Experimental study and model predictions on helium release in an enclosure with single or multiple vents

This paper presents experiments performed at Canadian Nuclear Laboratories (CNL) to examine the dispersion behaviour of helium in a polycarbonate enclosure that was representative of a residential parking garage. The purpose was to gain a better understanding of the effect of buoyancy- or wind-driven natural ventilation on hydrogen dispersion behaviour. Although hydrogen dispersion studies have been reported extensively in the literature, gaps still exist in predictive methods for hazard analysis. Helium, a simulant for hydrogen, was injected near the centre of the floor with a flow rate ranging from 5 to 75 standard litres per minute through an upward-facing nozzle, resulting in an injection Richardson number ranging between 10^?1 and 10². The location of the nozzle varied from the bottom of the enclosure to near the ceiling to examine the impact of the nozzle elevation on the development of a stratified layer in the upper region of the enclosure. When the injection nozzle was placed at a sufficiently low elevation, the vertical helium profile always consisted of a homogenous layer at the top overlaying a stratified layer at the bottom. To simulate outdoor environmental conditions, a fan was placed in front of each vent to examine the effect of opposing or assisting wind on the dispersion. The helium transients in the uniform layer predicted with analytical models were in good agreement with the measured transients for most tests. Model improvements are required for adequately predicting transients with primarily stratified profiles or strong opposing wind.     

Dispersion and behavior of hydrogen for the safety design of hydrogen production plant attached with nuclear power plant

Development of nuclear energy and hydrogen energy both as renewable energy open up a vast range of prospects. The scheme for hydrogen generation station in nuclear power plant has been carried out in china. However, Nuclear Energy is expected to encourage a safety culture that prevents serious accidents while dispersion of hydrogen from a container produces a risk of combustion. The dispersion and behavior of hydrogen production plant attached with nuclear power plant are still poorly understood. In this paper, a dispersion of hydrogen model is established and is calculated under two typical condition with corrected ideal gas state equation. The flammability of hydrogen after dispersion is studied. The range of flammability of dispersion of hydrogen production plant with different pressures, positions and temperatures is obtained. This work could contribute to the marginal hydrogen safety design for hydrogen production station and lay the foundation for the establishment of a safe distance standard that it's necessary to prevent hydrogen explosion.     

Numerical and experimental investigation of buoyant gas release: Application to hydrogen jets

The physics of hydrogen release from compressed storage vessels is investigated for flow conditions corresponding to a subsonic turbulent jet venting into atmosphere at a Mach number ∼0.3 using large eddy simulations (LES) and particle image velocimetry (PIV) measurements. A major focus of the work is investigation of the dynamic features of the flow and mixing processes by analyzing the detailed instantaneous data available from LES. Simulations are first conducted for a helium jet with flow parameters corresponding closely to hydrogen jet release at low Mach number in order to allow direct comparison with PIV measurements. The simulations and measurements are consistent, and show that though exact self-similarity of the turbulent jet is not achieved due to buoyancy effects, the normalized averaged velocity and concentration profiles collapse reasonably well and can be approximated by a Gaussian distribution. Analysis of the instantaneous concentration fields shows that, due to the complex dynamics of the jet, which includes flapping and vortex shedding, concentration levels above the flammability threshold occur intermittently in regions where average concentrations are low. The spatial extent of regions where flammability thresholds are exceeded is nearly twice as large when considering transient effects compared to the mean (time-averaged) flow; the possible implications for safety guidelines are important. LES results are also presented for a hydrogen jet during the transient phase following the onset of release. Compared to the stationary jet flow, the radial extent of concentration above the flammability threshold is found to increase by ∼30% during transients as a result of the strong vortex ring that forms when the hydrogen jet penetrates stationary ambient air.     

