Canada

The use of high-fidelity simulation methods based on large-eddy simulation (LES) are proving useful for understanding and mitigating the safety hazards associated with hydrogen releases from nuclear power plants. However accurate modelling of turbulent premixed hydrogen flames via LES can require very high resolution to capture both the large-scale turbulence and its interaction with the flame fronts. Standard meshing strategies can result in impractically high computational costs especially for the thin fronts of hydrogen flames. For these reasons the use of a recently formulated integral length scale approximation (ILSA) subfilter-scale model in combination with an efficient anisotropic block-based adaptive mesh refinement (AMR) technique is proposed and examined herein for performing LES of turbulent premixed hydrogen flames. The anisotropic AMR method allows dynamic and solution-dependent resolution of flame fronts and the grid-independent properties of the ILSA model ensure that numerical errors associated with implicitly-filtered LES techniques in regions with varying resolution are avoided. The combined approach has the potential to allow formally converged LES solutions (direct numerical simulation results are typically reached in the limit of very fine meshes with standard subgrid models). The proposed LES methodology is applied to combustion simulations of lean premixed hydrogen-air mixtures within closed vessels: a problem relevant to hydrogen safety applications in nuclear facilities. A progress variable-based method with a multi-phenomena burning velocity model is used as the combustion model. The present simulation results are compared to the available experiment data for several previously studied THAI vessel cases and the capabilities of the proposed LES approach are assessed.

Computational Fluid Dynamics (CFD) is applied to investigate the near exit jet behavior of high pressure hydrogen release into quiescent ambient air through different types of orifices. The size and geometry of the release hole can affect the possibility of auto-ignition. Therefore the effect of release geometry on the behavior and development of hydrogen jet issuing from non-axisymmetric (elliptical) and expanding orifices is investigated and compared with their equivalent circular orifices. A three-dimensional in-house code is developed using the MPI library for parallel computing to simulate the flow based on an inviscid approximation. Convection dominates viscous effects in strongly underexpanded supersonic jets in the vicinity of release exit justifying the use of the Euler equations. The transport (advection) equation is applied to calculate the concentration of hydrogen-air mixture. The Abel-Nobel equation of state is used because high pressure hydrogen flow deviates from the ideal gas assumption. This work effort is conducted to fulfill two objectives. First two types of circular and elliptic orifices with the same cross sectional area are simulated and the flow behavior of each case is studied and compared during the initial stage of release. Second the comparative study between expanding circular exit and its fixed counterpart is carried out. This evaluation is conducted for different sizes of nozzle with different aspect ratios.

This paper examines the flame dynamics of vented deflagration in stratified hydrogen layers. It also compares the measured combustion pressure transients with 3D GOTHIC simulations to assess GOTHIC’s capability in simulating the associated phenomena. The experiments were performed in the Large-Scale Vented Combustion Test Facility at the Canadian Nuclear Laboratories. The stratified layer was formed by injecting hydrogen at a high elevation at a constant flow rate. The dominant parameters for vented deflagrations in stratified layers were investigated. The experimental results show that significant overpressures are generated in stratified hydrogen–air mixtures with local high concentration although the volume-averaged hydrogen concentration is non-flammable. The GOTHIC predictions capture the overall pressure dynamics of combustion very well but the peak overpressures are consistently over-predicted particularly with higher maximum hydrogen concentrations. The measured combustion overpressures are also compared with Molkov’s model prediction based on a layer-averaged hydrogen concentration.

An accident modelling approach is used to assess the safety of a hydrogen station as part of a ground transportation network. The method incorporates prevention barriers associated to human factors management and organizational failures in a risk assessment framework. Failure probabilities of these barriers and end-states events are predicted using Fault Tree Analysis and Event Tree Analysis respectively. Results from the case study considered revealed the capability of the proposed method in estimating the likelihood of various outcomes as well as predicting the future probability. In addition the scheme offers opportunity to provide dynamic adjustment by updating the failure probability with actual plant data. Results from the analysis can be used to plan maintenance and management of change as required by the plant condition.

Previous experiments demonstrated that the accidental release of high pressure hydrogen into air can lead to the possibility of spontaneous ignition. It is believed that this ignition is due to the heating of the mixing layer between hydrogen and air that is caused by the shock wave driven by the pressurized hydrogen during the release. Currently this problem is poorly understood and not amenable to direct numerical simulation. This is due to the presence of a wide range of scales between the sizes of the blast wave driven and the very thin mixing layer. The present study addresses this fundamental ignition problem and develops a solution framework in order to predict the ignition event for given hydrogen storage pressures and dimension of the release hole. In this problem only the mixing layer between the hydrogen and air is considered. This permits us to use much higher resolution than previous studies. This mixing layer at the jet head is advected as a Lagrangian fluid particle. The key physical processes in the problem are identified to be the mixing of the two gases at the mixing layer the initial heating by the shock wave and a cooling effect due to the expansion of the mixing layer. The results of the simulations indicate that for every storage pressure there exists a critical hole size below which ignition is prevented during the release process. Close inspection of the results indicate that this limit is due to the competition between the heating provided by the shock wave and the cooling due to expansion. Furthermore the results also indicate that the details of the mixing process do not play a significant role to leading order. The limiting ignition criteria were found to be well approximated by the Homogeneous Ignition Model of Cuenot and Poinsot supplemented by a heat loss term due to expansion. Therefore turbulent mixing occurring in reality is not likely to affect the ignition limits derived in the present study. Comparison with existing experiments showed very good agreement.

