Safety

In the present work CFD simulations of a hydrogen deflagration experiment are performed. The experiment carried out by KIT was conducted in a 1 m3 enclosure with a square vent of 0.5 m2 located in the center of one of its walls. The enclosure was filled with homogeneous hydrogen-air mixture of 18% v/v before ignition at its back-wall. As the flame propagates away from the ignition point unburned mixture is forced out through the vent. This mixture is ignited when the flame passes through the vent initiating a violent external explosion which leads to a rapid increase in pressure. The work focuses on the modeling of the external explosion phenomenon. A new approach is proposed in order to predict with accuracy the strength of external explosions using Large Eddy Simulation. The new approach introduces new relations to account for the interaction between the turbulence and the flame front. CFD predictions of the pressure inside and outside the enclosure and of the flame front shape are compared against experimental measurements. The comparison indicates a much better performance of the new approach compared to the initial model.

Previous studies have shown that the two-layer model more accurately predicts hydrogen dispersion than the conventional notional nozzle models without significantly increasing the computational expense. However the model was only validated for predicting the concentration distribution and has not been adequately validated for predicting the velocity distributions. In the present study particle imaging velocimetry (PIV) was used to measure the velocity field of an underexpanded hydrogen jet released at 10 bar from a 1.5 mm diameter orifice. The two-layer model was the used to calculate the inlet conditions for a two-dimensional axisymmetric CFD model to simulate the hydrogen jet downstream of the Mach disk. The predicted velocity spreading and centerline decay rates agreed well with the PIV measurements. The predicted concentration distribution was consistent with data from previous planar Rayleigh scattering measurements used to verify the concentration distribution predictions in an earlier study. The jet spreading was also simulated using several widely used notional nozzle models combined with the integral plume model for comparison. These results show that the velocity and concentration distributions are both better predicted by the two-layer model than the notional nozzle models to complement previous studies verifying only the predicted concentration profiles. Thus this study shows that the two-layer model can accurately predict the jet velocity distributions as well as the concentration distributions as verified earlier. Though more validation studies are needed to improve confidence in the model and increase the range of validity the present work indicates that the two-layer model is a promising tool for fast accurate predictions of the flow fields of underexpanded hydrogen jets.

With the development of the hydrogen economy it requires a better understanding of the potential for fires and explosions associated with the unintended release of hydrogen within a partially confined space. In order to mitigate the hydrogen fire and explosion risks effectively accurate predictions of the hydrogen transport and mixing processes are crucial. It is well known that turbulence modelling is one of the key elements for a successful simulation of gas mixing and transport. GASFLOW-MPI is a scalable CFD software solution used to predict fluid dynamics conjugate heat and mass transfer chemical kinetics aerosol transportation and other related phenomena. In order to capture more turbulence information the Large Eddy Simulation (LES) model and LES/RANS hybrid model Detached Eddy Simulation (DES) have been implemented and validated in 3-D CFD code GASFLOW-MPI. The standard Smagorisky SGS model is utilized in LES turbulence model. And the k-epsilon based DES model is employed. This paper assesses the capability of algebraic k-epsilon DES and LES turbulence model to simulate the mixing and transport behavior of highly buoyant gases in a partially confined geometry. Simulation results agree well with the overall trend measured in experiments conducted in a reduced scale enclosure with idealized leaks which shows that all these four turbulent models are validated and suitable for the simulation of light gas behavior. Furthermore the numerical results also indicate that the LES and DES model could be used to analysis the turbulence behavior in the hydrogen safety problems.

The states of Regulations Codes and Standards (RCS) of hydrogen refueling stations (HRSs) in Japan and France are compared and specified items to understand correspondence and differences among each RCSs for realizing harmonization in RCS. Japan has been trying to reform its RCSs to reduce HRS installation and operation costs as a governmental target. Specific crucial regulatory items such as safety distances mitigation means materials for hydrogen storage and certification of anti-explosion proof equipments are compared in order to identify the origins of the current obstacles for disseminating HRS.

It is well-known that deflagration to detonation transition (DDT) may be a significant threat for hydrogen explosions. This paper presents a summary of the work carried out for the development of models in order to enable the industrial computational fluid dynamic (CFD) tool FLACS to provide indications about the possibility of a deflagration-to-detonation transition (DDT). The likelihood of DDT has been expressed in terms of spatial pressure gradients across the flame front. This parameter is able to visualize when the flame front captures the pressure front which is the case in situations when fast deflagrations transition to detonation. Reasonable agreement was obtained with experimental observations in terms of explosion pressures transition times and flame speeds for several practical geometries. The DDT model has also been extended to develop a more meaningful criterion for estimating the likelihood of DDT by comparison of the geometric dimensions with the detonation cell size. The conclusion from simulating these experiments is that the FLACS DPDX criterion seems robust and will generally predict the onset DDTs with reasonable precision including the exact location where DDT may happen. The standard version of FLACS can however not predict the consequences if there is DDT as only deflagration flames are modelled. Based on the methodology described above an approach for predicting detonation flames and explosion loads has been developed. The second part of the paper covers new paradigms associated with risk assessment of a hydrogen infrastructure such as a refueling station. In particular approaches involving one-to-one coupling between CFD and FEA modelling are summarized. The advantages of using such approaches are illustrated. This can have wide-ranging implications on the design of things like protection walls against hydrogen explosions.

