Enclosure Fire Dynamics Chapter 1: Introduction Chapter 2: Qualitative description of enclosure fires Chapter 3: Energy release rates Chapter 4: Plumes and flames Chapter 5: Pressure and vent flows Chapter 6: Gas temperatures (Chapter 7: Heat transfer) Chapter 8: Smoke filling (Chapter 9: Products of combustion) Chapter 10: Computer modeling Each course unit represents breaking down the problem into individual pieces

Content Classification of fire models Computer fire models Zone models (Example CFAST) CFD models (Example FDS, Fire Dynamics Simulator) Egress models Application of fire models Conclusions

Before we start… • Learning theory vs. learning software – It is better to learn zone model or CFD theory • You can then learn how to use any zone model – If you just learn about one program, that is all you know! • Two hours of lecture does not make a fire modeler – To become good takes much practice – The technical reference for CFAST alone is 250 pages

Probabilistic and deterministic fire models Nearly all fire models used in practice are deterministic models, and we will concentrate on those. Probabilistic models provide quantitative measure of the probability of occurrence – Attempt to provide the most probable (range of) answers – Associated with risk analysis and reliability engineering We will not talk more about probabilistic models

Deterministic models • Deterministic = to determine, figure out. Same input always gives same output • Model fire physics such as: – plume flow – generation of heat, smoke, etc. – heat transfer – other fluid flows • Simulate transport of smoke and heat in enclosures

Specialized deterministic models • Heat transfer • Structural response • Glass breakage • Detector/sprinkler actuation • Evacuation/egress • Hydraulic/water supply • Explosion venting

Classification of fire models • Type: deterministic or probabilistic • Complexity: single equation to hundreds of thousands • Fire type: steady state, quasi-steady, transient • Applications: pre-fire, post-fire

We will look at two main types of deterministic fire models Zone models – Tend to be specialized for fire Computational fluid dynamics models (CFD) – You may have seen an older term for this type of model called “field models” – Many CFD models used for fire are general purpose computer codes - Design airplanes, pumps, ships, etc.

Content Classification of fire models Computer fire models Zone models (Example CFAST) CFD models (Example FDS, Fire Dynamics Simulator) Egress models Application of fire models Conclusions

Zone models Divides enclosure into a “small” number of zones (usually two zones): Upper layer, lower layer – Plume, ceiling jet – Boundary – Objects (fuels)

Usual zone model assumptions • Typically, two zone models: – Homogeneous: uniform, well-mixed – No (or little) mixing between layers – Combustion in upper layer may not be allowed – Regular compartment shape (a box) – Implies local effects are unimportant or nonexistent (at least from a practical point of view), => Modeling global effects Sacrifice some accuracy for: – Ease of use and setup – Short computation time

Two zone model OK?

How about this case?

Different zone models HAZARD I WPI/Harvard fire code BRANZFIRE CFAST combined with tenability and egress models Hazard analysis package FAST/CFAST etc… WPI/Harvard fire code One of the first models to include flame spread (couple radiation to solid heating) FIRST version of code has 6 plume models BRANZFIRE From New Zealand, multi-room zone model, comprehensive, flame spread 30+ different models available

Currently supported or commonly used zone models • ASET - US • BRI2 - Japan • CFAST -US • CFIRE-X - Germany/Norway collaboration • CiFi - France • COMPBURN - US • COMPF2 -US • DSLAYV - Sweden • FIRST (HARVARD V) - US • FISBA -France • MAGIC -France • NRCC 1 and 2 (a component of FIRECAM) - Canada • RADISM -UK • RVENT - Norway • Sfire - Sweden

CFAST programs • CFAST is the fire model – FAST, FASTLite, FireCAD, FireWalk and all of the other programs are basically data editors – They all run CFAST • CFAST is an acronym for the Consolidate model of Fire Growth and Smoke Transport.

