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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]

Sustainable technologies to replace current fossil solutions are essential to meet future CO2 emission reduction targets. Therefore, this paper compares key performance indicators of a hydrogen- and a methane-flame-fired crucible furnace with computational fluid dynamics simulations at identical firing powers, aiming to fully decarbonize the process. Validated numerical models from the literature were used to compare temperatures, radiation fields, radiation parameters and heat transfer characteristics. As a result, we observed higher combustion temperatures and a 19.0% higher fuel utilization rate in the hydrogen case, indicating more efficient operating modes, which could be related to the increased radiant heat flux and temperature ranges above 1750 K. Furthermore, higher scattering of the heat flux distribution on the crucible surface could be determined indicating more uneven melt bath temperatures. Further research could focus on quantifying the total fuel consumption required for the heating up of the furnace, for which a transient numerical model could be developed. [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]

Large eddy simulations (LES) with flamelet and presumed filtered density function closure are used to simulate turbulent premixed and partially premixed hydrogen flames. Different approaches to model differential diffusion are investigated and compared. In particular, two existing models are extended to the LES framework to correct the resolved diffusive flux of the controlling variables due to differential diffusion. A lean premixed turbulent hydrogen flame in a slotted burner configuration is simulated first to compare the capability of the considered models in capturing local mixture fraction redistribution, super-adiabatic temperatures and thermo-diffusive instabilities. Results show that both models describe the formation of cellular burning structures. Next, a partially premixed lifted hydrogen flame in vitiated hot coflow is simulated to gain insight on the relevance of differential diffusion modelling at a higher turbulence level, a different combustion mode and in the presence of a complex stabilisation mechanism. Good predictions of the turbulent mixing and temperature fields are observed. Moreover, results show that the flame lift-off height has an appreciable sensitivity to the differential diffusion model. When differential diffusion is included only in the thermochemistry database, only mild effects on the predicted temperature fields, mixing and flame height are observed. On the contrary, a considerable shift of the flame base is observed when corrections are applied in the LES at the resolved level, depending on what controlling variables are considered. Further analyses reveal how the corrections of diffusive fluxes in the thermochemistry and at the LES level affect differently the flame burning mode, whose details are given throughout the paper. [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 (NH 3), hydrogen (H 2) and methane (CH 4) as zero- or low-carbon fuels are considered as highly promising alternative fuels. Turbulent non-premixed jet combustion is a prospective way in practical combustion devices for ammonia, hydrogen and methane, but the relevant experimental researches are rare, while such information is critical for the development and validation of numerical models. In this paper, the laser diagnostic studies are carried out in a constant volume combustion chamber to investigate the characteristics of the 50%NH 3 +50%H 2 blend jet with or without ignition in comparison to H 2 , CH 4 and NH 3 jets. Firstly, the morphological developments and concentration distributions of the four gas jets without ignition are compared. Then, the OH or NO distributions in the non-premixed gas jet flames are clarified by the planar laser-induced fluorescence techniques. The generation and consumption processes of OH and NO are elucidated by the one-dimensional simulation. The results show that the four jets exhibit a similar morphological development with the nearly same jet angles, except for a little slower tip penetration of 50%NH 3 +50%H 2 blend. The equivalence ratio of the 50%NH 3 +50%H 2 blend and NH 3 jets along the jet axis are lower than those in the CH 4 and H 2 jets due to the difference in the flow rate and stoichiometric number. The H 2 jet exhibits the highest combustion intensity, featuring faster flame propagation, the shortest lift-off length and an apparently higher OH radical intensity, while the NH 3 jet fails to develop a stable flame. The mixture in the 50%NH 3 +50%H 2 blend jet is too lean, resulting in a weaker combustion intensity than the H 2 and CH 4 jets. However, the NO intensity in the 50%NH 3 +50%H 2 jet flame is remarkably stronger than those of the H 2 and CH 4 jet, due to the much higher amount of fuel-type NO converted from NH 3 than that of thermal NO. A self-reducing (SR) zone can be observed where OH is produced and then consumed in the 50%NH 3 +50%H 2 blend, H 2 and CH 4 jet flames. However, the NO SR zone is only observable in the 50%NH 3 +50%H 2 blend jet flames, probably owing to the consumption by the NH 2 and NH radicals. • The laser diagnostic was used to obtained the OH and NO distributions. • The 50%NH 3 +50%H 2 , H 2 , CH 4 and NH 3 jets exhibit a similar morphologic development. • The equivalence ratios of the 50%NH 3 +50%H 2 blend and NH 3 jets are the lowest. • The fuel-type NO makes the strongest NO intensity in 50%NH 3 +50%H 2 blend flames. • For the first time a self-reducing zone was observed for the OH and NO. [ABSTRACT FROM AUTHOR]

With the development of hydrogen fuel cell vehicles, hydrogen refuelling stations are required to operate at a higher pressure. However, the safety issue of high pressure hydrogen is always considered as the barrier of the wide application of such transportation infrastructures. In this paper, the safety evaluation is performed on a 70 MPa hydrogen dispenser, as one of the most vulnerable equipment to safety events in hydrogen refuelling stations. The numerical simulations are conducted for the hydrogen leakage and combustion for the prototype of the dispenser. The results show the high radiation during the combustion, and the environmental wind can reduce it by promoting the diffusion of hydrogen. Accordingly, the lethal ranges in different cases are manifested by radiation heat flux at the concerned height for the safety of humans. Furthermore, an improved design of the hydrogen dispenser is proposed to cancel the lower ventilation holes based on the analysis. As a result, the radiation heat fluxes due to the hydrogen combustion after leakage is significantly reduced and consequently lead to smaller lethal ranges, indicating the enhanced safety performance of the hydrogen dispenser. • A numerical study is conducted for the hydrogen combustion from a 70 MPa hydrogen dispenser. • The safety distance is evaluated for hydrogen leakage and combustion of dispensers under different wind conditions. • An improved design of the hydrogen dispenser is proposed and proved to be effective in enhancing the safety performance. [ABSTRACT FROM AUTHOR]

Blending hydrogen with natural gas is useful for reducing CO 2 emissions, but this alters the flame structure, creating challenges for flame stability. Numerous researches on methane-hydrogen flame structure have been carried out based on two-dimensional combustion diagnostic techniques such as flame chemiluminescence and planar laser-induced fluorescence. In this paper, a low-cost OH* chemiluminescence imaging system consisting of 8 ultraviolet (UV) cameras is proposed to reconstruct the three-dimensional (3D) structure of methane-hydrogen flames, and validated through use of the correlation coefficient. Further experimental investigations of the low-swirl methane-hydrogen flames are performed by this computed tomography of chemiluminescence (CTC) system. The flame properties are determined to evaluate the effects of various fractions of blended hydrogen on the flame structure, including the flame center of mass, flame surface density and fractal dimension. Results showed that the flame initially has bowl-shape structure. The flame height gradually decreases and the flame cross-sectional area increases as the blended hydrogen fraction increases. Meanwhile, the bottom of the flame moves toward the nozzle outlet. Nevertheless, when the blended hydrogen fraction reaches 50%, the flame transforms into a cup-shaped flame and shows a double-layer flame structure. When the flame is bowl-shaped, the flame center of mass moves away from the nozzle center axis with increasing blended hydrogen fraction, but the fractal dimension and flame surface density are not sensitive to the blended hydrogen fraction. In contrast, when the flame is cup-shaped, the flame center of mass moves towards the nozzle center axis, and the flame surface density and fractal dimension increase significantly, indicating the increase of flame wrinkling. These results reveal that blending hydrogen leads to two distinct flame structures and characteristics in low-swirl flame, also demonstrate that the proposed 3D low-cost CTC system based on multiple UV cameras is well-suited for combustion diagnostics. • A low-cost 3D computed tomography of OH* chemiluminescence system is proposed. • Two distinct low-swirl CH 4 –H 2 flame shapes are observed and reconstructed. • The change in flame shape is accompanied by a significant change in flame wrinkling. [ABSTRACT FROM AUTHOR]

