Abstract
An essential part of fire risk assessment is the analysis
of fire hazards and fire propagation. In this work,
models and tools for two different aspects of numerical
fire simulation have been developed. The primary
objectives have been firstly to investigate the
possibility of exploiting stateoftheart fire models
within probabilistic fire risk assessments and secondly
to develop a computationally efficient solver of thermal
radiation for the Fire Dynamics Simulator (FDS) code.
In the first part of the work, an engineering tool for
probabilistic fire risk assessment has been developed.
The tool can be used to perform Monte Carlo simulations
of fires and is called the Probabilistic Fire Simulator
(PFS). In Monte Carlo simulation, the simulations are
repeated multiple times, covering the whole range of
variability of the input parameters and thus resulting in
a distribution of results covering what can be expected
in reality. In practical applications, advanced
simulation techniques based on computational fluid
dynamics (CFD) are needed because the simulations cover
large and complicated geometries and must address the
question of fire spreading. Due to the high computational
cost associated with CFDbased fire simulation,
specialized algorithms are needed to allow the use of CFD
in Monte Carlo simulation. By the use of the TwoModel
Monte Carlo (TMMC) technique, developed in this work, the
computational cost can be reduced significantly by
combining the results of two different models. In TMMC,
the results of fast but approximate models are improved
by using the results of more accurate, but
computationally more demanding, models. The developed
technique has been verified and validated by using
different combinations of fire models, ranging from
analytical formulas to CFD.
In the second part of the work, a numerical solver for
thermal radiation has been developed for the Fire
Dynamics Simulator code. The solver can be used to
compute the transfer of thermal radiation in a mixture of
combustion gases, soot particles and liquid droplets. The
radiative properties of the gassoot mixture are computed
using a RadCal narrowband model and spectrally averaged.
The threedimensional field of radiation intensity is
solved using a finite volume method for radiation. By the
use of an explicit marching scheme, efficient use of
lookup tables and relaxation of the temporal accuracy,
the computational cost of the radiation solution is
reduced below 30% of the total CPU time in engineering
applications. If necessary, the accuracy of the solution
can be improved by dividing the infrared spectrum into
discrete bands corresponding to the emission bands of
water and carbon dioxide, and by increasing the number of
angular divisions and the temporal frequency. A new model
has been developed for the absorption and scattering by
liquid droplets. The radiative properties of droplets are
computed using a Mietheory and averaged locally over the
spectrum and presumed droplet size distribution. To
simplify the scattering computations, the singledroplet
phase function is approximated as a sum of forward and
isotropic components. The radiation solver has been
verified by comparing the results against analytical
solutions and validated by comparisons against
experimental data from pool fires and experiments of
radiation attenuation by water sprays at two different
length scales.
Original language  English 

Qualification  Doctor Degree 
Awarding Institution 

Supervisors/Advisors 

Award date  6 Jun 2008 
Place of Publication  Espoo 
Publisher  
Print ISBNs  9789513870997 
Electronic ISBNs  9789513871000 
Publication status  Published  2008 
MoE publication type  G5 Doctoral dissertation (article) 
Keywords
 fire simulation
 Monte Carlo simulation
 probabilistic risk assessment
 thermal radiation
 verification
 validation
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Hostikka, S. (2008). Development of fire simulation models for radiative heat transfer and probabilistic risk assessment: Dissertation. VTT Technical Research Centre of Finland. http://www.vtt.fi/inf/pdf/publications/2008/P683.pdf