Dispersion of cryogenic hydrogen through high-aspect ratio nozzles

Liquid hydrogen is increasingly being used as a delivery and storage medium for stations that provide compressed gaseous hydrogen for fuel cell electric vehicles. In efforts to provide scientific justification for separation distances for liquid hydrogen infrastructure in fire codes, the dispersion characteristics of cryogenic hydrogen jets (50–64 K) from high aspect ratio nozzles have been measured at 3 and 5 bar_abs stagnation pressures. These nozzles are more characteristic of unintended leaks, which would be expected to be cracks, rather than conventional round nozzles. Spontaneous Raman scattering was used to measure the concentration and temperature field along the major and minor axes. Within the field of interrogation, the axis-switching phenomena was not observed, but rather a self-similar Gaussian-profile flow regime similar to room temperature or cryogenic hydrogen releases through round nozzles. The concentration decay rate and half-widths for the planar cryogenic jets were found to be nominally equivalent to that of round nozzle cryogenic hydrogen jets indicating a similar flammable envelope. The results from these experiments will be used to validate models for cryogenic hydrogen dispersion that will be used for simulations of alternative scenarios and quantitative risk assessment.     

Simulation of small-scale releases from liquid hydrogen storage systems

Knowledge of the concentration field and flammability envelope from small-scale leaks is important for the safe use of hydrogen. These small-scale leaks may occur from leaky fittings or o-ring seals on liquid hydrogen-based systems. The present study focuses on steady-state leaks with large amounts of pressure drop along the leak path such that hydrogen enters the atmosphere at near atmospheric pressure (i.e. Very low Mach number). A three-stage buoyant turbulent entrainment model is developed to predict the properties (trajectory, hydrogen concentration and temperature) of a jet emanating from the leak. Atmospheric hydrogen properties (temperature and quality) at the leak plane depend on the storage pressure and whether the leak occurs from the saturated vapor space or saturated liquid space. In the first stage of the entrainment model ambient temperature air (295 K) mixes with the leaking hydrogen (20-30 K) over a short distance creating an ideal gas mixture at low temperature (∼65 K). During this process states of hydrogen and air are determined from equilibrium thermodynamics using models developed by NIST. In the second stage of the model (also relatively short in distance) the radial distribution of hydrogen concentration and velocity in the jet develops into a Gaussian profile characteristic of free jets. The third and by far the longest stage is the part of the jet trajectory where flow is fully developed. Results show that flammability envelopes for cold hydrogen jets are generally larger than those of ambient temperature jets. While trajectories for ambient temperature jets depend solely on the leak densimetric Froude number, results from the present study show that cold jet trajectories depend on the Froude number and the initial jet density ratio. Furthermore, the flammability envelope is influenced by the hydrogen concentration in the jet at the beginning of fully developed flow.     

Experimental and numerical investigation of DDT in hydrogen–Air behind a single obstacle

Two-dimensional numerical simulations of deflagration-to-detonation transition (DDT) in hydrogen–air mixtures are presented and compared with experiments. The investigated geometry was a 3 m long square channel. One end was closed and had a single obstacle placed 1 m from the end, and the other end was open to the atmosphere. The mixture was ignited at the closed end. Experiments and simulations showed that DDT occurred within 1 m behind the obstacle. The onset of detonation followed a series of local explosions occurring far behind the leading edge of the flame in a layer of unburned reactants between the flame and the walls. A local explosion was also seen in the experiments, and the pressure records indicated that there may have been more. Furthermore, local explosions were observed in the experiments and simulations which did not detonate. The explosions should have sufficient strength and should explode in a layer of sufficient height to result in a detonation.     

