Evaluation of an improved CFD model against nine vented deflagration experiments

In the present work, a newly developed CFD deflagration model incorporated into the ADREA-HF code is evaluated against hydrogen vented deflagrations experiments carried out by KIT and FM-Global in a medium (1 m³) and a real (63.7 m³) scale enclosure respectively. A square vent of 0.5 m² and 5.4 m² respectively is located in the center of one of side walls. In the case of the medium scale enclosure the 18% v/v homogeneous hydrogen-air mixture and back-wall ignition case is examined. In the case of the real scale enclosure the examined cases cover different homogeneous mixture concentrations (15% and 18% v/v), different ignition locations (back-wall and center) and different levels of initial turbulence. The CFD model accounts for flame instabilities that develop as the flame propagates inside the chamber and turbulence that mainly develops outside the vent. Pressure predictions are compared against experimental measurements revealing a very good performance of the CFD model for the back-wall ignition cases. For the center ignition cases, the model overestimates the maximum overpressure. The opening of the vent cover is identified as a possible reason for the overprediction. The analysis indicates that turbulence is the main factor which enhances external explosion strength causing the sudden pressure increase, confirming previous findings.     

An inter-comparison exercise on CFD model capabilities to simulate hydrogen deflagrations with pressure relief vents

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 m³ vessel with vents of different size (0.2 m² and 0.3 m²) 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, b0b 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.     

Hydrogen deflagrations in stratified flat layers in the large-scale vented combustion test facility

This paper examines the flame dynamics of vented deflagration in stratified hydrogen layers. It also compares the measured combustion pressure transients with 3D GOTHIC simulations to assess GOTHIC's capability to simulate the associated phenomena. The experiments were performed in the Large-Scale Vented Combustion Test Facility at the Canadian Nuclear Laboratories. The stratified layer was formed by injecting hydrogen at a high elevation at a constant flow rate. The dominant parameters for vented deflagrations in stratified layers were investigated. The experimental results show that significant overpressures are generated in stratified hydrogen–air mixtures with local high concentration even though the volume-averaged hydrogen concentration is non-flammable. The GOTHIC predictions capture the overall pressure dynamics of combustion very well, but the peak overpressures are consistently over-predicted, particularly with higher maximum hydrogen concentrations. The measured combustion overpressures are also compared with Molkov's model prediction based on a layer-averaged hydrogen concentration.     

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.     

Detailed simulations of the transient hydrogen mixing,leakage and flammability in air in simple geometries

During an accidental release, hydrogen disperses very quickly in air due to a relatively high density difference. A comprehensive understanding of the transient behavior of hydrogen mixing and the associated flammability limits in air is essential to support the fire safety and prevention guidelines. In this study, a buoyancy diffusion computational model is developed to simultaneously solve for the complete set of equations governing the unsteady flow of hydrogen. A simple vertical cylinder is considered to investigate the transient behavior of hydrogen mixing, especially at relatively short times, for different release scenarios: (i) the sudden release of hydrogen at the cylinder bottom into air with open, partially open, and closed tops, and (ii) small hydrogen jet leaks at the bottom into a closed geometry. Other cases involving the hydrogen releases/leaks at the cylinder top are also explored to quantify the relative roles of buoyancy and diffusion in the mixing process. The numerical simulations display the spatial and temporal distributions of hydrogen for all the configurations studied. The complex flow patterns demonstrate the fast formation of flammable zones with implications in the safe and efficient use of hydrogen in various applications.     

An inter-comparison exercise on CFD model capabilities to simulate hydrogen deflagrations in a tunnel

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.     

CFD based simulation of hydrogen release through elliptical orifices

Computational Fluid Dynamics (CFD) is employed to investigate the hydrogen jet exiting through different shapes of orifices. The effect of orifice geometry on the structure, development and dispersion of a highly under-expanded hydrogen jet close to the exit is numerically investigated. Various shapes of orifices are evaluated, including holes with constant areas such as elliptical and circular openings, as well as, enlarging circular orifices. A three-dimensional in-house parallel code is exploited to simulate the flow using an unstructured tetrahedral finite volume Euler solver. The numerical simulations indicate that, for a high pressure reservoir hydrogen release, the area of the orifice is the main geometric parameter influencing the centerline pressure at the hydrogen-air interface and the transient peak temperature, while the elliptical or expanding orifices slightly mitigate the auto-ignition risks associated with the accidental release of hydrogen. Therefore, circular openings represent the most conservative geometry for the study of auto-ignition of hydrogen.     

Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 1: Experimental study

If the general public is to use hydrogen as a vehicle fuel, customers must be able to handle hydrogen with the same degree of confidence, and with comparable risk, as conventional liquid and gaseous fuels. Since hydrogen is stored and used as a high-pressure gas, a jet release in a confined or congested area can create an explosion hazard. Therefore, hazards associated with jet releases from leaks in a vehicle-refuelling environment must be considered. As there was insufficient knowledge of the explosion hazards, a study was initiated to gain a better understanding of the potential explosion hazard consequences associated with high-pressure leaks from hydrogen vehicle refuelling systems. Our first paper [1] describes the release and subsequent ignition of a high-pressure hydrogen jet in a simulated dispensing area of a hydrogen vehicle refuelling station. In the present paper, an array of dummy storage cylinders with confining walls (to represent isolation from the forecourt area) was used to represent high-pressure hydrogen cylinder storage congestion. Experiments with ignition of premixed 5.4 m × 6.0 m × 2.5 m hydrogen-air clouds and hydrogen jet releases up to 40 MPa pressures were performed. The results are presented and discussed in relation to the conditions giving the highest overpressures. We concluded from the study that the ignition of a jet release gives much higher local overpressure than in the case of ignition of a homogeneous mixture inside the cylinder storage congestion area. The modelling of these results will be presented in Part 2 of this paper.     

Advancing the hydrogen safety knowledge base

The International Energy Agency's Hydrogen Implementing Agreement (IEA HIA) was established in 1977 to pursue collaborative hydrogen research and development and information exchange among its member countries. Information and knowledge dissemination is a key aspect of the work within IEA HIA tasks, and case studies, technical reports and presentations/publications often result from the collaborative efforts. The work conducted in hydrogen safety under Task 31 and its predecessor, Task 19, can positively impact the objectives of national programs even in cases for which a specific task report is not published. The interactions within Task 31 illustrate how technology information and knowledge exchange among participating hydrogen safety experts serve the objectives intended by the IEA HIA.     

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.     

CFD model for charge and discharge cycle of adsorptive hydrogen storage on activated carbon

High surface area activated carbons and other microporous adsorbents have generated a significant amount of interest over the past decade as storage media for hydrogen and natural gas, due to their high storage capacity at low temperatures and their use in gas purification processes. This paper uses computational fluid dynamics (CFD) to simulate the charging and discharging of a sorption-based hydrogen storage system. The CFD model is based on the mass, momentum and energy conservation equations of a system formed of gaseous and adsorbed hydrogen, an activated carbon bed and steel tank walls. The adsorption process is modeled using the Dubinin–Astakov adsorption isotherms extended to the supercritical regime. The model is implemented using Fluent. In our study, we can obtain accuracy peak temperature of simulation due to a non-constant isosteric heat of adsorption is used, derived from the model isotherms. We adopt piecewise heat capacity to consider the heat capacity of the adsorbed phase of hydrogen. We can make a conclusion that the simulated temperatures without consideration of heat capacity for hydrogen in adsorbed phase (c_pa), rise faster and reach higher peaks than the simulated temperatures with consideration of c_pa, and diverge more from experimental results. Also, we study the changes of temperature, pressure and adsorption during the charging and discharging processes as well as when the system is idle (which we define as dormancy) in the case of room temperature water cooling. The results are compared with experimental data from a storage unit cooled with room temperature water. The simulated pressure is in a good agreement with the experimental values. The simulated temperature profiles are also generally in good agreement with the experimental values, except close to the inlet and the wall. In addition, we have studied the effect of quality of the mesh on the accuracy and stability of the numerical computation and the influence of the mass flow rates on temperature and adsorption capacity.     

Modelling the mitigation of hydrogen deflagrations in a vented cylindrical rig with water fog and nitrogen dilution

Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of mitigating the overpressure rise, should a hydrogen deflagration in a vented enclosure occur. A numerical CFD gas explosion code (FLACS) has been used to simulate the pressure-time curves and the rate of pressure rise generated following the ignition of different hydrogen–oxygen–nitrogen mixtures in a small scale vented cylindrical explosion rig. This has allowed the potential mitigating effect of nitrogen dilution (reduced oxygen) and very fine water fog, used both alone and in combination, to be explored and permitted their direct comparison with corresponding experimental test data.     

A modelling evaluation of an ammonia-fuelled microchannel reformer for hydrogen generation

Hydrogen production from an ammonia-fuelled microchannel reactor is simulated in a three-dimensional (3D) model implemented via Comsol Multiphysics™. The work described in this paper endeavours to obtain a mathematical framework that provides an understanding of reaction-coupled transport phenomena within the microchannel reactor. The transport processes and reactor performance are elucidated in terms of velocity, temperature, and species concentration distributions, as well as local reaction rate and NH₃ conversion profiles. The baseline case is first investigated to comprehend the behaviour of the microchannel reactor, then microstructural design and operating parameters are methodically altered around the baseline conditions to explore the optimum values. The simulation results show that an optimum NH₃ space velocity (GHSV) of 65,000 Nml g_cat⁻¹ h⁻¹ yields 99.1% NH₃ conversion and a power density of 32 kW_e L⁻¹ at the highest operating temperature of 973 K. It is also shown that a 40-μm-thick porous washcoat is most desirable at these optimum conditions. Finally, a low channel hydraulic diameter (225 μm) is observed to contribute to high NH₃ conversion. Mass transport limitations in the porous-washcoat and gas-phase are negligible as depicted by the Damköhler and Fourier numbers, respectively. The experimental microchannel reactor yields 98.2% NH₃ conversion and a power density of 30.8 kW_e L⁻¹ when tested at the optimum operating conditions established by the model. Good agreement with experimental data is observed, so the integrated experimental-modelling approach developed in this paper may well provide an incisive step toward the efficient design of ammonia-fuelled microchannel reformers.     

