Flame Acceleration Simulator Research Articles

CFD-based AHP-TOPSIS decision-making model for gas explosion overpressure mitigation measures in underground utility tunnels

Gas explosion is a highly destructive form of disaster, and it seriously threatens the safety of structures and personnel. The reduction and mitigation measures of gas explosions have been extensively investigated, such as blast walls, safety gaps, and inert gases. Among these numerous measures, how to select the most economical, feasible, and effective measure to reduce the consequences of gas explosions is one of the most important problems faced by decision-makers. Therefore, a CFD-based analytic hierarchy process–Technique for Order Preference by Similarity to an Ideal Solution (AHP-TOPSIS) decision-making model was developed in this study. It combines the subjective experiences of decision-makers with the objective results of CFD numerical simulations, which makes the selected measure more reasonable and effective. The selection of mitigation measures for gas explosion overpressures in the UUT was taken as an example to verify the effectiveness of the proposed decision-making model. Three effective measures proposed in this study (i.e., yve = 10 m, D = 300 μm, yve = 10 m – D = 400 μm) were evaluated using the CFD software Flame Acceleration Simulator (FLACS) and used as target decision measures for the model. The results of the CFD-based AHP-TOPSIS decision-making model show that the measure of yve = 10 m – D = 400 μm is the most appropriate mitigation measure for gas explosion overpressures in the UUT under the evaluation index system, including feasibility, effectiveness, and economy. The results will support the selection of the appropriate overpressure mitigation measure of gas explosions and the blast-resistant design of UUTs.

Temporal and spatial evolution of hydrogen leakage and diffusion from tube fittings on fuel cell vehicles under the effect of ambient wind

Hydrogen has become a competitive energy carrier owing to its high fuel value, cleanliness, and renewability. Fuel cell vehicles (FCVs) are crucial in achieving global carbon neutrality in the transportation sector. This study used Flame Acceleration Simulator (FLACS) software to model hydrogen leaks and diffusion from tube fittings of hydrogen systems with the numerical model validated by experimental data. The effect of wind direction on hydrogen leakage and diffusion in the chassis was investigated. Hydrogen leakage rates from the tube fittings under various pressures, tightening angles, and torques were determined; a release rate of 0.6 NL/min is considered a typical value for hydrogen leakage from the tube fittings of a 1.2-MPa hydrogen supply line. The results demonstrate that in windless conditions, the high concentration region with 1–2% vol. was located around the group of hydrogen storage tanks and that there was no flammable region in the entire chassis area. Wind accelerated hydrogen diffusion and reduced the extension range of the hydrogen envelope by 1–2% vol. Compared with longitudinal wind conditions, a crosswind can more effectively reduce the hydrogen concentration. Cutting off the hydrogen supply resulted in rapid contraction of the hydrogen envelope of 1–2% vol. within 5 s. These results have implications for installation of hydrogen sensors in the chassis of FCVs and the location of the blower for forced ventilation by first responders in the event of hydrogen leakage.

Experimental and Numerical Study on Deflagration Characteristics of Large-Scale Propane-Air Mixture.

Seven deflagration tests of a propane-air mixture were carried out in a 22.5 m3 large-scale chamber. The effects of initial volume, gas concentration, and initial turbulence intensity on deflagration characteristics were analyzed. The main frequency of the explosion wave was quantitatively determined by the combination of the wavelet transform and energy spectrum analysis. The results show that the explosive overpressure is formed by the discharge of combustion products and secondary combustion, and the effects of turbulence and gas concentration on the explosive overpressure are higher than the initial volume. Under the condition of weak initial turbulence, the main frequency of gas explosion wave is between 32.13 and 48.33 Hz. Under strong initial turbulence conditions, the main frequency of the gas explosion wave increases with the increase of overpressure, and the empirical formula of the relationship between the main frequency and overpressure is summarized, which could provide theoretical support for the design of mechanical metamaterials for oil and gas explosion. Finally, the flame acceleration simulator numerical model was calibrated through tests, and the overpressure simulation values were in good agreement with the experimental data. The leakage, diffusion, and explosion of a liquefied hydrocarbon loading station in a petrochemical enterprise were simulated. The lethal distance and explosion overpressure at key buildings are predicted for different wind speed conditions. The simulation results can provide a technical basis for evaluating personnel injury and building damage.

