The time dependent thermal behavior is analyzed with FLUENT for the fluid as well as the solid region (concrete component) in case of a 100 MW n-heptane fire. To study the effect of parameters in mathematical-physical models several sensitivity studies were carried out to investigate the effect on the fluid flow as well as on the component. The influence of soot was additionally considered. FDS simulations as well as empirical calculations considering underlying assumptions are additionally used to examine the plausibility of results from the FLUENT simulations. This is an appropriate method if no experimental results are available. Recommendations are given for choosing parameters in mathematical-physical models e.g. radiation models. The results of the CFD investigations show that considering the influence of soot provides maximum temperatures which were 200 K lower than without soot.
Nach DIN 18230-1 erfolgt in Deutschland die Brandlastberechnungen unter Anwendung
des Abbrandfaktors m. Der Abbrandfaktor m ist ein dimensionsloser Beiwert mit dem die
Brandlast aus einem Stoff oder Stoffgemisch bewertet wird. Neben dem Brandverhalten
des Stoffes berücksichtigt er das Temperatur-Zeit-Verhalten im Bauteil. Die einzige
Prüfapparatur, mit der Abbrandfaktoren bestimmt wurden, ist jedoch nicht mehr existent.
Ein Wiederaufbau der abgeschafften Prüfapparatur wurde wegen apparateabhängigen
Messunsicherheiten und auch aus wirtschaftlichen Gründen als nicht sinnvoll angesehen.
Die Erarbeitung eines äquivalenten Verfahrens zum Abbrandfaktor m unter Verwendung
der Verbrennungseffektivität und unter Berücksichtigung der Bauteilerwärmung war daher
Gegenstand dieses Forschungsvorhabens. Ziel war es für Brandlastberechnungen
nach DIN 18230-1 die Verbrennungseffektivität anzuwenden. Die Verbrennungseffektivität
beschreibet jedoch nur den verringerten Energieumsatz von Stoffen im Brandraum.
Es wurde daher ein Verfahren erarbeitet, dass ergänzend zur Brandlastberechnung unter
Anwendung der Verbrennungseffektivität das Temperatur-Zeit-Verhalten in einem
brandbelastenden Bauteil berücksichtigt.
Mittels Propan-Brennern mit einer Brennerleistung 750 kW wurden in einer U-Bahn-Station Heißgasversuche durchgeführt und die zeitlichen und örtlichen Verteilungen der physikalischen Größen (Stoffkonzentration, Temperatur und Rauchgasdichte) erfasst. Laborversuche sowie die Feldversuche sind Validierungsgrundlage für ANSYS CFX und FDS und die später im Rechenmodell der U-Bahn-Station zur Untersuchung der Grundströmung und Rauchausbreitung verwendeten mathematisch-physikalischen Modelle. Der Aufsatz stellt das Projekt ORPHEUS vor und diskutiert die ersten Ergebnisse.
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).