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Hydrogen embrittlement (HE) threatens the safety of hydrogen transportation pipelines. This study investigates HE susceptibility in X52M and X52MH steels, intended for hydrogen transport. Slow strain rate tensile (SSRT) tests are conducted in N₂ and H₂ at 6.3 MPa, with fracture surfaces analyzed using SEM and 3D X-ray micro-CT. Results reveal that X52MH steel is more susceptible to HE than X52M, attributed to its ferrite/bainite (F/B) duplex-phase microstructure, unlike the acicular ferrite (AF) structure of X52M. Segregation of Cu, Cr, and Ni in X52MH also promotes crack initiation and propagation, as hydrogen-induced cracks on the specimen surface and localised concentrations at the F/B interface from uncoordinated deformation increase HE susceptibility in X52MH. [Display omitted] • X52MH has higher hydrogen embrittlement due to its ferrite/bainite structure. • Cu, Cr, Ni segregation in X52MH speeds up hydrogen-induced crack growth. • Uncoordinated ferrite/bainite deformation in X52MH accelerates crack initiation. • Surface cracks in X52MH raise hydrogen embrittlement susceptibility vs X52M. [ABSTRACT FROM AUTHOR]

In the transition towards green energy, long distance transportation of gaseous hydrogen by pipeline is considered to be an economically attractive solution. For safe transportation of hydrogen, it is important to conduct relevant mechanical tests in representative hydrogen environments and working conditions of pipelines to demonstrate acceptable hydrogen embrittlement resistance of the steel. One important issue is the testing condition for the ratio of gaseous hydrogen volume over specimen surface area required in mechanical testing since this has not been specified in any testing standards and codes or investigated. This study investigated this issue by performing slow strain rate tensile (SSRT) testing of an X52 pipeline steel in gaseous hydrogen with different ratios of hydrogen volume/specimen surface area, R v/s , ml/mm2. It was found that, with increasing R v/s ratio, the area reduction of the steel decreased and hydrogen embrittlement sensitivity increased in the range of the R v/s ratios investigated. The saturating R v/s ratio appeared to be ∼2.0 beyond which the effect of R v/s ratio on hydrogen embrittlement sensitivity was relatively small. The macroscopic and microscopic observations on both the fracture and side surfaces of the SSRT tested specimens also found that the tendency to cleavage fracture increased with increasing R v/s ratio. The results of the ab initio molecular dynamics (AIMD) simulations performed also predicted a dependence of hydrogen embrittlement on hydrogen volume when R v/s ratio is relatively small. • The effect of the ratio of gaseous hydrogen volume over specimen surface area, R v/s , on HE was investigated the first time in the world. • The threshold R v/s ratio, beyond which the influence of the R v/s ratio on hydrogen embrittlement sensitivity is small, was determined. • Tendency to cleavage fracture increased with increasing R v/s ratio. • The increased tendency to hydrogen embrittlement under higher R v/s ratios was verified by the AIMD simulations. [ABSTRACT FROM AUTHOR]

In this paper, the characteristics of hydrogen-doped natural gas combustion in industrial boilers were investigated by means of numerical simulations. Initially, a jet flame test bench was constructed to conduct single-variable experiments at various hydrogen doping ratios and excess air coefficients, thus analyzing the impact on flame combustion patterns. Computational Fluid Dynamics (CFD) numerical simulations were performed using the Fluent software, focusing on the combustion zones within the burner and boiler chamber. Simulations were based on actual operating conditions to set parameters, employing the k-ε turbulence model. The SIMPLE algorithm facilitated examination of the effect of hydrogen doping ratios on the combustion temperature and concentration of pollutants, with a particular focus on the concentration of the flow field, temperature field, NOx generation rate, and the concentrations of O 2 and CO. The results show that: As the hydrogen doping percentage increases, the fuel flame propagation speed also increases, leading to a faster combustion process that yields a shorter, wider flame. Conversely, as the excess air coefficient rises, the flame becomes narrower, transitioning from a clearly-layered structure to a uniform one. Furthermore, an increase in the hydrogen doping ratio results in higher combustion temperatures and accelerated combustion reaction rates, accompanied by a decrease in CO concentration and an increase in NOx emission concentration. These results indicate that hydrogen doping enhances the combustion reaction and reduces pollutant emissions from gas combustion. This study provides a theoretical foundation for the practical application of hydrogen-doped natural gas in industrial boilers. • Experimental study of the effect of hydrogen doping ratio on flame morphology. • Effect of hydrogen doping ratio on combustion characteristics of natural gas. • Effect of excess air coefficient on combustion characteristics of natural gas. • Combining experimental and simulation to analyze combustion characteristics. • Analysis of combustion temperatures and pollutant generation characteristics. [ABSTRACT FROM AUTHOR]

