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Pohjoismaisen puutavaran pyrolyysi ja halkeilu ulkoisen lämpöaltistuksen aikana
No full textTimber as a construction material has been gaining renewed interest in recent times. However, wood is inherently associated with an increased risk to fire safety. To address the fire risks of wood construction, regulators impose prescriptive building codes that define the limits for use of timber in construction. Performance-based fire safety design is the only method to realize timber construction projects that go beyond the boundaries set by prescriptive building codes. Computational fluid dynamics (CFD) -based fire simulation is a common tool in performancebased fire safety engineering. In realistic CFD fire simulations involving wooden surfaces, wood is defined by a material model that predicts char front progress and release of combustible volatiles, thus interacting with flaming in the gaseous phase. This thesis proposes material models for spruce and pine woods for use in performance-based fire safety design. The thesis also provides an experimental dataset of direct observations of cracking on surface of charring timber to help in implementation of cracking effects to material models in later research.
During this work, two independent material models were developed for the studied wood species of spruce and pine: one that assumes wood decomposing through pyrolysis as a single component (single reaction), and another that observes pyrolysis as a sum of individual decomposition reactions of wood primary components: hemicellulose, cellulose and lignin (parallel reactions). Material properties and decomposition kinetics of wood were studied by use of various microscale experimental methods, such as thermogravimetry and differential scanning calorimetry. Cone calorimeter experiments were used in estimation of any remaining model parameters, and in material model validation. The single reaction model arises as the preferable option, offering a similar quality of fit to the experimental data as the more complex parallel reactions model, but without the associated increased model uncertainty. Validation using large-scale experiments revealed that the model is highly sensitive to the surrounding oxygen concentration of the surrounding atmosphere, and inclusion of the oxidation reaction is important for correct fire spread prediction.
Formation of cracks on charring spruce, pine and birch woods was studied in real time using an infrared camera. The experiments revealed a linear relationship between heat flux and the inverse of the square root of crack formation time, analogously to the time to ignition in ignition model for thermally thick solids. Contrary to expectations, the number of cracks did not grow consistently as a function of heat flux. The analytical formulation of a previously existing cracking model predicts the number of cracks in correct order of magnitude, but not accurately.Kiinnostus puuhun rakennusmateriaalina on ollut kasvussa viime aikoina. Puumateriaaliin kuitenkin liittyy olennaisesti kasvanut paloturvallisuusriski. Puurakentamisen paloriskien hallitsemiseksi on asetettu erilaisia taulukoituja rakennusmääräyksiä, jotka asettavat rajoituksia puun käytölle rakentamisessa. Toiminnallinen palosuunnittelu on ainoa keino toteuttaa puurakennuksia, jotka eivät mukaudu taulukoitujen rakennusmääräysten rajoituksiin. Laskennalliseen virtausmekaniikkaan perustuva tulipalomallinnus yleinen työkalu toiminnallisessa palosuunnittelussa. Realistisissa puuta palavana pintana sisältävissä virtausmekaanisissa palomallinnuksissa puun hiiltyminen ja palavien kaasujen vapauttaminen määritellään materiaalimallilla. Näin ollen puu on vuorovaikutuksessa kaasufaasissa tapahtuvan liekehtimisen kanssa. Tässä väitöskirjassa on kehitetty materiaalimallit kuusi-, ja mäntypuutavaralle käytettäväksi toiminnallisessa palosuunnittelussa. Väitöskirjassa on myös esitetty suorat kokeelliset havainnot hiiltyvän puun pinnan halkeilusta auttamaan halkeilun vaikutuksen huomioimisessa materiaalimallinnuksessa myöhemmässä tutkimuksessa.
