Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by Δμ_H(σ) = −1.124σ kJ/mol_H for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.     

Stability of LaNi5-xCox alloys cycled in hydrogen — Part 1 evolution in gaseous hydrogen storage performance

In this study, the evolution trends in various hydrogen storage properties as well as kinetics performance of LaNi_5−xCo_x (x = 0, 0.25, 0.50 and 0.75) alloys up to 1000 cycles are studied, and the effects of Co on the long-term hydrogen absorption/desorption properties are revealed. The alloys have single LaNi₅ phase structure. The cell volume increases with increasing Co content, resulting in a lower hydrogen absorption/desorption plateau pressure and a more stable hydride phase. The alloys have good kinetics performance and can fully absorb hydrogen within 300 s. The hydrogen absorption rate increases with either cycling or Co addition, but neither of them changes the rate limiting step which remains to be diffusion controlled model in the temperature range of 343–383 K. With cycling, an intermediate γ phase emerges with the coexistence of α and β phases indicated by an extra higher plateau in P-C-T curves. This γ phase acts as a buffer releasing the microstrains which is reflected from the slower capacity degradation, and the significant decrease in hysteresis and slope factor upon its appearance. Moreover, the c/a value increases after 1000 cycles which also relieves the microstrains in the alloys. Co addition not only enhances the buffer effect of γ phase by partially combining it with β phase, but also promotes the increasing degree of c/a value, contributing to a better crystal structure, bigger particle size and higher cycle stability during long-term cycling.     

Detonation wave propagation in semi-confined layers of hydrogen–air and hydrogen–oxygen mixtures

This paper presents results of an experimental investigation on detonation wave propagation in semi-confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen–air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H₂/air-mixtures, as well as experiments with uniform H₂/O₂ mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H₂/air-mixtures with concentration gradients are shown.     

Combustion of hydrogen–air in catalytic micro-combustors made of different material

Micro-combustors have low stability, thus catalyst is applied to improve it. In this experiment, the performances of catalytic micro-combustors made of different materials (quartz glass, alumina ceramic, copper) are compared. Asbestine threads are used as the catalyst supports of Pt, and installed in the combustors. According to the experimental results, the combustors have high stability, they keep working until the extreme equivalence ratio close to 0. The stability limits of homogeneous reaction in the quartz glass and alumina ceramic combustor range from 0.0907 to 8.69 and 0.158 to 7.31 on average, respectively. But the two combustors exhibit obvious hot spots, which are 1058 and 728 K at 0.2 L/min, respectively. Whereas the copper combustor has low and uniform temperature distribution on its surface. Moreover, the heat loss in the quartz glass combustor is 4.13 W higher than in the copper one at 0.2 L/min, which is opposite to the conventional situation that heat loss increases with the wall thermal conductivity. Computational fluid dynamic simulation reveals that the reaction modes inside the combustors differ. The higher wall thermal conductivity makes the heterogeneous reaction dominate, thus induces the temperature distribution and heat loss aforementioned.     

Production of hydrogen via hydrolysis of hydrides in Mg–La system

Hydrogen generation via hydrolysis of 250 mg hydrogenated Mg₃La and La₂Mg₁₇ in 100 ml water has been investigated at 298 K. Hydrolysis reactions of hydrogenated Mg₃La and La₂Mg₁₇ obtained by induction melting and then hydrogenated at 298 K is found to be fast when they are immersed in water. The hydrolysis reaction of hydrogenated Mg₃La almost completes within 21 min with faster kinetics and higher yield than those obtained of hydrogenated La₂Mg₁₇. The hydrogen production rate is 43.8 ml min⁻¹ g⁻¹ of hydrogen in the first 20 min of reaction compared to a conversion yield of 88% for hydrogenated Mg₃La and 40.1 ml min⁻¹ g⁻¹ of hydrogen in the first 20 min of reaction for hydrogenated La2Mg17. It is related to the catalytic effect of LaH₃ formed during the hydriding process, accentuating corrosion of MgH₂ greatly. The experimental curves of hydrogen generation kinetics at room temperature are well fitted by the Avrami–Erofeev equation. The reaction mechanism of hydrogenated Mg₃La and La₂Mg₁₇ was also discussed.     

Characterisation of laser ignition in hydrogen–air mixtures in a combustion bomb

Laser-induced spark ignition of lean hydrogen–air mixtures was experimentally investigated using nanosecond pulses generated by Q-switched Nd:YAG laser (wavelength 1064 nm) at initial pressure of 3 MPa and temperature 323 K in a constant volume combustion chamber. Laser ignition has several advantages over conventional ignition systems especially in internal combustion engines, hence it is necessary to characterise the combustion phenomena from start of plasma formation to end of combustion. In the present experimental investigation, the formation of laser plasma by spontaneous emission technique and subsequently developing flame kernel was measured. Initially, the plasma propagates towards the incoming laser. This backward moving plasma (towards the focusing lens) grows much faster than the forward moving plasma (along the direction of laser). A piezoelectric pressure transducer was used to measure the pressure rise in the combustion chamber. Hydrogen–air mixtures were also ignited using a spark plug under identical experimental conditions and results are compared with the laser ignition ones.     

