed, lean hydrogen-air deflagration, in a confined and obstructed channel is performed. The experimental configuration studied is the GraVent explosion channel (L. Boeck et al., Shock Waves, 2016). A complete methodology to perform LES of lean hydrogen, strongly compressible deflagrations is presented. The capability of LES to quantitatively reproduce the main Flame Acceleration (FA) mechanisms of the fast deflagration is illustrated. The physics of FA are analysed and the contribution of the unburnt mixture flow aerodynamics to the absolute flame propagation speed, is evaluated. This is made possible by the access to the complete reactive flow fields, which are not available in the experiments. It is shown that the flow contraction, at fence-type obstacles, and the flame/vortex interaction, between the flame front and the turbulent structures in the wake of the obstacles, interact constructively, driving FA. [Display omitted] • 3-D Large Eddy Simulation of a fast deflagration in the GraVent BR30hS300 channel. • Methodology for LES of lean hydrogen explosions with Adaptive Mesh Refinement. • Excellent agreement with experimental measurements of flame velocity and overpressure. • Flow contraction at obstacles and flame/vortex interaction drive flame acceleration. [ABSTRACT FROM AUTHOR]

To explore the combustion characteristics of ammonia‒hydrogen‒air flame, explosion venting experiments of ammonia‒hydrogen‒air mixtures with different equivalence ratios (ϕ), in the range of 0.6–1.7, are carried out in a 2 m long duct. The results show that the pressure‒time histories inside the duct have a three-peak structure (P b , P out , and P ext), which is caused by the burst of the vent cover, venting of burned mixtures, and counterflow flame generated by the external explosion, respectively. P out evolves into three pressure peak types with increasing ϕ. The pressure peak value P e is generated at the pressure measuring point outside the duct because of the external explosion. The change in P e with ϕ is consistent with P out and P ext , all of which increase first and then decrease with increasing ϕ and reach a maximum value at ϕ = 1.3. This study provides basic theoretical support for the promotion and application of ammonia mixed fuel. • The effects of the equivalence ratio, ϕ , on duct-vented ammonia‒hydrogen‒air deflagration were studied. • Counterflow flame generated by the external explosion was observed in all experiments. • The pressure‒time histories inside the duct under each operating condition present had a three-peak structure. • The maximum overpressure inside and outside duct first increased and thereafter decreased. [ABSTRACT FROM AUTHOR]

Ammonia's distinctive chemical makeup, coupled with its ability to combust fully without carbon emissions, position it as a promising carbon-free fuel but requires careful management to mitigate NO x emissions. In practice, supplementing ammonia with hydrogen addresses its slow combustion rate and high ignition energy requirements. Meanwhile, the introduction of a bluff body (BB) within the pipe can alter the way of flame propagation. This study examines the combustion dynamics of the premixed ammonia/hydrogen/air flame in a pipe with a variable cross-section. The experimental findings demonstrate that increasing both equivalence ratio (Φ) and hydrogen volume fraction (α H 2 ) significantly alters the flame front velocity (FFV) and maximum overpressure (P max), while also enhancing the symmetry of the flame across variable cross-section. The addition of BB creates a vortex in the flame, the formation of which intensifies the combustion of the flame behind the BB, which can promote the generation of Helmholtz oscillations, elevate the amplitude of the pressure, improve FFV, and shorten the flame propagation time in the pipe. With the increase of Φ and α H 2 , the time is shortened less. When Φ = 0.8, α H 2 = 40%, the flame propagation time was shortened by 709 ms. When Φ = 1.2, α H 2 = 60%, the flame propagation time was shortened by 5.67 ms only. In addition, P max and maximum flame front velocity (FFV max) are more sensitive to α H 2 than Φ. After the addition of BB, the increase amplitude of P max and FFV max is increased. However, when Φ = 1.0 and Φ = 1.2, the amplitude of FFV max increase decreases with the increase of α H 2 . This study can provide some suggestions for the application of NH 3 /H 2 fuels in variable cross-sections as well as under obstacles. • Ammonia-hydrogen premixed flames were studied. • The BB makes the propagation time is shortened by 709 ms at Φ = 0.8, α H 2 = 40%. • The BB promotes the generation of Helmholtz oscillations. • The addition of BB creates a vortex in the flame, the formation of which intensifies the combustion of the flame behind the BB. • The BB makes the propagation time is shortened by only 5.67 ms at Φ = 1.2, α H 2 = 60%. [ABSTRACT FROM AUTHOR]

The intensification of global warming is leading the exploration of low-carbon fuels, with ammonia emerging as a prominent hydrogen carrier and a research hotspot. This paper summarizes recent literature on turbulent combustion of ammonium-based fuels and analyzes the combustion dynamics, turbulent combustion characteristics and application in industrial facilities. It is found that in turbulent combustion, the simplified mechanism has a wider application. The enhancement of turbulent disturbances leads to intensified molecular diffusion and thermal diffusion in ammonia combustion, resulting in variations in flame surface and differences in heat release rate (HRR) distribution, and impacts on nitrogen oxide (NO x) emissions. Recent advances in the use of ammonia in industrial equipment demonstrate a promising prospect. In future applications, guided by the latest research findings, the combustion efficiency, stability and pollutants reduction of ammonia in industrial equipment can be further enhanced, thereby facilitating the low-carbon transition of industrial systems. • The characteristics and applications of ammonia mechanisms are compared thoroughly. • The effects on turbulent flame structure of ammonia-based fuels are described in details. • Effects of flame structure, molecular distribution and reaction path on NO x are discussed. • Application of ammonia combustion in industry is introduced. [ABSTRACT FROM AUTHOR]

Studying the synergistic effects of dilution gases and hydrogen on the laminar combustion characteristics and NO X -emission concentrations of methane/air can contribute to achieving efficient and clean natural-gas combustion. This study quantitatively analyzes the impact of N 2 and CO 2 as dilution gases on the laminar combustion characteristics and NO X emissions of methane/air under different equivalence ratios. The virtual gases FN 2 and FCO 2 are introduced to distinguish between the physical and chemical effects of the dilution gases. Subsequently, the effect of hydrogen addition on the laminar combustion characteristics and NO X emissions of a methane/5% dilution gas/air mixture is investigated. Finally, the synergistic effects of the dilution gases and hydrogen on the laminar flame speed and NO X -emission concentrations of methane/air under various blending conditions are discussed. The results indicate that CO 2 exhibits a more substantial reduction in laminar flame speed, flame temperature, key radical concentrations, and NO X -emission concentrations than N 2 , primarily because of its physical effects. N 2 has minimal chemical effects, but marginally increases NO X emissions. The addition of hydrogen increases the laminar flame speed and key radical concentrations of the methane/dilution gas/air mixture. however, significant differences in the NO X -concentration trends with increasing hydrogen-blending ratios are observed for the three equivalence ratios (Φ = 0.7,1.0,1.4). Selecting appropriate blending ratios of dilution gases and hydrogen can simultaneously enhance the laminar flame speed of methane/air while reducing the NO X -emission concentrations. This study provides valuable insights into the optimization of the blending ratio of hydrogen-enriched natural-gas engines coupled with exhaust-gas recirculation. • Effects of N 2 and CO 2 on combustion characteristics of CH 4 /air are compared. • Physical and chemical effects of N 2 and CO 2 are distinguishedd. • Coupling H 2 with dilution gases enables clean and efficient combustion of CH 4. • Optimal blending ratios of dilution gases and hydrogen are discussed. [ABSTRACT FROM AUTHOR]

A new design of plasma-assisted injector to ignite or stabilize combustion is presented. This design is intended to produce discharges that penetrate the combustion chamber instead of staying at the wall. This plasma-assisted injector, simple and robust, allows compact designs for a weakly intrusive implementation in engines, and is compatible with either air or fuel, depending on the process or application. A prototype was manufactured, implemented and tested with quasi-DC discharges on the LAPCAT2 Dual Mode Ramjet combustor at LAERTE facility in ONERA. Combustion tests showed earlier ignition and increased pressure when the plasma was turned on with a measured plasma power of 1.4 kW. • Presentation of the design and principle of a new plasma assisted injector. • Demonstration of its ability to generate discharges penetrating the flow. • Characterization of the discharge dynamics and its interaction with the gas flow. • Comparison of the discharge behavior in hydrogen vs air. • Ability of the plasma assistance to enhance supersonic combustion. [ABSTRACT FROM AUTHOR]