Numerical validation of pressure peaking from an ignited hydrogen release in a laboratory-scale enclosure and application to a garage scenario

This work focuses on the overpressures arising from the rapid ignited release of hydrogen in an enclosure, specifically the peak in overpressure that may result in the initial period of the release, dependent on the level of ventilation. Two volumes are considered: a 1 m³ laboratory scale enclosure for which experimental data exists, and a real scale residential garage. Various vent configurations are considered for each scenario for leak rates typical of those from a fuel cell (laboratory scale enclosure) and from onboard hydrogen storage tanks through a thermally activated pressure relief device (TPRD) in the garage-like enclosure. A validation study has been performed for the laboratory scale enclosure and the modelling approach which gives optimum results has been identified. The influence of heat transfer on the pressure peak has been highlighted, particularly, the importance of radiation in predicting the pressure peak. The validated modelling approach has been applied to a range of experiments and garage scenarios. Both the laboratory and real scale simulations demonstrate the complex relationship between vent size and release rate and indicate the significant overpressures that can result through pressure peaking following an ignited release in an enclosure. The magnitude of the pressure peak as a result of an ignited release has been found to be two orders of magnitude greater than that for the corresponding unignited release. The work indicates that TPRDs currently available for hydrogen-powered vehicles may result in a dangerous situation for the specific scenario considered which should be accounted for in regulations, codes and standards. The application of this work extends beyond TPRDs and is relevant where there is a rapid, ignited release of hydrogen in an enclosure with ventilation.     

Measurements of effective diffusion coefficients of helium and hydrogen through gypsum

An experimental apparatus, which was based on the ¼-scale garage previously used for studying helium release and dispersion in our laboratory, was used to obtain effective diffusion coefficients of helium and hydrogen (released as forming gas for safety reasons) through gypsum panel. Two types of gypsum panel were used in the experiments. Helium or forming gas was released into the enclosure from a Fischer burner¹ located near the enclosure floor for a fixed duration and then terminated. Eight thermal-conductivity sensors mounted at different vertical locations above the enclosure floor were used to monitor the temporal and spatial gas concentrations. An electric fan was used inside the enclosure to mix the released gas to ensure a spatially uniform gas concentration to minimize stratification. The temporal variations of the pressure difference between the enclosure interior and the ambience were also measured. An analytical model was developed to extract the effective diffusion coefficients from the experimental data.     

Numerical simulation of leakage and diffusion distribution of natural gas and hydrogen mixtures in a closed container

The transportation and utilization of hydrogen blended natural gas have received extensive attention. The dangerous characteristics of hydrogen such as high diffusivity and wide flammability/explosion limit also increase the leakage risk of hydrogen blended natural gas. In this paper, a numerical model is established for the leakage and diffusion of hydrogen blended natural gas in a closed container. The evolution of the distribution, diffusion law and flammable area of different proportions of hydrogen blended natural gas after leaking into a closed container is investigated. The results show that the flammable area with low hydrogen ratios (20% and below) will disappear within 2.7 s–11.1 s after the leakage, which is relatively safer, while the high hydrogen ratio (80% and above) reaches 3875 s–4555 s with a significant increase in risk duration. After the 50% hydrogen ratio leakage, the thickness of the flammable area is higher than 15.67% for the 80% hydrogen ratio and 30.25% higher than pure hydrogen at 120 s after leakage, and the risk is higher in a short time. Due to the difference in the diffusion rates between methane and hydrogen, hydrogen diffuses to the middle and lower part of the enclosed container faster, and the risk in the middle and lower part also deserves attention.     