Technologies used in the transport sector have a substantial impact on air pollution and global warming. Due to the immense impact of air pollution on Earth, it is crucial to investigate novel ways to reduce emissions. One way to reduce pollution from ICE is to use alternative fuels. However, blends of alternative fuels in different proportions are known to improve some emissions' parameters, while others remain unchanged or even worsen. It is therefore necessary to find ways of reducing all the main pollutants. For SI engines, mixtures of hydrogen and natural gas can be used as alternative fuels. The use of such fuel mixtures makes it possible to reduce CO, HC, and CO2 emissions from the engine, but the unique properties of hydrogen tend to increase NOx emissions. One way to address this challenge is to use port water injection (PWI). This paper describes studies carried out under laboratory conditions on an SI engine fuelled with CNG and CNG + H2 mixtures (H2 = 5, 10, 15% by volume) and injected with 60 and 120 mL/min of water into the engine. The tests showed that the additional water injection reduced CO and NOx emissions by about 20% and 4–5 times, respectively. But, the results also show that water injection at the rate of 120 mL/min increases fuel consumption by between 2.5% and 7% in all cases. [ABSTRACT FROM AUTHOR]

The development of large energy systems based on renewable energy sources is accompanied by an increase in the input capacities of large grid storages. Taking into account the vector for decarbonization and maneuverability, the use of hydrogen as a fuel in the cycle of a diabatic compressed air energy storage (CAES) is a promising scientific and technical direction. This paper analyzes the key performance indicators of a compressed air energy storage in the presence and absence of thermal energy recovery within the cycle. In addition, an assessment was made of the prospects for the use of a methane-hydrogen mixture in gas turbines. At the end of the paper, a calculation of the process of plasma pyrolysis of methane is given as one of the possible methods for the production of hydrogen for use in energy purposes. [ABSTRACT FROM AUTHOR]

With the advantages of simple structure, high power density, and better fuel adaptability, the hydrogen FPE is one of the ideal onboard power devices for new energy vehicles in the future. The combustion system is essential for forming the in-cylinder mixture and the stable operation of the engine. This paper compares combustion, injection, and performance at different ignition energy using high-energy ignition methods and applying computational fluid dynamics simulations. In order to further optimize the combustion system, the influence of ignition timing was discussed. The results show that with the increase of ignition energy, the flame kernel size increases continuously, the speed of flame propagation increases, and the mixture participating in combustion increases simultaneously. When the ignition energy reaches 300 mJ, the fuel combustion efficiency rises to 90.22%, and the indicated thermal efficiency rises to 32.76%. Finally, a comparison was made with a gasoline FPE; the hydrogen FPE in this paper reduces the NO emission value to less than 20 ppm without any after-treatment device, while no other pollutants are generated. The performance stability of pure hydrogen combustion under FPE is preliminarily revealed. [Display omitted] • A prototype of hydrogen FPE with a high-energy ignition system was designed. • Establish a CFD model for hydrogen FPE. • Set up the bench test and calibrate the experimental data. • Early ignition timing has the advantages of a fast heat release rate and low emission. • The performance of hydrogen FPE with different ignition energy was compared. [ABSTRACT FROM AUTHOR]

This paper investigates an intelligent strut design scheme for achieving high-performance combustion of a combustion chamber under wide-ranging flow conditions. The design process involves optimizing the base strut configuration for the design point and obtaining the Pareto front. The subsequent selection of the optimal configuration is followed by analyzing and interpreting the optimization results using single-factor analysis rules. The findings reveal that the selected optimal configuration enhances the combustion efficiency by 8.22% and reduces the force by 66.12% compared to the traditional DLR combustion chamber. Additionally, a surrogate model is constructed to investigate the influence of the rotation angle of the intelligent strut's rotating blades on the combustion efficiency and force at a Mach number range of 2–3.5. Results indicate that the intelligent strut is capable of improving combustion efficiency via regulation, and the efficiency can quickly reach a desirable level. These findings suggest that the proposed design scheme has potential applications in achieving high-performance combustion in combustion chambers exposed to wide-ranging flow conditions. • This paper investigates an intelligent strut design scheme for achieving high performance combustion. • The design process involves optimizing the base strut configuration for the design point and obtaining the Pareto front. • This design scheme aims to achieve the best comprehensive combustion. Performance based on the design point. • Results indicate that the intelligent strut is capable of improving combustion efficiency. via regulation. [ABSTRACT FROM AUTHOR]

The article presents the authors' comments on a paper by E. N. Aleksandrov regarding initiation of Chain and Thermal Explosions by the Reactor Surface. According to the authors, the said paper has an erroneous statement regarding gas combustion. Moreover, the authors also analyzes assertations in the paper that contradict the chain-branching processes theory. Among the assertions includes the reaction chain in hydrogen combustion process.

The vision of a carbon-neutral world implies the shift from fossil to clean fuels for combustion-driven processes and machines like gas turbines. Green hydrogen is a promising alternative to substitute natural gas and other fossil fuels. In the H2mGT project, funded by the German BMWK, a microgas turbine (mGT) burner with 100% hydrogen firing is developed and validated. The project is collaboration between Technische Universität Berlin (TUB) and the manufacturer Euro-K GmbH. The project consists of three phases: (1). Atmospheric pressure tests with a fused silica combustion chamber; (2). Atmospheric pressure tests with counterflow-cooled steel combustion chamber and secondary air injection; (3). Validation of the burner in the micro gas turbine at elevated pressure levels. This paper will present the results of Phase 1. The hydrogen burner is based on a swirl-stabilized burner of TUB and was scaled to match the requirements of the mGT with its 130 kW thermal power. The burner design features multiple geometrical parameters to enable the optimization of the flame towards low NOx emissions. Therefore, a variable swirl intensity, additional axial momentum of air in the mixing tube, a movable center-body and different fuel injection locations are implemented. Phase 1 investigates the parameter space in terms of flame stability, operational range, and parameter impact on flame shape and emissions. Therefore, temperature, pressure, and emission measurements as well as OH* imaging are carried out. It is found that the flame can be operated over a large range of equivalence ratios and preheating temperatures up to 500 °C for many parameter settings. However, at some configurations, flashback into the mixing tube is triggered. As expected, the NOx emissions are mainly influenced by the equivalence ratio, the fuel distribution, and the swirl intensity. Single-digit emissions are reached up to an equivalence ratio of 0.4 at atmospheric pressure conditions. Furthermore, at low air mass flow, the burner can be operated at 100% natural gas or 100% hydrogen without any geometry changes. The fuel switch, thereby, does not change the NOx emissions significantly if reasonable normalization is used. [ABSTRACT FROM AUTHOR]