Hydrogen addition has been widely used to improve the combustion performance of low-reactivity fuels such as NH 3. In this study, a series of two-dimensional NH 3 /H 2 flames in mixing layers were investigated numerically. The aim is to assess and interpret the effects of H 2 addition on the structure and dynamics of ammonia flames. Detailed chemistry and transport models were considered in the simulations. It was shown that for pure hydrogen or ammonia flames, a classical tribrachial structure was observed. The non-premixed branch in the tribrachial flame of NH 3 combustion features high heat release rate (HRR), while the two premixed branches are weak. The mixture fraction corresponding to the maximum HRR of NH 3 combustion is significantly lower than the stoichiometric mixture fraction. When blending ammonia with hydrogen, a tetrabrachial flame structure consisting of two premixed branches and two non-premixed branches was observed, where one non-premixed branch corresponds to the reaction of NH 3 , while the other is formed due to the reaction of H 2. It was suggested that the formation of the tetrabrachial flame is related to the preferential diffusion of hydrogen. After decreasing artificially the diffusivity of hydrogen to suppress the preferential diffusion effect, the flame exhibits only three branches with the merging of the two non-premixed branches. By analyzing the flame dynamics at the leading edge, it was found that the flame speed (S d ∗) is negatively correlated with the flame stretch (κ) and scalar dissipation rate (χ), with S d ∗ decreasing almost linearly with κ or χ , consistent with previous studies of methane and hydrogen flames. As the H 2 concentration is increased, the values of S d ∗ increase due to the enhanced reactivity. • Effect of H 2 addition on NH 3 flame structure and dynamics in a mixing layer is studied. • Tetrabrachial flame of NH 3 /H 2 combustion is identified for the first time. • Influence of preferential diffusion of H 2 on the structure of NH 3 flame is analyzed. [ABSTRACT FROM AUTHOR]

This study suggests a combustion strategy and introduces a new geometry for the combustor of a micro gas turbine fed by ammonia as fuel. Ammonia is essential for making the energy supply chain more eco-friendly because it is seen as an excellent way to transport green hydrogen and as a renewable, carbon-free fuel. This study has considered different cases with a wide range of overall and primary equivalence ratios to find the optimum geometry regarding lower NO X emission, lower unburned NH 3 , lower unburned H 2 , and temperature distribution. A comprehensive case study of CFD simulation was conducted considering earlier research that showed the NO reduction level in an NH 3 -air swirl burner is dependent on the equivalence ratio of the primary combustion zone. The CFD simulation's boundary conditions were set by running a one-dimensional cycle simulation of the micro gas turbine at various loads. Results indicate that by modifying the geometry to include a staged rich-lean strategy with a primary equivalence ratio of 1.07, the NO mole fraction in the combustor can be reduced to one-third of its original level despite the significantly higher Combustor Inlet Temperature (CIT) compared to previous studies. The study found that the unburnt NH 3 and H 2 levels are low enough in the modified design, which shows the fuel is completely consumed and the overall combustion efficiency is optimized. Also, the analysis of the fluid dynamic behavior of the combustor shows that streamlines are uniform, leading to improved heat transfer and better overall system performance. These findings highlight the staged rich-lean strategy's potential by tuning the combustion stages' equivalence ratio as a promising approach to mitigate NO X emissions in ammonia combustion systems while maintaining high energy efficiency. • Study of ammonia combustion in a 3 kWe MGT's combustor. • Proposing a novel design approach for the combustor. • Higher combustor inlet temperature investigation in smaller combustor. • Implementation of a staged rich-lean combustion strategy to reduce NOx emissions by one-third. • Investigation of various primary ERs and identification of the optimal value at 1.07. [ABSTRACT FROM AUTHOR]

This paper focuses on a dual-fuel injector to characterize the ignition delay time (IDT), flame propagation, and flame spectral distribution of diesel and methane combustion. To gain an insight into the combustion characteristics of diesel pilot-ignited methane dual-fuel combustion with a high-pressure direct injection (HPDI) strategy under various conditions, the ignition and combustion processes are obtained by a high-speed color camera. The variables such as diesel injection pressure (DIP), diesel injection duration (DID), fuel relative injection interval time (RIT), ambient temperature (AT), and ambient pressure (AP) are varied according to engine-like conditions for a better understanding of the relation between the operating conditions and combustion characteristics. The results show that methane jets inhibited the IDT of diesel, while the conditions and ignition of diesel also determined the ignition of methane. Premixed flame always exists at the initial combustion, the development of the flame propagation indicates that the entrainment between diesel and methane plays a crucial role in the flame velocity and combustion intensity. According to sensitivity analysis, the IDT, Flame tip penetration (P), Flame tip velocity (V), and Flame area (FA) are most sensitive to the AT, and at least 4, 3, 2, and 7 times to the other variables each. However, the area ratio of chemiluminescence to soot incandescence (B/R) is most sensitive to AP, which is at least 14 times the other variables. • The effects of 5 variables on diesel ignition high-pressure methane combustion characteristics are studied. • A segmentation method for chemiluminescence and soot incandescence has been proposed. • The sensitivity of combustion characteristic parameters to variables is quantitatively analyzed. [ABSTRACT FROM AUTHOR]

In this study, we used the numerical simulation tool FLUENT to numerically simulate the combustion of ammonia-hydrogen fuel in the combustion chamber of a 9E heavy-duty gas turbine, and analysed the effects of different hydrogen blending ratios on the stability of ammonia combustion and NOx emissions. We found that an increase in the hydrogen mixing ratio in the fuel during the ammonia-hydrogen mixture combustion process contributes to the reduction of NOx emissions and has a positive effect on combustion stability. The key highlights of our research include. (see Fig. 3–6 and 10 and 11) • Ammonia-hydrogen combustion simulation of the 9E heavy duty gas turbine. • Increase in the proportion of hydrogen leads to an increase in the maximum temperature produced by combustion. • The high temperature zone moves closer to the fuel inlet as hydrogen increases. • The increase in hydrogen has little effect on the flow field in the combustion chamber. • Increased hydrogen ratio effectively reduces NOx emissions. [ABSTRACT FROM AUTHOR]

The vehicle sector has seen an upsurge in energy demand due to population expansion. Due to the escalating costs and exhaustion of fossil fuels, researchers are now dedicating more of their endeavors to exploring other options. The objective of this investigation, which is being conducted on a single-cylinder DI diesel engine propelled by hydrogen and diesel, is to optimize engine load. We evaluated the investigation on hydrogen/diesel composites against unadulterated diesel (D100). Different variations of fuels with varying percentages of hydrogen supplementation, viz., DH5 (diesel and 5% hydrogen), DH10 (diesel and 10% hydrogen), and DH15 (diesel and 15% hydrogen), were analyzed. In the experimental investigation, we used an injection pressure of more than 220 bar at 18:1 compression ratio for load optimization, and we used a single fuel injector with a diameter of 0.25 mm, The results showd improved brake thermal efficiency of 1.8% for DH5, 5.2% for DH10, and 17.6% for DH15, along with a drop in fuel consumption of 1.7% for DH5, 14.5% for DH10, and 31.6% for DH15. In addition, the addition of hydrogen to diesel demonstrates a promising reduction in smoke emissions of 10.5%, carbon monoxide (CO), and hydrocarbon (HC) emissions by DH15 under full load conditions. The results were subjected to regression analysis using an artificial intelligence network (ANN), to enhance the performance and reduce emissions of fuel mixtures. The ANN proves to be a very good method for the regression of data and prediction. The ANN provides an excellent fit for all the parameters, with a fitting value of more than 99%. [Display omitted] • Diesel-H2 mixes outperformed pure diesel fuel in terms of BTE. • Blends of diesel with H2 lowered BSFC and smoke emissions. • NOx emissions are increased by diesel-H2 mixes. • The engines performance and emission characteristics were predicted using ANN. [ABSTRACT FROM AUTHOR]

For hydrogen internal combustion engines (H 2 ICEs), the existing researches mainly focus on their thermodynamic aspects, while the engine acoustics are paid less attention. The objective of this research is to investigate the combustion noise characteristics of H 2 ICEs and find the biggest lever for combustion noise mitigation. Based on the data from previous thermodynamic measurements of various H 2 ICEs, the influences of different parameters for the combustion noise and knocking are studied with sensitivity analysis and machine learning based method. The results show that increasing Air–fuel equivalence ratio (λ), ignition delay and exhaust-gas recirculation rate (EGR) are acoustically beneficial, where λ is the most effective one with possible noise reduction up to 20 dB. Further, the comparison results between the H 2 ICEs and conventional engines indicates that the combustion noise of the H 2 ICE is kept at similar level as the respective gasoline variant if the H 2 ICE is operated withλ >2. [ABSTRACT FROM AUTHOR]

