The introduction of hydrogen to the commercial market as alterative fuel brings up safety concerns. Its storage in liquid or cryo-compressed state to achieve gravimetric efficiency involves additional risks and their study is crucial. This work aims to investigate the behavior of cryogenic hydrogen release and to study the factors that greatly affect vapor dispersion. We focus on the effect of ambient humidity and air’s components (nitrogen and oxygen) freezing, in order to indentify the conditions under which these factors have considerable influence. The study reveals that humidity reduces conspicuously the longitudinal distance of the Lower Flammability Limit (LFL). 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 much smaller than in hydrogen spills and almost negligible.

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Consequence modelling of high potential risks of usage and transportation of cryogenic liquids yet requires substantial improvements. Among the cryogenics, liquid hydrogen (LH2) needs especial treatments and a comprehensive understanding of spill and spread of liquid and dispersion of vapor. Even though many of recent works have shed lights on various incidents such as spread, dispersion and explosion of the liquid over land, less focus was given on spill and spread of LH2 onto water. The growing trend in ship transportation has enhanced risks such as ships’ accidental releases and terrorist attacks, which may ultimately lead to the release of the cryogenic liquid onto water. The main goal of the current study is to present a computational fluid dynamic (CFD) approach using OpenFOAM to model release and spread of LH2 over water substrate, and discuss previous approaches. It also includes empirical heat transfer equations due to boiling, and computation of evaporation rate through an energy balance. The results of the proposed model will be potentially used within another coupled model that predicts gas dispersion [1]. This work presents a good practice approach to treat pool dynamics and appropriate correlations to identify heat flux from different sources. Furthermore, some of the previous numerical approaches to redistribute, or in some extend, manipulate the LH2 pool dynamic are brought up for discussion, and their pros and cons are explained. In the end, the proposed model is validated by modelling LH2 spill experiment carried out in 1994 at the Research Centre Juelich in Germany [2,3].

As hydrogen-air mixtures are flammable in a wide range of concentrations and the minimum ignition energy is low compared to hydrocarbon fuels, the safe handling of hydrogen is of utmost importance. Additional hazards may arise with the inadvertent spill of liquid hydrogen. An accidental release of LH2 leads to a formation of a cryogenic pool, a dynamic vaporization process, and consequently a dispersion of gaseous hydrogen into the environment. Several LH2 release experiments as well as modeling approaches address this phenomenology. Different model approaches have been validated in the past against the existing experimental data. These models can be divided into two sections:

1. Models calculating cryogenic pool propagation and vaporization rates,

2. Models calculating gaseous hydrogen dispersion using pre-calculated evaporation rates and pool surface areas as source term.

This leads to uncertainties if LH2 pool models lack relevant processes for vaporization, and in the gas distribution models the source term represents only an approximation of the real source term. At Forschungszentrum Jülich, a transient 3D multicomponent-multiphase model has been developed, using the commercial code ANSYS CFX 15.0 and including the additional sub-models of the rates of vaporization on solid ground, volume vaporization, humidity, and the influence of changing wind conditions. This new modeling approach is capable to simulate the release of LH2, its spreading and vaporization, and the gas distribution in the atmosphere under realistic environmental conditions (e.g. humidity and changing wind conditions). The model has been validated against recent LH2 spill experiments conducted by HSL and the NASA.

We have analyzed vacuum insulation failure in an automotive cryogenic pressure vessel (also known as cryo-compressed vessel) storing hydrogen (H2). Vacuum insulation failure increases heat transfer into cryogenic vessels by about a factor of 100, potentially leading to rapid pressurization and venting to avoid exceeding maximum allowable working pressure (MAWP). H2 release to the environment may be dangerous if the vehicle is located in a closed space (e.g. a garage or tunnel) at the moment of insulation failure. We therefore consider utilization of the hydrogen in the vehicle fuel cell and electricity dissipation through operation of vehicle accessories or battery charging as an alternative to releasing hydrogen to the environment. We consider two strategies: initiating hydrogen extraction immediately after vacuum insulation failure, or waiting until MAWP is reached before extraction. The results indicate that cryogenic pressure vessels have thermodynamic advantages that enable slowing down hydrogen release to moderate levels that can be consumed in the fuel cell and dissipated onboard the vehicle, even in the worst case when the vacuum fails with a vessel storing hydrogen at maximum refuel density (70 g/L at 300 bar). The two proposed strategies are therefore feasible and the best alternative can be chosen based on economic and/or implementation constraints.

Laboratory measurements were made on the concentration and temperature fields of cryogenic hydrogen jets. Images of spontaneous Raman scattering from a pulsed planar laser sheet were used to measure the concentration and temperature fields from varied releases. Jets with up to 5 bar pressure, with near-liquid temperatures at the release point, were characterized in this work. This data is relevant for characterizing unintended leaks from piping connected to cryogenic hydrogen storage tanks, such as might be encountered at a hydrogen fuel cell vehicle fueling station. The average centerline mass fraction was observed to decay at a rate similar to room temperature hydrogen jets, while the half-width of the Gaussian profiles of mass fraction were observed to spread more slowly than for room temperature hydrogen. This suggests that the mixing and models for cryogenic hydrogen may be different than for room temperature hydrogen. Results from this work were also compared to a one-dimensional (streamwise) model. Good agreement was seen in terms of temperature and mass fraction. In subsequent work, a validated version of this model will be exercised to quantitatively assess the risk at hydrogen fueling stations with cryogenic hydrogen on-site.

