Gas Stratification Research Articles

Stratification characteristics of hydrogen-blended natural gas for gas risers inside residential users

ABSTRACT In this paper, a CFD model is developed to examine the stratification characteristics of hydrogen-blended natural gas in residential gas risers. Meanwhile, the accuracy of the CFD model is validated by field experiments. The effects of relevant parameters on stratification are analyzed, including hydrogen blending ratio, riser internal diameter, riser height, media temperature, and media pressure. The relative importance of relevant parameters for stratification is identified using the orthogonal experiment design and improved grey relational analysis. Furthermore, a device is designed to mitigate the impact of blended gas stratification. The results show that the stratification of hydrogen-blended natural gas inside residential gas risers is significant in the direction of gravity. The maximum stratification height in standing for 8 h can reach up to 17.4 cm. The stratification height presents a negative correlation with riser internal diameter, riser height, and media temperature, while that represents a positive correlation with hydrogen blending ratio and media pressure. Furthermore, the effects of these parameters on stratification height could be ranked as follows: media pressure > hydrogen blending ratio > riser internal diameter > riser height > media temperature. These achievements provide effective guidance for residential users on safely using hydrogen-blended natural gas.

Simulation and Experimental Verification of Dispersion and Explosion of Hydrogen–Methane Mixture in a Domestic Kitchen

Hydrogen is a carbon-free energy source that can be obtained from various sources. The blending of hydrogen represents a transitional phase in the shift from natural gas systems to hydrogen-based systems. However, concerns about the safety implications of introducing hydrogen have led to extensive discussions. This paper utilizes Fluent 17.0 numerical simulation software to simulate the leakage of hydrogen-blended natural gas in a closed domestic kitchen and analyze the concentration distribution and its variation pattern after a leakage. An experimental platform is set up, and a mixture of nitrogen and helium gas is used as a substitute for hydrogen-blended natural gas for the simulations and experiments. The simulation results demonstrate that the leaked gas spreads and accumulates towards the top of the space, gradually filling the entire area as the leak persists. As the hydrogen content in the leaked gas increases, the dispersion capacity of the gas in the confined space also increases. Furthermore, as the flow rate of the leaked gas increases, the average concentration of the leaked gas rises, and the gas stratification in the confined kitchen diminishes. The concentration distribution observed in the experiments aligns with the simulation results. After establishing the feasibility conditions of the model, the dispersion of the hydrogen-blended natural gas in the kitchen is further simulated. The results suggest that blending hydrogen into the gas enhances the dispersion of the gas after a leak, leading to a wider distribution within the kitchen and an increased risk in the event of a leak. Additionally, this paper employs the CASD module of FLACS 11.0 software to construct a three-dimensional geometric model of the domestic kitchen for simulation studies on the explosion of hydrogen-blended natural gas in a confined space. By adjusting the hydrogen ratio in the combustible gases present in the space and examining the variations in hydrogen concentration and external conditions, such as opening or closing the door, the influence on parameters including the peak explosion pressure, explosion overpressure, explosion flame temperature, and explosion response time are examined. Furthermore, the extent of the explosion area is determined, and the effect of hydrogen on the blast is clarified.

Modeling of the influence of local heat sources on a light gas stratification formation and erosion in a large-scale experimental facility using eddy resolving numerical approach

The main objective of this work is to evaluate the results of application of Implicit Large Eddy Simulation (ILES) turbulence modeling approach to simulate the complex hydrogen safety problem. The paper presents the results of numerical simulation of the OECD/NEA HYMERES (HYdrogen Mitigation Experiments for Reactor Safety) HP2 series tests in the PANDA facility, aimed at studying the thermal effect of passive autocatalytic recombiner (PAR) operation on mixing the containment atmosphere, using CABARET-SC1 code. The code is based on the eddy resolving CABARET technique, which allows implicit modeling of the subgrid turbulence scales without using tuning parameters (ILES approximation). The natural convection flow inside the PAR model was calculated explicitly using a simplified model of the heating section and the housing. Good agreement of calculated transient with experimental data was obtained, which indicates the effectiveness of such approach.

