Experimental data and CFD performance for cloud dispersion analysis: The USP-UPC project

Forecasting the behaviour of a flammable or toxic cloud is a critical challenge in quantitative risk analysis. Recent literature shows that empirical and integral models are unable to model complex dispersion scenarios, like those occurring in semi-confined spaces or with the presence of physical barriers. Although CFD simulators are promising tools in this regard, they still need to be fully validated with comprehensive datasets coming from experimental campaigns designed ad-hoc. In this paper, we present an experimental campaign carried out by a joint venture between University of São Paulo and Universitat Politècnica de Catalunya to investigate CFD tools performance when used to analyse clouds dispersion. The experiments consisted on propane cloud dispersion field tests (unobstructed and with the presence of a fence obstructing the flow) of releases up to 0.5 kg s⁻¹ of 40 s of duration in a discharge area of 700 m². We provide a full 1-s averaged propane concentration evolution dataset of two experiments, extracted from 29 points located at different positions within the cloud, with which we have tested FLACS^® CFD-software performance. FLACS reproduces successfully the presence of complex geometry, showing realistic concentration decreases due to cloud dispersion obstruction by the existence of a fence. However, simulated clouds have not represented the whole complex accumulation dynamics due to wind variation.     

Quantification of source-level turbulence during LNG spills onto a water pond

The use of computational fluid dynamics (CFD) models to simulate LNG vapor dispersion scenarios has been growing steadily over the last few years, with applications to LNG spills on land as well as on water. Before a CFD model may be used to predict the vapor dispersion hazard distances for a hypothetical LNG spill scenario, it is necessary for the model to be validated with respect to relevant experimental data. As part of a joint-industry project aimed at validating the CFD methodology, the LNG vapor source term, including the turbulence level associated with the evaporation process vapors was quantified for one of the Falcon tests.This paper presents the method that was used to quantify the turbulent intensity of evaporating LNG, by analyzing the video images of one of the Falcon tests, which involved LNG spills onto a water pond. The measured rate of LNG pool growth and spreading and the quantified turbulence intensity that were obtained from the image analysis were used as the LNG vapor source term in the CFD model to simulate the Falcon-1 LNG spill test. Several CFD simulations were performed, using a vaporization flux of 0.127 kg/m² s, radial and outward spreading velocities of 1.53 and 0.55 m/s respectively, and a range of turbulence kinetic energy values between 2.9 and 28.8 m²/s². The resulting growth and spread of the vapor cloud within the impounded area and outside of it were found to match the observed behavior and the experimental measured data.The results of the analysis presented in this paper demonstrate that a detailed and accurate definition of the LNG vapor source term is critical in order for any vapor cloud dispersion simulation to provide useful and reliable results.     

Validation of CFD-model for hydrogen dispersion

To be able to perform proper consequence modelling as a part of a risk assessment, it is essential to be able to model the physical processes well. Simplified tools for dispersion and explosion predictions are generally not very useful. CFD tools have the potential to model the relevant physics and predict well, but without proper user guidelines based on extensive validation work, very mixed prediction capability can be expected. In this article, recent dispersion validation effort for the CFD tool FLACS–HYDROGEN is presented. A range of different experiments is simulated, including low-momentum releases in a garage, subsonic jets in a garage with stratification effects and subsequent slow diffusion, low momentum and subsonic horizontal jets influenced by buoyancy, and free jets from high-pressure vessels. LH₂ releases are also considered. Some of the simulations are performed as blind predictions.     

Evaluation of multi-phase atmospheric dispersion models for application to Carbon Capture and Storage

A dispersion model validation study is presented for atmospheric releases of dense-phase carbon dioxide (CO₂). Predictions from an integral model and two different Computational Fluid Dynamics (CFD) models are compared to data from field-scale experiments conducted by INERIS, as part of the EU-funded CO₂PipeHaz project.The experiments studied consist of a 2 m³ vessel fitted with a short pipe, from which CO₂ was discharged into the atmosphere through either a 6 mm or 25 mm diameter orifice. Comparisons are made to measured temperatures and concentrations in the multi-phase CO₂ jets.The integral dispersion model tested is DNV Phast and the two CFD models are ANSYS-CFX and a research and development version of FLACS, both of which adopt a Lagrangian particle-tracking approach to simulate the sublimating solid CO₂ particles in the jet. Source conditions for the CFD models are taken from a sophisticated near-field CFD model developed by the University of Leeds that simulates the multi-phase, compressible flow in the expansion region of the CO₂ jet, close to the orifice.Overall, the predicted concentrations from the various models are found to be in reasonable agreement with the measurements, but generally in poorer agreement than has been reported previously for similar dispersion models in other dense-phase CO₂ release experiments. The ANSYS-CFX model is shown to be sensitive to the way in which the source conditions are prescribed, while FLACS shows some sensitivity to the solid CO₂ particle size. Difficulties in interpreting the results from one of the tests, which featured some time-varying phenomena, are also discussed.The study provides useful insight into the coupling of near- and far-field dispersion models, and the strengths and weaknesses of different modelling approaches. These findings contribute to the assessment of potential hazards presented by Carbon Capture and Storage (CCS) infrastructure.     

