Safety

GASFLOW is a finite-volume computer code that solves the time-dependent two-phase homogeneous equilibrium model compressible Navier–Stokes equations for multiple gas species with turbulence. The fluid-dynamics algorithm is coupled with conjugate heat and mass transfer models to represent walls floors ceilings and other internal structures to describe complex geometries such as those found in nuclear containments and facilities. Recent applications involve simulations of cryogenic hydrogen tanks at elevated pressures. These applications which often have thermodynamic conditions near the critical point require more accurate real-gas Equations-of-State (EoS) and transport properties than the standard ideal gas EoS and classical kinetic-theory transport properties. This paper describes the rigorous implementation of the generalized real-gas EoS into the GASFLOW CFD code as well as the specific implementation of respective real-gas models (Leachman's NIST hydrogen EoS a modified van der Waals EoS and a modified Nobel-Abel EoS); it also includes a logical testing procedure based upon a numerically exact benchmark problem. An example of GASFLOW simulations is presented for an ideal cryo-compressed hydrogen tank of the type utilized in fuel cell vehicles.

Computational fluid dynamics (CFD) is used to numerically solve the sudden release of hydrogen from a high pressure tank (up to 70MPa) into air. High pressure tanks increase the risk of failure of the joints and pipes connected to the tank which results in release of Hydrogen. The supersonic flow caused by high pressure ratio of reservoir to ambient generates a strong Mach disk. A three dimensional in-house code is developed to simulate the flow. High pressure Hydrogen requires a real gas law because it deviates from ideal gas law. Firstly Beattie-Bridgeman and Abel-Noble real gas equation of states are applied to simulate the release of hydrogen in hydrogen. Then Abel-Noble is implied to simulate the release of hydrogen in air. Beattie-Bridgeman has stability problems in the case of hydrogen in air. A transport equation is used to solve the concentration of Hydrogen-air mixture. The code is second order accurate in space and first order in time and uses a modified Van Leer limiter. The fast release of Hydrogen from a small rupture needs a very small mesh therefore parallel computation is applied to overcome memory problems and to decrease the solution time. The high pressure ratio of the reservoir to ambient causes a very fast release which is accurately modelled by the code and all the shocks and Mach disk happening are observed in the results. The results show that the difference between real gas and ideal gas models cannot be ignored.

The competition between chemical energy release rate and volumetric expansion related to shock wave’s dynamics is of primary importance for a number of situations relevant to explosion safety. While studies have been performed on this topic over the years they have been limited to mixtures with monotonous energy release profile. In the present study the ignition of H2-NO2/N2O4 mixtures which exhibit a single-step or a two-step energy release rate profile depending on the equivalence ratio has been investigated under volumetric expansion conditions. The rate of expansion has been calculated using the Taylor-Sedov solution and accounted for using 0-D numerical simulations with time-dependent specific volume. The results were analyzed in terms of a Damkohler number defined as the ratio of the expansion to ignition times. For mixtures with non-monotonous energy release rate profiles two critical Damkohler numbers can be identified one for each of the steps of energy release. It was also shown that the fluid element which is the most likely to ignite corresponds to the one behind a shock propagating at the Chapman-Jouguet velocity. The thermo-chemical dynamics have been analyzed about the critical conditions using energy release rate per reaction rate of production and sensitivity analyses.

The current study addresses the spontaneous ignition of hydrogen jets released into a confined oxidizer environment experimentally. The experiments are conducted in a shock tube where hydrogen gas is shock-accelerated into oxygen across a perforated plate. The operating conditions and hole dimension of the plate were varied in order to identify different flow field and ignition scenarios. Time resolved Schlieren visualization permitted to reconstruct the gasdynamic evolution of the release and different shock interactions. Time resolved self-luminosity records permitted us to record whether ignition was achieved and also to record the dimension of the turbulent mixing layer. The ignition limits determined experimentally in good agreement with the 1D diffusion ignition model proposed by Maxwell and Radulescu. Nevertheless the experiments demonstrated that the mixing layer is two to three orders of magnitude thicker than predicted by molecular diffusion which can be attributed to the observed mixing layer instabilities and shock-mixing layer interactions which provide a much more intense mixing rate than anticipated from previous and current numerical predictions. These observations further clarify why releases through partly confined geometries are more conducive to jet ignition of the jets.

The paper will present results of hydrogen/oxygen mixtures ignited by using electric sparks electrostatic discharges a heating element and a flame. Measurements of the lower flammability limit (LFL) was done for each ignition method. The hydrogen mixtures of different concentrations were ignited at the bottom of a combustion chamber leading to an upward propagation of the resulting flame. At some level of concentration the combustion was partial due to the limited upward propagation. The complete combustion of the whole mixture was observed at concentration limits higher than the known LFL of 4% vol. for hydrogen in air. The paper will describe the test facilities and the resulting ignition probabilities for different ignition methods.

