Explosion of
Hydrocarbon Fuels
Aviation Fire Dynamics – Spring 2013
Final Presentation
Derick Endicott
What is an ‘explosion’?
An explosion can be defined as:
-“the sudden conversion of potential (chemical in
this case) into kinetic energy including the
production and release of gases under
pressure.”[1]
-“the process of the rapid release of energy
involving spontaneous and vigorous reactions
with rapid production of very large volumes of
gases and heat fluxes, having destructive effects
on nearby surroundings”[1]
Why study explosions?
• Explosions can be caused by accidental or deliberate
sources including but not limited to failure of electronic
components, fuel supply lines, and fuel storage tanks.
• Explosions can produce not only extreme temperatures
which can comprise structures but devastating pressure
waves that can annihilate anything in close proximity.
• In order to predict these temperature and pressures an
explosion may produce and the subsequent damage,
one must have a deep multidisciplinary knowledge.
Important Definitions
• Combustion/Fire: chemical reaction in which a
substance combines with an oxidant and releases
energy. Part of the energy released is used to sustain
the reaction. [9]
• Ignition: ignition of a flammable mixture may be caused
by the mixture coming in contact with a source of ignition
with sufficient energy or the gas reaches a high enough
temperature to cause the gas to autoignite.[9]
• Fire Point: the lowest temperature at which a vapor
above a liquid fuel will continue to burn once ignited;
higher than the flash point.[9]
Important Definitions
• Mechanical Explosion: an explosion resulting from the
sudden failure of a vessel containing high-pressure
nonreactive gas.[9]
• Overpressure: the pressure on an object as a result of
an impacting shock (or pressure) wave. This pressure is
in excess of the ambient value.[9]
• Peak Overpressure: maximum pressure minus the
ambient pressure.
Types of HC Explosions:
Deflagration vs. Detonation
The term ‘explosion’ does NOT imply detonation! Type is determined from the
pressure wave developed by the explosion:
Deflagration
• Pressure wave (expansion)
travels at the speed of sound in
unburned gaseous fuel-air
mixture, while the reaction front
travels slower than the speed
of sound.
• Simple burning chemistry,
involving turbulent flame speed
(or laminar to simplify).
• Rapid oxidation
• 5-10 atm pressure rise
Detonation
• Pressure wave (now a shock
wave) travels faster than the
speed of sound in unburned
gaseous fuel-air mixtures.
• More complicated reaction, not
just simple HC burning chemistry,
sometimes involving highly
unstable agents.
• Usually associated with fuels like
TNT. CAN OCCUR in HC/Air
mixtures.
• 15-50+ atm pressure rise (HC
Fuel)
Fire or Explosion?
• For deflagrations, only depends upon the rate of
energy release!
– No real definition or limit which describes what level
of release rate is an explosion and what level is just a
fire.
– An ‘explosion’ must have a sudden enough energy
release such that the energy ‘builds up’ at the site of
explosion.[10]
• This energy may be dissipated by pressure waves,
projectiles, thermal radiation, acoustic energy, etc. [10]
Fire or Explosion?
The maximum pressure achieved and the
maximum rate of pressure increase are
important parameters when characterizing
an explosion and what effects it will have,
and the possible subsequent damage.
[6]
[9]
Detonation vs. Deflagration
Applied – PDE
http://www.youtube.com/watch?v=DdLjHJuFWQ
Parameters that Determine
Explosion Type
•
•
•
•
•
•
•
•
•
Ambient temperature
Ambient pressure
Composition of explosive material
Physical properties of explosive material
Ignition source
Un/Confined fuel-air mixture
Turbulence
Amount of combustible material available
Rate at which combustible material is introduced
Flammability/Explosive Limits
• It is not always the initial component failure that causes
an explosion!
• Is there exists a source of ignition within an area where
the air fuel mixture is within flammability/explosive limits?
• These fuels will have both Upper and Lower Explosive
Limits (UEL and LEL).
• High temperatures and pressures can expand these
limits.
Focus of Study
•
•
•
•
Fire/Combustion
Hydrocarbon Deflagrations
Hydrocarbon Detonations
Short Intro to Detonations of high
explosives
• How to prevent explosions
Combustion
Combustion is the exothermic oxidation reaction between a fuel (HC) and
an oxidizer (AirOxygen).
Or more simply:
[12]
Combustion
• The previous slide describes the global reaction,
or the simplest way to approach the reaction.
