Vented confined explosions involving methane/hydrogen mixtures

The EC funded Naturalhy project is assessing the potential for using the existing gas infrastructure for conveying hydrogen as a mixture with natural gas (methane). The hydrogen could then be removed at a point of use or the natural gas/hydrogen mixture could be burned in gas-fired appliances thereby providing reduced carbon emissions compared to natural gas. As part of the project, the impact on the safety of the gas system resulting from the addition of hydrogen is being assessed. A release of a natural gas/hydrogen mixture within a vented enclosure (such as an industrial housing of plant and equipment) could result in a flammable mixture being formed and ignited. Due to the different properties of hydrogen, the resulting explosion may be more severe for natural gas/hydrogen mixtures compared to natural gas. Therefore, a series of large scale explosion experiments involving methane/hydrogen mixtures has been conducted in a 69.3 m³ enclosure in order to assess the effect of different hydrogen concentrations on the resulting explosion overpressures. The results showed that adding up to 20% by volume of hydrogen to the methane resulted in a small increase in explosion flame speeds and overpressures. However, a significant increase was observed when 50% hydrogen was added. For the vented confined explosions studied, it was also observed that the addition of obstacles within the enclosure, representing congestion caused by equipment and pipework, etc., increased flame speeds and overpressures above the levels measured in an empty enclosure. Predictions of the explosion overpressure and flame speed were also made using a modified version of the Shell Global Solutions model, SCOPE. The modifications included changes to the burning velocity and other physical properties of methane/hydrogen mixtures. Comparisons with the experimental data showed generally good agreement.     

Hydrogen permeation from CGH2 vehicles in garages: CFD dispersion calculations and experimental validation

The time and space evolution of the distribution of hydrogen in confined settings was investigated computationally and experimentally for permeation from typical compressed gaseous hydrogen (CGH2) storage systems for buses or cars. The main goal was to examine whether hydrogen is distributed homogeneously within a garage-like facility or whether stratified conditions are developed, under certain conditions. The nominal hydrogen flow rate considered was 1.087 L/min in a bus facility with a volume of 681 m³. The release was assumed to be directed upwards from a 0.15 m diameter hole located at the middle part of the bus cylinders casing. Ventilation rates up to 0.03 air changes per hour (ACH) were considered. Simulated time periods extended up to 20 days. The CFD simulations performed with the ADREA-HF code showed that fully homogeneous conditions exist for low ventilation rates, while stratified conditions prevail for higher ventilation rates. Regarding flow structure it was found that the vertical concentration profiles can be considered as the superposition of the concentration at the floor (driven by diffusion) plus a concentration difference between floor and ceiling (driven by buoyancy forces). In all cases considered this concentration difference was found to be less than 0.5%. The dispersion experiments were performed in a large scale garage-like enclosure of 40 m³ using helium (GARAGE facility). Comparison between CFD simulations and experiments showed that the predicted concentrations were in good agreement with the experimental data. Finally, simulations were performed using two integral models: the fully homogeneous model and a two-layer model and the results were compared both against CFD and the experimental data.     

The rapid formation and dispersion of flammable zones within cylindrical vertical enclosures following the release of a fixed mass of hydrogen and other gaseous fuels into air

The release of a certain mass of fuel gas into the ambient atmosphere with negligible pressure difference whether deliberately or inadvertently results in the transient formation of flammable mixture zones for a period of time that represent a potential fire and explosion hazard. A numerical model based on the simultaneous solution of the equations of conservation of mass, momentum and energy has been developed to describe the development of such flammable zones when a finite quantity of fuel is released into the overlaying air within cylindrical vertical enclosures open to the outside atmosphere. Hydrogen disperses into the air extremely quickly with a strong temporal dependency on both horizontal and vertical directions. Comparison of the typical behavior of hydrogen dispersion with that of the lighter than air methane, the nearly buoyancy neutral ethylene and the much heavier than air propane is made. Some guidelines for reducing the fire and explosion hazards in such situations are presented.     

