Blind-prediction: Estimating the consequences of vented hydrogen deflagrations for homogeneous mixtures in 20-foot ISO containers

This paper summarises the results from a blind-prediction study for consequence models used for estimating the reduced explosion pressure and structural response in vented hydrogen deflagrations. The work is part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA). The scenarios selected for the blind-prediction entailed vented explosions with homogeneous hydrogen-air mixtures in a 20-foot ISO (International Organization for Standardization) container. The test program included two configurations and six experiments, i.e. three repeated tests for each scenario. The comparison between experimental results and model predictions reveals reasonable agreement for some of the models, and significant discrepancies for others. The results from the first blind-prediction study in the HySEA project should motivate developers to improve and validate their models, as well as to update documentation and guidelines for users of the models.     

Simulating vented hydrogen deflagrations: Improved modelling in the CFD tool FLACS-hydrogen

This paper describes validation of the computational fluid dynamics tool FLACS-Hydrogen. The validation study focuses on concentration and pressure data from vented deflagration experiments performed in 20-foot shipping containers as part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA), funded by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU). The paper presents results for tests involving inhomogeneous hydrogen-air clouds generated from realistic releases performed during the HySEA project. For both experiments and simulations, the peak overpressures obtained for the stratified mixtures are higher than those measured for lean homogeneous mixtures with the same amount of hydrogen. Using an in-house version of FLACS-Hydrogen with the numerical solver Flacs3 and improved physics models results in significantly improved predictions of the peak overpressures, compared to the predictions by the standard Flacs2 solver. The paper includes suggestions for further improvements to the model system.     

Numerical study of vented hydrogen explosions in a small scale obstructed chamber

There is a growing need to understand and estimate the explosion hazards associated with hydrogen storage and utilisation. This paper presents a comprehensive numerical study on the explosion characteristics of a lean hydrogen-air mixture in a small-scale obstructed vented chamber. The large eddy simulation (LES) technique is employed to study the highly unsteady turbulence-driven explosion when the flame propagates past successive obstructions. A dynamic flame surface density (DFSD) model is applied to the filtered chemical source term in the LES to account for the progressive wrinkling of the deflagrating flame. The driving mechanism of pressure rise and the underlying physics of flame-obstacle interactions are illustrated using the detailed LES results. The paper considers 11 individual flow experimental configurations of various obstacle number, size and location. They are further classified into six groups to investigate the influence of the level of blockage and the separation distance between adjacent obstructions. Critical safety-related parameters including the maximum overpressure and its incidence time are analysed. A comparison with propane is also made to highlight the substantial overpressure and flame acceleration of hydrogen deflagrations. Satisfactory agreements have been obtained between the LES and the experimental data, and this confirms the capability of the developed computational models in capturing essential explosion features and information for the study of vented hydrogen explosions.     

Performance evaluation of empirical models for vented lean hydrogen explosions

This paper aims to provide a comprehensive review of available empirical models for overpressures predictions of vented lean hydrogen explosions. Empirical models and standards are described briefly, with discussion on salient features of each model. Model predictions are then compared with the available experimental results on vented hydrogen explosions. First comparison is made for standards tests, with empty container and quiescent starting conditions. Comparisons are then made for realistic cases with obstacles and initial turbulent mixture. Recently, a large number of experiments are carried out with standard 20-foot container for the HySEA project. Results from these tests are also used for model comparison. Comments on accuracy of model predictions, their applicability and limitations are discussed.A new model for vented hydrogen explosion is proposed. This model is based on external cloud formation, and explosion. Available experimental measurements of flame speed and vortex ring formation are used in formulation of this model. All assumptions and modelling procedure are explained in detail. The main advantage of this model is that it does not have any tuning parameter and the same set of equations is used for all conditions. Predictions using this model show a reasonably good match with experimental results.     