Numerical study of hydrogen leakage, diffusion, and combustion in an outdoor parking space under different parking configurations

As the number of hydrogen fuel cell vehicles (HFCVs) being used is increasing, confirming that their safety level is the same as that of gasoline vehicles is becoming important. HFCVs are commonly parked in outdoor spaces. Thus, the safety of HFCVs in garage scenarios should be ensured and related regulations, codes, and standards for preventing accidents must be developed. In this study, the Flame Acceleration Simulator (FLACS) software was utilized to evaluate the behavior of hydrogen release, dispersion, and flame propagation in the presence of multiple HFCVs in an outdoor parking space through the activation of thermally activated pressure relief devices (TPRDs). The effects of different leak diameters (2, 3, and 4 mm), release directions (upward and downward), and parking configurations (vertical and parallel) on the formation of a flammable gas cloud (FGC) and safety distances were examined, by considering tank blowdown. The results showed that larger TPRD diameters resulted in a larger envelope of the FGC; however, it also dissipated faster in outdoor parking spaces. Compared with the vertical parking configuration, the fire-origin HFCV had less impact on adjacent vehicles in the parallel parking configuration. The direction of an upward release was found to contribute to a decrease in the volume of the FGC in outdoor parking spaces owing to low congestion. This study highlights the significance of the TPRD diameters and parking configurations, providing guidance for the safe introduction of HFCVs in existing outdoor parking spaces and infrastructure.

FLACS-Based Simulation of Combustible Gases Leaked from the Pressure Device for the Optimizing of Gas Detectors’ Setup

The leakage and diffusion of hazardous gases from steam methane reforming (SMR) equipment are investigated by Flame Acceleration Simulator (FLACS) software to optimize the layout of combustible gas detectors. A typical accident scenario, with the gases leaked from converter tubes with leak apertures of 5 mm, 25 mm, and 100 mm and medium pressure of 0.1 MPa, 1 MPa, and 10 MPa, is established. At the same time, the influence of the environment wind speeds from 0.2 m·s−1 to 6 m·s−1 on the diffusion process is also investigated. The research results show that the leakage source concentration and diffusion distance positively correlate with the leakage aperture. Suggestion on the distance between combustible gas detectors and possible leak point is within 5 m, 10 m, and 15 m in the scenario of the leak aperture of 5 mm (small-hole leak aperture), 25 mm (middle-hole leak aperture), and 100 mm (big-hole leak aperture). The most dangerous scenario is at the static ambient wind speed, and the diffusion process strengthens with the raising of wind speed. The turning point scenario occurs at a wind speed of 1 m·s−1, where the flammable area is minimal. The medium pressure relates to the jet speed of the combustible gases. The wind speed should be comprehensively determined when considering the layout of the combustible gas detectors affected by this factor. The orthogonal experimental design shows that the most significant influence factor on the diffusion process of the combustible gas is the leak aperture, followed by the medium pressure and, finally, by the ambient wind speed. Recommendations are listed for the optimization of the layout of gas detectors in related enterprises.

Peripheric sensors-based leaking source tracking in a chemical industrial park with complex obstacles

Hazardous gas leakage can cause irreversible damage to the environment and human health. When it happens, it's necessary to find the accurate position of the leaking source efficiently and take effective measures to reduce or prevent more irreversible losses. However, source tracking in the scenario with complex obstacles faces the challenge caused by turbulent wind flow. In this paper, ethane leak scenarios with different leaking sources and environmental conditions are simulated using the Flame acceleration simulator (FLACS). Considering that sensors are often deployed at the boundaries of industrial parks for the detection of hazardous gas leakage, the concentration information of these peripheric sensors is mapped to images, which serve as inputs to a convolutional neural network (CNN) to determine the location of the leaking source and wind direction in a chemical industrial park with complex obstacles. The results show the effectiveness of the proposed method. In addition, fixed failure rates of the sensor along with additional meteorological conditions are considered to evaluate the performance of generalization.

