Simo Hostikka

Probabilistic simulation of fire scenarios

A risk analysis tool is developed for computation of the distributions of fire model output variables. The tool, called Probabilistic Fire Simulator (PFS), combines Monte Carlo simulation and CFAST, a two-zone fire model. In this work, the tool is used to estimate the failure probability of redundant cables in a cable tunnel fire, and the failure and smoke filling probabilities in an electronics room during an electronics cabinet fire. Sensitivity of the output variables to the input variables is calculated in terms of the rank order correlations. The use of the rank order correlations allows the user to identify both modelling parameters and actual facility properties that have the most influence on the results. Various steps of the simulation process, i.e. data collection, generation of the input distributions, modelling assumptions, definition of the output variables and the actual simulation, are described.

Numerical Modeling of Pool Fires Using LES and Finite Volume Method for Radiation

The thermal environment in small and moderate-scale pool flames is studied by Large Eddy Simulation and the Finite Volume Method for radiative transport. The spectral dependence of the local absorption coefficient is represented using a simple wide band model. The predicted radiative heat fluxes from methane/natural gas flames as well as methanol pool burning rates and flame temperatures are compared with measurements. The model can qualitatively predict the pool size dependence of the burning rate, but the accuracy of the radiation predictions is strongly affected by even small errors in prediction of the gas phase temperature.

Improved Radiation And Combustion Routines For A Large Eddy Simulation Fire Model

Improvements have been made to the combustion and radiation routines of a large eddy simulation fire model maintained by the National Institute of Standards and Technology. The combustion is based on a single transport equation for the mixture fraction with state relations that reflect the basic stoichiometry of the reaction. The radiation transport equation is solved using the Finite Volume Method, usually with the gray gas assumption for large scale simulations for which soot is the dominant emitter and absorber. To make the model work for practical fire protection engineering problems, some approximations were made within the new algorithms. These approximations will be discussed and sample calculations presented.

Computational fluid dynamics modelling of fire

An overview of a methodology for simulating fires and other thermally-driven, low-speed flows is presented. The model employs a number of simplifications of the governing equations that allow for relatively fast simulations of practical fire scenarios. The hydrodynamic model consists of the low Mach number large-eddy simulation subgrid closure with either a constant or dynamic coefficient eddy diffusivity. Combustion is typically treated as a mixing-controlled, single-step reaction of fuel and oxygen. The radiation transport equation is written in terms of a spectrally-averaged grey gas. Applications of the model include the design of fire protection systems in buildings and the reconstruction of actual fires.

FDS+Evac: An Agent Based Fire Evacuation Model

In this paper, an evacuation simulation method is presented, which is embedded in a CFD based fire modelling programme. The evacuation programme allows the modelling of high crowd density situations and the interaction between evacuation simulations and state-of-the-art fire simulations. The evacuation process is modelled as a quasi-2D system, where autonomous agents simulating the escaping humans are moving according to equations of motion and decision making processes. The space and time, where the agents are moving, is taken to be continuous, but the building geometry is discretized using fine meshes. The model follows each agent individually and each agent has its own personal properties, like mass, walking velocity, familiar doors, etc. The fire and evacuation calculations interact via the smoke and gas concentrations. A reaction function model is used to select the exit routes. The model is compared to other evacuation simulation models using some test simulations.

Estimation of Pyrolysis Model Parameters for Solid Materials Using Thermogravimetric Data

Determination of the material parameters is one of the key challenges of numerical fire simulation attempting to predict, rather than prescribe the heat release rate. In this work, we use common fire simulation software and genetic algorithms to estimate the kinetic reaction parameters for wood components, birch wood, PVC and black PMMA. Parameters are estimated by modelling thermogravimetric experiments and minimizing the error between the experimental and numerical results. The implementation and choice of the parameters for the genetic algorithm as well as the scheme to describe wood pyrolysis are discussed.

CFD Fire Simulation Using Mixture Fraction Combustion and Finite Volume Radiative Heat Transfer

A computational fluid dynamics (CFD) model of fire and smoke transport is described. Combustion is represented by means of a single conserved scalar known as the mixture fraction. Radiation transport is approximated in the gray gas limit. The algorithms have been incorporated in the Fire Dynamics Simulator (FDS), a computer program maintained by the National Institute of Standards and Technology. Sample calculations are presented demonstrating the performance of the new algorithms, especially as compared to earlier versions of the model.

A Proposal for the Goals and New Techniques of Modelling Pedestrian Evacuation in Fires

In this article, we propose the goals for evacuation simulations in the context of the fire safety engineering. It is proposed that the safety of a building design should be measured using F-N plots that are based on the fire statistics. A new evacuation code is developed that allows the modelling of ‘panic’ situations and interaction between evacuation simulation and the state-of-the-art fire simulation. The major features of the new code are described and first preliminary results are shown. The method presented was found to run satisfactorily, and fast enough for practical purposes. When the results were compared against the results obtained using Simulex and buildingExodus codes, a good agreement was found in two of the three cases but for a case with congested corridor considerable differences occurred.

Modeling Evacuees’ Exit Selection with Best Response Dynamics

We present a model for occupants’ exit selection in emergency evacuations. The model is based on the game theoretic concept of best response dynamics, where each player updates his strategy periodically according to other players’ strategies. A fixed point of the system of all players’ best response functions defines a Nash equilibrium of the game. In the model the players are the occupants and the strategies are the possible target exits. We present a mathematical formulation for the model and analyze its properties with simple test simulations.

Generalized direct method for pyrolysis kinetic parameter estimation and comparison to existing methods

Solid-phase pyrolysis is often modelled using the Arrhenius degradation equation with three unknown parameters: reaction order, activation energy and pre-exponential factor. Since the parameters are model dependent and not directly measurable, several estimation methods have been developed over the years for extracting them from the experimental small-scale data. Lately, the most commonly used methods have been based on optimization and curve fitting. These methods are very efficient for complex problems with multiple reactions but may require significant computational time. Direct (analytic) methods are simpler and faster but often have more restrictions and limited accuracy. This article presents a new, generalized direct method and its performance evaluated along with other commonly used estimation methods. The real usability of the methods is tested also in the presence of small noise.