Boundary layer theory approach to the concentration layer adjacent to a ceiling wall of a hydrogen leakage: Far region

The field of the hydrogen leakage in partially open space can be divided into two main regions according to the importance of the hydrogen concentration distribution and the flow behavior. These two regions are the jet region and the boundary layer region which are adjacent to the ceiling wall of the space, resulting from impinging the hydrogen jet to the wall. The boundary layer region in turn can be divided into two regions, according to the modeling of the flow. These regions are the stagnation-point boundary layer region and the far boundary layer region. Previously, we studied the region of stagnation-point flow (Hiemenz flow) [El-Amin MF, Kanayama H. Boundary layer theory approach to the concentration layer adjacent to a ceiling wall at impinging region of a hydrogen leakage. Int J Hydrogen Energy, in press.]. The current paper is devoted to analyze the far region of the boundary layer adjacent to the ceiling wall using the boundary layer theory. Also, an experiment has been conducted on the hydrogen leakage in partially open space to estimate the concentration distribution vertically at the center of the domain under the ceiling wall. In order to verify the boundary layer theory approach, a comparison between the measurements and the boundary layer theory approximations is investigated and the results showed a good agreement. The wall shear stress, the local friction factor, the friction drag and the non-dimensional drag coefficient of the ceiling wall are calculated. Also, both momentum and concentration boundary layer thicknesses are estimated.     

Modeling of hydrogen dispersion from hydrogen fuel cell vehicles in an underground parking garage

Hydrogen is a promising energy carrier that will become competitive in the near future. The present study modeled hydrogen leaks and diffusion in an actual size underground parking garage with the numerical model validated by scale experimental data. The results show that the hydrogen concentration distributions are not uniform in the gas mixture layer along the ceiling and the initial front velocity of the gas mixture layer decays with horizontal distance from the leaking car. The vertical filling front velocity for times after 600 s remain constant in the near field but increases linearly with distance in the far field. The corner walls did not significantly affect the far-field concentration distributions and the ventilation layout with vents in the garage corners provided better hydrogen removal. These results can be used to predict the hydrogen concentration buildup in large confined spaces and to help design underground parking garage ventilation systems.     

Spontaneous ignition of high-pressure hydrogen and boundary layer characteristics in tubes

Hydrogen is efficient and environmentally friendly, but the danger of self-ignition resulted from the leakage of high-pressure hydrogen cannot be ignored. In this work, the self-ignition of high-pressure hydrogen released through different conditions was studied. 700-mm-length tubes with different diameters were adopted in our experiments. It summarized the characteristics of shock waves' attenuation and evolution process of hydrogen jets in tubes. In addition, effects of the boundary layer on the leading shock waves, the contact surface, and expansion waves were discussed. Results indicate that minimum pressure when self-ignition occurs for 15-mm-diameter tube is similar to 10-mm-diameter. And they have closely velocity of shock waves. Simulations show that the greater the release pressure, the more ignition products of hydrogen. Higher release pressure and smaller diameter can create a thicker boundary layer in micro shock tubes, and the boundary layer can lead to a change in the velocity of shock waves’ structures.     

Integral solutions for selected turbulent quantities of small-scale hydrogen leakage: A non-buoyant jet or momentum-dominated buoyant jet regime

In this paper, the integral method is used to derive a complete set of results and expressions for selected physical turbulent properties of a non-buoyant jet or momentum-dominated buoyant jet regime of small-scale hydrogen leakage. Several quantities of interest, including the cross-stream velocity, Reynolds stress, velocity-concentration correlation (radial flux), dominant turbulent kinetic energy production term, turbulent eddy viscosity and turbulent eddy diffusivity are obtained. In addition, the turbulent Schmidt number is estimated and the normalized jet-feed material density and the normalized momentum flux density are correlated. Throughout this paper, experimental results from Schefer et al. [Schefer RW, Houf WG, Williams TC. Investigation of small-scale unintended releases of hydrogen: momentum-dominated regime. Int J Hydrogen Energy 2008;33(21):6373–84] and other works for the momentum-dominated jet resulting from small-scale hydrogen leakage are used in the integral method. For a non-buoyant jet or momentum-dominated regime of a buoyant jet, both the centerline velocity and centerline concentration are proportional with z⁻¹. The effects of buoyancy-generated momentum are assumed to be small, and the Reynolds number is sufficient for fully developed turbulent flow. The hydrogen–air momentum-dominated regime or non-buoyant jet is compared with the air–air jet as an example of non-buoyant jets. Good agreement was found between the current results and experimental results from the literature. In addition, the turbulent Schmidt number was shown to depend solely on the ratio of the momentum spread rate to the material spread rate.     

