Hydrogen Stratification Research Articles

Split injection strategies for a high-pressure hydrogen direct injection in a small-bore dual-fuel diesel engine

Hydrogen-diesel dual direct-injection (H2DDI) engines present a promising pathway towards cleaner and more efficient transportation. In this study, hydrogen split injection strategies were explored in an automotive-size single-cylinder compression ignition (CI) engine, with a focus on varying the injection timings and energy fractions. The engine was operated at an intermediate load with fixed combustion phasing through adjustments of pilot diesel injection timing. An energy substitution principle guided the variation in energy fraction between the two hydrogen injections and then diesel injection while keeping the total energy input constant. The findings demonstrate that early first hydrogen injection timings lead to characteristics indicative of premixed combustion, reflecting a high homogeneity of the hydrogen-air mixture. In contrast, hydrogen stratification levels were predominantly influenced by later second injection timings, with mixing-controlled combustion behaviour apparent for very late injections near top dead centre or when the second hydrogen injection held high energy fractions, which led to decreased nitrogen oxides (NOx: NO and NO2) emissions. The carbon dioxide (CO2) emissions did not show high sensitivity to the hydrogen split injection strategies, exhibiting about 77 % reduction compared to the diesel baseline due primarily to increased hydrogen energy fraction of up to 90 %.

Possibility of Hydrogen Stratification under Accident Conditions with Loss of Coolant from the Cooling System into the Tokamak Vacuum Vessel

Possibility of Hydrogen Stratification under Accident Conditions with Loss of Coolant from the Cooling System into the Tokamak Vacuum Vessel

Combustion Characteristics of a Novel Ammonia Combustor equipped with Stratified Injection for Low Emissions

Ammonia – a carbon-free fuel- is being considered by various research groups and organizations as a potential alternative fuel to tackle some global warming issues of the 21st century. However, ammonia faces some challenges as a fuel, particularly, the production of unwanted NOx and unburned ammonia traces at the back end of combustion systems accompanied by unstable flame regimes. Therefore, new concepts are currently being developed to address these challenges whilst also providing stable, high-power operation. This work presents a newly designed stratified combustion system for gas turbines capable of operating with both premixed and stratified modes. The system has been initially evaluated using these two modes employing solely hydrogen and ammonia. Increase of hydrogen stratification resulted in the reduction of NO and NO2, while N2O slightly increased due to the reduction of fuel in the premixed system. Further tests were conducted to evaluate the potential of the systems to replace fossil fuels. Previous work done using methane as the fuel-to-be-replaced has shown interesting trends in terms of emissions and stability. Thus, to progress on the subject, the work conducted here attempted propane/ammonia/hydrogen using this innovative strategy. Results denoted how stratification has a strong impact on NOx production at the flame front, whilst the mechanism of flame stabilization of the new burner enables large operability regions under a great variety of fuel combinations. Strategic injection setups were identified, whilst the best propane/ammonia/hydrogen blends for stability and emissions reduction are also detailed for further implementation in larger systems.

Numerical study on settlement behaviors of slush hydrogen in cryogenic tank

AbstractSlush hydrogen can be used as a promising propulsion fuel for deep space exploration vehicles because of its more excellent thermodynamic properties compared with liquid hydrogen. To study stratification and settlement characteristics of slush hydrogen in cryogenic tanks, a transient model based on Eulerian–Eulerian two‐fluid method was developed and was verified by experiment data. Many numerical simulations were conducted to determine the influence of settlement, including particle diameter (dp = 0.5–0.015 mm), initial volume fraction of solid hydrogen (αs = 30%–50%), and heat leakage of cryogenic tank (qleak = 15–45 W m−2). It was found that there is a critical particle diameter for settlement of slush hydrogen, which is about 0.02 mm. Meanwhile, a large range of natural convection circulation has been observed in the entire tank. Numerical results of this paper could improve uniform distribution of slush hydrogen for highly efficient storage and application.

