Heavy Gas Dispersion Research Articles

An innovative three-dimensional computational fluid dynamics-iterative ensemble Kalman filter model for the prediction of heavy gas leakage and dispersion in enclosed workplaces: Case study for hydrogen sulfide leakage

An innovative three-dimensional computational fluid dynamics-iterative ensemble Kalman filter model for the prediction of heavy gas leakage and dispersion in enclosed workplaces: Case study for hydrogen sulfide leakage

Experimental and numerical study of heavy gas dispersion in presence of obstacle motion

Most of the current research on heavy gas dispersion focuses on how fixed obstacles affect gas spreading. The turbulence caused by the movements of people or traffic will significantly affect heavy gas dispersion. Accordingly, this study presents an investigation of heavy gas dispersion under the influence of moving obstacles, which is based on the dynamic mesh approach using computational fluid dynamics (CFD). A turbulence model suitable for such a condition has been developed, and we have designed a small-scale experiment to verify the model's viability. Consequently, the verified model is further applied to a full-scale scenario for a chemical factory where a chlorine leak triggers an evacuation movement. On the basis of this scenario, we simplify the massive escape crowd to a regular geometric model, and the effects of the speed and size of movable obstacles on gas dispersion have been investigated by means of the RNG k-ε model and the layering updating approach in the dynamic mesh.

Extension of the EFFECTS dispersion model for buoyant plume rise including lift-off

Alternative energy carriers such as hydrogen and ammonia play an important role for the energy transition to reduce global CO2 emissions substantially. These substances will be produced and used in much larger amounts in the near future to replace hydrocarbons that are widely used nowadays. Therefore, consequences and risks of bulk storage, handling and transport of these materials must be addressed accurately. Because these substances have a low molecular weight, pressurised or liquefied gases released by accident may undergo a transition from heavy gas to rising plume behaviour during dispersion depending on the storage, release and atmospheric conditions, as well as surface effects such as heat transfer. The plume lift-off, plume rise, and the entrainment of ambient air play a significant role in the effect distances and the volume of a flammable or toxic cloud. The consequence modelling software tool EFFECTS contains different dispersion models for neutral and heavy gas dispersion, which are based on a 1-D discretisation method. Since the code for these dispersion models was developed in the past with the focus on heavy or neutral gas conditions, the simulation of buoyant plumes so far was limited. The aim of the present research was to improve the accuracy of the EFFECTS dispersion model for strongly buoyant plumes and predict the transition from the momentum dominated to the buoyant regime correctly. The paper describes the theoretical background of the extensions implemented in the dispersion model in EFFECTS. These extensions are aimed at improving the modelling approach for lighter than air plumes. The validation of the models incorporating these improvements has been performed and is based on results compared against experimental data including instantaneous and continuous releases, jets and low momentum releases from the ground for different materials including helium and hydrogen.

Investigation of heavy gas dispersion characteristics in a static environment: Spatial distribution and volume flux prediction

The unexpected release of hazardous gases due to natural/industrial accidents or man-made disasters can have tragic consequences. Compared with other gaseous pollutants, heavy gases easily accumulate in personnel working areas and are more difficult to eliminate through existing mechanical ventilation systems. In this paper, to improve air quality in enclosed environments, the spatial characteristics of heavy gas dispersion were obtained based on an analysis of the basic properties of heavy gas dispersion. Schlieren experiments and image processing were conducted with shadow correction and noise reduction. The computational fluid dynamics (CFD) was used for predicting the dispersion of heavy gas. The numerical simulations cover a wide range of the densimetric Froude number (Fr) from 6.45 to 25.79, and a best-fit relation of Fr and the height of heavy gas fountain was provided. The radial velocity, radial density and axial density at cross-section of heavy gas dispersion were normalized using the unified scaling law. The axial density reached a minimum value at a height of 10 times the inlet diameter. Further decreases in the inertial force or decreases in the buoyant force can reduce the dilution of heavy gas by ambient fluids, which means that heavy gases diffuse faster and farther. An alternative method for estimating volume flux during gas dispersion using Fr was also proposed.