The present work proposes the application of CH 4 and H 2 synergy to improve the performance of the Wankel rotary engine (WRE). The whole work was experimentally conducted at 1500 r/min. The results indicate that CH 4 WRE can achieve similar power and significantly higher thermal efficiency than gasoline WRE based on quantitative control, with a maximum absolute efficiency improvement of 11.4%. At the same time, it also can achieve 64% and 77% maximum reduction of CO and NO emissions, respectively. CH 4 –H 2 synergy can further improve the performance of CH 4 -fueled WRE, which can broaden the lean flammability limits and make qualitative control application possible. Compared with CH 4 WRE with quantitative control, qualitative control hybrid fuel WRE can achieve a maximum relative improvement of 6.54% in brake thermal efficiency and significantly reduced NO and CO emissions. However, the extent of qualitative control is limited by cyclic variation. Overall, CH 4 –H 2 synergy can be a potential alternative to elevate the performance of WRE, the key to which is the reasonable match of synergy level and qualitative control extent. • Comparison of performances between CH4 and gasoline Wankel rotary engines. • CH4 has significant advantages in efficiency, CO and NO emission than gasoline. • CH4–H2 synergy can further improve the performance of the CH4 Wankel rotary engine. • Blending H2 coupling qualitative control is a good load control model in CH4 WRE. [ABSTRACT FROM AUTHOR]

The issue of spontaneous ignition has become a bottleneck for hydrogen applications. This comprehensive review delves into the intricate mechanisms governing the spontaneous ignition of pressurized hydrogen released through tubes. Beginning with an exploration of fundamental concepts, including the properties of hydrogen and various theoretical frameworks, the study meticulously examines the development of diffusion ignition theory. Methodologically, both experimental and simulation approaches are scrutinized for their contributions to understanding this complex phenomenon. Moving deeper into the core of the investigation, the review dissects the spontaneous ignition mechanisms occurring inside pipes, unraveling the influence of different initial conditions as well as the impact of varying pipe structures. Subsequently, the review shifts its focus to the flame morphology at the pipe exit, analyzing combustion behavior, shock wave dynamics, flow structures, and the formation of jet flames, while exploring influential factors affecting combustion. Through a comprehensive synthesis of findings, this review provides valuable insights into the spontaneous ignition dynamics of pressurized hydrogen within tube systems, offering a foundation for future research and engineering applications in safety and energy sectors. • Introduced theories of self-ignition and development of diffusion ignition theory. • Summarized ignition mechanisms of pressurized hydrogen released inside pipes. • Summarized flame morphology at pipe exit based on vortex position. • Identified deficiencies in reviewed studies and areas needing further research. [ABSTRACT FROM AUTHOR]

NH 3 is a carbon-free fuel with good storage and transport characteristics. Applying Moderate or Intense Low-oxygen Dilution (MILD) combustion to NH 3 /syngas blended fuel is expected to achieve clean and low-carbon combustion. This work explores the impacts of changing the NH 3 /syngas ratio and O 2 content in co-flow on the combustion characteristics and the NO x formation and transformation mechanisms. As the proportion of NH 3 increases, the flame is gradually lifted, while the rise of H 2 in syngas reduces the lifted height. As the O 2 proportion is increased to 15%, almost the entire computational domain is transformed from MILD to high-temperature combustion when the ratio of NH 3 to syngas is 3:7. NH that is derived from the decomposition of NH 3 participates more actively in the consumption process of NO and N 2 O. N radicals are more likely to generate N 2 when O 2 content in hot co-flow increases. [Display omitted] • Higher NH 3 ratio results in a shorter flame length and higher flame lifted-height. • The MILD combustion boundary of NH 3 /syngas mixed fuel is depicted. • The increase in syngas promotes the consumption of NH 3. • Detailed NH 3 conversion routes of NH 3 /syngas in JHC burner are explored. • Patterns of NO formation routes with the rise of NH 3 and O 2 ratio are revealed. [ABSTRACT FROM AUTHOR]

In the study of self-ignition during pressurized hydrogen release, the morphology of the rupture disk alters the flow and mixing processes of the gas jet, as well as the development of shock waves, thereby affecting the occurrence of self-ignition. This study employs the LES method based on the RNG-SGS model, EDC model, and NUIMech1.1 H 2 combustion mechanism to investigate the flow mixing, shock wave development, and self-ignition phenomena under three different the rupture disk morphologies. The research findings indicate that as the growth trend of the rupture disk rupture ratio increases, the propagation speed of shock waves within the tube and the compression speed of the air and hydrogen-air mixing region accelerate, resulting in an earlier occurrence of self-ignition. A more complex rupture morphology leads to higher mixing rates between hydrogen and air and increased complexity of shock waves. Self-ignition initially occurs in the fuel-lean region of the boundary layer, followed by ignition at the center of the tube. The characteristic number of rupture disk is proposed to describe its impact on the self-ignition time. The competitive relationship between the two elementary reaction pathways of oxygen influences the development of the flame. • A CFD model for hydrogen release and self-ignition is established and validated. • The development process of multidimensional shock waves is described in detail. • The process of self-ignition and entire flame development is fully captured. • The reaction mechanism and HRR of self-ignition are analyzed. • The impact of rapture disk morphology on flow and self-ignition is fully obtained. [ABSTRACT FROM AUTHOR]

AlH 3 is a high-capacity hydrogen storage material but is prone to explosion. Thermodynamic and structural properties of AlH 3 -nanoparticles at different CO 2 inerting systems from ignition to combustion are studied using ReaxFF-MD method. The influence of CO 2 on AlH 3 -nanoparticles ignition and combustion is investigated by comparing the changes of surface O adsorption rates, particle charge, temperature rise rates and so on. Results indicate that an increase in CO 2 is crucial for suppressing AlH 3 -nanoparticles ignition and combustion by impeding the formation of an Al-rich environment. In contrast to three stages of AlH 3 -nanoparticles combustion in pure O 2 system: surface combustion, slow temperature rise, and self-sustaining combustion, the process of CO 2 inerting system is different, with third stage is slow temperature rise at a constant rate. When CO 2 concentration reaches to 90%, the dehydrogenation rate reduces by 70.0%, the temperature rise rate decreases by 78.6%, and the ignition delay time reaches 45 ps. [Display omitted] • The mechanism of CO 2 inhibition on ignition and combustion of AHNP is clarified. • The influence of CO 2 on AHNP dehydrogenation is also elucidated. • Revealing mechanisms at the atomic scale. [ABSTRACT FROM AUTHOR]

Hydrogen, which emits large amounts of heat and has harmless products, is considered a potential alternative fuel. However, its wide flammability range and low ignition energy make it hazardous during storage and transportation. Therefore, in the event of a hydrogen leak in a confined space, a certain amount of inert gas can be added to inhibit combustion. Nonetheless, the addition of inert gas can also lead to an increase in the initial pressure as well. The aim of this research is to analyze the characteristics of laminar flame propagation and the inherent instability of laminar flames composed of 30% H 2 -air under various initial pressures and different types or quantities of inert gas dilution. In the experimental setup, hydrogen and air are pressurized into a 20 L spherical chamber to create the required mixed gas, following which inert gases such as CO 2 , N 2 , and He are added to achieve the desired pressure levels. Initially, the study focuses on elucidating the impacts of different variables on parameters such as flame propagation speed, flame stretching rate, un-stretched flame propagation speed, Markstein Length, and un-stretched flame burning speed. Additionally, it delves into investigating the effective Lewis number, density ratio, and flame thickness under varying conditions to understand the mechanisms influencing inherent flame instability. The findings indicate that increasing p 0 or augmenting the amount of dilute gas mixing both exacerbate flame front instability and reduce the critical radius of the flame. Among the three admixed species, CO 2 addition exhibits the most significant effect. [Display omitted] • Experiments study on the inherent flame instability of H 2 -air with combined effects of p 0 and inert gas dilution. • For p 0 ≥ 150 kPa, CO 2 addition≥ 20%, or N 2 addition≥ 25%, the stretching rate enhances flame propagation speed. • Inert gas dilution alters the structure of the H 2 -air premixed flame front. [ABSTRACT FROM AUTHOR]

