HySafe Standard benchmark Problem SBEP-V11: Predictions of hydrogen release and dispersion from a CGH2 bus in an underpass

One of the tasks of the HySafe Network of Excellence was the evaluation of available CFD tools and models for dispersion and combustion in selected hydrogen release scenarios identified as “standard benchmark problems” (SBEPs). This paper presents the results of the HySafe standard benchmark problem SBEP-V11. The situation considered is a high pressure hydrogen jet release from a compressed gaseous hydrogen (CGH2) bus in an underpass. The bus considered is equipped with 8 cylinders of 5 kg hydrogen each at 35 MPa storage pressure. The underpass is assumed to be of the common beam and slab type construction with I-beams spanning across the highway at 3 m centres (normal to the bus), plus cross bracing between the main beams, and light armatures parallel to the bus direction. The main goal of the present work was to evaluate the role of obstructions on the underside of the bridge deck on the dispersion patterns and assess the potential for hydrogen accumulation. Four HySafe partners participated in this benchmark, with 4 different CFD codes, ADREA-HF, CFX, FLACS and FLUENT. Four scenarios were examined in total. In the base case scenario 20 kg of hydrogen was released in the basic geometry. In Sensitivity Test 1 the release position was moved so that the hydrogen jet could hit directly the light armature on the roof of the underpass. In Sensitivity Test 2 the underside of the bridge deck was flat. In Sensitivity Test 3 the release was from one cylinder instead of four (5 kg instead of 20). The paper compares the results predicted by the four different computational approaches and attempts to identify the reasons for observed disagreements. The paper also concludes on the effects of the obstructions on the underside of the bridge deck

Large Eddy Simulation study on the structure of turbulent flow in a complex city

Abstract Large Eddy Simulation (LES) of atmospheric flows has become an increasingly popular modelling approach within the last years, as it has the potential to provide deeper insight into unsteady flow phenomena. LES can be improved and validated using specifically designed and well documented wind tunnel datasets. In this work, we evaluate the performance of LES against a wind tunnel experiment in a semi-idealized city (Michel-Stadt; CEDVAL-LES database) and use the LES results to study the structure of the turbulent flow at the particular urban area. The first, second and third order statistics are presented, as well as velocity frequency distributions and energy spectra. The results compare well with the experimental values. Information about special features of the flow field is also provided. A particular focus of this work is put on the influence of grid resolution on the results. Five different grids are examined and the required resolution for turbulent flow within the canopy layer is evaluated. This study reveals the strong potential of LES for urban flow simulations. It is shown that LES can assess highly non-Gaussian flow behaviour in street canyons, which has implications for urban ventilation, wind comfort assessment and urban design

The evolution and structure of ignited high-pressure cryogenic hydrogen jets

The anticipated upscaling of hydrogen energy applications will involve the storage and transport of hydrogen at cryogenic conditions. Understanding the potential hazard arising from leaks in high-pressure cryogenic storage is needed to improve hydrogen safety. The manuscript reports a series of numerical simulations with detailed chemistry for the transient evolution of ignited high-pressure cryogenic hydrogen jets. The study aims to gain insight of the ignition processes, flame structures and dynamics associated with the transient flame evolution. Numerical simulations were firstly conducted for an unignited jet released under the same cryogenic temperature of 80 K and pressure of 200 bar as the considered ignited jets. The predicted hydrogen concentrations were found to be in good agreement with the experimental measurements. The results informed the subsequent simulations of the ignited jets involving four different ignition locations. The predicted time series snapshots of temperature, hydrogen mass fraction and the flame index are analyzed to study the transient evolution and structure of the flame. The results show that a diffusion combustion layer is developed along the outer boundary of the jet and a side diffusion flame is formed for the near-field ignition. For the far-field ignition, an envelope flame is observed. The flame structure contains a diffusion flame on the outer edge and a premixed flame inside the jet. Due to the complex interactions between turbulence, fuel-air mixing at cryogenic temperature and chemical reactions, localized spontaneous ignition and transient flame extinguishment are observed. The predictions also captured the experimentally observed deflagration waves in the far-field ignited jets

Computational fluid dynamics modeling of vapor cloud explosion, cold bleve and hot bleve in a large scale tunnel

