Hazard Assessment of Large-Scale Releases of Combustible Chemicals

Greg Jackson and Arnaud TrouvéUniversity of Maryland, College Park

e-mail contact: gsjackso@eng.umd.edu

Tom McGrath & Bill HinckleyNSWC – Indian Head Division

CECD Overview MeetingMay 14, 2007

• Safe dispersion

• Basic scenario:

Accidental release of gaseous fuelin ambient air

Turbulent mixing of fuel and air(delayed ignition)

Ignition(in flammable region of vapor cloud)

Combustion

• Explosion: detonation (blast)• Flash fire: deflagration (no blast)• Fireball: diffusion flame

• Formation of a large flammable cloud• Formation of a large ultra-rich cloudFuelAir Fuel

Fuel + Air

Air Fuel

Ultra-lean

Air Fuel

Flammable

Ultra-lean

Ultra-rich

Air Fuel

Ultra-lean

Ultra-rich

Air Fuel

Ignition

Fuel

PremixedFlame

Fuel

Flammable

Ultra-lean

Ultra-rich

Air Fuel

Ultra-lean

Ultra-rich

Air

Air Fuel

DiffusionFlame

Formation, Ignition, and Combustion of Fuel Vapor Clouds

Advanced Modeling Approach• Start to finish modeling of release, dispersion, and explosion or fire of

large scale chemical release requires multiple physical models – Spill modeling not yet implemented although explored– Dispersion requires convective/diffusive modeling– Detonation requires convective/reactive modeling with shock capturing

– Deflagration requires convective/diffusive/reactive models

• Approach to integrate/modify existing codes – Dispersion: Fire Dynamic Simulator (FDS) developed by NIST– Detonation: GEMINI by NSWC IHDIV and enhanced

• Enhanced by McGrath et al. to support gas-phase reactions/detonations– Deflagration: Fire Dynamic Simulator (FDS) by NIST

• Enhanced by Trouvé et al. for premixed and partially premixed combustion– Structural Response: In-house development of UMCP based on existing pressure-

impulse techniques

• Experiments in attempt to validate submodels

Detonation – Large Scale Model TestingPropane Vapor Release

• Pressure contours plotted at selected times during event

• Detonation wave travels through fuel/air dispersion and continues to propagate as a blast wave

• Blast reflects off target structure

• Detonation fails to consume all fuel present in domain

– Fuel mass fraction plotted at final simulation time

– Significant portions exist outside of detonability limits and do not react

– Remaining fuel may burn as a large fireball upon mixing with air

Dispersion – Simulation of Large Scale LNG Spill Vaporization

• Simulation of LNG spill

– 1.0 m/s crosswind over 10 m high ship

– 30 X 30 m LNG spill downstream of ship

– Constant heat transfer coefficient (155 W/m2*K)

• Fuel remains along surface but flammable regions rise well off surface

• Structure of dispersion shows buoyant plumes but hesitancy to interpret this as physically realistic

• Temperature remains cold in fuel rich regions

• Risk of large-scale fire possible but likely not flash fire due to strong gradients of fuel concentration.

Gas Temperature (°C)

pool location

pool location

CH4 mole fractions

~ flammability region

Dispersion – Flammable Mass as a Function of Wind Speed

0

50

100

150

200

250

300

0 100 200 300 400 500

Time fom initial vaporization (s)

Fla

mm

able

mas

s (k

g)

Uwind = 0.5 m/sUwind = 1.0 m/sUwind = 2.0 m/s

• Simulation of LNG spill vaporization downstream of a structure similar to a ship show that flammable mass created decreases with winds over the structure > 1 m/s.

• Results like this make a strong statement on hazard assessment.

Large Eddy Simulations for Formation, Ignition, and Transient Combustion of Fuel Vapor Clouds:

Arnaud Trouvé – University of Maryland

• Trouvé et al. have been developing sub-grid models that can couple turbulent premixed flame ignition and transition to partially premixed and fully non-premixed turbulent combustion.

• This capability is absolutely critical for large-scale fire hazard modeling where large chemical releases must be initially ignited as premixed mixtures and may likely transition over to a non-premixed steady fire scenario.

ign

cLu

jF

TcLu

jj

j

ccs

x

c

Sc

s

xuc

xc

t

~1~6

4

)~

)/616

(()~~(~

• Premixed combustion models added to FDS to capture deflagration propagation as well as cloud ignition processes

• Combustion model based on governing equations for the LES-filtered progress variable c:(Boger et al., Proc. Combust. Inst. 1998; Boger & Veynante, 2000)

• Variations of laminar flame speed with mixture strength are described using a presumed polynomial function of fuel properties including flammability limits

UnburntReactants

1~ c0~ c

Burnt Products

TurbulentFlame Front

Enhanced FDS Combustion Models – Premixed Flamesfor ignition and deflagration propagation

stZLFLZ

UFLZ

stLS ,

Flammable domain

cm 5.2)( 3/1 zyx

MJ 55kg 2.1

heptane :Fuel

F

F

Em

• Model problem: ignition of a fuel vapor cloud in a sealed compartment

– Uniform mesh (160×160×120) = 3,072,000

Coupled Combustion Models in Turbulent Flame Simulations

• Location and structure of premixed and non-premixed flames at t = 2.5 s

– Initiation of partially-premixed combustion

Premixed Non-premixed

Buoyant puff(vertical spread)

Expanding flame kernel (horizontal spread)

Coupled Combustion Models in Turbulent Flame Simulations

• Location and structure of premixed and non-premixed flames at t = 3 s

– Partially-premixed combustion

Premixed Non-premixed

Buoyant puff impinging on ceiling wall

Coupled Combustion Models in Turbulent Flame Simulations

• Time variations of global heat release rate of mixed premixed/diffusion flame

ignition

diffusion burningpartially-premixed

combustion

short time dynamics

Coupled Combustion Models in Turbulent Flame Simulations

• Continued efforts to seek further support– Meeting May 10, 2007 with ONI, DOE, FERC, and Coast Guard– Ongoing discussions (led by Bob Kavetsky) with DHS– Other efforts including collaborations with NIST

• Further improvements to deflagration modeling in FDS– Improved radiation modeling for coupling heat feedback to vaporization

(Jackson)– Testing of code to evaluate models ability to capture large-scale premixed to

diffusion flame transitions (Trouvé)• Further improvements to detonation modeling in Gemini

– Implementation of multi-phase detonations to evaluate high explosive threats as well as initial conditions of fuel vapor blasts (McGrath / Jackson)

• Efforts to establish dedicated computational facility at UMD for this project

Activities Going Forward