Modeling the diffusion of flammable hydrogen cloud under different liquid hydrogen leakage conditions in a hydrogen refueling station

Constructing hydrogen refueling stations will be popular for hydrogen energy use in the future, and investigating the diffusion characteristics of hydrogen in a leakage incident is quite significant. The instantaneous evolution of flammable hydrogen clouds arising from liquid hydrogen leakage in a hydrogen refueling station is predicted using Ansys Fluent, and parametric analyses are conducted to reveal the effects of storage pressure, source height, and leakage direction on the distributions of the flammable regions. In addition, the feasibilities of heating the ceiling or the ground of the station after the leakage of liquid hydrogen to accelerate the hydrogen dilution are examined. The results show that the flammable region is stabilized at 90 s, the corresponding flammable hydrogen cloud volume is about 333 m³, and the extensions of downwind and vertical directions reach 10 m and 9.3 m. Storage pressure has a finite effect on the downwind diffusion distance of the flammable cloud. A lower source height tends to format the high-concentration hydrogen cloud near the ground while a higher source height helps separate the flammable clouds from the ground. The upward leakage direction leads to the maximum downwind diffusion distance of about 10.2 m while the downward leakage direction makes the high hydrogen concentration region confined below the ceiling. Just maintaining the ceiling at the initial temperature of 300 K is effective for accelerating the hydrogen dilution in the upward leakage. The maximum hydrogen concentration and the flammable volume can be reduced at rates of 0.35 vol % and 8% for every 50 K increase in heating temperature. For the downward leakage, keeping the ground at the initial temperature just works for the first 40 s in reducing the maximum hydrogen concentration, while increasing the heating temperature receives a gradually declined effect on reducing the flammable volume.     

Planar momentum-dominated regime of small-scale hydrogen leakage

The geometry of the source of hydrogen leakage is essential in forming hydrogen flow and distribution in air. For example, if the geometry of the source is circular, the behavior of leakage flow becomes axisymmetric and a radial jet is constructed. On the other hand, if the geometry of the source is planar, the behavior of hydrogen leakage becomes planar and a planar jet is constructed. Throughout this article, the problem of momentum-dominated regime of a planar slow-leak hydrogen–air jet resulting from a hydrogen leakage from a planar source is considered. We derive a set of analytical expressions for selected physical turbulent properties. Several quantities of interest are obtained, including the cross-stream velocity, the Reynolds stresses, the velocity-concentration correlation, the dominant turbulent kinetic energy production term, the turbulent eddy viscosity, the turbulent eddy diffusivity, and the turbulent Schmidt number. Moreover, the normalized jet-feed material density and the normalized momentum flux density are correlated.     

Dual solutions in boundary layer stagnation-point flow and mass transfer with chemical reaction past a stretching/shrinking sheet

In this paper, an analysis is presented to study dual nature of solution of mass transfer with first order chemical reaction in boundary layer stagnation-point flow over a stretching/shrinking sheet. The governing equations are transformed into a set of self-similar ordinary differential equations by similarity transformations. The transformed equations are solved numerically using very efficient shooting method. The study reveals that the dual solutions of velocity and concentration exist for certain values of velocity ratio parameter (the ratio of stretching/shrinking rate and straining rate). The concentration boundary layer thickness decreases with increasing values of Schmidt number and reaction-rate parameter for both solutions.     