Liquid hydrogen (LH2) storage is a viable approach to assuring sufficient hydrogen capacity at commercial fuelling stations. Presently, LH2 is produced at remote facilities and then transported to the end-use site by road vehicles (i.e., LH2 tanker trucks). Venting of hydrogen to depressurize the transport storage tank is a routine part of the LH2 delivery process. However, the behaviour of cold hydrogen plumes has not been well characterized because empirical field data are essentially non-existent. The National Fire Protection Association (NFPA) Standard 2 Hydrogen Storage Safety Task Group, which consists of hydrogen producers, safety experts, and computational fluid dynamics modellers, has identified the lack of understanding of hydrogen dispersion during LH2 venting of storage vessels as a critical gap for establishing safety distances at LH2 facilities, especially commercial hydrogen fuelling stations. To address this need, the National Renewable Energy Laboratory sensor laboratory, in collaboration with the NFPA 2 Safety Task Group, developed the Cold Hydrogen Plume Analyzer to empirically characterize the hydrogen plume formed during LH2 storage tank venting. A prototype analyzer was developed and field deployed at an actual LH2 venting operation. Critical findings included:

• Hydrogen (H2) was detected as much as 2 m lower than the release point, which is not predicted by existing models.

• A small and inconsistent correlation was found between oxygen depletion and the hydrogen concentration.

• A negligible to non-existent correlation was found between in-situ temperature and the hydrogen concentration.

The analyzer is currently being upgraded for enhanced metrological capabilities, including improved realtime spatial and temporal profiling of the plume and tracking of prevailing weather conditions. Additional deployments are planned to monitor plume behaviour under different wind, humidity, and temperature conditions. The data will be shared with the NFPA 2 Safety Task Group and ultimately will be used support theoretical models and code requirements prescribed in NFPA 2.

The thermal hazards from ignited under-expanded cryogenic releases are not yet fully understood and reliable predictive tools are missing. This study aims at validation of a CFD model to simulate flame length and radiative heat flux for cryogenic hydrogen jet fires. The simulation results are compared against the experimental data by Sandia National Laboratories on cryogenic hydrogen fires from storage with pressure up to 5 bar abs and temperature in the range 48-82 K. The release source is modelled using the Ulster’s notional nozzle theory. The problem is considered as steady-state. Three turbulence models were applied and their performance compared. The realizable κ-ε model demonstrated the best performance in reproduction of experimental flame length and radiative heat flux. Therefore, it has been employed in the CFD model along with Eddy Dissipation Concept for combustion and Discrete Ordinates (DO) model for radiation. A parametric study has been conducted to assess the effect of selected numerical and physical parameters on the simulations capability to reproduce experimental data. DO model discretization is shown to strongly affect simulations, indicating 10x10 as minimum number of angular divisions to provide a convergence. The simulations have shown sensitivity to experimental parameters such as humidity and exhaust system volumetric flow rate, highlighting the importance of accurate and extended publication of experimental data to conduct precise numerical studies. The simulations correctly reproduced the radiative heat flux from cryogenic hydrogen jet fire at different locations.

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. The hazards associated with jet releases from accidental leaks in a vehicle-refuelling environment must be considered if hydrogen is stored and used as a high-pressure gas since a jet release can result in a fire or explosion. This paper describes the work done by us in modelling some of the consequences of accidental releases of hydrogen, implemented in our Fire Explosion Release Dispersion (FRED) software. The new dispersion model is validated against experimental data available in the open literature. The model predictions of hydrogen gas concentration as a function of distance are in good agreement with experiments. In addition, FRED has been used to model the consequence of the bursting of a vessel containing compressed hydrogen. The results obtained from FRED, i.e. overpressure as a function of distance, match well in comparison to experiments. Overall, it is concluded that FRED can model the consequences of an accidental release of hydrogen and the blast waves generated from bursting of vessel containing compressed hydrogen.

At least once, air filling a piping from main hydrogen pipe line to an individual home end should be replaced with hydrogen gas to use the gas in the home. Special attention is required to complete the replacing operation safely, because air and supplied hydrogen may generate flammable/explosive gas mixture in the piping. The most probable method to fulfill the task is that, at first an inert gas is used to purge air from the piping, then hydrogen will be supplied into the piping. It is easily understood that the amount of the inert gas consumed by this method is much to purge whole air, especially in long piping system. Hence, to achieve more economical efficiency, an alternative method was considered. In this method, previously injected nitrogen between air and hydrogen prevents them from mixing. The key point is that how much the gas is required to prevent mixing and keep the condition in the piping safe. The authors investigated to find the minimum amount of the inert gas required to keep the replacing operation safe. The main objective of this study is to assess the effect of the nitrogen and estimate a pipe length that the safety is maintained under various conditions by using computational fluid dynamic (CFD). The effects of the amount of the injected nitrogen, the hydrogen-supply conditions and the structure of piping system are discussed.