A Modified k-ε Turbulence Model for Heavy Gas Dispersion in Built-Up Environment

For hazard assessment purposes, the dispersion of gases in complex urban areas is often a scenario to be considered. However, predicting the dispersion of heavy gases is still a challenge. In Germany, the VDI Guideline 3783, Part 1 and 2 is widely used for gas dispersion modelling. Whilst Part 1 uses a gauss model for calculating the dispersion of light or neutrally buoyant gases, Part 2 uses wind tunnel experiments to evaluate the heavier-than-air gas dispersion in generic built up areas. In practice, with this guideline, it is often not possible to adequately represent the existing obstacle configuration. To overcome this limitation, computational fluid dynamics (CFD) methods could be used. Whilst CFD models can represent obstacles in the dispersion area correctly, actual publications show that there is still further research needed to simulate the atmospheric flow and the heavy gas dispersion. This paper presents a modified k-ε-turbulence model that was developed in OpenFOAM v5.0 (England, London, The OpenFOAM Foundation Ltd Incorporated) to enhance the simulation of the atmospheric wind field and the heavy gas dispersion in built-up areas. Wind tunnel measurements for the dispersion of neutrally buoyant and heavy gases in built-up environments were used to evaluate the model. As a result, requirements for the simulation of the gas dispersion under atmospheric conditions have been identified and the model showed an overall good performance in predicting the experimental values.

Can a linear superposition relationship be used for transport of heavy gas delivered by supply air in a ventilated space?

In emergency events where hazardous heavy gases are injected into supply air, the rapid prediction of heavy gas dispersion is significantly important. The linear superposition relationship based on a fixed flow field offers the advantage of fast predictions; however, the buoyancy due to the density difference destabilizes the flow field. In this work, the applicability of a linear relationship based on transient accessibility index in predicting heavy gas dispersion delivered from supply air was studied. The dimensionless transient concentrations predicted by the linear model were compared with those of CFD (computational fluid dynamics) simulation. The numerical results from two heavy gases, carbon dioxide (CO2) and hydrogen sulfide (H2S); two supply mass fraction concentrations, 4E-4 and 4E-2; and two air distributions, ceiling supply side down return (CSD) and side up supply side down return (SUSD), were analyzed. The results showed that the flow field and heavy gas concentrations at a low supply concentration of 4E-4 were slightly different from those of the passive gas. The air jet exhibited sinking characteristics at a high supply concentration of 4E-2. Significant prediction deviations using the linear model mainly occurred at a few positions surrounding the supply air jet and in the upper space for the CSD and SUSD. An acceptable accuracy was achieved with average deviations ranging from 7.8%–15.5%. High heavy gas density and heavy gas concentration in supply air increased the prediction deviation. This study provides support for the rapid assessments of emergency scenarios in the context of ventilation decisions.

Study of Heavy Gas Pollutants’ Dispersion in Street Canyon Terrain

This study focused on heavy gas dispersion under the terrain conditions of street canyons. The effects of street aspect ratio and height ratio were investigated, and the influence of environmental wind speed in the typical ideal street canyon terrain was explored. The results indicated that the surrounding flow field distributions in street terrains were dominated by higher buildings. In addition, when the building height was held constant, the flow field was affected by the joint influence of the two isolated buildings. The interception effect of the street canyon on upstream pollutants declined with the decrease in the street canyon’s aspect ratio. In addition, when the height ratios were different, a large quantity of upstream pollutants accumulated on the windward side of higher buildings. The relative concentration per unit area inside the canyon was affected by the air circulation inside and outside the canyon and the size of the dispersion space. The increase in the environmental wind speed promotes the entry of pollutants into the street while aggravating the overall dispersion of the pollutants. Therefore, the emergence of the most unsafe wind speeds caused most of the pollutants to gather in the street canyons.

