A numerical study of hydrogen leakage and diffusion in a hydrogen refueling station

Studies focused on the behavior of the hydrogen leakage and diffusion are of great importance for facilitating the large scale application of the hydrogen energy. In this paper, the hydrogen leakage and diffusion in six scenarios which including comparison of different leakage position and different wind effect are analyzed numerically. The studied geometry is derived from the hydrogen refueling station in China. Due to the high pressure in hydrogen storage take, the hydrogen leakage is momentum dominated. The hydrogen volume concentration with the variation of the leakage time in different scenarios is plotted. More importantly, profiles of the flammable gas cloud at the end of the leakage are quantitatively studied. Results indicate that a more narrow space between the leakage hole and the obstacle and a smaller contact area with the obstacle make the profile of the flammable gas cloud more irregular and unpredictable. In addition, results highlight the wind effect on the hydrogen leakage and diffusion. Comparing with scenario which the wind direction consistent with the leakage direction, the opposite wind direction may result in a larger profile of the flammable gas cloud. With wind velocity increasing, the profile of the flammable gas cloud is confined in a smaller range. However, the presence of the wind facilitates the form of the recirculation zone near the obstacle. With an increase of the wind velocity, the recirculation zone moves downward along the obstacle. Thus, the hydrogen accumulation is more prominent near the obstacle.     

Simulation and risk assessment of hydrogen leakage in hydrogen production container

In this paper, the hydrogen leakage and diffusion characteristics analysis and risk assessment are carried out on the container where a 2 Nm³/h alkaline hydrogen production device is located. Firstly, the transient and steady process of hydrogen leakage from hydrogen production container is analyzed. Secondly, the dynamic balance of combustible hydrogen cloud is analyzed, the concept of critical ventilation flow is put forward. It was found that in order to reduce the flammable volume by 85%, the installed ventilation can only cover the leakage flow of 1.0 Nm³/min. Finally, TNT equivalent method is used to evaluate the hazard degree of hydrogen leakage. It is found that the existed hydrogen production container ventilation device can only exhaust the hydrogen-air cloud with small flow leakage, while the accumulation of gas cloud still exists in large flow leakage. Under the critical ventilation flow, the minor injury radius can be reduced from 4.8 m to 2.78 m. The effect of critical ventilation flow was verified.     

Study on leakage and explosion consequence for hydrogen blended natural gas in urban distribution networks

It appears to be the most economical means of transporting large quantities of hydrogen over great distances by the existing natural gas pipeline network. However, the leakage and diffusion behavior of urban hydrogen blended natural gas and the evolution law of explosion characteristics are still unclear. In this work, a Computational Fluid Dynamics three-dimensional simulation model of semi-confined space in urban streets is developed to study the diffusion process and explosion characteristics of hydrogen-blended natural gas. The influence mechanism of hydrogen blending ratio and ambient wind speed on the consequences of explosion accident is analyzed. And the dangerous area with different environmental wind effects is determined through comparative analysis based on the most dangerous scenarios. Results indicate that the traffic flow changes the diffusion path of the jet, the flammable gas cloud forms a complex profile in many obstacles, high congestion level lead to more serious explosion accidents. Wind effect keeps the flammable gas cloud near the vehicle flow, the narrow gaps between the vehicles aggravate the expansion of the flammable gas cloud. When the wind direction is consistent with the leakage direction, hydrogen blended natural gas is gathered in the recirculation zone due to the vortex effect, which results in more serious accident consequences. With the increase in hydrogen blending ratio, the higher content of H and OH in the gas mixture significantly increases the premixed burning rate, the maximum overpressure rises rapidly when the hydrogen blend level increases beyond 40%. The results can provide a basis for construction safety design, risk assessment of leakage and explosion hazards, and emergency response in hydrogen blended natural gas distribution systems.     

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.     

Surface effects on flammable extent of hydrogen and methane jets

Studies on the effect of surfaces on the extent of the flammable cloud of high-pressure horizontal and vertical jets of hydrogen and methane are performed using CFD numerical simulations. For the horizontal jets, two scenarios pertaining to the location of the surface are studied: horizontal surface (the ground), and vertical surface (side wall). For a constant flow rate release, the extent of the flammable cloud is determined as a function of time. Effects of the proximity of the surface on the flammable extent along the axis of the jet and its impact on the maximum extent of the flammable cloud is explored and compared for both hydrogen and methane. The results are also compared to the predictions of the Birch correlations for flammable extents. It is found that the presence of a surface and its proximity to the jet centerline result in a pronounced increase in the extent of the flammable cloud compared to a free jet.     

