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Transport Phenomena in Fire Security of the Built Infrastructure

Arvind Atreya

Department of Mechanical EngineeringUniversity of Michigan, Ann Arbor

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Background and Societal Impact (Fire Security == Life Safety)• 9/11 – Unprecedented death and destruction.

Never before have 2,830 people lost theirlives including 400 emergency responders in any single incident.

The Towers could have sustained the impactof the planes but the resulting fires caused a total collapse.

• We are unprepared for fire in general and fire induced collapse in particular.

• There is a critical and urgent national need to: Produce the technical basis for cost-effective

Fire Safety Design and Retrofit of Structures.

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Background and Societal Impact (cont…)• More recent incident: Bay area fire-induced overpass

collapse – 4/29/2007.Problem: Current design practices do not consider fire as a design load inthe prediction and evaluation of structural performance. At present, there is no accepted science-based set of verified tools to evaluate the performance of the entire structure under realistic fire conditions.

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Relationship to Transport phenomena1. Fires involve both convective & diffusive transport of heat,

species, and momentum. Further,2. Transport of heat by radiation plays a dominant role.3. Fire-structure interaction requires heat transfer to the structural

member through some kind of fireproof material—such as gypsum board or a spray-on material.

– Aspects of the problem that can not be included under the transport phenomena heading are: • Structural deformation• Structural failure and collapse• Condition of the structural fireproofing material

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Technical Areas: Fire-Structure Interaction [Ref. 4]Coupled analysis very difficult – different scales

Fire Dynamics Thermal Response Structural ResponseThese Concern Transport Phenomena Research

l ~ 102cm; t ~ 10-2sl ~ 1cm t ~ 10s

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Challenges (structure thermal

analysis)

The technical disciplines of

Structures, Fire Dynamics,

& Thermal Analysis

have evolved separately.

It is necessary

to couple them.

Structural Analysis:Structural Analysis: Calculations of Calculations of

Displacements, stresses, Displacements, stresses, and loss of load carrying and loss of load carrying capacity of the structurecapacity of the structure

Fire Dynamics:Fire Dynamics: Simulations of Simulations of

combustion, fluid combustion, fluid mechanics, heat mechanics, heat & mass transport& mass transport

in the gas.in the gas.

Thermal Analysis:Thermal Analysis: Simulations of Simulations of

heating and heating and Cooling of Cooling of

condensed phase condensed phase materials.materials.

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Challenges (fire dynamics)• Purpose: To predict the evolution of gas temperature,

composition & motion.– gas motion good predictive capability.– gas T & χ require what is burning models inadequate.– Need models for burning of solid fuels (charring and

thermoplastic) – Need models for fire spread and growth over solid fuels.– Need improvements in suppression models (e.g. sprinklers)– Predictions become less accurate with time – due to changing

boundary conditions & fuels.

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Challenges (fire dynamics cont…)• Primary mode of heat transfer Radiation

– Require time-dependent solution for radiative transport.

– Need Soot, CO2, H2O, CO, etc. concentrations from fire dynamics calculations – require better subgrid models.

• Fire vs. Combustion simulation– Combustion simulation typically small scale.

• Detailed numerical modeling of the physical and chemical processes at highly resolved temporal and spatial scales possible.

– Fire simulation typically Large scale. • Modeling of transport processes at dominant hydrodynamic length

and time scales with mixture fraction combustion. However, need enough chemical detail to predict toxicity & radiation.

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Another important unsolved problem– Prediction of flashover

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Challenges (structure thermal analysis)– Coupled analysis very difficult – disparate scales.– Fire boundary conditions continuously changing.– Geometry is changing with time due to large structural

deformations. – Material properties often not known as function of

temperature– Unknown condition of the fireproofing material– Primary mode of heat transfer Radiation – Current codes: 1D time dependent heat conduction through

surfaces.

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Fire protecting composite wall board

Ablation products – gas and solid particles

aCO3 + HEAT = CaO + CO2

Deposited CaCO3 particles held with a binder that chars and decomposes at ~ 600K

Fiberglass or ceramic substrate with fibrous strands to integrate the ablating solid particles and binder

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Benefits of this fire protection:– CO2 helps extinguish the flames;

– Particles protect the structure from flame radiation;

– Charring prevents heat transfer to the structure.

– In a room with such wall boards, the mass of CO2 evolved is about 4 times the mass of air in the room. Hence the room will be completely flooded with CO2.

– The amount of heat consumed is about 5 times larger than that produced by the fire.

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Recommendations (not including structure improvements)• Better submodels for burning of various solids & their assemblies.• Better Fire Spread and Growth Models with prediction capability for arbitrary fuel

geometry and type.• Better subgrid models to predict temperature and concentrations of soot and toxic

species for tenability and radiation prediction.• Better Structural Fire Protection – non-brittle SFRM and intumescent materials with

better adhesion properties. • Wall boards with passive fire suppression capability.• Analytical models wherever possible for coupling fire dynamics & structure thermal

response to alleviate the scale problem.

Recommendations for Life Safety• Firefighting technologies and practices for large structures/buildings• Occupant behavior and evacuation technologies and practices for tall buildings• Command, control, and communication systems for fire service response and safe

egress.

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References1. McGrattan, K.B., Baum, H.R., Rehm, R.G., Hamins, A., Forney, G.P., Floyd,

J.E., Hostikka, S., and Prasad, K., “Fire Dynamics Simulator (Version 3) – Technical Reference Guide” National Institute of Standards and Technology Report NISTIR 6783, Nov. 2002.

2. McGrattan, K. B., Baum, H. R., Rehm, R., Forney, G. and Prasad, K., “The Future of Fire Simulation,” Fire Protection Engineering, 13: 24-36, 2002.

3. Atreya, A. and Baum, H. “A model of opposed-flow flame spread over charring materials,” Proceedings of the Combustion Institute, v 29, p 227-236, 2002

4. Prasad, K. and Baum, H R, “Coupled Fire Dynamics and Thermal Response of Complex Building Structures,” Proceedings of the Combustion Institute 30, 2005, pp. 2255-2262.

5. Baum, H. R. “Simulating Fire Effects on Complex Building Structures,” Howard W. Emmons Invited Plenary Lecture, Eighth IAFSS Symposium, Beijing, 2005.

6. NIST and the World Trade Center Analysis, http://wtc.nist.gov/, 2006.