HySafe standard benchmark Problem SBEP-V11: Predictions of hydrogen release and dispersion from a CGH2 bus in an underpass

One of the tasks of the HySafe Network of Excellence was the evaluation of available CFD tools and models for dispersion and combustion in selected hydrogen release scenarios identified as “standard benchmark problems” (SBEPs). This paper presents the results of the HySafe standard benchmark problem SBEP-V11. The situation considered is a high pressure hydrogen jet release from a compressed gaseous hydrogen (CGH2) bus in an underpass. The bus considered is equipped with 8 cylinders of 5 kg hydrogen each at 35 MPa storage pressure. The underpass is assumed to be of the common beam and slab type construction with I-beams spanning across the highway at 3 m centres (normal to the bus), plus cross bracing between the main beams, and light armatures parallel to the bus direction. The main goal of the present work was to evaluate the role of obstructions on the underside of the bridge deck on the dispersion patterns and assess the potential for hydrogen accumulation. Four HySafe partners participated in this benchmark, with 4 different CFD codes, ADREA-HF, CFX, FLACS and FLUENT. Four scenarios were examined in total. In the base case scenario 20 kg of hydrogen was released in the basic geometry. In Sensitivity Test 1 the release position was moved so that the hydrogen jet could hit directly the light armature on the roof of the underpass. In Sensitivity Test 2 the underside of the bridge deck was flat. In Sensitivity Test 3 the release was from one cylinder instead of four (5 kg instead of 20). The paper compares the results predicted by the four different computational approaches and attempts to identify the reasons for observed disagreements. The paper also concludes on the effects of the obstructions on the underside of the bridge deck.     

On the use of hydrogen in confined spaces: Results from the internal project InsHyde

The paper presents an overview of the main achievements of the internal project InsHyde of the HySafe NoE. The scope of InsHyde was to investigate realistic small-medium indoor hydrogen leaks and provide recommendations for the safe use/storage of indoor hydrogen systems. Additionally, InsHyde served to integrate proposals from HySafe work packages and existing external research projects towards a common effort. Following a state of the art review, InsHyde activities expanded into experimental and simulation work. Dispersion experiments were performed using hydrogen and helium at the INERIS gallery facility to evaluate short and long term dispersion patterns in garage like settings. A new facility (GARAGE) was built at CEA and dispersion experiments were performed there using helium to evaluate hydrogen dispersion under highly controlled conditions. In parallel, combustion experiments were performed by FZK to evaluate the maximum amount of hydrogen that could be safely ignited indoors. The combustion experiments were extended later on by KI at their test site, by considering the ignition of larger amounts of hydrogen in obstructed environments outdoors. An evaluation of the performance of commercial hydrogen detectors as well as inter-lab calibration work was jointly performed by JRC, INERIS and BAM. Simulation work was as intensive as the experimental work with participation from most of the partners. It included pre-test simulations, validation of the available CFD codes against previously performed experiments with significant CFD code inter-comparisons, as well as CFD application to investigate specific realistic scenarios. Additionally an evaluation of permeation issues was performed by VOLVO, CEA, NCSRD and UU, by combining theoretical, computational and experimental approaches with the results being presented to key automotive regulations and standards groups. Finally, the InsHyde project concluded with a public document providing initial guidance on the use of hydrogen in confined spaces.     

Hydrogenation of C14 Laves phase alloy: CaLi2

The CaLi₂ alloy which was prepared by the induction melting method has been successfully hydrogenated. The CaLi₂ alloy synthesized had a hexagonal C14-type Laves phase structure and absorbed 6.8–7.1 mass% hydrogen under the temperature range from 273 K to 393 K. The hydrogenated CaLi₂ which consisted of CaH₂ and LiH hydride phases did not desorb hydrogen under 10 kPa-H₂ at the same temperatures. The hydrogen absorption kinetics measured under 3.1 MPa-H₂ at room temperature showed that the hydrogen content reached to 6 mass% in 10 s. No obvious hydrogen desorption from the hydrogenated CaLi₂ was observed even after evacuation for 20 h at 623 K.     