Blending a low ratio of hydrogen into traditional hydrocarbon fuels can reduce the modification in structure of existing combustors. However, even a low hydrogen ratio can increase the propensity of flashback due to the extremely high reactivity of hydrogen. Swirl number is a key parameter of swirl combustors which determines the flow structure. To design reliable gas turbine combustors for hydrogen-blended fuels, the study in the effect of swirl strength on flashback characteristic and the underlying mechanism is necessary. In this study, the flashback characteristics of premixed 30% H 2 /70% CH 4 /air flame in a bluff-body swirl burner is investigated via both experiment and large-eddy simulations with the flamelet-generated-manifold method (FGM-LES). The flashback mode and flame-flow interaction are analyzed based on the LES results. It is found that the increase in swirl strength will increase the propensity of flashback and promote the flashback speed of premixed 30% H 2 /70% CH 4 /air flame in the bluff-body swirl burner. The turns from Mode II (small-scale flame bulges leading flashback) to Mode I (large-scale swirling flame tongue leading flashback) during flashback of hydrogen-blended flames due to the production of stronger reversed flow by swirling flow. It is also found that stronger swirl strength will result in a smoother flame front and narrower strain rate distribution range which suppresses the local hydrogen enrichment due to preferential diffusion of hydrogen, thus suppressing the flame propagating speed. However, this effect is not dominant in flashback process compared to the promotion of reversed flow by increased swirl strength. • Efficient FGM tabulation including heat loss and differential diffusion for premixed hydrogen/methane flame is applied. • The flashback processes of premixed hydrogen-blended flame in experiment were reproduced well through FGM-LES. • The interaction between swirl strength and preferential diffusion of hydrogen was investigated. [ABSTRACT FROM AUTHOR]

• Quantitatively evaluated equilibrium and diffusion behavior of mixed hydrogen-methane gas in gravity fields. • Effects of height, temperature, and mixing ratio on stratification and diffusion behavior of the mixture are revealed. • The gravitational stratification of hydrogen-methane mixture requires either an extremely strong gravity field or very large drops in altitude, being negligible for realistic pipes. • Temporary interruptions in pipeline gas transportation will not cause visible stratification due to slow diffusion of gases. Repurposing existing natural gas pipelines is a promising solution for large-scale transportation of mixed hydrogen-methane gas. However, it remains debatable whether gravitational stratification can notably affect hydrogen partial pressure in the gas mixture. To address this issue, we combined molecular dynamics simulation with thermodynamic and diffusion theories. Our study systematically examined the equilibrium distribution of hydrogen-methane mixtures in gravity fields. We demonstrated that partial pressures of both gases decrease with altitude, with hydrogen showing a slower decrease due to its smaller molar mass. As a result, the volume fraction of hydrogen is maximized at the top end of pipes. The stratification is more favorable at low temperatures and large altitude drops, with notable gas stratification only occurring at extremely large drops in altitude, being generally negligible even at a drop of 1500 m. Furthermore, we showed that the diffusion time required to achieve the equilibrium distribution is proportional to gas pressure and the square of pipeline height. This requires approximately 300 years for a 1500 m pipeline at 1 bar. Therefore, temporary interruptions in pipeline gas transportation will not cause visible stratification. Our work clarifies the effect of gravity on hydrogen-methane gas mixtures and provides quantitative insights into assessing the stratification of gas mixtures in pipelines. [ABSTRACT FROM AUTHOR]