Hydrogen stands out as a compelling alternative to fossil fuels for combustion engines. Predictive combustion models are instrumental in developing hydrogen-fuelled engines. A fundamental metric of these models is the laminar burning velocity (LBV), which can be precisely determined through laminar flame propagation simulations. In this context, the selection of an appropriate combustion mechanism is critical. This paper aims to propose the appropriate combustion mechanism for calculating LBV in predictive combustion models of hydrogen-fuelled engines. 15 state-of-the-art combustion mechanisms were applied to reproduce the LBV measurements in engine-like conditions, especially considering the application of lean combustion and water injection. The FFCM 1.0 mechanism was identified from them and further optimised to improve its prediction accuracy at elevated pressures for the lean mixture. The maximum deviation of LBV was reduced from 17.6 % to 8.7 % by this optimisation, in comparison to 10.5 % for its closest competitor mechanism, ELTE (Varga et al., 2015). • 15 combustion mechanisms were selected as the candidates for optimisation. • Test cases consider applications of lean combustion and water injection. • Many mechanisms predict laminar burning velocities at elevated pressure inadequately. • FFCM 1.0 mechanism was optimised to a new mechanism for modelling H2-fuelled engines. • Prediction errors of laminar burning velocity by the new mechanism are within 8.7 %. [ABSTRACT FROM AUTHOR]

This paper aims to numerically investigate the effects of high compression ratio (CR) on the performance of ammonia-hydrogen engines. In this work, four CRs from 10.7 to 13.7 with scanning spark timing (ST) from 28°CA to 0°CA BTDC were analyzed. The main results are as follows: As the CR increases, there is a trade-off relationship between the dissipation rate of turbulence and the turbulent kinetic energy (TKE). Initially, the TKE rises as the CR increases. As the CR continues to rise, the tendency for an increase in TKE diminishes, while the turbulent dissipation rate consistently rises. Additionally, there is an escalation in heat transfer loss. Therefore, there is a trend of rising and then falling in the flow velocity and turbulence intensity with the increase of the CR. Ammonia-hydrogen flame propagation is susceptible to temperature and flow field, and a high CR can improve ignition stability, shorten combustion duration, minimize cooling loss, and enhance output power. Unfortunately, the emission of NOx gradually rises as the CR increases. At high CR, the combustion performance is optimized by adjusting ST, and the maximum IMEP and ITE are 4 bar and 38.3 %, respectively. The ST for maximum braking torque (MBT) should be gradually delayed toward TDC as the CR increases. • As the compression ratio increases, the flow velocity first rises and then decreases. • The turbulent dissipation rate increases with increasing compression ratio. • A high compression ratio improves ignition stability and combustion velocity. • The NOx gradually increases with the increase of the compression ratio. • A high compression ratio improves the power and economy of NH3-H2 engines. [ABSTRACT FROM AUTHOR]

Land-based gas turbines (GTs) are continuous-flow engines that run with permanent flames once started and at stationary pressure, temperature, and flows at stabilized load. Combustors operate without any moving parts and their substantial air excess enables complete combustion. These features provide significant space for designing efficient and versatile combustion systems. In particular, as heavy-duty gas turbines have moderate compression ratios and ample stall margins, they can burn not only high- and medium-BTU fuels but also low-BTU ones. As a result, these machines have gained remarkable fuel flexibility. Dry Low Emissions combustors, which were initially confined to burning standard natural gas, have been gradually adapted to an increasing number of alternative gaseous fuels. The paper first delivers essential technical considerations that underlie this important fuel portfolio. It then reviews the spectrum of alternative GT fuels which currently extends from lean gases (coal bed, coke oven, blast furnace gases...) to rich refinery streams (LPG, olefins) and from volatile liquids (naphtha) to heavy hydrocarbons. This "fuel diet" also includes biogenic products (biogas, biodiesel, and ethanol) and especially blended and pure hydrogen, the fuel of the future. The paper also outlines how, historically, land-based GTs have gradually gained new fuel territories thanks to continuous engineering work, lab testing, experience extrapolation, and validation on the field. [ABSTRACT FROM AUTHOR]

Abstract This short communication points to some dubious and physically unattainable experimental data and results reported by Yue et al. [1] on the effect of hydrogen enrichment of low calorific value gas on engine performance. [ABSTRACT FROM AUTHOR]

In this paper, the CFD simulation model of a high-efficiency X-type rotary engine using premixed gasoline and hydrogen direct injection is established. The effect of hydrogen injection timing on combustion, pollutant emissions, and thermodynamic performances is investigated under hydrogen single-stage direct injection (HSDI). Furthermore, a novel hydrogen two-stage direct injection (HTDI) strategy using different first injection mass fractions (FIMF) and the same total injection pulse width of 3.5 °CA is proposed for improving combustion performance in the later phase under HSDI. The results show that the hydrogen injection timing of 290 °CA presents a 6.09% higher indicated thermal efficiency than that of 270 °CA under HSDI. The delay of the hydrogen injection timing leads to a decrease in soot emission. The optimal HTDI strategy is obtained at the FIMF of 60%, which shows a 6.35% higher combustion efficiency and a 0.89% higher indicated thermal efficiency than the optimal HSDI. • Effect of hydrogen injection timing on X-type rotary engine performance is studied. • Novel HTDI method with different FIMFs and same total hydrogen amounts is proposed. • Combustion efficiency, thermal efficiency, emission of soot, NOx, CO are evaluated. • 0.89% higher thermal efficiency is found at optimum FIMF of 60% in HTDI than HSDI. [ABSTRACT FROM AUTHOR]

Hydrogen, an ideal carbon-free energy carrier with favorable combustion properties, has drawn worldwide attention as a high-expectation way to enhance fossil fuels' combustion performance. However, currently, limited investigations are about enhancing n-butanol hybrid gasoline engines through hydrogen addition. Thus, in this paper, the hydrogen direct injection (HDI) was put forward to improve an SI engine fueled with n-butanol/gasoline, and the synergic effect of hydrogen and n-butanol on combustion and emission characteristics was studied. Experimental results indicated the favorable influence of a low n-butanol mixing ratio (ν N , ν N = 25%) on enhancing combustion can relatively weaken the similar promoting effect of hydrogen. And the sequential increase in hydrogen addition ratio (φ H , φ H = 15–20%) can compensate for the negative impact of unfavorable atomization and vaporization quality caused by higher n-butanol blending (ν N = 75–100%), namely the usage amount of n-butanol in engine can be raised. Further, compared with pure gasoline, the percentage change in BMEP (brake mean effective pressure) at ν N = 25% and 100% can be increased from 1.79% to 5.04% and −5.38%–2.01% respectively by φ H from 0 to 20%. Besides, the HDI can also shorten the combustion duration, accelerate the heat release process and improve the combustion stability of each ν N , while the amplitude of promotion exhibits a relative downtrend with the increase of φ H. As for emission characteristics, the HC, CO and particulate emissions decreased monotonously with the increasing φ H under all ν N , but there was a visible increase in NO X emissions. • A hydrogen direct injection SI engine fueled with n-butanol/gasoline is proposed. • The engine performance with five hydrogen/n-butanol ratios is investigated. • A small n-butanol has a similar enhanced influence on combustion as hydrogen. • The usage of high n-butanol hybrid fuel can be achieved by increasing hydrogen. [ABSTRACT FROM AUTHOR]