CFAST Transient calculation of smoke and fire gas spread throughout a number of compartments based on a user defined fire (18 rooms possible) Download for free at www.fire.nist.gov See latest editions of Users Manual and Technical Reference from www.fire.nist.gov

CFAST has features similar to most zone models • Multi-room fire model • Input: room description, what’s burning, etc • Output: heat release rates, fluxes, temperatures, flows, species (gas and smoke), • Consequences: smoke detector operation, heat detector (including sprinkler) operation

Control volumes are written for each compartment Conservation of mass and conservation of energy equations written for each control volume

Choosing input Choose scenarios Geometrical aspects (rectangular rooms, which door/window is open, etc), define a few scenarios Fire scenario (where does ignition occur, what will the fire growth be, what is the maximum heat release), define a few scenarios People and egress (how many people, what type of occupancy, where are exits), define a few scenarios This will usually lead to many scenarios, but these will generally be reduced to a handful

Typical apartment layout

What about irregular rooms?

Sub-models in zone models • Provide source terms to conservation equations – Mass of fuel consumed – Heat transfer to walls – Plume entrainment – Vent flows • These are treated as “source terms” for the conservation equations

Conservation equations Conservation of mass Conservation of species (fuel, O2, products) Conservation of energy

Types of plumes in CFAST

Types of vent flows • Horizontal flow (doors, windows , ...) • Vertical flow (holes in ceilings/floors) • Forced flow (mechanical ventilation) • The pressure in the enclosure is uniform with respect to the energy equation – Hydrostatic pressure differences lead to vent flows • Generally follow the vent flow equations we derived in Chapter 5

Zone model heat transfer Typically 1-D conduction through walls Different materials permitted for walls, ceiling and floor Can also have different layers of materials (up to 3 layers in CFAST) CFAST uses constant material properties Bounding surfaces and upper layer are gray Lower layer is transparent in wall/layer interchange calculations Flame radiates to the upper layer and to walls No radiation interchange between rooms

Example, temperature prediction

Advantages of zone models • Easy to learn and use software – Can generate results for a number of scenarios in a short time • Important for design work • Most fire deaths result from smoke movement outside the compartment of origin – Especially at NIST, zone models have been developed to describe the movement of smoke away from the fire toward occupants • Real time calculations – Allows many combinations to be investigated

What zone models do not do • Fire models (as a rule) DO NOT model fire – HRR not accelerated (increased) due to conditions inside compartment – Difficult to account for increase in HRR as other fuels become involved (HRR under predicted) – (Exception: Harvard, BRANZFIRE, etc.) • Fire models DO predict the effects of a userspecified fire – User specified fire typically based on understanding of fire dynamics – Most zone models should simply be called smoke filling models

Content Classification of fire models Computer fire models Zone models (Example CFAST) CFD models (Example FDS, Fire Dynamics Simulator) Egress models Application of fire models Conclusions

CFD (Computational Fluid Dynamics) • At some point in time, the number of equations and complexity of the zone model reach a point where it may be better to use a CFD code – For example: • Very large buildings or tall atria • Detailed wall heat transfer calculations • Radiation transfer for flame spread • Sprinkler suppression • As we will see, just because we are using a CFD code does not mean we have the right (correct) answer!

Computation fluid dynamics (CFD) models Solve conservation equations over a large number of control volumes – Navier-Stokes equations – Field equations • The big problem… – Combustion time scales down to 10-3 – 10-6 seconds – Length scales down to 10-6 m – But our problem dimensions are in 10’s of meters and minutes – Computer resources not (yet) ready

General transport equation • φ is any one of the solution variables (u,v,w, enthalpy, concentration) • Linear partial differential equation • Source can include combustion, radiation etc.

Discretization • Discretization = divide the area of interest (domain) into many control volumes • Type of discretization can have a big impact on how the equations are solved • Solving conservation equations only at a limited number of points

CFD computational domain with structured grid

Unstructured grid

Closure Models Closure model = a simplification to allow solution of the conservation equations – Discretization not small enough for all fire physics – Models sub-grid scale physics – Computationally expensive • Radiation transfer – Radiation properties • Combustion – Soot production • Turbulence – This is the big one!