A CFD-based approach for gas detectors allocation

Accidental gas releases are detected by allocating sensors in optimal places to prevent escalation of the incident. Gas release effects are typically assessed based on calculating the dispersion from releasing points. In this work, a CFD-based approach is proposed to estimate gas dispersion and then to obtain optimal gas sensors allocation. The Ansys-Fluent commercial package is used to estimate concentrations in the open air by solving the governing equations of continuity, momentum, energy and species convection-diffusion combined with the realizable κ-ε model for turbulence viscosity effects. CFD dynamic simulations are carried out for potential gas leaks, assuming worst-case scenarios with F-stability and 2 m/s wind speed during a 4 min releasing period and considering 8 wind directions. The result is a scenario-based methodology to allocate gas sensors supported on fluid dynamics models. The three x–y–z geographical coordinates for the sensor allocation are included in this analysis. To highlight the methodology, a case study considers releases from a large container surrounded by different types of geometric units including sections with high obstacles, low obstacles, and no obstacles. A non-redundant set of perfect sensors are firstly allocated to cover completely the detection for all simulations releases. The benefits of redundant detection via a MooN voting arranging scheme is also discussed. Numerical results demonstrate the capabilities of CFD simulations for this application and highlight the dispersion effects through obstacles with different sizes.     

Forced ventilation effect by Air-Fin-Cooler on flammable gas cloud dispersion

This paper is the second installment of a paper published on Process Safety and Environment Protection in 2013, which evaluates the Air-Fin-Cooler (AFC) forced ventilation effect over natural ventilation inside congested LNG process train, i.e., modularized LNG, considering the Air Change per Hour (ACH) using Computational Fluid Dynamics (CFD) analysis. This second paper evaluates the effect of forced ventilation on gas cloud dispersion using CFD in order to evaluate possible design measures, such as safety distance in trains and whether to shut down the AFC in case of releases. The results of this evaluation show that gas cloud accumulation is reduced by AFC induced air flow in the case of shorter separation distances between modules. Based on the results, two design measures are proposed, i.e., keep AFC running during emergency and train orientation against prevailing wind direction.     

Validation of FLACS against experimental data sets from the model evaluation database for LNG vapor dispersion

The siting of facilities handling liquefied natural gas (LNG), whether for liquefaction, storage or regasification purposes, requires the hazards from potential releases to be evaluated. One of the consequences of an LNG release is the creation of a flammable vapor cloud, that may be pushed beyond the facility boundaries by the wind and thus present a hazard to the public. Therefore, numerical models are required to determine the footprint that may be covered by a flammable vapor cloud as a result of an LNG release. Several new models have been used in recent years for this type of simulations. This prompted the development of the “Model evaluation protocol for LNG vapor dispersion models” (MEP): a procedure aimed at evaluating quantitatively the ability of a model to accurately predict the dispersion of an LNG vapor cloud.This paper summarizes the MEP requirements and presents the results obtained from the application of the MEP to a computational fluid dynamics (CFD) model – FLACS. The entire set of 33 experiments included in the model validation database were simulated using FLACS. The simulation results are reported and compared with the experimental data. A set of statistical performance measures are calculated based on the FLACS simulation results and compared with the acceptability criteria established in the MEP. The results of the evaluation demonstrate that FLACS can be considered a suitable model to accurately simulate the dispersion of vapor from an LNG release.     

Spill-over characteristics of peroxy-fuels: Two-phase CFD investigations

Two-phase CFD (Computational Fluid Dynamics) model for characterising the spill-over/dispersion of peroxy-fuels is presented. The model is independent of type and burning rate of the spilled/dispersed fuel and considers only overflow Reynolds number (Re) to characterise the spill/dispersion behaviour. Additional simulations are performed for LNG (Liquified Natural Gas) dispersion and it is found that the model can be used for different fuels within a defined range of Re. Different scenarios with Re = 100 to 3 × 10⁵ are investigated covering a wide range of mass flow rates, opening sizes and viscosities. Depending on Lower Flammability Limits (LFL) of the fuels spill/dispersion (vapour cloud) diameters (D_CFD) and heights (h_CFD) are predicted. A generalised correlation between D_CFD and Re is established to predict the dispersion occurring at varying scales. The model is validated by: (1) conducting an extensive grid independent study; (2) comparing the results with the existing analytical methods and (3) comparing against the standard field test data on LNG dispersions.     