Climate change and the energy crisis urge the transition from fossil to renewable fuels. Ammonia and hydrogen are the most versatile carbon-free chemical energy carriers. Hydrogen-diluted ammonia combustion can drastically lower the emission of nitrogen oxides. The present paper focuses on the numerical analysis of ammonia/hydrogen combustion in a lean-premixed swirl burner, highlighting the effect of hydrogen share and equivalence ratio. The hydrogen share notably influences the axial position of the heat release maximum due to the higher reactivity, while it marginally influences the global velocity field. Unburned ammonia was observed at lean condition and low hydrogen concentration. At ultra-lean conditions, nitrogen oxide emission increases with hydrogen content, while a maximum can be reached by increasing the equivalence ratio. The values are compared to experimental data from the literature. Dinitrogen oxide emission decreases with both hydrogen concentration and equivalence ratio. Non-reacting flow field was validated by Particle Image Velocimetry measurements. [Display omitted] • H 2 /NH 3 flames were modeled by k-ω SST turbulence and EDC models. • The heat release pattern was influenced by the H 2 share. • A maximum in NO emission was observed for richer mixtures by increasing H 2. • A compromise should be made between NO and N 2 O emissions. • The numerical results were validated by PIV for non-reacting airflow. [ABSTRACT FROM AUTHOR]

The present paper deals with the influence of different hydrogen enrichment on oxy-fuel combustion of natural gas, enabling important insights regarding decarbonization of high-temperature processes. Experiments were conducted using a high-impulse multifuel burner for industrial applications for hydrogen contents between 0 and 100%. The fuel input varied between 100 and 140 kW, while water-cooled lances were used to simulatethermal loads. The analysis included temperature measurement points, atomic and energy balances and the cooling capacity of the thermal load for steady-state operation. The obtained results show that the flue gas temperature decreases by up to 126 K for pure hydrogen, while the cooling capacities of the thermal load increased by up to 16.3% at constant calorific energy input. The flames for different hydrogen enrichments were also compared visually. Overall, all investigated fuel blends were highly interchangeable with the investigated burner and showed high potential for a timely decarbonization of industrial heating. • Oxy-fuel combustion of hydrogen-enriched natural gas with 0–100% H 2. • Experimental analysis of a semi-industrial scale furnace at steady state conditions. • How the flame length, heat distribution and flow rates depend on H 2 content in NG. • Analysis of mass and energy fluxes with positive pressure within the furnace. • Promising solution for timely CO 2 reduction in high-temperature processes. [ABSTRACT FROM AUTHOR]

In this paper, we examine the combustion of a hydrogen microjet outflowing from a curved channel with a round micronozzle. Jet flows that are generated using rectilinear and curved channels differ in that, in the second case, Dean vortices make a noticeable contribution to the formation of the jet and its combustion. The interaction of the latter with Kelvin–Helmholtz vortices, the formation of which is typical for flows with a velocity shift, causes changes in the combustion characteristics. They include spatial distortions of the laminar flame zone near the nozzle exit, the area of turbulent combustion downstream, as well as the turbulent flame under the conditions of its separation from the nozzle exit and the cessation of laminar combustion in the initial section of the flow. The results of these studies provide the possibility to understand better the combustion features of hydrogen microjets under conditions of their hydrodynamic instability. [ABSTRACT FROM AUTHOR]

In this paper, the effect of H 2 addition to CH 4 on the NO emission of oxy-fuel, oxygen-enriched, and air-fuel MILD combustion is studied by numerical and chemical calculations. In the CFD section, the University of Lisbon MILD furnace is modeled. The results show that H 2 added to CH 4 is associated with a higher NO emission in the same oxidant composition. It is due to higher NO formation by NNH and thermal mechanisms. It can be found that by transformation from air-fuel to oxy-fuel MILD regimes, NO emission is reduced significantly. Moreover, the addition of H 2 prevents flame extinction that often occurs under MILD conditions when CO 2 replaces N 2. Reaction pathway analysis displays that dominant routes of NO emission in the H 2 -lean are N 2 O-intermediate for the air-fuel and oxygen-enriched conditions and NNH for the oxy-fuel MILD regime. In the H 2 -rich condition, the NNH mechanism plays the most important role in the NO production. • X H2,cr is known that in the higher values of it, NO emission increases dramatically. • H 2 added to CH 4 prevents flame extinction due to the replacement of CO 2 with N 2. • Change of diluent form N 2 to CO 2 reduces significantly NO emission under MILD regime. • H 2 -lean shows N 2 O-intermediate as main NO mechanism of air-fuel and oxygen-enriched. • NNH is the main route in the NO emission of CH 4 /H 2 -rich MILD regime. [ABSTRACT FROM AUTHOR]

Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of hydrogen-fueled, spark-ignited, single-cylinder Cooperative Fuel Research using experimental and numerical approaches. This study examines the effect of the air–fuel ratio on engine performance, emissions, and knock behaviour across different compression ratios. The results indicate that λ significantly affects both engine performance and emissions, with a λ value of 2 yielding the highest efficiency and lowest emissions for all the tested compression ratios. Combustion analysis reveals normal combustion at λ ≥ 2, while knocking combustion occurs at λ < 2, irrespective of the tested compression ratios. The Livenwood–Wu integral approach was evaluated to assess the likelihood of end-gas autoignition based on fuel reactivity, demonstrating that both normal and knocking combustion possibilities are consistent with experimental investigations. Combustion analysis at the ignition timing for maximum brake torque conditions demonstrates knock-free stable combustion up to λ = 3, with increased end-gas autoignition at lower λ values. To achieve knock-free combustion at those low λs, the spark timings are significantly retarded to after top dead center crank angle position. Engine-out NOx emissions consistently increase in trend with a decrease in the air–fuel ratio of up to λ = 3, after which a distinct variation in NOx is observed with an increase in the compression ratio. [ABSTRACT FROM AUTHOR]

The utilization of hydrogen for reciprocating internal combustion engines remains a subject that necessitates thorough research and careful analysis. This paper presents a study on the co-combustion of hydrogen with diesel fuel and biodiesel (RME) in a compression-ignition piston engine operating at maximum load, with a hydrogen content of up to 34%. The research employed engine indication and exhaust emissions measurement to assess the engine's performance. Engine indication allowed for the determination of key combustion stages, including ignition delay, combustion time, and the angle of 50% heat release. Furthermore, important operational parameters such as indicated pressure, thermal efficiency, and specific energy consumption were determined. The evaluation of dual-fuel engine stability was conducted by analyzing variations in the coefficient of variation in indicated mean effective pressure. The increase in the proportion of hydrogen co-combusted with diesel fuel and biodiesel had a negligible impact on ignition delay and led to a reduction in combustion time. This effect was more pronounced when using biodiesel (RME). In terms of energy efficiency, a 12% hydrogen content resulted in the highest efficiency for the dual-fuel engine. However, greater efficiency gains were observed when the engine was powered by RME. It should be noted that the hydrogen-powered engine using RME exhibited slightly less stable operation, as measured by the COVIMEP value. Regarding emissions, hydrogen as a fuel in compression ignition engines demonstrated favorable outcomes for CO, CO2, and soot emissions, while NO and HC emissions increased. [ABSTRACT FROM AUTHOR]

The paper is devoted to the analysis of two different approaches for the numerical simulation of gaseous combustion. The first one is based on a full system of Navier-Stokes equations describing the dynamics of the compressible reactive medium, while the second one utilizes low-Mach number approximation. The compressible model is realized by the traditional low-order numerical scheme and the contemporary CABARET method. The low-Mach approach is implemented on the base of a widely known FDS numerical scheme. The benefits and disadvantages of compressible and low-Mach approaches are discussed and demonstrated on a specially developed set of problem setups, applicable for validation and verification of the numerical methods for combustion analysis. In particular, the laminar flame velocity test, spherical bomb test, and multidimensional modeling of combustion development inside the rectangular closed vessel are performed via both techniques that allowed to determine the applicability limits of the low-Mach number approximation. [ABSTRACT FROM AUTHOR]