Reynolds stress turbulence models • Model Reynolds “apparent” stresses • κ-ε turbulence model – Strategy is to solve time averaged Navier- Stokes equations – Conservation equations for turbulent kinetic energy, κ, and dissipation of turbulent kinetic energy, ε – Very common in current codes – There are a number of well documented problems with this model • Unfortunately many of them apply to fire

More detailed turbulence models Large eddy simulation FDS (Fire Dynamics Simulater) is the CFD model you will use in your computer lab, and FDS uses Large eddy simulation Solve a simplified form of Navier Stokes equations Solves large eddies directly Only model turbulence on scales less than grid Artificial fluid viscosity to dissipate remaining energy Things tend to be more uniform at this scale, thus (somewhat) easier to model correctly Direct numerical simulation (give a few years) Grid (discretization) fine enough to calculate important flow features 2-D calculations and low Re number 3-D

Finite Volume Radiation Transfer • Integrations for each direction and for all wavelengths – Computationally expensive

Computational domain extends beyond the room of interest

Cross section showing gas velocities

Competent use of CFD models • Characterization of Fire – Heat source versus combustion source – Fire plume temperature and flame height • Grid design and solution convergence • 2D versus 3D solution • See Manuals and Best Practice Guidelines now available

Summary on CFD Conservation equations based on first principles Offers a universal modeling tool Developed for wide range of problems including: indoor air movement, smoke movement, flame spread, fire resistance furnace, atmospheric dispersion More complex than simple engineering correlations and zone models Increased chance for misuse Input becoming much easier – so it is possible to run CFD codes with no knowledge of CFD!

Summary on CFD Powerful tool if in right hands Requires significant education Requires proper simulation of physical and chemical processes with appropriate initial and boundary conditions Still solving conservation equations at a limited number of points Realistic treatment of fire source is important Rapid implementation (use) in consulting

Content Classification of fire models Computer fire models Zone models (Example CFAST) CFD models (Example FDS, Fire Dynamics Simulator) Egress models Application of fire models Conclusions

Egress models Predict egress (exit) time for occupants to exit a structure Time = distance / walking speed May include occupant behavior Research shows different people respond differently during a fire Should include impact of fire Results from fire model feed into egress model A few models have even been specifically developed for ships, airplanes, tunnels

Predicting fire impact on people is difficult People complicate things by moving Impact of smoke may change people’s movement While moving they are exposed to different levels of smoke/heat Example models EXITT Evacnet SIMULEX EXODUS

Example: Simulex

Content Classification of fire models Computer fire models Zone models (Example CFAST) CFD models (Example FDS, Fire Dynamics Simulator) Egress models Application of fire models Conclusions

Application of fire models Post-Fire Investigation Reconstruction Pre-Fire Design Analysis Variances or equivalencies During-Fire Predict possible growth of fire (very seldom)

Post fire analysis • Generally looking at a number of scenarios – Ignition location – Position of doors – Status of windows – Actual materials (and their properties) – Location of contents and occupants – Fire department activities • Fire destroys evidence necessary for modeling

Post-fire example: Göteborg disco fire, 65 died

Post flashover burning

Pre fire analysis and design Many input parameters to vary: Ventilation conditions Material properties Placement of fuel packages Initial ignition location Number and location of occupants Future changes in building

Using fire models “Given” a scenario Complexity depends on model being used Geometry Develop input data set room dimensions door, window, other vent locations Physics and chemistry material properties fuel characteristics Most importantly, the (input) fire

Looking at the results Do the predictions make sense? Anything with flame temperature greater than 1300oC should be examined closely Compare the predictions of flame, flame height, and plume entrainment with empirical correlations.

Looking at the results There is no One answer. Rule of thumb: The user must give a range of likely answers. This means for every problem, the user must do sensitivity analyses involving many runs. Rule of thumb: Zone model, most simple problems: 15-30 runs Zone model, more complicated 30-100 runs Probably fewer, but still many for CFD

How good are your results? Uncertainty Analysis Single point representation of a distribution Accounting for “unknown randomness” Sensitivity Analysis How much does the result change when an input parameter value is changed? Ideally, check all “assumed” input parameters, but practically difficult What are the significant assumed input parameter values?

Content Classification of fire models Computer fire models Zone models (Example CFAST) CFD models (Example FDS, Fire Dynamics Simulator) Egress models Application of fire models Conclusions

Conclusions Models supplement engineering judgment They should never be used instead of engineering judgment No one run of any model will give THE correct answer! Many people assume that CFD and zone models are easy to use Reality is it takes a significant amount of work to use such models properly So, they are actually easy to misuse

Conclusions Rapid development in the building industry, larger and more complex buildings, more complex technologies, design and materials New building regulations based on performance requirements Progress in the understanding in fire phenomena, risk concepts and human behaviour has been rapidly increasing Many models available for simulating fires and simulating movement of humans But, there is a lack of education in Fire Safety Engineering