The Study on the Shock Wave Propagation Rule of a Gas Explosion in a Gas Compartment

Combined with the k-ε turbulence model of general application, a refined finite element model of a utility tunnel’s gas compartment filled with the methane/air mixture is developed. A series of analyses are made by using the powerful industry-leading computational fluid dynamics (CFD) software flame acceleration simulator (FLACS) to study the shock wave propagation rule in the gas compartment. The longitudinal and transversal distribution laws of the explosion shock wave are gained taking into consideration the spatial characteristics of the gas compartment. The influences of a few parameters, such as initial conditions and section size of the gas compartment, on the shock wave propagation rule are further discussed. The basic procedure for predicting the peak pressure of the blast wave is provided by considering the initial conditions and the gas compartment, and the corresponding injury effect of the explosion wave on the living beings is assessed. The investigation demonstrates that the peak pressure by the coupled effect between the initial conditions is significantly influenced, especially at the upper and lower gas explosion limits. The peak pressure increases gradually as the width or height increases, and both basically meet the linear relation. The proposed method can forecast the peak pressure of the explosion shock wave in the gas compartment accurately. According to the peak pressure longitudinal and transversal distributions of the blast wave, the peak pressure is far greater than the killing pressure threshold in the underground and closed space; consequently, it is not safe for the living beings in the gas compartment.

CFD implementation to mitigate the LNG leakage consequences: A review of explosion accident calculation on LNG-fueled ships

Liquefied Natural Gas (LNG) has a significant benefit in reducing air pollutants such as sulfur oxide, nitrogen oxide, and particulate matter emitted by Diesel Oil or Heavy Fuel Oil. The International Maritime Organization (IMO) regulates these pollutants in MARPOL Protocol Annex VI. It was implemented by establishing an Emission Control Area (ECA) in 2010 that restricts exhaust gas of ships. This regulation has resulted in new shipbuilding orders or conversions to adopt the LNG-fueled systems. Besides the benefits of LNG fuel, storing, transporting, and distributing this fuel has a different problem with conventional fuels. LNG is volatile, and it has a low flashpoint that could lead to spontaneous ignition. To assess the safety and mitigation for LNG-powered ships, Computational Fluid Dynamics (CFD), such as Kameleon Fire Ex (KFX) and Flame Acceleration Simulator (FLACS), is considered to simulate the LNG release. This technical measure is taken to observe the possible damage or loss due to the cryogenic effect, the extreme temperature of the jet fire, or Vapor Cloud Explosion (VCE). These consequences could happen sequentially that known as the “Domino Effect”. The effect of the leak and environment-related parameters could influence the severity of the damage. The applications of CFD for the LNG release cases are discussed and highlighted in this review.

Numerical studies on an accidental flashfire at a water fun park by FLACS software

A severe accidental fire with ?explosion? resulting from spraying colored corn flour powder occurred at a water fun park in Taiwan. The possible fire scenarios were studied in this paper using flame acceleration simulator (FLACS). Environmental conditions including wind action and solid boundary conditions were deduced based on government fire investigation and video records, and were used as the input parameters in simulation. Simulation results indicate that upon ignition of the sprayed powder, the maximum overpressure in the open space was only 0.03 bar gauge, with a dust flash fire generated without having an explosion. These environmental conditions or parameters and the simulation results together would give a fire scene that agrees with the accident observed, indicating that appropriate environmental parameters had been identified. Therefore, CFD simulation with carefully selected parameters can be used to reproduce explosion scenarios and to identify key factors in supporting accident investigations.

