Initial Hydrogen Concentration Research Articles

Predicting the effect of cooling rates and initial hydrogen concentrations on porosity formation in Al‐Si castings

Predicting the effect of cooling rates and initial hydrogen concentrations on porosity formation in Al‐Si castings

Simulation of gas porosity formation and interaction with dendrite and eutectic structures during solidification of Al-Si alloys

A two-dimensional (2-D) multi-phase cellular automaton (CA)-finite difference (FD)-lattice Boltzmann (LB) coupled model is developed to simulate the hydrogen pore formation during dendritic and eutectic solidification of hypoeutectic Al-Si alloys. In this model, the dendrite and eutectic structures are simulated using a CA technique. The solute diffusion is solved by the FD method. The nucleation, growth, and motion behavior of gas pore, and hydrogen transport are simulated by a LB Shen-Chen scheme. It is found that the pores can grow fully but compete in the primary dendrite growth stage. Conversely, pores newly nucleated in residual inter-dendrite liquid during the eutectic solidification stage grow restrictedly and independently. Pore shape severely deformed by the compression of the surrounding solid structures. Moreover, the effects of the initial Si composition, initial hydrogen concentration and cooling rate on the pore formation are qualitatively and quantitatively studied. It is found that the initial Si composition significantly impacts on the pore size and distribution, but little effect on the porosity percentage which decreases markedly with the decreasing initial hydrogen concentration and increasing cooling rate. Comparison reveals that the simulated microstructure morphological characteristics and the predicted porosity percentage agree well with the experimental observation reported in the literature.

A comparative investigation of the explosion mechanism of metal hydride AlH3 dust and Al/H2 mixture

Aluminum hydride (AlH3) is an excellent hydrogen storage material, whereas hydrogen is easily released because of the low-temperature dehydrogenation characteristic. Therefore, the explosion mechanism of AlH3 dust is explored by comparison with the Al/H2 mixture. The findings indicate that since the residual aluminum after hydrogen evolution combusts more completely, AlH3 has a higher explosion pressure value than Al/H2 mixture. However, the explosion pressure rise rate and average flame propagation velocity of AlH3 dust are lower than those of Al/H2 mixture. It is attributed to the initial free hydrogen state and initial higher hydrogen concentration of Al/H2 mixture. The key radicals that generate main explosion residues Al2O3 are AlO and O. AlH3(+M) ≤> AlH + H2(+M) has the greatest sensitivity on the formation of O and AlO. Hydrogen combustion expedites the phase transition process of aluminum dust, while their competition for oxygen is not conducive to the formation of Al2O3.

Bioactivity of crude extract produced by endophytic fungi isolated from Ziziphus spina-christi (Nabq) leaves for antimicrobial evaluation as well as optimization of culture medium conditions

Twenty fungal endophytes were isolated from Nabq, and plug diffusion screening revealed four of those isolates to be promising. These isolates were identified molecularly as Aspergillus sp. of Sect. Flavi (ZSEFL2) and Sec. Niger (ZSEFL14), and Penicillium spp. closest related to Penicillum crustosum (ZSEFL15, 19). Investigations were conducted to determine the effects of culture parameters, such as the type of culture medium, incubation temperature, initial pH and agitation, on the production of antimicrobial metabolites in a culture filtrate extract. Using the disk diffusion technique, antimicrobial metabolite synthesis was evaluated. In comparison to the other three distinct culture media (PDB-y, rice, and Cz), PDB demonstrated to be the best culture medium for the four tested fungal endophytes, exhibiting high levels of the Antimicrobial Index (AI). Also, a restricted range of incubation temperatures resulted in the formation of bioactive metabolites, but raising the incubation temperatures from 25 to 30 °C increased the amount of bioactive metabolites produced. Moreover, the best initial hydrogen concentration typically ranged from pH 5 to 6. Finally, when the conditions changed from stationary to shaking, the bioactive secondary metabolite's antimicrobial efficacy reduced. The behavior of the four examined fungal isolates was consistent, and they all required the same conditions to produce the most bioactive metabolites. Moreover, ethyl acetate extract's MIC values against the investigated bacterial and fungal human pathogens ranged from 0.19 to 6.25 mg/mL.

