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.     

Co-design and aerodynamic study on a two-step high pressure reducing system for hydrogen decompression: From hydrogen refueling station to hydrogen fuel cell vehicle

Fuel for hydrogen fuel cell vehicles comes from hydrogen refueling stations. During the hydrogen filling process, a high-pressure gradient from 35 MPa (hydrogen storage pressure) to 0.16 MPa (fuel cell pressure) is generated. Such a large pressure gradient posed a challenge to the design of the pressure reducing system. Traditional system is difficult to reduce hydrogen pressure from 35 MPa to 0.16 MPa without accompanying large noise and energy consumption. This work is exploring a new concept to combine the multi-stage continuous resistance perforated components and the Tesla valve to design a two-step high pressure reducing system for hydrogen decompression. To validate the superiority of the developed system, a detailed aerodynamic study on the new system is performed, since aerodynamic performance directly affects the operating flexibility and stability. Finally, the optimized co-design of the system is achieved. Results show that the new system is well-designed for hydrogen decompression with the function of control noise and energy consumption. Larger orifice radius (r₁/r₀) and orifice ratio (k) contribute the better aerodynamic performance. Angle α = 45° is considered the best for better aerodynamic performance. The descending order of the effects on better aerodynamic performance is angle (α), row (m), sleeve stage (N), orifice radius (r₁/r₀) and width (t₁/t₀). This study provides basic support for experts to achieve throttling design of related pressure control systems in hydrogen industry.     

Mechanism of hydrogen-restrained crack propagation and practical application research of thermohydrogen treatment in a TiAl-based alloy

The mechanism of hydrogen-restrained crack propagation and practical application of thermohydrogen treatment in a TiAl-based alloy was investigated in this study. Hydrogenated and unhydrogenated alloys were subjected to high-temperature compression test, with a temperature range 1050–1200 °C and strain rate range 0.001–1 s⁻¹. The results showed that crack propagation was restrained due to hydrogen addition. The main mechanism of hydrogen-restrained crack propagation of such alloy was revealed that hydrogen-promoted lamella bending and hydrogen-decreased Young's modulus induced inter-lamellar cracks transforming into trans-lamellar cracks, decreasing cracks in the hydrogenated alloy. Additionally, hydrogen-induced mechanical twinning in γ-phase lamellae partly restrained inter-lamellar crack propagation. In the two-step forging process, the optimum forging parameters were determined. It was found that hydrogen could effectively restrain crack propagation during the two-step forging process. Hydrogen refined grains of the forged billets, which improved toughness of such billets. The hydrogen content of the forged hydrogenated billets could be decreased to a desired value, and the phase composition and content were basically identical to those of the initial unhydrogenated alloy.     

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.     

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.     

Numerical study of hydrogen dispersion in a fuel cell vehicle under the effect of ambient wind

In the rescue of hydrogen-fueled vehicle accidents, once accidental leakage occurs and hydrogen enters the cabin, the relatively closed environment of the vehicle is prone to hydrogen accumulation. Excessive hydrogen concentration inside the vehicle cabin may cause suffocation death of injured passengers and rescue crews, or explosion risk. Based on hydrogen fuel cell vehicle (HFCV) with hydrogen storage pressure 70 MPa, four different scenarios (i. with opened sunroof, ii. opened door windows, iii. opened sunroof and door windows and iv. opened sunroof, door windows and rear windshield) under the condition of accidental leakage were simulated using computational fluid dynamics (CFD) tools. The hydrogen concentration inside the vehicle and the distribution of flammable area (>4% hydrogen mole fraction) were analyzed, considering the effect of ambient wind. The results show that in the case of convection between interior and exterior of the vehicle via the sunroof, door windows or rear windshield, the distribution of hydrogen inside the vehicle is strongly affected by the ambient wind speed. In the least risk case, ambient wind can reduce the hydrogen mole fraction in the front of the vehicle to less than 4%, however the rear of the vehicle is always within flammable risk.     

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.     

