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Beim Ansprechen von Sicherheitseinrichtungen werden unter Druck stehende Gase als Freistrahl freigesetzt. Die Abmessung des daraus resultierenden Gefährdungsbereiches ergibt sich durch den Vergleich mit einem Grenzwert, wie z. B. der unteren Zündgrenze.
Die Berechnung des Freistrahles wird im Allgemeinen durch empirische oder Integralmodelle durchgeführt. Hierbei wird jedoch angenommen, dass die Freisetzungsdauer ausreichend lang ist, damit der Freistrahl einen stationären Zustand erreicht. Ist die Freisetzungsdauer durch ein kurzes Ansprechen der Druckentlastungseinrichtung oder einer geringen zur Verfügung stehenden Gasmenge nur kurz, stellt sich die Frage, inwiefern der resultierende Freistrahl die durch die stationäre Annahme in den Modellen berechneten Längen überhaupt noch erreicht, bzw. ab welcher Freisetzungsdauer von einem stationären Strahl ausgegangen werden kann.
Um diese Fragestellung zu untersuchen wurden CFD Berechnungen durchgeführt. Variiert wurden hierbei die Gasdichte (unterschiedliche Stoffe), die Austrittsgeschwindigkeit, der Austrittsdurchmesser und die Freisetzungsdauer. Basierend auf diesen Untersuchungen wird diskutiert, ob eine Korrelation zwischen den Austrittsparametern und der Dauer bis zum Erreichen der Stationarität abgeleitet werden kann. Diese Korrelation ermöglicht dann aus den Ergebnissen der stationären Freistrahlmodelle, Aussagen für sehr kurze Freisetzungszeiträume zu treffen.
Digital image processing for the advanced characterization and simulation of experimental fire tests
(2025)
The fire engulfment of storage tanks of hazardous materials is among the most critical scenarios in hazard assessment of industrial value chains. Laboratory and full-scale experimental trials are used to test the tank performance and integrity in such scenarios. However, strong uncertainties usually affect the actual fire load experienced by the tank, in particular when large-scale experiments are carried out in open test fields. Such uncertainties arise from several factors difficult to control during experimental tests, such as the atmospheric conditions as there is the influence of wind drifts influencing the actual fire engulfment, the flame temperature and the flame dynamic distribution around the target. Consequently, verifying the concordance of an experimental test with standard test criteria and defining accurate boundary conditions in correlated model simulations is challenging. In this study, the development of a novel method for the analysis of fire conditions based on image processing is presented. The approach allows identifying the flame coverage on the target surface during the test and provides an accurate map of the flame distribution on the equipment over time. The approach is tested using experimental data from a full-scale fire test campaign carried out on liquid hydrogen cryogenic tanks. The results prove to be accurate in replicating the experimental temperatures measured on the outer tank shell during the test. The proposed methodology can be used to better understand the results of experimental fire tests and to characterize realistic fire scenarios, also supporting the definition of fire test requirements. Moreover, the approach produces results that can be implemented as advanced space-time-varying boundary conditions in simulation models, improving their accuracy in reproducing real cases.
In einer Reihe von Experimenten wurden die möglichen Folgen der Freisetzung von verflüssigtem Wasserstoff (LH2) auf, bzw. unter Wasser untersucht. Die Experimente zielten darauf ab, eine unbeabsichtigte Freisetzung von LH2 (z.B. durch Schlauchabriss) insbesondere bei der Betankung eines Schiffes zu simulieren. Für verflüssigtes Erdgas (LNG) wurden dabei sog. RPT’s (rapid phase transition) nachgewiesen, bei denen die spontane Verdampfung relevante Druckwellen erzeugt. Es kann nicht ausgeschlossen werden, dass RPTs auch im Falle von LH2 möglich sind. Die Versuche wurden auf dem Testareal Wasserstoffsicherheit auf dem Testgelände Technische Sicherheit der Bundesanstalt für Materialforschung und -prüfung (BAM-TTS) in Horstwalde, im Rahmen einer Forschungskooperation zwischen der BAM und Gexcon im Rahmen des SH2IFT-Programms durchgeführt. Die LH2-Freisetzungen erfolgten direkt aus einem LH2-Tanklastwagen über eine lange, flexible, vakuumisolierte Transferleitung. Während die Freisetzung oberhalb und unterhalb der Wasseroberfläche jeweils vertikal orientiert war, wurde bei der Unterwasserfreisetzung zusätzlich eine horizontale Ausströmung, parallel zur Wasseroberfläche realisiert. Zur Bestimmung des Massenstromes, wurde ein Wägesystem unter dem Tankwagen eingesetzt. Spezielle Drucksensoren wurden verwendet, um die durch die Freisetzungsvorgänge erzeugten Stoßwellen sowohl im Wasser als auch in der Luft zu messen. Die Gaskonzentrationen über dem Wasserbecken wurden an verschiedenen Positionen gemessen. Hochgeschwindigkeits-, Infrarot- (IR) und normale Kameras wurden eingesetzt, um die Phänomenologie der Freisetzung aufzuzeichnen und das Verhalten der Gaswolke im Zeitverlauf zu verfolgen. Neben den fest installierten Systemen an Land, kamen auch Unterwasserkameras sowie eine Drohne mit Normal- und IR-Kameras zum Einsatz.
