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- 3 Gefahrgutumschließungen; Energiespeicher (4)
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Aufgrund gestiegener Rechnerleistung finden heutzutage
numerische Strömungssimulationen aus dem
Bereich der CFD (Computational Fluid Dynamics)
immer mehr Eingang in die Sicherheitstechnik.
Bisherige Berechnungsverfahren zur Schwergasausbreitung
basieren zum Teil auf empirischen
Annahmen und sind in ihrem Anwendungsbereich
eingeschränkt. Die zugrunde liegenden Formeln
und Theorien erlauben außerdem meist nur
eindimensionale Aussagen bezüglich der Konzentrationsverteilung
in der Umgebung. Anhand von
Literaturdaten und eigenen Freifeldversuchen zur
Schwergasausbreitung soll die Leistungsfähigkeit
von CFD-Berechnungen hinsichtlich ihres Kosten/
Nutzen-Verhältnisses sowie im Vergleich zu herkömmlichen
Modellen bewertet werden. Während
die eigenen Freifeldversuche hauptsächlich zur
Validierung der numerischen Simulationen dienen,
können anhand der in der Literatur veröffentlichten
Daten Vergleiche mit z. B. der VDI-Richtlinie 3783,
Blatt 2 durchgeführt werden.
In this work, results of numerical simulations and experimental investigations of the mass flow from evaporating liquid pools are presented. Numerical simulations are based on the boundary layer equations combined with an algebraic turbulence model. The experiments were carried out on open air test sites within flat and very rough topographies, at different temperatures with ethanol and cyclohexane as the evaporating liquids. An experimental investigation of the influence of the topography on the evaporation of a liquid pool is conducted. The results of the simulation are validated against experimental data from the open air experiments. Furthermore, a comparison of the simulation results with empirical prediction models has been made.
Betreiber, deren Betriebsbereiche den erweiterten Pflichten der Störfall-Verordnung unterliegen, müssen einen Sicherheitsbericht anfertigen, in dem Szenarien möglicher Störfälle beschrieben sind. Im Rahmen dieser Störfallauswirkungsbetrachtungen sind auch oftmals Szenarien zu untersuchen, bei denen Flüssigkeiten freigesetzt werden. Der Verdunstungsmassenstrom aus der Flüssigkeitslache ist der maßgebliche Eingangsparameter für die Gasausbreitungsberechnung mit der die Gefährdungsradien ermittelt werden. Für die Berechnung des Verdunstungsmassenstroms werden meist empirische Modelle eingesetzt. Anhand von experimentellen Daten aus eigenen Freilandversuchen werden die Leistungsfähigkeit sowie die Grenzen des Anwendungsbereichs diverser bekannter Modelle untersucht.
A steel pipeline segment of 2.5 m length was subjected to quasi-static four-point bending load in three steps for studying the initial cracking and damage accumulation based on the Acoustic Emission (AE) technique and by the direct current potential drop (DCPD) technique. For the latter, a new post-test analysis method was established.
AE is found more sensitive to crack initiation than DCPD. Formation of mesoscopic and macroscopic cracks as well as their closure and the resulting friction generate weighted peak frequencies below 400 kHz, whereas microscopic cracking produces AE with broad band spectra identifiable by weighted peak frequencies above 400 kHz. Critical states alike the maximum load level and the leak opening were accompanied by peak amplitudes above 85 dBAE. This rather fundamental study provides a data base for possibly developing advanced strategies of detection and alarm systems based on acoustic monitoring of pipelines, or at least, steel structures.
CFD simulations of near-ground gas dispersion depend significantly on the accuracy of the wind field. When simulating wind fields with conventional RANS turbulence models, the velocity and turbulence profiles specified as inlet boundary conditions change rapidly in the approach flow region. As a result, when hazardous materials are released, the extent of hazardous areas is calculated based on an approach flow that differs significantly from the boundary conditions defined. To solve this problem, a turbulence model with consistent boundary conditions was developed to ensure a horizontally homogeneous approach flow. Instead of the logarithmic vertical velocity profile, a power law is used to overcome the problem that with the logarithmic profile, negative velocities would be calculated for heights within the roughness length. With this, the problem that the distance of the wall-adjacent cell midpoint has to be higher than the roughness length is solved, so that a high grid resolution can be ensured even in the near-ground region which is required to simulate
gas dispersion. The evaluation of the developed CFD model using the German guideline VDI 3783/9 and wind tunnel experiments with realistic obstacle configurations showed a good agreement between the calculated and the measured values and the ability to achieve a horizontally homogenous approach flow.