Hydrogen permeation from CGH2 vehicles in garages: CFD dispersion calculations and experimental validation

The time and space evolution of the distribution of hydrogen in confined settings was investigated computationally and experimentally for permeation from typical compressed gaseous hydrogen (CGH2) storage systems for buses or cars. The main goal was to examine whether hydrogen is distributed homogeneously within a garage-like facility or whether stratified conditions are developed, under certain conditions. The nominal hydrogen flow rate considered was 1.087 L/min in a bus facility with a volume of 681 m³. The release was assumed to be directed upwards from a 0.15 m diameter hole located at the middle part of the bus cylinders casing. Ventilation rates up to 0.03 air changes per hour (ACH) were considered. Simulated time periods extended up to 20 days. The CFD simulations performed with the ADREA-HF code showed that fully homogeneous conditions exist for low ventilation rates, while stratified conditions prevail for higher ventilation rates. Regarding flow structure it was found that the vertical concentration profiles can be considered as the superposition of the concentration at the floor (driven by diffusion) plus a concentration difference between floor and ceiling (driven by buoyancy forces). In all cases considered this concentration difference was found to be less than 0.5%. The dispersion experiments were performed in a large scale garage-like enclosure of 40 m³ using helium (GARAGE facility). Comparison between CFD simulations and experiments showed that the predicted concentrations were in good agreement with the experimental data. Finally, simulations were performed using two integral models: the fully homogeneous model and a two-layer model and the results were compared both against CFD and the experimental data.     

HySafe SBEP-V20: Numerical studies of release experiments inside a naturally ventilated residential garage

This work presents the results of the Standard Benchmark Exercise Problem (SBEP) V20 of Work Package 6 (WP6) of HySafe Network of Excellence (NoE), co-funded by the European Commission, in the frame of evaluating the quality and suitability of codes, models and user practices by comparative assessments of code results. The benchmark problem SBEP-V20 covers release scenarios that were experimentally investigated in the past using helium as a substitute to hydrogen. The aim of the experimental investigations was to determine the ventilation requirements for parking hydrogen fuelled vehicles in residential garages. Helium was released under the vehicle for 2 h with 7.200 l/h flow rate. The leak rate corresponded to a 20% drop of the peak power of a 50 kW fuel cell vehicle. Three double vent garage door geometries are considered in this numerical investigation. In each case the vents are located at the top and bottom of the garage door. The vents vary only in height. In the first case, the height of the vents is 0.063 m, in the second 0.241 m and in the third 0.495 m. Four HySafe partners participated in this benchmark. The following CFD packages with the respective models were applied to simulate the experiments: ADREA-HF using k–? model by partner NCSRD, FLACS using k–? model by partner DNV, FLUENT using k–? model by partner UPM and CFX using laminar and the low-Re number SST model by partner JRC. This study compares the results predicted by the partners to the experimental measurements at four sensor locations inside the garage with an attempt to assess and validate the performance of the different numerical approaches.     

Natural and forced ventilation of buoyant gas released in a full-scale garage: Comparison of model predictions and experimental data

An increase in the number of hydrogen-fueled applications in the marketplace will require a better understanding of the potential for fires and explosion associated with the unintended release of hydrogen within a structure. Predicting the temporally evolving hydrogen concentration in a structure, with unknown release rates, leak sizes and leak locations is a challenging task. A simple analytical model was developed to predict the natural and forced mixing and dispersion of a buoyant gas released in a partially enclosed compartment with vents at multiple levels. The model is based on determining the instantaneous compartment over-pressure that drives the flow through the vents and assumes that the helium released under the automobile mixes fully with the surrounding air. Model predictions were compared with data from a series of experiments conducted to measure the volume fraction of a buoyant gas (at 8 different locations) released under an automobile placed in the center of a full-scale garage (6.8 m × 5.4 m × 2.4 m). Helium was used as a surrogate gas, for safety concerns. The rate of helium released under an automobile was scaled to represent 5 kg of hydrogen released over 4 h. CFD simulations were also performed to confirm the observed physical phenomena. Analytical model predictions for helium volume fraction compared favorably with measured experimental data for natural and forced ventilation. Parametric studies are presented to understand the effect of release rates, vent size and location on the predicted volume fraction in the garage. Results demonstrate the applicability of the model to effectively and rapidly reduce the flammable concentration of hydrogen in a compartment through forced ventilation.     

Simulation of detonation after an accidental hydrogen release in enclosed environments

An accidental hydrogen release in equipment enclosures may result in the presence of a detonable mixture in a confined environment. To assess the value of CFD techniques as a tool for damage assessment, numerical simulation of detonation was performed for two realistic scenarios. The first scenario starts with a pipe failure in an electrolyzer, resulting in a leak of 42 g of hydrogen. The second scenario deals with a failure in a reformer where 84 g of hydrogen is released. Dispersion patterns were first obtained from separate numerical simulation and detonative ignition was simulated by adding energy to a narrow region. Impulse values reached 600 Ns/m² in the electrolyzer scenario while they reached 1100 Ns/m² in the reformer. For all cases studied, the consequences are more serious in the reformer than the electrolyzer.     

Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures

Experimental data obtained for hydrogen mixtures in a room-size enclosure are presented and compared with data for propane and methane mixtures. This set of data was also used to develop a three-dimensional gasdynamic model for the simulation of gaseous combustion in vented enclosures. The experiments were performed in a 64 m³ chamber with dimensions of 4.6 × 4.6 × 3.0 m and a vent opening on one side and vent areas of either 2.7 or 5.4 m² were used. Tests were performed for three ignition locations, at the wall opposite the vent, at the center of the chamber or at the center of the wall containing the vent. Hydrogen-air mixtures with concentrations close 18% vol. were compared with stoichiometric propane-air and methane-air mixtures. Pressure data, as function of time, and flame time-of-arrival data were obtained both inside and outside the chamber near the vent. Modeling was based on a Large Eddy Simulation (LES) solver created using the OpenFOAM CFD toolbox using sub-grid turbulence and flame wrinkling models. A comparison of these simulations with experimental data is discussed.     

Hydrogen storage of nanostructured TiO2-impregnated carbon nanotubes

Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO₂-nanorods and nanotubes has been performed at room temperature and moderate hydrogen pressures of 8–18 atm. Under hydrothermal synthesis conditions, nanorods (NRs) and nanoparticles (NPs) are found to form either of the two polymorphic phases, i.e., nanorods are formed of predominantly anatase phase while nanoparticles are formed of rutile phase. NRs and NPs are introduced into the CNT matrix via the wetness-impregnation method. These composites store up to 0.40 wt.% of hydrogen at 298 K and 18 atm, which is nearly five times higher the hydrogen uptake of pristine CNTs. The excess amount of hydrogen stored in TiO₂-impregnated CNTs is determined from the amount of TiO₂ in the sample and the measured hydrogen uptake of TiO₂ nanoparticles. Higher hydrogen uptake of NP-impregnated CNTs when compared pristine CNTs is accounted for by considering initial binding of hydrogen on TiO₂ and subsequent spillover in CNT–TiO₂-NPs.     

Hydrogen storage properties of Mg–50 vol.%V7.4Zr7.4Ti7.4Ni composite prepared by spark plasma sintering

Hydrogen storage properties of Mg–50 vol.%V7.4Zr7.4Ti7.4Ni composite prepared by spark plasma sintering were investigated based on the PCT measurements, kinetics and DSC estimations and microstructure observations. The results showed that the composite consisted of Mg phase and V-based solid solution, with a small amount of sintering phase at their interface, and could absorb and desorb hydrogen at 303 K and 573 K, with a maximum hydrogen storage capacity of 3.05 wt.% and 2.55 wt.%, respectively. At 573 K it was found that the Mg phase was the basis for the hydrogen absorption/desorption, but with the combination of the V-based solid solution its kinetics was greatly improved, and its hydrogen desorption temperature decreased by about 117 K, which made it possible for hydrogen desorption of Mg phase at 573 K. Meanwhile the sintering phase was considered to be a key factor in improving hydriding properties of the Mg phase, which might act as a catalyst and offer preferable paths for hydrogen diffusion from V-based solid solution to the Mg phase.     

Numerical studies of dispersion and flammable volume of hydrogen in enclosures

This paper presents a numerical study of dispersion and flammable volume of hydrogen in enclosures using a simple analytical method and a computational fluid dynamics (CFD) code. In the analytical method, the interface height and hydrogen volume fraction of the upper layer are obtained based on mass and buoyancy conservation while the centreline hydrogen volume fraction is derived from similarity solutions for buoyant jets. The two methods (CFD and analytical) are used to simulate an experiment conducted by INERIS, consisting of a 1 g/s hydrogen release for 240 s through a 20 mm diameter orifice into an enclosure. It is found that the predicted centreline hydrogen concentration by both methods agrees with each other and is also in good agreement with the experiment. There are however differences in the calculated total flammable volume because the analytical method does not consider local mixing and diffusion in the upper layer which is assumed uniformly well mixed. The CFD model, in comparison, incorporates the diffusion and stratification phenomena in the upper layer during the mixing stage.     

