High-pressure release and dispersion of hydrogen in a partially enclosed compartment: Effect of natural and forced ventilation

The study of compressed hydrogen releases from high-pressure storage systems has practical application for hydrogen and fuel cell technologies. Such releases may occur either due to accidental damage to a storage tank, connecting piping, or due to failure of a pressure release device (PRD). Understanding hydrogen behavior during and after the unintended release from a high-pressure storage device is important for development of appropriate hydrogen safety codes and standards and for the evaluation of risk mitigation requirements and technologies. In this paper, the natural and forced mixing and dispersion of hydrogen released from a high-pressure tank into a partially enclosed compartment is investigated using analytical models. Simple models are developed to estimate the volumetric flow rate through a choked nozzle of a high-pressure tank. The hydrogen released in the compartment is vented through buoyancy induced flow or through forced ventilation. The model is useful in understanding the important physical processes involved during the release and dispersion of hydrogen from a high-pressure tank into a compartment with vents at multiple levels. Parametric studies are presented to identify the relative importance of various parameters such as diameter of the release port and air changes per hour (ACH) characteristic of the enclosure. Compartment overpressure as a function of the size of the release port is predicted. Conditions that can lead to major damage of the compartment due to overpressure are identified. Results of the analytical model indicate that the fastest way to reduce flammable levels of hydrogen concentration in a compartment is by blowing through the vents. Model predictions for forced ventilation are presented which show that it is feasible to effectively and rapidly reduce the flammable concentration of hydrogen in the compartment following the release of hydrogen from a high-pressure tank.     

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.     

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.     

A comparative CFD assessment study of cryogenic hydrogen and LNG dispersion

The introduction of hydrogen to the commercial market as alternative fuel brings up safety concerns. Its storage in liquid or cryo-compressed state to achieve volumetric efficiency involves additional risks and their study is crucial. This work aims to investigate the behavior of cryogenic hydrogen release and to study factors that affect the vapor dispersion. We focus on the effect of ambient humidity and air's components (nitrogen and oxygen) freezing, in order to identify the conditions under which these factors have considerable influence. The study reveals that the level of influence depends highly on the release conditions and that humidity can reduce conspicuously the longitudinal distance of the Lower Flammability Limit (LFL). Low Froude (Fr) number (<1000) at the release allows the generated by the humidity phase change buoyancy to affect the dispersion, while for higher Fr number - that usually are met in cryo-compressed releases - the momentum forces are the dominant forces and the buoyancy effect is trivial. Simulations with liquid methane release have been also performed and compared to the liquid hydrogen simulations, in order to detect the differences in the behavior of the two fuels as far as the humidity effect is concerned. It is shown that in methane spills the buoyancy effect in presence of humidity is smaller than in hydrogen spills and it can be considered almost negligible.     

Dispersion and catalytic ignition of hydrogen leaks within enclosed spaces

An experimental investigation is conducted into the nature of catalytic ignition of leaked hydrogen gas within an enclosure, and the nature of hydrogen dispersion under varied venting conditions. Using a 1/16th linear scale two-car garage as a model, and a platinum foil as a catalytic surface, it is found that for all conditions tested, catalytic ignition is observed after the leaked hydrogen comes in contact with the catalytic surface, which is initially at or near room temperature. After ignition, these surface reactions lead to steady-state surface temperatures in the range of 600–800 K, dependent on inlet conditions in terms of mixture composition and flow rate. In addition, varying the venting opportunities from the garage walls suggests that not only total area, but also the number and position of vents may impact the nature of hydrogen accumulation within an enclosed structure.     

Release and dispersion modeling of cryogenic under-expanded hydrogen jets

In the present work performed within the framework of the SUSANA EC-project, we address the release and dispersion modeling of hydrogen stored at cryogenic temperatures and high pressures. Due to the high storage pressures the resulting jets are under-expanded. Due to the low temperatures the choked conditions can be two-phase. For the release modeling the homogeneous equilibrium model (HEM) was used combined with NIST equation of state for hydrogen. For the dispersion modeling the 3d CFD methodology was used combined with a) a notional nozzle approach to bridge the expansion to atmospheric pressure region that exists near the nozzle, b) the ideal gas assumption for hydrogen and air and c) the standard (buoyancy included) k–ε turbulence model. Predicted release choked mass fluxes are compared against 37 experiments from literature. Predicted steady state hydrogen concentrations along the jet axis are compared against five dispersion experiments from literature as well as the Chen and Rodi correlation and the behavior of the proposed release and dispersion modeling approaches is assessed.     