Dilution of flammable vapor cloud formed by liquid hydrogen spill

The flammable vapor cloud formed by liquid hydrogen spill can severely threaten the safety of life and property, which is one of the primary concerns during handling liquid hydrogen. Mixture model and Realizable k-ε model are adopted in ANSYS Fluent to predict the two-phase flow of liquid hydrogen spill. Dilution of the hydrogen vapor cloud formed by liquid hydrogen spill is analyzed, including the turbulent disturbance during cloud dispersion, and the cloud dilution characteristics especially its behavior under different wind speeds. The results show that the turbulent disturbance in the upper part of flammable vapor cloud can be 7.21 times of that in the background atmosphere, due to the mixing of the vapor cloud with the incoming wind and the upper air. The dilution time, needed for the vapor cloud to be diluted below the lower flammable limit since the end of the spill, increases firstly and then turns to decrease with the increasing wind speed. The atmospheric turbulence, the turbulent disturbance induced by the upward movement of vapor cloud, and the cloud frontal area all affect the dilution behavior.     

Study on vapour dispersion and explosion from compressed hydrogen spill: Risk assessment on a hydrogen plant

Hydrogen produced from renewable resources is one of the cleanest fuels and could be used to store intermittent solar, wind and other energies. The main concern about using hydrogen is its hazards, such as high storage pressure, wide-range flammability, low mass density, and high diffusion. This study investigated the hazards of compressed hydrogen storage by developing a CFD model to understand the gas dispersion behaviour. The model was validated using the past experimental data and showed a good agreement, which could demonstrate the diffusion characteristics and gas stratification of a buoyant gas. A case study of an accidental release of compressed hydrogen from a storage tank was investigated to evaluate the risk of a hydrogen plant. A mathematical model of the jet spill was used to account for the choking effect from a high-pressure release to ensure the input velocity in CFD simulation is suitable for modelling gas dispersion using verified spatial and temporal scales, then the simulation results were used as inputs of vapour cloud explosions (VCEs) to investigate the potential overpressure effect. It was found the CFD model could predict a more reasonable flammable gas amount in cloud than using the bulk hydrogen release rate. The safety distance based on the overpressure prediction was reduced by 35%. The method proposed in this study can provide more validity for the consequence analysis as part of risk assessment.     

Effect of wind condition on unintended hydrogen release in a hydrogen refueling station

Ambient condition, especially the wind condition, is an important factor to determine the behavior of hydrogen diffusion during hydrogen release. However, only few studies aim at the quantitative study of the hydrogen diffusion in a wind-exist condition. And very little researches aiming at the variable wind condition have been done. In this paper, the hydrogen diffusion in different wind condition which including the constant wind velocity and the variable wind velocity is investigated numerically. When considering the variable wind velocity, the UDF (user defined function) is compiled. Characteristics of the FGC (flammable gas cloud) and the HMF (hydrogen mass fraction) are analyzed in different wind condition and comparisons are made with the no-wind condition. Results indicate that the constant wind velocity and the variable wind velocity have totally different effect for the determination of hydrogen diffusion. Comparisons between the constant wind velocity and the variable wind velocity indicate that the variable wind velocity may cause a more dangerous situation since there has a larger FGC volume. More importantly, the wind condition has a non-negligible effect when considering the HMF along the radial direction. As the wind velocity increases, the distribution of the HMF along the radial direction is not Gaussian anymore when the distance between the release hole and the observation line exceeds to a critical value. This work can be a supplement of the research on the hydrogen release and diffusion and a valuable reference for the researchers.     