Simulation of Possible Fire and Explosion Hazards of Clean Fuel Vehicles in Garages

Clean fuel is advocated to be used for sustainability. The number of liquefied petroleum gas (LPG) and hydrogen vehicles is increasing globally. Explosion hazard is a threat. On the other hand, the use of hydrogen is under consideration in Hong Kong. Explosion hazards of these clean fuel (LPG and hydrogen) vehicles were studied and are compared in this paper. The computational fluid dynamics (CFD) software Flame Acceleration Simulator (FLACS) was used. A car garage with a rolling shutter as its entrance was selected for study. Dispersion of LPG from the leakage source with ignition at a higher position was studied. The same garage was used with a typical hydrogen vehicle leaking 3.4 pounds (1.5 kg) of hydrogen in 100 s, the mass flow rate being equal to 0.015 kgs−1. The hydrogen vehicle used in the simulation has two hydrogen tanks with a combined capacity of 5 kg. The entire tank would be completely vented out in about 333 s. Two scenarios of CFD simulation were carried out. In the first scenario, the rolling shutter was completely closed and the leaked LPG or hydrogen was ignited at 300 s after leakage. The second scenario was conducted with a gap height of 0.3 m under the rolling shutter. Predicted results of explosion pressure and temperature show that appropriate active fire engineering systems are required when servicing these clean fuel vehicles in garages. An appropriate vent in an enclosed space such as the garage is important in reducing explosion hazards.

Hole/pore-scale investigation of gas explosions in a coal-mine gob

The characteristics of a gas explosion were investigated with regard to the hole/pore scale in a coal-mine gob. A 3-D hole-pore single-layer network model (3-D HPSLNM) was developed, and the effects of the length-to-diameter ratio (L/D), ignition location, and gas concentration on the explosion flame and overpressure were determined with the FLACS flame acceleration simulator (Gexcon). The flame propagation speed was positively correlated with L/D and was lower in the hole than in the pores. The overpressure increased linearly with L/D. The flame propagation shape stretched and developed in different ways depending on the ignition location. The maximum flame speed and maximum overpressure for ‘end ignition’ were 56.5% and 25.2% higher than for ‘edge ignition’, respectively. The findings of this study will help further research into the hole/pore-scale effects of gas explosions in coal-mine gobs.

Numerical Simulation of Hydrogen Leakage from Fuel Cell Vehicle in an Outdoor Parking Garage

It is significant to assess the hydrogen safety of fuel cell vehicles (FCVs) in parking garages with a rapidly increased number of FCVs. In the present work, a Flame Acceleration Simulator (FLACS), a computational fluid dynamics (CFD) module using finite element calculation, was utilized to predict the dispersion process of flammable hydrogen clouds, which was performed by hydrogen leakage from a fuel cell vehicle in an outdoor parking garage. The effect of leakage diameter (2 mm, 3 mm, and 4 mm) and parking configurations (vertical and parallel parking) on the formation of flammable clouds with a range of 4–75% by volume was considered. The emission was assumed to be directed downwards from a Thermally Activated Pressure Relief Device (TPRD) of a 70 MPa storage tank. The results show that the 0.7 m parking space stipulated by the current regulations is less than the safety space of fuel cell vehicles. Compared with a vertical parking configuration, it is safer to park FCVs in parallel. It was also shown that release through a large TPRD orifice should be avoided, as the proportion of the larger hydrogen concentration in the whole flammable domain is prone to more accidental severe consequences, such as overpressure.

A risk-based optimal pressure relief opening design for gas explosions in underground utility tunnels

A gas explosion is one of the main hazards of underground utility tunnels (UUTs), and the explosion overpressure can have catastrophic consequences. A pressure relief opening (PRO) can be an effective method of reducing the risk of an explosion. However, most existing UUTs in China are not configurated with a PRO. Therefore, the present study proposes a risk-based PRO design to explore the optimal layout for UUTs. Both dispersion and explosion simulations using Flame ACceleration Simulator (FLACS) were conducted to calculate the maximum overpressures, and then probabilistic studies were applied for risk analysis. The FLACS simulation results show that PROs can significantly reduce the maximum explosion overpressure from 2.23 bar to 1.97 bar. An ALARP (As Low As Reasonably Practicable) concept was applied to provide a risk reference for the design of a PRO in UUTs.

