LES modelling of hydrogen release and accumulation within a non-ventilated ambient pressure garage using the ADREA-HF CFD code

Computational Fluid Dynamics (CFD) has already proven to be a powerful tool to study the hydrogen dispersion and help in the hydrogen safety assessment. In this work, the Large Eddy Simulation (LES) recently incorporated into the ADREA-HF CFD code is evaluated against the INERIS-6C experiment of hydrogen leakage in a supposed garage, which provides detailed experimental measurements, visualization of the flow and availability of previous CFD results from various institutions (HySafe SBEP-V3). The short-term evolution of the hydrogen concentrations in this confined space is examined and comparison with experimental data is provided, along with comments about the ability of LES to capture the transient phenomena occurring during hydrogen dispersion. The influence of the value of the Smagorinsky constant on the resolved and on the unresolved turbulence is also presented. Furthermore, the renormalization group (RNG) LES methodology is also tested and its behaviour in both highly-turbulent and less-turbulent parts of the flow is highlighted.     

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.     

Design layout of hydrogen research and development garage

Research and development programs toward fuel cells and other hydrogen technologies have increased significantly during the past two decades. These programs require appropriate facilities to undertake the research and development programs. This paper discusses the design layout of one such facility, the “Missouri S&T EcoCAR Hydrogen Vehicle Garage”, which can be used as a model while designing a hydrogen R&D garage. The Missouri S&T EcoCAR garage is a 12.2 m × 7.6 m garage situated at the Missouri University of Science and Technology (Missouri S&T) and serves as the headquarters for the Missouri S&T EcoCAR team. Within the garage, students will gain real-world, hands-on experience by transforming a standard production vehicle into a hydrogen Fuel Cell Plug-in Hybrid Electric Vehicle (FC-PHEV). The garage is classified as a Class 1 Division 2, Group B hazardous location and is equipped to safely test and integrate the vehicle prototype. Specifically, the design includes (i) a hydrogen gas detection system, (ii) hazardous location electrical service, heating, ventilation and air-conditioning, lighting, and compressed air systems, and (iii) emergency backup electric power system with alarms/monitors/security cameras for the hydrogen R&D facility. The garage will be connected to an external backup power supply unit which will be powered by a PEM fuel cell.     

Non-homogeneous hydrogen deflagrations in small scale enclosure. Experimental results

University of Pisa performed hydrogen releases and deflagrations in a 1.14 m³ test facility, which shape and dimensions resemble a gas cabinet. Tests were performed for the HySEA project, founded by the Fuel Cells and Hydrogen 2 Joint Undertaking with the aim to conduct pre-normative research on vented deflagrations in enclosures and containers used for hydrogen energy applications. The test facility, named Small Scale Enclosure (SSE), has a vent area of 0,42 m² which can host different types of vent; plastic sheet and commercial vent were tested. Realistic levels of congestion are obtained placing a number of gas bottles inside the enclosure. Releases are performed from a buffer tank of a known volume filled with hydrogen at a pressure ranging between 15 and 60 bar. Two nozzles of different diameter and three different release directions were tested, being the nozzle placed at a height where in a real application a leak has the highest probability to occur. Three different ignition locations were investigated as well. This paper is aimed to summarize the main features of the experimental campaign as well as to present its results.     

HySafe SBEP-V20: Numerical studies of release experiments inside a naturally ventilated residential garage

This work presents the results of the Standard Benchmark Exercise Problem (SBEP) V20 of Work Package 6 (WP6) of HySafe Network of Excellence (NoE), co-funded by the European Commission, in the frame of evaluating the quality and suitability of codes, models and user practices by comparative assessments of code results. The benchmark problem SBEP-V20 covers release scenarios that were experimentally investigated in the past using helium as a substitute to hydrogen. The aim of the experimental investigations was to determine the ventilation requirements for parking hydrogen fuelled vehicles in residential garages. Helium was released under the vehicle for 2 h with 7.200 l/h flow rate. The leak rate corresponded to a 20% drop of the peak power of a 50 kW fuel cell vehicle. Three double vent garage door geometries are considered in this numerical investigation. In each case the vents are located at the top and bottom of the garage door. The vents vary only in height. In the first case, the height of the vents is 0.063 m, in the second 0.241 m and in the third 0.495 m. Four HySafe partners participated in this benchmark. The following CFD packages with the respective models were applied to simulate the experiments: ADREA-HF using k–? model by partner NCSRD, FLACS using k–? model by partner DNV, FLUENT using k–? model by partner UPM and CFX using laminar and the low-Re number SST model by partner JRC. This study compares the results predicted by the partners to the experimental measurements at four sensor locations inside the garage with an attempt to assess and validate the performance of the different numerical approaches.     