Acceleration of hydrogen forced ventilation after leakage ceases in a partially open space

This paper treats the real-time sensing-based risk-mitigation control of hydrogen dispersion and accumulation in a partially open space with low-height openings by forced ventilation. A hunting-preventive control scheme that we previously proposed (Matsuura et al., Int J Hydrogen Energy, 2012;37(2):1972–84) has parameters such as the monitoring period of hydrogen sensors T_p, a unit increment in the exhaust flow rate per area from a roof vent α, and a threshold ε for the change in the exhaust flow rate. Through parametric simulations of the hydrogen exhaust after leakage ceases, we clarify the effects of the parameters on the rate of exhaust flow from the roof vent and the amount of hydrogen accumulating near the roof, which are critical for ventilation performance. With a selected combination of (T_p, α, ε) for which the ventilation system has a quick response and reasonable original performance, we first introduce two acceleration methods separately to the original hunting-preventive scheme to improve the ventilation performance after hydrogen leakage ceases. Ventilation performance employing the two methods is compared with that employing the original scheme. From the results, a hybrid method is finally proposed. The effectiveness of the proposed method is computationally validated for leak flow rates of 9.44 × 10⁻⁴, 4.72 × 10⁻⁴ and 2.36 × 10⁻⁴ m³/s.     

Parametric analysis on multi-stage high pressure reducing valve for hydrogen decompression

Hydrogen fuel cell electric vehicle (FCEV) can reduce air pollution as well as achieve efficient use of hydrogen energy. Farther travel distance requires larger hydrogen storage pressure, thereby imposing more demanding working conditions on the pressure reducing system. In this paper, a multi-stage high pressure reducing valve (MSHPRV) for hydrogen decompression in FCEV is proposed, and the effects of different structural parameters on its internal flow characteristics are investigated to achieve a better hydrogen decompression process. Results show that compared with perforated plate, multi-stage perforated sleeves and valve core hold the dominant position in hydrogen throttling process. Larger multi-stage perforated sleeve diameter, perforated plate diameter and pressure ratio relate to larger hydrogen kinetic energy, turbulence vortex and energy consumption. However, with the increase of perforated plate stage and perforated plate radius, the turbulent intensity and energy consumption inside MSHPRV decreases correspondingly. This study can provide some technical supports for achieving hydrogen decompression in FCEV when facing harsh working conditions, or help with dealing energy conversion during decompression process.     

Parametric study on Tesla valve with reverse flow for hydrogen decompression

Recharge mileage is of great importance for a hydrogen fuel cell electric vehicle. High pressure hydrogen storage can increase the recharge mileage significantly. Before hydrogen flows into the fuel cell, a decompression process is necessary. To overcome the seal of the piping system and realize the decompression, Tesla valve can be well used, since it is a type of check valve without moving parts, and when there is a reverse flow, large pressure drop appears between the inlet and outlet. In order to obtain a better pressure drop performance for a Tesla valve, in this paper, the structural parameters including the hydraulic diameter, the valve angle, and the inner curve radius are investigated for a large range of inlet velocities. The results indicate that a small hydraulic diameter and small inner curve radius but large valve angle can provide a higher pressure drop under a large inlet velocity, while the pressure drop under different structural parameters barely changes under a small inlet velocity (less than 100 m/s). Besides, there is a low-pressure zone behind the outlet of the bend channel, which should be paid attention. This work can be referred by the further applications of Tesla valves in hydrogen fuel cell electric vehicles for hydrogen decompression.     

Strain rate and hydrogen effects on crack growth from a notch in a Fe-high-Mn steel containing 1.1 wt% solute carbon

Effects of strain rate and hydrogen on crack propagation from a notch were investigated using a Fe-33Mn-1.1C steel by tension tests conducted at a cross head displacement speeds of 10⁻² and 10⁻⁴ mm/s. Decreasing cross head displacement speed reduced the elongation by promoting intergranular crack initiation at the notch tip, whereas the crack propagation path was unaffected by the strain rate. Intergranular cracking in the studied steel was mainly caused by plasticity-driven mechanism of dynamic strain aging (DSA) and plasticity-driven damage along grain boundaries. With the introduction of hydrogen, decrease in yield strength due to cracking at the notch tip before yielding as well as reduction in elongation were observed. Coexistence of several hydrogen embrittlement mechanisms, such as hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) were observed at and further away from the notch tip resulting in hydrogen assisted intergranular fracture and cracking which was the key reason behind the ductility reduction.     

