Safety
Combustion Characteristics of Premixed Hydrogen/Air in an Undulate Microchannel
Jan 2022
Publication
This work reports a numerical investigation of microcombustion in an undulate microchannel using premixed hydrogen and air to understand the effect of the burner design on the flame in order to obtain stability of the flame. The simulations were performed for a fixed equivalence ratio and a hyperbolic temperature profile imposed at the microchannel walls in order to mimic the heat external losses occurred in experimental setups. Due to the complexity of the flow dynamics combined with the combustion behavior the present study focuses on understanding the effect of the fuel inlet rate on the flame characteristics keeping other parameters constant. The results presented stable flame structure regardless of the inlet velocity for this type of design meaning that a significant reduction in the heat flux losses through the walls occurred allowing the design of new simpler systems. The increase in inlet velocity increased the flame extension with the flame being stretched along the microchannel. For higher velocities flame separation was observed with two detected different combustion zones and the temperature profiles along the burner centerline presented a non-monotonic decrease due to the dynamics of the vortices observed in the convex regions of the undulated geometry walls. The geometry effects on the flame structure flow field thermal evolution and species distribution for different inlet velocities are reported and discussed.
Safety Standard for Hydrogen and Hydrogen Systems Guidelines for Hydrogen System Design, Materials Selection, Operations, Storage and Transportation
Jan 1997
Publication
The NASA Safety Standard which establishes a uniform process for hydrogen system design materials selection operation storage and transportation is presented. The guidelines include suggestions for safely storing handling and using hydrogen in gaseous (GH2) liquid (LH2) or slush (SLH2) form whether used as a propellant or non-propellant. The handbook contains 9 chapters detailing properties and hazards facility design design of components materials compatibility detection and transportation. Chapter 10 serves as a reference and the appendices contained therein include: assessment examples; scaling laws explosions blast effects and fragmentation; codes standards and NASA directives; and relief devices along with a list of tables and figures abbreviations a glossary and an index for ease of use. The intent of the handbook is to provide enough information that it can be used alone but at the same time reference data sources that can provide much more detail if required.
Health and safety in the new energy economy
Dec 2010
Publication
Over the next decade and beyond the UK is set to take significant steps towards a new energy economy. This will be an economy where the technologies meeting<br/>our electricity heat and fuel needs have to deliver against three key criteria: sustainability security and affordability.<br/><br/>In this context a wide range of emerging energy technologies are expected to play an important role in reshaping the way we satisfy our energy requirements. The extent to which they do so however will depend fundamentally on their ability to be harnessed safely.<br/><br/>Compiled by HSE’s Emerging Energy Technologies Programme this report provides a current assessment of the health and safety hazards that key emerging energy technologies could pose both to workers and to the public at large. (Nuclear energy technologies fall outside the scope of this report.) But it also highlights how an appropriate framework can be and is being put in place to help ensure that these hazards are managed and controlled effectively – an essential<br/>element in enabling the technologies to make a major contribution to the UK’s energy future.
Releases of Unignited Liquid Hydrogen
Jan 2014
Publication
If the hydrogen economy is to progress more hydrogen fuelling stations are required. In the short term in the absence of a hydrogen distribution network these fuelling stations will have to be supplied by liquid hydrogen road tanker. Such a development will increase the number of tanker offloading operations significantly and these may need to be performed in close proximity to the general public.<br/>The aim of this work is to identify and address hazards relating to the storage and transport of bulk liquid hydrogen (LH2) that are associated with hydrogen refuelling stations located in urban environments. Experimental results will inform the wider hydrogen community and contribute to the development of more robust modelling tools. The results will also help to update and develop guidance for codes and standards.<br/>The first phase of the project was to develop an experimental and modelling strategy for the issues associated with liquid hydrogen spills; this was documented in HSL report XS/10/06[1].<br/>The second phase of the project was to produce a position paper on the hazards of liquid hydrogen which was published in 2009 XS/09/72[2]. This was also published as a HSE research report RR769 in 2010[3].<br/>This report details experiments performed to investigate spills of liquid hydrogen at a rate of 60 litres per minute. Measurements were made on unignited releases which included concentration of hydrogen in air thermal gradient in the concrete substrate liquid pool formation and temperatures within the pool. Computational modelling of the unignited releases has been undertaken at HSL and reported in MSU/12/01 [4]. Ignited releases of hydrogen have also been performed as part of this project; the results and findings from this work are reported in XS/11/77[5].