Numerical predictions of cryogenic hydrogen vertical jets

Comparison of Computational Fluid Dynamics (CFD) predictions with measurements is presented for cryo-compressed hydrogen vertical jets. The stagnation conditions of the experiments are characteristic of unintended leaks from pipe systems that connect cryogenic hydrogen storage tanks and could be encountered at a fuel cell refueling station. Jets with pressure up to 5 bar and temperatures just above the saturation liquid temperature were examined. Comparisons are made to the centerline mass fraction and temperature decay rates, the radial profiles of mass fraction and the contours of volume fraction. Two notional nozzle approaches are tested to model the under-expanded jet that was formed in the tests with pressures above 2 bar. In both approaches the mass and momentum balance from the throat to the notional nozzle are solved, while the temperature at the notional nozzle was assumed equal to the nozzle temperature in the first approach and was calculated by an energy balance in the second approach. The two approaches gave identical results. Satisfactory agreement with the measurements was found in terms of centerline mass fraction and temperature. However, for test with 3 and 4 bar release the concentration was overpredicted. Furthermore, a wider radial spread was observed in the predictions possibly revealing higher degree of diffusion using the k-ε turbulence model. An integral model for cryogenic jets was also developed and provided good results. Finally, a test simulation was performed with an ambient temperature jet and compared to the cold jet showing that warm jets decay faster than cold jets.     

Hydrogen jet fires in a passively ventilated enclosure

This paper describes a combined experimental, analytical and numerical modelling investigation into hydrogen jet fires in a passively ventilated enclosure. The work was funded by the EU Fuel Cells and Hydrogen Joint Undertaking project Hyindoor. It is relevant to situations where hydrogen is stored or used indoors. In such situations passive ventilation can be used to prevent the formation of a flammable atmosphere following a release of hydrogen. Whilst a significant amount of work has been reported on unignited releases in passively ventilated enclosures and on outdoor hydrogen jet fires, very little is known about the behaviour of hydrogen jet fires in passively ventilated enclosures. This paper considers the effects of passive ventilation openings on the behaviour of hydrogen jet fires. A series of hydrogen jet fire experiments were carried out using a 31 m³ passively ventilated enclosure. The test programme included subsonic and chocked flow releases with varying hydrogen release rates and vent configurations. In most of the tests the hydrogen release rate was sufficiently low and the vent area sufficiently large to lead to a well-ventilated jet fire. In a limited number of tests the vent area was reduced, allowing under-ventilated conditions to be investigated. The behaviour of a jet fire in a passively ventilated enclosure depends on the hydrogen release rate, the vent area and the thermal properties of the enclosure. An analytical model was used to quantify the relative importance of the hydrogen release rate and vent area, whilst the influence of the thermal properties of the enclosure were investigated using a CFD model. Overall, the results indicate that passive ventilation openings that are sufficiently large to safely ventilate an unignited release will tend to be large enough to prevent a jet fire from becoming under-ventilated.     

The interaction of hydrogen jet releases with walls and barriers

It has been suggested that separation or safety distances for pressurised hydrogen storage can be reduced by the inclusion of walls or barriers between the hydrogen storage and vulnerable plant or other items. Various NFPA codes [1] suggest the use of 60° inclined fire barriers for protection against jet flames in preference to vertical ones. Work by Sandia National Laboratories [2] included experiments and modeling aimed at characterisation of the effectiveness of barrier walls at reducing hazards.This paper describes a series of experiments performed in order to compare the performance of 60° barriers with that of 90° barriers. Their relative efficiency at giving protection from thermal radiation and blast overpressure was measured together with the propensity for the thermal radiation and blast overpressure to be reflected back to the source of the leak.The work was primarily focused on compressed H₂ storage for stationary fuel cell systems, which may be physically separated from a fuel cell system or could be on board such a system. Different orifice sizes were used to simulate different size leaks; all releases were made from storage at 200 bar.Overall conclusions on barrier performance were made based on the recorded measurements.     

Prediction of the liftoff,blowout and blowoff stability limits of pure hydrogen and hydrogen/hydrocarbon mixture jet flames

The paper presents experimental studies of the liftoff and blowout stability parameters of pure hydrogen, hydrogen/propane and hydrogen/methane jet flames using a 2 mm burner. Carbon dioxide and Argon gas were also used in the study for the comparison with hydrocarbon fuel. Comparisons of the stability of H₂/C₃H₈, H₂/CH₄ and H₂/CO₂ flames showed that H₂/C₃H₈ produced the highest liftoff height and H₂/CH₄ required highest liftoff, blowoff and blowout velocities. The non-dimensional analysis of liftoff height was used to correlate liftoff data of H₂, H₂/C₃H₈, H₂/CO₂, C₃H₈ and H₂/Ar jet flames tested in the 2 mm burner. The suitability of extending the empirical correlations based on hydrocarbon flames to both hydrogen and hydrogen/hydrocarbon flames was examined.     