In the present work, Computational Fluid Dynamics (CFD) explosion simulations are performed in a large scale tunnel. The length of the tunnel is approximately equal to one kilometer. Three different cases of explosion are studied: Vapor Cloud Explosion (VCE), Cold BLEVE (Boiling Liquid Expanding Vapor Explosion) and Hot BLEVE. The main purpose of this study is the calculation of the generated overpressures inside the tunnel and the comparison of the pressure dynamics among these type of explosions. Realistic scenarios are chosen for each explosion based on the traffic of the tunnel. In the Vapor Cloud Explosion case, the release and dispersion of 23100 kg propane into the atmosphere are simulated in order to calculate the concentration distribution in the tunnel. Both external wind and piston effect due to vehicles’ movement was taken under account in the dispersion process. Then the mixture is ignited and the deflagration process is simulated in order to calculate the generated overpressures. In the Cold BLEVE case the total loss of confinement of a 29 m3 high pressure (57 bar) carbon dioxide storage tank is simulated, whereas in the Hot BLEVE case the total loss of confinement of a 46 m3 propane storage tank (at 18 bar) is considered. The total loss of confinement of the tanks lead to a violent expansion due to evaporation. As a result high overpressures are generated. The transient three dimensional Navier-Stokes equations of the multispecies mixture along with the continuity equation, the conservation equation of species and the energy equation are solved. Turbulence is modelled with the standard k-ε model. In the Vapor Cloud Explosion case a Multi- Phenomena turbulent burning velocity combustion model is used. In the Hot BLEVE case, fire is modeled using the Eddy Dissipation Concept (EDC) model. The simulation results reveal that the modeling approach that is used is capable of reproducing physical realistic results. Differences in pressure dynamics among the scenarios are revealed due to the different physics of the explosions.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Source, Dispersion and Combustion Modelling of an Accidental Release of Hydrogen in an Urban Environment.

Abstract not availableJRC.H-Institute for environment and sustainability (Ispra

DISPLAY-2: a Two-Dimensional Shallow Layer Model for Dense Gas Dispersion Including Complex Features.

Abstract not availableJRC.H-Institute for environment and sustainability (Ispra

CFD Modelling of Hydrogen Release, Dispersion and Combustion for Automotive Scenarios

The paper describes the analysis of the potential effects of releases from compressed gaseous hydrogen systems on commercial vehicles in urban and tunnel environments using computational fluid dynamics (CFD). Comparative releases from compressed natural gas systems are also included in the analysis. This study is restricted to typical non-articulated single deck city buses. Hydrogen releases are considered from storage systems with nominal working pressures of 20, 35 and 70 MPa, and a comparative natural gas release (20 MPa). The cases investigated are based on the assumptions that either fire causes a release via a thermally activated pressure relief device(s) (PRD) and that the released gas vents without immediately igniting, or that a PRD fails. Various release strategies were taken into account. For each configuration some worstcase scenarios are considered. By far the most critical case investigated in the urban environment, is a rapid release of the entire hydrogen or natural gas storage system such as the simultaneous opening of all PRDs. If ignition occurs, the effects could be expected to be similar to the 1983 Stockholm hydrogen accident [Venetsanos, A. G., Huld, T., Adams, P., & Bartzis, J. G. (2003). Source, dispersion and combustion modelling of an accidental release of hydrogen in an urban environment. Journal of Hazardous Materials, A105, 1¿25]. In the cases where the hydrogen release is restricted, for example, by venting through a single PRD, the effects are relatively minor and localised close to the area of the flammable cloud. With increasing hydrogen storage pressure, the maximum energy available in a flammable cloud after a release increases, as do the predicted overpressures resulting from combustion. Even in the relatively confined environment considered, the effects on the combustion regime are closer to what would be expected in a more open environment, i.e. a slow deflagration should be expected. Among the cases studied the most severe one was a rapid release of the entire hydrogen (40 kg) or natural gas (168 kg) storage system within the confines of a tunnel. In this case there was minimal difference between a release from a 20MPa natural gas system or a 20MPa hydrogen system, however, a similar release from a 35MPa hydrogen system was significantly more severe and particularly in terms of predicted overpressures. The present study has also highlighted that the ignition point significantly affects the combustion regime in confined environments. The results have indicated that critical cases in tunnels may tend towards a fast deflagration, or where there are turbulence generating features, e.g. multiple obstacles, there is the possibility that the combustion regime could progress to a detonation.JRC.F.4-Nuclear design safet