The development of a set of safety codes and standards for hydrogen facilities is necessary to ensure they are designed and operated safely. To help ensure that a hydrogen facility meets an acceptable level of risk code and standard development organizations (SDOs) are utilizing risk-informed concepts in developing hydrogen codes and standards. Two SDOs the National Fire Protection Association (NFPA) and the International Organization for Standardization (ISO) through its Technical Committee (TC) 197 on hydrogen technologies have been developing standards for gaseous hydrogen facilities that specify the facilities have certain safety features use equipment made of material suitable for a hydrogen environment and have specified separation distances. Under Department of Energy funding Sandia National Laboratories (SNL) has been supporting efforts by both of these SDOs to develop the separation distances included in their respective standards. Important goals in these efforts are to use a defensible science-based approach to establish these requirements and to the extent possible harmonize the requirements. International harmonization of regulations codes and standards is critical for enabling global market penetration of hydrogen and fuel cell technologies.

In order to improve risk analyses and influence the design of the future H2 systems an experimental study on “real” leaks qualification and quantification was performed. In H2 energy applications fittings appeared as a significant leakage potential and subsequently explosion and flame hazards. Thus as a part of the “Horizon Hydrogène Energie” French program four kinds of commercial fittings usually employed on H2 systems were tested thanks to a new high pressure test bench – designed setup and operated by INERIS – allowing experiments to be led for H2 pressures until 700 bar. The fittings underwent defined stresses representative of H2 systems lifetime and beyond. The associated leaks – when existing – are characterized in terms of flow rate.

Sandia National Laboratories is working with stakeholders to develop scientific data for use by standards development organizations to create hydrogen codes and standards for the safe use of liquid hydrogen. Knowledge of the concentration field and flammability envelope for high-pressure hydrogen leaks is an issue of importance for the safe use of liquid hydrogen. Sandia National Laboratories is engaged in an experimental and analytical program to characterize and predict the behaviour of liquid hydrogen releases. This paper presents a model for computing hydrogen dilution distances for cold hydrogen releases. Model validation is presented for leaks of room temperature and 80 K high-pressure hydrogen gas. The model accounts for a series of transitions that occurs from a stagnate location in the tank to a point in the leak jet where the concentration of hydrogen in air at the jet centerline has dropped to 4% by volume. The leaking hydrogen is assumed to be a simple compressible substance with thermodynamic equilibrium between hydrogen vapor hydrogen liquid and air. For the multi-phase portions of the jet near the leak location the REFPROP equation of state models developed by NIST are used to account for the thermodynamics. Further downstream the jet develops into an atmospheric gas jet where the thermodynamics are described as a mixture of ideal gases (hydrogen–air mixture). Simulations are presented for dilution distances in under-expanded high-pressure leaks from the saturated vapor and saturated liquid portions of a liquid hydrogen storage tank at 10.34 barg (150 PSIG).

Conditions are examined under which mechanical stimuli produced by striking controlled blows can result in sparking and ignition of hydrogen in air mixtures. The investigation principally concerns magnesium thermite reaction as the ignition source and focuses on the conditions and thermomechanical parameters that are involved in determining the probability of ignition. It is concluded that the notion of using the kinetic energy of impact as the main criterion in determining whether an ignition event is likely or not is much less useful than considering the parameters which determine the maximum temperature produced in a mechanical stimuli event. The most influential parameter in determining ignition frequency or probability is the velocity of sliding movement during mechanical stimuli. It is also clear that the kinetic energy of a moving hammer head is of lesser importance than the normal force which is applied during contact. This explains the apparent discrepancy in previous studies between the minimum kinetic energy thought to be necessary to allow thermite sparking and gas ignition to occur with drop weight impacts and glancing blow impacts. In any analysis of the likelihood of mechanical stimuli to cause ignition the maximum surface temperature generated should be determined and considered in relation to the temperatures that would be required to initiate hot surface reactions sufficient to cause sparking and ignition.

The U.S. Department of Energy supports the implementation of hydrogen fuel cell technologies by providing hydrogen safety and emergency response training to first responders. A collaboration was formed to develop and deliver a one-day course that uses a mobile fuel cell vehicle (FCV) burn prop designed and built by Kidde Fire Trainers. This paper describes the development of the training curriculum including the design and operation of the FCV prop; describes the successful delivery of this course to over 300 participants at three training centers in California; and discusses feedback and observations received on the course. Photographs and video clips of the training sessions will be presented.