Prankul Middha

CFD calculations of gas leak dispersion and subsequent gas explosions: validation against ignited impinging hydrogen jet experiments.

Computational fluid dynamics (CFD) tools are increasingly employed for quantifying incident consequences in quantitative risk analysis (QRA) calculations in the process industry. However, these tools must be validated against representative experimental data, involving combined release and ignition scenarios, in order to have a real predictive capability. Forschungszentrum Karlsruhe (FZK) has recently carried out experiments involving vertically upwards hydrogen releases with different release rates and velocities impinging on a plate in two different geometrical configurations. The dispersed cloud was subsequently ignited and resulting explosion overpressures recorded. Blind CFD simulations were carried out prior to the experiments to predict the results. The simulated gas concentrations are found to correlate reasonably well with observations. The overpressures subsequent to ignition obtained in the blind predictions could not be compared directly as the ignition points chosen in the experiments were somewhat different from those used in the blind simulations, but the pressure levels were similar. Simulations carried out subsequently with the same ignition position as those in the experiments compared reasonably well with the observations. This agreement points to the ability of the CFD tool FLACS to model such complex scenarios even with hydrogen as a fuel. Nevertheless, the experimental set-up can be considered to be small-scale. Future large-scale data of this type will be valuable to confirm ability to predict large-scale accident scenarios.

Predicting deflagration to detonation transition in hydrogen explosions

Because of the development in computational resources, Computational Fluid Dynamics (CFD) has assumed increasing importance in recent years as a tool for predicting the consequences of accidents in petrochemical and process industries. CFD has also been used more and more for explosion predictions for input to risk assessments and design load specifications. The CFD software FLACS has been developed and experimentally validated continuously for more than 25 years. As a result, it is established as a tool for simulating hydrocarbon gas deflagrations with reasonable precision and is widely used in petrochemical industry and elsewhere. In recent years the focus on predicting hydrogen explosions has increased, and with the latest release the validation status for hydrogen deflagrations is considered good.

CFD-based risk assessment for hydrogen applications

Computational fluid dynamics calculations for gas explosion safety have been widely used for doing risk assessments within the oil and gas industry for more than a decade. On the basis of predicted consequences of a range of potential accident scenarios a risk level is predicted. The development of applications using hydrogen as a clean energy carrier has accelerated in recent years, and hydrogen may be used widely in the future. Because of the very high reactivity of hydrogen, safe handling is critical. For most applications it is not realistic to perform an extensive risk assessment similar to what is done for large petrochemical installations. On the other hand, simplified methods, like venting guidelines, may have a questionable validity for hydrogen. The use of simple methods, if these actually are conservative, will in general predict too high consequences for the majority of scenarios, as these are not able to represent actual geometry and physics of the explosion.

Prioritisation of Research and Development for modelling the safe production, storage, delivery and use of hydrogen

