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Organisationseinheit der BAM
The number of cars which use a battery for propulsion, like BEV (battery electric vehicle) or PHEV (plug-in hybrid electric vehicle), is increasing. Following, the probability of these car types being involved in car accidents will increase. In a worst-case accident scenario, the installed batteries could go under thermal runaway (TR). A TR could be initiated by various causes, e.g., high temperatures from outside, mechanical damage of the cells/modules/batteries or internal and external short circuits. During such an event, large amounts of heat and toxic and/or flammable gases are released. This poses a great hazard to people and property in the immediate vicinity. The paper describes a large-scale test series on the lithium-ion battery TR consequences of automobile cells and modules up to E = 6.85 kWh. Near field temperatures of ΔT > 800°C in a distance of l = 2 m were measured. During an overcharge test, fragment throwing distances of l > 30 m were detected. Subsequent gas explosions of the released gases were documented. Hydrogen fluoride (HF) was measured in all tests, from cell to module. The highestmeasured concentration was cHF = 76 ppm, which is significantly higher than the 30-min-level for AEGL 2 (acute exposure guideline level 2). Based on the experimentally determined consequences, concrete measures for the accident analysis are subsequently drawn.
Seit langer Zeit ist bekannt, dass ungünstige Lagerungsbedingungen
bei brennbaren Schüttgütern und Stäuben zur
Selbstentzündung führen können. Die meisten Studien zu
diesem Thema beziehen sich allerdings auf Umgebungsbedingungen
(d. h. mit einem Sauerstoff-Volumenanteil von
21 % in der Schüttungsumgebung). In einigen technischen
Anwendungen jedoch wird brennbares Schüttgut oder Staub
unter einer Gasatmosphäre mit reduziertem Sauerstoff-
Volumenanteil gelagert und verarbeitet (Inertisierung). Dabei
ist zunächst nicht klar, bis zu welchem Ausmaß der Sauerstoff-
Volumenanteil reduziert werden muss, um bei einer
vorgegebenen Lagerungstemperatur Selbstentzündung auszuschließen.
Neben dem Sauerstoffvolumenanteil können
auch der Feuchtegehalt der Schüttung sowie die relative Luftfeuchtigkeit
in der Schüttungsumgebung den Vorgang der
Selbstentzündung beeinflussen.
Die hier vorgestellten experimentellen Untersuchungen beschreiben
zum einen den Einfluss des Sauerstoff-Volumenanteils
auf die Selbstentzündungstemperatur (SET) von
Schüttgütern und Stäuben. Es wurde beobachtet, dass die
SET mit abnehmendem Sauerstoff-Volumenanteil in der
Schüttungsumgebung anstieg. Jedoch traten auch bei sehr
kleinem Sauerstoff-Volumenanteil noch signifikante Temperaturerhöhungen
in den untersuchten Proben auf.
Zum zweiten wurde der Einfluss der Feuchte auf die SET
untersucht. Dabei konnte gezeigt werden, dass unterkritisch
gelagerte Schüttungen durch Kondensation von Wasserdampf
oder Zugabe von Wasser auf die Oberfläche der
Schüttung in einen überkritischen Zustand gebracht werden
konnten.
Neben den experimentellen Untersuchungen wurde ein numerisches
Modell entwickelt, mit dessen Hilfe sich der
Selbsterwärmungsprozess bis hin zur Zündung von Schüttgütern
berechnen lässt. Anwendungen findet dieses Modell
für die Lagerung von Schüttgütern, Stäuben, Feststoffen
sowie Abfällen.
In the frame of the European harmonization, new European technical standards (Eurocodes) have been developed in recent years. Classical methods, like tables and simplified analytical procedures, as well as general engineering techniques are allowed by the Eurocodes for the fire protection design. The modeling and calculation of fire scenarios with CFD (Computational Fluid Dynamics) numerical methods is one of the general engineering methods. It is nowadays still difficult to check and evaluate the CFD results for their use as technical documents for fire safety design.
Analytical engineering techniques, zone models and CFD-models have been used and compared in the present work for the prediction of the fire development in a building.
To solve the conservation equation for the CFD-model, the CFD-program FDS, with the mixture fraction model, and the CFD-program FLUENT, with the one step reaction model as well as with the volumetric source term model, have been used.
The combustion of polyurethane is modeled in FDS by specifying the heat release rate and the stoichiometry. For the combustion in volumetric source term model, the heat release rate and the smoke release were specified with respect to the stoichiometry. The input parameter for the one step reaction model is the pyrolysis mass flow.
In the one step reaction model, the transport equations for polyurethane, H₂O, N₂, O₂, CO₂, CO and C (soot) are solved and the heat of combustion is determined from the standard formation enthalpy of all the components. In volumetric source term model, the transport equation is solved for air and smoke. FDS solves the transport equation for the mixture fraction.
To model the fire development, and where no literature data was available, the required material characteristics like specific heat capacity, absorption coefficient and heat of combustion were measured.
In all the investigated CFD-models the heat- and species transport equation has been solved and the absorption coefficient of soot has been considered.
Furthermore, the fire development has also been investigated using zone models with the programs CFAST and MRFC.
Results from analytical engineering techniques (plume calculations), which were design criteria in the past, have been used as plausibility checks for the present work. The calculation results from the investigations were compared to measurements in the same building performed by the National Institute for Standards and Technology (NIST).