Validation of geometry modelling approaches for offshore gas dispersion simulations

Computational Fluid Dynamics (CFD) codes are widely used for gas dispersion studies on offshore installations. The majority of these codes use single-block Cartesian grids with the porosity/distributed-resistance (PDR) approach to model small geometric details. Computational cost of this approach is low since small-scale obstacles are not resolved on the computational mesh. However, there are some uncertainties regarding this approach, especially in terms of grid dependency and turbulence generated from complex objects. An alternative approach, which can be implemented in general-purpose CFD codes, is to use body-fitted grids for medium to large-scale objects whilst combining multiple small-scale obstacles in close proximity and using porous media models to represent blockage effects. This approach is validated in this study, by comparing numerical predictions with large-scale gas dispersion experiments carried out in DNV GL's Spadeadam test site. Gas concentrations and gas cloud volumes obtained from simulations are compared with measurements. These simulations are performed using the commercially available ANSYS CFX, which is a general-purpose CFD code. For comparison, further simulations are performed using CFX where small-scale objects are explicitly resolved. The aim of this work is to evaluate the accuracy and efficiency of these different geometry modelling approaches.     

Ammonia large scale atmospheric dispersion experiments in industrial configurations

A programme of large-scale experiments for atmospheric dispersion was carried out by INERIS over a period extending from December 1996 to April 1997. The objectives of the test campaign were to measure anhydrous ammonia concentrations in a range of few meters to 2 km from the release, in order to generate data to be used to improve 2-phase discharge and dispersion modelling.

The discharges were released from a 6-tonne storage tank of pressurised liquid ammonia and through a discharge device with an outlet diameter of 2 in. Fifteen trials were carried out with various release configurations corresponding to industrial situations (impinging jets on the ground and on a wall at various distances, release through a flange without seal…). The quantity of ammonia discharged from the liquid phase varied according to the tests, from 1.4 to 3.5 tons for durations between 7 and 14 min and, therefore, at flow rates between 2 and 4.5 kg/s. Approximately 200 sensors were settled downwind to measure ammonia concentrations and temperature in the plume. These tests showed that for discharges with identical flow rates the distances corresponding to the same concentration vary a lot according to the configurations. These distances tend to be reduced by the presence of obstacles or retention dikes that collected liquid ammonia. In the paper, the main experimental results are presented. In order to enable the comparisons with numerical predictions, more detailed information are given in [Bouet R. (1999). Ammoniac—Essais de dispersion atmosphérique à grande échelle. INERIS rapport, ref INERIS-DRA-RBo-1999-20410 (available at http://www.ineris.fr/recherches/recherches.htm).     

Validation of discharge and atmospheric dispersion for unpressurised and pressurised carbon dioxide releases

This paper discusses the validation of discharge and subsequent atmospheric dispersion for both unpressurised and pressurised carbon dioxide releases using the consequence modelling package Phast.The paper first summarises the validation of the Phast dispersion model (UDM) for unpressurised releases. This includes heavy gas dispersion from either a ground-level line source (McQuaid wind-tunnel experiments) or an area source (Kit-Fox field experiments). For the McQuaid experiments minor modifications of the UDM were made to support line sources. For the Kit Fox experiments steady-state and 20-s finite-duration releases were simulated for both neutral and stable conditions. Most accurate predictions of the concentrations for finite duration releases were obtained using the UDM Finite Duration Correction method.Using experiments funded by BP and Shell and made available via DNV's CO2PIPETRANS JIP, the paper secondly summarises the validation of the Phast discharge and dispersion models for pressurised CO₂ releases. This modelling accounted for the possible presence of the solid CO₂ phase following expansion to atmospheric pressure. These experiments included both high-pressure steady-state and time-varying cold releases (liquid storage) and high-pressure time-varying supercritical hot releases. Both the flow rate and the concentrations were found to be predicted accurately.The above validation was carried out with no fitting whatsoever of the Phast extended discharge and dispersion models.     