Experimental investigations on steam-air-hydrogen condensation and corresponding component separation phenomenon

Under severe accident conditions of a pressurized water reactor, hydrogen will be released into the containment, forming a mixture of steam-air-hydrogen, and then transported to the surroundings. This process can be coupled with complex mass transfer behavior caused by wall condensation. Under the coupling effect of condensation and buoyancy introduced by hydrogen, the mixture gas will form different types of transport behaviors. This will be closely related to the stratification and flammability risk of high-concentration hydrogen. To quantitatively determine these phenomena and assess the potential causing reasons, experiments were performed with helium instead of hydrogen based on the COAST (condensation and species transport) facility. The tests cover a broad parameter range with steam concentrations of 8.4– 89%, helium concentrations of 10– 66% (XHe/XNon), and pressures of 0.4– 1.3 MPa. The phenomena of steam condensation induced component separation were firstly confirmed in the tests, and the component concentration threshold was clarified at XHe/XNon = 50%. The three flow patterns of steam-air- helium mixture gas was identified by Shapiro-Moffette ternary diagram. Moreover, the temperature inverse stratification caused by the high-concentration helium-air layer was discovered in the experiments, and its influence on heat transfer was clarified.

Hydrogen Dispersion and Ventilation Effects in Enclosures under Different Release Conditions

Hydrogen is an explosive gas, which could create extremely hazardous conditions when released into an enclosure. Full-scale experiments of hydrogen release and dispersion in the confined space were conducted. The experiments were performed for hydrogen release outflow of 63 × 10−3 m3/s through a single nozzle and multi-point release way optionally. It was found that the hydrogen dispersion in an enclosure strongly depends on the gas release way. Significantly higher hydrogen stratification is observed in a single nozzle release than in the case of the multi-point release when the gas concentration becomes more uniform in the entire enclosure volume. The experimental results were confirmed on the basis of Froud number analysis. The CFD simulations realized with the FDS code by NIST allowed visualization of the experimental hydrogen dispersion phenomenon and confirmed that the varied distribution of hydrogen did not affect the effectiveness of the accidental mechanical ventilation system applied in the tested room.

Green hydrogen leaking accidentally from a motor vehicle in confined space: A study on the effectiveness of a ventilation system

Green hydrogen can play a considerable role in helping the world achieve net zero emissions by forming a bridge between the energy sector and transportation. Currently, it is, at the initial phase of development, due to the low demand for hydrogen (H2) fuel for transportation in general. However, an increase in the H2 fuel demand in the future will require high investments in the infrastructure sector. In this regard, parking hydrogen vehicles in residential garages pose a potential safety hazard because of the accidents that could arise from hydrogen leaks. The diffusion of hydrogen in a ventilated garage has been investigated using FLUENT software. This study provides some insight into the hydrogen extraction efficiency of a natural ventilation system. We studied the influence of important ventilation parameters (shape and aspect ratio R of opening ventilation) on hydrogen stratification. First, results show a remarkable efficiency in the case of the square shape. Second, when the aspect ratio R decreases, there is an augmentation in fresh air drawn and hydrogen evacuation which generates the decrease of hydrogen concentration inside the garage and increases the extraction efficiency. Highlights This article develops a hot issue in the context of scarce resources, namely the use of hydrogen as a more environmental-friendly source of energy. Eventual leakage of hydrogen in confined spaces frequented by motor vehicles poses a significant hazard. The flow structure and molar fraction of hydrogen is strongly influenced by ventilation parameters The square shape has the lower concentration values; it presents the highest extraction efficiency. The hydrogen extraction of the transverse rectangular shape (R = 0.5) is more efficient and it presents better results than the others do.

Experimental study on gas mixing and transport induced by external cooling on large-scale facility

Migration and distribution of gas mixtures of hydrogen generated in severe accidents are related to thermal hydraulic behavior of the containment, which is a key issue for safety analysis. A series of experiments are conducted on a large-scale test facility, to study the effects of external cooling on stratification and mixing behavior of gas mixtures. Different concentration distributions are formed by changing injection conditions, including injection momentum, initial pressure and composition of gas mixtures. Results show that the increase of injection momentum or initial pressure will forming a more uniform distribution of helium, while the increase of hydrogen fraction of injection will promote stratification. During external cooling period, hydrogen concentration increases caused by condensation, and hydrogen stratification is weakened by natural circulation. The mixing behavior caused by natural circulation will be influenced by initial concentration distribution. The experimental data can be helpful for codes assessment.