Effect of helium implantation on the hydrogen retention of Cr2O3 films formed in an ultra-low oxygen environment

To simulate and investigate the irradiation damage of neutron and transmutation effect (He production) on the hydrogen isotope trapping behavior, in the present study, a new monoclinic hydrogen permeation barrier composed of Cr₂O₃ was fabricated and helium implantation with different fluences was tentatively employed. First, pure chromium samples were oxidized in an ultra-low oxygen partial pressure (1.7 × 10⁻²³ Pa) environment to obtain a single Cr₂O₃ layer. Then a dense layer of helium bubbles was formed in a substrate using a helium ion implantation method. Finally, the samples were treated in a hydrogen plasma environment at 500 °C. The damage, vacancy, and helium distribution in these samples were then simulated by SRIM. The morphology, phase, surface characteristics, thermal desorption, and electrochemical properties were subsequently characterized and evaluated. Thermal desorption spectrum analysis (TDS) was used to study the thermal desorption of hydrogen and helium at different temperatures. Our results showed that the inhibitory effect of the composite hydrogen barrier layer on the hydrogen diffusion in the substrate first increased and then decreased with the increase of the helium ion implantation fluence.     

Evidence of catalytic production of hot atomic hydrogen in RF generated hydrogen/helium plasmas

A study of the line shapes of hydrogen Balmer series lines in RF generated low pressure He/H₂ plasmas produced results suggesting a catalytic process between helium and hydrogen species results in the generation of ‘hot’ (ca. 28 eV) atomic hydrogen. Even far from the electrodes ‘hot’ atomic hydrogen was predominant in He/H₂ plasmas. Line shapes, relative line areas of cold and hot atomic hydrogen (hot/cold > 2.5), were very similar for areas between the electrodes and far from the electrodes for these plasmas. In contrast, in Xe/H₂ only ‘warm’ (<5 eV) hydrogen (warm/cold < 1.0) was found between the electrodes, and only cold hydrogen away from the electrodes. Earlier postulates that preferential hydrogen line broadening in plasmas results from the acceleration of ionic hydrogen in the vicinity of electrodes, and the special charge exchange characteristics of Ar/H₂+ are clearly belied by the present results that show atomic hydrogen line shape are similar for He/H₂ plasmas throughout the relatively large cylindrical (14 cm ID × 36 cm length) cavity.     

Numerical investigation of subsonic hydrogen jet release

A buoyant round vertical hydrogen jet is investigated using Large Eddy Simulations at low Mach number (M = 0.3). The influence of the transient concentration fields on the extent of the gas envelope with concentrations within the flammability limits is analyzed and their structure are characterized. The transient flammable region has a complex structure that extends up to 30% beyond the time-averaged flammable volume, with high concentration pockets that persist sufficiently long for potential ignition. Safety envelopes devised on the basis of simplified time-averaged simulations would need to include a correction factor that accounts for transient incursions of high flammability concentrations.     

Thermodynamic assessments of a novel integrated process for producing liquid helium and hydrogen simultaneously

Liquid helium and hydrogen are two precious cryogens with advanced applications in various energy research fields. However, producing these cryogens generally come with high-cost processes. In this research, Liquid hydrogen is obtained in two stages with the aid of a mixed refrigeration subprocess and helium cryogen. Also, liquid helium is obtained in three stages with the aid of helium upgrader, pressure swing adsorption, and helium liquefier subprocesses. The liquid helium is produced at 19.42 K, 195 kPa, and 6161 kgmole/h. Also, the liquid hydrogen is produced at 3.69 K, 110.3 kPa, and 17,970 kgmole/h. The novelties of this research can be described as the production of liquid helium and hydrogen simultaneously, low SEC, novel configuration, and production of liquid helium and hydrogen at near ambient pressure. Thermodynamic analyses show that the specific energy consumption, coefficient of performance, and figure of merit are equal to 18.96 kW h/kg, 0.03, and 0.37, respectively. Also, the exergy analysis shows that the exergy efficiency and exergy destruction in the whole process are equal to 67% and 4471 MW, respectively. Also, sensitivity analysis shows that increasing the PSA process efficiency positively impacts all process parameters like SEC, COP, FOM, and exergy efficiency.     