Adapted Galvanostatic Charge Method for cell-individual in-situ characterization of PEM fuel cell stacks and systems

Increasingly challenging lifetime requirements for automotive Polymer-Electrolyte-Membrane (PEM) fuel cells call for onboard characterization methods to enable state-of-health (SOH) adaptive operation strategies. In this work, an adapted Galvanostatic Charge Method (GCM) is presented, that allows for cell-individual electrochemical characterization without nitrogen flush, making it system- and potentially onboard-compatible. Key innovation is to utilize the system’s standard shutdown procedure, i.e. oxygen depletion with closed air valves, to create an inert atmosphere in the cathode volume in which the characterization takes place. The electrochemical processes triggered during the characterization with GCM are now embedded in a closed-volume-problem instead of a steady-flow-environment. This causes the hydrogen concentration difference between anode and cathode to be dynamic instead of constant during the characterization. In this work, a 0D-simulation model of the closed-volume-problem and complementary measurements on a system testbench are presented. Additionally, a novel model-based evaluation method is introduced which extracts the SOH-indicators electrochemical active surface area (ECSA), double layer capacity, hydrogen permeability of the membrane and short circuit resistance from the measurements by fitting simulated to measured voltage curves. The simulation model is thoroughly analyzed and a systematic error, evoked by the fluidic coupling of the individual cells, is found to influence the results obtained for membrane permeability. The influence of different initial conditions (among them initial hydrogen concentration) on the evaluation results is experimentally investigated and found to have no significant influence. For further studies, improvement potential was found in modeling, measurement procedure and evaluation method.

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

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

Optimization of Pore Formation Process of Lotus Aluminum by Phase Field Simulation

Abstract We have used multi-phase field simulation to mimic the actual process of lotus aluminum production, i.e., unidirectional solidification of aluminum in a hydrogen atmosphere. We used PD control and PID control, conventional methods of feedback control, to control the pore width precisely in the simulation. The pore width was shown to decrease slowly in PID control due to the slow feedback of I-control, and PD control was shown to produce stable long pores, although the pore width became larger than the target pore width. The initial hydrogen concentration in the liquid, which was controlled by atmospheric hydrogen pressure, had a significant effect on the pore growth; the higher the hydrogen concentration, the thicker the pore width and vice versa. It was also clarified that the pore width and the elongation length of the pore took various values depending on the combination of PD parameters even if the same target value was set.

Hierarchical Bayesian modeling to quantify fracture limit uncertainty of high-burnup advanced fuel cladding tubes under loss-of-coolant accident conditions

For realizing a highly reliable fracture limit evaluation of fuel cladding tubes during loss-of-coolant accidents (LOCAs) in light-water reactors, we developed a method to quantify the fracture limit uncertainty of high-burnup advanced fuel cladding tubes. This method employs a hierarchical Bayesian model that can quantify uncertainty even with limited experimental data. The hierarchical Bayesian model was an extended version of the fracture probability estimation model developed in our previous study. The fracture limit uncertainty was quantified as a probability using the amount of oxidation (Equivalent cladding reacted: ECR) and the initial hydrogen concentration (the hydrogen concentration in the fuel cladding tubes before the LOCA-simulated tests) as explanatory variables. We divided the regression coefficients of this model into a hierarchical structure with an overall average term common to all types of fuel cladding tubes and a term representing differences among various types of fuel cladding tubes. This hierarchical structure enabled us to quantify the fracture limit uncertainty through the effective use of prior knowledge and data, even for high-burnup advanced fuel cladding tubes with a small number of data points. The fracture limits representing a 5% fracture probability with 95% confidence of the high-burnup advanced fuel cladding tubes evaluated by the hierarchical Bayesian model were higher than 15% ECR for the initial hydrogen concentrations of up to 700–900wtppm and restraint loads below 535N. These fracture limits were comparable to the limit of the unirradiated Zircaloy-4 cladding tube, indicating that the burnup extension and use of the advanced fuel cladding tubes do not significantly lower the fracture limit of fuel cladding tubes. Further, we proposed a method to reduce the fracture limit uncertainty by using non-binary data, instead of the binary data, depending on the condition of the fuel cladding tube specimens after performing the LOCA-simulated test, thereby increasing the amount of information in the data.