Computational fluid dynamics modeling of liquid hydrogen release and dispersion in gas refuelling stations

The hydrogen consumption is expected to grow in the near future, and a forecasted energy transition after the COVID-19 pandemic may increment such growth. For this reason, there is the need for a solution to increase the hydrogen density for both storage and transportation purposes. The release of hydrogen from its handling equipment is a scenario that must be assessed to define the technology feasibility. Both confined and unconfined hydrogen releases have been broadly studied in the scientific literature. However, the focus has been placed mainly on the release and dispersion of compressed gaseous hydrogen. Hydrogen distribution to the future gas refuelling stations in liquid (cryogenic) phase rather than compressed gas is one of the options to increase the truck payload. For this reason, potential liquid hydrogen (LH2) release with a consequent pool formation and gas dispersion is one of the scenarios to consider by the associated risk assessment. The aim of this study is to comprehend the hydrogen behaviour after a liquid release in a refuelling station, which represents a semi-confined space, by means of a commercial computational fluid dynamics (CFD) tool: the FLame ACceleration Simulator (FLACS). The LH2 pool formation as well as the dispersion of the hydrogen gas cloud in the surrounding were investigated. Different parameters such as the variation in density of the extremely cold gas and the lower flammability limit (LFL) of the gas cloud were measured. As expected, the wind speed and direction significantly affect the position and dilution of the flammable gas cloud within and outside the facility. Few solutions to prevent further consequences from the LH2 releases such as a vapour cloud explosion were proposed to spark the interest on future studies on safety barriers for this type of accident scenario. Copyright © 2021, AIDIC Servizi S.r.l.

Computational Fluid Dynamics (CFD) Modeling and Analysis of Hydrocarbon Vapor Cloud Explosions (VCEs) in Amuay Refinery and Jaipur Plant Using FLACS

Process safety helps prevent the unexpected and unplanned release of flammable and toxic chemicals, leading to poisonous gas clouds, fires, and explosions. Vapor cloud explosions (VCEs) are among the most severe hazards to humans and the environment in process facilities. Therefore, process safety demands to use best and reliable techniques to model VCEs in process industries and storage tanks of flammable chemicals. In this regard, the Computational Fluid Dynamics (CFD) models are more appropriate, as these models provide three-dimensional (3D) modeling of all sequences of events in an accident. In this study, CFD is used to model VCE in two industrial accidents: the Amuay refinery disaster (happened in 2012) and the Indian Oil Corporation’s (IOC) Jaipur terminal (2009). This work studies 3D CFD modeling of flammable cloud explosion in the real-time configuration for both accidents. FLACS (FLame ACceleration Simulator), a CFD software, is used to simulate the loss of hydrocarbon containment, cloud formation, and explosion in both industrial case studies. The ignition locations and grid sizes were varied to analyze their influence on explosion overpressure, temperature, vapor velocity, and fuel mass. This work also investigated the effect of geometry complexity on the explosion. Results showed that, as opposed to the coarse grid, the fine grid provides more precision in the analysis. The study also reveals an explosion overpressure of the order 4–15 bar (g) for the given case studies. This study’s results can help perform a qualitative and quantitative risk assessment of the Amuay refinery accident and Jaipur fire. The simulation of different scenarios can help develop and improve safety guidelines to mitigate similar accidents.

