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Buoyant unstable behavior in initially spherical lean hydrogen-air premixed flames within a center-ignited combustion vessel have been studied experimentally under a wide range of pressures (including reduced, normal, and elevated pressures). The experimental observations show that the flame front of lean hydrogen-air premixed flames will not give rise to the phenomenon of cellular instability when the equivalence ratio has been reduced to a certain value, which is totally different from the traditional understanding of the instability characteristics of lean hydrogen premixed flames. Accompanied by the smoothened flame front, the propagation mode of lean hydrogen premixed flames transitions from initially spherical outwardly towards upwardly when the flames expand to certain sizes. To quantitatively investigate such buoyant instability behaviors, two parameters, “float rate (ψ)” and “critical flame radius (Rcr)”, have been proposed in the present article. The quantitative results demonstrate that the influences of initial pressure (Pint) on buoyant unstable behaviors are different. Based on the effects of variation of density difference and stretch rate on the flame front, the mechanism of such buoyant unstable behaviors has been explained by the competition between the stretch force and the results of gravity and buoyancy, and lean hydrogen premixed flames will display buoyant unstable behavior when the stretch effects on the flame front are weaker than the effects of gravity and buoyancy.

Flame surface instability has a strong effect on the flame speed of turbulent flames in the flamelet and corrugated flamelet regimes. These effects can either increase the burning rate or cause local extinction. In this work, direct numerical simulations are used to evaluate the linear and nonlinear growth of hydrodynamic and thermo-diffusive instabilities caused by a spatial perturbation pattern along a lean hydrogen/air premixed front. The numerical simulations use detailed chemistry and transport in a two-dimensional channel geometry with periodic boundary conditions along the vertical boundaries. Both stabilized Bunsen flames and transient flames are simulated. The increase in local combustion rate due to flame stretch in both cases is compared, revealing the transient effects. Then, a diagram of the dispersion relation for the transient flame is calculated, showing the growth rate of the disturbance and the corresponding wavenumbers. The results show that in regions where the stretch is weaker, the spectrum of curvature and wavelengths tends to follow the relationships between flame speed and curvature as predicted by linear theory, despite the chaotic behavior of the fold front. On the other hand, when the curvature of the flame front tends to high negative values, significant deviations from the linear model are observed. The results demonstrate the role of hydrodynamic and thermo-diffusive instabilities in enhancing the combustion rate of turbulent lean hydrogen flames. This enhancement of combustion rate is driven by the increased flame surface area resultant from these instabilities. Hydrodynamic instability induces wrinkles on the flame surface, thus increasing its area. Similarly, thermo-diffusive instability, resulting from differential diffusion effects between heat and species, leads to variations in flame surface area, consequently affecting the combustion rate. These phenomena collectively boost the combustion process by maximizing the reactive surface area.

The replacement of hydrocarbon fuels by ammonia in industrial systems is challenging due to its low burning velocity, its narrow flammability range, and a large production of nitric oxide and nitrogen dioxide when burned close to stoichiometric conditions. Cracking a fraction of ammonia into hydrogen and nitrogen prior to injection in the combustion chamber is considered a promising strategy to overcome these issues. This paper focuses on evaluating how different levels of ammonia cracking affect the overall burning velocity, the lean blow-off limit, the concentration of nitric oxide and nitrogen dioxide, and the flame response to acoustic perturbations. Swirl stabilized premixed flames of pure ammonia–air and ammonia–hydrogen–nitrogen–air mixtures mimicking 10%, 20%, and 28% of cracking are experimentally investigated. The results show that even though ammonia cracking is beneficial for enhancing the lean blow-off limit and the overall burning velocity, its impact on pollutant emissions and flame stability is detrimental for a percentage of cracking as low as 20%. Based on an analysis of the flame dynamics, reasons for these results are proposed.

Numerical simulations of flame structure and laminar burning velocity S L are performed for a lean (12%) hydrogen–air mixture under standard conditions. An analysis of the concentration profiles of intermediate species shows that a change in the kinetic mechanism that controls heat release dynamics occurs with increasing temperature. Thus, heat release in the flame consists of two stages. In the region of maximum temperature gradient, the concentrations of H 2 O 2 and HO 2 reach their peak values. The subsequent decrease in H 2 O 2 and HO 2 concentrations is accompanied by a concurrent increase in H, O, and OH concentrations. Variation of the rate constants for the reactions responsible for heat release results in changes in both temperature gradient and the value of S L . The value of S L is most sensitive to the reaction in which molecular hydrogen combines with hydroxyl radical to produce water vapor.

A series of experiments were performed to investigate the effect of ignition energy (Eig) and hydrogen addition on the laminar burning velocity (Su0), ignition delay time (tdelay), and flame rising time (trising) of lean methane−air mixtures. The mixtures at three different equivalence ratios (ϕ) of 0.6, 0.7, and 0.8 with varying hydrogen volume fractions from 0 to 50% were centrally ignited in a constant volume combustion chamber by a pair of pin-to-pin electrodes at a spark gap of 2.0 mm. In situ ignition energy (Eig ∼2.4 mJ ÷ 58 mJ) was calculated by integration of the product of current and voltage between positive and negative electrodes. The result revealed that the Su0 value increases non-linearly with increasing hydrogen fraction at three equivalence ratios of 0.6, 0.7, and 0.8, by which the increasing slope of Su0 changes from gradual to drastic when the hydrogen fraction is greater than 20%. tdelay and trising decrease quickly with increasing hydrogen fraction; however, trising drops faster than tdelay at ϕ = 0.6 and 0.7, and the reverse is true at ϕ = 0.8. Furthermore, tdelay transition is observed when Eig > Eig,critical, by which tdelay drastically drops in the pre-transition and gradually decreases in the post-transition. These results may be relevant to spark ignition engines operated under lean-burn conditions.