Risk analysis of a cross-regional toxic chemical disaster by using the integrated mesoscale and microscale consequence analysis model

The rapidly growing capacity and scale of the world's petrochemical industries have forced many plants to have an even larger amount of hazardous substances. Once a serious leak occurs, the outcome of the effect zone could be very large or even uncontrollable just like the Bhopal disaster. In order to assess the risk of a cross-regional damage, this study aims to develop a model that can combine the benefits of both CFD model of the microscale simulation and the Gaussian dispersion model of the mesoscale simulation.The developed integrated model is employed on a toxic chemical tank leak accident of a process plant within an industrial park in order to explore the consequences and the risk of the toxic gas dispersion on three different scopes; one is the accident site, the second is the long-distance transmission route of the mesoscale area and the third is a target city. According to the simulation's results, it is obvious that the complexity of the structure surrounding the leaking tank will eventually affect the maximum ground concentration, the cloud shapes and cloud dilution rate, while the released gas is under dispersion. On the other hand, since the simple Gaussian dispersion model doesn't consider the above impacts, its calculation results will have many differences as compared to the realistic situation. This integrated model can be used as a tool for estimating the risk on a microscale or mesoscale areas and it can produce better results when an environmental impact analysis is required for a larger hazardous chemical process.     

3-D dispersion model for simulation of accidental toxic gas releases in a metropolitan area

Computational fluid dynamic (CFD) simulations were performed to assess the potential chlorine leak scenario in the super-urban area of South Korea, where the human population density is very high and numerous buildings exist near operational water treatment facilities. Flame acceleration simulator (FLACS) was used to predict the consequence from accidental chlorine releases out of one of the water treatment facilities for the nearby area having a size of 5 km × 3 km approximately. The ability to precisely implement 3-D geometries is crucial for a successful 3-D simulation. Thus, a method was proposed to rapidly and accurately implement geometry by importing computer aided-design (CAD) files provided by a government agency, and processing them using Auto CAD and MicroStation software programs. An accidental release from an 18-ton tank was simulated with three different wind directions to determine the expected evacuation distances. Results from the study showed that the endpoint distances varied depending on the density and arrangement of the buildings. Moreover, we employed physical barriers with varying heights for mitigating the effects of toxic gas releases and simulated how effectively they decreased the concentration of released chlorine.     

Numerical analysis of toxic cloud generation and dispersion: A case study of the ethylene oxide spill

The present study examined the accidental spill of ethylene oxide, and a sensitivity analysis of the corresponding consequences was conducted using computational fluid dynamics (CFD). A validation of the gas dispersion CFD model against the experimental data sets included in the model evaluation protocol (MEP) was performed. The effect of the variability of the wind velocity on the extension of the hazardous areas and pool evaporation characteristics was evaluated. Additionally, the mitigation effects of the dike walls surrounding a spill were discussed. CFD simulation results have shown that the mitigation effect of dike walls is determined by their influence on both gas dispersion and pool evaporation and depends strongly on wind velocity in terms of toxic impact distances.     

Application of CFD on the sensitivity analyses of some parameters of the modified Hartmann tube

The aim of this work is to determine the influence of operating parameters such as the dispersion pressure, the ignition delay and height on the dust flammability. A Computational Fluid Dynamics (CFD) simulation, based on an Euler–Lagrange approach, was developed with Ansys Fluent™ and validated experimentally. Such analysis will facilitate the choice of the most conservative conditions for a flammability test. This paper is focused on a case study performed on wheat starch with the modified Hartmann tube. The dispersion process of the powder was studied with granulometric analyses performed in situ and high speed videos. Tests were performed with injections at gas pressure ranging from 3 to 6 bars and the evolution of the particle size distribution (PSD) was recorded at different ignition heights (5, 10 and 15 cm over the dispersion nozzle). The observations highlighted the presence of agglomeration/deagglomeration processes and dust segregation. Besides, a CFD simulation analysis was aimed at evaluating the impact of a set of parameters on the PSD and the local turbulence, which are closely linked to some flammability parameters. For this computational analysis, the CFD simulation was coupled with a collision treatment based on a Discrete Element Method (DEM) in order to consider the cohesive behavior of the combustible dust. Thus the results suggest performing the injection of the gases at approximately 5 bars for the flammability tests of wheat starch in order to obtain the finest PSD at a given ignition height. It is also shown that the finest PSD are obtained at 5 cm over the dispersion nozzle. However, the local instabilities and turbulence levels are so high during the first stages of the dispersion that the flame growth can be disturbed for short ignition delays. Moreover, the stabilization of the bulk of the dust cloud requires longer periods of time when the ignition sources are located at 15 cm. As a result, the recommended height to perform a flammability test is 10 cm in this case. Finally, this study proposes some tools that might improve the procedure of dust flammability testing.     