The simulate of the stratification of an explosive mixture of hydrogen and air in the RBMK-1000 spent-fuel storage pool

Currently, in Russia, codes with concentrated parameters are used as software tools to justify the hydrogen explosion safety of nuclear power units, which use the concept of “average concentration of hydrogen” in the calculated volume and do not take into account the stratification of hydrogen, due to which local concentrations of hydrogen can significantly exceed the “average”. This paper presents the hydrogen explosion safety analysis results during a beyond-design accident in the RBMK-1000 spent-fuel storage pool taking into account local concentrations of gas components using the gas dynamic code ANSYS FLUENT. The vapor-gas mixture propagation problem in the spent-fuel storage pool was solved in a two-dimensional approximation. The following sources of hydrogen were considered radiolysis hydrogen generation, hydrogen generation by reducing its solubility in water and the vapor-zirconium reaction. As a result of modeling, the possible accumulation areas of an explosive mixture and their sizes were determined. It is shown that under certain combinations of boundary conditions, due to the presence of convective flows, an explosive concentration is not achieved even if the mixture components stratification is taken into account.

Experimental Study on a Hydrogen Stratification Induced by PARs Installed in a Containment

Hydrogen can be produced in undesired ways such as a high temperature metal oxidation during an accident. In this case, the hydrogen must be carefully managed. A hydrogen mitigation system (HMS) should be installed to protect a containment of a nuclear power plant (NPP) from hazards of hydrogen produced by an oxidation of the fuel cladding during a severe accident in an NPP. Among hydrogen removal devices, passive auto-catalytic recombiners (PARs) are currently applied to many NPPs because of passive characteristics, such as not requiring a power supply nor an operators’ manipulations. However, they offer several disadvantages, resulting in issues related to hydrogen control by PARs. One of the issues is a hydrogen stratification in which hydrogen is not well-mixed in a compartment due to the high temperature exhaust gas of PARs and accumulation in the lower part. Therefore, experimental simulation on hydrogen stratification phenomenon by PARs is required. When the hydrogen stratification by PARs is observed in the experiment, the verification and improvement of a PAR analysis model using the experimental results can be performed, and the hydrogen removal characteristics by PARs installed in an NPP can be evaluated using the improved PAR model.

Improvements of turbulence model for analysis of hydrogen stratification erosion by turbulent buoyant jet

Light gas stratification erosion by steam turbulent buoyant jet is the typical phenomenon about containment thermal hydraulics and hydrogen safety. Overprediction of stratification erosion rate has been observed in computational fluid dynamics (CFD) analysis by using current turbulence model based on simple gradient diffusion hypothesis (SGDH). To solve this problem, the effects of buoyancy term, coefficient and turbulent Schmidt number are discussed and a modified k-ε model with buoyancy term based on generalized gradient diffusion hypothesis (GGDH) which contains vertical and horizontal density gradient is developed. In modified k-ε model, besides the value of coefficient, the buoyancy term based on SGDH is modified into GGDH. The strain tensor is modified into rotating tensor to suppress the overestimated local turbulent kinetic energy generated in flow stagnation zone. Comparison between analysis results and experimental data shows that improvement effect of modified model on prediction of light gas stratification erosion rate is significant.

