Transcript Hazard Assessment of Commercial Maritime Flammable and

Hazard Assessment of Large-Scale
Releases of Combustible Chemicals
Greg Jackson and Arnaud Trouvé
University of Maryland, College Park
e-mail contact: [email protected]
Tom McGrath & Bill Hinckley
NSWC – Indian Head Division
CECD Overview Meeting
May 14, 2007
Formation, Ignition, and Combustion of Fuel Vapor Clouds
Ultra
Ignition
-lean
• Basic scenario:
 Accidental release of gaseous fuel
in ambient air
Flammable
Ultra-lean
Fuel
+ Air
Ultra-rich
Ultra
-rich
 Turbulent mixing of fuel and air
(delayed ignition)
Air
•• Formation
Safe dispersion
of a large ultra-rich
flammablecloud
cloud
 Ignition
(in flammable region of vapor cloud)
Fuel
Premixed
Diffusion
Flame
 Combustion
• Explosion: detonation (blast)
• Flash fire: deflagration (no blast)
• Fireball: diffusion flame
Air
Fuel
Fuel
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 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
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)
CH4 mole fractions
pool location
• Fuel remains along surface but
~ flammability region
flammable regions rise well off surface
• Structure of dispersion shows buoyant Gas Temperature (°C)
plumes but hesitancy to interpret this as
physically realistic
• Temperature remains cold in fuel rich
regions
• Risk of large-scale fire possible but
pool location
likely not flash fire due to strong
gradients of fuel concentration.
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
Flammable mass (kg)
250
200
150
100
Uwind = 0.5 m/s
Uwind = 1.0 m/s
Uwind = 2.0 m/s
50
0
0
100
200
300
Time fom initial vaporization (s)
400
50
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.
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
u sL  c
 T c~
 ~


~
~
c   ( c u j )  ((

)
)
t
x j
x j 16 6 / 
ScF x j
S L ,st
6 c~1  c~ 

 u sL    4
  ign
 c
Unburnt
Reactants
Turbulent
Flame Front
~ 1
c
Burnt Products
~0
c
Flammable domain
ZUFL
Z LFL Z st
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
(xyz)1/ 3  2.5 cm
Fuel : heptane
mF  1.2 kg
EF  55 MJ
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
short time dynamics
ignition
partiallypremixed
combustion
diffusion burning
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