The report describes the findings of a workshop that was held at the Institute for Energy and Transport (JRC) in Petten Netherlands, on the topic “Gap analysis of CFD modelling of hydrogen release and combustion”. The main topic was divided in 6 sub-topics: release and dispersion,auto-ignition, fires, deflagrations, detonations and DDT, and accident consequences. For each sub-topic, the main gaps in CFD modelling were identified and prioritised.Hydrogen is expected to play an important role in the energy mix of a future low carbon society, (the European Strategic Energy Technology Plan of the European Commission (COM 2007 - 723) and in the Hydrogen, Fuel Cells & Infrastructure Technologies Program-Multi-Year Research, Development, and Demonstration Plan of the USA Department of Energy (DoE 2007).Hydrogen safety issues must be addressed in order to ensure that the wide spread deployment and use of hydrogen and fuel cell technologies can occur with the same or lower level of hazardsand associated risk compared to the conventional fossil fuel technologies. Hydrogen safety is a EUPolicy relevant issue as it is stated in the priority 3 Action 2 (Continuous improvement in safety andsecurity) of the EU “Energy 2020 A strategy for competitive, sustainable and secure energy”: “Thesame security and safety considerations will also be upheld in the development and deployment of new energy technologies (hydrogen safety, safety of CO2 transportation network, CO2 storage, etc…)”.Computational Fluid Dynamics (CFD) is one of the tools to investigate safety issues related to theproduction, storage, delivery and use of hydrogen. CFD techniques can provide a wealthy amountof information on the dynamics of hypothetical hydrogen accident and its consequences. The CFD based consequence analysis is then used in risk assessments. This report describes the output of aworkshop organised at the Institute for Energy and Transport (JRC) in Petten, Netherlands to identify the gaps and issues in CFD modelling of hydrogen release and combustion.A hydrogen accident usually follows a typical sequence of events: an unintended release, themixing of hydrogen with air to form a flammable mixture, the ignition of the flammable cloud anddepending on the conditions, and a fire or an explosion (deflagration or/and detonation). Foreach stage of the accident, the critical CFD issues have been identified and prioritised. Beyond thespecific issues of CFD modelling that are described for each accident stage in the report, some general modelling issues can be found in all stages:• lack of an extensive validation of CFD codes/models that covers all the relevant range of conditions that can be found in hypothetical accident scenarios e.g. in terms of geometrical layout,leak flow rates.• lack of a CFD validation protocol for hydrogen like it exists for Liquefied Natural Gas (LNG): theModel Evaluation Protocols (MEP) for assessment of models for accident consequences, withguidance on evaluating models in terms of scientific assessment, verification and validation.• lack of a database of experiments for validation of hydrogen models.• in some cases, lack of complete and accurate experimental data for the CFD validation.The goals of this work were to perform a state of the art review in CFD modelling of hypotheticalaccidents scenarios related to hydrogen technologies and identify and prioritise the gapsin the field.The report is based on a dedicated workshop organised in Petten with the participation of externalexperts an extensive literature review performed by experts in the field and the direct expertise andexperience of the experts. The experts were carefully selected according to their experience/expertise, number of scientific publications and participations to International Conferences, seminars, workshops and to international and/or European co-fundedprojects such as HySafe (Hydrogen Safety), HyApproval (Approval of Hydrogen Re-fuellingStations), European Integrated Hydrogen Projects.By performing a state of the art review of CFD modelling for hydrogen safety issues, a consensuswas reached among the scientific experts as to the main gaps in the field and on the priority of theresearch needs.Potential impact:Identifying the modelling gaps is a first necessary step for making decisions on the next steps tocarry out the full and safe utilization of hydrogen. The document aims to become a referencedocument for researchers/scientists and technical experts working in the area. It is also a welcomedcontribution for the Fuel Cell and Hydrogen Joint Undertaking and for other funding bodies/organizations that must make decisions on research programmes and during the selection/choice of projects to be financially supported pursuing the safe use of hydrogen. The protocol could work as a catalyst to accelerate both the improvements of existing codes and models and the developments of new models/codes with increased predictive capabilities.Attendees:Experts who attended the workshop andcontributed to the report• Daniele Baraldi, Efthymia Papanikolaou, Matthias Heitsch, Pietro Moretto (JRC)• Stewart Cant (Cambridge University, UK)• Dirk Roekaerts (Delft University of Technology, NL)• Alexei Kotchourko (KIT/FZK, Germany)• Prankul Middha (GexCon, Norway)• Andrei V. Tchouvelev (H2Can, Canada)• Jennifer Wen (Kingston University, UK)• Alexander Venetsanos (National Center Scientific Research Demokritos, Greece)• Vladimir Molkov (University of Ulster, UK)Experts who contributed to the report (but did not attend the workshop)• Stefan Ledin (HSL/HSE, Health and Safety Laboratory, Health and safety Executive,UK)• Sergey Dorofeev (FM Global, USA)