Analysis of transient supersonic hydrogen release,dispersion and combustion

A hydrogen leak from a facility, which uses highly compressed hydrogen gas (714 bar, 800 K) during operation was studied. The investigated scenario involves supersonic hydrogen release from a 10 cm² leak of the pressurized reservoir, turbulent hydrogen dispersion in the facility room, followed by an accidental ignition and burn-out of the resulting H₂-air cloud. The objective is to investigate the maximum possible flame velocity and overpressure in the facility room in case of a worst-case ignition. The pressure loads are needed for the structural analysis of the building wall response. The first two phases, namely unsteady supersonic release and subsequent turbulent hydrogen dispersion are simulated with GASFLOW-MPI. This is a well validated parallel, all-speed CFD code which solves the compressible Navier-Stokes equations and can model a broad range of flow Mach numbers. Details of the shock structures are resolved for the under-expanded supersonic jet and the sonic-subsonic transition in the release. The turbulent dispersion phase is simulated by LES. The evolution of the highly transient burnable H₂-air mixture in the room in terms of burnable mass, volume, and average H₂-concentration is evaluated with special sub-routines. For five different points in time the maximum turbulent flame speed and resulting overpressures are computed, using four published turbulent burning velocity correlations. The largest turbulent flame speed and overpressure is predicted for an early ignition event resulting in 35–71 m/s, and 0.13–0.27 bar, respectively.     

Large-scale hydrogen release in an isothermal confined area

INERIS has set up large-scale fully instrumented experiments to study the formation of flammable clouds resulting from a finite duration leakage of hydrogen in a quiescent room (80 m³ chamber). Concentration, temperature and mass flow measurements were monitored during the release period and several hours after. Experiments were carried out for mass flow rates ranging from 0.2 g/s to 1 g/s. The instrumentation allowed the observation and quantification of rich hydrogen layers stratification effects. This paper presents both the experimental facility and the test results. These experimental results have been used to assess and benchmark CFD tools capabilities [1].     

Validation of CFD modelling of LH2 spread and evaporation against large-scale spill experiments

Hydrogen is widely recognized as an attractive energy carrier due to its low-level air pollution and its high mass-related energy density. However, the safety characteristics of hydrogen are a concern, primarily due to its wide flammability range and high burning velocity. A significant fraction of hydrogen is stored and transported as a cryogenic liquid. Therefore, loss of hydrogen containments may lead to the formation of a pool on the ground. In general, very large spills will give a pool, whereas moderate sized spills may evaporate immediately. Accurate hazard assessments of storage systems require a proper prediction of the liquid hydrogen pool evaporation and spreading when conditions are conducive to the formation of a pool.A pool model handling the spread and the evaporation of liquid spills on different surfaces has recently been implemented in the 3D-Computational Fluid Dynamics (CFD) tool FLACS [1], [2], [3] and [4]. As the influence of geometry on the liquid spread is taken into account in the pool model, realistic industrial scenarios can be investigated. The model has been extensively validated for Liquefied Natural Gas (LNG) spills [5] and [6]. The model has previously been tested for LH₂ release in the framework of the EU-sponsored Network of Excellence HySafe where experiments carried out by BAM were modelled. In the large-scale BAM experiments [7], 280 kg of liquid hydrogen was spilled in 6 tests adjacent to buildings. In these tests, the pool spreading, the evaporation, and the cloud formation were investigated. Simulations of these tests are found to compare reasonably well with the experimental results.In the present work, the liquid hydrogen spill experiments carried out by NASA are simulated with the pool model. The large-scale NASA experiments [8] and [9] consisted of 7 releases of liquefied hydrogen at White Sand, New Mexico. The release test 6 is used. During these experiments, cloud concentrations were measured at several distances downwind of the spill point. With the new pool model feature, the FLACS tool is shown to be an efficient and accurate tool for the investigation of complex and realistic accidental release scenarios of cryogenic liquids.     

Structural and hydrogenation properties of SrAl2−xNix alloys

In this paper, Al was partially substituted by Ni in the Zintl phase alloy SrAl₂ and the structural and hydrogenation characteristics of the SrAl_2−xNi_x (0 ≤ x ≤ 0.4) alloys were studied by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and hydrogenation measurements. The alloy consisted of a single Zintl phase of SrAl₂ when x = 0. However, partial substitution of Al by Ni resulted in multiphase structure of the alloys. When x = 0.1, the alloy was composed of SrAl₂, Sr₅Al₉, SrAl and AlNi phases. With the increase of x, the amount of SrAl₂ and Sr₅Al₉ phases decreased, while the amount of SrAl and AlNi phases increased. Hydrogenation measurements were made at 473 K under 3 MPa hydrogen pressure. It was interesting to find that the hydriding kinetics of the alloy was improved greatly after Ni substitution, which could be attributed to the catalysis of AlNi phase.     