The search for new combustion systems makes the two-stage combustion system (TJI) widely used when burning poor mixtures of methane and, increasingly, hydrogen. This paper presents the thermodynamic conditions for the combustion of methane and hydrogen when using a passive pre-combustion chamber. Thermodynamic analyses were carried out on a single-cylinder AVL 5804 research engine at varying values of excess air ratio (CH 4 : λ = 1.0–1.3; H 2 : λ = 2.16–2.70) and by changing the position of the center of combustion (CoC = 6; 8; 10; 12; 14; 16 deg TDC). As a result of the analyses carried out, the ignition advance was found to be significantly smaller when burning hydrogen than methane to maintain the same CoC value. A larger range of changes in ignition advance is required to determine CoC when burning hydrogen, indicating hydrogen's greater sensitivity to changes in ignition angle. During the combustion of the two fuels, different values of overall efficiency were determined: about 48% when burning hydrogen, and about 33% when methane combustion (in both cases, the maximum efficiency was obtained at the highest value of the λ, for methane and hydrogen, respectively: 1.3 and 2.7). • Combustion of Methane and Hydrogen in the TJI System. • Different Thermodynamic Combustion of Hydrogen and Methane. • 2.5-Times Higher λ-Value when Burning Hydrogen than Methane. • Higher Combustion Efficiency of Hydrogen than Methane. [ABSTRACT FROM AUTHOR]

This study investigates the stable combustion range and CO/NO emissions of laminar premixed CH 4 -NH 3 impinging flames experimentally. Effects of flame impingement on the flame stability and CO/NO formations are analyzed quantitatively. The result shows that the flame impingement improves the flame stability considerably through decelerating the unburned gases velocity in the stagnation region, with smaller nozzle-to-wall distance (H) exerting stronger improvement on the flame stability. The NO emission is increased initially but dropped finally with the increased H. This trend is ascribed to the suppressed NO productions at both small and large H due to the intensive cooling effects of cold wall and ambient air entrainment respectively in consideration of the relatively inferior reactivity of NH 3. Additionally, due to the insufficient oxidizer in the fuel-rich flame, the more NH 3 molecule can also facilitate the NO destruction via the DeNOx pathways at small or large H. In contrast, the CO emissions show similar variation trends with H like the NO emission for the fuel-lean and stoichiometric flames, while the CO emission has a "N" type variation trend with H for the fuel-rich flames. Since the CH 4 reactivity can be improved by the NH 3 addition at low temperatures, a low-temperature environment at small H due to the strong cooling effect, as well as more available NH 3 in the flame, can effectively highlight the improvement of NH 3 on the CH 4 reactivity in the fuel-rich flame. This makes for the CO production at small H combined with the insufficient oxidizer and eventually results in the higher CO emission of the fuel-rich flame at small H. Additionally, the decreased CO emission at large H results from the improved CO oxidation that is caused by the increased residence time and more oxidizer from ambient air. • The laminar premixed CH 4 -NH 3 impinging flame is investigated experimentally. • The stability maps of the CH 4 -NH 3 free and impinging flames are obtained. • The CH 4 -NH 3 impinging flame has the better stability in a wider range of conditions. • NO emissions are influenced considerably by the cooling effects of wall and air. • The NH 3 addition increases the CO emissions of fuel-rich flames at small H. [ABSTRACT FROM AUTHOR]

Free-piston engine linear generator (FPELG) is an innovative energy conversion device, offering multifuel adaptability and a simple structure. With the piston motion characteristics of FPELGs strongly correlated with the combustion process, the diverse combustion processes associated with various fuels lead to distinct piston dynamics and in-cylinder combustion characteristics. Therefore, this paper proposes a coupled simulation and iterative computation approach, integrating a zero-dimensional dynamic model with a multi-dimensional computational fluid dynamic model, then quantitatively analyses the piston dynamics and engine combustion characteristics of FPELGs using four typical hydrogen-rich fuels. The result reveals that hydrogen- and methane-fuelled FPELGs exhibit the highest and lowest operating frequencies, respectively. Furthermore, hydrogen-fuelled FPELGs showcase the highest peak pressure and temperature in-cylinder, the shortest combustion duration of the free-piston engine, the highest peak heat release rate, increased diffusion in the region of elevated temperature in-cylinder, and the lowest concentration of nitrogen oxide emissions. • The piston dynamics and combustion characteristics of FPELG with hydrogen-rich fuel are quantitatively analysed. • The coupled simulation and iterative computation approach that integrates a zero-dimensional dynamic model with a multi-dimensional computational fluid dynamic model is proposed. • The unique benefits of FPELGs operated with hydrogen-rich fuels are elucidated. [ABSTRACT FROM AUTHOR]

The hydrogen fuel free-piston engine (FPE) is an ideal onboard power device for new energy vehicles due to its multiple advantages, including better fuel adaptability. However, the periodic cyclic variations of FPE pose a significant challenge in controlling its stable operation. To address this issue, a secondary fuel injection (SFI) control strategy based on the inner dead center (IDC) has been designed. It categorizes and manages IDC by exploring the relationship between different positions of IDC and IMEP. The controller calculates the SFI quantity to control the IMEP and reduce cycle fluctuations effectively based on the classification results and the difference between the current IMEP and the target value. In practice, a rapid simulation combustion control model based on the prototype was established to validate the effectiveness of the control strategy. After implementing active control, IMEP achieved the step response to the target value in just one cycle instantaneously, and the variation range stabilized from 0.3 bar to 0.1 bar. The degree of fluctuation in the PV curve of FPE significantly decreases before and after control, indicating more stable FPE operation. The results demonstrate that this control strategy can effectively achieve precise IMEP control and reduce cycle variations. This means a more reliable and efficient operation of hydrogen fuel FPEs in new energy vehicles, enhancing their performance and viability. • An in-cycle controlled secondary fuel injection control strategy to reduce cycle variations was designed. • The mathematical model of single cylinder free-piston engine based on the prototype was established. • The required secondary injection volume can be calculated based on the initial injection and the inner dead center. • The secondary fuel injection control strategy can instantly achieve step response and stabilize near the set point. • The stability of free-piston engine operation under two control methods, PID and secondary fuel injection, was compared. [ABSTRACT FROM AUTHOR]

To achieve sustainable transportation, ammonia (NH 3) and hydrogen (H 2) should be applied in internal combustion engines. The adoption of turbulent jet ignition (TJI) can improve ignition and combustion stability. The present study aims to explore the combustion characteristics of NH 3 /H 2 /air adopting TJI, and the effect of nitrogen (N 2) dilution on factors such as ignition mechanism and flame propagation process was analyzed. The results indicate that N 2 dilution worsens the combustion of NH 3 /H 2 /air. When the H 2 volume fraction is 50%, a dilution ratio of 30% can increase the ignition delay and combustion duration by 3–6 times. With the increase of the dilution ratio, sensitivities of ignition delay and combustion duration to dilution ratio are enhanced in both passive TJI and active TJI modes. Compared to passive TJI, the injection of auxiliary gas in active TJI mode reduces the sensitivity of jet velocity to dilution, which can accelerate the early stage of flame development. However, the high turbulence intensity provided by active TJI makes the ignition position closer to the bottom and causes an extended final stage of combustion. For ultra-low reactivity mixtures, the active TJI with the scavenging system can provide sufficient ignition energy, but ignition occurs after prolonged accumulation of heat and radicals, as well as dissipation of turbulence. • Effects of N 2 dilution on NH 3 /H 2 /air combustion with TJI is studied. • H 2 -assisted TJI reduces the sensitivity of ignition performance to dilution. • High-speed jet suppresses flame propagation near the orifice in active TJI mode. • Scavenging can improve the flammability limit for N 2 diluted NH 3 /H 2. • Weak flame may occur in the early combustion for low reactivity mixture. [ABSTRACT FROM AUTHOR]

Developing highly efficient catalysts for oxygen evolution reactions using abundant earth-rich and non-precious metals poses a critical and indispensable challenge. In this study, Fe: 1, 3, and 5% and Cr: 2% doped ZnO nanoparticles (NPs) were synthesized by employing a simple solution combustion method (SCM) for oxygen evaluation reaction (OER). Using a variety of methods, the synthesized transition metal-doped metal oxides were thoroughly examined. The electrochemical behavior of the Fe and Cr-doped ZnO electrodes in an alkaline solution was investigated through LSV, EIS, CV, and CA. The electrochemical studies showed that ZnO/Cr/Fe:5 (η = 305 mV and Tafel slop = 46 mV/dec) demonstrates significant oxygen evolution reaction (OER) activity under alkaline conditions. All prepared samples exhibit good stabilities for up to 10 h. • Developing electrode materials with cost-effective for energy conversion. • Fe and Cr doped nanoparticles were synthesized by the solution combustion method. • ZnO/Cr/Fe:5 exhibited superior OER performance due to a reduced overpotential of 305 mV@10mAcm−2. • All electrocatalysts exhibited excellent long-term stability over 10 h. [ABSTRACT FROM AUTHOR]

