CFD modelling of hydrogen release, dispersion and combustion for automotive scenarios

Wind Impact Assessment of a Sour Gas Release in an Offshore Platform

Complex installations that involve dangerous substances, such as oil and gas or nuclear plants, must mandatorily undergo a quantitative risk assessment (QRA) according to current regulations. This requires, among others, the simulation of hundreds of accidental scenarios, which are typically carried out using empirical tools due to their fast response. Nonetheless, since they are not able to guarantee sufficient accuracy, especially when complex geometries are involved, computational fluid dynamics (CFD) tools are increasingly used. In this work, a high-pressure accidental release of a sour gas (CH4-H2S) in an offshore platform under several wind conditions is considered. A methodology used to perform a wind sensitivity analysis via CFD, while avoiding high computational costs, is presented. The wind intensity impact on some risk-related figures of merit, such as the high lethality or irreversible injuries areas, is discussed in relation to the flammability and toxicity limits of the released mixture. The results show that even a very low amount of H2S in the released mixture can strongly affect the threat zones. A progressive decrease in the toxic and flammable volumes in the platform is observed as the wind velocity increases; nonetheless, a saturation effect appears in high wind speed scenarios.

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.

Quantitative Risk Assessment on the Transport of Dangerous Goods Vehicles Through Unidirectional Road Tunnels: An Evaluation of the Risk of Transporting Hydrogen

A quantitative risk analysis (QRA) concerning dangerous goods vehicles (DGVs), including also vehicles for the transport of liquid hydrogen (LH₂ TVs), running through unidirectional motorway tunnels was performed. An event tree was built, and a wide parametric analysis based on different geometric and traffic characteristics of tunnels was carried out. The effects of the annual average daily traffic (AADT) per lane, the tunnel length (L), the percentage both of heavy goods vehicles (HGVs) and DGVs (for a given 7% of LH₂ TVs) were investigated. The results in terms of social risk, as expressed by F/N curves and the expected value (EV), show an increased risk level with the presence of the hydrogen transported, and with certain F/N curves that might also lie above the acceptability limit. This means that additional safety measures should be implemented in order to reduce the risk level or that, alternatively, appropriate strategies of traffic control systems should be taken. A statistical modeling for developing a predictive method of the EV is also performed. The outcomes show that the regression coefficients have the signs expected. In particular, the EV increases with the tunnel length (L), the AADT, and the percentage both of HGVs and DGVs. However, the magnitude of estimated coefficients indicates that the expected value EV increases more with the traffic (AADT per lane, HVGs, or DGVs) than the tunnel length. The application of the approximate method might help the Tunnel Management Agencies (TMAs) in making quick decisions, at a preliminary stage, about temporarily allowing, forbidding or limiting the circulation of DGVs and/or LH₂ TVs through tunnels; and subsequently investigating in greater depth the potential hazards due to the transport of hydrogen in the worst cases individualized.

Numerical Simulation of the Deflagration-to-Detonation Transition in Inhomogeneous Mixtures

In this study the hazardous potential of flammable hydrogen-air mixtures with vertical concentration gradients is investigated numerically. The computational model is based on the formulation of a reaction progress variable and accounts for both deflagrative flame propagation and autoignition. The model is able to simulate the deflagration-to-detonation transition (DDT) without resolving all microscopic details of the flow. It works on relatively coarse grids and shows good agreement with experiments. It is found that a mixture with a vertical concentration gradient can have a much higher tendency to undergo DDT than a homogeneous mixture of the same hydrogen content. In addition, the pressure loads occurring can be much higher. However, the opposite effect can also be observed, with the decisive factor being the geometric boundary conditions. The model gives insight into different modes of DDT. Detonations occurring soon after ignition do not necessarily cause the highest pressure loads. In mixtures with concentration gradient, the highest loads can occur in regions of very low hydrogen content. These new findings should be considered in future safety studies.