Tämän väitöstyön aikana kehitettiin kaksi erillistä materiaalimallia tutkituista puulajeista molemmille, sekä kuuselle että männylle. Näistä ensimmäinen on yhden reaktion malli, joka olettaa puun yhtenäisesti pyrolysoituvaksi materiaalikomponentiksi, ja toinen rinnakkaisten reaktioiden malli, joka tarkkailee kunkin puun pääkomponentin (hemiselluloosa, selluloosa, ligniini) pyrolyysiä erillisenä reaktiona. Materiaaliominaisuudet ja kineettiset parametrit selvitettiin käyttäen eri mikroskaalan koemenetelmiä, kuten termogravimetriaa ja differentiaalista pyyhkäisykalorimetriaa. Kartiokalorimetrikokeita käytettiin loppujen malliparametrien arvioinnissa ja materiaalimallin validoinnissa. Yhden reaktion malli osoittautui suositeltavaksi vaihtoehdoksi, sen tarjotessa yhtäläisen sopivuuden koetuloksiin kuin monimutkaisempi rinnakkaisten reaktioiden malli, mutta ilman monimutkaisemman mallin suurempaa epävarmuutta. Validointi käyttäen suuren mittakaavan palotestejä paljasti, että malli on huomattavan herkkä ympäröivälle happipitoisuudelle, ja hapettumisen huomioiminen on tärkeää palon leviämisen oikeellisen ennustamisen kannalta.
Halkeamien muodostumista hiiltyvän kuusen, männyn ja koivun pinnoilla tutkittiin reaaliaikaisesti infrapunakameralla. Kokeet paljastivat lineaarisen riippuvuuden ulkoisen lämpövirrantiheyden ja halkeamien muodostumisajan neliöjuuren käänteisluvun välillä. Termisesti paksujen kiinteiden aineiden syttymismalli ennustaa vastaavan riippuvuuden syttymisajankohdalle. Vastoin odotuksia halkeamien lukumäärä ei johdonmukaisesti kasvanut ulkoisen lämpövirrantiheyden funktiona. Aiemmin olemassa olleen halkeilumallin analyyttinen muotoilu ennusti halkeamien määrän oikeassa suuruusluokassa kykenemättä kuitenkaan tarkkaan ennusteeseen
Full-Scale Fire Modeling and Simulation
No full textAbstract Full-scale fire modelling using Computational Fluid Dynamics is used extensively to investigate the fire consequences and the performance of building materials and fire protection systems in domestic and industrial environments. The chapter summarizes the current modelling approaches for the key physical phenomena like turbulent flow, combustion, thermal radiation, and condensed phase heating and degradation. The emphasis is put on the model features that are needed for capturing the coupled nature of the fire growth simulations. Reported simulations of vertical flame spread, burning electrical cables, and fire retardant polymers are briefly reviewed. © 2024 selection and editorial matter, Alexander B. Morgan and Charles A. Wilkie. All rights reserved.Peer reviewe
Development of fire simulation models for radiative heat transfer and probabilistic risk assessment:Dissertation
No full textAn 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 state-of-the-art 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 CFD-based fire simulation, specialized algorithms are needed to allow the use of CFD in Monte Carlo simulation. By the use of the Two-Model 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 gas-soot mixture are computed using a RadCal narrow-band model and spectrally averaged. The three-dimensional field of radiation intensity is solved using a finite volume method for radiation. By the use of an explicit marching scheme, efficient use of look-up 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 Mie-theory and averaged locally over the spectrum and presumed droplet size distribution. To simplify the scattering computations, the single-droplet 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
Development of fire simulation models for radiative heat transfer and probabilistic risk assessment:Dissertation
No full textAn 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 state-of-the-art 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 CFD-based fire simulation, specialized algorithms are needed to allow the use of CFD in Monte Carlo simulation. By the use of the Two-Model 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 gas-soot mixture are computed using a RadCal narrow-band model and spectrally averaged. The three-dimensional field of radiation intensity is solved using a finite volume method for radiation. By the use of an explicit marching scheme, efficient use of look-up 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 Mie-theory and averaged locally over the spectrum and presumed droplet size distribution. To simplify the scattering computations, the single-droplet 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
Tulipalon simuloinnissa käytettävän säteilylämmönsiirtomallin ja riskianalyysimenetelmän kehittäminen
No full textAn 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 state-of-the-art 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 CFD-based fire simulation, specialized algorithms are needed to allow the use of CFD in Monte Carlo simulation. By the use of the Two-Model 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 gas-soot mixture are computed using a RadCal narrow-band model and spectrally averaged. The three-dimensional field of radiation intensity is solved using a finite volume method for radiation. By the use of an explicit marching scheme, efficient use of look-up 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 Mie-theory and averaged locally over the spectrum and presumed droplet size distribution. To simplify the scattering computations, the single-droplet 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.Paloriskien arvioinnissa on olennaista palon seurausten ja leviämismahdollisuuksien analysointi. Tässä työssä on kehitetty tulipalojen numeerisen simuloinnin malleja ja työkaluja. Työn päätavoitteita ovat olleet palosimuloinnin parhaimpien laskentamallien hyödyntäminen todennäköisyyspohjaisessa paloriskien arvioinnissa sekä laskennallisesti tehokkaan säteilylämmönsiirron ratkaisijan kehittäminen Fire Dynamics Simulator -ohjelmaan.