Hydrogen dispersion phenomenon in nominally closed spaces

The hydrogen dispersion phenomenon in an enclosure depends on the ratio of the gas buoyancy-induced momentum and diffusive motions. Random diffusive motions of individual gas particles become dominative when the release momentum is low, and a uniform hydrogen concentration appears in the enclosure instead of the gas cumulation below the ceiling. The expected hydrogen behavior could be projected by the Froude number, which value ~1 predicts a decline of buoyancy. This paper justifies this hypothesis by demonstrating full-scale experimental results of hydrogen dispersion within a confined space under six different release variations. During the experiments, hydrogen was released into the test room of 60 m³ volume in two methods: through a nozzle and through 21 points evenly distributed on the emission box cover (multi-point release). Each release method was tested with three volume flow rates (3.2 × 10⁻³ m³/s, 1.6 × 10⁻³ m³/s, 3.3 × 10⁻⁴ m³/s). The tests confirm the decrease of hydrogen buoyancy and its stratification tendencies when the Mach, Reynolds, and Froud number values decrease. Because the hydrogen dispersion phenomenon would impact fire and explosive hazards, the presented experimental results could help fire protection systems be in an enclosure designed, allowing their effectiveness optimization.     

Pressure peaking phenomenon: Model validation against unignited release and jet fire experiments

The aim of this study is validation of pressure peaking phenomenon models for unignited and ignited releases of hydrogen in enclosures with limited ventilation, e.g. residential garages. The existence of “unexpected” peak in the pressure transient during release of a lighter than air gas in a vented enclosure was observed by Brennan et al. (2010) by carrying out theoretical and numerical research. The amplitude and duration of this pressure peak vary depending on the enclosure volume, vent size and leak flow rate. The peak can significantly exceed the steady-state overpressure, which is reached when the enclosure is fully occupied by leaking with a constant rate gas. The pressure peaking phenomenon can jeopardise a civil structure integrity in the case of accident if it is ignored at the design stage of hydrogen-powered vehicles. This could cause serious life safety and property protection issues that requires development of prevention and mitigation strategies and innovative safety engineering solutions. The experimental validation of the phenomenon was absent up to this work. The previous model for unignited release and developed in this study model for ignited release (jet fire) have been validated against experiments performed in a vented enclosure of 1 m³ volume with three different gases: air, helium, and hydrogen. The model for unignited release reproduces closely the experimental pressure peak and the pressure dynamics within the enclosure. The model for ignited release reproduces the pressure peak with acceptable engineering accuracy, and the simulation of pressure dynamics after the peak requires the increase of the discharge coefficient due to the change of vent flow from heavier air at the start to lighter hot combustion products afterwards and ultimately hydrogen. The methodology to calculate the pressure peaking phenomenon in two steps is described in detail. Examples of pressure peaking phenomenon calculation for typical hydrogen applications are presented. The phenomenon is relevant to most of indoor applications, when release of lighter than air gas is possible in an enclosure with limited ventilation. It must be considered when performing safety engineering design of inherently safer hydrogen systems and infrastructure.     

Natural hydrogen and blend gas: a dynamic model of accumulation

Natural hydrogen is accumulating in the form of “blend gases”, with the association mainly of methane, nitrogen, and helium. The extreme variation of proportion between these four gas compounds is explained through a model of dynamic accumulation, where some compounds as helium are chemically inert in the reservoir, nitrogen almost inert, methane is altered into CO₂, and hydrogen is severely altered mainly in protons, and in a smaller degree in methane. All the gas compounds may also leak out of the reservoir, either through advection without any chemical fractionation, or through solubilization in water and diffusion, inducing a variable composition for the residual gas remaining in the accumulation.The model can explain the concomitance of high concentration of helium, clearly accumulated through geological time, with hydrogen, supposedly renewable in human time scales. With the best choice of parameters for the model, the ages of hydrogen accumulation known in the World are small, the hydrogen being diluted more and more in nitrogen and methane through geological time. Diffusive leakage through water appears negligeable compared to advective leakage and compared to hydrogen reactivity in the reservoir. The example of the hydrogen field of Bourakébougou would present an age of 500 years according to the model. Diffusive leakage through water solubilization has a small effect on gas composition. Advective leakage, even not fractionating, has a significant effect on accumulated gas composition, because of its mixing with active deep fluxes. During future production of natural hydrogen accumulations, it is predicted that the proportion of hydrogen should increase during production time, whereas the helium and nitrogen concentrations should decrease.     