Risk assessment methodology for high-pressure CO2 pipelines incorporating topography

This paper presents a risk assessment methodology for high-pressure CO₂ pipelines developed at the Health and Safety Laboratory as part of the EU FP7 project CO2Pipehaz.Traditionally, consequence modelling of dense gas releases from pipelines at major hazard impact levels is performed using integral models with limited or no consideration being given to weather bias or topographical features of the surrounding terrain. Whilst dispersion modelling of CO₂ releases from pipelines using three-dimensional CFD models may provide higher levels of confidence in the predicted behaviour of the cloud, the use of such models is resource-intensive and usually impracticable. An alternative is to use more computationally efficient shallow layer or Lagrangian dispersion models that are able to account for the effects of topography whilst generating results within a reasonably short time frame.In the present work, the proposed risk assessment methodology for CO₂ pipelines is demonstrated using a shallow-layer dispersion model to generate contours from a sequence of release points along the pipeline. The simulations use realistic terrain taken from UK topographical data. Individual and societal risk levels in the vicinity of the pipeline are calculated using the Health and Safety Laboratory's risk assessment tool QuickRisk.Currently, the source term for a CO₂ release is not well understood because of its complex thermodynamic properties and its tendency to form solid particles under specific pressure and temperature conditions. This is a key knowledge gap and any subsequent dispersion modelling, particularly when including topography, may be affected by the accuracy of the source term.     

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.     

Concentration model for gas releases in buildings and the mitigation effect

In case of accidents involving releases of hazardous materials, calculating the gas dispersion is essential for assessing risks. In general, the leaked chemical is assumed to be instantly dispersed to the atmosphere if the leak occurs in the outdoor location. However, a different approach should be made for the incidents when sources are located inside a building. For the indoor release, the gas will be diluted prior to the release to the atmosphere and the gas release from a building to the atmosphere demands the application of another model before the dispersion calculation. The indoor release model calculates average indoor concentration and volumetric flowrate to the exterior. The model is fast and reasonably accurate compared to rigorous but time-consuming computational fluid dynamics (CFD) models. The model results were compared with experimental data, and CFD simulation results both with simple geometry to demonstrate validation and assess the performance of the indoor release model. Lastly, the behavior and effect of mitigation of indoor release were demonstrated by using the model results.     

On the non-monotonic wind influence on flammable gas cloud from CFD simulations for hazardous area classification

Hazardous areas are defined as a result of a variety of variables as storage temperature, pressure, leak orifice size, physical properties of flammable substance, and wind characteristics. The potential formation of an explosive atmosphere must be accurately assessed to ensure process safety. Therefore, computational fluid dynamics (CFD) arises as an important tool for accurate predictions as recommended by the international standard IEC 60079-10-1 (2015). This study aims to analyze the influence of wind velocity magnitude and direction on the hazardous area classification. The authors evaluated the extent and volume for methane, propane, and hydrogen leakages from a CFD model. For each flammable gas, the wind velocity magnitude and direction were regularly varied. The outcomes show that the behavior of the plume size as the wind varies mainly depends on the gas concentration. Counter-flow wind directions lead to zero relative velocity closer to the release point, which concentrates the gas, and wind in the release direction promotes a higher dilution of the gas cloud increasing the hazardous extent while decreases the volume. As a consequence, the wind also influences the zone type, which was accurately predicted from CFD simulations and significant differences were found when compared to the standard analyses. These differences are, to some extent, related to the consideration of wind velocity effects on the gas jet release.     