Wind-tunnel experiments on atmospheric heavy gas dispersion: Metrological aspects

We propose a new experimental protocol to investigate the atmospheric dispersion of a dense gas in wind-tunnel experiments. Simultaneous measurements of concentration and velocity are performed by coupling a flame ionisation detector (FID) and a hot-wire anemometer (HWA), whose probes are located sufficiently close to each other so that the respective measurements correspond to a same sampling volume. The heavy gas emission consists in a mixture of air, carbon dioxide, and ethane. Carbon dioxide is used as a buoyant agent, and ethane is used as a tracer, its concentration being detected by the FID. In this context, the main metrological issue of the setup concerns the response of the FID and the HWA to different proportions of the two gases in the mixture. Notably, the presence of carbon dioxide affects the linear response of the FID, which can attain a saturation level, as well as the response of the HWA, due to the varying density of the gas. This can be compensated by determining a non-linear calibration curve of the FID, which is used to determine the instantaneous fluid density, and to apply a time-dependent correction to the HWA response. To analyse the performances of the coupled HWA–FID system we realised wind-tunnel experiments of tracer dispersion in a turbulent boundary layer. The estimate of mass fluxes through different sections downwind the source enlightens the reliability of the experimental results, and show that the measurements within a negatively buoyant gas plume are as accurate as those within a passive scalar plume.

Computational fluid dynamics study of CO2 dispersion with phase change of water following the release of supercritical CO2 pipeline

Accidental release of pressurized CO2 pipeline in carbon capture and storage involves the interaction of phase change and heavy-gas dispersion. However, the effect of the phase change of water vapor in air on the performance of cold CO2 dispersion is usually neglected. In this study, a three-dimensional two-phase computational fluid dynamics (CFD) model is developed to evaluate the cold CO2 dispersion by considering the phase change of water. A phase-change model based on the homogeneous relaxation model is used to describe the evaporation and condensation of water. The effects of terrain roughness, atmospheric stability, and turbulence models on the dispersion are also considered. The numerical results show that the model that uses the k–ω turbulent equations is superior to the other models. The results in which the phase change of water is considered exhibit a better agreement with the data from the experiments than those that do not consider it. The model is subsequently used in urban areas, which results in over-predicted CO2 concentration in the near field and under-predicted CO2 concentration in the far field when the phase change of water vapor is considered than that when it is neglected. Therefore, we proposed that the phase change of water vapor in the atmosphere should not be overlooked in the more accurate modeling of cold CO2 dispersion.

Study on convection and dispersion characteristics of dense gases in urban environment considering trees

Although several studies on the dispersion of heavy toxic gas released from ruptured tanks on vehicles during transportation have considered complex terrain such as urban buildings, the influence of trees on the flow field in urban areas during gas dispersion tends to be ignored. In this study, a Computational Fluid Dynamics (CFD) model was proposed to investigate the characteristics of gas release and dispersion from loaded vehicle in the urban environment. In this model, the tree crown was treated as a porous medium, and the influence of drag due to the crown was incorporated into the model by a momentum source term through a user-defined function. In this study, the dynamic characteristics of chlorine (Cl2) dispersion under the conditions of building distribution, tree species and porosities were comprehensively analysed, to cover the influence of urban complexity, leaf density, and tree planting configuration. The results show that compared with flat terrain, the presence of urban buildings will prolong the dense gas retention time and increase the dangerous distance. It is found that the horizontal dispersion distance can increase by 63% and the isosurface of 25 ppm hazardous gas can increase by 130% with the introduction of buildings. Compared with the terrain with only buildings, the introduction of arbors or shrubs can result in a 147% or 359% increase in the maximum concentration. Also, trees will prolong the dispersion duration. It is also found that the higher the porosity, the less the wind blocking effect, and the weaker the ability of capturing gas. The wind field affected by arbores and shrubs are different in height, and arbores capture more Cl2. Planting short shrubs around buildings can effectively reduce the spread of harmful gases.