Hydrogen dispersion phenomenon in nominally closed spaces

The hydrogen dispersion phenomenon in an enclosure depends on the ratio of the gas buoyancy-induced momentum and diffusive motions. Random diffusive motions of individual gas particles become dominative when the release momentum is low, and a uniform hydrogen concentration appears in the enclosure instead of the gas cumulation below the ceiling. The expected hydrogen behavior could be projected by the Froude number, which value ~1 predicts a decline of buoyancy. This paper justifies this hypothesis by demonstrating full-scale experimental results of hydrogen dispersion within a confined space under six different release variations. During the experiments, hydrogen was released into the test room of 60 m³ volume in two methods: through a nozzle and through 21 points evenly distributed on the emission box cover (multi-point release). Each release method was tested with three volume flow rates (3.2 × 10⁻³ m³/s, 1.6 × 10⁻³ m³/s, 3.3 × 10⁻⁴ m³/s). The tests confirm the decrease of hydrogen buoyancy and its stratification tendencies when the Mach, Reynolds, and Froud number values decrease. Because the hydrogen dispersion phenomenon would impact fire and explosive hazards, the presented experimental results could help fire protection systems be in an enclosure designed, allowing their effectiveness optimization.     

The rapid formation and dispersion of flammable zones within cylindrical vertical enclosures following the release of a fixed mass of hydrogen and other gaseous fuels into air

The release of a certain mass of fuel gas into the ambient atmosphere with negligible pressure difference whether deliberately or inadvertently results in the transient formation of flammable mixture zones for a period of time that represent a potential fire and explosion hazard. A numerical model based on the simultaneous solution of the equations of conservation of mass, momentum and energy has been developed to describe the development of such flammable zones when a finite quantity of fuel is released into the overlaying air within cylindrical vertical enclosures open to the outside atmosphere. Hydrogen disperses into the air extremely quickly with a strong temporal dependency on both horizontal and vertical directions. Comparison of the typical behavior of hydrogen dispersion with that of the lighter than air methane, the nearly buoyancy neutral ethylene and the much heavier than air propane is made. Some guidelines for reducing the fire and explosion hazards in such situations are presented.     

Experimental investigation of highly pressurized hydrogen release through a small hole

The dispersion characteristics of hydrogen leaking through a small hole from a high-pressure source were investigated experimentally to develop guidelines for determining safety distances for hydrogen stations. Tests were carried out for leaking holes with diameters of 0.5, 0.7 and 1.0 mm and for release pressures of 100, 200, 300 and 400 bar. For these realistic hydrogen leaking conditions, the Froude numbers are so large that the buoyancy effect, manifested by the hydrogen jets bending upward, can be expected to be negligible. Flow visualization was performed using an Nd-YAG laser to confirm that the buoyancy effect was negligible. By letting a thin laser sheet penetrate the center line of a hydrogen jet conveying Al₂O₃ particles, the particles were illuminated and the hydrogen jet was visualized. The hydrogen concentration was measured by sampling hydrogen at five points along the jet centerline, based on the large Froude number. The measured data were always lower than the isentropic prediction.     

Comparison of helium and hydrogen releases in 1 m3 and 2 m3 two vents enclosures: Concentration measurements at different flow rates and for two diameters of injection nozzle

This work presents a parametric study on the similitude between hydrogen and helium distribution when released in the air by a source located inside of a naturally ventilated enclosure with two vents. Several configurations were experimentally addressed in order to improve knowledge on dispersion. Parameters were chosen to mimic operating conditions of hydrogen energy systems. Thus, the varying parameters of the study were mainly the source diameter, the releasing flow rate, the volume and the geometry of the enclosure. Two different experimental set-ups were used in order to vary the enclosure's height between 1 and 2 m. Experimental results obtained with helium and hydrogen were compared at equivalent flow rates, determined with existing similitude laws. It appears, for the plume release case, that helium can suitably be used for predicting hydrogen dispersion in these operating designs. On the other hand – when the flow turns into a jet – non negligible differences between hydrogen and helium dispersion appear. In this case, helium – used as a direct substitute to hydrogen – will over predict concentrations we would get with hydrogen. Therefore, helium concentration read-outs should be converted to obtain correct predictions for hydrogen. However such a converting law is not available yet.     