In this series of two papers, we present a novel physically sound kinetic submechanism aimed at modeling the formation and decay of chemiluminescent (electronically excited) OH ∗ molecules in the ignition and combustion of hydrogen and hydrogen-based mixtures over a wide range of temperatures and pressures. The present Part describes the construction of this detailed reaction submechanism based on the groundwork and findings of the preceding paper. In so doing, we relied both on critical evaluation of known and at times conflicting experimental and theoretical data on the elementary reaction kinetics of OH ∗ and on our quantum chemical calculations and rate constant estimates for the key processes. The kinetic model under development was validated against the representative dataset of the observed OH( A 2 Σ + → X 2 Π) chemiluminescent emission accompanying the high-temperature (post-shock) oxidation of various H 2 -containing mixtures. To improve its performance against the collected OH ∗ emission data, the rate constants of some reactive and quenching processes, to which the OH ∗ kinetics is the most sensitive and for which there is a particular uncertainty, were jointly varied within the limits imposed by theoretical considerations and/or the scatter in the available experimental evidence. Thus, by refining the rate constants of these elementary processes, we obtained a reasonable reaction submechanism for OH ∗ that describes the known experimental data better than previous models. Novelty and significance statement Although ultraviolet OH ∗ chemiluminescence as an optical signature of the combustion process and the underlying chemistry is known to provide unique capabilities for combustion diagnostics, the development of the detailed reaction mechanisms intended for quantitative interpretation of the OH ∗ emission measurements has now almost stagnated. Indeed, existing kinetic models contain a discouragingly small number of elementary processes and, apparently, do not account for the full variety of possible reaction pathways of OH ∗ formation and depletion. Therefore, in this serial work, we have intended to revise the current ideas about the kinetics of OH ∗ production and consumption during ignition and combustion and focused first on the OH ∗ kinetics in a reacting H 2 /O 2 /Ar/N 2 mixture. The principal outcome of this Part of the series is a new physically based kinetic submechanism aimed at modeling the formation and decay of electronically excited OH ∗ molecules in the ignition and combustion of hydrogen and hydrogen-containing mixtures. Its incorporation into any conventional kinetic model of hydrogen oxidation enables quantitative interpretation of the OH( A 2 Σ + − X 2 Π) chemiluminescent emission observed experimentally in flames and shock-heated reacting flows with an accuracy higher than that of previous analogous mechanisms. No less importantly, the rate constant approximations for a number of individual elementary processes with OH ∗ , recommended here based on the critical review of literature data and on our quantum chemical calculations, are also of independent interest for the kinetics of electronically excited OH ∗ particles. Finally, the OH ∗ reaction subset we suggested for hydrogen flames can form the basis of more complex OH ∗ chemiluminescence modeling reaction mechanisms for other fuels and fuel compositions. [ABSTRACT FROM AUTHOR]

Combined with need of the carbon emissions, the feasibility of Moderate or Intense Low-oxygen Dilution (MILD) combustion fueled with hydrogen/methane blends needs to be investigated. This paper discusses the pollutant emissions, the stable operating range and the flame morphology for a jet-induced MILD model combustor. The hydrogen/methane volume ratios range 0:10 to 5:5. The NO x emissions are less than 5 ppm@15%O 2 when the hydrogen content is less than 50% by volume in the atmospheric conditions. The calculation using chemical reactor network (CRN) model demonstrates that the effect of heat loss on NO x emissions increases as the adiabatic combustion temperature increases, which is consistent with the experimental results. The maximum OH∗ signal intensity increased at higher hydrogen content, especially when the hydrogen content exceeds 30% by volume. Due to the increase in turbulent burning velocity and the enhancement in the reaction intensity, the reaction zones shrink with increasing hydrogen content. In addition, with increasing hydrogen content, the stable operation range of the combustor becomes narrower, and the stable combustion is not maintained when the hydrogen content exceeds 50% by volume. The findings of the paper help to further understand the effect of hydrogen content on the formation of MILD combustion in the jet-induced combustor. • NOx emissions are ultralow when the combustor outlet temperature is below 1800K fueled with hydrogen/methane blends. • Effect of heat loss on NO X emissions increases when the adiabatic combustion temperature increases. • The stable operation range of jet-induced MILD model combustor becomes narrower increasing hydrogen content. [ABSTRACT FROM AUTHOR]

This paper describes the pulsed injection of hydrogen fuel with an incoming Mach number of 2.5 in a directly connected pulse combustion wind tunnel. The hydrogen pulse injection test with 33 Hz–62hz frequencies was carried out by using a mechanical pulse injector.The effects of different injection frequencies on the ignition delay and combustion performance of the hydrogen fuel are analyzed by means of flame self-luminescence photography and wall pressure measurements, and the results are compared with those of steady injection. The results shows that there is an optimal pulsed injection frequency--39 Hz in the frequency range studied in this paper, at this frequency, the ignition delay of hydrogen is much shorter than that of steady injection.In addition, the study also found that pulsed injection improves the combustion performance of hydrogen, so that the energy released by hydrogen combustion at lower equivalence ratios is equal to that released by steady hydrogen injection at a higher equivalence ratio. For both pulsed and steady injection, the development of the hydrogen flame can be attributed to the propagation theory of diffusion flame. The pulse frequency is not observed to have any significant effects on hydrogen combustion performance. • Effects of steady injection and pulsed injection on ignition and combustion were studied. • The results show that 39 Hz pulsed injection can greatly shorten hydrogen ignition delay. • Penetration depth of pulsed injection is higher than that of steady injection. • Pulsed injection can improve the combustion performance of hydrogen. [ABSTRACT FROM AUTHOR]

Hydrogen energy is gaining greater attention because of the energy crisis and CO2 emissions, and knock combustion has become the main obstacle to improving thermal efficiency and power performance of hydrogen engines, which is an important method of hydrogen energy application. In this paper, the knock characteristic parameters and the factors affecting knock tendency of a 2.0 L DI hydrogen engine are investigated experimentally. The results reveal the variation in knock intensity is not linear with the retarding of SOI, which is related to the cylinder mixture distribution. Furthermore, methods such as increasing injection pressures can be useful in reducing the knock intensity. Equivalence ratio has a greater impact on knock compared with other parameters. The conclusions can be used in the further exploration of the knock combustion mechanism in DI hydrogen engines. Effect of changes in parameters on knock intensity [Display omitted] • Combustion knock in DI hydrogen engines is different from PFI hydrogen engines. • Retarding of SOI leads to the nonlinear variation of detonation intensity. • Equivalence ratio has a greater effect on knock compared with other variables. [ABSTRACT FROM AUTHOR]