Unsteady 2D-mathematical model for predicting the dynamic behavior of a metal hydride water pump

In this paper, an unsteady mathematical model for evaluating the performance of a metal hydride water pump was developed, along with a computation code that was successfully validated against experimental data. A variety of pump operating conditions were simulated numerically using this code. Results show that increasing the heating fluid temperature from 313 K to 363 K, the heat transfer coefficient from 10 Wm−2K−1 to 500 Wm−2K−1, and the initial concentration of hydrogen from 0.9 to 0.98 can reduce pumping time by 93 %, 75 %, and 18 %, respectively. Additionally, it has been found that there is an optimal pumping head that provides the highest pumping efficiency for a particular heating fluid temperature. There is also an optimal initial hydrogen concentration and pumping head point that results in the best performance of the pump.

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

Visualization of the external flow field during a vented explosion for hydrogen-air mixtures: Effects of hydrogen concentrations and vent areas

To study the effects of hydrogen concentrations and vent areas on the vented hydrogen-air explosion, series of experiments were performed in a cylindrical tube under the conditions of 1 atm and 10 ℃. The pressure oscillation frequency is proportional to the hydrogen concentration, while the Helmholtz oscillation disappeared gradually with the increase of Kv. The position inside the container which was farther away from the vent cover would experience the higher dynamic loading. The volume concentration of the hydrogen at the external field (cExt) was always lower than the initial hydrogen concentration, that increased with the increase of the initial concentration, while it was zero at some concentrations with the increase of Kv. The external flame velocity of rich fuel was always larger than that of lean fuel. The deflagration to detonation (DDT) in the outer field was easier to achieve for the concentration near to 50 vol% and for lower Kv. The secondary explosion wave (PExt) increased firstly and then decreased with the increase of hydrogen concentration, which decreased with the increase of Kv in general. The maximum surface temperature rose with the increase of concentration and dropped with the increase of Kv in general. The high-temperature field was closer to the vent for lower concentration and higher Kv.

Modeling of Hydrogen Diffusion in Inhomogeneous Steel Welded Joints.

Hydrogen is a main factor in cold cracking or hydrogen-induced cracking. The most crack susceptible region of a steel welded joint is the heat affected zone (HAZ). The formulation and functional-analytical solution of the one-dimensional problem of hydrogen diffusion in an inhomogeneous butt-welded joint considering weld and joint dimensions and initial hydrogen distribution as well as hydrogen diffusion coefficients and solubilities are presented. It is shown that the peak hydrogen concentration in the HAZ of inhomogeneous joints varies in direct proportion to the initial hydrogen concentration in the weld metal. It is inversely proportional to the ratio of hydrogen solubilities in the weld metal and the HAZ metal and is nonlinear in the diffusion coefficient ratio of these metals. The peak hydrogen concentration in the HAZ can exceed 170% of the average initial concentration in the joint if martensitic steel is welded using low-carbon low-alloy welding consumables. The utilization of austenitic consumables leads to a dramatic reduction in the hydrogen concentration in the HAZ in comparison with the non-austenitic consumables. No direct relationship was found between the hydrogen concentration in the HAZ and the hydrogen evolution from the joint surface.

Numerical simulation of dendritic growth and porosity evolution in solidification of Al-Cu alloy with lattice Boltzmann – Cellular automata method

A multi-phase lattice Boltzmann method-cellular automata (LBM-CA) model is developed for the microstructural morphologies of dendritic growth and porosity evolution in the solidified Al-Cu alloy. The proposed model can simulate the dendritic growth and porosity evolution during solidification taking account for the solidification conditions such as cooling rates and initial hydrogen concentration. LB and CA were adopted to simulate the diffusion of alloy solute and hydrogen in liquid melt and phase transition during solidification, respectively. Microstructural morphologies including the formation of gas porosity and the growth of solid phase were visualized with different solidification conditions. The simulation results were validated by comparing with experiment data reported in the literature in terms of the percentage of porosity and morphology tendency with respect to the solidification conditions. The pore size decreased when applying the higher cooling rate due to the insufficient growth time. Apparently, the higher initial hydrogen concentration led to the higher percentage of the porosity. The proposed model can be utilized to optimize the solidification conditions for reducing porosity defect in solidified material including 3D printing and welding process as well as casting process.