In this study, the differences in the combustion process of ammonia-premixed ignition with conventional diesel and kerosene as pilot fuels were investigated based on flame chemiluminescence, and further study on the effects of ammonia ratio (AMR) and direct injection timing (DIT) on the ammonia-kerosene dual-fuel combustion (AKDC) process was carried out. The results show that the kerosene with stronger volatility is more conducive to promoting rapid and stable combustion of ammonia. A composite flame structure is observed in dual-fuel modes. With the increase of AMR, the capacity to ignite ammonia is lessened, the flame area and luminosity are stabilized and then reduced, and the flame hue is transformed from blue-green to yellow. Higher combustion rates are observed when the DIT lies between −9° and −12° CA ATDC, resulting in higher flame area and luminosity. The overly advanced or delayed DIT leads to the deterioration of ammonia combustion. • An optical study of the AKDC and ADDC mode was conducted. • A composite flame structure is observed in AKDC and ADDC modes. • Kerosene is more conducive to the ignition of ammonia than diesel. • As AKDC phases are roughly the same, the combustion state of pilot fuel dominates. • Adjusting the phase of AKDC towards TDC via DIT helps the rapid combustion of it. [ABSTRACT FROM AUTHOR]

The study investigates the effect of tube shrinkage rate on the explosion characteristics of premixed hydrogen-air in a confined space using Large Eddy Simulation (LES). The wrinkle factor of the flame surface density model is dynamically modeled based on the Fureby and Muppala (FurebyM) model. The numerical results from this model can be better verified with previous experimental results. The results indicate that a lower shrinkage rate results in longer flame propagation time from the reducer region. The peak flame front velocities were 68.6 m/s, 83.6 m/s, 96.4 m/s, and 103.6 m/s for tube shrinkage rates of 0.25, 0.5, 0.75, and 1.0, respectively. There is also a negative correlation between explosion overpressure and varying shrinking rates. The split tulip flame forms at a shrinkage rate of 0.25. Analysis of the flame structure and velocity flow field suggests that this phenomenon may be related to the formation of multiple fish-tail vortex structures on the left side of the flame front. Moreover, the effect of different shrinkage rates on the flame structure and the velocity flow field before the propagation of the premixed flame to the reducer region is relatively insignificant for tube structures with different shrinkage rates. This study provides a theoretical basis for protection against confined space explosions with premixed hydrogen-air mixtures. • The wrinkle factor in the flame surface density model is dynamically modeled and the accuracy of the model is verified. • The effect of tube shrinkage on the deflagration characteristics of hydrogen-air was investigated. • The flame structure evolves into a split tulip flame for a shrinkage of 0.25. [ABSTRACT FROM AUTHOR]

The chemistry mechanism has great significance in the simulations of supersonic combustion flow. A pre-partitioned adaptive chemistry with pre-exponent adjustment (PPAC-PEA) method has been proposed to develop an efficient and accurate reaction mechanism for simulating supersonic hydrogen combustion. The main idea is to adjust the pre-exponents of reactions based on the local gas state, minimizing the error in ignition delay time between a simplified mechanism and a detailed one. The PPAC-PEA method reproduced the detailed mechanism's ignition delay time with errors less than 0.08 orders of magnitude across a wide range of 900–1800 K temperatures and 0.5–10 atm pressures. Importantly, the PPAC-PEA mechanism achieves significant computational speedups of 3.77 to 5.91 times in several cases while maintaining excellent accuracy. In the case of the 2D oblique detonation, the PPAC-PEA mechanism shows only a 2.2% deviation in detonation wave position. These promising results demonstrate the PPAC-PEA method as an attractive solution for efficient and accurate numerical simulations of supersonic reactive flows. [Display omitted] • A PPAC-PEA method for efficient and accurate supersonic combustion simulations. • The magnitude of average ignition delay time error decreases from above 0.31 to below 0.08. • 2.2% deviation in the detonation wave position for a 2D oblique detonation simulation. • The optimized mechanism achieves speedups of 3.77 to 5.91 times compared to the detailed mechanism. [ABSTRACT FROM AUTHOR]

This study focuses on numerically exploring the effect of coupling factors on enhanced combustion performance in a two-strut scramjet combustor. The impact of five geometric variables, namely d (the vertical distance between the two struts), ur (lr) (the radius of the upper(lower) strut' tip), and ul (ll) (the length of the upper(lower) strut), is investigated through univariate analysis. While increasing ur and lr intensifies shocks at the cost of great total pressure loss, decreasing ul (ll) strengthens the upper(lower)flame but compresses the other. Besides, changing d influences the interaction of shocks and shear layers. To investigate the coupling effect of these variables, the combustion efficiency of the two-strut combustor is optimized through the SEGA algorithm based on a Kriging model built by small sampling points in 5D design space, which increases combustion efficiency by 14% and thrust by 27%. The optimum design features small ur and zero lr , larger d and ll and smaller ul. Further analysis performed on it revealed that the mechanism for enhancing combustion performance involves coupling the five geometry variables to assist combustion of the fuel closer to the upper expanding wall via stronger shock waves. Simultaneously, it weakens the shocks induced by the other strut to balance the total pressure loss. • Study coupling factors for efficient combustion in a two-strut scramjet combustor. • Enhance combustion in an asymmetry two-strut configuration via optimization. • Alternate shocks to enhance ignition and decrease total pressure loss. [ABSTRACT FROM AUTHOR]

To limit the negative impact on the environment, increasingly stringent regulations are being introduced regarding the reduction of pollutant emissions. In automotive applications, reciprocating internal combustion engines (ICE) are under immense pressure due to emissions. Society aims to move away from fossil fuels. However, it is acknowledged that ICE will continue to play a significant role in electricity generation systems and certain transportation sectors. To achieve zero emissions or drastically reduce emissions, alternative carbon-free fuels are being pursued. Hydrogen (H 2) is a natural substitute for fossil fuels, and recently, ammonia (NH 3) has been considered an intriguing fuel. Ammonia is easier to transport and store compared to hydrogen. NH 3 is a good hydrogen carrier with significant potential for use in ICE. However, ammonia combustion in such applications exhibits challenges related to its low burning rate. Here, hydrogen emerges as a high reactivity fuel, used as a combustion enhancer for NH 3. Hydrogen-assisted ammonia combustion shows great promise for future of ICE. This paper presents a summary of research findings on the NH 3 /H 2 combustion process in a spark-ignition ICE. Concepts of dual-fuel NH 3 /H 2 engine operation, the influence of fuel proportions on cylinder pressure profiles, heat release, combustion stages, and engine emissions aspects are discussed. The directions of development of SI piston engines powered by NH 3 /H 2 are given. • Assessment of NH 3 and NH 3 /H 2 combustion strategies in the SI engine • Direct injection of ammonia reduces its content in exhaust gases. • SI engine powered by NH 3 requires high ignition energy. • The challenge is to reduce N 2 O emissions. • Hydrogen expands the spectrum of application of NH 3 as a fuel for the SI engine. [ABSTRACT FROM AUTHOR]

NH 3 can be decomposed into H 2 at high temperatures to improve combustion and reduce NO x emissions. In this work, NH 3 decomposition experiments were carried out in a flow reactor, the effects of temperature (1273K–1773K), residence time [300/ T (K)s∼600/ T (K)s] and O 2 involvement (concentration: 0.1%∼0.5%) were investigated. Besides, numerical simulation was conducted using several kinetic mechanisms. The experimental results showed that the decomposition rate of NH 3 is positively correlated with temperature and residence time. O 2 involvement promoted decomposition at low temperatures (<1473K) and low concentrations (<0.2%), inhibited at high temperatures (≥1473K) or high concentrations (≥0.3%). The Konnov mechanism only accurately predicts NH 3 decomposition without O 2. The Glarborg mechanism overpredicts the decomposition onset temperature and therefore performs poorly. The reaction NH 2 +NH 3 N 2 H 3 +H 2 was added to the Glarborg mechanism for modification, this modified mechanism improves the predictability of NH 3 decomposition by lowering the onset temperature to better align with experiments. • Effects of residence time. • Wide temperature range. • NH 3 decomposition reaction with/without O 2. • Detailed reaction mechaninsms. • Validation of the modified mechanism. [ABSTRACT FROM AUTHOR]