Simulations of the effect of addition of H, O, OH, HO 2 , and H 2 O 2 on the structure and propagation of laminar flames in lean (12 and 15%) hydrogen-air flames are performed at pressures of 1 and 6 bar. It is found that impurities in concentrations of no more than 0.1% do not have any significant effect on laminar burning velocity. When initial temperature is increased to 400 K, the effect of impurities becomes even weaker. Among the impurities under study, only the addition of OH reduces the laminar flame velocity. The weak effect of the impurities is attributed to fast formation of intermediate products via reactions involving O and H atoms without noticeable change in heat release rate. An increase in initial pressure to 6 bar does not change the effect of impurities.

Flame stability limits controlled by flame blowout and flashback were calculated and experimentally determined in a model burner device with various hydrogen addition to a premixed methane–hydrogen flame. The experimental data obtained are used to verify the mathematical model of methane–hydrogen mixture combustion, considering the dependence of the laminar flame propagation speed on temperature, pressure, and composition of air-fuel mixture (AFM), and also using the verified chemico–kinetic Wang 2018 mechanism. It is shown that the mathematical model of methane–hydrogen mixture combustion predicts flame blowout and flashback upstream with satisfactory accuracy in comparison with experimental data. It is shown that the mathematical model of methane–hydrogen mixture combustion predicts flame blowout and flashback limits upstream, observed in the model burner device with satisfactory accuracy, and can be used to determine stable operation limits of developed combustion chambers of gas turbine power plants transferred to hydrogen–containing mixtures.

This study introduces an innovative approach involving the injection of hydrogen into a low-swirl, non-premixed flame, which operates with gaseous fuels derived from an air-blast atomizer designed for aero-engine applications. The aim is to characterize how hydrogen enrichment influences flame structures while maintaining a constant thermal output of 4.6 kW. Using high-speed chemiluminescence imaging, three fueling conditions were compared: the first involved pure methane/air, while the second and third conditions introduced varying levels of hydrogen to an air–methane mixture. The results reveal significant effects of hydrogen enrichment on flame characteristics, including a slightly shorter length and a wider angle attributed to heightened expansion within the Combustion Recirculation Zone. Moreover, the emission of UV light underwent considerable changes, resulting in a shifted luminosity zone and reduced variance. To delve deeper into the underlying mechanisms, the researchers employed Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD) analyses, showing coherent structures and energetic modes within the flames. Hydrogen enrichment led to the development of smaller structures near the nozzle exit, accompanied by longitudinal oscillations and vortex shedding phenomena. These findings contribute to an advanced understanding of hydrogen’s impact on flame characteristics, thereby propelling efforts toward improved flame stability. Additionally, these insights hold significance in the exploration of hydrogen as an alternative energy source with potential environmental benefits.

Hydrogen-fueled internal combustion engines present a promising pathway for reducing carbon emissions in urban transportation by allowing for the reuse of existing vehicle platforms while eliminating carbon dioxide emissions from the exhaust. However, operating these engines with lean air–fuel mixtures—necessary to reduce nitrogen oxide emissions and improve thermal efficiency—leads to significant reductions in power output due to the low energy content of hydrogen per unit volume and slower flame propagation. This study investigates whether integrating a mild hybrid electric system, operating at 48 volts, can mitigate the performance losses associated with lean hydrogen combustion in a small passenger vehicle. A complete simulation was carried out using a validated one-dimensional engine model and a full zero-dimensional vehicle model. A Design of Experiments approach was employed to vary the electric motor size (from 1 to 15 kW) and battery capacity (0.5 to 5 kWh) while maintaining a fixed system voltage, optimizing both the component sizing and control strategy. Results showed that the best lean hydrogen hybrid configuration achieved reductions of 18.6% in energy consumption in the New European Driving Cycle and 5.5% in the Worldwide Harmonized Light Vehicles Test Cycle, putting its performance on par with the gasoline hybrid benchmark. On average, the lean H2 hybrid consumed 41.2 kWh/100 km, nearly matching the 41.0 kWh/100 km of the gasoline P0 configuration. Engine usage analysis demonstrated that the mild hybrid system kept the hydrogen engine operating predominantly within its high-efficiency region. These findings confirm that lean hydrogen combustion, when supported by appropriately scaled mild hybridization, is a viable near-zero-emission solution for urban mobility—delivering competitive efficiency while avoiding tailpipe CO2 and significantly reducing NOx emissions, all with reduced reliance on large battery packs.

This paper presents a state‐of‐the‐art literature review on noise, vibration, and harshness (NVH) in hydrogen‐fuelled internal combustion engines. Studies published between 2011 and 2025 were screened, covering fundamental flame physics, test‐bench work, and recent prototype vehicles. The review links hydrogen's core properties—high flame speed, wide flammability, low ignition energy, strong diffusivity—to specific NVH outcomes such as rapid pressure rise, knock, back‐fire, and block resonance. For each pathway we summarise measured noise levels, vibration signatures, and psycho‐acoustic findings. Mitigation methods are then grouped: lean premixing, direct injection, adaptive ignition timing, exhaust tuning, and structural damping. Results show that, with these measures, hydrogen engines can approach the NVH envelope of modern gasoline units. Remaining gaps lie in long‐term durability under high‐frequency loading and in full‐vehicle sound quality. Overall, the review clarifies current knowledge, highlights consistent trends, and points to research still needed for quiet, smooth hydrogen mobility.