Pressure analysis on two-step high pressure reducing system for hydrogen fuel cell electric vehicle

Hydrogen fuel cell electric vehicle (FCEV) can achieve zero exhaust emission and zero pollution. In order to make FCEV reach a farther travel distance, greater demands are put on its pressure reducing system. In this paper, a two-step high pressure reducing system for FCEV is proposed. The system is made up of two parts, a new high multi-stage pressure reducing valve (HMSPRV) and a multi-stage muffler. As a new system, its feasibility has to be verified. Since the valve opening condition has a great effect on hydrogen flow, pressure reduction and energy consumption, different valve opening conditions are taken as the research point. The flow field analysis of the new HMSPRV is conducted on three aspects: pressure field, velocity field and energy consumption. It can be found that both the pressure reducing and velocity increasing gradients mainly reflect at those throttling components for all valve openings. For energy consumption, in the comprehensive study of flow vortexes and turbulent dissipation rate, it can be found that the larger of the valve opening, the larger of energy consumption. Then, a thermo-fluid-solid coupling analysis is conducted on the new HMSPRV, and it is concluded that the new system meets strength requirement. Furthermore, as the second step of the high pressure reducing system, the flow and pressure fields of multi-stage muffler are investigated. The five-stage muffler is exactly designed to complete the whole pressure reducing process. This study can provide technological support for achieving pressure regulation in the hydrogen transport system of FCEV when facing complex conditions, and it can also benefit the further research work on energy saving and multi-stage flow of pressure reducing devices.     

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.     

Mass transport limitations in microchannel methanol-reforming reactors for hydrogen production

The potential of methanol reforming systems to greatly improve productivity in chemical reactors has been limited, due in part, to the effect of mass transfer limitations on the production of hydrogen. There is a need to determine whether or not a microchannel reforming reactor system is operated in a mass transfer-controlled regime, and provide the necessary criteria so that mass transfer limitations can be effectively eliminated in the reactor. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the essential characteristics of mass transport processes in a microchannel reforming reactor and to develop criteria for determining mass transfer limitations. The reactor was designed for thermochemically producing hydrogen from methanol by steam reforming. The mass transfer effects involved in the reforming process were evaluated, and the role of various design parameters was determined for the thermally integrated reactor. In order to simplify the mathematics of mass transport phenomena, use was made of dimensionless numbers or ratios of parameters that numerically describe the physical properties in the reactor without units. The results indicated that the performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness and through adjusting feed composition to minimize or reduce mass transfer limitations in the reactor. There is not an effective method to reduce channel dimensions if the flow rate remains constant, or to reduce fluid velocities if the residence time is kept constant. The rate of the reforming reaction is limited by mass transfer near the entrance of the reactor and by kinetics further downstream, when the heat transfer in the autothermal system is efficient. Finally, the criteria that can be used to distinguish between different mass transport and kinetics regimes in the reactor with a first-order reforming reaction were presented.     

Numerical investigations on flow characteristics and energy separation in a Ranque Hilsch vortex tube with hydrogen as working medium

In the last decades, the theory of energy separation in vortex tubes is debated broadly based on the heat transfer and work transfer between core and peripheral flow layers. Many parameters were considered in the literature. However, the present study involves the inlet energy considered collectively towards energy separation. In this paper, three-dimensional computational fluid dynamic simulations are discussed in vortex tube to analyze the energy separation phenomena in different cases by varying the working medium such as hydrogen and air having specific heat variation. The energy at the inlet is maintained same in both cases by adjusting the inlet mass flow rate. The results from this study are validated with recently published literature using hydrogen as a working medium. Vortex tube with hydrogen as working medium yields a temperature separation of 8 K lower than air as working medium. Further studies on vortex tube with hydrogen as a working fluid is explored at different inlet temperatures relative to the room temperature. Vortex tube with hydrogen at an inlet temperature of 400 K gives better temperature separation as compared to other inlet temperatures considered in this study.     

Investigation of concentration measurement for hydrogen leakage with a new calibration visual approach

Hydrogen leakage concentration rapid measurement is the key issue for hydrogen application as hydrogen leakage is easy to cause hydrogen safety issues such as hydrogen explosions. Non-invasive visual measurement method such as the schlieren measurement technique is the prospective solution. However, the specific relationship between the hydrogen leakage concentration and schlieren image gray remains unclear, which leads that the schlieren technique procedure is developed for visualization and acquiring qualitative information only. This paper aims to decouple the hydrogen leakage concentration from the complicated schlieren image information, and find the mapping relationship between the hydrogen leakage concentration and schlieren image gray, hence realizing a quantitative hydrogen leakage concentration analysis. Therefore, a hydrogen leakage visualization experimental bench is established to simulate and measure hydrogen leakage by a series of experiments under different leakage concentrations. The mapping relationship between the hydrogen leakage concentration and schlieren image gray is obtained by the experiments using the schlieren technique. Then, a new calibration schlieren technique with the function of visually measuring hydrogen leakage concentration is developed and verified under 80% hydrogen leakage concentration. Results of the trials demonstrated the ability of the proposed technique successfully measure concentration distributions with satisfactory accuracy.     