Intelligent Mobile Wireless Network for Toxic Gas Cloud Monitoring and Tracking.

Intelligent wireless networks that comprise self-organizing autonomous vehicles equipped with punctual sensors and radio modules support many hostile and harsh environment monitoring systems. This work’s contribution shows the benefits of applying such networks to estimate clouds’ boundaries created by hazardous toxic substances heavier than air when accidentally released into the atmosphere. The paper addresses issues concerning sensing networks’ design, focussing on a computing scheme for online motion trajectory calculation and data exchange. A three-stage approach that incorporates three algorithms for sensing devices’ displacement calculation in a collaborative network according to the current task, namely exploration and gas cloud detection, boundary detection and estimation, and tracking the evolving cloud, is presented. A network connectivity-maintaining virtual force mobility model is used to calculate subsequent sensor positions, and multi-hop communication is used for data exchange. The main focus is on the efficient tracking of the cloud boundary. The proposed sensing scheme is sensitive to crucial mobility model parameters. The paper presents five procedures for calculating the optimal values of these parameters. In contrast to widely used techniques, the presented approach to gas cloud monitoring does not calculate sensors’ displacements based on exact values of gas concentration and concentration gradients. The sensor readings are reduced to two values: the gas concentration below or greater than the safe value. The utility and efficiency of the presented method were justified through extensive simulations, giving encouraging results. The test cases were carried out on several scenarios with regular and irregular shapes of clouds generated using a widely used box model that describes the heavy gas dispersion in the atmospheric air. The simulation results demonstrate that using only a rough measurement indicating that the threshold concentration value was exceeded can detect and efficiently track a gas cloud boundary. This makes the sensing system less sensitive to the quality of the gas concentration measurement. Thus, it can be easily used to detect real phenomena. Significant results are recommendations on selecting procedures for computing mobility model parameters while tracking clouds with different shapes and determining optimal values of these parameters in convex and nonconvex cloud boundaries.

Computational fluid dynamics simulation of gas dispersion in complex facilities using Kit Fox field experiments: Validation and statistical evaluation

Gas release and its dispersion is a major concern in chemical industries. In order to manage and mitigate the risk of gas dispersion and its consequences, it is necessary to predict gas dispersion behavior and its concentration at various locations upon emission. Therefore, models and commercial packages such as Phast and ALOHA have been developed. computational fluid dynamics (CFD) can be a useful tool to simulate gas dispersion in complex areas and conditions. The validation of the models requires the employment of the experimental data from filed and wind tunnel experiments. It appears that the use of the experimental data to validate the CFD method that only includes certain monitor points and not the entire domain can lead to unreliable results for the intended areas of concern. In this work, some of the trials of the Kit Fox field experiment, which provided a wide-range database for gas dispersion, were simulated by CFD. Various scenarios were considered with different mesh sizes, physical conditions, and types of release. The results of the simulations were surveyed in the whole domain. The data matching each scenario was varied by the influence of the dominant displacement force (wind or diffusivity). Furthermore, the statistical parameters suggested for the heavy gas dispersion showed a dependency on the lower band of gas concentration. Therefore, they should be used with precaution. Finally, the results and computation cost of the simulation could be affected by the chosen scenario, the location of the intended points, and the release type.

Risk mapping of Chlorine gas dispersion from a storage plant in a dense urban area of Lahore

In recent times, with the increasing population in urban areas, the adverse events happening due to explosion and dispersion of toxic chemicals has increased. Events like toxic gas dispersion can cause severe environmental issues that endanger human safety and health. Consequence hazard modeling and vulnerable population assessment are critical to predict and minimize the losses. Chlorine is not only utilized in water treatment plants as a disinfectant, but it ca n also lead to some serious concerns to human health. In this paper, the modeling of chlorine release has been investigated for chlorine storage plant by using Heavy gas dispersion model. The modeling results showed that the cloud of chlorine is about 1.9 miles across just downwind of the release for accidental release of chlorine in the summer; however, the probability of fatality is 100 % in a whole year in a distance of 0.36 miles from the Storage Place. As a significant result, the land around the chlorination unit covering a range of approximately 1.9 miles is vulnerable in all wind directions and in the case of South-West direction of the wind, vulnerable population is highly dense, risk prevention in that region should be accounted for. Affected Population and areas at risk are calculated, which illustrates the toxically impacted areas and the population in need of immediate help and evacuation. Such studies can serve as a useful tool for decision-makers to prepare an emergency plan in case of accidental releases.