Green hydrogen, produced using renewable energy, is nowadays one of the most promising alternatives to fossil fuels for reducing pollutant emissions and in turn global warming. In particular, the use of hydrogen as fuel for internal combustion engines has been widely analyzed over the past few years. In this paper, the authors show the results of some experimental tests performed on a hydrogen-fueled CFR (Cooperative Fuel Research) engine, with particular reference to the combustion. Both the air/fuel (A/F) ratio and the engine compression ratio (CR) were varied in order to evaluate the influence of the two parameters on the combustion process. The combustion duration was divided in two parts: the flame front development (characterized by laminar flame speed) and the rapid combustion phase (characterized by turbulent flame speed). The results of the hydrogen-fueled engine have been compared with results obtained with gasoline in a reference operating condition. The increase in engine CR reduces the combustion duration whereas the opposite effect is observed with an increase in the A/F ratio. It is interesting to observe how the two parameters, CR and A/F ratio, have a different influence on the laminar and turbulent combustion phases. The influence of both A/F ratio and engine CR on heat transfer to the combustion chamber wall was also evaluated and compared with the gasoline operation. The heat transfer resulting from hydrogen combustion was found to be higher than the heat transfer resulting from gasoline combustion, and this is probably due to the different quenching distance of the two fuels. [ABSTRACT FROM AUTHOR]

Non-stationary modes of gaseous fuel combustion are of paramount importance for their efficient and safe usage in the space industry. The paper analyzes experimental and numerical data on the early stages of flame propagation in channels of different widths. It is demonstrated that there is a self-similar behavior of flame development at least at the considered stages. Qualitatively flame acceleration proceeds exponentially in channels of different widths, though the particular acceleration increment is inversely proportional to the width of the channel. Exponential acceleration is related to the positive feedback between the flame and flow dynamics, while the previous stage is associated mainly with the flame propagation induced by the expansion of combustion products. Based on the analysis of experimental data, one can estimate both the time instant at which the exponential stage starts and the increment of exponential acceleration, which in turn depends unambiguously on the normal burning rate. As a result, a novel technique for normal burning rate determination with the use of a channel reactor can be proposed. • Flame evolution in a channel goes from spherically expanding to finger flame modes. • Channel width defines exponential increment of finger flame velocity increase. • Flame acceleration process proceeds similarly in channels of different widths. • Transition point between flame modes can be used to estimate laminar burning velocity. [ABSTRACT FROM AUTHOR]

The present paper analyzes experimentally the morphology of a set of non-premixed diffusion reactions of hydrogen enriched natural gas (NG). For the analysis the H 2 content was varied from 0 to 100 vol.-% and the mixture issues in a co-flow at variable preheating temperatures, from ambient conditions of 25 °C and up to 400 °C. For the various NG-H 2 blends the thermal power input was held constant at 0.8 kW. For processing the images, a bitmap filter was used to isolate visible flame height, visible width variations, and soot free length fraction (SFLF). The results show that visible flame height always decreases when increasing H 2 content in NG at any preheating temperature. The effects of air preheating summarize as follows: for NG enriched up to 75 vol.-% H 2 , the visible flame height reduces when increasing the air preheating temperature from 25 °C to 200 °C, then at 300 °C the visible flame height increases when rising the air temperature up to 400 °C. For the 100% H 2 non-premixed flame the visible height presents a linear trend to increase as air temperature rises. Roper's theory for non-premixed flames in circular ports correlates reaction length to air preheating temperature and fuel composition variations, however those trends neglect buoyancy and diffusion phenomena, which results in significant error on predicting the effects of hydrogen addition to NG on the reaction length. A modified correlation is proposed to predict accurately the effects of air preheating and H 2 fraction on visible flame height. Finally, a multidimensional scaling analysis shows that hydrogen addition up to 50 vol.-% does not modifies the visible flame morphology significantly when compared to pure NG. • Adding H 2 up to 50 v% to NG does not significantly change the flame shape. • NG-H 2 non-premixed flame length has a parabolic trend with air preheating. • Air preheating and hydrogen addition make non-premixed flames shorter and thinner. • The soot free length fraction decreases when increasing air preheating temperature. • Accurate correlation for predicting visible flame height of NG-H 2 blends. [ABSTRACT FROM AUTHOR]

In this paper, a novel hydrogen-doping X-type rotary engine (XRE) using dual-ignitors is proposed to improve its high-altitude performance. Firstly, the effects of hydrogen doping fraction and operation altitude on the combustion, thermodynamic and environmental performance of XRE are revealed by establishing CFD model. Furthermore, the novel dual-ignition strategy using different ignition intervals is raised based on the eddy distribution characteristics. Finally, the comprehensive performance of the dual-ignition strategies is analyzed by comparing with the traditional single-ignition strategy under high-altitude conditions. The results show that the thermal efficiency increases firstly and then drops with the increasing hydrogen doping fraction. When hydrogen doping fraction increase by 6 %, the NO x emission increases by 2.9 times at 0 km altitude, while it increases by 4.2 times at 6 km altitude. Auxiliary ignitor arranged at back side can generate backward flame and promote combustion. At 0 km altitude, the thermal efficiency of dual-ignition reaches the maximum of 0.330, which is 3.12 % higher than that of single-ignition. However, the synchronous dual-ignition is the best strategy at high-altitude, in which the thermal efficiency is improved by 2.59 % and CO 2 emission is reduced by 3.99 % compared with the asynchronous ignition using an interval of 10 °EA. • A novel hydrogen-doping X-type rotary engine (XRE) using dual-ignitors is proposed. • Effects of hydrogen on the high-altitude performance of novel XRE are investigated. • Dual-ignition strategy of XRE is raised based on eddy distribution characteristics. • Performance differences of XRE using single-ignitor and dual-ignitors are revealed. [ABSTRACT FROM AUTHOR]

• Staged and MILD combustion are combined to achieve ultra-low/near-zero NO x emissions. • NO emission reduction mechanisms of CH 4 /H 2 staged MILD combustion are investigated. • Air-staged MILD combustion is suggested to maximize NO reduction for H 2 -rich fuels. • Staged MILD combustion can inhibit NO formation via prompt/N 2 O-intermediate/NNH. • More NO reduction by HCCO/CH i =0-3 and H is achieved in air-staged MILD combustion. Hydrogen is a promising carbon-free fuel and its co-firing with natural gas achieves rapid CO 2 emission reduction but aggravates NO x formation in industrial furnaces. This paper reports a strategy of moderate or intense low-oxygen dilution (MILD) combustion coupled with staged combustion (i.e., staged MILD combustion) to realize ultra-low and even near-zero NO x emissions for hydrogen-enriched natural gas. Meanwhile, the effectiveness and mechanism of NO emission reduction in fuel/air-staged MILD combustion of CH 4 /H 2 mixtures over a wide range of hydrogen-blending ratios from 0 to 100 % are investigated in a serial perfectly stirred reactor network model. Also, the effect of oxygen concentration ( X O 2 = 3–6 %), inlet reactant temperature (T in = 1073–1373 K), and residence time in the primary reaction zone (τ pri = 0.1–0.9 s) on NO formation/reduction via thermal, N 2 O-intermediate, NNH, and H reburning pathways during pure hydrogen fuel/air-staged MILD combustion are further analyzed. The results show that further NO emission reduction for MILD combustion can be achieved by combining fuel/air-staged combustion. A larger peak NO reduction efficiency can be reached up to 67.4 % for hydrogen when using air-staged MILD combustion but up to 52.4 % for methane when using fuel-staged MILD combustion at a secondary air/fuel ratio of 25 %. In staged MILD combustion, NO emission reduction is attributed to less NO formation via prompt and N 2 O-intermediate for methane but via N 2 O-intermediate, NNH, and thermal for hydrogen; also, another important factor is that the role of HCCO/CH i =0-3 and H radicals in NO reduction is enhanced when using air staging. In addition, the efficiency of NO emission reduction can be enhanced from 63.7 to 73.1 % for pure hydrogen air-staged MILD combustion when τ pri is increased from 0.1 to 0.9 s. In conclusion, to maximize NO emission reduction, fuel-staged MILD combustion is preferable when burning methane and its blends with a low percentage of hydrogen, whereas air-staged MILD combustion is suggested when it comes to pure hydrogen or fuels enriched with a high percentage of hydrogen. Also, a longer τ pri can achieve more NO emission reduction in air-staged MILD combustion. [ABSTRACT FROM AUTHOR]