Experimental study on hydrogen explosions in a full-scale hydrogen filling station model

In order for fuel cell vehicles to develop a widespread role in society, it is essential that hydrogen refuelling stations become established. For this to happen, there is a need to demonstrate the safety of the refuelling stations. The work described in this paper was carried out to provide experimental information on hydrogen outflow, dispersion and explosion behaviour. In the first phase, homogeneous hydrogen–air mixtures of a known concentration were introduced into an explosion chamber and the resulting flame speed and overpressures were measured. Hydrogen concentration was the dominant factor influencing the flame speed and overpressure. Secondly, high-pressure hydrogen releases were initiated in a storage room to study the accumulation of hydrogen. For a steady release with a constant driving pressure, the hydrogen concentration varied as the inlet airflow changed, depending on the ventilation area of the room, the external wind conditions and also the buoyancy induced flows generated by the accumulating hydrogen. Having obtained this basic data, the realistic dispersion and explosion experiments were executed at full-scale in the hydrogen station model. High-pressure hydrogen was released from 0.8 to 8.0 mm nozzle at the dispenser position and inside the storage room in the full-scale model of the refuelling station. Also the hydrogen releases were ignited to study the overpressures that can be generated by such releases. The results showed that overpressures that were generated following releases at the dispenser location had a clear correlation with the time of ignition, distance from ignition point.     

CFD modeling of hydrogen dispersion under cryogenic release conditions

The use of hydrogen as a fuel should always be accompanied by a safety assessment concerning the case of an accidental release. To evaluate the potential hazards in a spill accident both experiments and simulations are performed. In the present work, the CFD code, ADREA-HF, is used to simulate the liquefied hydrogen (LH2) spill experiments (test 5, 6, 7) conducted by the Health Safety Laboratory (HSL). Two horizontal releases, the one along the ground and the other one at a distance above the ground, and one vertical release are examined with spill rate 60 lt/min. The main focus of this study is on the presence of humidity in the atmosphere and its effect on the vapor dispersion. When humidity is present is cooled, condenses and freezes due to the low prevailing temperature (∼20 K near the release), and releases heat. In addition, during the release hydrogen droplets are formed due to mechanical and flashing break up, and water droplets and ice crystals due to humidity phase change. Therefore, two models are tested: the hydrodynamic equilibrium model, which assumes that the phases are in thermodynamic and kinematic equilibrium and the non hydrodynamic equilibrium model (slip model), which assumed that the phases are in thermodynamic equilibrium but they can obtain different velocities. The fluctuating wind direction was also taken into account, since it greatly affects the hydrogen dispersion. The computational results are compared with the experimental measurements, and it is concluded that humidity along with the slip effect influences the buoyancy of the cloud to a great extent. The best simulation case (humidity and slip effect) is consistent with the experiment for all three tests for the majority of the sensors.     

Experimental and analytical analyses of the mechanisms governing the dispersion of flammable clouds formed by liquid hydrogen spills

This paper presents the initial findings of hydrogen vapor cloud dispersion experiments conducted by NASA. The experiments were performed to obtain basic information regarding the physical phenomena governing the dispersion of flammable clouds formed as the result of spills of large quantities of liquid hydrogen. The experiments consisted of ground spills of up to 5.7 m³ (1500 gal) of liquid hydrogen, with spill durations of approx. 35 s. Instrumented towers, located downwind of the spill site, gathered data on the temperature, hydrogen concentration and turbulence levels as the hydrogen vapor cloud drifted downwind. Visual phenomena were recorded by motion picture and still cameras. Preliminary results of the experiments indicate that, for rapid spills, thermal and momentum-induced turbulences cause the cloud to disperse to safe concentration levels and become positively buoyant long before mixing due to normal atmospheric turbulence becomes a major factor. An adiabatic mixing model has been developed to deduce hydrogen-air mixture ratios for temperature measurements obtained within the cloud formed by liquid hydrogen spills. The model appears to be a most useful tool for describing the hydrogen concentration within the cloud.     

Numerical Simulation of Enclosure Fires with Horizontal Vents

The buoyancy-induced flow generated by a heat source, such as fire in a long square enclosure with single or multiple horizontal vents, has been of interest in the modeling of enclosure fires and heat removal systems for electronic equipment. This flow is studied using numerical methods. A two-dimensional laminar natural convection flow is investigated with the buoyancy term represented by the Boussinesq approximation. The governing equations are solved in the stream function and vorticity formulation using high accuracy finite difference schemes. The effect of single or multiple horizontal vents of different sizes on the induced flow is studied in detail for different Grashof numbers. The results show a significant change in flow behavior for varying vent width at a fixed Grashof number. A bidirectional flow across the vent occurs due to buoyancy, as previously reported in the literature. The results show that the flow becomes more stable with a decrease in the vent width. The critical Grashof number is identified as beyond which the flow becomes unstable, leading to chaotic flow in the partial enclosure. The main focus is on the time-dependent flow, though steady state flow is also obtained at a longer duration of time in most cases. The implications of these results in the modeling of a fire in an enclosed space with horizontal vents are also discussed.     