In industry handling hydrogen explosion presents a potential danger due to its effects on people and property. In the nuclear industry this explosion which is possible during severe accidents can challenge the reactor containment and it may lead to a release of radioactive materials into the environment. The Three Mile Island accident in the United States in 1979 and more recently the Fukushima accident in Japan have highlighted the importance of this phenomenon for a safe operation of nuclear installations as well as for the accident management.<br/>In 2013 the French Research Agency (ANR) launched the MITHYGENE project with the main aim of improving knowledge on hydrogen risk for the benefit of reactor safety. One of the topics in this project is devoted to the effect of hydrogen explosions on solid structures. In this context CEA conducted a test program with its SSEXHY facility to build a database on deformations of simple structures following an internal hydrogen explosion. Different regimes of explosion propagation have been studied ranging from detonation to slow deflagration. Different targets were tested such as cylinders and plates of variable thickness and diameter. Detailed instrumentation was used to obtain data for the validation of coupled CFD models of combustion and structural dynamics.<br/>This article details the experimental set-up and the results obtained. A companion article focuses on the comparison between these experimental results and the prediction of CFD numerical models

The reproducibility of fire test protocol in the UN Global Technical Regulation on Hydrogen and Fuel Cell Vehicles (GTR#13) is not satisfactory. Results differ from laboratory to laboratory and even at the same laboratory when fires of different heat release (HRR) rate are applied. This is of special importance for fire test of tank without thermally activated pressure relief devise (TPRD) the test requested by firemen. Previously the authors demonstrated a strong dependence of tank fire resistance rating (FRR) i.e. time from fire test initiation to moment of tank rupture on the HRR in a fire. The HRR for complete combustion at the open is a product of heat of combustion and flow rate of a fuel i.e. easy to control in test parameter. It correlates with heat flux to the tank from a fire – the higher HRR the higher heat flux. The control of only temperature underneath a tank in fire test as per the current fire test protocol of UN GTR#13 without controlling HRR of fire source is a reason of poor fire test reproducibility. Indeed a candle flame can easily provide a required by the protocol temperature in points of control but such test arrangements could never lead to tank rupture due to fast heat dissipation from such tiny fire source i.e. insufficient and very localised heat flux to the tank. Fire science requires knowledge of heat flux along with the temperature to characterise fire dynamics. In our study published in 2018 the HRR is suggested as an easy to control parameter to ensure the fire test reproducibility. This study demonstrates that the use of specific heat release rate HRR/A i.e. HRR in a fire source divided by the area of the burner projection A enables testing laboratories to change freely a burner size depending on a tank size without affecting fire test reproducibility. The invariance of FRR at its minimum level with increase of HRR/A above 1 MW/m2 has been discovered first numerically and then confirmed by experiments with different burners and fuels. The validation of computational fluid dynamics (CFD) model against the fire test data is presented. The numerical experiments with localised fires under a vehicle with different HRR/A are performed to understand the necessity of the localised fire test protocol. The understanding of fire test underlying physics will underpin the development of protocol providing test reproducibility.

Hydrogen fuelling in the US is unattended activity although this precedent is not without several challenges that have been addressed in the past decade. This paper provides the recent history and the generic safety case which has established this precedent for hydrogen. The paper also explores the longer history of unattended gasoline fuelling and attempts to help place hydrogen fuelling into the longer history of fuelling personal vehicles.

The Pressure Peaking Phenomena (PPP) is the effect of introducing a light gas into a vented volume of denser gas. This will result in a nonequilibrium pressure as the light gas pushes the dense gas out at the vent. Large scale experiments have been performed to produce relevant evidence. The results were used to validate an analytical model. Pressure and temperature were measured inside a constant volume while the mass flow and vent area were varied. The analytical model was based on the conservation of mass and energy. The results showed that increasing the mass flow rate the peak pressure increases and with increasing the ventilation area the peak pressure decreases. Peak pressure was measured above 45 kPa. Longer combustion time resulted in higher temperatures increasing an underpressure effect. The experimental results showed agreement with the analytical model results. The model predicts the pressures within reasonable limits of+/-2 kPa. The pressure peaking phenomena could be very relevant for hydrogen applications in enclosures with limited ventilation. This could include car garages ship hull compartments as well as compressor shielding. This work shows that the effect can be modeled and results can be used in design to reduce the consequences.

Guidance on Sensor Placement was identified as the top research priority for hydrogen sensors at the 2018 HySafe Research Priority Workshop on hydrogen safety in the category Mitigation Sensors Hazard Prevention and Risk Reduction. This paper discusses the initial steps (Phase 1) to develop such guidance for mechanically ventilated enclosures. This work was initiated as an international collaborative effort to respond to emerging market needs related to the design and deployment equipment for hydrogen infrastructure that is often installed in individual equipment cabinets or ventilated enclosures. The ultimate objective of this effort is to develop guidance for an optimal sensor placement such that when integrated into a facility design and operation will allow earlier detection at lower levels of incipient leaks leading to significant hazard reduction. Reliable and consistent early warning of hydrogen leaks will allow for the risk mitigation by reducing or even eliminating the probability of escalation of small leaks into large and uncontrolled events. To address this issue a study of a real-world mechanically ventilated enclosure containing GH2 equipment was conducted where CFD modelling of the hydrogen dispersion (performed by AVT and UQTR and independently by the JRC) was validated by the NREL Sensor laboratory using a Hydrogen Wide Area Monitor (HyWAM) consisting of a 10-point gas and temperature measurement analyzer. In the release test helium was used as a hydrogen surrogate. Expansion of indoor releases to other larger facilities (including parking structures vehicle maintenance facilities and potentially tunnels) and incorporation into QRA tools such as HyRAM is planned for Phase 2. It is anticipated that results of this work will be used to inform national and international standards such as NFPA 2 Hydrogen Technologies Code Canadian Hydrogen Installation Code (CHIC) and relevant ISO/TC 197 and CEN documents.