• The diesel ignition and flame propagation characteristics are investigated. • Methane has an inhibitory effect on diesel ignition. • The combustion duration mainly CA50-90 can be shortened by hydrogen blending. • Hydrogen promotes methane combustion, especially at a low equivalence ratio. • The ignition and wall effects limit the promotion of hydrogen on flame propagation. Nowadays, internal combustion engine fuels need to shift from diesel and gasoline to clean fuels. As a low-carbon fuel, methane has attracted widespread attention. Diesel ignition of premixed methane is one of the measures to achieve effective application. However, limited by the equipment and the active reactivity of methane, there are few basic visualization studies on that, resulting in a lack of deep understanding of ignition and flame propagation processes. Therefore, this paper studies the diesel ignition and combustion characteristics of diesel-ignited methane and the promotion of 10 and 20% (volume fraction) hydrogen on combustion performance based on the rapid compression and expansion machine. The experimental results indicate that methane has a suppressive effect on the ignition of diesel. The combustion process can be mainly divided into the diesel ignition stage, the premixed combustion stage, and the fuel burnout stage. Blending 10% and 20% hydrogen has little effect on the diesel ignition stage, but has a relatively great impact on the premixed combustion stage. The flame propagation can be improved with hydrogen blending, but that is limited due to the effects of ignition and wall. The combustion duration can be shortened with hydrogen blending, mainly for the CA50-90, especially at a lower equivalence ratio. The combustion efficiency can be improved with hydrogen blending. Overall, the strategy of blending hydrogen is considered more suitable for large bore engines and the results can deepen the understanding of the combustion process of diesel-ignited methane/hydrogen/air mixtures and have value for the relevant engine development so that the energy shortage and environment pollution issues can be alleviated. [ABSTRACT FROM AUTHOR]

A novel approach for cost-effective and temporally resolved in-line combustion gas diagnostics based on spontaneous Stokes Raman spectroscopy is presented in this paper. The proposed instrument uses a multipass configuration designed to increase the scattering generation, giving information about gas species concentrations, including H2 and N2 that are not commonly available from analysis with absorption spectroscopy techniques. The system performs calibrated analysis providing both qualitative and quantitative information about the gas composition. Depending on the application, the device can work with spectra integration time from 0.15 s up to 10 s, with a Raman spectrum ranging from the H2 rotational peak at Raman shift of 587 cm−1 up to the H2 vibrational peak at 4156 cm−1, covering all the Raman emissions of major combustion species. The device response was characterized by a working pressure from 0.7 to 7.5 bar. The instrument prototype has been made completely transportable, designed to operate using a gas sampling system, and ready to be operated in relevant industrial in-line environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • The PC acting as igniter and oxidant supplier is used to control H2/O2 combustion. • The PC induced H2/O2 diffusive combustion is divided into four stages. • The ignition delay and combustion duration vary in opposite trend with φ G. • Transient of premix to diffusive combustion is decided by PC combustion intensity. • Enlarging orifice avails increasing pressure and combustion efficiency. The H2/O2 internal combustion is promising to be adopted as auxiliary power by combusting the wasted hydrogen in the upper stages under aerospace conditions. Concerning no inert gases participating the combustion, the control of H2/O2 heat release is a great challenge. This paper proposes to control the heat release by adopting an active pre-chamber (PC). All the combustion consumed oxygen is injected into the PC. The combustion of oxygen and small amount of pre-existed H2 in PC increases the PC pressure, enabling the hot reactant and massive oxygen to be injected into the main chamber (MC) to arrange the oxygen jet induced hydrogen diffusive combustion. The test was conducted on a constant volume combustion bomb (CVCB) to explore the effects of equivalence ratio (φ G) and orifice diameter on the diffusive H2/O2 combustion characteristics. The results showed that increasing φ G resulted in the decreased premix combustion fraction and prolonged combustion duration. The peak pressure of MC dropped with φ G indicating the ability of adjusting engine load by varying φ G. The heat release process could be controlled either by φ G or orifice diameter. Properly increasing the orifice diameter availed shortening the combustion duration and promoting the heat release efficiency. [ABSTRACT FROM AUTHOR]

The use of hydrogen as a fuel is a promising way to reduce the emissions of civil aviation but it requires the development of wholly new injectors for the combustion chamber. Thanks to the increase in available computing power, the application of optimization techniques combined with CFD computations is now possible to develop these injectors. Among the optimization approaches, Bayesian optimization is particularly relevant when the objective functions and constraints of the optimization problem are expensive to evaluate which is the case in CFD-based optimization. Besides, the use of a multifidelity strategy allows to reduce the simulation cost of the Bayesian method. Therefore, this paper investigates the application of a multifidelity and multi-objective Bayesian approach to improve the performances of a laboratory swirl injector using hydrogen and operating in conditions close to industrial targets. This optimization study combines LES simulations as high-fidelity model with 2D RANS simulations as low-fidelity. • A multifidelity and multi-objective optimization is performed for an H 2 -powered laboratory injector. • The operating point is representative of the cruise conditions of a SMR aircraft. • Large-eddy simulations are integrated in the optimization approach. • This optimization study is compared to a monofidelity approach and to two single-objective approaches. [ABSTRACT FROM AUTHOR]

This paper presents an overview of the past and present of low-emission combustor research with hydrogen-rich fuels at Aachen University of Applied Sciences (AcUAS). In 1990, AcUAS started developing the dry-low-NOx Micromix combustion technology. Micromix reduces NOx emissions using jet-in-crossflow mixing of multiple miniaturized fuel jets and combustor air with an inherent safety against flashback. At first, pure hydrogen as fuel was investigated with lab-scale applications. Later, Micromix combustor prototypes were developed for the use in an industrial gas turbine Honeywell/Garrett GTCP-36-300, proving low NOx characteristics during real gas turbine operation, accompanied by the successful definition of safety laws and control system modifications. Further, the Micromix was optimized for the use in annular and can-combustors as well as for fuel-flexibility with hydrogen-methane-mixtures and hydrogen-rich syngas qualities by means of extensive experimental and numerical simulations. In 2020, the latest Micromix application is demonstrated in a commercial 2 MW-class gas turbine can-combustor with full-scale engine operation. This paper discusses the advances in Micromix research over the last three decades. [ABSTRACT FROM AUTHOR]

