Hydrogen gas leaking from a hydrogen-powered vehicle in a residential garage may form a flammable mixture with air. Passive, buoyancy-driven ventilation is one approach to limiting the concentration to a safe level. We explored the relationship between leak rate, ventilation design, and hydrogen concentration through laboratory testing, an algebraic analysis, and CFD modeling. We used helium to test slow, steady, low-velocity leaks in a full-scale test room under nearly isothermal, steady conditions, and we report the results in sufficient detail that other modelers can use them. The results show the importance and variability of stratification. Our algebraic and CFD models agree very well with the experimental results. We describe our CFD approach in sufficient detail for use by others. We tested under nearly isothermal conditions, but also discuss indoor-outdoor temperature difference as an important risk factor. Information about realistic leakage scenarios is needed to apply these results as safety recommendations. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Evaluation of the hydrogen-oxygen global reaction rate was conducted to predict auto-ignition onset and delay. For this purpose, a global reaction rate was incorporated into a CFD simulation of hot hydrogen that flows in a laminar regime, in a simple geometry of a cylindrical enclosure where ambient air prevails. Auto-ignition was obtained for different inflow temperatures at atmospheric pressure. The results agree with the classical results of H-2-O-2 explosion limits, referred to as the "peninsula". Ignition onset was estimated at different pressures within the thermal branch of the peninsula (third limit). Results show that the current global reaction rate adequately represents ignition onset dependency on temperature and equivalence ratio, whilst dependency on pressure is poor. The ignition delay decreases with the increase in temperature; however, at higher temperatures (1000-1200 K), it remains almost constant. This may be explained by the fact that a non-premixed mixture is being examined, in which minimal time is needed to form the required conditions for ignition temperature and fuel to oxidizer ratio (FOR). The present study focuses on the transient hydrogen dispersion and examines the flammability limits of non-premixed H-2 air mixtures with no external ignition energy. Transient flows, that are often the subject of safety-related scenarios, are characterized by a rapid formulation of flammability zones. The present work allows the prediction of ignition zones directly, rather than predicting them by combining dispersion maps and flammability limits maps. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Computational Fluid Dynamics (CFD) simulations of hydrogen dispersion and combustion in accident scenarios in a mock-up liquid hydrogen (LH(2)) refuelling station have been performed with the codes ADREA-HF and REACFLOW. The assumed accident scenarios are caused by a hose break during LH(2) car refuelling. Two main configurations have been taken into account: a mitigated case and a non-mitigated case. For each configuration, 5 different ambient conditions are assumed: a case with no wind at all and four cases with a 5 m/s wind speed, coming from the south, north, east and west direction. The numerical simulations show the effect of the wind on the dispersion and explosion of the hydrogen flammable cloud. The effect of ignition position has also been investigated. The expectation that the wind plays a positive role in increasing the dispersion of the flammable cloud and therefore in reducing the overpressures in case of explosions does not hold true for all wind directions. Depending on the wind direction and on the station layout, the wind can have the effect of trapping the flammable cloud with more reactive concentrations and higher level of turbulence within the station, generating a similar level of overpressure as in the stagnant case although the amount of flammable mixture is much smaller than in the stagnant case. (C) 2008 Elsevier Ltd. All rights reserved.

In the frame of the European Commission co-funded Network of Excellence HySafe (Hydrogen Safety as an Energy Carrier, www.hysafe.org), five organizations with significant experience in explosion modelling have performed numerical simulations of explosions of stoichiometric hydrogen-air mixtures in a 78.5 m long tunnel. The five organizations are the Karlsruhe Research Centre, GexCon AS, the joint Research Centre, the Kurchatov institute Research Centre and the University of Ulster. Five CFD (Computational Fluid Dynamics) codes with different turbulence and combustion models have been used in this Standard Benchmark Exercise Problem (SBEP). Since tunnels are semi-confined environments, hydrogen explosions in tunnels can potentially be critical accident scenarios from the point of view of the accident consequences and CFD methods are increasingly employed to assess explosions hazards in tunnels. The objective of the validation exercise is to assess the accuracy of the theoretical and numerical models by comparisons of the simulation results with the experimental data. A very good agreement between experiments and simulations was found in terms of maximum overpressures. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