Ignitability limits for combustion of unintended hydrogen releases: Experimental and theoretical results

The ignition limits of hydrogen/air mixtures in turbulent jets are necessary to establish safety distances based on ignitable hydrogen location for safety codes and standards development. Studies in turbulent natural gas jets have shown that the mean fuel concentration is insufficient to determine the flammable boundaries of the jet. Instead, integration of probability density functions of local fuel concentration within the quiescent flammability limits, termed the flammability factor, was shown to provide a better representation of ignition probability. Recent studies in turbulent hydrogen jets showed that the envelope of ignitable gas composition (based on the mean hydrogen concentration), did not correspond to the known flammability limits for quiescent hydrogen/air mixtures. The objective of this investigation is to validate the flammability factor approach to the prediction of ignition in hydrogen leak scenarios. The ignition probability within a turbulent hydrogen jet was determined using a pulsed Nd:YAG laser as the ignition source. Laser Rayleigh scattering was used to characterize the fuel concentration throughout the jet. Measurements in methane and hydrogen jets exhibit similar trends in the ignition contour, which broadens radially until an axial location is reached after which the contour moves inward to the centerline. Measurements of the mean and fluctuating hydrogen concentration are used to characterize the local composition statistics conditional on whether the laser spark results in a local ignition event or complete light-up of a stable jet flame. The flammability factor is obtained through direct integration of local probability density functions. A model was developed to predict the flammability factor using a presumed probability density function with parameters obtained from experimental data and computer simulations. Intermittency effects that are important in the shear layer are incorporated in a composite probability density function. By comparing the computed flammability factor with the measured ignition probability we have validated the flammability factor approach for application to ignition of hydrogen jets.     

A method to characterize mixing and flammability of hydrogen-air mixtures in enclosures

Since the use of hydrogen as energy carrier is likely to increase in the future, the associated safety concerns have to be addressed. It is necessary that the mixing characteristics of hydrogen gas released into an enclosure be quantified with the help of numerical indices that qualitatively and quantitatively reflect the overall mixing process and hazard of formation of flammable cloud in the enclosure. Four indices meant to characterize mixing and flammability of hydrogen in enclosures, namely, Average mole fraction, Non-uniformity index, Deflagration Volume Fraction and Deflagration Pressure Ratio are proposed in this paper. Significance and utility of these indices in providing useful information on mixing and flammability have been demonstrated through numerical studies of hydrogen distribution in enclosures containing air. These indices can serve as useful tools in studies of hydrogen distribution in enclosures.     

URANS study of pulsed hydrogen jet characteristics and mixing enhancement in supersonic crossflow

In this paper, three-dimensional pulsed hydrogen jet in supersonic crossflow (PJISC) is investigated by the unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with the k-ω shear stress transport (SST) turbulence model. The numerical validation and mesh resolution have been carried out against experiment firstly. The effects of the pulsed frequency and amplitude on the jet flow field and mixing performance in supersonic cross-flow are all addressed. It significantly changes the distribution of the hydrogen jet flow by comparing with the steady jet in supersonic crossflow. The fuel jet penetration, mixing efficiency, decay rate of the maximum hydrogen mass fraction and total pressure losses are used to quantitatively analyze the mixing performance. The mixing of fuel and incoming air flow is enhanced by the pulsed jet, especially for the case of 50 kHz, which is the optimal pulsed frequency while considering the effects of jet excitation frequency in the present simulations. The decay rate of the maximum mass fraction of hydrogen in the far field downstream is related to the frequency of the pulse jet. Moreover, the pulsed frequency and amplitude have little effects on the total pressure recovery coefficient for the cases studied in the present simulations.     

Experiments on the distribution of concentration due to buoyant gas low flow rate release in an enclosure

Experiments on buoyant gas dispersion in an enclosure have been conducted in a facility of the typical size of a private garage. Helium is used as a model gas for hydrogen. For release flow rate of the order of 0.1 Nl/min to 10 Nl/min, the dispersion is studied in a tightly sealed configuration of the enclosure and for two vertical positions of a vent, near the bottom and near the top. Results are compared to existing simple analytical models. A good accordance is found in the tightly sealed case. With one vent, some significant differences with models are found for the highest flow rates due to a vertical stratification. However a good accordance is found in the limit of very low flow rates even for the simplest model based on a ventilation flow rate independent of the interior mixture density. The main properties of the equivalent flammable atmosphere formed with the vent are presented.     