Numerical simulation of high-pressure hydrogen jet flames during bonfire test

Characteristics of high-pressure hydrogen jet flames resulting from ignition of hydrogen discharge during the bonfire test of composite hydrogen storage vessels are studied. Firstly, a 3-D numerical model is established based on the species transfer model and SST k − ω turbulence model to study the high-pressure hydrogen jet flow. It is revealed that under-expanded jets are formed after the high-pressure hydrogen discharging from the vessel. Secondly, the mathematical methods are adopted to study the high-pressure hydrogen jet flames. The effects of pressure, initial temperature and the nozzle diameter on the jet flames are investigated. The results show that the jet flame length increases with the increase of discharge pressure, but decreases with the increase of nozzle diameter and temperature difference between the filling hydrogen temperature and the environment temperature. Finally, the simulation models are established to study the characteristics of hydrogen jet flames in an open space. The effects of barrier walls on the distribution of jet flames are also studied. The results show that the barrier walls can greatly reduce the damage from hydrogen jet flames to testers and properties around.     

Exergy analysis on the simulation of a small-scale hydrogen liquefaction test rig with a multi-component refrigerant refrigeration system

This study investigates the simulation of a proposed small-scale laboratory liquid hydrogen plant with a new, innovative multi-component refrigerant (MR) refrigeration system. The simulated test rig was capable of liquefying a feed of 2 kg/h of normal hydrogen gas at 21 bar and 25 °C to normal liquid hydrogen at 2 bar and −250 °C. The simulated power consumption for pre-cooling the hydrogen from 25 °C to −198 °C with this new MR cycle was 2.07 kWh/kg_GH2 from the ideal minimum of 0.7755 kWh per kilogram of feed hydrogen gas. This was the lowest power consumption available when compared to today’s conventional hydrogen liquefaction cycles, which are approximately 4.00 kWh/kg_GH2. Hence, the MR cycle’s exergy efficiency was 38.3%. Exergy analysis of the test rig’s cycle, which is required to find the losses and optimize the proposed MR system, was evaluated for each component using the simulation data. It was found that the majority of the losses were from the compressors, heat exchangers, and expansion valves. Suggestions are provided for how to reduce exergy in each component in order to reduce the exergy loss. Finally, further improvements for better efficiency of the test rig are explained to assist in the design of a future large-scale hydrogen liquefaction plant.     

High pressure hydrogen fires

Within the scope of the French national project DRIVE and European project HyPER, high pressure jet flames of hydrogen were produced and instrumented.The experimental technique and measurement strategy are presented. Many aspects are original developments like the direct measurement of the mass flow rate by weighing continuously the hydrogen container, the image processing to extract the flame geometry, the heat flux measurement device, the thermocouples arrangement…Flames were observed from 900 bar down to 1 bar with orifices ranging from 1 to 3 mm. An original set of data is now available about the main flame characteristics and about some thermodynamic aspects of hydrogen releases under high pressure.A brief comparison of some available models is presented.     

Liquid hydrogen production via hydrogen sulfide methane reformation

Hydrogen sulfide (H₂S) methane (CH₄) reformation (H₂SMR) (2H₂S + CH₄ = CS₂ + 4H₂) is a potentially viable process for the removal of H₂S from sour natural gas resources or other methane containing gases. Unlike steam methane reformation that generates carbon dioxide as a by-product, H₂SMR produces carbon disulfide (CS₂), a liquid under ambient temperature and pressure—a commodity chemical that is also a feedstock for the synthesis of sulfuric acid. Pinch point analyses for H₂SMR were conducted to determine the reaction conditions necessary for no carbon lay down to occur. Calculations showed that to prevent solid carbon formation, low inlet CH₄ to H₂S ratios are needed. In this paper, we analyze H₂SMR with either a cryogenic process or a membrane separation operation for production of either liquid or gaseous hydrogen. Of the three H₂SMR hydrogen production flowsheets analyzed, direct liquid hydrogen generation has higher first and second law efficiencies of exceeding 80% and 50%, respectively.     

Delivery of cold hydrogen in glass fiber composite pressure vessels

We are proposing to minimize hydrogen delivery cost through utilization of glass fiber tube trailers at 200 K and 70 MPa to produce a synergistic combination of container characteristics with properties of hydrogen gas: (1) hydrogen cooled to 200 K is ∼35% more compact for a small increase in theoretical storage energy (exergy); and (2) these cold temperatures (200 K) strengthen glass fibers by as much as 50%, expanding trailer capacity without the use of much more costly carbon fiber composite vessels.     