Effect of gas equation of state on CFD predictions for ignition characteristics of hydrogen escaping from a tank

This work involves the investigation of the sensitivity of computational fluid dynamics based models of auto ignition of hydrogen gas escaping into the surroundings to the use of an ideal gas and a real gas Noble–Abel equation of state. Ensuring consistent modeling techniques when the real gas equation of state is implemented, real gas based thermodynamic properties, real gas based property mixture models, and real gas based chemical equilibrium constant formulations are utilized. Within the standard computational fluid dynamics models, a customized chemical kinetic equation integrator is employed. An LES based turbulence model is implemented. For tank pressures of 40, 80, and 120 MPa, differences in the gas conditions, including gas pressures, temperatures, velocities, flow rates, energy, and chemical species mass fractions, are compared. The relationships between the local and time varying gas conditions, chemical reaction indicators, the tank pressure, and the equation of state captured in the simulations are described in detail. The results clearly show the increasing deviation between the ideal gas and Noble–Abel based results as the tank pressure increases, indicating the importance of the use of the proper material model and chemical equilibrium formulation for the conditions of interest.     

Modeling thermal response of polymer composite hydrogen cylinders subjected to external fires

With the anticipated introduction of hydrogen fuel cell vehicles to the market, there is an increasing need to address the fire resistance of hydrogen cylinders for onboard storage. Sufficient fire resistance is essential to ensure safe evacuation in the event of car fire accidents. The authors have developed a Finite Element (FE) model for predicting the thermal response of composite hydrogen cylinders within the frame of the open source FE code Elmer. The model accounts for the decomposition of the polymer matrix and effects of volatile gas transport in the composite. Model comparison with experimental data has been conducted using a classical one-dimensional test case of polymer composite subjected to fire. The validated model was then used to analyze a type-4 hydrogen cylinder subjected to an engulfing external propane fire, mimicking a published cylinder fire experiment. The external flame is modelled and simulated using the open source code FireFOAM. A simplified failure criteria based on internal pressure increase is subsequently used to determine the cylinder fire resistance.     

Evaluation of a combustion model for the simulation of hydrogen spark-ignition engines using a CFD code

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k–? turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations.     

3D risk management for hydrogen installations

This paper introduces the 3D risk management (3DRM) concept, with particular emphasis on hydrogen installations (Hy3DRM). The 3DRM framework entails an integrated solution for risk management that combines a detailed site-specific 3D geometry model, a computational fluid dynamics (CFD) tool for simulating flow-related accident scenarios, methodology for frequency analysis and quantitative risk assessment (QRA), and state-of-the-art visualization techniques for risk communication and decision support. In order to reduce calculation time, and to cover escalating accident scenarios involving structural collapse and projectiles, the CFD-based consequence analysis can be complemented with empirical engineering models, reduced order models, or finite element analysis (FEA). The paper outlines the background for 3DRM and presents a proof-of-concept risk assessment for a hypothetical hydrogen filling station. The prototype focuses on dispersion, fire and explosion scenarios resulting from loss of containment of gaseous hydrogen. The approach adopted here combines consequence assessments obtained with the CFD tool FLACS-Hydrogen from Gexcon, and event frequencies estimated with the Hydrogen Risk Assessment Models (HyRAM) tool from Sandia, to generate 3D risk contours for explosion pressure and radiation loads. For a given population density and set of harm criteria, it is straightforward to extend the analysis to include personnel risk, as well as risk-based design such as detector optimization. The discussion outlines main challenges and inherent limitations of the 3DRM concept, as well as prospects for further development towards a fully integrated framework for risk management in organizations.     

Forced ventilation for sensing-based risk mitigation of leaking hydrogen in a partially open space

This study performs the numerical simulation of hydrogen dispersion in a partially open space with a single roof vent. The effects of various roof vent positions, leak positions, leak flow rates and exhaust flow rates on the forced ventilation of leaking hydrogen, are shown and discussed. Based on the results, a proper roof vent position and the disadvantage of ventilation with constant exhaust flow rates are established. To overcome the disadvantage, a new control strategy to change exhaust flow rates with the roof vent fixed at the proper position is proposed. First a plot is constructed to show acceptable exhaust flow rates to various inflow rates and leak positions. Assuming real-time sensing of hydrogen concentration and height-direction velocity, volume flow rates of leaking hydrogen are then estimated. Based on the estimated leak flow rates and hydrogen sensor information near the roof, control is conducted considering the plot of acceptable exhaust flow rates to various inflow rates and leak positions. The proposed method is validated against various leak positions, leak flow rates and leak modes. This paper proposes an innovative approach to sensing-based risk mitigation control of hydrogen dispersion and accumulation in a partially open space by forced ventilation.     

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.