Zwei Wetterstationen wurden zur Messung von Windgeschwindigkeit, Windrichtung, Temperatur und Luftfeuchtigkeit während aller durchgeführten Tests eingesetzt. Des Weiteren kamen Bolometer zur Wärmestrahlungsmessung zum Einsatz. Zwar führten die Freisetzung zu einer hochturbulenten LH2/Wasser Mischzone, jedoch zu keinen nennenswerten Überdrücken durch RPT. Im Gegensatz dazu wurde unerwartet, aber reproduzierbar, eine Zündung der Gaswolke in freier Luft in einiger Entfernung von den Instrumenten und dem Ort der Freisetzung beobachtet. Die daraus resultierenden Gaswolkenexplosionen führten zu relevanten Überdrücken und zur Wärmeabstrahlung in die Umgebung.
For hazard assessment purposes, the dispersion of gases in complex urban areas is often a scenario to be considered. However, predicting the dispersion of heavy gases is still a challenge. In Germany, the VDI Guideline 3783, Part 1 and 2 is widely used for gas dispersion modelling. Whilst Part 1 uses a gauss model for calculating the dispersion of light or neutrally buoyant gases, Part 2 uses wind tunnel experiments to evaluate the heavier-than-air gas dispersion in generic built up areas. In practice, with this guideline, it is often not possible to adequately represent the existing obstacle configuration. To overcome this limitation, computational fluid dynamics (CFD) methods could be used. Whilst CFD models can represent obstacles in the dispersion area correctly, actual publications show that there is still further research needed to simulate the atmospheric flow and the heavy gas dispersion. This paper presents a modified k-ε-turbulence model that was developed in OpenFOAM v5.0 (England, London, The OpenFOAM Foundation Ltd Incorporated) to enhance the simulation of the atmospheric wind field and the heavy gas dispersion in built-up areas. Wind tunnel measurements for the dispersion of neutrally buoyant and heavy gases in built-up environments were used to evaluate the model. As a result, requirements for the simulation of the gas dispersion under atmospheric conditions have been identified and the model showed an overall good performance in predicting the experimental values.
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.
In the presented work, the potential of fiber-optic distributed acoustic sensing (DAS) for detection of small gas pipeline leaks (<1%) is investigated. Helical wrapping of the sensing fiber directly around the pipeline is used to increase the system sensitivity for detection of weak leak-induced vibrations. DAS measurements are supplemented with reference accelerometer data to facilitate analysis and interpretation of recorded vibration signals. The results reveal that a DAS system using direct fiber application approach is capable of detecting pipeline natural vibrations excited by the broadband noise generated by the leaking medium. In the performed experiment, pipeline vibration modes with acceleration magnitudes down to single ug were detected. Simple leak detection approach based on spectral integration of time-averaged DAS signals in frequency domain was proposed. Potential benefits and limitations of the presented monitoring Approach were discussed with respect to its practical applicability. We demonstrated that the approached is potentially capable of detection and localization of gas pipeline leaks with leak rates down to 0.1% of the pipeline flow volume and might be of interest for monitoring of short- and medium-length gas pipelines.
For hazard Assessment purposes we need models to predict what would happen in case of an accident.
The challenge in hazard assessment consists not only in choosing the right model, but beforehand to define the correct scenario. For hydrogen applications the validity of the models has to be checked, as well as the kind of scenario to assume for accidental releases of hydrogen. Open questions on these topics will be adressed and the newly founded competecne centre for Hydrogen Saftey at BAM will be presented.
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.
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).