A numerical model for segmented flow in a microreactor

A numerical model for segmented flow in a microreactor has been developed. The model is based on computational fluid dynamics (CFD) which means that the flow field and mass transfer are described by a set of partial differential equations. A general purpose CFD code was extended in order to predict the internal flow patterns of fluid segments and the transfer of dissolved chemical species within segments and across fluid segment interfaces. The model has been validated by comparing predicted data with experimental microreactor titrations.     

Study on hydrogen dispersion in confined space with complex air supply and exhaust system

The safety issue involving unplanned H₂ leakage has retarded promotion of Hydrogen fuel cell vehicles (HFCVs). When an accidental leakage occurs in a relatively closed space, H₂ tends to accumulate. It is necessary to obtain a better understanding of the H₂ dispersion characteristics in a relatively closed space. Most studies focus on the dispersion features in a small garage, whereas few consider large underground garages with complex ventilation systems. Herein, H₂ released from a pressurized tank in a real large underground garage was investigated by a two-stage computational fluid dynamics (CFD) study. An isentropic expansion model with a real gas equation of state was employed in stage (1) to estimate the source strength which can be used as inlet boundary conditions for stage (2), the dispersion stage. The CFD models were validated against the experimental results. The concentration field and flammability envelope were focused. This study provides a feasible method for assessing the risks associated with HFCVs.     

Numerical studies of premixed hydrogen/air flames in a small-scale combustion chamber with varied area blockage ratio

The increasing use of hydrogen as a renewable source of energy underlines the need to be able to assess the safety risks involved in the event of an accidental explosion. This paper presents numerical studies for hydrogen/air propagating flames at an equivalence ratio of 0.7 in a laboratory-scale combustion chamber equipped with turbulence generating baffles and a solid square cross section obstruction. The large eddy simulation (LES) modelling technique is used with an in-house computational fluid dynamics (CFD) model for compressible flows to study the flow turbulence and the flame propagation characteristics. The study is carried out using four different baffle arrangements and two different solid obstructions with area blockage ratios of 0.24 and 0.5. Results for the generated peak overpressure and the timing at which it occurs following ignition are considered as the primary safety factors. The time histories of the flame speed and position relative to the ignition source are validated against published experimental data. Good agreement is obtained between numerical results and experimental data which enables further predictions where measurements are limited in the study of vented hydrogen explosions. It was concluded that adding successive baffles and increasing the area blockage ratio escalates the maximum rate at which pressure rises and raises the generated peak explosion overpressure.     

Characterization and evaluation of a novel semi-industrial scale vertical shaft furnace for particle spheroidization

In this study, a novel furnace concept for in-flight particle spheroidization is presented, characterized and evaluated. A natural gas fired burner with a continued staged air principle and internal recirculation (COSTAIR), was used to provide the required temperature for the spheroidization process. Therefore, a numerically inexpensive CFD model for the calculation of combustion and multiphase flow is proposed. Particularly for the calculation of particle trajectories and particle peak temperatures of non-spherical (chiseled and flaky) slag particles, the presented CFD model differs in two modifications from the current state-of-the-art CFD models: first, a numerically efficient combustion model with a detailed chemical reaction mechanism was used in order to calculate the temperature profile of the furnace. While in most current state-of-the-art CFD models the numerically expensive and time consuming eddy dissipation concept (EDC) model or the insufficient eddy dissipation model (EDM) are used. Second, the discrete phase model, which is based on a numerically efficient Euler-Lagrangian approach, is used for multiphase modeling. Non-spherical particles are considered by application of a suitable particle drag model from literature. Although, the assumption of spherical particles is more common in current state-of-the-art CFD models for multiphase modeling. It was concluded that the presented furnace concept is applicable for the semi-industrial scale production of spherical boiler slag particles. The numerical results show that the assumption of non-spherical particles, compared to spherical particles, is more suitable for the calculation of particle trajectories and particle peak temperatures in the presented furnace.