Semi-empirical model of hydrogen distribution under presence of steam within a reactor building

Hydrogen energy is one of the green energy that may be potentially served for humans. However, it has not been widely used, due to the likelihood of lacking in understanding of hydrogen distribution within an enclosure. This kind of issue still remains unclear in reactor building for nuclear power plants, and has drawn many experts' attention, especially on the hydrogen stratification characteristics under different flow regimes, such as different Froude numbers. A large number of experimental and analytical studies have been conducted to figure out the single phase of hydrogen's dispersion mechanism, in order to evaluate the potential hydrogen risk in gradient layer, which may prevent the fluid from mixing evenly, thus increasing the form of combustible gas, with the wide-spread application of storage and transportation of hydrogen. However, in severe accident for nuclear power plants, build-up of hydrogen gradient layer is more complicated, due to the presence of steam. There is little research focusing on the injection and flow behaviors of both hydrogen and steam, which may occur in Loss-of-Coolant-Accidents (LOCAs), and moreover, with the scaling study by Peterson, it can be realized that the mixing process within an enclosure is governed not only by Froude number, but also by Reynolds number. Therefore, in this paper, a semi-empirical model of hydrogen distribution under the presence of steam is proposed in consideration of both the relative effect of momentum and buoyancy, and the effect of viscous force. The prediction results show better agreement with the experimental data obtained from an international benchmark test, THAI-HM2, and tests with high steam content based on a small scale facility and a modelled reactor building of CAP1400 built in China. After these validations, this analytical model is applied to basic conditions with different Re and Fr numbers. It can be discovered that the steam increases the inertness of ambient in the lower region of the modelled reactor building, and with the increase of the injection's steam content, the light gas will be diluted at the bottom, due to both effects of large density and good flowability characteristics of steam, leading to larger concentration and more stratified layer in upper region, corresponding to higher Re numbers. In order to figure out the relation between mass transfer process and complex flow regimes in gradient layer, a useful criterion is proposed, in consideration of the effect of Re, which can successfully identify the variations of flow regimes, corresponding to the build-up of gradient layer. The analytical model from this work can support the development of new CFD codes and help get new understandings of complex thermal-hydraulic phenomena in hydrogen gradient layer within any given enclosure.

Identification of the causes of non-uniformity of the condensate collector metal of the main gas pipeline

The complex of negative effects of hydrogen on metal, leading to destruction of metal structures, is commonly called hydrogen corrosion. The object of the study was a sample of steel A 516-55 condensate collector of the main gas pipeline, which was subjected to hydrogen stratification. The aim of the work was to establish a link between the non-uniformity of the metal and its tendency to hydrogen stratification. The work solved problems that were aimed at studying the physical and mechanical properties and microstructure of the analysed metal. X-ray fluorescence analysis was carried out to identify the causes of discontinuities within the metal. A light microscopy method was carried out to study the microstructure of the test sample taken from the focus zone of the condensate collector destruction. The microstructure was examined using the “METAM RV - 22 type EU LOMO metallographic microscope. An X-ray fluorescence analysis was carried out to determine the causes of the discontinuity within the metal. Tests were performed on a EDX-800HS spectrometer at various X-ray tube operations. It has been found that the inhomogeneities are due to the uneven distribution of manganese in the metal body. A post-publication change was made to this article on 11 Jun 2020 to correct the pdf so that it matched the webpage.

Numerical investigation on erosion of hydrogen stratification by steam jet within a local compartment

During hypothetical severe accidents in nuclear power plants, a large amount of hydrogen is generated rapidly as a result of zirconium-water reaction and released into local compartments. Deflagration or detonation risk is probably caused by the uneven hydrogen distribution. Generally, buoyancy drives light combustible gas up to establish gas stratification. On the other hand, stratification may be destroyed by convection flow from steam injection due to coolant's discharge from the primary loop. The mechanism of erosion of stratification can be divided into buoyancy dominated and momentum dominated regimes, the priority of which depends on a nondimensional parameter, named of interaction Froude number. However, jet velocity and the diameter on the interface between steam and stratification are difficult to measure in experiments. Therefore, CFD method is adopted to capture a detailed velocity profile to solve this problem. In this article, a CFD model of single local compartment facility is built and helium distribution behavior within the local compartment is fully investigated, as a substitute of hydrogen, which is usually used in experiments. Prior to determining the interaction Froude number, four flow models, including laminar flow model, algebraic model, k-ε turbulence model and shear stress transport (SST) k-ω model are evaluated under three typical experimental stages, including light gas injection, the formation of stratification, and the erosion of stratification. Results show that SST k-ω model can predict much better under these specific conditions in the small-scaled configuration, compared to the experimental results. Next, with the setup of a helium stratification structure with homogeneous layer and gradient layer, the interaction between steam and gradient layer is analyzed in detail. The mechanisms of erosion of stratification can be divided into complete penetration, destruction from the bottom level to the top and soft dissolution without penetration, corresponding to the interaction Froude numbers of being greater than 1, close to 1, and far less than 1. The simulation results are consistent with relevant literature findings. The present work can help studying the complex hydrogen distribution process in local compartment and provide data for evaluation of postinerting strategy in severe accident management.