Ammonia/hydrogen fuel mixture is considered to be a new type of sustainable and environmentally friendly energy. However, their explosive properties are not well understood. This study investigated the explosion of NH 3 /H 2 /air fuel mixture in a 1.67-litre constant-volume combustion chamber using hydrogen ratios (H 2 = 15, 20, 25, and 30%), equivalence ratios (ϕ = 0.8–1.4), starting temperatures (T i = 298, 363, and 428 K), and pressures (P i = 1, 2, and 3 bar). The explosivity of NH 3 /H 2 /air was characterized using the maximum explosion pressure (P max), maximum pressure rise rate (dP/dt max), and deflagration index (K G). Also, the maximum heat release rate (HRR max) and heat loss were used to account for variations in heat generated by the explosion. The findings indicate that P max decreases with increasing temperature but P max , dP/dt max , and K G increase with an increase in pressure. Overall, an increase in pressures, hydrogen ratios, and equivalence ratio enhances the explosion severity, but the pressure has a significant effect on the explosion severity. Furthermore, a cubic polynomial provides a superior match to the correlations between the P max , dP/dt max , K G , and HRR max and equivalence ratio. This study deeply explores the safe use mechanism of NH 3 /H 2 mixed fuel and provides an essential reference for the safe application of NH 3 /H 2 mixed fuel. • Studied confined explosion of NH 3 /H 2 mixed fuel under different initial conditions. • K p shows max explosion pressure sensitivity decreases then slightly increases with higher equivalence ratio. • Hydrogen content's impact on explosion parameters is more significant in lean fuel than rich fuel. • Deflagration index shows H 2 makes NH 3 more flammable with little increase in hazard. [ABSTRACT FROM AUTHOR]

In this study, three premixed swirl gas turbine nozzles with different fuel injection hole sizes were designed to optimize combustion characteristics influenced by changes in the hydrogen mixing ratio. Experiments involved increasing the hydrogen volume fraction of the fuel from 0 to 60%. The amount of fuel supplied through the pilot flow was increased to suppress combustion instability. The OH* chemiluminescence technique was introduced to analyze flame structure. To understand combustion instability, time delay and time-delay distribution were derived from flame structure analysis and applied to the 1D thermoacoustic model. The results indicated that the amount of fuel required for pilot injection with increasing hydrogen ratio varied with fuel hole size. It was also observed that changes in fuel hole size significantly affected flame characteristic length. Findings from the 1D thermoacoustic analysis confirmed that fuel hole size variation altered combustion instability characteristics by influencing time delay and time-delay distribution variation. [Display omitted] • This study investigated the impact of fuel hole size for H 2 co-firing. • Increase in H 2 fraction reduced flame length and heat release distribution. • Fuel hole size variation affected time delay and its distribution. • Changing the time delay and its distribution influenced combustion instability. [ABSTRACT FROM AUTHOR]

New graphene Schiff base iron nanocomplexes (S-792-Fe, S-550-Fe, and S-590-Fe) were designed and fabricated based on investigations of reactions mechanisms of AP pyrolysis, structural analysis of new materials and simulation results of quantum mechanics to deal with the long-standing contradiction between safety and performance for composite propellent combustion catalyst. By taking traditional catalyst Catocene as the benchmark, the S-550-Fe nanocomplexes exhibited competitive catalytic performances, much better anti-migration properties and significantly reduced impact and electrostatic sensitivities. After adding S-550-Fe, the burning rate of propellant enhanced from 13.87 to 19.77 mm s−1 at 15 MPa, and the impact and electrostatic sensitivities are improved to 50.2 cm and 172.12 mJ, respectively. The excellent catalytic performance of S-550-Fe is closely related to its molecular structure, which is manifested by an increase in charge density and a decrease in NH 3 oxidation energy barriers. The promotion effects of S-550-Fe on desorption and oxidation of NH 3 from the surface of AP is conducive to the rapid and complete pyrolysis of AP, which is reflected in the improvement of combustion rate and temperature gradient. The theory-driven design mode of this work may provide new pathways to tune combustion behaviors of composite propellants from the view of molecular-level insights. Effects and mechanisms of S-550-Fe on combustion and safety of AP-HTPB propellant were disclosed. [Display omitted] • Graphen Schiff base Fe nanocomplexes were first investigated in AP-HTPB propellant. • S-550-Fe is conducive to the combustion and safety performance of AP-HTPB propellant. • The structure-activity relationship of S-550-Fe at the molecular level was explored. • The influence of catalysts on NH 3 oxidation process was revealed. [ABSTRACT FROM AUTHOR]

The dielectric barrier discharge (DBD) plasma reforming of methane is ideal for fuel upgrading in rocket engine. However, the working conditions such as high flow rate, high pressure, and low temperature in the propellant supply system significantly affect the reforming performance. This research explores the performance of plasma methane reforming under extreme conditions, examining the characteristics of four kinds of structures, with aims to make reasonable engine design and settings based on actual needs. Simulation is conducted to investigate the effect of plasma reforming on methane ignition delay. The results indicate that prolonging the gas residence could promote methane decomposition. The methane conversion rate under Nanosecond pulsed (NP) excitation is 9.6% higher than that under AC excitation, and the energy utilization efficiency is 1.8% higher. However, the carbon deposition of NP excitation is more severe. As the pressure increases, the cracking reaction intensity decreases by 5%. NP excitation has ideal adaptability to high work pressure. Low temperature has little effect on methane plasma conversion but could affect product distribution, promoting the transformation towards H 2 and C 2 H 6. The ignition delay could be shortened by 71 ms with C 3 H 8 adding. The results of this study provide theoretical guidance for the ignition improving of the rocket engine. • Plasma reforming of methane in attitude and orbit control thrust. • 1.12s is ideal gas residence for plasma methane reforming and limited engine scale. • Working Pressure and Temperature has significant effect on production distribution. • Plasma reforming could shorten the ignition delay by up to 71 ms. • The order of ignition delay shortening effect is C 3 H 8 ＞C 2 H 6 ＞H 2. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of oxygen concentration on ammonia combustion, the effect of oxygen concentration on ammonia combustion and its mechanism were systematically investigated by using the ReaxFF molecular dynamics simulations and density functional theory (DFT) calculations. The results show that different oxygen concentrations significantly affect the concentration of reactants, free radicals, intermediates and products in the ammonia combustion process. High oxygen concentration significantly increases the concentration of free radicals, which effectively promotes the ammonia reaction. However, more free radicals such as ·H and ·OH are produced in the early stage of the reactions, which react with ·NH 2 radicals. It promotes the production of the intermediates HNO and indirectly inhibits the production of the intermediates N 2 H 4 and N 2 H 2. It promotes the production of NO X. The results of the DFT calculations show that the relative Gibbs free energy barrier of the reaction of ·OH with NH 3 is low at 1.00 kcal/mol, so the increase in ·OH concentration greatly facilitates the reaction of NH 3. These results not only identify the intrinsic effect of oxygen concentration on ammonia combustion and its mechanisms, but also provide some theoretical guidance for the study of efficient and clean ammonia utilization. • NH 3 combustion with different O 2 numbers was studied by ReaxFF MD simulations. • The main chain reactions of NH 3 combustion were studied by DFT calculations. • The effect of O 2 concentration on NH 3 combustion was studied by DFT calculations. • High oxygen concentration promotes the NH 3 combustion. • High oxygen concentration promotes the production of NO X of NH 3 combustion. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) and hydrogen (H 2) mixture are expected to replace traditional fossil fuels in the transportation industry to achieve zero-carbon emissions. Adopting turbulent jet ignition (TJI) is a reliable way to enhance the ignition and combustion of NH 3 /H 2. The present study aims to explore the effects of initial pressure and temperature on the ignition and combustion characteristics of NH 3 /H 2 ignited by TJI. It can be found that the increased initial pressure reduces the jet velocity. Due to the joint effect of weakened turbulence and inherent flame propagation characteristics caused by the elevated pressure, the combustion duration is extended. However, the ignition performance can be enhanced by increasing the initial pressure. Improvement is reflected in the emergence of flame ignition mechanisms and the obvious radial propagation of jet flames in early development. The increase in initial temperature slightly increases the peak jet velocity, while the auxiliary H 2 in active mode will reduce the sensitivity of jet velocity to the initial temperature. Although the increased initial temperature has shown the promotion for achieving rapid ignition and combustion, there is no significant improvement in the ignition mechanism, the ignition is caused by accumulated heat and radicals provided by the reacted jet. • Effects of pressure and temperature on NH 3 /H 2 combustion in TJI mode were studied. • Increased pressure is beneficial for achieving the flame ignition mechanism. • A more obvious radial jet flame propagation is promoted by the elevated pressure. • Injected auxiliary H 2 reduces the sensitivity of jet generation to temperature. [ABSTRACT FROM AUTHOR]

