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Knowledge gaps and recent progress

Simulations of hydrogen dispersion using the CFD methodology have increasingly grown in number during the last 10 years and are expected to grow even more in the near future. Prediction of the time and space distribution of the flammable hydrogen clouds evolving after accidental hydrogen leaks of various types in widely different environments is the main output necessary for subsequent risk assessment estimation of the various hydrogen applications. In this process, simulations have been performed using different CFD codes (commercial or research tools) and different modelling strategies (turbulence models,source treatment, discretization options, etc.).

To ensure the quality and trust in industrial CFD applications best practice guidelines have been developed in the past either of a general character like ERCOFTAC 2000 (CaseyM:2000) or more related to particular applications like HSL 2003 Invalid BibTex Entry!. No CFD guidelines specific to hydrogen dispersion applications have been proposed.

Taking the above in consideration a significant effort has been concentrated within the European Network of Excellence HYSAFE with aim to perform a systematic evaluation of the various CFD approaches (codes and models) in predicting hydrogen dispersion, based on a series of benchmark exercise problems, using existing and new state of the art experimental data.

The results of the first such hydrogen dispersion benchmark exercise (SBEP-V1) were reported by Gallego et al. (GallegoE:2005). The experiment simulated was that of Shebeko et al. (ShebekoYN:1988), who investigated the dispersion of hydrogen in an hermetically closed cylinder (20m3 volume) by measuring axial hydrogen concentrations (6 locations) at times from 2 to 250 minutes following an initial 60 s vertical subsonic jet release at a rate of 4.5 l/s from a 10mm nozzle. Large variations in predictions were monitored during this first benchmark (as expected), which could be attributed to variations in turbulence models, boundary conditions as well as discretization options.

The aim of the second hydrogen dispersion benchmark exercise (SBEP-V3) was three fold a) to further investigate on the ability of the models to predict the long term stratification/diffusion problem in a confined space, b) to test the ability of models to predict the concentration field of a vertical subsonic hydrogen jet release and c) to attempt to minimize or justify large variations between model predictions. Newly performed hydrogen dispersion experiments by INERIS at their gallery facility (garage like enclosure with dimensions 7.2x3.8x2.9 m) were used for this benchmark. The release was vertical upwards at a rate of 1 g/s from an orifice of 20 mm diameter and lasted for a period of 240 s. The total simulation time was 5400 s. The benchmark took part in two phases a blind pre-test phase and a post–test one.

Further benchmarks focus on testing the ability to predict free choked hydrogen flows, obstacle effects on hydrogen dispersion within confined spaces as well as hydrogen dispersion from LH2 releases.

Computer fluid dynamics simulations of chocked flows are difficult to tackle due to the presence of the shock waves. The simulations may require, for commercial solvers, resolving the Mach cone and the shock wave patterns to some degree of details. Since the extent of the flammability envelopes resulting from chocked releases from apertures of about 1 cm may reach 10 to 100 m depending on the storage pressure, length scales of up to five orders of magnitude must be covered by the mesh. In addition, convergence will usually be problematic.

The difficulties faced by direct CFD simulations of chocked releases may be alleviated by using effective diameter approaches/ The applicability of effective diameter approaches to horizontal releases of hydrogen should be investigated further, particularly for the large hydrogen releases resulting from high pressure flows, where the effects of buoyancy on the shape of the release remain an issue.

For choked hydrogen releases the fact that the molar concentration is proportional to the inverse distance has been observed experimentally, but given that significantly different proportionality constants have been reported, a systematic investigation both experimental and computational is still required to cover a wider range of storage pressures and orifice diameters.

Regarding obstacle effects on hydrogen dispersion it should be mentioned that steady-state flow rates can lead to unsteady behaviour of the dispersion pattern in some cases, particularly when impingement flows or external flows (over a surface) are considered, due to significant vortex shedding. Such situations may require a statistical definition of the constant (flammable) concentration envelope, based on the probability distribution of finding a given concentration of hydrogen at a specific location at a given time.

Finally as far as LH2 release and dispersion are concerned it seems that more experimental information is needed to trigger further physical understanding and model development/improvement. From the past experience it seems that these proposed tests should focus on better control over the experimental conditiosn (less uncertainty).


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Hydrogen Dispersion

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Page last modified on December 04, 2008, at 06:34 PM