Removing the Disrupting Wind Effect in Single Vented Enclosure Exposed to External Wind
Oct 2015
Publication
We are addressing hydrogen release into a single-vented facility with wind blowing onto the opposite side of the vent wall. Earlier work based on tests performed by HSL with wind (within the HyIndoor project) and comparative CFD simulations with and without wind ([1]within the H2FC project) has shown that the hydrogen concentrations inside the enclosure are increased compared to the case with no wind. This was attributed to the fact that wind is disrupting the passive ventilation. The present work is based on the GAMELAN tests (within the HyIndoor project) performed with one vent and no wind. For this enclosure simulations were performed with and without wind and reproduced the disrupting wind effect. In order to remove this effect and enhance the ventilation additional simulations were performed by considering different geometrical modifications near the vent. A simple geometrical layout around the vent is here proposed that leads to elimination of the disrupting wind effect. The analysis has been performed using the ADREA-HF code earlier validated both for the HSL and the GAMELAN tests. The current work was performed partly within HyIndoor project
A Study on the Effectivity of Hydrogen Leakage Detection for Hydrogen Fuel Cell
Sep 2017
Publication
Unlike four-wheel fuel-cell vehicles fuel-cell motorcycles have little semi-closure space corresponding to the engine compartment of four-wheel fuel-cell vehicles. Furthermore motorcycles may fall while parked or running. We conducted hydrogen concentration measurement and ignition tests to evaluate the feasibility of detecting leaks when hydrogen gas leaked from a fuel-cell motorcycle as well as the risk of ignition. We found that the installation of hydrogen leak detectors is effective because it is possible to detect minute hydrogen leaks by installing leak detectors at appropriate points on the fuel cell motorcycle and risks can be reduced by interrupting the hydrogen leak immediately after detection.
Mixing and Warming of Cryogenic Hydrogen Releases
Sep 2017
Publication
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 fuelling 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 fuelling stations with cryogenic hydrogen on-site.
Optimizing Mixture Properties for Accurate Laminar Flame Speed Measurement from Spherically Expanding Flame: Application to H2/O2/N2/He Mixtures
Sep 2019
Publication
The uncertainty on the laminar flame speed extracted from spherically expanding flames can be minimized by using large flame radius data for the extrapolation to zero stretch-rate. However at large radii the hydrodynamic and thermo-diffusive instabilities induce the formation of a complex cellular flame front and limit the range of usable data. In the present study we have employed the flame stability theory of Matalon to optimize the properties of the initial mixture so that transition to cellularity may occur at a pre-determined large radius. This approach was employed to measure the laminar flame speeds of H2/O2/N2/He mixtures with equivalence ratios from 0.6 to 2.0 at pressures of 50/80/100 kPa and a temperature of 300 K. For all the performed experiments the uncertainty related to the extrapolation to zero stretch-rate (performed with the linear curvature model) was below 2% as shown by the position of the data points in the (Lb/Rf;U Lb/Rf;L) plan where Lb is the burned Markstein length; and Rf;L and Rf;U are the flame radii at the lower and upper bounds of the extrapolation range. Comparison of the predictions of four chemical mechanisms with the present unstretched laminar flame speed data indicated an error below 10% for most conditions. In addition unsteady 1-D simulations performed with A-SURF demonstrated that the flame dynamical response to stretch rate could not be captured by the mechanisms. The present work indicates that although the stability theory of Matalon provides a well defined framework to optimize the mixture properties for improved flame speed measurement the uncertainty of some of the required parameters can result in largely over-estimated critical radius for cellularity onset which compromise the accuracy of the optimization procedure.