Numerical study of hydrogen dispersion in a fuel cell vehicle under the effect of ambient wind

In the rescue of hydrogen-fueled vehicle accidents, once accidental leakage occurs and hydrogen enters the cabin, the relatively closed environment of the vehicle is prone to hydrogen accumulation. Excessive hydrogen concentration inside the vehicle cabin may cause suffocation death of injured passengers and rescue crews, or explosion risk. Based on hydrogen fuel cell vehicle (HFCV) with hydrogen storage pressure 70 MPa, four different scenarios (i. with opened sunroof, ii. opened door windows, iii. opened sunroof and door windows and iv. opened sunroof, door windows and rear windshield) under the condition of accidental leakage were simulated using computational fluid dynamics (CFD) tools. The hydrogen concentration inside the vehicle and the distribution of flammable area (>4% hydrogen mole fraction) were analyzed, considering the effect of ambient wind. The results show that in the case of convection between interior and exterior of the vehicle via the sunroof, door windows or rear windshield, the distribution of hydrogen inside the vehicle is strongly affected by the ambient wind speed. In the least risk case, ambient wind can reduce the hydrogen mole fraction in the front of the vehicle to less than 4%, however the rear of the vehicle is always within flammable risk.     

Numerical investigation of flammable cloud on liquid hydrogen spill under various weather conditions

The investigations of hydrogen leakage during the latest decades, have developed our knowledge level. However, few studies concerned the whole dispersion history of the flammable cloud (from generating to disappearing in the atmosphere). A Non-Homogeneous Equilibrium Model (NHEM) was used, and validated by a large scale LH₂ spill experiment. The predicted data displayed good agreement with the experiment. Moreover, the experiment was further investigated on the colourless flammable cloud. Three primary questions of the hydrogen dispersion process were concerned: the maximum spreading range, the minimum distance above the ground, and the duration time of the flammable cloud in the atmosphere. Three major influence factors were selected to simulate various weather conditions, including ambient temperature (coupled with ground temperature), wind speed and atmospheric pressure. The hydrogen dispersion can be excited with the increased wind speed, and be impeded with the increased atmospheric pressure. The hydrogen dispersion process in four seasons of a year appears a different trend.     

Explosion-prevention strategies of airflow controlling and closed-inerting for hydrogen dilution in utility tunnel

Under the large-scale application of hydrogen energy and the acceleration of utility tunnel construction in China, the hydrogen pipeline would enter the utility tunnel soon. The explosion-prevention strategy is of important to hydrogen transportation in utility tunnel. Thus, a series of comparative studies were carried out in the present study, to explore the high efficiency dilution methods for unexpected hydrogen leak scenarios. (1) Effects of source type and domain extension were further validated based on our previous numerical model. The small-hole model was found to be simple and feasible for pipe release simulation, and has the same high accuracy like the notional nozzle model. The influence of unexpanded domain is acceptable. (2) Investigation of the explosion-prevention strategies was performed based on the finding of the worst leakage case, and hydrogen leaks at the middle of the upwind tunnel with upward leak direction was found to be the worst leakage case for the normal utility tunnel. (3) Compared with the layouts of lateral vents and the single horizontal air distributor, the middle exhaust vent was found to be the optimal airflow controlling strategy. (4) Furtherly, compared to the optimal airflow controlling strategy, closed-inerting was validated to be the most efficient dilution method. In the closed-inerting cases, the peak value of the flammable gas volume which is only ∼40% of the unimproved case appears. After that, the flammable cloud volume decays gradually, while the value increases steadily in the initial unimproved case. No obvious difference was found between the N₂ and CO₂ injection cases.     

Investigation of the hazardous area in a liquid hydrogen release with or without fence

Constructing a fence for potential liquid hydrogen leakage is a common protective measure to prevent the unrestricted spread, and critically evaluating the effects of a fence is essential for the safe use of hydrogen energy. Based on NASA's large-scale experiment of liquid hydrogen release, a 3D numerical model considering the existence of fence is proposed to study the dynamic spread behaviors of hydrogen flammable cloud and liquid hydrogen pool. The results show that the liquid hydrogen pool keeps a downwind radius of about 1.75 m most of the time during the release, and the hydrogen flammable cloud has a limit spread distance. Three cases are set to comparatively evaluate the effect of the fence, which are full fence, downwind half fence and no fence. Under the three cases, liquid hydrogen pools all evaporate completely within 5 s. Full fence scenario has the modest spread scale of hydrogen flammable cloud in the atmosphere but the longest duration for 86 s, and the upwind fence causes an easily overlooked hazardous region due to the vortex here, thus constructing a full fence is not an ideal strategy when the spread direction is constant. Just constructing a downwind half fence leads to the greatest spread distance of the flammable hydrogen cloud, which are 33.6 m and 68 m in vertical and downwind directions. No fence case corresponds to the smallest diffusion scale of flammable hydrogen cloud in atmosphere but the longest near-ground spread distance for 43.35 m. Four stages can be divided to summarize the entire life period of the flammable cloud for the cases with and without a fence, while three stages for the case with a downwind half fence.     