Experimental study on hydrogen explosions in a full-scale hydrogen filling station model

In order for fuel cell vehicles to develop a widespread role in society, it is essential that hydrogen refuelling stations become established. For this to happen, there is a need to demonstrate the safety of the refuelling stations. The work described in this paper was carried out to provide experimental information on hydrogen outflow, dispersion and explosion behaviour. In the first phase, homogeneous hydrogen–air mixtures of a known concentration were introduced into an explosion chamber and the resulting flame speed and overpressures were measured. Hydrogen concentration was the dominant factor influencing the flame speed and overpressure. Secondly, high-pressure hydrogen releases were initiated in a storage room to study the accumulation of hydrogen. For a steady release with a constant driving pressure, the hydrogen concentration varied as the inlet airflow changed, depending on the ventilation area of the room, the external wind conditions and also the buoyancy induced flows generated by the accumulating hydrogen. Having obtained this basic data, the realistic dispersion and explosion experiments were executed at full-scale in the hydrogen station model. High-pressure hydrogen was released from 0.8 to 8.0 mm nozzle at the dispenser position and inside the storage room in the full-scale model of the refuelling station. Also the hydrogen releases were ignited to study the overpressures that can be generated by such releases. The results showed that overpressures that were generated following releases at the dispenser location had a clear correlation with the time of ignition, distance from ignition point.     

Simulation of catalytic oxidation of lean hydrogen–methane mixtures

The combustion of preheated lean homogeneous mixtures of hydrogen with methane in air in a catalytic packed-bed reactor was modeled at atmospheric pressure. The non-equilibrium, one-dimensional model developed employs multi-step surface and gas-phase reactions and accounts for the three modes of heat transfer within the bed as well as for heat loss from the bed. The catalyst considered was platinum. It was demonstrated that the model could predict the effects of changes in operational conditions such as inlet mixture temperature, fuel composition and mixture equivalence ratio on the methane and hydrogen conversions, as well as species concentrations and gas temperature profiles along the bed. It was shown that the hydrogen is consumed completely within the early part of the reactor length in all the cases considered for simulations. It was also shown that the improving effect of hydrogen on methane conversion is particularly evident at relatively low inlet temperatures and for very lean mixtures. However, this effect diminishes significantly with increasing inlet temperature and equivalence ratio. It was also shown that the positive effect of hydrogen addition which is more pronounced at its low concentrations in the fuel mixture, decreases somewhat with a further increase of the hydrogen content. The displayed trends were in good agreement with the corresponding experimentally observed.     

Measurements of effective diffusion coefficients of helium and hydrogen through gypsum

An experimental apparatus, which was based on the ¼-scale garage previously used for studying helium release and dispersion in our laboratory, was used to obtain effective diffusion coefficients of helium and hydrogen (released as forming gas for safety reasons) through gypsum panel. Two types of gypsum panel were used in the experiments. Helium or forming gas was released into the enclosure from a Fischer burner¹ located near the enclosure floor for a fixed duration and then terminated. Eight thermal-conductivity sensors mounted at different vertical locations above the enclosure floor were used to monitor the temporal and spatial gas concentrations. An electric fan was used inside the enclosure to mix the released gas to ensure a spatially uniform gas concentration to minimize stratification. The temporal variations of the pressure difference between the enclosure interior and the ambience were also measured. An analytical model was developed to extract the effective diffusion coefficients from the experimental data.     

A correlation for heat transfer during laminar natural convection in an enclosure containing uniform mixture of air and hydrogen

In this paper, a correlation for heat transfer due to laminar natural convection in a rectangular enclosure containing a uniform mixture of hydrogen and air with vertical walls at different temperatures is proposed. This correlation is in terms of Nusselt and Rayleigh numbers evaluated by taking properties of air alone. Mixture properties, viz., density, viscosity, specific heat capacity and thermal conductivity are not needed in this correlation. A modification factor which is based on the mole fraction of hydrogen in the mixture accounts for the differences in heat exchange due to differences in the properties of mixture and pure air. Thus, this correlation is easier to use for a dilute mixture of hydrogen and air as compared to the conventional correlations that are based on mixture properties which may be cumbersome to evaluate. Further, the results highlight that heat transfer correlations for a mixture of gases can be expressed in terms non-dimensional numbers for dominant gas and a correction factor for the gas mixing with it. An in-house code HDS (Hydrogen Distribution Simulator), which has well validated modules for calculation of mixture properties, has been used to carry out numerical study and establish this heat transfer correlation.     