Risk Based Safety Distances for Hydrogen Refuelling Stations
Sep 2017
Publication
This paper introduces a risk-based methodology for hydrogen refuelling stations. Momentarily four stations are present in the Netherlands. This number is expected to increase to around twenty in the next years. For these stations a quantitative risk analysis (QRA) must be carried out to account for spatial planning. The presented method identifies the loss of containment scenarios and failure frequencies. Additionally the results of this study may be used in legislative context in the form of fixed generic safety distances. Using the risk analysis tool Safeti-NL safety distances are determined for three different kinds of hydrogen refuelling stations distinguished by the supply method of the hydrogen. For the hydrogen refuelling stations a maximum safety distance of 35 m is calculated. However despite the relatively small safety distances the maximum effect distances (distance to 1% lethality) can be very large especially for stations with a supply and storage of liquid hydrogen. The research was overseen by an advisory committee which also provided technical information on the refuelling stations.
A Simple Model for Calculating Peak Pressure in Vented Explosions of Hydrogen and Hydrocarbons
Apr 2019
Publication
The authors presented a basic mathematical model for estimating peak overpressure attained in vented explosions of hydrogen in a previous study (Sinha et al. [1]). The model focussed on idealized cases of hydrogen and was not applicable for realistic accidental scenarios like presence of obstacles initial turbulent mixture etc. In the present study the underlying framework of the model is reformulated to overcome these limitations. The flame shape computations are simplified. A more accurate and simpler formulation for venting is also introduced. Further by using simplifying assumptions and algebraic manipulations the detailed model consisting of several equations is reduced to a single equation with only four parameters. Two of these parameters depend only on fuel properties and a standard table provided in the Appendix can be used. Therefore to compute the overpressure only the two parameters based on enclosure geometry need to be evaluated. This greatly simplifies the model and calculation effort. Also since the focus of previous investigation was hydrogen properties of hydrocarbon fuels which are much more widely used were not accounted for. The present model also accounts for thermo-physical properties of hydrocarbons and provides table for fuel parameters to be used in the final equation for propane and methane. The model is also improved by addition of different sub-models to account for various realistic accidental scenarios. Moreover no adjustable parameters are used; the same equation is used for all conditions and all gases. Predictions from this simplified model are compared with experimentally measured values of overpressure for hydrogen and hydrocarbons and found to be in good agreement. First the results from experiments focussing on idealized conditions of uniformly mixed fuel in an empty enclosure under quiescent conditions are considered. Further the model applicability is also tested for realistic conditions of accidental explosion consisting of obstacles inside the enclosure non-uniform fuel distribution initial turbulent mixture etc. For all the cases tested the new simple model is found to produce reasonably good predictions.
Numerical and Experimental Investigation of H2-air and H2-O2 Detonation Parameters in a 9 m Long Tube, Introduction of a New Detonation Model
Sep 2017
Publication
Experimental and numerical investigation of hydrogen-air and hydrogen-oxygen detonation parameters was performed. A new detonation model was introduced and validated against the experimental data. Experimental set-up consisted of 9 m long tube with 0.17 m in diameter where pressure was measured with piezoelectric transducers located along the channel. Numerical simulations were performed within OpenFoam code based on progress variable equation where the detonative source term accounts for autoignition effects. Autoignition delay times were computed at a simulation run-time with the use of a multivariate regression model where independent variables were: pressure temperature and fuel concentration. The dependent variable was the autoignition delay time. Range of the analyzed gaseous mixture composition varied between 20% and 50% of hydrogen-air and 50%–66% of hydrogen in oxygen. Simulations were performed using LES one-equation eddy viscosity turbulence model in 2D and 3D. Calculations were validated against experimental data.
A New Approach to Vented Deflagration Modeling
Sep 2017
Publication
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.
The Residual Strength of Automotive Hydrogen Cylinders After Exposure to Flames
Sep 2017
Publication
Fuel cell vehicles and some compressed natural gas vehicles are equipped with carbon fiber reinforced plastic (CFRP) composite cylinders. Each of the cylinders has a pressure relief device designed to detect heat and release the internal gas to prevent the cylinder from bursting in a vehicle fire accident. Yet in some accident situations the fire may be extinguished before the pressure relief device is activated leaving the high-pressure fuel gas inside the fire-damaged cylinder. To handle such a cylinder safely after an accident it is necessary that the cylinder keeps a sufficient post-fire strength against its internal gas pressure but in most cases it is difficult to accurately determine cylinder strength at the accident site. One way of solving this problem is to predetermine the post-fire burst strengths of cylinders by experiments. In this study automotive CFRP cylinders having no pressure relief device were exposed to a fire to the verge of bursting; then after the fire was extinguished the residual burst strengths and the overall physical state of the test cylinders were examined. The results indicated that the test cylinders all recorded a residual burst strength at least twice greater than their internal gas pressure for tested cylinders with new cylinder burst to nominal working pressure in the range 2.67–4.92 above the regulated ratio of 2.25.