Overview of separate effect and integral system tests on the passive containment cooling system of SMART100

SMART100 has a containment pressure and radioactivity suppression system (CPRSS) for passive containment cooling system (PCCS). This prevents overheating and over-pressurization of a containment through direct contact condensation in an in-containment refueling water storage tank (IRWST) and wall condensation in a CPRSS heat exchanger (CHX) in an emergency cool-down tank (ECT). The Korea Atomic Energy Research Institute (KAERI) constructed scaled-down test facilities, SISTA1 and SISTA2, for the thermal-hydraulic validation of the SMART100 CPRSS. Three separate effect tests were performed using SISTA1 to confirm the heat removal characteristics of SMART100 CPRSS. When the low mass flux steam with or without non-condensable gas is released into an IRWST, the conditions for mitigation of the chugging phenomenon were identified, and the physical variables were quantified by the 3D reconstruction method. The local behavior of the non-condensable gas was measured after condensation inside heat exchanger using a traverse system. Stratification of non-condensable gas occurred in large tank of the natural circulation loop. SISTA2 was used to simulate a small break loss-of-coolant accident (SBLCOA) transient. Since the test apparatus was a metal tank, compensations of initial heat transfer to the material and effect of heat loss during long-term operation were important for simulating cooling performance of SMART100 CPRSS. The pressure of SMART100 CPRSS was maintained below the design limit for 3 days even under sufficiently conservative conditions of an SBLOCA transient.

Equilibrium distribution and diffusion of mixed hydrogen-methane gas in gravity field

Repurposing existing natural gas pipelines is a promising solution for large-scale transportation of mixed hydrogen-methane gas. However, it remains debatable whether gravitational stratification can notably affect hydrogen partial pressure in the gas mixture. To address this issue, we combined molecular dynamics simulation with thermodynamic and diffusion theories. Our study systematically examined the equilibrium distribution of hydrogen-methane mixtures in gravity fields. We demonstrated that partial pressures of both gases decrease with altitude, with hydrogen showing a slower decrease due to its smaller molar mass. As a result, the volume fraction of hydrogen is maximized at the top end of pipes. The stratification is more favorable at low temperatures and large altitude drops, with notable gas stratification only occurring at extremely large drops in altitude, being generally negligible even at a drop of 1500 m. Furthermore, we showed that the diffusion time required to achieve the equilibrium distribution is proportional to gas pressure and the square of pipeline height. This requires approximately 300 years for a 1500 m pipeline at 1 bar. Therefore, temporary interruptions in pipeline gas transportation will not cause visible stratification. Our work clarifies the effect of gravity on hydrogen-methane gas mixtures and provides quantitative insights into assessing the stratification of gas mixtures in pipelines.

Experimental study on gas explosions of methane-air mixtures in a full-scale residence building

The field tests were carried out in a full-scale residence building with volume of 105 m3 to reveal the hazards and mechanism of methane-air mixture explosions. The real layout and variations of gas concentration, initial state and ignition position are considered in the explosion tests. The pressure–time histories and flame development of each test were recorded and analyzed. The concentration distribution was also measured by using the fixed concentration sensors and mobile robots. It is found that when the methane diffuses freely in the resident buildings, concentration stratification can be observed in the vertical direction and the concentration of region with height of 0.64 ∼ 2.38 m is within the explosive limit in the 9.5 vol% case. In the real scenario, the maximum overpressure of gas explosions in residence buildings is less than 10 kPa due to the effective venting of doors and windows. The concentration of 9.5 vol%, uniform gas distribution and ignition position in room 2 can bring about more intensive gas explosions and in turn result in higher overpressure loads and stronger external flame propagation. What is more, the acoustic oscillation is significantly restrained by the disturbance on the acoustic field caused by the effective venting, gas stratification and obstacle distribution in residence buildings.