Stochastic modeling of autoignition in turbulent non-homogeneous hydrogen–air mixtures

The autoignition of turbulent non-homogeneous hydrogen–air mixtures is studied using the linear-eddy model (LEM). The initial solution consists of fully segregated regions of fuel and oxidizer mixtures. The relative size of these regions represents a measure of mixture heterogeneity, while the specified turbulence conditions determine the subsequent evolution of the dissipation rate field. Chemistry and transport are described accurately using a detailed mechanism for hydrogen–air chemistry and the CHEMKIN libraries. The simulations are implemented for a range of pressures and initial mixing conditions to identify the effects of mixing on the dominant autoignition chemistry. The simulations show that some of the salient features of the coupling between autoignition chemistry and mixing may adequately be captured by the LEM. This coupling includes the competing roles of mixing on this chemistry. Mixing can increase the volumetric rate of reaction and heat release by increasing the interface between ignition fronts and unburnt gases; it also contributes to homogenizing the mixture.     

A mechanistic study of Soret diffusion in hydrogen–air flames

The separate and combined effects of Soret diffusion of the hydrogen molecule (H₂) and radical (H) on the structure and propagation speed of the freely-propagating planar premixed flames, and the strain-induced extinction response of premixed and nonpremixed counterflow flames, were computationally studied for hydrogen–air mixtures using a detailed reaction mechanism and transport properties. Results show that, except for the conservative freely-propagating planar flame, Soret diffusion of H₂ increases the fuel concentration entering the flame structure and as such modifies the mixture stoichiometry and flame temperature, which could lead to substantial increase (decrease) of the flame speed for the lean (rich) mixtures respectively. On the other hand, Soret diffusion of H actively modifies its concentration and distribution in the reaction zone, which in turn affects the individual reaction rates. In particular, the reaction rates of the symmetric, twin, counterflow premixed flames, especially at near-extinction states, can be increased for lean flames but decreased for rich flames, whose active reaction regions are respectively located at, and away from, the stagnation surface. However, such a difference is eliminated for the single counterflow flame stabilized by an opposing cold nitrogen stream, as the active reaction zone up to the state of extinction is always located away from the stagnation surface. Finally, the reaction rate is increased in general for diffusion flames because the bell-shaped temperature distribution localizes the H concentration to the reaction region which has the maximum temperature.     

Analysis of entropy generation in counter-flow premixed hydrogen–air combustion

In this paper, entropy generation in counter-flow premixed hydrogen–air combustion confined by planar opposing jets is investigated for the first time. The effects of the equivalence ratio and the inlet Reynolds number (corresponding to the global stretch rate) on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, three interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for reactive flows, are revealed. Moreover, it is observed that the whole investigated domain can be divided into two parts according to the predominant irreversibilities. The total entropy generation number can be approximated as a linear increasing function of the equivalence ratio and the inlet Reynolds number for all the cases under the present study.     

Nanoindentation of palladium–hydrogen

Nanoindentation has been utilised to track the mechanical effects of hydrogen on palladium foils over a range of hydrogen concentrations. The miscibility gap in the palladium–hydrogen system yields discrete phases over a range of compositions. It is shown that nanoindentation can measure the extent of hydrogen-induced phase transformations across the film thickness after hydrogen removal, with the α → β → α phase transformations yielding a ∼50% increase in local hardness. Interstitial hydrogen was observed to promote work hardening in β phase regions, and a ∼75% increase in hardness was observed in regions where the α phase was saturated with hydrogen.     

Optimization of flame stabilization methods in the premixed microcombustion of hydrogen–air mixture

The premixed combustion of a lean hydrogen–air mixture is analyzed in this study to examine various properties and flame stabilization. A two-dimensional (2D) analysis of a microscale combustor is performed with various shapes of bluff bodies (e.g., circular and triangular). Nine bluff bodies are placed at the entrance of the microscale combustor and solved with 2D governing equations. The analysis is performed with the three velocities of 10, 20, and 30 m/s, but the equivalence ratio is fixed in all cases. The various characteristics of the microscale combustor are studied such as the temperature of the wall, difference in peak temperature, the mean velocity at the outlet, and temperature of the exhaust gases. Flame stabilization depends on various factors such as bluff body shape and size, and the velocity of the fuel–air mixture at the inlet and recirculation zone. In comparison to all bluff body cases, we observe that the wall blade bluff body is the most efficient (low exhaust gas temperature, large recirculation zone, low mean velocity at the outlet of the microcombustor, and high wall temperature) compared with all eight other bluff body cases. Combustion efficiency is directly proportional to the wall temperature, meaning that the microcombustor with wall blade bluff bodies is more efficient with a stabilized flame. The simulation results are compared with published data on an L/D ratio of 15.     