Assessment of an accidental hydrogen leak from a vehicle tank in a confined space

This study aims to characterize some safety aspects by examining the geometries of the infrastructure, currently used by societies, against the accumulation of hazardous hydrogen clouds during an accidental leak in areas with limited ventilation. Using ANSYS FLUENT as a modeling tool, the influence of garage roof shape; pyramidal and domed roof compared with the basic model (flat roof), for different leak times, on dispersion and stratification of hydrogen layers, is analyzed. As a result, the domed roof promotes to have a lower hydrogen concentration and presents two remarkable peaks of the Richardson number (Ri) with the highest value more than 2 × 10⁵, which is three times higher than the flat roof. Besides, the influence of the leak time on the dynamic of the flow, concentration, and stratification process are observed: the mole fraction of hydrogen is more than 0.25 after 1 h of leak, whereas it is lower than 0.05 after 100 s. The volume flow and therefore the flammable volume increase. This study highlights the importance of geometrical and sizing parameters on the characteristics of hydrogen leaks and subsequently gives insights to establish performance standards for the availability and reliability of safety critical systems.     

Modeling the development of hydrogen vapor cloud considering the presence of air humidity

A three-dimensional CFD model for large-scale liquid hydrogen spills is developed and validated by the experiments carried out by NASA. The effect of humidity on the development of hydrogen vapor cloud is emphasized, with the modified expressions of Lee model accounting for the phase changes of water and hydrogen. The results show that the numerical prediction is more consistent with the experiment considering the presence of air humidity. The condensation of water in the atmosphere increases the buoyancy of the vapor cloud, and promotes the diffusion of the cloud in vertical direction. The dimension of the cloud in streamwise direction changes little under different humidity, due to the balance between the height-dependent wind speed and the induced buoyancy. The scope of visible cloud indicated by the condensed water vapor expands with the increasing air humidity, and still lies within the flammable domain when the relative humidity approaching to 75%. Water vapor condensation induces the cloud temperature rise under the same concentration, and the leeward part is more influenced compared with the upwind part.     

A CFD simulation of hydrogen dispersion for the hydrogen leakage from a fuel cell vehicle in an underground parking garage

In the present study, the dispersion process of hydrogen leaking from an FCV (Fuel Cell Vehicle) in an underground parking garage is analyzed with numerical simulations in order to assess hazards and associated risks of a leakage accident. The temporal and spatial evolution of the hydrogen concentration as well as the flammable region in the parking garage was predicted numerically. The volume of the flammable region shows a non-linear growth in time with a latency period. The effects of the leakage flow rate and an additional ventilation fan were investigated to evaluate the ventilation performance to relieve accumulation of the hydrogen gas. It is found that expansion of the flammable region is delayed by the fan via enhanced mixing near the boundary of the flammable region. The present numerical results can be useful to analyze safety issues in automotive applications of hydrogen.     

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.     

Numerical modelling of release of subsonic and sonic hydrogen jets

For the general public to use hydrogen as a vehicle fuel, they must be able to handle hydrogen with the same degree of confidence as conventional liquid and gaseous fuels. For refuelling hydrogen cars, hydrogen is stored at high pressures up to 700 bar. The hazards associated with jet releases from accidental leaks of such highly pressurized storage must be considered since a jet release and dispersion can result in a fire or explosion. Therefore, it is essential to understand the dispersion characteristics of hydrogen to determine the extent of the flammable cloud when released from a high-pressure source. These parameters are very important in the establishment of the safety distances and sizes of hazardous zones. This paper describes the work done by us in modelling of dispersion of accidental releases of hydrogen, using the FRED (Fire Explosion Release Dispersion) software. The dispersion module in FRED is validated against experimental data available in the open literature for steady release and dispersion of cold and ambient hydrogen gas. The validation is performed for a wide range of hole sizes (0.5–4 mm), pressure (1.7–400 bar) and temperature (50–298 K).The model predictions of hydrogen gas jet velocity, concentration decay as a function of distance as well as radial concentration distribution are in good agreement with experiments. Overall, it is concluded that FRED can accurately model accidental release and dispersion of hydrogen in unconfined and open configurations.     