Data-driven diagnosis of the high-pressure hydrogen leakage in fuel cell vehicles based on relevance vector machine

The hydrogen pressure inside tanks and its adjacent pipes can reach up to 70 MPa in fuel cell vehicles. This is the weak links of hydrogen leakage. The diagnosis time of mainstream hydrogen leakage diagnosis method based on hydrogen concentration sensors (HCSs) is easily affected by the number and location of installed sensors. In this study, a data-driven diagnosis method is proposed for the high-pressure hydrogen leakage. Fisher discrimination analysis and linear least squares fitting are used for data preprocessing, relevance vector machine is used for pattern recognition. When the total volume of tanks is 82 L and the hydrogen leakage flow rate is larger than 2 g/s, the diagnosis accuracy of the proposed method is higher than 95% and the diagnosis time is constant. When the leakage location is far away from HCSs, the proposed method can the diagnose hydrogen leakage in a shorter time than mainstream method.     

Response to “Commentary by Dr. Henryk Drulis”

One safety aspect of a material-based hydrogen storage system is the exposure of such system to a high-temperature environment (e.g., a fire) causing an increase in pressure. A simple analysis, based on a material balance, is provided to estimate the effect of temperature on equilibrium storage pressure in a material-based hydrogen storage system. Jiann C. Yang in Int J Hydrogen Energy 33 (2008) 4424–4426.     

Production of hydrogen from purge gases of ammonia plants in a catalytic hydrogen-permselective membrane reactor

The present study investigates hydrogen production in a hydrogen-permselective membrane reactor from purge gases of an ammonia plant. Hydrogen which initially exists in the purge gases and hydrogen that is produced from decomposition of ammonia on nickel–Alumina catalyst bed simultaneously permeate from reaction side to shell side through a thin layer of palladium–silver membrane. A sweep gas can be used in the shell side for increasing driving force. The amount of hydrogen that can be gained annually and effect of pressure, temperature, thickness of Pd–Ag layer, configuration of flow in the membrane reactor and sweep gas flow ratio have been studied. This study shows that the countercurrent mode is better than co-current mode of operation. The rate of hydrogen permeation increases with increasing of temperature, pressure and sweep gas flow rate. This approach produces and separates large amounts of hydrogen and decreases environmental impacts owing to ammonia emission.     

Improvement in high temperature stability of Pd coating on Nb by Nb2C intermediate layer

Niobium subcarbide (Nb₂C) was chosen as a material for non-porous intermediate layer to improve the high temperature durability of Pd–Nb composite membranes for hydrogen separation. A layer of Nb₂C was prepared between Nb substrate and thin Pd films (100 nm), and the stability of Pd coating at elevated temperatures (573–773 K) was examined by hydrogen absorption experiments. Hydrogen permeability through the Nb₂C layer appeared to be sufficiently high, and no noticeable deterioration was observed in hydrogen absorption rate under as-prepared conditions. The degradation in coating effect of Pd at elevated temperatures was substantially mitigated by Nb₂C layer. Such improved durability was ascribed to retardation of open porosity development by Nb₂C caused as a consequence of impeded interdiffusion between Pd and Nb.     

Effect of hydrogen on superplastic deformation of (TiB + TiC)/Ti–6Al–4V composite

Ti–6Al–4V matrix composite reinforced with TiB plus TiC was prepared and hydrogenated. The phases were identified by X-ray diffraction (XRD). Microstructures were examined by optical microscopy (OM). Dependence of transus temperatures of the composite on hydrogen concentration was determined. The result shows hydrogen decreases transus temperatures of the composite significantly and increases the amount of β phase. Superplastic deformation of the hydrogenated composites was performed. Hydrogen decreases the optimum superplastic temperatures and increases the optimum superplastic strain rate. The flow stresses of hydrogenated composites decrease greatly compared to unhydrogenated composite. The strain rate sensitivity index m of the composite increases with increasing hydrogen concentration and reaches maximum 0.327 at 0.85 wt.% H. Meanwhile, the activation energy Q decreases with increasing hydrogen concentration and ranges in 488–382 kJ/mol. Hydrogen promotes recrystallization of the composite and refines the microstructure during superplastic deformation.     