Simulation-based Safety Investigation of a Hydrogen Fueling Station with an On-site Hydrogen Production System Involving Methylcyclohexane
Jan 2017
Publication
Adequate safety measures are crucial for preventing major accidents at hydrogen fuelling stations. In particular risk analysis of the domino effect at hydrogen fuelling stations is essential because knock-on accidents are likely to intensify the consequences of a relatively small incident. Several risk assessment studies have focused on hydrogen fuelling stations but none have investigated accidental scenarios related to the domino effect at such stations. Therefore the purpose of this study is to identify a domino effect scenario analyze the scenario by using simulations and propose safety measures for preventing and mitigating of the scenario. In this hazard identification study we identified the domino effect scenario of a hydrogen fuelling station with an on-site hydrogen production system involving methylcyclohexane and investigated through simulations of the scenario. The simulations revealed that a pool fire of methylcyclohexane or toluene can damage the process equipment and that thermal radiation may cause the pressurized hydrogen tanks to rupture. The rupture-type vent system can serve as a critical safety measure for preventing and mitigating the examined scenario.
Understanding and Mitigating Hydrogen Embrittlement of Steels: A Review of Experimental, Modelling and Design Progress from Atomistic to Continuum
Feb 2018
Publication
Hydrogen embrittlement is a complex phenomenon involving several lengthand timescales that affects a large class of metals. It can significantly reduce the ductility and load-bearing capacity and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Despite a large research effort in attempting to understand the mechanisms of failure and in developing potential mitigating solutions hydrogen embrittlement mechanisms are still not completely understood. There are controversial opinions in the literature regarding the underlying mechanisms and related experimental evidence supporting each of these theories. The aim of this paper is to provide a detailed review up to the current state of the art on the effect of hydrogen on the degradation of metals with a particular focus on steels. Here we describe the effect of hydrogen in steels from the atomistic to the continuum scale by reporting theoretical evidence supported by quantum calculation and modern experimental characterisation methods macroscopic effects that influence the mechanical properties of steels and established damaging mechanisms for the embrittlement of steels. Furthermore we give an insight into current approaches and new mitigation strategies used to design new steels resistant to hydrogen embrittlement.<br/>*Correction published see Supplements section
Hydrogen Dispersion in a Closed Environment
Sep 2017
Publication
The highly combustible nature of hydrogen poses a great hazard creating a number of problems with its safety and handling. As a part of safety studies related to the use of hydrogen in a confined environment it is extremely important to have a good knowledge of the dispersion mechanism.<br/>The present work investigates the concentration field and flammability envelope from a small scale leak. The hydrogen is released into a 0.47 m × 0.33 m x 0.20 m enclosure designed as a 1/15 – scale model of a room in a nuclear facility. The performed tests evaluates the influence of the initial conditions at the leakage source on the dispersion and mixing characteristics in a confined environment. The role of the leak location and the presence of obstacles are also analyzed. Throughout the test during the release and the subsequent dispersion phase temporal profiles of hydrogen concentration are measured using thermal conductivity gauges within the enclosure. In addition the BOS (Background Oriented Schlieren) technique is used to visualise the cloud evolution inside the enclosure. These instruments allow the observation and quantification of the stratification effects.