Dispersion and burning behavior of hydrogen released in a full-scale residential garage in the presence and absence of conventional automobiles

Experiments are described in which hydrogen was released at the center of the floor of a real-scale enclosure having dimensions of a typical two-car residential garage. Real-time hydrogen concentrations were monitored at a number of locations. The hydrogen/air mixtures were ignited at pre-determined local volume fractions ranging from 8% to 29%. The combustion behavior and structural effects were monitored using combinations of high-speed pressure transducers and ionization gauges, standard thermocouples, hydrogen sensors, and digital, infrared, and high-speed video cameras. Experiments were performed both for empty garages and garages with conventional automobiles parked above the hydrogen release location.     

Numerical study on the influence of different boundary conditions on the efficiency of hydrogen recombiners inside a car garage

Passive auto-catalytic recombiners (PARs) have the potential to be used in the future for the removal of accidentally released hydrogen inside confined areas. PARs could be operated both as stand-alone or backup safety devices, e.g. in case of active ventilation failure.Recently, computational fluid dynamics (CFD) simulations have been performed in order to demonstrate the principal performance of a PAR during a postulated hydrogen release inside a car garage. This fundamental study has now been extended towards a variation of several boundary conditions including PAR location, hydrogen release scenario, and active venting operation. The goal of this enhanced study is to investigate the sensitivity of the PAR operational behavior for changing boundary conditions, and to support the identification of a suitable PAR positioning strategy. For the simulation of PAR operation, the in-house code REKO-DIREKT has been implemented in the CFD code ANSYS-CFX 15.In a first step, the vertical position of the PAR and the thermal boundary conditions of the garage walls have been modified. In a subsequent step, different hydrogen release modes have been simulated, which result either in a hydrogen-rich layer underneath the ceiling or in a homogeneous hydrogen distribution inside the garage. Furthermore, the interaction of active venting and PAR operation has been investigated.As a result of this parameter study, the optimum PAR location was identified to be close underneath the garage ceiling. In case of active venting failure, the PAR efficiently reduces the flammable gas volume (hydrogen concentration > 4 vol.%) for both stratified and homogeneous distribution. However, the simulations indicate that the simultaneous operation of active venting and PAR may in some cases reduce the overall efficiency of hydrogen removal. Consequently, a well-matched arrangement of both safety systems is required in order to optimize the overall efficiency. The presented CFD-based approach is an appropriate tool to support the assessment of the efficiency of PAR application for plant design and safety considerations with regard to the use of hydrogen in confined areas.     

Large-scale hydrogen release in an isothermal confined area

INERIS has set up large-scale fully instrumented experiments to study the formation of flammable clouds resulting from a finite duration leakage of hydrogen in a quiescent room (80 m³ chamber). Concentration, temperature and mass flow measurements were monitored during the release period and several hours after. Experiments were carried out for mass flow rates ranging from 0.2 g/s to 1 g/s. The instrumentation allowed the observation and quantification of rich hydrogen layers stratification effects. This paper presents both the experimental facility and the test results. These experimental results have been used to assess and benchmark CFD tools capabilities [1].     

An inter-comparison exercise on the capabilities of CFD models to predict the short and long term distribution and mixing of hydrogen in a garage

The paper presents the results of the CFD inter-comparison exercise SBEP-V3, performed within the activity InsHyde, internal project of the HySafe network of excellence, in the framework of evaluating the capability of various CFD tools and modelling approaches in predicting the short and long term mixing and distribution of hydrogen releases in confined spaces. The experiment simulated was INERIS-TEST-6C, performed within the InsHyde project by INERIS, consisting of a 1 g/s vertical hydrogen release for 240 s from an orifice of 20 mm diameter into a rectangular room (garage) of dimensions 3.78 × 7.2 × 2.88 m in width, length and height respectively. Two small openings at the bottom of the front side of the room assured constant pressure conditions. During the test hydrogen concentration time histories were measured at 12 positions in the room, for a period up to 5160 s after the end of release, covering both the release and the subsequent diffusion phases. The benchmark was organized in two phases. The first phase consisted of blind simulations performed prior to the execution of the tests. The second phase consisted of post-calculations performed after the tests were concluded and the experimental results made available. The participation in the benchmark was high: 12 different organizations (2 non-HySafe partners), 10 different CFD codes and 8 different turbulence models. Large variation in predicted results was found in the first phase of the benchmark, between the various modelling approaches. This was attributed mainly to differences in turbulence models and numerical accuracy options (time/space resolution and discretization schemes). During the second phase of the benchmark the variation between predicted results was reduced.     