Study on hydrogen stratification and erosion process in Chinese SMR containment under severe accident scenario

During postulated severe accident, large amount of hydrogen-vapor mixture will be released into the containment and inflammable gas mixture may form. Once the gas will be ignited, the generated pressure loads can jeopardize the integrity of the internal structures. As the local accumulation of high content hydrogen related much to the stratification and erosion mechanism, it is therefore of interest whether a stratification of gas layers will form within small containment. In this study, a stratification/erosion experiment is performed and the criteria under which the steady stratification will be destroyed by steam jet discussed. Then the simulation method is validated by the experimental data. At last, several severe accident scenarios for Chinese SMR performed and one of them is adopted for containment hydrogen CFD analysis. It is shown that high content of hydrogen will form in the upper part of the containment, and clear gas stratification will form, such stratification will be enhanced due to the condensation effect of steel containment wall.

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

This paper summarises the results from a blind-prediction benchmark study for models used for estimating the consequences of vented hydrogen deflagrations, as well as for users of such models. The work was part of the HySEA project that received funding from the Fuel Cells and Hydrogen Joint Undertaking (FCH JU) under grant agreement no. 671461. The first blind-prediction benchmark exercise in the HySEA project focused on vented explosions with homogeneous hydrogen-air mixtures in 20-foot ISO containers. The scenarios selected for the second blind-prediction study focused on vented deflagrations in inhomogeneous hydrogen-air mixtures resulting from continuous stratification of hydrogen during vertical jet releases inside 20-foot ISO containers. The deflagrations were vented through commercial vent panels located on the roof of the containers.The test program included two configurations and four experiments, i.e. two repeated tests for each scenario. The paper compares experimental results and model predictions and discusses the implications of the findings for safety related to hydrogen applications. Several modellers predicted the stratification of hydrogen inside the container during the release phase with reasonable accuracy. However, there is significant spread in the model predictions, especially for the maximum reduced explosion pressure, and including predictions from different modellers using the same model system. The results from the blind-prediction benchmark studies performed as part of the HySEA project constitute a strong incentive for developers of consequence models to improve their models, implement automated procedures for scenario definition and grid generation, and update training and guidelines for users of the models.

Numerical simulation of dispersion and distribution behaviors of hydrogen leakage in the garage with a crossbeam

A three-dimensional computational fluid dynamics simulation model resolved by the unsteady Reynolds-Averaged Navier–Stokes equations was developed to predict hydrogen dispersion in an indoor environment. The effect of the height of the crossbeam (Hc) on hydrogen dispersion and distribution behaviors in a four-car garage was numerically investigated under fully confined and natural ventilation conditions. For the fully confined condition, the garage was almost completely filled with a flammable hydrogen cloud at t=600 s. In addition, the volumetric ratio of the flammable region, thickness of the hydrogen stratification, and hydrogen mole fraction all increased as Hc increased. When two symmetric ventilation openings were set up, the volumetric ratio of the flammable region decreased by 50% at t=600 s. Moreover, Hc had evident influence on the vertical distribution of hydrogen mole fraction. In addition, there existed little explosion hazard under the height of 1.6 m. The results show that Hc was a non-negligible factor for the safety design of hydrogen in the garage and Hc=0.12 m was the optimal height of the crossbeam. Furthermore, the ventilation system in the present study cannot completely eliminate the risk of hydrogen explosion. The present risk assessment results can be useful to analyze safety issues in automotive applications of hydrogen.