A dynamic approach for evaluating the consequences of toxic gas dispersion in the chemical plants using CFD and evacuation modelling

Accidental releases of toxic gas in the chemical plants have caused significant harm to the exposed occupants. To evaluate the consequences of these accidents, a dynamic approach considering the gas dispersion and behavior evacuation modelling has been proposed in this paper. This approach is applied to a hypothetical scenario including an accidental chlorine release in a chemical plant. CFD technique is utilized to calculate the time-varying concentration filed and evacuation modelling is used to obtain the evacuation routes. The exposure concentrations in the evacuation routes are calculated by using the code of data query. The integrated concentration toxic load model and probit model are used to calculate the probability of mortality of each occupant by using the exposure concentrations. Based on this dynamic approach, a new concept of average probability of mortality (APM) has been proposed to quantify the consequences of different accidental scenarios. The results show that APM decreases when the required detection time decreases or emergency evacuation mode is implemented. The impact of the detection time on APM becomes small as the wind speed increases. The effect of emergency evacuation mode is more obvious when the release occurs in an outdoor space.     

Design of a system for real-time modelling of the dispersion of hazardous gas releases in industrial plants 2. Accidental releases from storage installations

This paper is the second part of a research programme concerning the modelling capabilities of accidental releases of heavier-than-air toxic gases. The existing theory, which includes the strength of the source and the subsequent development of the released cloud under representative environmental conditions, is described. Comparison of the ZZB-2 system predictions with field data from the Desert Tortoise and Lyme Bay V, ammonia and chlorine releases, shows excellent agreement at distances between ≈ 200 m and a few kilometres from the source. The correlation between observed and predicted cloud concentrations, was in all cases significant at a confidence level better than 95%.     

Phast validation of discharge and atmospheric dispersion for pressurised carbon dioxide releases

The consequence modelling package Phast examines the progress of a potential incident from the initial release to the far-field dispersion including the modelling of rainout and subsequent vaporisation. The original Phast discharge and dispersion models allow the released substance to occur only in the vapour and liquid phases. The latest versions of Phast include extended models which also allow for the occurrence of fluid to solid transition for carbon dioxide (CO₂) releases.As part of two projects funded by BP and Shell (made publicly available via CO2PIPETRANS JIP), experimental work on CO₂ releases was carried out at the Spadeadam site (UK) by GL Noble Denton. These experiments included both high-pressure steady-state and time-varying cold releases (liquid storage) and high-pressure time-varying supercritical hot releases (vapour storage). The CO₂ was stored in a vessel with attached pipework. At the end of the pipework a nozzle was attached, where the nozzle diameter was varied.This paper discusses the validation of Phast against the above experiments. The flow rate was predicted accurately by the Phast discharge models (within 10%; considered within the accuracy at which the BP experimental data were measured), and the concentrations were found to be predicted accurately (well within a factor of two) by the Phast dispersion model (UDM). This validation was carried out with no fitting whatsoever of the Phast extended discharge and dispersion models.     

大气扩散计算流体力学模型综合评估方法研究

模型评估方法研究是模型研发工作的重要组成部分。基于科学性验证与统计性确认提出了大气扩散CFD模型的综合确认评估方法,通过示例的方式展示了方法的使用效果。综合确认评估方法可以提高模型筛选的速度,降低统计性评估对确认性试验数据的需求量,从总体上提高模型确认的效率。评估理论的研究有助于提高基于模型的大气扩散研究的准确性,也有利于高精度模型和试验的设计、开发与遴选。     

大气扩散模型统计性确认评估

使用计算模型预测物理过程之前,必须对该计算模型进行论证,确认评估通过后才能用于物理状态和过程的计算和预测。讨论了基于数据统计属性的确认评估对面向大气扩散物理现象的计算模型之适用性;探究了模型评估工作的基本内容、关键概念和通用原则;给出了确认评估参数的定义、选择原则及示例;分析了评估度量指标的内涵和适用条件;探讨了可接受模型需要满足的指标准则及统计性确认评估指标的选用;最后,指出了基于统计度量的确认评估方法的改进方向。扩散模型评估理论的探讨有助于提高基于模型仿真的大气扩散研究的准确性,也有利于高精度模型和基准试验的设计、开发与遴选。     