Helium dispersion following release in a 1/4-scale two-car residential garage

A series of experiments are described in which helium was released at a constant rate into a 1.5 m × 1.5 m × 0.75 m enclosure designed as a 1/4-scale model of a two-car garage. The purpose was to provide reference datasets for testing and validating computational fluid dynamics (CFD) models and to experimentally characterize the effects of a number of variables on the mixing behavior within an enclosure and the exchange of helium with the outside surroundings. Helium was used as a surrogate for hydrogen, and the total volume released was scaled as the amount that would be released by a typical hydrogen-fueled automobile with a full tank. Temporal profiles of helium were measured at seven vertical locations within the enclosure during and following 1-h and 4-h releases. Idealized vents in one wall sized to provide air exchange rates typical of actual garages were used. The effects of vent size, number, and location were investigated using three different vent combinations. The dependence on leak location was considered by releasing helium at three different points within the enclosure.     

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.     

Experimental study of vented hydrogen deflagration with ignition inside and outside the vented volume

Experiments were carried out inside a 25 m³ vented combustion test facility (CVE) with a fixed vent area sealed by a plastic sheet vent. Inside the CVE, a 0.64 m³ open vent box, called RED-CVE was placed. The vent of the RED-CVE was left open and three different vent area were tested. Two different mixing fans, one for each compartment, were used to establish homogeneous H₂ concentrations. This study examined H₂ concentrations in the range between 8.5% vol. to 12.5% vol. and three different ignition locations, (1) far vent ignition, (2) inside the RED-CVE box ignition and (3) near vent ignition (the vent refers to the CVE vent). Peak overpressures generated inside the test facility and the smaller compartment were measured. The results indicate that the near vent ignition generates negligible peak overpressures inside the test facility as compared to those originated by far vent ignition and ignition inside the RED-CVE box. The experiments with far vent ignition showed a pressure increase with increasing hydrogen concentration which reached a peak value at 11% vol. concentration and then decreased showing a non-monotonic behaviour. The overpressure measured inside the RED-CVE was higher when the ignition was outside the box whereas the flame entered the box through the small vent.     

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.     

Hydrogen related risks within a private garage: Concentration measurements in a realistic full scale experimental facility

The next generation of hydrogen energy based vehicles is expected to come into widespread use in the near future. Various topics related to hydrogen including, production, storage, and application of hydrogen as an energy carrier, have become subjects of discussion in the framework of various European and international projects. Safety information is vital to support the successful introduction into mainstream and public acceptance of hydrogen as an energy carrier. One of such issues, which is seeking major attention is related to hydrogen powered vehicles parked inside a confined area (such as in a private garage). It is of utmost importance to predict, if uncontrolled release of hydrogen from a vehicle parked inside a confined area can create an explosive atmosphere. Subsequently, how preventive measures can be implied to control these explosive atmospheres, if present inside a confined area? There is little guidance currently developed for confined areas accommodating hydrogen fuelled vehicles. It is essential that mitigation measures for such conditions become established.     

Experimental studies on wind influence on hydrogen release from low pressure pipelines

At the DIMNP (Department of Mechanical, Nuclear and Production Engineering) laboratories of University of Pisa (Italy) a pilot plant called HPBT (Hydrogen Pipe Break Test) was built in cooperation with the Italian Fire Brigade Department. The apparatus consists of a 12 m³ tank connected with a 50 m long pipe. At the far end of the pipeline a couple of flanges have been used to house a disc with a hole of the defined diameter. The plant has been used to carry out experiments of hydrogen release. During the experimental activity, data have been acquired about the gas concentration and the length of release as function of internal pressure and release hole diameter. The information obtained by the experimental activity will be the basis for the development of a new specific normative framework arranged to prevent fire and applied to hydrogen. This study is focused on hydrogen concentration as function of wind velocity and direction. Experimental data have been compared with theoretical and computer models (such as CFD simulations).     