Electrochemical properties of titanium-based hydrogen storage alloy prepared by solid phase sintering

The structure and electrochemical properties of titanium-based hydrogen storage alloy prepared by solid phase sintering at 1123 K were investigated. The result of X-ray diffraction (XRD) showed that the sintered alloy mainly consists of Ti₂Ni phase coexisting with TiNi, TiNi₃ and Ni phases. The alloy had a maximum discharge capacity of 205 mAh/g at a discharge current density of 60 mA/g and showed a discharge capacity of 146 mAh/g at 150 mA/g. The results of linear polarization (LP) and potential-step measurement presented that the exchange current density and hydrogen diffusion efficient of the alloy were 100 mA/g and 4.2 × 10⁻⁹ cm²/s, respectively. The electrochemical performance of the alloy could be effectively improved by using solid phase sintering.     

Zirconia supported catalysts for bioethanol steam reforming: Effect of active phase and zirconia structure

Three new catalysts have been prepared in order to study the active phase influence in ethanol steam reforming reaction. Nickel, cobalt and copper were the active phases selected and were supported on zirconia with monoclinic and tetragonal structure, respectively. To characterize the behaviour of the catalysts in reaction conditions a study of catalytic activity with temperature was performed. The highest activity values were obtained at 973 K where nickel and cobalt based catalysts achieved an ethanol conversion of 100% and a selectivity to hydrogen close to 70%. Nickel supported on tetragonal zirconia exhibited the highest hydrogen production efficiency, higher than 4.5 mol H₂/mol EtOH fed. The influence of steam/carbon (S/C) ratio on product distribution was another parameter studied between the range 3.2–6.5. Nickel supported on tetragonal zirconia at S/C = 3.2 operated at 973 K without by-product production such as ethylene or acetaldehyde. In order to consider a further application in an ethanol processor, a long-term reaction experiment was performed at 973 K, S/C = 3.2 and atmospheric pressure. After 60 h, nickel supported on tetragonal zirconia exhibited high stability and selectivity to hydrogen production.     

CFD modeling and experimental study of multi-walled carbon nanotubes production by fluidized bed catalytic chemical vapor deposition

A parametric study investigating the impact of temperature, gas velocity, and composition of the gaseous phase on the catalytic growth of multi-walled carbon nanotubes (CNT) has been performed. CNTs have been produced by catalytic chemical vapor deposition from methane decomposition over Co-Mo/MgO with average diameter of 188 μm with spherical shape in a fluidized bed reactor. The computational fluid dynamics (CFD) method was used for simulating the hydrodynamics of the reactor and investigating the operational and best velocity for producing high quality CNTs by this system. The operational and best velocities obtained by simulation were 0.015 to 0.05 m/s and near 0.015 m/s. Then the results used in the experiments with different temperature and gas compositions. CNTs products were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results showed that temperature of 900 °C, methane to hydrogen volume ratio 1:4 and 0.02 m/s are the best quantities of the parameters for CNTs growth.     

Rehydrogenation performance of an MgH2–Nb2O5 system modified by heptane and acetone

An MgH₂ + 1 mol% Nb₂O₅ system was modified by heptane and acetone through a high-energy ball milling process, and their rehydrogenation performances were investigated. XRD results indicated that except MgH₂ and Nb₂O₅ phases Mg and MgO phases existed after ball milling. The rehydrogenation results showed that after modification by heptane the capacity increased from 3.0 wt% and 4.2 wt% to 5.0 wt% and 5.5 wt% within 110 s at 523 K and 573 K, respectively. The hydriding rate increased from 0.08 wt%/s after 20 s to 0.22 wt%/s after 10 s at 523 K. However, after modification by acetone it only absorbed 1.8 wt% and 2.0 wt% of hydrogen even within 8000 s at 523 K and 573 K, respectively. Rietveld refinement results indicated that after modification by the heptane the content of MgO was reduced from 6.8 wt% to 4.2 wt%, while after the modification by the acetone the content of MgO was significantly increased from 6.8 wt% to 23.8 wt%. The difference in the rehydrogenation performance was believed to be attributed to the different contents of the MgO phase, which led to the difference in the contents of the MgH₂ phase. It implied that the heptane acted as a solvent without oxygen element in it to prevent the MgH₂ + Nb₂O₅ system from aggregation, crystallization and oxidation. It suggested heptane was suitable for the improvement of the rehydrogenation performance of MgH₂ system.     

Hydrogen adsorption behavior of graphene above critical temperature

We evaluate the hydrogen adsorption behavior of the pristine single-layer graphene sheets in a powder form, which was performed at cryogenic and room temperatures. Only 0.4 wt.% and <0.2 wt.% hydrogen uptake was obtained at 77 K under 100 kPa and room temperature under 6 MPa, respectively, for the pristine graphene sample with a BET specific surface area of 156 m² g⁻¹. Structure/property investigations suggest that the low specific surface area and weak binding to hydrogen should be responsible for the small gravimetric uptake of pristine graphene sample.