Hyper experiments on catastrophic hydrogen releases inside a fuel cell enclosure

In the frame of the EC-funded project HYPER [1] Pro-Science GmbH performed distribution and combustion experiments on the hazard potential of a severe hydrogen leakage inside a fuel cell cabinet using a generic enclosure model with the dimensions of a commercially available fuel cell application. Hydrogen amounts from 1.5 to 15 g were released within 1 s into the enclosure. In distribution experiments the effects of different venting characteristics and different amounts of internal enclosure obstruction on the hydrogen concentrations measured at fixed positions in- and outside the model were investigated. Subsequently combustion experiments with ignition positions in- and outside the enclosure and two different ignition times were performed. BOS (Background-Oriented-Schlieren) observation combined with pressure and light emission measurements were performed to describe characteristics and hazard potential of the induced hydrogen combustions. The experiments provide new experimental data on the distribution and combustion behaviour of hydrogen releases into a partly vented and partly obstructed enclosure with different venting characteristics.     

Effect of nitrogen on deflagration characteristics of hydrogen / methane mixture

In order to study the influence of nitrogen on the deflagration characteristics of premixed hydrogen/methane, the explosion parameters of premixed hydrogen/methane within various volume ratios and different dilution ratios were studied by using a spherical flame method at room temperature and pressure. The results are as follows: The addition of nitrogen makes the upper limit of explosion of hydrogen/methane premixed gas drop, and the lower limit rises. For explosion hazard (F-number), hydrogen/methane premixed fuel with a hydrogen addition ratio of 10% has the lowest risk, and nitrogen has a greater impact on the dangerous degree of hydrogen and methane premixed gas whose hydrogen addition ratio does not exceed 30%. In terms of flame structure, the spherical flame was affected by buoyancy instability as the percentage of nitrogen dilution increased, but the buoyancy instability gradually decreased as the percentage of hydrogen addition increased. The addition of diluent gas reduces the spreading speed of the stretching flame and reduces the stretching rate in the initial stage of flame development. The laminar flame propagation velocity calculated by the experiment in this paper is consistent with the laminar flow velocity of the hydrogen/methane premixed gas calculated by GRI Mech 3.0. Considering the explosion parameters such as flammability limit, laminar combustion rate and deflagration index, when hydrogen is added to 70%, it is the turning point of hydrogen/methane premixed fuel.     

环境风对燃气轮机通风系统影响的数值模拟研究

工业燃气轮机在全封闭的箱装体中运行时,需要设置通风系统。当外界环境风速较高时,会对通风系统的进口和出口条件造成影响,进而影响燃气轮机通风系统的整体性能。本文以某型燃气轮机通风系统为研究对象,采用数值模拟方法,研究不同风速(0.5～55.2 m/s)和风向(顺风、逆风、侧风)对通风性能的影响,并与现场实际测得的数据进行对比,验证数值模拟方法的准确性。通过分析不同风况下的结果发现,当机组侧向来风时对通风性能的影响较大。对此的解决方案为将迎风一侧的排风口封死,使通风系统单侧排风。计算结果表明：这种方案可保证通风系统在7级风以上情况下以额定工况的76%继续工作。     

Hydrogen non-premixed combustion in enclosure with one vent and sustained release: Numerical experiments