Työn ensimmäisessä osassa on kehitetty insinöörikäyttöön soveltuva, Probabilistic Fire Simulator (PFS) -niminen työkalu paloriskien arviointiin. PFS-työkalulla tulipaloa voidaan tutkia Monte Carlo -menetelmällä, jossa simulointeja toistetaan useita kertoja satunnaisilla syöteparametrien arvoilla, jolloin yksittäisen numeroarvon sijaan tuloksena saadaan tulosten jakauma. Käytännön sovelluksissa tarvitaan numeeriseen virtauslaskentaan perustuvia simulointimenetelmiä, koska simuloitavat tilavuudet ovat suuria ja monimutkaisia ja koska niissä pitää pystyä simuloimaan palon leviämistä. Monte Carlo -menetelmän toteutuksessa on tällöin käytettävä tehtävään sopivia erikoismenetelmiä, koska virtauslaskenta on laskennallisesti raskasta ja aikaa vievää. Tässä työssä kehitetyn Kahden mallin Monte Carlo -menetelmän avulla laskentaa voidaan nopeuttaa yhdistämällä kahden eritasoisen mallin tulokset. Nopeasti ratkaistavan mutta epätarkan mallin tuottamia tuloksia parannetaan hitaammin ratkaistavan mutta tarkemman mallin avulla. Menetelmää on testattu erilaisilla palomallien yhdistelmillä aina analyyttisistä kaavoista virtauslaskentaan asti.
Työn toisessa osassa on kehitetty säteilylämmönsiirron numeerinen ratkaisija Fire Dynamics Simulator -ohjelmaan. Ratkaisija laskee säteilyn etenemistä palokaasuja, nokea ja nestepisaroita sisältävässä väliaineessa. Palokaasujen ja noen muodostaman seoksen säteilyominaisuudet lasketaan keskiarvoistamalla RadCal-kapeakaistamallin tulokset aallonpituuden yli. Lämpösäteilyn eteneminen ratkaistaan säteilylämmönsiirron kontrollitilavuusmenetelmällä. Säteilyratkaisijan vaatima laskenta-aika saadaan alle 30 %:iin kokonaislaskenta-ajasta käyttämällä eksplisiittistä ratkaisumenetelmää ja tehokkaita taulukkohakuja sekä luopumalla ratkaisun aikatarkkuudesta. Tarkkuutta voidaan tarvittaessa parantaa jakamalla tarkasteltava aallonpituusalue veden ja hiilidioksidin tärkeimpiä absorptiokaistoja vastaaviin osiin sekä tihentämällä diskretointia avaruuskulman ja ajan suhteen. Työssä on kehitetty uusi laskentamalli nestepisaroiden ja säteilyn vuorovaikutukselle. Pisaroiden säteilyominaisuudet lasketaan Mie-teorian avulla ja keskiarvoistetaan sekä spektrin että pisarakokojakauman yli. Yksittäisen nestepisaran sirottaman energian vaihefunktiota approksimoidaan eteenpäin siroavien ja isotrooppisten komponenttien summana. Säteilyratkaisijaa on testattu vertaamalla laskettuja tuloksia analyyttisiin ja kokeellisiin tuloksiin.reviewe
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