Experiments have been performed to determine the consequences of a storage vessel containing liquified hydrogen (LH2) is engulfed by a fire. The tests were performed at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Germany within a research cooperation between BAM and Gexcon as part of the SH2IFT program. Three tests were performed using double-walled vacuum insulated vessels of 1 m3 volume varying the orientation of the vessel and the effect of the insulation material used (perlite or multi-layer insulation (MLI)). The degree of filling of the vessel was approximately 35 % in each of the tests performed. The fire load was provided by a propane fed burner positioned under the storage vessel and designed to give a homogeneous fire load. In one of the tests a rupture of the storage vessel occurred causing a blast, a fireball and fragments. Apart from measuring these consequences, the conditions in the vessel (e.g. temperatures and pressure) during the heating process were monitored in all three tests. The work described was undertaken as part of the project Safe Hydrogen fuel handling and Use for Efficient Implementation (SH2IFT).
Large-scale experiments have been performed to investigate the possible consequences of realistic amounts of liquified hydrogen (LH2) encountering water. The experiments aimed at simulating an accidental release of LH2 onto water, for instance during the fuelling of a ship. For liquified natural gas (LNG), it has been demonstrated that physical explosions may occur when it is spilled onto water. These phenomena are referred as rapid phase transitions (RPTs). It cannot be excluded that RPTs are also possible in the case of LH2. The tests were performed at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Horstwalde, Germany. The tests were performed in a 10 m x 10 x 1.5 m basin filled with water. LH2 releases of up to about 1 kg/s were established releasing directly from a trailer carrying LH2. The releases occurred from a height of 50 cm above the water surface pointing downwards, 30 cm under the water surface pointing downwards and 30 cm under the water surface pointed along the water surface. All release configurations resulted in a very chaotic LH2-water mixing zone, causing considerable evaporation and resulting in minor over pressures. No RPTs were observed. The main phenomenon to be observed is, however, an ignition of the released gas cloud resulting in significant blast wave overpressures and heat radiation to the surroundings. The ignition occurred in all under-water releases and in about 90 % of the releases above the water surface.
Hazardous substances with a boiling point close to ambient temperatures will evaporate at higher vapour pressures, so that the evaporation takes places in the smooth transition between the evaporation at boiling point and below boiling point, representing the transition between two different physical phenomena. Whilst the evaporation at boiling point is driven by the available heat flux, the evaporation below boiling point is driven by the concentration gradient between the pool surface and the ambient air. Available evaporation models usually focused on the correct description of the mass transfer coefficient for temperatures below boiling point. A formulation of the correct equation for the mass flow is rarely documented. Whilst the mass transfer coefficient formulation is more or less equivalent in most models, the main difference occurs in the formulation of the mass flow equation. In Fact two types of models can be identified: the models with a linear pressure term and the models with a logarithmic pressure term. Whilst the logarithmic formulations result in an infinite mass flow near boiling point, which is not plausible, the linear formulations reach (different) finite values. Due to a lack of published experimental data it was not possible to determine whether the linear approach is conservative, under predicting or more or less accurate close to the boiling point. To evaluate the accuracy of each type of formulation, test series on liquid pools have been carried out at BAM for substances like Water, Ethanol, Cyclohexane, and Acetone. The tests were done under ambient conditions with a heatable, 90 cm diameter pool, so that the vapour pressures investigated ranged from 0 to close to 1 bar. The experimental data showed that neither of the linear nor the logarithmic formulation of the evaporation models is able to predict correctly the mass flow close to the boiling point. The logarithmic approach heavily over predicts the mass flow, while the linear approach is not conservative anymore when the vapour pressure exceeds 0.7 bar.