Safety assessment of unignited hydrogen discharge from onboard storage in garages with low levels of natural ventilation

This study is driven by the need to understand requirements to safe blow-down of hydrogen onboard storage tanks through a pressure relief device (PRD) inside a garage-like enclosure with low natural ventilation. Current composite tanks for high pressure hydrogen storage have been shown to rupture in 3.5–6.5 min in fire conditions. As a result a large PRD venting area is currently used to release hydrogen from the tank before its catastrophic failure. However, even if unignited, the release of hydrogen from such PRDs has been shown in our previous studies to result in unacceptable overpressures within the garage capable of causing major damage and possible collapse of the structure. Thus, to prevent collapse of the garage in the case of a malfunction of the PRD and an unignited hydrogen release there is a clear need to increase blow-down time by reducing PRD venting area. Calculations of PRD diameter to safely blow-down storage tanks with inventories of 1, 5 and 13 kg hydrogen are considered here for a range of garage volumes and natural ventilation expressed in air changes per hour (ACH). The phenomenological model is used to examine the pressure dynamics within a garage with low natural ventilation down to the known minimum of 0.03 ACH. Thus, with moderate hydrogen flow rate from the PRD and small vents providing ventilation of the enclosure there will be only outflow from the garage without any air intake from outside. The PRD diameter, which ensures that the pressure in the garage does not exceed a value of 20 kPa (accepted in this study as a safe overpressure for civil structures) was calculated for varying garage volumes and natural ventilation (ACH). The results are presented in the form of simple to use engineering nomograms. The conclusion is drawn that PRDs currently available for hydrogen-powered vehicles should be redesigned along with either a change of requirements for the fire resistance rating or innovative design of the onboard storage system as hydrogen-powered vehicles are intended for garage parking. Further research is needed to develop safety strategies and engineering solutions to tackle the problem of fire resistance of onboard storage tanks and requirements to PRD performance. Regulation, codes and standards in the field should address this issue.     

Experimental study of ignited unsteady hydrogen releases from a high pressure reservoir

In order to simulate an accidental hydrogen release from the high pressure pipe system of a hydrogen facility a systematic study on the nature of transient hydrogen jets into air and their combustion behavior was performed at the KIT hydrogen test site HYKA. Horizontal unsteady hydrogen jets from a reservoir of 0.37 dm³ with initial pressures of up to 200 bar have been investigated. The hydrogen jets released via round nozzles 3, 4, and 10 mm were ignited with different ignition times and positions. The experiments provide new experimental data on pressure loads and heat releases resulting from the deflagration of hydrogen–air clouds formed by unsteady turbulent hydrogen jets released into a free environment. It is shown that the maximum pressure loads occur for ignition in a narrow position and time window. The possible hazard potential arising from an ignited free transient hydrogen jet is described.     

Numerical simulation of temperature rise within hydrogen vehicle cylinder during refueling

Basing on the Spallart-Allmaras turbulence model and the real gas equation of state, a numerical model is proposed in this paper to study the mechanism of temperature rise within hydrogen vehicle cylinder during refueling. The model is validated by comparing calculated results with experimental data. With the validated model, the effect of mass filling rate, initial pressure within cylinder and ambient temperature on the maximum temperature rise during refueling are investigated. The study shows that the maximum temperature rise increases with the growth in mass filling rate and ambient temperature, while it descends as the initial pressure increases. Finally, an empirical formula is obtained by fitting numerical results and effective methods for temperature control is given.     

Large scale passive ventilation trials of hydrogen

This paper describes the investigation of a passive ventilation solution to manage the hydrogen concentration within a large ullage space (0.9–3 m deep) above a liquid (free surface area of ∼40 m²) containing a hydrogen source. The aim of the ventilation is to maintain the hydrogen concentration within the ullage space below 25% of the Lower Explosive Limit (LEL). The programme of tests involved examination of the ventilation performance in terms of sensitivity to chimney position, hydrogen release rate, hydrogen release point, ullage height and chimney diameter.The tests carried out lasted many hours, and the hydrogen concentration was monitored at a number of points within the ullage space. Pairs of ventilation chimneys with associated instrumentation systems were used to control and monitor the hydrogen concentration within the ullage space.This paper describes the approach to the testing, the results obtained and their analysis.