An experimental study of an LPG tank at low filling level heated by a remote wall fire

This paper describes an experimental study of 2300 L pressure vessels exposed to remote fire heating by a natural gas fuelled wall fire simulator. The tanks were filled to 15% capacity with commercial liquid propane. The flame intensity and distance were varied to study the effect of different heating levels on the tank and its lading.The fire simulator is first characterized with tests including fire thermocouples, radiative flux meters and thermal imaging. With the appropriate positioning of a target tank it is possible to get very realistic fire heat fluxes at the tank surface.Three tests were conducted with the 2300 L tanks filled to 15% capacity with propane. The tanks were positioned at three different distances from the wall fire resulting in measured average peak heat flux at the tank surfaces ranging between 24 and 43 kW m^?2. The data shows rapid rise in vapour space wall temperatures, significant temperature stratification in the vapour space, and moderate rate of pressure rise. These results provide excellent data for the validation of computer models used to predict the response of pressure vessels exposed to moderate heating from a remote fire.     

A CFD based explosion risk analysis methodology using time varying release rates in dispersion simulations

A full probabilistic Explosion Risk Analysis (ERA) is commonly used to establish overpressure exceedance curves for offshore facilities. This involves modelling a large number of gas dispersion and explosion scenarios. Capturing the time dependant build up and decay of a flammable gas cloud size along with its shape and location are important parameters that can govern the results of an ERA. Dispersion simulations using Computational Fluid Dynamics (CFD) are generally carried out in detailed ERA studies to obtain these pieces of information. However, these dispersion simulations are typically modelled with constant release rates leading to steady state results. The basic assumption used here is that the flammable gas cloud build up rate from these constant release rate dispersion simulations would mimic the actual transient cloud build up rate from a time varying release rate. This assumption does not correctly capture the physical phenomena of transient gas releases and their subsequent dispersion and may lead to very conservative results. This in turn results in potential over design of facilities with implications on time, materials and cost of a project.In the current work, an ERA methodology is proposed that uses time varying release rates as an input in the CFD dispersion simulations to obtain the fully transient flammable gas cloud build-up and decay, while ensuring the total time required to perform the ERA study is also reduced. It was found that the proposed ERA methodology leads to improved accuracy in dispersion results, steeper overpressure exceedance curves and a significant reduction in the Design Accidental Load (DAL) values whilst still maintaining some conservatism and also reducing the total time required to perform an ERA study.     

Numerical simulation of water curtain application for ammonia release dispersion

Using water curtain system to forced mitigate ammonia vapor cloud has been proven to be an effective measure. Currently, no engineering guidelines for designing an effective water curtain system are available, due to lack of understanding of complex interactions between ammonia vapor cloud and water droplets, especially the understanding of ammonia absorption into water droplets. This paper presents numerical calculations to reproduce the continuous ammonia release dispersion with and without the mitigating influence of a downwind water curtain using computational fluid dynamic (CFD) software ANSYS Fluent 14.0. The turbulence models k–ɛ and RNG were used to simulate the ammonia cloud dispersion without downwind water curtain. The simulated results were compared with literature using the statistical performance indicators. The RNG model represents better agreement with the experimental data and the k–ɛ model generates a slightly lesser result. The RNG model coupled with Lagrangian discrete phase model (DPM) was used to simulate the dilution effectiveness of the water curtain system. The ammonia absorption was taken into account by means of user-defined functions (UDF). The simulated effectiveness of water curtains has good agreements with the experimental results. The effectiveness of water mitigation system with and without the ammonia absorption was compared. The results display that the effectiveness mainly depends on the strong air entrainment enhanced by water droplets movement and the ammonia absorption also enhances the effectiveness of water curtain mitigation system. The study indicates that the CFD code can be satisfactorily applied in design criteria for an effective mitigation system.     

Measurements and predictions of harmful releases of the gathering station over the mountainous terrain

Having a risk analysis of harmful releases over mountainous terrains through wind tunnel experiment is a frontier problem in China. In this paper, a straight-flow wind tunnel is applied to simulate the atmospheric boundary layer and research the motion of high-sulfur gas released to atmosphere when accidental releases occur in a gathering station over the mountainous terrain. After an analysis of hourly concentration in the field accident for eight wind directions, experimental results reveal that nearby concentration fields are dominated by wind and far-field concentration distribution is dominated by topography, which leads to complete levels of consequence impact for the personnel risk inside and around the gathering station. Based on CFD techniques, a three-dimensional modelling was established in comparison with the wind tunnel experiment, which suggests that CFD prediction had underestimated the near-field gas concentration and the performance could not precisely match actual risks the gathering station causes to the mountainous terrain, which leads to a modified equation for numerical prediction. Instead of proposing a lower personnel risk evaluation obtained through the use of CFD techniques, the wind tunnel experiment offers a new choice for the consequence impact analysis for the petrochemical industry in China.