Two-way Eulerian-Lagrangian coupling approach in GASFLOW code for containment spray modelling

Spray droplets are considered to have significant effects on the gas mixing and depressurization in the containment of nuclear power plants during hypothetical accident scenarios. The Eulerian-Lagrangian approach, which enables the two-way mass, momentum, and energy coupling of the continuous gas and the dispersed spray droplets, is promising to model the behaviours of spray droplets in 3D simulations of the full-scale containment. Two-way heat and mass transfer models have been developed in the 3D CFD code GASFLOW. The particle group method is applied in the Lagrangian approach to track the droplets. The ordinary differential equations which couple the local heat and mass exchanges between each droplet group and the surrounding gas mixtures have been solved using the Runge-Kutta method. The GASFLOW coupling this new approach is validated by TOSQAN water spray experiments, which demonstrate the good feasibility of implementing the approach. The analysis of the impact of the spray on containment atmosphere indicates that: 1) the droplet swarm entrains and mixes the surrounding gas, breaking up the gas stratification, with turbulent diffusion dominating momentum mixing in dead zones; 2) the thermal and mass exchanges between the spray droplet swarm and gas jointly determine the atmosphere depressurization, with the evaporation of the spray-formed film/sump on heat structures of great importance for the containment pressure development; and 3) these spray thermodynamic and dynamic effects highly depend on the droplet size distribution and the spray shape.

Numerical research of the in-cylinder natural gas stratification in a natural gas-diesel dual-fuel marine engine

The urgent need for decarbonization and emission reduction for marine engines is driving the development of alternative low-carbon fuels. One of the best alternative fuels for engines is natural gas, a clean energy source with vast reserves and a low price. However, low-pressure injection dual-fuel engines exhibit poor combustion characteristics at low engine loads, and extinction is likely to occur in the fuel mixture's dilution region. Thus, the Low Pressure Post Injection (LPPI) strategy was proposed in this paper. The major objective of the LPPI mode is to increase monocirculation combustion stability under low to moderate engine loads by achieving a reasonable distribution of NG in the combustion chamber. LPPI was compared with the Low Pressure Injection (LPI) strategy. Results indicate that the LPPI mode could successfully raise the swirl ratio in the cylinder upto 60.7 percent while perfectly avoiding the NG leakage phenomena. Additionally, with the aid of radial-wall distribution of NG and an improved swirl ratio in LPPI mode, the combustion duration is reduced by 33.6 percent, and the lean burn limit of the dual-fuel marine engine can be expanded to 0.30. Although local higher combustion temperature caused an large increase in NOx emission which is more than three times than LPI NOx emission, LPPI mode still meets Tier III NOx emission requirements.

Study on the stratification of the blended gas in the pipeline with hydrogen into natural gas

When blending hydrogen into existing natural gas pipelines, the non-uniform concentration distribution caused by the density difference between hydrogen and natural gas will result in the fluctuations of local hydrogen partial pressure, which may exceed the set one, leading to pipeline failure, leakage, measurement error, and terminal appliance. To solve the problem, the H2–CH4 stratification in the horizontal and undulated pipe was investigated experimentally and with numerical simulations. The results show that in the gas stagnant situation, hydrogen-methane blending process will cause an obvious stratification phenomenon. The relations between the elevation, pressure, hydrogen fraction, etc., and the gas stratification are figured out. Moreover, even when the blended gas flows at a low rate, the hydrogen-caused stratification should also be considered. Thereafter, the blended gas should be controlled into a situation with low pressure and high speed, which could help to set the pressure, speed, the fraction of H2.

Study on vapour dispersion and explosion from compressed hydrogen spill: Risk assessment on a hydrogen plant

Hydrogen produced from renewable resources is one of the cleanest fuels and could be used to store intermittent solar, wind and other energies. The main concern about using hydrogen is its hazards, such as high storage pressure, wide-range flammability, low mass density, and high diffusion. This study investigated the hazards of compressed hydrogen storage by developing a CFD model to understand the gas dispersion behaviour. The model was validated using the past experimental data and showed a good agreement, which could demonstrate the diffusion characteristics and gas stratification of a buoyant gas. A case study of an accidental release of compressed hydrogen from a storage tank was investigated to evaluate the risk of a hydrogen plant. A mathematical model of the jet spill was used to account for the choking effect from a high-pressure release to ensure the input velocity in CFD simulation is suitable for modelling gas dispersion using verified spatial and temporal scales, then the simulation results were used as inputs of vapour cloud explosions (VCEs) to investigate the potential overpressure effect. It was found the CFD model could predict a more reasonable flammable gas amount in cloud than using the bulk hydrogen release rate. The safety distance based on the overpressure prediction was reduced by 35%. The method proposed in this study can provide more validity for the consequence analysis as part of risk assessment.