Numerical experiments are performed to understand different regimes of hydrogen non-premixed combustion in an enclosure with passive ventilation through one horizontal or vertical vent located at the top of a wall. The Reynolds averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) model with a reduced chemical reaction mechanism is described in detail. The model is based on the renormalization group (RNG) k-ε turbulence model, the eddy dissipation concept (EDC) model for simulation of combustion coupled with the 18-step reduced chemical mechanism (8 species), and the in-situ adaptive tabulation (ISAT) algorithm that accelerates the reacting flow calculations by two to three orders of magnitude. The analysis of temperature and species (hydroxyl, hydrogen, oxygen, water) concentrations in time, as well as the velocity through the vent, shed a light on regimes and dynamics of indoor hydrogen fires. A well-ventilated fire is simulated in the enclosure at a lower release flow rate and complete combustion of hydrogen within the enclosure. Fire becomes under-ventilated at higher release flow rates with two different modes observed. The first mode is the external flame stabilised at the enclosure vent at moderate release rates, and the second mode is the self-extinction of combustion inside and outside the enclosure at higher hydrogen release rates. The simulations demonstrated a complex reacting flow dynamics in the enclosure that leads to formation of the external flame or the self-extinction. The air intake into the enclosure at later stages of the process through the whole vent area is a characteristic feature of the self-extinction regime. This air intake is due to faster cooling of hot combustion products by sustained colder hydrogen leak compared to the generation of hot products by the ceasing chemical reactions inside the enclosure and hydrogen supply. In general, an increase of hydrogen sustained release flow rate will change fire regime from the well-ventilated combustion within the enclosure, through the external flame stabilised at the vent, and finally to the self-extinction of combustion throughout the domain.     

High-pressure release and dispersion of hydrogen in a partially enclosed compartment: Effect of natural and forced ventilation

The study of compressed hydrogen releases from high-pressure storage systems has practical application for hydrogen and fuel cell technologies. Such releases may occur either due to accidental damage to a storage tank, connecting piping, or due to failure of a pressure release device (PRD). Understanding hydrogen behavior during and after the unintended release from a high-pressure storage device is important for development of appropriate hydrogen safety codes and standards and for the evaluation of risk mitigation requirements and technologies. In this paper, the natural and forced mixing and dispersion of hydrogen released from a high-pressure tank into a partially enclosed compartment is investigated using analytical models. Simple models are developed to estimate the volumetric flow rate through a choked nozzle of a high-pressure tank. The hydrogen released in the compartment is vented through buoyancy induced flow or through forced ventilation. The model is useful in understanding the important physical processes involved during the release and dispersion of hydrogen from a high-pressure tank into a compartment with vents at multiple levels. Parametric studies are presented to identify the relative importance of various parameters such as diameter of the release port and air changes per hour (ACH) characteristic of the enclosure. Compartment overpressure as a function of the size of the release port is predicted. Conditions that can lead to major damage of the compartment due to overpressure are identified. Results of the analytical model indicate that the fastest way to reduce flammable levels of hydrogen concentration in a compartment is by blowing through the vents. Model predictions for forced ventilation are presented which show that it is feasible to effectively and rapidly reduce the flammable concentration of hydrogen in the compartment following the release of hydrogen from a high-pressure tank.     

Comparison of the explosion characteristics of hydrogen,propane, and methane clouds at the stoichiometric concentrations

Numerical simulations were performed to study explosion characteristics of the unconfined clouds. The examined cloud volume was 4 m × 4 m × 2 m. The build-in obstruction inside the cloud was the 8 × 8 × 4 perpendicular rod array. The obstacle volume blockage ratio was 0.74. Three gases were considered: hydrogen/air at the stoichiometric concentrations, propane/air at the stoichiometric concentrations, and methane/air at the stoichiometric concentrations. The hydrogen/air cloud explosion has higher peak overpressure and the overpressure rises locally at the nearby region of the cloud boundary. The explosion overpressures of both methane/air and propane/air are lower, compared with the hydrogen/air, and decreases with distance. The maximum peak dynamic pressure is reached beyond the original cloud, which is clearly different from the explosion peak overpressure tends. Furthermore, dynamic pressure of a cloud explosion is of the same order as overpressure. The explosion flame region for the hydrogen/air cloud is approximately 1.25 times of the original width of the cloud. The explosion flame regions for propane/air or methane/air clouds are approximately 1.4 times of the original width of the cloud. Unlike the explosion overpressures, the explosion temperatures have little difference between the three mixture examined in this study. The higher energy of explosive mixture generates a high temperature hazard effect, but the higher energy of explosive mixture may not generate a larger overpressure hazard effect in a gas explosion accident.     