Performance Evaluation of Empirical Models for Vented Lean Hydrogen Explosions
Sep 2017
Publication
Explosion venting is a method commonly used to prevent or minimize damage to an enclosure caused by an accidental explosion. An estimate of the maximum overpressure generated though explosion is an important parameter in the design of the vents. Various engineering models (Bauwens et al. 2012 Molkov and Bragin 2015) and European (EN 14994 ) and USA standards (NFPA 68) are available to predict such overpressure. In this study their performance is evaluated using a number of published experiments. Comparison of pressure predictions from various models have also been carried out for the recent experiments conducted by GexCon using a 20 feet ISO container. The results show that the model of Bauwens et al. (2012) predicts well for hydrogen concentration between 16% and 21% and in the presence of obstacles. The model of Molkov et al. (2015) is found to work well for hydrogen concentrations between 10% and 30% without obstacles. In the presence of obstacles as no guidelines are given to set the coefficient for obstacles in the model it was necessary to tune the coefficient to match the experimental data. The predictions of the formulas in NFPA 68 show a large scatter across different tests. The current version of both EN 14994 and NFPA 68 are found to have very limited range of applicability and can hardly be used for vent sizing of hydrogen-air deflagrations. Overall the accuracy of all the engineering models was found to be limited. Some recommendations concerning their applicability will be given for vented lean-hydrogen explosion concentrations of interest to practical applications.
Non-steady Characteristics of Dispersion and Ignitability for High-pressurized Hydrogen Jet Discharged From a Pinhole
Sep 2017
Publication
Hydrogen gas concentrations and jet velocities were measured downstream by a high response speed flame ionization detector and PIV (Particle Image Velocimetry) in order to investigate the characteristics of dispersion and ignitability for 40–82 MPa high-pressurized hydrogen jet discharged from a nozzle with 0.2 mm diameter. The light emitted from both OH radical and water vapor species yielded from hydrogen combustion ignited by an electric spark were recorded by two high speed cameras. From the results the empirical formula concerning the relationships for time-averaged concentrations concentration fluctuations and ignition probability were obtained to suggest that they would be independent of hydrogen discharge pressure.
Response Time Measurement of Hydrogen Sensors
Sep 2017
Publication
The efficiency of gas sensor application for facilitating the safe use of hydrogen depends considerably on the sensor response to a change in hydrogen concentration. Therefore the response time has been measured for five different-type commercially available hydrogen sensors. Experiments showed that all these sensors surpass the ISO 26142 standard; for the response times t90 values of 2 s to 16 s were estimated. Results can be fitted with an exponential or sigmoidal function. It can be demonstrated that the results on transient behaviour depend on both the operating parameters of sensors and investigation methods as well as on the experimental conditions: gas change rate and concentration jump.
Ignited Releases of Liquid Hydrogen
Jan 2014
Publication
If the hydrogen economy is to progress more hydrogen fuelling stations are required. In the short term in the absence of a hydrogen distribution network these fuelling stations will have to be supplied by liquid hydrogen (LH2) road tanker. Such a development will increase the number of tanker offloading operations significantly and these may need to be performed in close proximity to the general public.<br/>Several research projects have been undertaken already at HSL with the aim of identifying and addressing hazards relating to the storage and transport of bulk LH2 that are associated with hydrogen refuelling stations located in urban environments.<br/>The first phase of the research was to produce a position paper on the hazards of LH2 (Pritchard and Rattigan 2009). This was published as an HSE research report RR769 in 2010. <br/>The second phase developed an experimental and modelling strategy for issues associated with LH2 spills and was published as an internal report HSL XS/10/06. The subsequent experimental work is a direct implementation of that strategy. LH2 was first investigated experimentally (Royle and Willoughby 2012 HSL XS/11/70) as large-scale spills of LH2 at a rate of 60 litres per minute. Measurements were made on unignited releases which included the concentration of hydrogen in air thermal gradients in the concrete substrate liquid pool formation and temperatures within the pool. Computational modelling on the un-ignited spills was also performed (Batt and Webber 2012 HSL MSU/12/01).<br/>The experimental work on ignited releases of LH2 detailed in this report is a direct continuation of the work performed by Royle and Willoughby.<br/>The aim of this work was to determine the hazards and severity of a realistic ignited spill of LH2 focussing on; flammability limits of an LH2 vapour cloud flame speeds through an LH2 vapour cloud and subsequent radiative heat and overpressures after ignition. The results of the experimentation will inform the wider hydrogen community and contribute to the development of more robust modelling tools. The results will also help to update and develop guidance for codes and standards.
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