Modeling Heavy-Gas Dispersion in Air with Two-Layer Shallow Water Equations

Computation of gas dispersal in urban places or hilly grounds requires a large amount of computer time when addressed with conventional multidimensional models. Those are usually based on two-phase flow or Navier-Stokes equations. Different classes of simplified models exist. Among them, two-layer shallow water models are interesting to address large-scale dispersion. Indeed, compared to conventional multidimensional approaches, 2D simulations are carried out to mimic 3D effects. The computational gain in CPU time is consequently expected to be tremendous. However, such models involve at least three fundamental difficulties. The first one is related to the lack of hyperbolicity of most existing formulations, yielding serious consequences regarding wave propagation. The second is related to the non-conservative terms in the momentum equations. Those terms account for interactions between fluid layers. Recently, these two difficulties have been addressed in Chiapolino and Saurel (2018) and an unconditional hyperbolic model has been proposed along with a Harten-Lax-van Leer (HLL) type Riemann solver dealing with the non-conservative terms. In the same reference, numerical experiments showed robustness and accuracy of the formulation. In the present paper, a third difficulty is addressed. It consists of the determination of appropriate drag effect formulation. Such effects also account for interactions between fluid layers and become of particular importance when dealing with heavy-gas dispersion. With this aim, the model is compared to laboratory experiments in the context of heavy gas dispersal in quiescent air. It is shown that the model accurately reproduces experimental results thanks to an appropriate drag force correlation. This function expresses drag effects between the heavy and light gas layers. It is determined thanks to various experimental configurations of dam-break test problems.

A Review of Leakage and Dispersion of LNG on the Ground

Based on the analysis of the whole process of LNG spill on land, the research methods of LNG pool expansion and heavy gas diffusion are summarized and analyzed. This paper reviews the experimental and analytical work performed to data on spill of LNG. Specifically, experiments on the spill of LNG onshore, as well as experiments and numerical study on heavy gas dispersion. Pool boiling and turbulence model are described and discussed, as well as models used to predict dispersion. Although there have been significant progress in understanding the behavior of LNG spills, technical knowledge gaps to improve hazard prediction are still identified. Some of the gaps can be addressed with current modeling and testing capabilities. Finally, a discussion of the state of knowledge, and recommendations to further improvement the understanding of the behavior of LNG spills onshore.

A dense gas dispersion model based on revised meteorological parameters and its performance evaluation

Heavy gas dispersion is significantly impacted by meteorological conditions. In this study, the widely-used heavy gas dispersion model SLAB is revised by using alternative parametrizations for the meteorological parameters, mixing layer height, and wind speed profile. The Nozaki method and the stability modified logarithmic wind profile evolved from the Monin-Obukhov similarity theory are adopted to calculate the mixing layer height and wind speed profile in the improved SLAB-M model, respectively. Dense gas dispersion model evaluation protocols are used to evaluate SLAB-M, and the datasets from the Jack Rabbit II (JR II) 2015 Trials are used to compare the accuracy of the simulations to observed data. The statistical performance measures (SPMs) of SLAB-M are overall a little better than that of SLAB, and SLAB-M effectively simulates heavy gas dispersion under the different meteorological conditions of the JR II 2015 Trials. Thus, it can better satisfy the requirements of consequence assessment and emergency response for the leakage and dispersion of hazardous gasses.