In this paper the detonation propensity of different compositions of mixtures of hydrogen propane and methane with air has been evaluated over a wide range of compositions. We supplement the conventional calculations of the induction delay with calculations of the characteristic acceleration parameter recently suggested by Radulescu Sharpeand Bradley(RSB) to characterize the instability of detonations. While it is well established that the ignition delay provides a good measure for detonability the RSB acceleration or its non-dimensionalform provides a further discriminant between mixtures with similar ignition delays. The present assessment of detonability reveals that while a stoichiometric mixture of hydrogen-air has an ignition delay one and two orders of magnitude shorter than respectively propane and methane hydrogen also has a parameter smaller by respectively one and two orders of magnitude. Its smaller propensity for instability is reflected by an RSB acceleration parameter similar to the two hydrocarbons. The predictions however indicate that lean hydrogen mixtures are likely to be much more unstable than stoichiometric ones. The relation between the parameter and potential to amplify an unstable transverse wave structure has been further determined through numerical simulation of decaying reactive Taylor-Sedov blast waves. Using a simplified two-step model calibrated for these fuels we show that methane mixtures develop cellular structures more readily than propane and hydrogen when observed on similar induction time scales. Future work should be devoted towards a quantitative inclusion of the RSB parameter in assessing the detonability of a given mixture.

We have implemented a portable large-scale shadowgraph system for use in flow visualization relating to hydrogen safety. Previous large-scale shadowgraph and schlieren implementations have often been limited to background- oriented techniques which are subject to noise. The system built is based on a large-scale shadowgraph technique developed by Settles which allows for high-quality visualization. We have applied the shadowgraph system to complex phenomena and current issues in hydrogen safety including DDT in long channels jet releases and unconfined deflagrations. Shadowgrams taken are compared to a Z-schlieren system. This shadowgraph system allows analysis of these phenomena at longer length scales.

The safety and convenience of hydrogen storage are significant for fuel cell vehicles. Based on mass conservation equation and energy conservation equation two thermodynamic models (single zone model and dual zone model) have been established to study the hydrogen gas temperature and tank wall temperature for compressed hydrogen storage tank. With two models analytical solution and Euler solution for single zone (gas zone) charge-discharge cycle have been compared Matlab/Simulink solution and Euler solution for dual zone (gas zone wall zone) charge-discharge cycle have been compared. Three charge-discharge cycle cases (Case 1 constant inflow temperature; Case 2 variable inflow temperature; Case 3 constant inflow temperature variable outflow temperature) and two compressed hydrogen tanks (Type III 25L Type IV 99L) charge-discharge cycle are studied by Euler method. Results show Euler method can well predict hydrogen temperature and tank wall temperature.

The current study addresses the spontaneous ignition of hydrogen jets released into a confined oxidizer environment experimentally. The experiments are conducted in a shock tube where hydrogen gas is shock-accelerated into oxygen across a perforated plate. The operating conditions and hole dimension of the plate were varied in order to identify different flow field and ignition scenarios. Time resolved Schlieren visualization permitted to reconstruct the gasdynamic evolution of the release and different shock interactions. Time resolved self-luminosity records permitted us to record whether ignition was achieved and also to record the dimension of the turbulent mixing layer. The ignition limits determined experimentally in good agreement with the 1D diffusion ignition model proposed by Maxwell and Radulescu. Nevertheless the experiments demonstrated that the mixing layer is two to three orders of magnitude thicker than predicted by molecular diffusion which can be attributed to the observed mixing layer instabilities and shock-mixing layer interactions which provide a much more intense mixing rate than anticipated from previous and current numerical predictions. These observations further clarify why releases through partly confined geometries are more conducive to jet ignition of the jets.

Experiments with a hydrogen jet were performed at two different pressures 96 psig (6.6 bars) and 237 psig (16.3 bars). The hydrogen leak was generated at two different hole sizes 1/16 inch (1.6 mm) and 1/32 inch (0.79 mm). The flammable shape of the plume was characterised by numerous measurements of the hydrogen concentration inside of the jet. The effect of the nearby horizontal surface on the shape of the plume was measured and compared with results of CFD numerical simulations. The paper will present results and an interpretation on the nature of the plume shape.