基于BSMPKU的高分辨率大气扩散数值模拟研究

为了检验北京大学街区尺度模式BSMPKU(Block Scale Model of Peking University)在城市大气环境研究中的适用性,首先利用Thompson风洞试验的数据集对BSMPKU模式进行了验证,并将其模拟结果与OpenFOAM(Open Source Field Operation and Manipulation)的模拟结果进行比较,然后将BSMPKU模式应用在复杂的实际建筑物群中,进行了3种不同交通线源排放的理想数值模拟研究.结果表明:1)对于单个建筑物,随建筑物宽度增加,建筑物迎风面回流区和建筑物背风面的尾流涡区范围增大;2)BSMPKU和OpenFOAM对单个建筑物周围的流场及浓度场有较好的模拟能力;3)与基于高斯扩散理论的AERMOD相比,BSMPKU和OpenFOAM能更好地模拟出建筑物周围的浓度场,但两个模式的模拟结果都与实测值存在一定误差;4)在实际小区中,受建筑物群影响,建筑物群内的流场分布比较复杂,大部分地区风速大幅下降,建筑物群内污染物浓度场的分布受排放源位置和走向的影响很大;5)BSMPKU能较好地模拟出实际城区的流场和浓度场分布,具有一定模拟和预报复杂城区污染物扩散过程的能力.     

An alternative CFD tool for gas dispersion modelling of heavy gas

The numerical simulation of gas dispersion is of great importance in various areas of engineering such as optimisation, synthesis of chemical process, petroleum industry and process safety. The OpenFOAM (Open Field Operation and Manipulation) code is a free and open source computational fluid dynamics (CFD) program. The current research is focused on the development and customisation of a computational tool for handling gas dispersion of heavy gases, such a LNG and CO₂. The novel CFD tool relies on OpenFOAM framework. The core of the work is based on the OpenFOAM solver rhoReactingBuoyantFoam to handle gas dispersion. A series of CFD simulations has been performed for methane and CO₂. The source term of the former is modelled by HSM (Hybrid Switch Model). The model comprises contribution from HEM (Homogeneous Equilibrium Model) approach, frozen model and non-equilibrium model for CO₂ leak. The novel approach switches between equilibrium and non-equilibrium conditions based on the meta-stable parameter on the grounds of thermodynamics and experimental observations. Good agreement with experimental data is observed. Numerical findings for methane leakage from the proposed CFD tool are compared with experimental data and FLACS. Good agreement is observed.     

随机游动模型在大气污染扩散分析中的应用探讨

应用随机游动扩散模拟方法,并结合天津市的实际气象数据,对某区域的大气污染扩散情况进行了模拟计算.由模拟过程和结果可以看出,相比于高斯模型等其他常用方法而言,随机游动扩散模拟适用的范围较广,可用于瞬时以及非均匀的浓度扩散分布状况,也能够不受污染物质的特性所限制(气态物和重粒子都可进行模拟).模拟结果对有关部门的污染防护与事故处理有一定的参考意义.     

大气扩散在防化领域中的研究进展

回顾了大气扩散研究的发展历程,根据动力学原理将大气扩散模式的发展分为高斯扩散模式、准静力模式、非静力模式、CFD软件、单向耦合模式和双向耦合模式6代.从核生化武器效应、核生化武器销毁危害预测与评价、核生化事故危害预测与评价几方面剖析了防化领域的大气扩散问题,论述了大气扩散在防化领域中的研究现状.提出了大气扩散数值模拟在核生化安全和核生化防护领域中需要进一步发展的研究方向.     

Sensitivity analysis of Phast’s atmospheric dispersion model for three toxic materials (nitric oxide, ammonia, chlorine)

We present the results of a parametric sensitivity analysis of a widely used model for atmospheric dispersion of toxic gases, in order better to understand the influence of user-adjustable parameters on model outputs. We have studied 60 min continuous release scenarios for three different products (nitric oxide, ammonia and chlorine), chosen to cover a range of physical characteristics and storage conditions. For each product, we have broken down base-case scenarios into a number of sub-scenarios corresponding to different release conditions which determine physical phenomena (flow rate, release angle, release elevation and atmospheric stability class). The use of statistical tools to analyze the results of a large number of model executions allows us to rank model parameters according to their influence on the variability of a number of model outputs (distances and concentrations), on a per-scenario and per-product basis. Analysis of the results allows us to verify our understanding of the modeling of cloud dispersion.     