A numerical study of the release and dispersion of a buoyant gas in partially confined spaces

Development of the hydrogen economy will require a better understanding of the potential for fires and explosions associated with the unintended release of hydrogen within a structure. The ability to predict the mixing and dispersion behavior of hydrogen, when accidentally released in a partially confined space (e.g. hydrogen leak from automobiles parked in a residential garage) is critical to the safe use of hydrogen products. Hydrogen release and dispersion in a garage can be simulated using computational fluid dynamic (CFD) tools. However, CFD software needs to be validated with experimental data before it can be used reliably for development of codes and standards appropriate for hydrogen fire safety. This paper assesses the capability of a CFD software package to simulate a set of experiments on the mixing and dispersion behavior of highly buoyant gases in a partially confined geometry. Simulation results accurately captured the overall trend measured in experiments conducted in a reduced scale enclosure with idealized leaks. The difference between experimentally measured peak concentrations and numerical simulation results, averaged over various heights was 2.3%. Sensitivity of the computed results on various model parameters was determined. Results indicate that the size of the leak has a small effect on the predicted concentrations, but the location of the leaks in the garage has a very significant effect on the computed results. This result has important implications on future modeling efforts as well as codes and standards related to hydrogen fire safety.     

Hydrogen gas dispersion studies for hydrogen fuel cell vessels I: Vent Mast releases

We report modeling results for hydrogen releases associated with deploying hydrogen fuel cell technology on vessels. This first paper (I) considers hydrogen releases through the vessel Vent Mast from 250-bar hydrogen gas storage tanks, the type of tanks being used for the first hydrogen vessels. A manifolded 10-tank hydrogen storage system, holding 278 kg of hydrogen, can be emptied in ～10 min for maintenance purposes, with a pressure reduction to half the original pressure (125 bar) realized in 2 min if a rapid pressure reduction is needed, for example in the event of a fire. The time profile for filling a tank is also of interest so as not to exceed the tank thermal limits. The calculations show that a manifolded 10-tank array can be filled with hydrogen to 250-bar pressure in ～2 h from a 350-bar hydrogen refueling trailer without exceeding the 85 °C temperature limit typical of Type IV hydrogen tanks.Computational fluid dynamic (CFD) modeling shows that when the hydrogen is released out of the 10-tank array and into the Vent Mast in a 5-mph wind blowing horizontally, the effect of the wind on the hydrogen dispersion strongly depends on the hydrogen exit speed. For high release speeds (～800–900 m/s), the hydrogen flow is strongly momentum-driven, and there is modest cross-wind influence. For low hydrogen exit speeds (～10 m/s), the hydrogen is readily entrained in the wind flow and blown sideways, with the downstream flammable envelope rising at a positive angle to the horizontal due to buoyancy. To capture the influence of a wind with a downward component (e.g., created by a downdraft near a building), a calculation of a low-velocity (8.6 m/s) hydrogen release was performed with a 5-mph wind pointed downward at a 45° angle. The results show that despite the buoyancy of hydrogen, the wind blows the hydrogen substantially downward for low hydrogen speeds exiting the Vent Mast.     

Detached Eddy Simulation of hydrogen turbulent dispersion in nuclear containment compartment using GASFLOW-MPI