Modeling the development of hydrogen vapor cloud considering the presence of air humidity

A three-dimensional CFD model for large-scale liquid hydrogen spills is developed and validated by the experiments carried out by NASA. The effect of humidity on the development of hydrogen vapor cloud is emphasized, with the modified expressions of Lee model accounting for the phase changes of water and hydrogen. The results show that the numerical prediction is more consistent with the experiment considering the presence of air humidity. The condensation of water in the atmosphere increases the buoyancy of the vapor cloud, and promotes the diffusion of the cloud in vertical direction. The dimension of the cloud in streamwise direction changes little under different humidity, due to the balance between the height-dependent wind speed and the induced buoyancy. The scope of visible cloud indicated by the condensed water vapor expands with the increasing air humidity, and still lies within the flammable domain when the relative humidity approaching to 75%. Water vapor condensation induces the cloud temperature rise under the same concentration, and the leeward part is more influenced compared with the upwind part.     

Fluid, heat and contaminant transport structures of laminar double-diffusive mixed convection in a two-dimensional ventilated enclosure

The objective of the present work is to investigate the characteristics of the airflow and heat/contaminant transport structures in the indoor air environment by means of a convection transport visualization technique. Laminar double-diffusive mixed convection in a two-dimensional displacement ventilated enclosure with discrete heat and contaminant sources is numerically studied. Based on the governing equations, the fluid, heat, and contaminant transport processes are respectively described by the corresponding streamfunction, heatfunction, and massfunction. Attentions are given to analyze the effects of the main factors—the strength of heat source indicated by the Grashof number (Gr), the strength of contaminant source by the buoyancy ratio (Br), the strength of ventilation by the Reynolds number (Re), and the ventilation mode—on the indoor air environment. Numerical results, presented by the contour function lines, namely, streamlines, heatlines, and masslines, illustrated that the indoor air, heat and contaminant transport structures are mainly determined by the interaction between the internal buoyancy natural convection induced by the discrete heat/contaminant sources and the external forced convection driven by the mechanical ventilation. It is found that the convection transport method could explicitly disclose the complicated philosophy of indoor air environment, and thus provides a simple but practical approach to see the indoor airflow and heat and contaminant transport structures.     

Characteristics of hydrogen production by a plasma–catalyst hybrid converter with energy saving schemes under atmospheric pressure

This paper investigated the production of hydrogen from methane under atmospheric pressure using a plasma–catalyst hybrid converter with emphasis on energy conservation. A spark discharge was used to ionize the hydrocarbon fuel and air mixture with a catalyst to enhance hydrogen production using two energy saving schemes, namely, heat recycling and heat insulation. The experimental results showed that higher methane feeding rate resulted in higher reformate gas temperature and a corresponding increase in methane conversion efficiency. The energy saving systems also enabled the oxygen/carbon ratio to be decreased to reduce oxidation of hydrogen and carbon monoxide and thereby improving the concentrations of hydrogen and carbon monoxide. By heat recycling, a lower methane feeding rate showed an 8.7% improvement in methane conversion efficiency whilst improvement was not apparent with higher methane supply rates due to the already high conversion efficiency. Moreover, it was shown that hydrogen production increased significantly with the reaction from water–gas shifting under the same operation parameters but with high methane selectivity. The best combination resulting in a total thermal efficiency of 77.11% was 10 L/min methane feeding rate and 0.8 O₂/C ratio. With water–gas shifting (S/C ratio=0.5), an 86.26% hydrogen yield, equating to 17.25 L/min hydrogen production rate could be achieved. The equilibrium production rate was calculated using the commercialized HSC Chemistry software (^©ChemSW Software, Inc.). Good correlation was obtained between the calculations and the experimental results.     

Helium dispersion following release in a 1/4-scale two-car residential garage

A series of experiments are described in which helium was released at a constant rate into a 1.5 m × 1.5 m × 0.75 m enclosure designed as a 1/4-scale model of a two-car garage. The purpose was to provide reference datasets for testing and validating computational fluid dynamics (CFD) models and to experimentally characterize the effects of a number of variables on the mixing behavior within an enclosure and the exchange of helium with the outside surroundings. Helium was used as a surrogate for hydrogen, and the total volume released was scaled as the amount that would be released by a typical hydrogen-fueled automobile with a full tank. Temporal profiles of helium were measured at seven vertical locations within the enclosure during and following 1-h and 4-h releases. Idealized vents in one wall sized to provide air exchange rates typical of actual garages were used. The effects of vent size, number, and location were investigated using three different vent combinations. The dependence on leak location was considered by releasing helium at three different points within the enclosure.