Comparing tools of varying complexity for calculating the gas dispersion

When handling flammable and/or toxic liquids or gases, the gas dispersion following a release of substance is a scenario to be considered in the risk assessment to determine the lower flammability distance (LFD) and toxicity thresholds. In this work a comparison of different gas dispersion tools of varying complexity ranging from a simple Gaussian model over a boundary layer model (BLM) and a Lagrangian model to CFD (in this case ANSYS CFX v14) is presented. The BLM covers the special case of liquid releases with formation of a pool. It does not only solve the gas dispersion but also calculates the evaporating mass flow out of the pool. The simulation values are compared to each other and to experimental data resulting mainly from our own open air experiments covering the near field and carried out on the Test Site Technical Safety of BAM (BAM-TTS) for different release types (pool evaporation, gas release) and topologies. Other validation data were taken from literature and cover large scale experiments in the range of several 100 m.     

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.     

Modelling of discharge and atmospheric dispersion for carbon dioxide releases

This paper discusses the modelling of the discharge and subsequent atmospheric dispersion for carbon dioxide releases using extensions of models in the consequence modelling package Phast. Phast examines the progress of a potential incident from the initial release to the far-field dispersion including the modelling of rainout and subsequent vaporisation. The original Phast discharge and dispersion models allow the released chemical to occur only in the vapour and liquid phases. As part of the current work these models have been extended to also allow for the occurrence of liquid to solid transition or vapour to solid transition. This applies both for the post-expansion state in the discharge model, as well as for the thermodynamic calculations by the dispersion model. Solid property calculations have been added where necessary. The above extensions are generally valid for fluid releases including CO₂. Using the extended dispersion formulation, a sensitivity study has been carried out for mixing of solid CO₂ with air, and it is demonstrated that solid effects may significantly affect the predicted concentrations.     

Heavy gas dispersion by water spray curtains: A research methodology

The mitigation of the consequences of accidental releases of dangerous toxic and/or flammable cloud is a serious concern in the petro-chemical and gas industries. Nowadays, the water-curtain is recognized as a useful technique to mitigate a heavy gas cloud. The paper presents a research methodology, which has been established and undertaken to quantify the forced dispersion factor provided by a water-curtain with respect to its configuration.

The method involves medium-scale field tests, Wind-Gallery tests and numerical simulations. These different approaches are discussed and exemplified by typical results emphasizing the observed concentration reduction due to the water-curtain.     

基于LabVIEW的固体火箭发动机直列式点火实验系统设计

固体火箭发动机点火装置是固体火箭的重要组成部分,它必须在极短的时间内可靠地点燃发动机主燃药,使发动机安全稳定的工作.直列式点火系统应用于固体火箭发动机上,极大地提高了发动机的可靠性.完整的直列式钝感点火系统由直列式全电子安全与解除保险装置、脉冲功率装置、冲击片点火管组成.针对这一系统,利用Labview设计一个可视化的固体火箭发动机直列式点火实验控制系统.该系统通过PC机可与单片机进行串口通信.单片机按照安伞解保时序向直列式全电子安全与解除保险装置传输上电信号,当解保时序正确且接收到高压监测信号时触发点火管,同时采集功率脉冲装置中高压储能电容的电压.试验表明,该系统可准确地按照固体火箭发动机直列式点火时序完成点火实验.     

一种瞬间泄漏重气扩散模型的探讨

在对现有盒子模型进行分析研究的基础上,结合高斯模型,建立了一种新的瞬间泄漏重气扩散模型（BOX GM）。BOX GM模型具有形式简单、容易编程和运算时间短等特点。将BOX GM模型模拟结果与Throney Island Trials系列试验的7组代表不同类型大气环境下的现场试验数据进行了比较,同时与IIT Heavy Gas Model 模型的模拟结果也进行了对比,结果表明,BOX GM模型的模拟精度较高。