Split Injection Strategy Optimization to Achieve Ideal Stratified‐Charge Mixture and Reinforce Ultralean Burn in a High Compression Ratio Direct Injection Spark Ignition Engine

Ultralean burn is considered an effective way to enhance the thermal efficiency of direct injection spark ignition engines. Herein, under ultralean burn condition, the split injection strategy is induced to investigate the in‐cylinder mixture distribution, combustion characteristics, and emissions by simulation in a high compression ratio engine model. First, the homogeneity index shows that compared with the single injection method, the split injection strategy shows better stratification of the mixture gas at the early stage of the compression stroke, which results in higher in‐cylinder pressure and heat release. Meanwhile, the results of heat balance show that split injection shows a slight effect on ignition delay, but significantly shortens combustion duration and advances CA50, and thus the exhaust loss is decreased. Second, with the decrease of a second injection fuel mass, the rich mixture is formed near the spark plug, thus reducing the ignition delay and combustion duration. Third, the split injection strategy presents faster flame kernel growth and propagation. A larger flame area is observed with the second injection mass decreasing. Finally, as the second fuel injection mass is reduced, NOx emissions increases continuously, while the soot emission decreases.

Failure analysis of ammonium chloride salt coagulation corrosion of U-tube heat exchanger in diesel hydrogenation unit

Diesel hydrogenation unit is an important equipment for processing inferior oil in petroleum refining enterprises. Among them, the U-tube heat exchanger has been in high temperature and corrosive environment containing chlorine for a long time. Serious problem of tube bundle corrosion failure caused by ammonium chloride salt deposit. Anatomy of the diesel hydrogenation heat exchanger showed that the leakage part mainly appeared in the outermost bundle of the heat exchanger tube near the exit section, and the local area showed a large area of gully damage. Metallographic and XRD results show that the failure tube bundle material does not have manufacturing defects, and the corrosion location scale contains NH4Cl and iron oxides. The process flow analysis based on PR-NRTL model showed that the heat exchanger was in the NH4Cl crystallization temperature range, and the crystallization temperature increased with the increase of N and Cl contents in raw materials. Fluid simulation analysis of inlet tube box and single U-tube is carried out by using component transport equation and condensation model. Computational fluid dynamics (CFD) results indicate that there is a low velocity zone with high volume fraction of gas phase in the tube box. The higher gas volume fraction results in the formation of more NH4Cl particles into the U-tube bundle during heat transfer cooling. With the decrease of temperature, liquid water content in U-tube increases gradually and mainly distributes at both sides and bottom of tube bundle. The gas phase gradually fills the top of the tube bundle, and the oil and gas stratification becomes more obvious. The mass transfer rate and corrosion rate were the highest at the bottom and both sides of the tube bundle. With the temperature decreases, the NH4Cl particles generated in the tube continue to increase. The particles were deposited at 2000–3000 mm near the outlet section of the U-tube tube. NH4Cl particles absorbed liquid water, resulting in under-deposit corrosion of the tube bundle, and eventually led to the thinning and leakage of the heat exchanger tube bundle.

Effect of light gas on condensation heat transfer of steam–air mixture and gas stratification

Condensation has been considered for various industrial applications. However, the characteristics of condensation heat transfer have not been fully elucidated. In this study, we identify the characteristics of condensation heat transfer deterioration due to light gas. We measure the condensation heat transfer coefficient by varying the mass fraction of noncondensable gas and the mole fraction of helium to noncondensable gas. We discuss parameters that can affect the condensation heat transfer coefficient, i.e., the gas density, diffusion coefficient, and mole fraction of helium. Gradual deterioration of condensation heat transfer coefficient by increasing mole fraction of helium to noncondensable gas is observed. Furthermore, conditions for helium stratification under condensation are identified. Models for predicting condensation heat transfer are evaluated by varying an effective diffusion coefficient model based on experimental data.