Similarity consideration of the buoyant jet resulting from hydrogen leakage

In this paper, the similarity form is developed of a hydrogen–air buoyant jet ‘forced plume’ resulting from an unignited small-scale hydrogen leakage in the air. As the domain temperature is assumed to be constant and therefore the density of the mixture is a function of the concentration only, the hydrogen–air mixture is assumed to be of a linear mixing type. The rate of entrainment is assumed to be a function of the plume centerline velocity and the ratio of the mean plume and ambient densities. The local rate of entrainment may be considered to be consisted of two components; one is the component of entrainment due to jet momentum while the other is the component of entrainment due to buoyancy. Consequently, the entrainment coefficient is taken as a variable not constant. The density will be considered as a variable in all terms of the equations of continuity, momentum and concentration. The integral models of the mass, momentum and concentration fluxes are obtained. Non-dimensional transformations known as similarity transformations and used to transform the integral model to a set of ordinary differential equations called similarity equations which are solved numerically. Under various values of Froude number, the hydrogen centerline mass fraction, jet width and centerline velocity are introduced and compared with published experimental results for a slow leak vertical hydrogen jet.     

Feasibility and sustainability analyses of carbon dioxide – hydrogen separation via de-sublimation process in comparison with other processes

Hydrogen (H₂) plays a vital role both as a reactant in petrochemical processes and as an energy carrier and storage medium. When produced from carbon-containing feed stocks, such as fossil fuels and biomass, hydrogen is typically produced as a mixture with carbon dioxide (CO₂), and must be subsequently separated by the associated energy, with an invertible energy penalty. In this study, the process for the removal of carbon dioxide from CO₂ - H₂ mixtures by de-sublimation was analysed. This process is particularly relevant to the production of liquid hydrogen (LH₂) at cryogenic temperatures, for which cooling of the H₂ stream is already necessary. The solid – gas equilibrium of CO₂ - H₂ was studied using the Peng-Robinson equation of state which provided a wide range of operating conditions for process simulation. The de-sublimation process was compared with selected conventional separation processes, including amine-based absorption, pressure swing adsorption and membrane separation. In the scenario in which the resulting products, carbon dioxide and hydrogen, were subsequently liquefied for transportation and storage at 10 bar and −46 °C, and 1 bar and −251.8 °C, respectively. The overall energy consumption per kg of CO₂ separated (MJ/kgCO₂₎, was found to follow the order: 8.19–11.21 for monoethanolamine (MEA) absorption; 1.81–8.93 for membrane separation; 1.53–5.69 for pressure swing adsorption; and 0.81–3.35 de-sublimation process. Each process was evaluated and compared on the bases of electricity demand, cooling water usage, high-pressure steam usage, and refrigeration energy requirements. Finally, the advantages and disadvantages were discussed and the feasibility and sustainability of the processes for application in the production of liquid hydrogen were assessed.     

Ignition of hydrogen jet fires from high pressure storage

Self-ignition behaviour of highly transient jets from hydrogen high pressure tanks were investigated up to 26 MPa. The jet development and related ignition/combustion phenomena were characterized by high speed video techniques and time resolved spectroscopy. Video cross correlation method BOS, brightness subtraction and 1-dimensional image contraction were used for data evaluation. Results gained provided information on ignition region, flame head jet velocity, flame contours, pressure wave propagation, reacting species and temperatures. On burst of the rupture disc, the combustion of the jet starts close to the nozzle at the boundary layer to the surrounding air. Combustion velocity decelerated in correlation to an approximated drag force of constant value which was obtained by analysing the head velocity. The burning at the outer jet layer develops to an explosion converting to a nearly spherical volume at the jet head; the movement of the centroid is nearly unchanged and follows the jet front in parallel. The progress of the nearly spherical explosion could be evaluated by assuming an averaged flame ball radius. An apparent flame velocity could be derived to be about 20 m/s. It seems to increase slightly on the pressure in the tank or the related initial jet momentum. Self-initiation is nearly always achieved especially induced the interaction of shock waves and their reflections from the orifice. The combustion process is composed of shell combustion of the jet cone at the bases with a superimposed explosion of the decelerating jet head volume.