Numerical investigation on the dispersion of hydrogen vapor cloud with atmospheric inversion layer

Characterization of the dispersion behaviors of spilled liquid hydrogen is necessary from the safety prospective. In this study, the two-phase flow of liquid hydrogen spill is predicted with the mixture multiphase model, Lee model and Realizable k-ε model, and the dispersion of hydrogen vapor cloud with the atmospheric inversion layer is numerically analyzed. The inversion layer restrains the upward movement of the cloud and the mixing of the cloud with air, increasing its ground-level hydrogen concentration. The heights of the flammable cloud can be reduced by 5.71%, 10.49% and 12.86%, respectively, with the temperature lapse rates of 0.03, 0.06 and 0.10 K/m in our investigated scenarios. Besides, the restraining effect is strengthened by increasing the ground air temperature if the temperature lapse rate remains unchanged.     

Sensing-based risk mitigation control of hydrogen dispersion and accumulation in a partially open space with low-height openings by forced ventilation

This paper presents the real-time sensing-based risk-mitigation control of hydrogen dispersion and accumulation in a partially open space with low-height openings by forced ventilation. In the partially open space we previously considered (Matsuura et al., Int J Hydrogen Energy, 35(10), p. 4776–4786 (2010)), a hydrogen buoyant plume is subjected to cross flows during forced ventilation, and hydrogen travels over a long distance in the lower part of the space, which enhances the hydrogen concentration there. On the basis of those results, we alternatively propose in this paper a new partially open space that permits the almost vertical rising of hydrogen from a leak source, and exhausts, by forced ventilation, hydrogen temporarily accumulated near the roof. We first describe computational geometries and scenarios, mathematical models and numerical procedures.     

Effects of suction/blowing on steady boundary layer stagnation-point flow and heat transfer towards a shrinking sheet with thermal radiation

In this paper, the effects of suction/blowing and thermal radiation on steady boundary layer stagnation-point flow and heat transfer over a porous shrinking sheet are investigated. The existence of dual solutions, unique solution and non-existence of solution for self-similar equations of the flow and heat transfer are analyzed numerically. It is noted that the range of velocity ratio parameter where the solution exists increases/decreases with increasing suction/blowing. With increasing suction, temperature at the wall is found to increase (decrease) for the first (second) solution. Due to increasing Prandtl number and thermal radiation parameter the thermal boundary layer thickness becomes thinner.     

Characteristics of SOFC single cells with anode active layer via tape casting and co-firing

SOFC (solid oxide fuel cell) single cells with anode active layers of various thicknesses were fabricated successfully via tape casting and co-firing in order to improve their electrochemical performance and long-term stability. The mercury porosimeter and the gas permeability were measured so as to examine the effects of the anode active layer while under a gaseous flow. It was found that the anode active layers affected the microstructural characteristics as a result of the pore distribution and the gas permeation behavior. The anode active layers improved the cell performance by increasing the number of active sites in the anode. The thickness of the anode active layer was optimized at 20 μm in this work through a combination of the power density, the ohmic ASR (area specific resistance), and the cell ASR. SOFCs with the optimized active layer showed good electrochemical performance at 600–700 °C in hydrogen or hydrocarbon fuel (methane) and excellent long-term stability for 500 h.     

