Fluid Mechanics Aspects of Fire and Smoke Dynamics in Enclosures

This book aims at fulfilling the need for a handbook at undergraduate and starting researcher level on fire and smoke dynamics in enclosures, giving fluid mechanics aspects a central role. Fluid mechanics are essential at the level of combustion, heat transfer and fire suppression, but they are described only cursorily in most of the existing fire safety science literature, including handbooks. The scope of this handbook ranges from the discussion of the basic equations for turbulent flows with combustion, through a discussion on the structure of flames, to fire and smoke plumes and their interaction with enclosure boundaries. Using this knowledge, the fire dynamics and smoke and heat control in enclosures are discussed. Subsequently, a chapter is devoted to the effect of water and the related fluid mechanics aspects. The book concludes with a chapter on CFD (Computational Fluid Dynamics), the increasingly popular calculation method in the field of fire safety science.

Introduction The candle flame The importance of chemistry, heat transfer and fluid mechanics in fires ChemistryHeat transfer Fluid mechanics and turbulence Combustion and fire Fire modelling Turbulent flows with chemical reactionFluid properties – state properties – mixtures Fluid propertiesMass densityViscosity Specific heat Conduction coefficient Diffusion coefficient State properties Pressure TemperatureInternal energyEnthalpy EntropyEquation of state MixturesCombustion Chemical reaction Thermodynamics EnthalpyTemperatureChemical kinetics Transport equations Conservation of mass Momentum equations Conservation of energyConvectionConductionRadiation Transport of species Mixture fraction Bernoulli Hydrostatics Buoyancy Non-dimensional numbers Fluid properties Flow properties Scaling laws Turbulence Reynolds number Reynolds averaging Turbulence modeling Energy cascade Turbulent scales Turbulence modelling Boundary layer flow Internal flows – pressure losses Entrainment Impinging flow Evaporation Pyrolysis Turbulent flames and fire plumesFlammabilityFlammability limits – threshold temperature Addition of gasesFlammability of liquid fuels Premixed flamesLaminar premixed flame structure Laminar burning velocity The effect of turbulence Diffusion flames Laminar diffusion flame structureThe effect of turbulenceJet flamesExtinction of flamesPremixed flames Diffusion flamesFire plumes Free fire plumes Average flame height Temperature evolutionKelvin-Helmholtz instability The effect of wind 1Transition from buoyancy-driven to momentum-driven jets Correlations Interaction with non-combustible walls Interaction with non-combustible ceiling The effect of ventilationReduced oxygen at ambient temperature Oxygen-enriched fire plumes Vitiated conditions Fire whirlsFlame spread Flame spread velocity – a heat balanceOpposed flow flame spread over a thermally thick fuel Opposed flow flame spread over a thermally thin fuelConcurrent flow flame spread over a thermally thick fuelConcurrent flow flame spread over a thermally thin fuel Gas phase phenomena Horizontal surfaceNatural convection Concurrent airflow Counter-current airflow Vertical surfaceInclined surfaceParallel vertical plates configurationCorner configuration Smoke plumesIntroductionAxisymmetric plume Theory and mathematical modelling Model development under the Boussinesq approximation ExperimentsLine plume Description of the configuration Conservation equationsExperimental studiesTransition from line to axisymmetric plume Wall and corner interaction with plumes Detailed example: line plume bounded by an adiabatic wall General correlations for wall and corner configurations Interaction of a plume with a ceilingDescription of a ceiling-jet Alpert’s Integral model Simplified correlationsAdditional considerationsSmoke layer build-up in a room Balcony and window spill plumesBalcony spill plumes Window plumes Scaling laws and buoyant releasesExercisesAnalytical solution for the Line plume problem Design of a reduced-scale helium/air mixture experiment of a car fire in a tunnelFire and smoke dynamics in enclosures Some fundamentals on flows through openingsGrowing fireFire source Fuel-controlled growing fire Ventilation-controlled growing fire Smoke dynamicsFlows through openings Horizontal openings Vertical openings Natural and mechanical ventilation Zone modelingFully developed fireFire source Smoke dynamics Flows through openingsHorizontal openings Vertical openings Natural and mechanical ventilation Zone modeling Pulsating fire Backdraft Fires in well-confined enclosures Driving forces in smoke and heat control Buoyancy – the stack effect Natural stack effect Fire-induced buoyancy Pressurization Natural ventilation Mechanical ventilation Vertical ventilation Horizontal ventilation Tunnels Other underground structures Smoke extractionThe effect of wind Positive pressure ventilationAir curtains Exercises Impact of water on fire and smoke dynamics Individual evaporating water dropletHeat and mass transfer Flow equations Sprays of water droplets Characterization of sprays Region near the nozzle Water flow rate Droplet size and velocity distribution Spray cone angle Spray-induced momentum Water curtains Heat absorption by waterInteraction of water with smokeSprinkler and water mist sprays Water curtain Fire fighting Interaction of water with flames Water as fire suppressant Introduction to fire modelling in computational fluid dynamics Introduction Laminar diffusion flames Instantaneous transport equations Combustion modellingInfinitely fast chemistryFinite-rate chemistry Turbulence modelling DNS RANS LES Turbulent non-premixed combustion Infinitely fast chemistry with a presumed PDF Flame sheet model Chemical equilibrium model Steady Laminar Flamelet Modelling (SLFM) Finite rate chemistry Eddy Break-Up (EBU) model and Eddy Dissipation Model (EDM) Eddy Dissipation Concept (EDC)Conditional Moment Closure (CMC) Transported PDF modelsRadiation modelling Models for radiative transferThe P-1 Radiation Model The Finite Volume Method (FVM) Models for the absorption coefficient Turbulence Radiation Interaction (TRI) The soot problem Soot nature, morphology and general description of its chemistry Importance of soot modelling Sootiness and radiationInteraction of soot with carbon monoxide The sootiness of fuels The laminar smoke point height The Threshold Sooting Index (TSI) Soot modelling Laminar flames Turbulent flamesBasics of numerical discretization Discretization schemesDescription of a 1-D exampleExplicit schemeImplicit scheme Initial and boundary conditions Properties of numerical methodsConsistency Stability Convergence Conservativeness Boundedness Pressure-velocity coupling The importance of the computational mesh Boundary conditions Fire source Gaseous fuelLiquid fuel Solid fuel Turbulence inflow boundary conditions Walls Velocity Temperature Open boundary conditions (natural ventilation) Velocity and scalars Pressure Mechanical ventilation and pressure effects Fixed velocity Fan curves and pressure effects Examples of cfd simulations Non-reacting buoyant plumeTest case description Simulation set-up Results Hot air plume impinging on a horizontal plate Test case description Simulation set-up Results Free-burning turbulent buoyant flameTest case description Simulation set-up Results Over-ventilated enclosure fireTest case description Simulation set-up and ResultsInteraction of a hot air plume with a water sprayTest case description Simulation set-upResultsUnderventilated enclosure fire with mechanical ventilation Test case description Simulation set-up Results Fire spread modelling References Subject Index