Formation of flammable hydrogen–air clouds from hydrogen leakage

The possibilities of the formation of a flammable cloud over the ground in an open atmosphere from the leakage of hydrogen stored at different temperatures are studied. The dispersion of hydrogen in the stable and unstable atmospheric conditions is determined using the Gaussian dispersion model. The efflux of hydrogen from the storage vessel is considered at velocities between 1 m/s and 1500 m/s, the latter corresponding to the upper limit of velocities arising from the choked flow. The dispersion analysis shows that flammable hydrogen–air clouds would not be formed over the ground under unstable atmospheric conditions for all efflux velocities and leakage areas and for the different temperatures of the hydrogen leak. However, under strongly stable atmospheric conditions, such as those associated with clear sky winter nights with low winds and temperature inversion in the planetary boundary layer, a flammable cloud is seen to be formed. This is particularly true for low temperature hydrogen efflux and very low velocities of the efflux.     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 1: Effects of increase in congestion

Experimental studies were conducted with the objective of gaining a better understanding of the potential explosion hazard consequences that could be associated with a high-pressure leak from a hydrogen vehicle refuelling system. The first part of the study, described in this paper, was a series of experiments designed to establish hydrogen–air explosion overpressures in a well-defined and well understood 3 m × 3 m x 2 m (high) repeated pipe congestion. The results of the experiments are discussed in terms of the conditions leading to the greatest overpressures. It is concluded from the study that stoichiometric ratio in the range of 1.2–1.3 gives highest overpressure. Moreover, it was observed that increasing the congestion from 4-gate to 9-gate congestion leads to significant increase in the overpressure. In addition, it was concluded that, explosion in a hydrogen-air mixture is significantly more severe than the explosion in an ethane-air, methane-air or propane-air mixtures. This is attributed to higher laminar flame speed of hydrogen-air mixtures.     

Effects of hydrogen concentration on premixed laminar flames of hydrogen–methane–air

The unstretched laminar burning velocities and Markstein numbers of spherically propagating hydrogen–methane–air flames were studied at a mixture pressure of 0.10 MPa and a mixture temperature of 350 K. The fraction of hydrogen in the binary fuel was varied from 0 to 1.0 at equivalence ratios of 0.8, 1.0 and 1.2. The unstretched laminar burning velocity increased non-linearly with hydrogen fraction for all the equivalence ratios. The Markstein number varied non-monotonically at equivalence ratios of 0.8 and 1.0 and increased monotonically at equivalence ratio of 1.2 with increasing hydrogen fraction. Analytical evaluation of the Markstein number suggested that the trends could be due to the effective Lewis number, which varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0 and increased monotonically at 1.2. The propensity of flame instability varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0.     

Kinetics of hydrogen in Zr–H and Zr–D systems

We measured dependences of the electrical resistance on time of isothermal annealing for Zr rods saturated electrolytically by hydrogen or deuterium. The annealing of samples was carried out at temperatures 305–498 K. The resistance of inhomogeneously saturated samples increased with the time of annealing. The model of diffusion of the hydrogen from the surface of the sample into its volume described this increase adequately. The resistance of homogeneously saturated samples had a minimum at some time of annealing. We showed that the decrease of the resistance during annealing obeyed the exponential law, and that the characteristic time of the decrease obeyed the Arrhenius law with the activation energy about 0.16 eV. We supposed that the resistance decreases due to the formation of the hydride in the saturated layer or on the boundaries of grains.     

Effect of hydrogen on butanol–biodiesel blends in compression ignition engines

Research suggests that there is a dramatic reduction in CO and particulate matter (PM) emissions when butanol is blended with biodiesel derived from rapeseed oil (RME), but a small increase in THC emissions. The addition of hydrogen as a combustion enhancer can be used to counteract the increase in THC emissions seen with the butanol fuel blends and further reduce CO and PM emissions. The emission benefits with hydrogen addition were shown to be further improved for RME-butanol fuel blends. The penalty for using hydrogen is an increase in NO_x emissions due to the increase in NO₂ formation during combustion, but this is expected to have significant benefits in the function of aftertreatment systems. In this study, it is shown that the increase in engine-out NO_x emissions can be effectively controlled through exhaust gas recirculation (EGR) without an excessive PM penalty thanks to the low PM concentration in the EGR (with an impeding PM recirculation penalty).