Calculation of the upper flammability limit of methane/hydrogen/air mixtures at elevated pressures and temperatures

The results of three different numerical methods to calculate flammability limits—namely (1) the calculation of planar flames with the inclusion of a (radiation) heat loss term in the energy conservation equation, and the application of (2) a limiting burning velocity and of (3) a limiting flame temperature—are compared with experimental data on the upper flammability limit (UFL) of methane/hydrogen/air mixtures with hydrogen fuel molar fractions of 20% and 40%, at initial pressures up to 10 bar and initial temperatures up to 200 °C. The application of a limiting burning velocity is found to predict the pressure dependence of the UFL well, while the application of a limiting flame temperature generally is found to slightly underestimate the temperature dependence of the UFL.     

CFD modelling of accidental hydrogen release from pipelines

Although nowadays hydrogen is distributed mainly by trailers, in the future distribution by means of pipelines will be more suitable if larger amounts of hydrogen are produced on industrial scale. Therefore from the safety point of view it is essential to compare hydrogen pipelines to natural gas pipelines, whose use is well established today. Within the paper we compare safety implications in accidental situations. In the analysis we do not consider technological aspects such as compressors or seals.     

Numerical simulation of high-pressure hydrogen jet flames during bonfire test

Characteristics of high-pressure hydrogen jet flames resulting from ignition of hydrogen discharge during the bonfire test of composite hydrogen storage vessels are studied. Firstly, a 3-D numerical model is established based on the species transfer model and SST k − ω turbulence model to study the high-pressure hydrogen jet flow. It is revealed that under-expanded jets are formed after the high-pressure hydrogen discharging from the vessel. Secondly, the mathematical methods are adopted to study the high-pressure hydrogen jet flames. The effects of pressure, initial temperature and the nozzle diameter on the jet flames are investigated. The results show that the jet flame length increases with the increase of discharge pressure, but decreases with the increase of nozzle diameter and temperature difference between the filling hydrogen temperature and the environment temperature. Finally, the simulation models are established to study the characteristics of hydrogen jet flames in an open space. The effects of barrier walls on the distribution of jet flames are also studied. The results show that the barrier walls can greatly reduce the damage from hydrogen jet flames to testers and properties around.     

Predicting radiative heat fluxes and flammability envelopes from unintended releases of hydrogen

The prediction of the radiative heat flux from a turbulent-jet flame issuing from a damaged, high-pressure hydrogen storage system is an issue of importance for the safe use of hydrogen. Information about the variation of the thermal radiation exposure with distance from the hydrogen jet flame as well as the length and duration of the flame is important in determining safe distances for the handling and storage of hydrogen. An equally important issue is the determination of the concentration decay of an unignited hydrogen jet in the surrounding air, and the envelope of locations where the concentration falls below the lower flammability limit for hydrogen.     

Comparisons of experimental measurements and large eddy simulations for a helium release in a two vents enclosure

This work takes place in the context of potential hazards in the use of hydrogen in fuel cells. The present article describes comparisons between PIV measurements performed on a two vented cavity with an helium injection and Large Eddy Simulation of the same configuration. A two vented cavity is chosen because a quasi state is reached rapidly and it facilitates both CFD calculations by reducing the CPU costs and also enables statistical treatment of the data, the temporal averaging being possible at steady state. At the same time, this configuration is close to fuel cell designs, except for the set-up reduced size. We also describe the experimental set-up and the care which has to be taken to produce Particle Image Velocimetry velocity fields. The final goal of the paper is to validate a L.E.S approach as a good replacement to experiments, since access to both velocity and concentration fields is required to improve existing simplified models. Indeed, most of the 2 vents models rely on simplified assumptions such as a constant entrainment coefficient, a bi-layer formation which is not always the case in real situations.