Prof. Bart Merci obtained his PhD, entitled ‘Numerical Simulation and Modelling of Turbulent Combustion’, at the Faculty of Engineering at Ghent University in the year 2000. As postdoctoral fellow of the Fund for ScientificResearch – Flanders (FWOVlaanderen), he specialized in numerical simulations of turbulent non-premixed combustion, with focus on turbulence – chemistry interaction and turbulence – radiation interaction. He reoriented his research towards fire safety science, taking the fluid mechanics aspects as central research topic. He became lecturer at Ghent University in 2004 and Full Professor in 2012. He is the head of the research unit ‘Combustion, Fire and Fire Safety’ in the Department of Flow, Heat and Combustion Mechanics. Since 2009, Bart Merci coordinates the ‘International Master of Science in Fire Safety Engineering’, with Lund University and The University of Edinburgh as partners. He has been the President of The Belgian Section of The Combustion Institute since 2009 and Associate Editor of Fire Safety Journal since 2010. He is member of the Executive Committee of the International Association for Fire Safety Science. He is author of more than 100 journal papers. Dr. Tarek Beji obtained his PhD, entitled "Theoretical and Experimental Investigation on Soot and Radiation in Fires", at the University of Ulster in 2009. He joined Ghent University in 2011 as a post-doctoral researcher in the department of Flow, Heat and Combustion Mechanics and worked on the novel topic of fire forecasting. Since 2012 he has been very active in a large international collaborative research program called PRISME, focusing on mechanical ventilation and fire dynamics in nuclear facilities. Since he joined Ghent University he participated actively in the 'International Master of Science in Fire Safety Engineering' as lecturer and member of the program steering committee.