A Physically Based Model Predicting the Degradation of Hydrogen on Crack Growth Critical Stress Intensity Factor of Metals

A simple, physically based model is developed to quantitatively predict the degradation of hydrogen on the crack growth critical stress intensity factor (CSIF) of metals. The model is formulated by combining a microscopically shielded Griffith criterion (MSGC) model for plasticity-induced cleavage fracture and thermodynamics decohesion (TDD) theory for hydrogen-enhanced interface decohesion. The hydrogen-influenced CSIF is described as a function of the intrinsic CSIF (hydrogen-free), initial hydrogen concentration (solubility), hydrogen trap binding energy and crack tip stress. All parameters in the model can be determined with a physical basis and the model is successfully validated by comparison with published experimental data.

Analysis of Hydrogen-Assisted Brittle Fracture Using Phase-Field Damage Modelling Considering Hydrogen Enhanced Decohesion Mechanism

This study proposes a hydrogen-assisted fracture analysis methodology considering associated deformation and hydrogen transport inside a phase-field-based formulation. First, the hydrogen transport around a crack tip is calculated, and then the effect of hydrogen enhanced decohesion (HEDE) is modeled by defining the critical energy release rate as a function of hydrogen concentration. The proposed method is based on a coupled hydrogen mechanical damage under phase-field and implemented through a user subroutine in ABAQUS software. The test using compact tension (CT) sample is investigated numerically to study the hydrogen embrittlement on 45CrNiMoVA steel. Experimentally, the microstructural fracture presents a mixed brittle fracture mode, consisting of quasi-cleavage (QC) and intergranular (IG) fracture with hydrogen. This fracture mode is consistent with the suggested HEDE mechanism in the model. The simulation results show that hydrogen accumulates at the crack tip where positive hydrostatic stress is located. Moreover, the model estimates the initial hydrogen concentration through iterations. The simulated load-line displacement curves show good agreement with the experimental plots, demonstrating the predictive capabilities of the presented scheme.

Effects of aluminum particle size distributions on the explosion behaviors of hydrogen/aluminum dust hybrid mixtures

The effects of aluminum particle size on the explosion behaviors of hydrogen/aluminum dust hybrid explosion were studied. The results show that due to the large specific surface area of small particles, the Pex and the (dP/dt)ex increased significantly with the decrease of aluminum particle size. The maximum value of the (dP/dt)ex can reach 239 MPa/s. When the particle size of aluminum dust decreased from 56.18 μm to 5.07 μm, the flame propagation displacement changed from double peaks to a single one. The actual burn time can be fitted as a function of initial hydrogen concentration. The maximum flame propagation velocities increased significantly with the decrease of aluminum particle size. When 56.18 μm and 30.82 μm aluminum dusts were burned in hydrogen/air mixtures, the expanded porous oxide layers appeared in the residues, while there was no porous oxide layer in the explosion residues of 5.07 μm aluminum dust.

End-gas autoignition and knocking combustion of ammonia/hydrogen/air mixtures in a confined reactor

End-gas autoignition and detonation development in ammonia/hydrogen/air mixtures in a confined reactor is studied through detailed numerical simulations, to understand the knocking characteristics under IC engine relevant conditions. One-dimensional planar confined chamber filled with ammonia/hydrogen/air mixtures is considered. Various initial end-gas temperature and hydrogen concentration in the binary fuels are considered. Homogeneous ignition of stochiometric ammonia/hydrogen/air mixtures is firstly calculated. It is found that H 2 addition significantly promotes autoignition, even if the amount of addition is small. For ammonia/air mixtures and ammonia/hydrogen/air mixtures with low hydrogen mole ratios, it is found from chemical explosive mode analysis results that NH 2 and H 2 NO are most important nitrogen-containing species, and R49 (NH 2 +NO<=>N 2 +H 2 O) is a crucial reaction during thermal runaway process. When the hydrogen mole ratio is high, the nitrogen-containing species and reactions on chemical explosive mode becomes less important. Moreover, a series of one-dimensional simulations are carried out. Three end-gas autoignition and combustion modes are observed, which includes forcibly ignited flame propagation, autoignition (no detonation), and developing detonation. These modes are identified within wide ranges of hydrogen contents and initial end-gas temperatures. Furthermore, chemical kinetics at the reaction front and autoignition initiation locations are also studied with chemical explosive mode analysis. Finally, different thermochemical conditions on knocking intensity and timing are investigated. It is found that a higher initial temperature or a higher H 2 content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas. • Detailed simulations on end-gas knocking combustion are performed. • Ammonia and hydrogen binary fuel blends with different ratios are considered. • Different end-gas combustion modes are observed from parametric studies. • Evolutions of thermochemical states in end-gas combustion are discussed. • Knocking intensity and timing in ammonia/hydrogen blending fuel are studied.