In this paper, the combustion characteristics of a methane-fueled free-piston linear engine (FPLE) with glow-assisted ignition (GAI) under different hydrogen fractions were studied by numerical simulations. The results showed that the chain-branching reactions start to process after the ignition source appears. The fuel-air mixture can be successfully ignited under the compression of the piston. Hydrogen addition to the fuel accelerates the elementary reaction and helps enhance the ignition and combustion processes. Compared with that of pure methane, the maximum HRR increases by 9.4%, 49.4%, 75.6%, 124.7%, and 241.4% with hydrogen fractions of 1.0%, 3.0%, 5.0%, 7.0%, and 9.0%, respectively. High hydrogen concentration (greater than 7.0%) can bring the occurring of over-advanced combustion, resulting in higher compression negative work. With the hydrogen fractions changing from 0% to 9.0%, the CA0–10 first decreases and then increases, with its minimum value at α H 2 = 7.0 % , and the CA10–90 is gradually decreased. The hydrogen addition raises the combustion temperature, which in turn, results in higher NOx emission. High combustion temperature helps convert carbon monoxide into carbon dioxide, decreasing the emission amount of CO. Hydrogen addition to methane has competition effect on soot emission. On one hand, it reduces the soot emission by soot oxidation. Another hand it causes chemical effect leading to higher soot nucleation and surface growth rates. In this paper, the result of competition suppresses soot formation. • A 3D computational model on in-cylinder combustion of glow-assisted ignition free-piston linear engine was established. • Effects of hydrogen addition on the performance of the free-piston linear engine were investigated. • The in-cylinder ignition, combustion, and emission characteristics under the glow-assisted ignition were analyzed. [ABSTRACT FROM AUTHOR]

• NG is considered a cleaner burning fossil fuel than oil or coal. • Hydrogen is a sustainable and environmentally friendly energy source that should be more widely used. • Experimental findings have concluded that hydrogen is better suited for use in gasoline engines due to its high self-ignition temperature. • Hydrogen addition to diesel fuel leads to a reduction in some exhaust emissions and fuel consumption, however NO x emissions were increased. This paper provides a comprehensive overview of the physical properties and applications of natural gas (NG) and hydrogen as fuels in internal combustion (IC) engines. The paper also meticulously examines the use of both NG and hydrogen as a fuel in vehicles, their production, physical characteristics, and combustion properties. It reviews the current experimental studies in the literature and investigates the results of using both fuels. It further covers the challenges associated with injectors, needle valves, lubrication, spark plugs, and safety requirements for both fuels. Finally, the challenges related to the storage, production, and safety of both fuels are also discussed. The literature review reveals that NG in spark ignition (SI) engines has a clear and direct positive impact on fuel economy and certain emissions, notably reducing CO 2 and non-methane hydrocarbons. However, its effect on other emissions, such as unburnt hydrocarbons (UHC), nitrogen oxides (NO x), and carbon monoxide (CO), is less clear. NG, which is primarily methane, has a lower carbon-to-hydrogen ratio than diesel fuel, resulting in lower CO 2 emissions per unit of energy released. In contrast, hydrogen is particularly well-suited for use in gasoline engines due to its high self-ignition temperature. While increasing the hydrogen content of NG engines reduces torque and power output, higher hydrogen input results in reduced fuel consumption and the mitigation of toxic exhaust emissions. Due to its high ignition temperature, hydrogen is not inherently suitable for direct use in diesel engines, necessitating the exploration of alternative methods for hydrogen introduction into the cylinder. The literature review suggests that hydrogen in diesel engines has shown a reduction in specific exhaust emissions and fuel consumption and an increase in NO x emissions. Overall, the paper provides a valuable and informative overview of the challenges and opportunities associated with using hydrogen and NG as fuels in IC engines. It highlights the need for further research and development to address the remaining challenges, such as the development of more efficient combustion chambers and the reduction of NO x emissions. [ABSTRACT FROM AUTHOR]

Free-piston linear generator (FPLG) engines currently gained great attention due to their capability to operate with variable fuel and compression ratio. This paper presents an experimental study on the effect of aspect ratio on the performance characteristics of the FPLG engine fueled by hydrogen. Three aspect ratios (i.e. 1.0, 1.5, and 2.0) are used to identify the engine combustion and performance parameters. The injection position is fixed in the middle of the stroke, while the equivalence ratio is kept at 1.0. The results indicate that the aspect ratio 2.0 produces the highest pressure, heat release, and shortest combustion duration. Whereas the aspect ratio 1.0 produces higher combustion efficiency and operating frequency. The piston speed decreases with the decrease in aspect ratio, which gives a negative effect on the indicated mean effective pressure and power output of the PFLG. Overall, the aspect ratio has a significant influence on engine performance characteristics. • This paper focuses on the experimental study on the FPLG engine performance. • The effect of aspect ratio on the performance of FPLG was also investigated. • Three aspect ratios (i.e. 1.0, 1.5, and 2.0) were used and tested in this study. • The long-stroke produces the highest combustion pressure, IMEP, and Prms. • Overall engine combustion and performance improves with increasing aspect ratio. [ABSTRACT FROM AUTHOR]

In the present paper, an experimental investigation has been performed under variable CR and 1400&1800RPM speed at a fixed spark timing of 24ºCA BTDC under wide-open throttle conditions. The hydrogen blending is performed based on energy fractions from 5% to 21% of the total fuel energy. With increasing compression ratio (CR), the flame development gets faster, and the flame propagation speed improves, leading to a short combustion period. Similarly, increasing hydrogen fraction improves combustion, resulting in a rapid rise in pressure and temperature. Despite a 13.64% decrease in volumetric efficiency from 5% to 21% hydrogen fraction at 1400 and 1800 RPM, BP and BTE increased by 16.89% and 33%, respectively. The slow-burning properties of NH 3 extend the combustion period, resulting in a long-delayed burning period. As a result, the temperature of the low-hydrogen fraction of the exhaust gas is higher. As the hydrogen fraction and CR increase, this effect decreases, resulting in lower EGT. The hydrogen addition increases the peak temperature; therefore, NO x increases continuously with increasing hydrogen despite reducing ammonia. Ammonia is a key element used to reduce NO x from vehicles. A practical solution for controlling the NO x due to the ammonia/hydrogen blend is selective catalytic reduction (SCR). • Ammonia is an excellent hydrogen energy carrier, ensures good fuel qualities. • H 2 improves the slow burning drawbacks of NH 3 at negligible volumetric losses. • High-octane number of NH 3 /H 2 allows high CR, increasing efficiency without knock. • With no greenhouse gas emissions, there is no impact on global warming. • NO x emissions are found to increase as the CR and H 2 energy fractions increase. [ABSTRACT FROM AUTHOR]

The target of net-zero emissions set by the 2015 Paris Agreement has strongly commissioned the energy production sector to promote decarbonization, renewable sources exploitation, and systems efficiency. In this framework, the utilization of hydrogen as a long-term energy carrier has great potential. This paper is concerned with the combustion characterization in a non-premixed gas turbine burner, originally designed for natural gas, when it is fed with NG-H2 blends featuring hydrogen content from 0 to 50% in volume. The final aim is to retrofit a 40 MW gas turbine. Starting from the operational data of the engine, a CFD model of the steady-state combustion process has been developed, with reference to the base load NG conditions, by reducing the fuel mass-flow rate by up to 17% to target the baseline turbine inlet temperature. When the fuel is blended with hydrogen, for a given temperature at turbine inlet, an increase in the peak temperature up to 800 K is obtained, if no countermeasures are taken. Furthermore, the flame results are more intense and closer to the injector in the case of hydrogen blending. The results of this work hint at the necessity of carefully analyzing the possible NOx compensation strategies, as well as the increased thermal stresses on the injector. [ABSTRACT FROM AUTHOR]

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio. • Flame characteristics of CH 4 /Air with H 2 addition in the micro space were studied. • Adding H 2 can broaden the flame stabilization zone and reduce the quenching distance. • H 2 enhanced flame stabilization is due to the increased SDR of local flame extinction. • Hydrogen addition leads to a faster mixing of the fuel/air mixture. [ABSTRACT FROM AUTHOR]