Experimental investigation on the dynamic responses of vented hydrogen explosion in a 40-foot container

To investigate the structural dynamics of a container subjected to a vented hydrogen explosion, 48 field tests were conducted in a 40-foot container with roof vents and an end vent. The effects of the hydrogen concentration, ignition position, and obstacles on the evolution of the dynamic responses were investigated. Three stages were generally observed for displacements: (1) At the stage of the vent rupture, the displacement could be approximated as a quasi-static response, and there was a linear relationship between the peaks of positive overpressure and displacement. (2) Structural deformation appeared as reciprocating vibration at the stage of Helmholtz oscillation. (3) The structure exhibited relatively weak irregular fluctuation when high-frequency acoustic oscillation occurred. Two types of the structural acceleration with low and high amplitudes resulting from Helmholtz oscillation and acoustic oscillation, respectively, were clearly observed. For the end-vented explosion, multiple peaks were observed for the displacement at the quasi-static stage due to the rupture, discharge, and external explosion. Moreover, the displacement was sensitive to hydrogen concentration, whereas the number of obstacles and the ignition position had significant influences on the peak acceleration for roof venting. This work conducted the fundamental explanation for the evolution law of structural responses induced by vented hydrogen explosions from the perspective of structural dynamics and enriched the experimental accumulation in a large-scale container with congestion in this field.     

An improved CFD model for vented deflagration simulations – Analysis of a medium-scale hydrogen experiment

Hydrogen is a promising alternative fuel which is expected to gain a significant portion of the energy market in the near future. However, it is a flammable gas with significant safety concerns as it can easily cause deflagrations or even detonations. The aim of this work is twofold: firstly, to derive a new CFD model for vented deflagration simulations improving an existing one and secondly, to better understand the physics of the phenomenon. A recent experiment conducted in Karlsruhe Institute of Technology is used for that purpose. The new model improves substantially the predictions representing successfully the experiment. The analysis indicates that turbulence generated outside the vent is responsible for the violence of the external explosion which leads to the sudden pressure increase. Rayleigh-Taylor instability on the other hand does not seem to contribute to burning velocity increase. From the analysis that is made combustion models are formulated highlighting the important components for a successful CFD deflagration model.     

A simple model for calculating peak pressure in vented explosions of hydrogen and hydrocarbons

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.     

Effects of nitrogen addition on vented hydrogen–air deflagration in a vertical duct

In this paper, experiments were performed to investigate the coupling effects of venting and nitrogen addition ratio (χ) on flame behavior and pressure evolution during hydrogen–air deflagration within and outside a 1-m-high vertical duct with a vent on its top. Experimental results reveal that χ has significant effects on the pressure–time histories in the duct. Helmholtz oscillations of the internal overpressure were observed in all tests, and acoustic type oscillations appears in the tests only for χ = 25% and 30%. For a certain χ, the maximum overpressure (Pmax) increased with the distance to the vent, i.e., the highest overall explosion overpressure was attained near the duct bottom; however, the difference in Pmax between various measuring points decreases with an increase in χ. In all tests, a pressure peak in the duct was observed shortly after external explosion. The maximum internal and external overpressure decreased as χ was increased.     

Vented hydrogen deflagrations in containers: Effect of congestion for homogeneous and inhomogeneous mixtures

This paper summarises the results from 66 vented hydrogen deflagration experiments performed in 20-foot shipping containers: 42 tests with initially homogeneous and quiescent mixtures, and 24 tests with inhomogeneous mixtures. Other parameters investigated include hydrogen concentration, vent area, type of venting device, ignition position, and the level and type of congestion inside the container. The results confirm that internal congestion can increase the maximum reduced explosion pressure in vented deflagrations significantly, compared to vented deflagrations in empty enclosures. As such, it is important to incorporate the effect of congestion in the theoretical and/or empirical correlations recommended in standards and guidelines for explosion protection. The work reported here is a deliverable from work package 2 (WP2) in the project “Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations” (HySEA). The project received funding from the Fuel Cells and Hydrogen Joint Undertaking (FCH JU).     