Numerical simulation of hydrogen distribution around a crack tip in a high-Mn steel

Hydrogen transport ahead of a crack tip is critical to understand the hydrogen-assisted cracking in steels. With this objective, a micromechanics-based model (Taylor relation) has been applied for finite element simulations of hydrogen transport in high-Mn steel. A sensitivity analysis regarding the influence of boundary condition, crack-tip radius, and initial hydrogen concentration on the hydrogen distribution is performed.The results indicate that stress-dependent boundary condition realistically presents the hydrogen transport, which is influenced by hydrostatic stress gradient and trap multiplication during plastic deformation. With strong plastic strain (small crack-tip radius) or deep traps (deformation twin), the total hydrogen peak moves towards the crack tip due to the dominant role of trapped hydrogen. However, the effect of trapped hydrogen will decrease with an increase of initial hydrogen concentration in the specimen. The simulations show useful implications for hydrogen embrittlement in high-Mn steels.

Hydrogen-Assisted Brittle Fracture Behavior of Low Alloy 30CrMo Steel Based on the Combination of Experimental and Numerical Analyses.

Compact-tension (CT) specimens made of low alloy 30CrMo steels were hydrogen-charged, and then subjected to the fracture toughness test. The experimental results revealed that the higher crack propagation and the lower crack growth resistance (CTOD-R curve) are significantly noticeable with increasing hydrogen embrittlement (HE) indexes. Moreover, the transition in the microstructural fracture mechanism from ductile (microvoid coalescence (MVC)) without hydrogen to a mixed quasi-cleavage (QC) fracture and QC + intergranular (IG) fracture with hydrogen was observed. The hydrogen-enhanced decohesion (HEDE) mechanism was characterized as the dominant HE mechanism. According to the experimental testing, the coupled problem of stress field and hydrogen diffusion field with cohesive zone stress analysis was employed to simulate hydrogen-assisted brittle fracture behavior by using ABAQUS software. The trapezoidal traction-separation law (TSL) was adopted, and the initial TSL parameters from the best fit to the load-displacement and J-integral experimental curves without hydrogen were calibrated for the critical separation of 0.0393 mm and the cohesive strength of 2100 MPa. The HEDE was implemented through hydrogen influence in the TSL, and to estimate the initial hydrogen concentration based on matching numerical and experimental load-line displacement curves with hydrogen. The simulation results show that the general trend of the computational CTOD-R curves corresponding to initial hydrogen concentration is almost the same as that obtained from the experimental data but in full agreement, the computational CTOD values being slightly higher. Comparative analysis of numerical and experimental results shows that the coupled model can provide design and prediction to calculate hydrogen-assisted fracture behavior prior to extensive laboratory testing, provided that the material properties and properly calibrated TSL parameters are known.

Analysis of Characteristics of Spent Fuels on Long-Term Dry Storage Condition

Currently, the interim storage pools of spent fuels in South Korea are expected to become saturated from 2024. It is required to prepare an operation plan of a domestic dry storage facility during a long-term period, with the researches on safety evaluation methods. This study modified the FRAPCON code to predict the spent fuel integrity evaluation such as the axial cladding temperature, the hoop stress and hydrogen distribution in dry storage. The cladding temperature in dry storage was calculated using the COBRA-SFS code with the burnup information which was calculated using the FRAPCON code. The hoop stress was calculated using the ideal gas equation with spent fuel information such as rod internal pressure. Numerical analysis method was used to calculate the degree of hydrogen diffusion according to the hydrogen concentration and temperature distribution during a dry storage period. Before 50 years of dry storage, the cladding temperature and hoop stress decreased rapidly. However, after 50 years, they decreased gradually and the cladding temperature was below 400 K. The initial temperature distribution and hydrogen concentration showed a parabolic line, but hydrogen was transferred by the hydrogen concentration and temperature gradient over time.