During the severe accident in nuclear power plants (NPPs), hydrogen is generated due to the zirconium-water reaction and released from the breaks in coolant pipe forming a locally high concentration hydrogen cloud in the steam generator (SG) compartment, which plays a key role for hydrogen safety analysis in NPPs. Accurate prediction of the turbulent dispersion process of hydrogen-steam gas mixture is a critical topic for a successful simulation of the flammable cloud distribution in SG compartment. In this study, the high-fidelity temporal evolution of the hydrogen turbulent dispersion in a SG compartment is performed using the Detached Eddy Simulation (DES) based on the parallel CFD code GASFLOW-MPI to capture more detailed unsteady turbulent information. Firstly, the newly developed DES turbulence model is validated using two turbulent benchmarks, a backward-facing step turbulent flow and a hydrogen turbulent jet. The simulation results are consistent well with the experimental data. Then a SG compartment model including one steam generator, two coolant pumps, a hot leg and two cold legs is built using the specialized auto-mesh generation module. There are two modes of turbulent dispersion behavior due to the turbulent driven force in the containment, i.e. jet dominated by initial monument and plume dominated by buoyancy. The simulation result shows that the decay rate for centerline velocity obeys $1/z$ law as well as hydrogen volume fraction, indicating a turbulent jet during the steam dominated phase. There is also a relatively long potential core region which could impinge on the bottom concrete floor for the downwards jet. While the hydrogen release transfers from a turbulent jet to a turbulent plume outside the region near the inlet during the hydrogen dominated phase. Different from the turbulent jet, the centerline velocity at the plume region decays with the slope $1/z^{1/3}$, and the decay rate for the centerline hydrogen volume fraction is $1/z^{5/3}$ during this phase. Compared with the jet flow, the potential core region of the plume flow is relatively short, forming a hydrogen cloud near the inlet. The combustibility evaluation shows that the combustion clouds can be generated in the source compartment at the hydrogen dominated phase. However, they will be diluted by the following persistent steam injection from the break. This can provide technical support for the design of hydrogen mitigation system.     

Modelling and numerical simulation of permeated hydrogen dispersion in a garage with adiabatic walls and still air

The dispersion of permeated hydrogen from a storage tank in a typical garage with adiabatic walls and still air is studied analytically and numerically. Numerical simulations are performed based on an original approach of a hydrogen mass source term introduction in the hydrogen conservation equation in control volumes around the tank surface. The maximum hydrogen concentration in an enclosure is always on the top surface of the tank and never reaches 100% by volume. Both the analytical analysis and numerical simulations have demonstrated that diffusion and buoyancy contributions to the hydrogen transport from the tank surface are balanced within 1 min from the start of the process. The quasi-steady state conditions within the enclosure with approximately linear distribution of hydrogen from the top to the bottom are established in about 1 h for both considered permeation rates: 1 NmL/hr/L of tank capacity and 45 NmL/hr/L the last being an equivalent to the SAE J2578 requirements. Finally, the numerical simulations demonstrated that the difference in hydrogen concentration between the garage ceiling and floor is negligible compared to the lower flammability limit of 4% by volume of hydrogen.     

The sensing-based adaptive risk mitigation of leaking hydrogen in a partially open space

This study performs the numerical simulation of hydrogen dispersion in a partially open space. The space under investigation measures 2.9 m × 1.22 m × 0.74 m and a leak flow rate of 2 standard cubic feet per minute is assumed. The effects of various roof vent positions and their areas on the ability to recognize the dispersion and accumulation of hydrogen, and its natural ventilation, are shown and discussed. Based on the results, this paper proposes an innovative approach to the sensing-based adaptive risk mitigation control of hydrogen dispersion and accumulation in a partially open space. By adaptively opening roof vents near the leak source, and closing other neighboring roof vents, concentrated hydrogen is exhausted rapidly and efficiently. It is also shown that cases exist where sufficient area of the proposed adaptive roof vent can be determined by the finite-time sensing of hydrogen concentration near the roof. The effects of the delay time of a sensor, and a method to cancel these effects, are also discussed.     

Analytical and numerical predictions of hydrogen gas flow induced by wall and corner leakages in confined space

This contribution addresses a newly developed semi-analytical model coupling the zone model, virtual point source buoyancy plume theory and mirror theory to predict the gas flow behaviors of leaked hydrogen restricted by a wall or a corner in confined space with an opening. The effects of leaked hydrogen mass flux, opening geometry and the leakage location on interface height, outflow velocity and hydrogen molar fraction in upper layer, were thoroughly investigated at steady stage. A computational fluid dynamics tool, FLACS, was employed to simulate the dispersion process in different leakage scenarios and validate the capability of the derived analytical model. The results show that in all center, wall and corner leakage circumstances, the interface height declines with larger leakage mass flux, whereas the outflow velocity and hydrogen molar fraction change inversely. The interface height, outflow velocity and hydrogen molar fraction are positively, negatively and negatively correlated with the opening dimension, respectively. The opening height plays a more important role in determining the interface height and hydrogen molar fraction but hardly affects the outflow velocity. The interface height keeps unchanged with varying leakage locations when other parameters remain constants. However, according to the mirror theory the outflow velocities in corner and wall leakage conditions are 0.63 and 0.4 times of those in center leakage case. Meanwhile, the hydrogen molar fractions of corner and wall leakages are 1.59 and 2.52 times of the ones in center leakage. All these ratios are validated by the corresponding analytical and numerical predictions. The credibility of the analytical model is verified by the good agreement with the numerical estimations.