Scaling effects of vented deflagrations for near lean flammability limit hydrogen-air mixtures in large scale rectangular volumes

Combustion-generated overpressures in nuclear containment buildings during a severe accident may be relieved by venting gases to adjacent compartments through relief panels or existing openings to avoid compromising a containment breach. Experimental studies on the dynamics of vented hydrogen-air combustion were extensively performed using vessels varied in shape and size at the Canadian Nuclear Laboratories. In this paper, the scaling effects are examined for near lean flammability hydrogen-air mixtures (6–12 vol.% H₂) with tests performed in rectangular volumes (25, 57 and 120 m³) with a scaled vent area (A_v/V^2/3) of 0.02–0.05 under both initially quiescent and fan-induced turbulent conditions. This study has found that the maximum peak overpressure of all quiescent tests are dominated by the acoustic coupled effect for the hydrogen concentration greater than 8 vol.%, and the acoustic effect becomes insignificant under turbulent conditions. The measured peak over-pressures are generally over-predicted for the quiescent tests and better predicted for the turbulent tests by the well-known Bradley–Mitcheson and Molkov correlations.     

Numerical simulation and validation of flame acceleration and DDT in hydrogen air mixtures

Combustion of hydrogen can take place in different modes such as laminar flames, slow and fast deflagrations and detonations. As these modes have widely varying propagation mechanisms, modeling the transition from one to the other presents a challenging task. This involves implementation of different sub-models and methods for turbulence-chemistry interaction, flame acceleration and shock propagation. In the present work, a unified numerical framework based on OpenFOAM has been evolved to simulate such phenomena with a specific emphasis on the Deflagration to Detonation Transition (DDT) in hydrogen-air mixtures. The approach is primarily based on the transport equation for the reaction progress variable. Different sub-models have been implemented to capture turbulence chemistry interaction and heat release due to autoignition. The choice of sub-models has been decided based on its applicability to lean hydrogen mixtures at high pressures and is relevant in the context of the present study. Simulations have been carried out in a two dimensional rectangular channel based on the GraVent experimental facility. Numerical results obtained from the simulations have been validated with the experimental data. Specific focus has been placed on identifying the flame propagation mechanisms in smooth and obstructed channels with stratified initial distribution. In a smooth channel with stratified distribution, it is observed that the flame surface area increases along the propagation direction, thereby enhancing the energy release rate and is identified to be the key parameter leading to strong flame acceleration. When obstacles are introduced, the increase in burning rate due to turbulence induced by the obstacles is partly negated by the hindrance to the unburned gases feeding the flame. The net effect of these competing factors leads to higher flame acceleration and propagation mechanism is identified to be in the fast deflagration regime. Further analysis shows that several pressure pulses and shock complexes are formed in the obstacle section. The ensuing decoupled shock-flame interaction augments the flame speed until the flame coalesces with a strong shock ahead of it and propagates as a single unit. At this point, a sharp increase in propagation speed is observed thus completing the DDT process. Subsequent propagation takes place at a uniform speed into the unburned mixture.     

Simulation and risk assessment of hydrogen leakage in hydrogen production container

In this paper, the hydrogen leakage and diffusion characteristics analysis and risk assessment are carried out on the container where a 2 Nm³/h alkaline hydrogen production device is located. Firstly, the transient and steady process of hydrogen leakage from hydrogen production container is analyzed. Secondly, the dynamic balance of combustible hydrogen cloud is analyzed, the concept of critical ventilation flow is put forward. It was found that in order to reduce the flammable volume by 85%, the installed ventilation can only cover the leakage flow of 1.0 Nm³/min. Finally, TNT equivalent method is used to evaluate the hazard degree of hydrogen leakage. It is found that the existed hydrogen production container ventilation device can only exhaust the hydrogen-air cloud with small flow leakage, while the accumulation of gas cloud still exists in large flow leakage. Under the critical ventilation flow, the minor injury radius can be reduced from 4.8 m to 2.78 m. The effect of critical ventilation flow was verified.     

