1. Hazard Assessment of Large-Scale Releases of Combustible Chemicals
Greg Jackson and Arnaud Trouvé
University of Maryland, College Park
contact:
Tom McGrath & Bill Hinckley
NSWC – Indian Head Division
CECD Overview Meeting
May 14, 2007

2. Formation, Ignition, and Combustion of Fuel Vapor Clouds
Ultra
-lean
Ultra-rich
Air
Fuel
Flammable
Ultra
-lean
-rich
Air
Fuel
Ultra-lean
Air
Fuel
Air
Fuel
Ignition
Fuel
Ultra
-lean
Ultra-rich
Air
Fuel
Flammable
Ultra
-lean
-rich
Air
Fuel
Fuel + Air
Air
Basic scenario:
Accidental release of gaseous fuel
in ambient air
Turbulent mixing of fuel and air
(delayed ignition)
Formation of a large ultra-rich cloud
Formation of a large flammable cloud
Safe dispersion
Ignition
(in flammable region of vapor cloud)
Air
Fuel
Diffusion
Flame
Fuel
Premixed
Flame
Combustion
Explosion: detonation (blast)
Flash fire: deflagration (no blast)
Fireball: diffusion flame

3. 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

4. Detonation – Large Scale Model Testing Propane 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

5. Dispersion – Simulation of Large Scale LNG Spill Vaporization
CH4 mole fractions
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.
pool location
~ flammability region
Gas Temperature (°C)
pool location

6. Dispersion – Flammable Mass as a Function of Wind Speed
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.
300
250
200
Flammable mass (kg)
150
100
Uwind = 0.5 m/s 50
Uwind = 1.0 m/s
Uwind = 2.0 m/s
100
200
300
400
500
Time fom initial vaporization (s)

7. 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.

8. Enhanced FDS Combustion Models – Premixed Flames for ignition and deflagration propagation
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
Flammable domain
Unburnt
Reactants
Burnt Products
Turbulent
Flame Front

9. Coupled Combustion Models in Turbulent Flame Simulations
Model problem: ignition of a fuel vapor cloud in a sealed compartment
Uniform mesh (160×160×120) = 3,072,000

10. 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)

11. 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

12. Coupled Combustion Models in Turbulent Flame Simulations
Time variations of global heat release rate of mixed premixed/diffusion flame
ignition
diffusion burning
partially-
premixed
combustion
short time dynamics

13. Activities Going Forward
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