Ignition propensity of hydrogen/air mixtures impinging on a platinum stagnation surface

An experimental investigation into the ignition characteristics of lean pre-mixed hydrogen/air mixtures is conducted using a stagnation-point flow configuration against a platinum surface, with a special emphasis on the determination of potential fire safety hazards associated with hydrogen release in the presence of a catalyst. Two distinct regimes are observed for this system – catalytic surface reactions and gas-phase ignition. It is demonstrated that depending on mixture equivalence ratio, catalytic surface reactions can be initiated with or without surface heating. When significant surface heat is released via catalytic reactions, gas-phase ignition can be induced, greatly increasing the apparent danger of hydrogen leaks in the presence of a platinum surface. The critical surface temperatures leading to catalytic ignition for hydrogen/air mixtures over a platinum surface are further investigated over a range of equivalence ratios and stretch rates. It is shown that ultra-lean hydrogen/air mixtures can be catalytically ignited even in the absence of external heat addition, suggesting that hydrogen leakage in the presence of a platinum surface may pose a fire safety risk even at room temperature. Furthermore, even without a transition to gas-phase ignition, the surface temperature that can be sustained with surface reactions alone may contribute to component degradation or itself pose a safety hazard.     

Hydrogen production by methanol–water solution electrolysis

The production of hydrogen by methanol–water solution electrolysis was investigated. Hydrogen and carbon dioxide were contained in the cathode exhaust gas and the hydrogen concentration was 95.5–97.2 mol%. The hydrogen flow rate in the cathode exhaust gas increased in proportion to the current density and almost agreed with the theoretical hydrogen-production rate. The voltage and electrical energy needed to produce hydrogen were less than that for water electrolysis. The electrical energy needed in methanol–water solution electrolysis was less than 60% of that required in water electrolysis. Permeation of methanol, water and carbon dioxide from the anode to the cathode of the electrolytic cell occurred with hydrogen production. The permeation rate of methanol became greater than that of water as the current density increased. When the current density was constant, the permeation rate of water did not depend on the methanol concentration in the methanol–water solution supplied to the anode, and that of methanol increased while that of carbon dioxide decreased as the methanol concentration increased.     

Modeling and analysis of hydrogen diffusion in an enclosed fuel cell vehicle with obstacles

Hydrogen has been utilized in FCV and leakage can cause safety issues. In this study, the vehicle is simplified as a cuboid enclosure with obstacles. The influence of obstacle locations on the hydrogen diffusion behavior is investigated with the iso-surfaces of 1% and 4% volume fraction of hydrogen. The time of the iso-surface of 4% volume fraction to reach the ceiling and the sidewalls without any obstacles is 1.42 and 1.25 times of that with an obstacle, respectively. Both height and the width of the flammable zone in the enclosure with an obstacle are greater than that without obstacles. The distance between the obstacle and the leakage affects significantly the hydrogen diffusion and the influence of the obstacle on the hydrogen diffusion strengthens with the decrease of the distance. Yet as the distance keeps same, the obstacle position relative to the leakage has no significant effect on hydrogen diffusion, and it makes little effect difference whether the obstacle is located in front of leakage or side of leakage.     

Acceleration of hydrogen forced ventilation after leakage ceases in a partially open space

This paper treats the real-time sensing-based risk-mitigation control of hydrogen dispersion and accumulation in a partially open space with low-height openings by forced ventilation. A hunting-preventive control scheme that we previously proposed (Matsuura et al., Int J Hydrogen Energy, 2012;37(2):1972–84) has parameters such as the monitoring period of hydrogen sensors T_p, a unit increment in the exhaust flow rate per area from a roof vent α, and a threshold ε for the change in the exhaust flow rate. Through parametric simulations of the hydrogen exhaust after leakage ceases, we clarify the effects of the parameters on the rate of exhaust flow from the roof vent and the amount of hydrogen accumulating near the roof, which are critical for ventilation performance. With a selected combination of (T_p, α, ε) for which the ventilation system has a quick response and reasonable original performance, we first introduce two acceleration methods separately to the original hunting-preventive scheme to improve the ventilation performance after hydrogen leakage ceases. Ventilation performance employing the two methods is compared with that employing the original scheme. From the results, a hybrid method is finally proposed. The effectiveness of the proposed method is computationally validated for leak flow rates of 9.44 × 10⁻⁴, 4.72 × 10⁻⁴ and 2.36 × 10⁻⁴ m³/s.