Effect of igniter type and number of igniters on vented deflagrations for near lean flammability limit hydrogen-air mixtures in a large scale rectangular volume

This paper examines the effect of igniter type (glow plug vs. spark igniter) and number of igniters on the dynamics of vented combustion under both initially quiescent and fan-induced turbulent conditions. These experiments are a subset of many series of tests performed in a 120 m³ large scale vented combustion test facility at the Canadian Nuclear Laboratories using near lean flammability hydrogen-air mixtures (8–12% H₂). One of the objectives of these studies was to have a better understanding of the effectiveness of deliberate ignition for mitigation of hydrogen during a postulated accident and to provide data for code validation. The test results of the current study show that the two types of ignition sources have no significant influence on the maximum combustion overpressure except that the initial burning rate is slightly faster using the spark igniter. Under either the quiescent or turbulent conditions, the maximum combustion overpressure always increases with an increase in the number of igniters, but under the current experimental conditions, the turbulent combustion overpressure with one igniter is always higher than quiescent combustion with multiple igniters.     

Modelling the mitigation of lean hydrogen deflagrations in a vented cylindrical rig with water fog

The wide flammability range of hydrogen–air mixtures means that the generation and presence of significant quantities of hydrogen in a confined space will always present some likelihood that a deflagration or explosion might occur. Very fine water mist fogs have been suggested as a possible method of mitigating the overpressure rise should a hydrogen–air deflagration occur.     

Effects of hydrogen concentration and film thickness on the vented explosion in a small obstructed rectangular container

The explosion venting is an effective way to reduce hydrogen-air explosion hazards, but the explosion venting has been less touched in an obstructed container. The present study mainly focused on the effects of hydrogen concentration and film thickness on the explosion venting in a small obstructed rectangular container. High speed schlieren photography was employed to obtain the flame fine structure and velocity. Pressure transducers were used to measure the overpressure nearby the obstacle. The experimental results show that the obstacle has a significant effect on the flame shape, tip speed and overpressure. In the process of flame evolution, the flame surface becomes more wrinkled with time after the tulip flame. Compared with the cases without the obstacle, the flame surface becomes more distorted and wrinkled downstream of the obstacle under the influence of obstacle enhanced turbulence and flow instability. Upstream of the obstacle, the lower part of the flame surface becomes concave while the upper part shows convex. The pressure histories show that the maximum overpressure increases with the hydrogen concentration in the range of 11.8%–23.7%. Two main pressure peaks were observed for all hydrogen concentrations in the presence of the obstacle. The Helmholtz oscillations appear after the second pressure peak and its duration increases slightly when the hydrogen concentration increases. The combined effect of the obstacle and hydrogen concentration on the second peak overpressure is more significant than on the first peak overpressure. Moreover, the maximum overpressure shows a monotonic increase with the film thickness.     

Trace level analysis of reactive ISO 14687 impurities in hydrogen fuel using laser-based spectroscopic detection methods

Hydrogen fuelled vehicles can play a key role in the decarbonisation of transport and reducing emissions. To ensure the durability of fuel cells, a specification has been developed (ISO 14687), setting upper limits to the amount fraction of a series of impurities. Demonstrating conformity with this standard requires demonstrating by measurement that the actual levels of the impurities are below the thresholds. Currently the industry is unable to do so, for measurement standards and sensitive dedicated analytical methods are lacking. In this work, we report on the development of such measurement standards and methods for four reactive components: formaldehyde, formic acid, hydrogen chloride and hydrogen fluoride. The primary measurement standard is based on permeation, and the analytical methods on highly sensitive and selective laser-based spectroscopic techniques. Relative expanded uncertainties at the ISO 14687 threshold level in hydrogen of 4% (formaldehyde), 8% (formic acid), 5% (hydrogen chloride), and 8% (hydrogen fluoride) have been achieved.     