During the transportation of hydrogen-blended natural gas (HBNG) through the natural gas pipeline network, the accuracy of the physical property model for real gas greatly influences the reliability of aerodynamic performance of compressor systems and the hydrothermal characteristics of pipeline networks. In this study, the adaptability of various equations of state (EOS) in predicting physical property parameters including compressibility factor, enthalpy difference, entropy difference, specific heat capacity, specific heat ratio and viscosity of natural gas (NG) and HBNG is evaluated based on the experimental data and NIST REFPROP firstly. The properties calculated using NIST REFPROP are compared with those from the EOSs, such as cubic EOSs (Redlich–Kwong EOS (RK), Soave–Redlich–Kwong EOS (SRK), Peng–Robinson EOS (PR)) and Soave-Benedict-Webb-Rubin (BWRS) EOS. After confirming the reliability of NIST REFPROP as the calculation benchmark, the accuracy of the four EOSs in calculating the physical properties of HBNG is assessed within the temperature range of 250∼350 K, pressure range of 1∼25 MPa, and hydrogen doping ratio of 0%–30%. Besides, the mixing rules of different equations of state (EOSs) are compared based on the NIST REFPROP calculation results. The results indicate that, except for specific heat capacity calculation, the mixing rule has little influence on the prediction accuracy of cubic EOS models, and the maximum calculation error between different mixing rules of the same EOS model does not exceed 0.5%. Under specific conditions, the traditional mixing rule exhibits better prediction accuracy and can be prioritized due to its simplicity. Furthermore, the pressure has a significant impact on the accuracy of EOS models, with the prediction accuracy decreasing as the pressure increased. The influence of temperature on the prediction accuracy is less significant than that of pressure. Generally, lower temperatures exhibit lower prediction accuracy, while higher temperatures show higher prediction accuracy. Among the cubic EOS models, the RK equation performs better than the SRK and PR EOS models. While the cubic EOSs show minimal differences in predicting specific heat ratio with an average relative error below 1%, the RK EOS shows significant advantages in predicting other physical parameters. The simple structure of RK EOS meets the calculation requirements for HBNG engineering. The comprehensive comparisons reveal that, the BWRS EOS has higher accuracy in predicting various physical properties parameters, with the maximum average relative error not exceeding 5% for each physical property parameter, and its stability is also better compared to the cubic EOSs. The further analysis shows that, the calculation of viscosity is influenced not only by the EOS but also by the viscosity model. When calculating viscosity using the LU-Model, the PR EOS provides the most accurate results. In contrast, when utilizing the LGE-Model or LBC-Model, the BWRS EOS demonstrates superior accuracy compared to other EOSs. • Mixing rules slightly effect state equations' accuracy across different x-H2, besides heat capacity calculations. • Unlike other parameters, the viscosity calculation depends on both the viscosity model and the equations. • Apart from viscosity calculations, Redlich-Kwong equation performs the best among the cubic state equations. • The Benedict-Webb-Rubin-Starling equation is more accurate than the cubic state equations overall. • The impact of pressure on equations' accuracy for hydrogen-blended natural gas is more obvious than that of temperature. [ABSTRACT FROM AUTHOR]

To reveal the deflagration hazard and propagation law of hydrogen-enriched natural gas (H 2 NG) in closed spaces, this paper studied the deflagration characteristic parameters under different H 2 blended ratio (λ) and inert gas dilution fraction (X dilution) in slender closed pipelines with steel circular hole obstacles. The results indicate that the positive feedback mechanism's sustained effect, failure, and re-action leads to a "three-zone" distribution of H 2 NG explosion overpressure along the pipeline, namely the acceleration, attenuation, and rebound zones. The maximum explosion overpressure (P max), maximum pressure rise rate ((dP/dt) max), and peak shock wave propagation velocity (V p) all increase with the increase of λ. The rise of λ has the most significant enhancement effect on (dP/dt) max , while the enhancement effect on V p is the smallest. This is because the main reasons affecting their increase are different. P max , (dP/dt) max , and V p exhibit exponential decay with increased N 2 and CO 2. However, CO 2 has a more significant inhibitory effect on the H 2 NG deflagration process. When X CO2 = 25% and X CO2 = 30%, the deflagration process of H 2 NG with λ = 20% and λ = 80% can be inhibited entirely, respectively, which has important guiding significance for the safe application of H 2 NG. • The evolution law of deflagration shock waves in slender closed pipelines by H 2 NG was researched. • Effects of H 2 blended ratio and N 2 /CO 2 dilution fraction on H 2 NG deflagration characteristics were revealed. • The effects of N 2 /CO 2 dilution fraction on the P max , (dP/dt) max , and V p were predicted by an exponential model. [ABSTRACT FROM AUTHOR]