Modeling the effect of hypothetical chlorine leakage from Malay-Sino Chemical Industries using ALOHA software and development of an emergency evacuation route around Teluk Kalong industrial area

Malay-Sino Chemical Industries in Teluk Kalong is the largest chlor-alkali manufacturer in Malaysia, which may potentially affect the safety of plant workers and nearby residential area in the event accidental chlorine leakage. The dispersion of a hypothetical chlorine leakage from Malay-Sino Chemical Industries in Teluk Kalong industrial area was modelled using ALOHA. The effect of different wind direction, wind speed and time in the meteorological mesoscale area was evaluated. The wind speed ranging from 1.55 to 11.1 m/s obtained from a local weather station was considered. Heavy gas dispersion model was used to determine the concentration of chlorine plume within the area of concern. The leakage rate was set at 702 kg/h. The result showed that the residential area R5 (Kampung Bukit Kuang, and Kampung Sungai Terjun) is affected by the plume dispersed by northern wind at night. A greater hazard zone can be formed under the wind speed below 1.55 m/s. A suitable safety evacuation routes were proposed to reduce the risk of exposure to hazardous gas. Finding from this work is useful to understand the risk of hazardous gas dispersion around Teluk Kalong industrial area.

Influence of monsoon seasons on accidental chlorine leak and dispersion around Gebeng industrial area

This paper presents the prediction of hypothetical chlorine leak dispersion around Gebeng industrial area using Areal Location of Hazardous Atmosphere (ALOHA) software. The effect of the different wind speed and direction to chlorine dispersion in the meteorological microscale area was investigated. The wind speed ranging from 1.32 to 10.7 m/s obtained from a local weather station was considered. Heavy gas dispersion model was used to determine the concentration of chlorine plume within the area of concern. The leakage rate was set to correspond to a leakage from the chlorine tank at 792 kg/h. The result showed that the residential areas R1 and R2 are affected by the plume dispersed by southwest and northern winds, respectively. A greater hazard area was found when the wind speed is below 2.3 m/s. Meanwhile, the hazard zone reduced to nearly half when the wind speed is above 5.4 m/s. The appropriate evacuation routes were developed to reduce risk of exposure to toxic gas. Finding from this work is useful to understand the risk of noxious gas dispersion around Gebeng industrial area.

Performance evaluation by computational fluid dynamics modelling of the heavy gas dispersion with a low Froude number in a built environment

To evaluate the dispersion of a heavy gas, such as sulphur hexafluoride, with a low Froude number in a built environment, an experimental and numerical simulation study was conducted. The experiment was carried out using seven different injection inlet configurations in an experimental chamber. The release rate was found to have a great effect on the concentration in the lower part of the chamber. The obstacle in the middle of the chamber could cause a non-uniform distribution of concentration, particularly due to variations in locations and angles of the release outlets. Additionally, numerical simulations were carried out to evaluate four turbulence models: the standard k- ε model, the realizable k- ε model, the re-normalization group (RNG) k- ε model and the shear stress transport (SST) k-ω model. Four indicators were used to evaluate the turbulent model performance. In general, the SST k-ω model performed the best, with geometric mean bias ( MG) = 0.968 and geometric variance ( VG) = 1.09 at 0.055 m height, and with MG = 0.384 and VG = 2.80 at 0.6 m height. The standard k- ε model was the next best in performance, followed by the realizable k- ε and the RNG k- ε model.

Numerical Investigation into the effects of obstacles on heavy gas dispersions in the atomsphere

Computational fluid dynamics (CFD) approach is applied to investigate heavy gas dispersion in the atmosphere, under the action of wind. Because of the effect of buoyancy, steady double peaks of the heavy gas concentrations in the downstream area are observed from the numerical results. The double peaks of the concentrations are a special pattern of heavy gas dispersion, which cannot be found in the neutral gas dispersions. Four types of obstacles are placed behind the leakage source to study the influences of these obstacles to the heavy gas dispersions. The numerical results show the detailed shapes and other contents of the heavy clouds under the obstacles.