Development, use, and validation of the CFD tool FLACS for hydrogen safety studies

Abstract

Computational Fluid Dynamics (CFD) calculations for gas explosion safety have been widely used for doing risk assessments within the oil and gas industry for more than a decade. Based on 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 future. Due to the very high reactivity of hydrogen, safe handling is critical. 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. 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. This PhD thesis deals with the development and validation of the Computational Fluid Dynamics (CFD) tool FLACS for hydrogen safety applications. Significant validation work against several experiments has been carried out in order to increase the confidence of predictions of scenarios relevant to hydrogen safety. The validation studies have included dispersion, explosion and combined dispersion and explosion studies. A range of different dispersion experiments is simulated, including low momentum releases in a garage, sub-sonic 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. FLACS uses a utility program in order to model releases from high-pressure reservoirs. Work has been carried out in order to extend the models in the utility program in order to include real gas effects. Validation against explosion experiments in geometries ranging from smooth and obstructed pipes, refuelling station, tunnel, vented vessels, jet-ignited lane, etc. have been successfully performed. However, CFD tools must be validated against representative experimental data, involving combined release and ignition scenarios, in order to have a real predictive capability in accidental situations. Therefore, a detailed study involving release and ignition experiments from FZK has been carried out. Work has also been done for developing risk analysis methods specific to hydrogen applications. Quantitative Risk Assessments (QRA) of hydrogen applications presents new challenges due to a large difference in properties of hydrogen and natural gas, namely in reactivity, flammability limits, buoyancy and transport properties. However, it is not realistic to perform an extensive risk assessment for all hydrogen applications similar to that carried out for petrochemical installations. On the other hand, simplified tools and techniques based on codes and standards likely have a limited applicability, as these are not able to represent actual geometry and physics of the explosion. A 3-step approach is proposed, in which the CFD-tool FLACS is used to estimate the risk. The initial approach is to carry out a “worst-case” calculation evaluating the consequences if a full stoichiometric gas cloud is ignited. Mitigation measures can also be considered. As a second step, if potential consequences of the initial approach are not acceptable, the assumptions are refined and more calculations are performed to make the evaluations more realistic and reduce unnecessary conservatism of the chosen worst-case scenarios. Typically a number of dispersion calculations are performed to generate likely gas clouds, which are subsequently ignited. If estimated consequences are still not acceptable, a more comprehensive study, including ventilation, dispersion and explosion, is performed to evaluate the probability for unacceptable events. Calculation examples have been used to illustrate the different approaches. The proposed approach is thus very flexible, and can be tailored to the scenario under consideration. However, in many of these scenarios, especially involving reactive gases such as hydrogen, deflagration to detonation transition (DDT) may be a significant threat. Another main part of this thesis has been the development of models in order to enable FLACS to provide indications about the possibility of a deflagration-to-detonation transition (DDT). The likelihood of DDT has been expressed in terms of spatial pressure gradients across the flame front. This parameter is able to visualize when the flame front captures the pressure front, which is the case in situations when fast deflagrations transition to detonation. Reasonable agreement was obtained with experimental observations in terms of explosion pressures, transition times, and flame speeds for several practical geometries. The DDT model has also been extended to develop a more meaningful criterion for estimating the likelihood of DDT by comparison of the geometric dimensions with the detonation cell size. In the end, several practical studies have been carried out. This includes a very detailed simulation study to examine what, if any, is the explosion risk associated with hydrogen vehicles in tunnels. Its aim was to further our understanding of the phenomena surrounding hydrogen releases and combustion inside road tunnels, and furthermore to demonstrate how a risk assessment methodology described above could be applied to the current task. A study to determine the relative risk of methane, hydrogen and hythane (a blend of hydrogen and methane) has also been performed. The work performed in the dissertation has resulted in 18 publications (journal articles + conference proceedings). 12 of them are included in the appendix and the complete list of publications (including 4 other conference papers connected with the work done in this thesis) is presented in the next pages. Thus, this dissertation presents the extensive work that has been carried out to develop and validate FLACS-Hydrogen to pave the way to use the tool for applications related to hydrogen safety.