Effects of hydrogen concentration on the vented deflagration of hydrogen-air mixtures in a 1-m3 vessel

We experimentally investigated the pressure buildup and flame behavior during the vented deflagration of hydrogen-air mixtures with concentrations ranging from 13% to 39% that were centrally ignited in a 1-m³ rectangular vessel with a 500 mm × 400 mm top vent. The performance of some available models for estimating the maximum reduced overpressure was experimentally evaluated. The maximum reduced overpressure increased from approximately 3 kPa to 100 kPa as hydrogen concentration increased from 13 to 39%. Turbulent pressure oscillations with frequencies of 200–300 Hz triggered by external explosions were observed in our tests with 22–39% hydrogen-in-air mixtures. Molkov's best-fit and conservative models predict the maximum reduced overpressure well for lean and rich hydrogen mixtures, respectively. The average speed of the external flame first decreases to a minimum value with the fireball expanding to its maximum size and then increases. As hydrogen concentration increases, the maximum length and duration of the external flame increases and decreases, respectively.     

Numerical simulation of the laminar hydrogen flame in the presence of a quenching mesh

Recent studies of J.H. Song et al. [1], and S.Y. Yang et al. [2] (see also references therein) have been concentrated on mitigation measures against hydrogen risk. The authors have proposed installation of quenching meshes between compartments or around the essential equipment in order to contain hydrogen flames. Preliminary tests were conducted which demonstrated the possibility of flame extinction using metallic meshes of specific size.Considerable amount of numerical and theoretical work on flame quenching phenomenon has been performed in the second half of the last century and several techniques and models have been proposed to predict the quenching phenomenon of the laminar flame system (see for example [3] and references therein). Most of these models appreciated the importance of heat loss to the surroundings as a primary cause of extinguishment, in particular, the heat transfer by conduction to the containing wall. The supporting simulations predict flame-quenching structure either between parallel plates (quenching distance) or inside a tube of a certain diameter (quenching diameter).In the present study the flame quenching is investigated assuming the laminar hydrogen flame propagating towards a quenching mesh using two-dimensional configuration and the earlier developed models. It is shown that due to a heat loss to a metallic grid the flame can be quenched numerically.     

Numerical modeling of hydrogen absorption measurements in Ni-catalyzed Mg

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this work, hydrogen absorption in Ni-catalyzed magnesium was measured in a wide range of pressure (500 kPa–5000 kPa) and temperature (498 K–573 K). Using this information, a model for the absorption kinetics and thermal behavior of the hydrogen storage system was proposed. This model could be used in the design of Ni-catalyzed magnesium storage tanks and other applications. It considers the independent contribution of three variables: temperature, pressure and reacted fraction to estimate the hydrogen absorption rate. An activation energy for the process was estimated and the value obtained (92 kJ/mol) was concordant with previous values reported in the literature.     

Probability of occurrence of ISO 14687-2 contaminants in hydrogen: Principles and examples from steam methane reforming and electrolysis (water and chlor-alkali) production processes model

According to European Directive 2014/94/EU, hydrogen providers have the responsibility to prove that their hydrogen is of suitable quality for fuel cell vehicles. Contaminants may originate from hydrogen production, transportation, refuelling station or maintenance operation. This study investigated the probability of presence of the 13 gaseous contaminants (ISO 14687-2) in hydrogen on 3 production processes: steam methane reforming (SMR) process with pressure swing adsorption (PSA), chlor-alkali membrane electrolysis process and water proton exchange membrane electrolysis process with temperature swing adsorption. The rationale behind the probability of contaminant presence according to process knowledge and existing barriers is highlighted. No contaminant was identified as possible or frequent for the three production processes except oxygen (frequent for chlor-alkali membrane process), carbon monoxide (frequent) and nitrogen (possible) for SMR with PSA. Based on it, a hydrogen quality assurance plan following ISO 19880-8 can be devised to support hydrogen providers in monitoring the relevant contaminants.