Gas Evolution in Inner Disk Cavities from a Synergic Analysis of IR-CO and UV-H2 Spectra

Abstract We are conducting a multi-wavelength analysis of high-resolution molecular spectra that probe the evolution of gas in the inner 10 au in protoplanetary disks. A sample of 15 disks has been combined to probe a range of inner disk structures including small and large dust cavities. Half of the sample has been observed in far-ultraviolet H2 emission with a new HST-COS program (GO-14703), that we have combined to near-infrared spectra of CO emission as observed with VLT-CRIRES and IRTF-iSHELL. This synergic dataset traces the evolution and depletion of CO and H2 in inner disk cavities and shows an evolving radial stratification of the molecular gas, where CO lines are narrower than H2 lines in disks with cavities. CO rotation diagrams also show significant evolution, suggesting a change in gas excitation as CO emission recedes to larger disk radii.

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.

On the settling of small grains in dusty discs: analysis and formulae

ABSTRACT Instruments achieve sharper and finer observations of micrometre-in-size dust grains in the top layers of young stellar discs. To provide accurate models, we revisit the theory of dust settling for small grains, when gas stratification, dust inertia, and finite correlation times for the turbulence should be handled simultaneously. We start from a balance of forces and derive distributions at steady state. Asymptotic expansions require caution since limits do not commute. In particular, non-physical bumpy distributions appear when turbulence is purely diffusive. This excludes very short correlation times for real discs, as predicted by numerical simulations.

Challenges in Containment Thermal Hydraulics

The containment of a nuclear reactor is a component whose loss in an accident has serious consequences on property, persons, and environment. The Fukushima accident reminded us of this reality. For more than 30 years, the French Nuclear Energy and Alternative Energies Commission has been conducting research on the failure modes of these enclosures, particularly on their slow pressurization during a steam release and hydrogen risk. Significant progress has been made on wall condensation and its spatial distribution, the occurrence and erosion of gas stratification, and the impact of mitigation systems, such as spraying and catalytic recombiners. This knowledge has been included in numerical tools and internationally recognized expertise. These tools have also been used for the safety of the hydrogen energy industry. The emergence of new systems, particularly passive systems and new light water reactor concepts, has led us to examine new questions that will have to be addressed in the coming years. This examination is done in view of current computational fluid dynamics code capabilities and limitations.

Analyses of Gas Stratification Erosion by a Vertical Jet in Presence of an Obstacle Using the GOTHIC Code

Abstract The GOTHIC code was validated using three experiments carried out in the PANDA facility in the framework of the OECD/NEA HYMERES project. These tests addressed the mixing of an initially stratified atmosphere by means of a vertical jet in the presence of on obstacle (circular plate). This paper reports on the simulations of three experiments, and complementary, quasi-steady-state tests without stratification, where the flow structure above the impingement plate could be observed by means of particle image velocimetry (PIV) velocity measurements in a region larger than that considered in the transient experiments. Moreover, simulations of similar tests without obstacle conducted during the OECD/SETH-2 project are also discussed. The reference, best-estimate model used for the analyses of the three experiments with different flowrates and initial and pressure boundary conditions was built on the base of a multistep approach. This was based on mesh and modeling sensitivity studies mostly performed for the complementary tests, to assess the capability to represent the flow structure produced by the jet–plate interaction with different meshes around the plate. Generally, the results show that the use of a coarse mesh and the standard k–ε turbulence model permits a reasonable representation of the erosion process, but with a systematic over prediction of the mixing time. The results with the reference model were more accurate for two experiments with two flowrates and same initial conditions and all complementary tests. For the third test with different initial and boundary conditions, however, poor results were obtained with the reference model, which could only be improved by further refining the mesh. These results indicate that a model “qualified” for certain conditions could be inadequate for other cases, and sensitivity studies are necessary for the specific conditions considered in the analyses.

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.