Turbojet engines play a crucial role in air transportation, providing superior thrust-to-weight ratios and lower NOx emissions compared to gas turbine engines, particularly at high speeds. Although they are efficient, environmental concerns and rising fuel costs have led to the exploration of alternative fuels. This study examines the use of green gaseous fuels, specifically hydrogen, in the SR-30 turbojet engine. Engine performance, combustion characteristics, and emissions were measured across a range of speeds from 40,000 to 75,000 rpm, without altering the combustion chamber. Hydrogen was injected into the engine air inlet manifold at a constant volume. The hydrogen used in the study was a mixture of 75% biogas and 25% hydrogen by volume. Due to hydrogen's high flammability, a flashback arrestor was employed to prevent backfire. The introduction of gaseous fuel increased maximum thrust by 4.5% and reduced fuel consumption by 2.2%. The addition of hydrogen improved thrust and combustion efficiency while lowering specific fuel consumption and unburned fuel. However, the presence of nitrogen and hydrogen may lead to increased NOx emissions. By optimizing fuel injection strategies, it is possible to mitigate NOx emissions, thereby improving engine efficiency and minimizing environmental impact. The study concludes that gaseous fuels like hydrogen provide a promising alternative for enhancing turbojet engine performance and reducing emissions. • Hydrogen was tested in the SR-30 turbojet engine without modifications with gaseous fuel. • Inclusion of gaseous fuel increases maximum thrust by 4.5% along with 2.2% reduction in fuel flow rate. • Hydrogen increases NOx emissions due to high combustion temperatures. • Injecting hydrogen via air intake is not effective. [ABSTRACT FROM AUTHOR]

The coal/ammonia co-firing technology for thermal power generation is a feasible method to effectively reduce CO 2 emissions. However, the easy formation of NO x is one of the challenges faced by this technology owing to the high nitrogen content in ammonia. This study proposes a novel swirl burner with a scalable dual-channel ammonia pipe, and evaluates the combustion and NO x formation under different extension lengths of the dual-channel ammonia pipe (i.e., 0 m, 0.4 m, 0.8 m, 1.0 m, 1.2 m). The results show that the extension length of the dual-channel ammonia pipe significantly affects the flow, combustion, and NO x formation. When the extension length of the dual-channel ammonia pipe is above 0.8 m, the ammonia stream can completely penetrate the internal recirculation zone into the high CO environment, which contributes to promoting the ammonia pyrolysis and inhibiting the ammonia oxidation to form NO x. NO x emissions significantly decrease from 846 ppm to 146 ppm as the extension length of the dual-channel ammonia pipe from 0 m to 0.8 m, but change slightly as further increases from 0.8 m to 1.2 m. The carbon content in fly ash decreases from 4.53% to 3.40% as the extension length of the dual-channel ammonia pipe increases from 0 m to 1.0 m. Still, it increases from 3.40% to 3.70% as the extension length of the dual-channel ammonia pipe rises to 1.2 m. Overall, it is reasonable to set the extension length of the dual-channel ammonia pipe at 1.0 m in this studied condition, which can obtain low NO x and low carbon content in fly ash. [Display omitted] • A novel swirl burner with a scalable dual-channel NH 3 pipe is proposed. • Effect of NH 3 pipe extension length on combustion and NO x formation is investigated. • NH 3 pipe extension length significantly affects flow, combustion, and NO x formation. • 1.0 m for NH 3 pipe extension length is reasonable with good performance. [ABSTRACT FROM AUTHOR]

Adopting ammonia (NH 3) is considered a viable way to reduce carbon emissions. The combustion of NH 3 /air can be enhanced through the fuel dissociation strategy and the use of turbulent jet ignition (TJI). This study investigated the combustion of partially dissociated NH 3 ignited by active TJI. It can be found that the hydrogen (H 2) pre-chamber effectively enhances the combustion of partially dissociated NH 3 , and the appropriate rich pre-chamber equivalence ratio is beneficial for the main chamber ignition. The lean main chamber mixtures realize the flame ignition mechanism and show a lower ignition delay. The increase in dissociation ratio enhances the tolerance of ignition to turbulence and leads to flame ignition mechanism. The increase in dissociation ratio also enhances the inhibiting effect of additional nitrogen (N 2) on combustion, but the ignition mechanism and flame shape are not sensitive to the additional N 2. [Display omitted] • Combustion of partially dissociated NH 3 under active TJI conditions was studied. • Rich pre-chamber equivalence ratio is conducive to rapid ignition and combustion. • Lean main chamber mixture is beneficial for achieving flame ignition mechanism. • Dissociation of NH 3 enhances the tolerance of ignition to turbulence. [ABSTRACT FROM AUTHOR]

Hydrogen/ammonia has a good prospect as an alternative fuel to traditional fossil fuels, and is one of the best ways to realize the goal of energy saving and emission reduction. However, due to hydrogen fuel's high flame propagation velocity and low ignition energy, abnormal combustion, such as backfire in port hydrogen injection engines, while ammonia fuel, characterized by high ignition energy and low laminar burning rate, thus the mixing of ammonia fuel as one of the effective methods to realize the hydrogen engine. This study endeavors to investigate the constraints of H 2 /NH 3 by utilizing a combined experimental and simulation approach to investigate the effect of the hydrogen/ammonia mixture ratio on engine combustion and emission performance at different injection timings. The results show that reasonable selection of injection timing can improve the fuel trapping ratio and reduce the fuel residue in the intake tract, which has an important effect on reducing the probability of backfire. For engine performance, the indicated mean effective pressure (IMEP), indicated thermal efficiency (ITE), and indicated power (IP) of the hydrogen internal combustion engine were reduced to 5.7 bar, 30%, and 5.1 kW, respectively, when ammonia was doped at a mass ratio of 50%. For combustion characteristics, ammonia addition reduced the in-cylinder pressure and exothermic rate, and extended the combustion duration. At the same time, chemical kinetic analysis showed that with the addition of ammonia, the concentration of H and OH radicals decreased, which would have a negative effect on the in-cylinder reaction rate, slowing down the reaction rate of the primitives, thus reducing the overall combustion rate and combustion efficiency. In terms of emissions characterisation, an increase in the ammonia content leads to higher nitrogen oxide (NO x) emissions, but these start to decrease once the ammonia content exceeds 40%. The results of the research could advance the development of hydrogen/ammonia engines and contribute to the realization of a zero-carbon combustion model for internal combustion engines. • The optimal hydrogen injection starting point should be selected 330° CA BTDC. • Ammonia doping reduces the speed and rate of combustion. • Ammonia addition reduces the maximum heat release rate and cylinder pressure. • As The ammonia in the fuel increases, the NOx emissions increase and then decrease. [ABSTRACT FROM AUTHOR]

Biogas is an efficient and environmentally friendly gaseous fuel. To ensure the safety of biogas applications, highly effective extinguishing technologies for suppressing biogas flames should be developed. In this paper, the extinguishing performance and kinetic mechanism of 2-BTP were studied experimentally and numerically. The results indicate that 2-BTP has excellent inhibition effect on biogas flames, but it depends on the ratio of hydrogen and oxygen in the reactants. For hydrogen-rich and oxygen-poor flames, 2-BTP shows efficient inhibition effect. However, it shows a little combustion promotion effect for hydrogen-poor and oxygen-rich flames at the addition of certain concentrations. The H and OH radical concentration variation and their production/consumption reactions are analyzed subsequently, and the simulation computation shows that some competing reactions involving Br-containing species lead to these results. The purpose of this research is to gain an in-depth understanding of the potential and effectiveness of 2-BTP in suppressing biogas fires, which can lead to better forecasting and optimization of its application, with the goal of enhancing the efficiency of fire extinguishment and minimizing possible adverse effects. • 2-BTP is particularly effective to slow the burning velocities of stoichiometric and fuel-rich biogas flames. • The effect of 2-BTP on combustion promotion or inhibition depends on the amount of hydrogen and oxygen in the flame. • The hydrogen contained in 2-BTP and the reaction HBr + O → Br + OH support the combustion of lean flame to some extent. [ABSTRACT FROM AUTHOR]