Visualization of hydrogen jet using deformation of the laser beam profile

Hydrogen has the widest flammable range, the fastest flame propagation speed, and the lowest ignition energy, so its safety needs special attention before the wide application of hydrogen energy. The main objective of this work is to propose a new method to evaluate hydrogen jet pressure by using a Helium–Neon laser through the jet. A mathematical model was proposed, which describes the deformation of the laser beam profile passing through an axisymmetric circular hydrogen jet pressure flow field in detail. This research attempts to apply the expression of density Gaussian distribution, ideal gas equation, and Gladstone Dale equation to disclose the deformation of laser beam profile under different outlet conditions. The experimental uncertainty is about 3 × 10^?3. A non-contact optical experimental system is established to visually measure the density gradient distribution of the gas jet. Our findings show that the hydrogen jet can be regarded as a gas-phase lens, and the deformation of the laser beam profile in the horizontal direction increases linearly with jet pressure. Finally, the preliminary results of calculations of the spot area with the theoretical model were presented and compared with the images of the laser beam profile passing through the jet in the experiment. The theoretical model gave similar results and the overall agreement with the experiment was satisfactory. Our technology exhibits high sensitivity in the measurement of hydrogen leakage pressure, providing a theoretical basis for non-contact, non-damaged, high-response, and high-sensitivity detection of hydrogen leakage.     

Numerical investigation of the flammable extent of semi-confined hydrogen and methane jets

The effect of surfaces on the extent of high pressure horizontal unignited jets of hydrogen and methane is studied using computer fluid dynamics simulations performed with FLACS Hydrogen. Results for constant flow rate through a 6.35 mm diameter pressure relief Device (PRD) orifice from 100 barg, 250 barg, 400 barg, 550 barg and 700 barg compressed gas systems are presented for both horizontal hydrogen and methane jets. To quantify the effect of a horizontal surface on the jet, the jet exit is positioned at various heights above the ground ranging from 0.1 m to 10 m. Free jet simulations are performed for comparison purposes. Also, for cross-validation purposes, a number of cases for 100 barg releases were simulated using proprietary models developed for hydrogen within commercial CFD software PHOENICS. It is found that the presence of a surface and its proximity to the jet centreline result in a pronounced increase in the extent of the flammable cloud compared to a free jet.     

A numerical simulation of the suppression of hydrogen jet fires on hydrogen fuel cell ships using a fine water mist

In this paper, in order to evaluate the reliability of a fine water mist for the suppression of fires on hydrogen fuel cell ships, the fire dynamics simulator (FDS) software was used to simulate the jet fire process and the action of a fine water mist on a fire caused by a hydrogen leakage in the hydrogen storage tank areas of hydrogen fuel cell ships. The fire scenario was classified into vertical or horizontal jet fires according to the location of the leakage in the hydrogen storage tank area, and the suppression effects of a fine water mist on hydrogen jet fires under a different droplet size, spray velocity, and ambient wind speed were compared and analyzed. The results indicate that a fine water mist is not effective in extinguishing hydrogen jet fires; however, by selecting suitable parameters (a spray velocity of 30 m/s and average droplet size of 30 μm), it can effectively reduce the fire field temperature of hydrogen jet fires and prevent the fire from developing further. Increasing the average droplet size of the fine water mist results in a gradual degradation of the suppression effect, while a higher spray velocity of the mist enhances the suppression effect to a certain extent. The ambient wind speed is an important factor that influences the suppression effect of a fine water mist on hydrogen jet fires, and when this speed is less than 4 m/s, a fine water mist with a higher spray velocity and smaller average droplet size is still a superior way of suppressing fires.