Deflagrations by design

AbstractLarge vapor cloud explosions (VCEs) and deflagration to detonation transition (DDT) events are of considerable interest to the petroleum refining and chemical processing industries. A detonation results in a very high flame speed and can significantly increase explosion energy and decrease standoff distance, increasing the blast load. Potential mitigation options to limit VCE severity include multiple simultaneous ignition sources to limit flame travel distance, suppressants to limit flame speed, and controlling the congested volume geometry to limit the flame travel distance. Another approach is minimizing the flame travel distance to free vent. Some VCE blast load prediction methods consider flame travel distance within a congested volume, but not the distance to free vent (DFV). Decreasing the DFV limits flame acceleration and decreases the potential for a DDT. High fuel reactivities and elevated congestion/confinement levels require smaller distances to free vent. This paper describes a test program to demonstrate this mitigation option using near‐stoichiometric ethylene‐air mixtures in an elongated test rig configured with a medium level of congestion to determine the free vent distance required to prevent a DDT. Test program results, associated Flame Acceleration Simulator predictions, and potential areas for future research are included.

Dynamic characteristics of chlorine dispersion process and quantitative risk assessment of pollution hazard.

The aim of this study was to analyze dispersion behavior characteristics and pollution hazard risk after a release of liquid chlorine. A full-scale model of liquid chlorine tanks in an area with a radius range of 3 km was established using FLACS (Flame Acceleration Simulator) code, and the chlorine dispersion characteristics of six leakage scenarios were calculated according to the POOL model, and the individual risk and social risk under different conditions as calculated quantitatively. The results show that leakage occurs in three stages: dynamic dispersion, gravity dispersion, and atmospheric dispersion. Variations in dispersion processes were expressed as "outward expansion" and "inward contraction." At the same time, dispersion was accompanied by the phenomenon of "cloud separation." In the six leakage scenarios, the total distance of chlorine dispersion was 84-1000 m for a concentration of 225 ppm, and 27.5-401.3 m for a concentration of 900 ppm. The corresponding times (duration) to the farthest dispersion distance were 235-1345 s and 185-680 s, respectively. Chlorine concentration and dispersion distance are consistent in trend; however, the farthest dispersion distance shows a "delay effect" in time. At 225 ppm and 900 ppm, the delay time was 125-1145 s and 75-480 s indifferent leakage scenarios. The installation of a safety instrument system (SIS) can effectively reduce the risk of chlorine dispersion.

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 studies on explosion hazards of vehicles using clean fuel in short vehicular tunnels

Short vehicular tunnels (SVT) with heavy traffic are commonly constructed in urban areas of densely populated cities of the Asia-Oceania Region. The introduction of vehicles using clean fuel poses new concerns on the fire safety of existing road tunnels. The explosion of a taxi using Liquefied Petroleum Gas (LPG) occurred in a garage, alarming the need for protecting existing road tunnels against hazards in using clean energy.In this paper, explosion hazards of an LPG taxi in a section of SVT were studied by using Computational Fluid Dynamics (CFD) Flame Acceleration Simulator (FLACS). The scenario of LPG leaking into the concealed space beneath the tunnel floor level of SVT through vents at the road side is simulated. A full tank of LPG would completely be released in about 231 s. The leaked LPG in the concealed space is ignited 19 s after the complete release, i.e. at t = 250 s. Predicted results on explosion pressure and temperature illustrate that appropriate protection measures should be provided to ensure the safety of SVT. Results also suggest that the existing fire safety codes for SVT should be revised by including an explosion analysis of LPG taxis.

Effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm determined using computational fluid dynamics

Storage tank separation distance, which considerably affects forestalling and mitigating accident consequences, is principally determined by thermal radiation modeling and meeting industry safety requirements. However, little is known about the influence of separation distance on gas dispersion or gas explosion, which are the most destructive types of accidents in industrial settings. This study evaluated the effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm. Experiments were conducted using Flame Acceleration Simulator, an advanced computational fluid dynamics software program. Codes governing the design of separation distances in China and the United States were compared. A series of geometrical models of storage tanks with various separation distances were established. Overall, increasing separation distance led to a substantial reduction in vapor cloud volume and size in most cases. Notably, a 1.0 storage diameter separation distance appeared to be optimal. In terms of vapor cloud explosion, a greater separation distance had a marked effect on mitigating overpressure in gas explosions. Therefore, separation distance merited consideration in the design of storage tanks to prevent gas dispersion and explosion.