Experimental investigation of gas transport induced by containment spray activation during a scaled severe accident scenario

Abstract We present the results of an experimental study on the activation of sprays representative of those used in a generic light water reactor containment for severe accident mitigation. Depending on the initial conditions, the spray nozzle type and the spray water flow rates, the spray may cause higher hydrogen concentration during depressurization due to steam condensation, or it may erode the hydrogen stratification by enhanced mixing. To investigate these phenomena, the tests were performed in the PANDA facility at Paul Scherrer Institut, Switzerland in the frame of the ERCOSAM-SAMARA project. The experiments followed a test sequence based on a postulated severe accident scenario scaled from a nuclear power plant design. The scaled scenario consisted of the formation of a stratified helium-rich (substitute for hydrogen) atmosphere in one of the PANDA vessels followed by the activation of a spray. Temporal evolution and spatial distribution of the gas temperature and the gas concentrations at selected positions were measured using thermocouples and mass spectrometers. Four tests were performed with two different spray nozzle types, two spray flow rates and two initial vessel wall temperatures of 105 °C and 135 °C, which created condensing and non-condensing environments, respectively. The different initial conditions lead to different density stratification. The effect of these different initial and boundary conditions on the transport and mixing of gases in the vessels due to the activation of the spray is revealed by these tests. The spray causes a fast breakup of the stratification and a complete mixing in the vessel in which it was injected. The concentration measurements in the adjacent connected vessel showed the presence of helium at the bottom of the vessel suggesting the possibility of hydrogen transport to the lower parts of the containment in a three gas mixture environment.

STUDY OF METAL HETEROGENEITY ASSOCIATED WITH HYDROGEN BONDAGE

The complex of the negative effects of hydrogen on the metal, leading to the destruction of metal structures, is called hydrogen corrosion. It should be noted that the interaction of hydrogen with metals and alloys is closely related to their electronic structure, with the electronic con- figuration of atoms located at the sites of the crystal lattice of the metal. The object of the study was a sample of steel 09G2S of gas pipeline condensate trap, subjected to hydrogen stratification. Aim is to establish the relationship between the heterogeneity of the metal and its tendency to hydrogen stratification. To achieve this aim the following tasks were solved: 1. Studying the physicomechanical properties and microstructure of the metal under study; 2. Conducting X-ray fluorescence analysis to identify the causes of the formation of discontinuities inside the metal. To study the microstructure of the test sample taken from the focal zone of destruction of the condensate collector, the method of light microscopy was carried out. The microstructure was studied using a LOMO EC metallographic microscope of the METAM PB-22 type. It was found that the steel sample under investigation has a ferritic- pearlite structure, in which heterogeneities are observed. To identify the causes of the formation of discontinuities inside the metal, an X-ray fluorescence analysis was carried out. The tests were carried out on an EDX-800HS spectrometer. Various results were obtained, on the basis of which it was revealed that inhomogeneities are due to the uneven distribution of manganese in the metal body. This is probably due to a violation of the technological process of metal production.

Investigation of the Hydrogen Stratification of the Metal of the Active Gas Pipeline

One of the most common types of metal destruction in the oil and gas industry is hydrogen embrittlement. Hydrogen corrosion is a complex of negative effects of hydrogen on steel, leading to the destruction of metal structures. Hydrogen passes through a defect-free metal, without lingering in it. In the presence of defects, hydrogen is retained in the metal forming a brittle solid solution, metal stratification along the segregation streamer, blistering. Studies of the metal of a gas pipeline made of steel 09G2S are presented in the article. The sample was selected from the local zone of destruction in the condensate collector, the metal of the pipe had typical for hydrogen corrosion stratifications. The scope of the study was identification of the most dangerous part of hydrogen corrosion on the sample taken from the local zone of destruction. Studies on the chemical composition and mechanical properties of 09G2S steel were also carried out. Stress-related characteristics of the metal microstructure of the failed gas pipeline were obtained and the character of the destruction progress was revealed. The presence of sulfides cluster in the metal of studied pipe was determined applying metallographic method for determining nonmetallic inclusions.