# What is a mixture fraction

Title:Modeling of fire scenarios in buildingsOther Titles:Modeling Fire Scenarios in BuildingsLanguage:GermanAuthors:Knaust, ChristianQualification level:DoctoralAdvisor:Schneider, UlrichAssisting Advisor:Krause, UlrichIssue Date:2009Number of Pages:169Qualification level:DoctoralAbstract:
As part of European harmonization, new design standards (Eurocodes) have been developed in recent years. In addition to the classic methods such as tables and simplified calculation methods, general engineering methods are also permitted in the Eurocodes for creating fire protection concepts. The general engineering methods include the modeling and calculation of fire scenarios using numerical methods CFD (Computational Fluid Dynamics).
Difficulties exist in the control and evaluation of the results produced with CFD programs, which have found their way into technical documents in the context of the application of engineering methods in submitted fire protection concepts.
To predict the course of the fire in a residential building, analytical calculation methods, zone models and field models were used in the present work. The results are compared.
In order to solve the balance equations of a CFD model, the CFD program FDS with mixture fraction model and the CFD program FLUENT both with the one-step reaction model (ERM) and with the volumetric source term model (VQM) were used.
The modeling of the combustion of polyurethane was carried out in FDS by specifying the heat release rate and the stoichiometry. In the VQM, the heat release rate had to be specified to model the combustion and the substance release rate for the flue gas, taking the stoichiometry into account. The starting parameter for the ERM is the pyrolysis gas mass flow of polyurethane to be specified.
The ERM solves the transport equations for polyurethane, H2O, N2, O2, CO2, CO and C (soot) and calculates the calorific value and thus the heat released as a result of the reaction from the standard enthalpies of formation of all components involved in the reaction. The transport equations for air and flue gas are solved in the VQM. FDS solves the transport equation for a mixture fraction.
The specific heat capacity, absorption coefficient and calorific value required to model the course of the fire were determined by measurement if these were not known from the literature.
In all CFD models, the radiation transfer equation is solved and the absorption coefficient of soot is taken into account.
In addition, the course of the fire in the same residential building was investigated by solving the balance equations of a zone model with the programs CFAST and MRFC.
Results from analytical calculation methods (plume calculations), which were regularly available as a basis for planning in the past, were used for plausibility checks in this work.
The calculation results from the investigations carried out were compared with measurement results from the National Institute for Standards and Technology from a fire test carried out in the same residential building.

Within the European Harmonization new European standards (Eurocodes) have been developed in recent years.
For the conception of fire safety designs, classical methods like tables and simplified arithmetic techniques as well as general engineering techniques are allowed by the Eurocodes.
The modeling and calculation fire scenarios with numerical methods CFD- (Computational Fluid Dynamics) is part of the general engineering methods.
There were difficulties in checking and evaluating of CFD results which are used as technical documents for fire safety designs. For the prediction of the fire development in a building analytical engineering techniques, zone models and CFD models are used in the present work and compared.
To solve the accounting 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 (ERM) as well as with the volumetric source term model (VQM) are used . The combustion of polyurethane is modeled in FDS by specifying the heat release rate and the stoichiometry. For the combustion in VQM the heat release rate and the smoke release were specified with respect to the stoichiometry. Input parameter for the ERM is the pyrolysis mass flow. In ERM 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 components. In VQM the transport equation is solved for air and smoke. FDS solves the transport equation for the mixture fraction. For modeling the fire development the required material characteristics like specific heat capacity, absorption coefficient and heat of combustion were measured, where no literature data were available.
In all CFD models the radiative transfer equation is solved and the absorption coefficient of soot is considered. In addition, the fire development was solved with zone models by the program CFAST and MRFC. Results from analytical engineering techniques (plume calculations) which were planning principles in the past, were used as plausibility checks within the present work.
The calculation results from the investigations were compared to measurements performed by the National Institute for Standards and Technology in the same building.
Keywords:Numerical flow simulation; CFD; Zone model; Plume model; FLUENT; FDS; Smoke; Fire progress simulation; Combustion; soot
computational fluid dynamics; CFD; zone model; plumemodel; FLUENT; FDS; smoke; simulation of fire development; combustion; soot
URI:https://resolver.obvsg.at/urn:nbn:at:at-ubtuw:1-23443
http://hdl.handle.net/20.500.12708/14732Library ID:AC05039827Organization:E206 - Institute for Building Materials, Building Physics and Fire ProtectionPublication Type:Thesis
University thesisAppears in Collections:Thesis