The comparison between experimental data and simulation results of hydrogen explosions in a vented vessel is described in the paper The validation exercise was performed in the frame of the European Commission co funded Network of Excellence HySafe (Hydrogen Safety as an Energy Carrier) that has the objective to facilitate the safe introduction of hydrogen technologies The mitigation effect of vents on the strength of hydrogen explosions is a relevant issue in hydrogen safety Experiments on stoichiometric hydrogen deflagrations in a 0 95 m(3) vessel with vents of different size (0 2 m(2) and 0 3 m(2)) have been selected in the available scientific literature in order to assess the accuracy of computational tools and models in reproducing experimental data in vented explosions Five organizations with experience in numerical modelling of gas explosions have participated to the code benchmarking activities with four CFD codes (COM3D, REACFLOW, bob and FLUENT) and one code based on a mathematical two zone model (VEX) The numerical features of the different codes and the simulations results are described and compared with the experimental measurements The agreement between simulations and experiments can be considered satisfactory for the maximum overpressure while correctly capturing some relevant parameters related to the dynamics of the phenomena such as the pressure rise rate and its maximum has been shown to be still an open issue (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved

Quantifying the risk of accidental ignition of flammable mixtures is extremely important in industry and aviation safety. The concept of a minimum ignition energy (MIE), obtained using a capacitive spark discharge ignition source, has traditionally formed the basis for determining the hazard posed by fuels. While extensive tabulations of historical MIE data exist, there has been little work done on ignition of realistic industrial and aviation fuels, such as gasoline or kerosene. In the current work, spark ignition tests are performed in a gaseous kerosene-air mixture with a liquid fuel temperature of 60 degrees C and a fixed spark gap of 3.3 mm. The required ignition energy was examined, and a range of spark energies over which there is a probability of ignition is identified and compared with previous test results in Jet A (aviation kerosene). The kerosene results are also compared with ignition test results obtained in previous work for traditional hydrogen-based surrogate mixtures used in safety testing as well as two hexane-air mixtures. Additionally, the statistical nature of spark ignition is discussed. (c) 2011 Elsevier Ltd. All rights reserved.

The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, the viewpoint of ignition as a statistical phenomenon appears to be more consistent with the inherent variability in engineering test data. We have developed a very low-energy capacitive spark ignition system to produce short sparks with fixed lengths of 1-2 mm, and the ignition system is used to perform spark ignition tests using a range of spark energies in lean hydrogen oxygen argon test mixtures used in aviation safety testing. The test results are analyzed using statistical tools to obtain probability distributions for ignition versus spark energy. A second low-energy spark ignition system was also developed to generate longer sparks with varying lengths up to 10 mm. A second set of ignition tests is performed in one of the test mixtures using a range of spark energies and spark lengths. The results are analyzed to obtain a probability distribution for ignition versus the spark energy per unit spark length. Preliminary results show that a single threshold MIE value does not exist, but rather that ignition is statistical in nature and highly dependent on mixture composition and spark length. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Nickel hydroxide/carbonate was chosen as the precursor to synthesize nickel nanopowders via an RF plasma-assisted hydrogen reduction route. Thermodynamic analysis revealed that the reaction could take place spontaneously. XRD patterns showed that metallic nickel powders could be obtained within the limited residence time. FESEM images indicated that the products consisted of well-dispersed spheres with an average diameter of about 60-100 nm. The obtained nickel powders exhibited high tap density. The use of nickel hydroxide/carbonate as the precursor guaranteed the products of high purity and the processing of environmental safety. The present plasma-assisted hydrogen reduction of nickel hydroxide/carbonate is an ideal route for large-scale synthesis of well-dispersed metallic nickel nanospheres used as electrode materials. (C) 2009 Elsevier B.V. All rights reserved.

Predictive models were built using neural network based Adaptive Neuro-Fuzzy Inference Systems for hydrogen flow rate, electrolyzer system-efficiency and stack-efficiency respectively. A comprehensive experimental database forms the foundation for the predictive models. It is argued that, due to the high costs associated with the hydrogen measuring equipment; these reliable predictive models can be implemented as virtual sensors. These models can also be used on-line for monitoring and safety of hydrogen equipment. The quantitative accuracy of the predictive models is appraised using statistical techniques. These mathematical models are found to be reliable predictive tools with an excellent accuracy of +/- 3%2compared with experimental values. The predictive nature of these models did not show any significant bias to either over prediction or under prediction. These predictive models, built on a sound mathematical and quantitative basis, can be seen as a step towards establishing hydrogen performance prediction models as generic virtual sensors for wider safety and monitoring applications. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.