Ammonia, as a renewable energy with broad future application prospects, can alleviate its deficiencies in combustion stability as well as NOx emission by swirl premixed combustion technology. To investigate the impacts of swirler configuration parameters on ammonia premixed combustion, vanes numbers, vanes angles, and inner/outer diameter ratios of the swirler are varied. The combustion characteristics and NOx emissions in the combustion chamber are analyzed using the ammonia combustion mechanism model proposed by Stagni, which is done by large eddy simulation and Flamelet generated manifold combustion model. The results indicate that a stronger inner recirculation and lower NO 2 /N 2 O emissions were obtained when the number of vanes is 10. Increasing the vanes angles has several benefits, including expanding the flame's main combustion region, enhancing the heat release rate and improving turbulence in the flow field. The highest combustion efficiency and the lowest NO emissions are achieved at a vanes angle of 35°. Additionally, a moderate inner/outer diameter ratio of 0.447 can ensure appropriate inner and outer circulation intensity and NOx emissions. Based on the investigation results, the references for swirler parameters design in premixed ammonia combustion are provided. • NH 3 /air premixed flames were modeled by LES and FGM model. • Effects of vanes number, angle, and inner/outer diameter ratio were analyzed. • Linked thermal performance and pollutant emissions to flow field features. [ABSTRACT FROM AUTHOR]

The creation of the internal combustion engine played a crucial role in the growth of the modern world to its current condition, which occurred at a faster speed than it had been previously. One of the numerous offshoots of a reciprocating engine is the source of inspiration for nearly every aspect of the modern society that we live in today. The relevance and necessity of engines, particularly in the transportation industry, have largely hidden the negative effects that engines have on the environment as a result of their emissions. This is especially true of the emissions that engines produce. A number of major issues have arisen as a consequence of this negligent application, including the release of greenhouse gases into the environment, the creation of situations that are hazardous to human health, and the depletion of fossil resources. Using methane-enriched hydrogen and spirulina microalgae biodiesel as low and high-reactive fuels, respectively, the objective of this work is to ascertain the characteristics of an RCCI engine that utilizes these fuels. Dimethyl ether, diesel, and microalgae spirulina biodiesel are combined in the following proportions in order to make eighty percent of the hydrogen refueling fuel (HRF): Dimethyl ether, diesel, and biodiesel make up 48% of the total. Variations will be made to the settings of the engine in order to explore the effects of using methane-enriched hydrogen as twenty percent of the low-reactive fuel. This fuel blend will be used. Methane to hydrogen ratios in the low reactive fuel blend can range anywhere from 1% to 5%, with concentrations of 1%, 2%, 3%, 4%, and 5% accordingly. The ratio can also be anywhere in between. The characteristics study is carried out by adjusting the speed at which the brakes are applied and the pressure at which the fuel is injected into the engine, all while maintaining the compression ratio at 17.5:1. The characteristics of the emissions of NO x , CO 2 , smoke, and hydrocarbon, as well as the features of the performances of BTE, BSFC, and EGT, are all included in this classification. The findings of the experiments indicate that the utilization of fuel mixtures has the potential to reduce the amount of emissions produced by engines while simultaneously enhancing the performance of the engines. • Discussed the role of hydrogen-methane blends on an RCCI engine. • Fuelled the engine spirulina microalgae biodiesel in dual-fuel mode. • Handled the performances of BTE, BSFC, and EGT, as well as the characteristics of the emissions of NO X, CO 2 , smoke, and HC. [ABSTRACT FROM AUTHOR]

This study investigated the flame combustion of gas-containing pipe cutting operations by combining experimental and numerical simulation techniques. Based on the empirical flame length formula and experimental and numerical simulation flame length data, a flame length formula for gas outlet with large aspect ratio was proposed. In the experimental phase, cutting experiments were conducted with cutting slit lengths of 1–11 cm for pipe pressures of 10–550 Pa and hydrogen doping ratios of 0%, 10%, and 20%. The flame lengths were then measured. A hydrogen-doped natural gas combustion model based on the vortex dissipation conceptual model and Grimech-3.0 mechanism was established to simulate the experimental conditions. The flame lengths derived from the numerical simulation were in good agreement with the experimentally measured flame lengths, with an average error of 7.02%. This indicates that the reliability of the numerical model can be verified. The effects of hydrogen doping ratio, tube pressure, and cutting slit length on flame length and thermal radiation range were investigated using the control variable method. The results demonstrated that hydrogen doping reduced flame length and flame length away from the flame. Furthermore, an increase in pipe pressure and single cut length also resulted in an increase in flame length. It was observed that under normal operating conditions, a safe intensity of thermal radiation could be maintained within the operating distance when the operator wore protective clothing. • The reveal that hydrogen blending enhances combustion stability and reduces the jet flame height and flame resistance height. [ABSTRACT FROM AUTHOR]

Unwanted consequences associated with entropy waves, such as increased NOx emission levels, combustion instability, and generated noise, have long been recognized in the field. Indirect noise or acceleration of entropy oscillations acts as a source of noise, that contributes to the instability of the combustion chamber. To gain a deeper understanding of these occurrences, an investigation was conducted to computationally explore the entropy noise in a lean-premixed green H 2 /ethylene burner. This research used the flamelet model and large eddy simulation methods and provided valuable insights into the behavior of entropy waves and their influence on combustion dynamics. The research specifically focused on investigating the impacts of diverse thermal and fluidic conditions on entropy noise in both circulation and adiabatic combustion chambers in terms of temperature. Factors such as inlet turbulence strength, inlet velocity, stoichiometric ratio, and preheating of the unburned mixture were meticulously scrutinized. The results of the research show that there are surprising correlations between the examined variables and the acoustic response of the system. It was noticed that the increase in inlet velocity, equivalence ratio, and temperature resulted in an escalation in the auditory response. These results indicate that higher inlet velocity, greater equivalence ratio, and increased temperature circumstances encourage the creation and propagation of entropy noise. On the other hand, the investigation showed that the intensity of the amplifier turbulence had an adverse influence on the conduction of entropy oscillations and consequently diminished the auditory response. This indicates that excessive turbulence intensity can disrupt the propagation of entropy waves and diminish their capability to generate a substantial acoustic response. • A mixture of hydrogen-ethylene fuel was assessed in terms of noise generation. • The generated acoustic noises are influenced by the variations in wave entropy dissipation. • Increasing the inlet excitation amplitude intensified the further entropy wave annihilation. • Preheating the unburned mixture improved entropy wave stability. • Realistic walls and inlet flow conditions resulted in a lower level of noise generated. [ABSTRACT FROM AUTHOR]

In response to the pressing need to mitigate greenhouse gas emissions, there is increased interest in the potential of hydrogen as an alternative fuel in compression ignition engines. Onboard hydrogen production emerges as a solution, especially for existing diesel engines. This strategy promises to enhance engine performance and solve challenges like hydrogen storage and transportation. The study aims to provide empirical insights into how small quantities of hydrogen influence combustion processes in diesel engines, thereby laying the foundation for onboard hydrogen production as a viable transitional fuel strategy. To this end, this study employs natural flame luminosity and OH* chemiluminescence techniques in an optical single-cylinder AVL 5402 research engine equipped with high-precision sensors and operated under both part- and full-load conditions at 2000 rpm, using diesel as the primary fuel and incorporating small amounts of hydrogen. In the case of part-load Natural Flame Luminosity, hydrogen addition slightly decreased early combustion luminosity and led to earlier combustion completion, attributed to hydrogen's properties and lean air-fuel mixture. At full-load in Natural Flame Luminosity, hydrogen showed increased luminous intensity in the premixed phase, suggesting increased energy release early in the expansion stroke that can lead to potential fuel efficiency improvements. However, later stages were dominated by soot incandescence. OH* chemiluminescence imaging at full load confirmed trends from Natural Flame Luminosity and revealed that hydrogen led to a more uniform flame, better radical distribution and higher luminoust intensity, enhancing all combustion phases as well as the post-combustion period compared to pure diesel. This is likely due to hydrogen's influence on OH* radical formation. Prolonged OH* radical emissions in post-combustion might suggest enhanced soot and CO oxidation, but possibly increased NO emissions. Even at low levels as minimal as 0.42 mg/s, hydrogen had a consistent positive impact on combustion quality. These findings substantiate the potential of hydrogen micro-addition as a transitional fuel strategy and set the stage for future studies focusing on advanced optical imaging techniques and quantifying this strategy's impact on diesel engine performance and emissions. • Small-scale H2 improves luminous intensity in premixed phase. • OH* chemiluminescence show more homogeneous and uniform flame. • Better radical distribution may lead to reduced soot formation and emissions. • Even minimal hydrogen (0.42 mg/s of H2) enhances combustion quality. [ABSTRACT FROM AUTHOR]