Abstract
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) 
Fingerprint
Keywords
 fire simulation
 Monte Carlo simulation
 probabilistic risk assessment
 thermal radiation
 verification
 validation
Cite this
}
Development of fire simulation models for radiative heat transfer and probabilistic risk assessment : Dissertation. / Hostikka, Simo.
Espoo : VTT Technical Research Centre of Finland, 2008. 185 p.Research output: Thesis › Dissertation › Collection of Articles
TY  THES
T1  Development of fire simulation models for radiative heat transfer and probabilistic risk assessment
T2  Dissertation
AU  Hostikka, Simo
PY  2008
Y1  2008
N2  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.
AB  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.
KW  fire simulation
KW  Monte Carlo simulation
KW  probabilistic risk assessment
KW  thermal radiation
KW  verification
KW  validation
M3  Dissertation
SN  9789513870997
T3  VTT Publications
PB  VTT Technical Research Centre of Finland
CY  Espoo
ER 