• Reality?  long chain of reactions in which
many radical intermediate species appear.
• Fires  flammable F/A mixture exposed to a
source of heat or ambient temperature at or
above the flash/fire point of the mixture.
Flame Spread
• Thermal/molecular diffusion
• Gravity (buoyancy) causes the heated
combustion products (less dense) to rise
and surrounding air/fuel to be entrained in.
• While this is happening the surrounding
air/fuel is being heated for reaction.
Flame Buoyancy
Effects
[36]
Focus of Study
•
•
•
•
Fire/Combustion
Hydrocarbon Deflagrations
Hydrocarbon Detonations
Short Intro to Detonations of High
Explosives
• How to prevent explosions
Deflagrations
[13]
Aircraft Deflagration Explosion
http://www.youtube.com/watch?v=W-wlyDQ7xvI
Deflagrations
• Subsonic combustion propagating through
an unburned mixture via thermal/molecular
diffusion.
• Reaction relatively very slow compared to
detonation.
• Most common type of HC explosion*
• Main danger: heat/thermal damage
Molecular (Thermal) Diffusion
• Thermal motion of particles at temperatures above
absolute zero.
Mass flux across
concentration gradient
Flame front
(serves as concentration gradient)
[Reactants]
Low
[Reactants]
High
[Products]
High
[Products]
Low
[ ] denotes concentration
Deflagrations – Flame Speed
• Flame speed, an important concept in the
physics of flames can be understood using
a basic 1-D tube assumption.
[15]
δ – flame thickness
SL – flame speed (laminar)
[15]
Predicting Laminar Flame Speed –
Theoretical Approaches
[21]
Maillard-LeChatelier theory:
Fundamental Burning Velocity:
Predicting Laminar Flame Speed Correlation
Often times correlations or experiments are used for greater accuracy
under specific conditions:
Flame speed correlation for a number of select fuels [7]
Predicting Laminar Flame Speed Correlation
Tref = 298 K
Pref = 1 atm
[7]
Predicting Laminar Flame Speed
Experimental – Case Western Reserve/Univ. of Connecticut
[16]
Predicting Laminar Flame Speed
Experimental – Case Western Reserve/Univ. of Connecticut
[16]
Reality: Turbulence
• Reality  Explosions highly turbulent!
• High turbulence  more surface area (of flame) increased flame
speed.
[7]
Turbulence model developed by Klimov [17] for a turbulent intensity >> 1
[20]
Flammability Limits (U/L-FL/EL)
• Upper and lower flammability/explosive limits
are critical to predicting an explosion.
• ‘flammability’ and ‘explosive’ are used
interchangeably and have the same meaning.
• Explosive limits widened with increasing
temperature.
UEL significantly with P, but pressure has
little effect on LEL.
Explosive Limits (U/L-FL/EL)
[18]
When will a deflagration be an
explosion?
•
•
•
•
•
•
High turbulence improves mixing  increase flame speed
High temperatures  increase flame speed
High pressure induce explosion in confined area
Stoichiometry close to 1.0  produces greatest flame speed
Well mixed F/A mixture rapid propagation
Fuel is introduced rapidly, while allowing the mixture to stay
within explosive limits
• Large volume of mixed F/A for a flame to propagate through
[23]
[24]
Focus of Study
•
•
•
•
Fire/Combustion
Hydrocarbon Deflagrations
Hydrocarbon Detonations
Short Intro to Detonations of High
Explosives
• How to prevent explosions
Detonations
500 ton TNT explosion [22]
Detonations
http://www.youtube.com/watch?v=PgLzgdbfeJE
Detonations
• Strong pressure wave (shock) compresses the
unreacted mixture in front of the reaction front
above its autoignition temperature  abrupt
pressure change in front of the reaction.
• Shock can travel at 5-7 times speed of sound.
• A detonation is a shock wave sustained by the
energy released by this combustion reaction of the
compressed mixture.
• Main danger: Overpressure!
[11]
[1]
Detonation of HC Fuels
• HC fuels hard to detonate!
• Direct detonation heavy HC fuels oxygenenrichment required very high Eign
– For practical use O2 must be stored on board, or a
generation system must be on board.
– Undesirable method for propulsion purposes because
of the added weight and complexity (not to mention
danger of storing pure O2).
Detonation of HC Fuels
• The ignition energy to directly detonate at
STP for practical HC-air stoichiometric
mixtures is on the order of 105 J [29].
– The typical spark plug can provide only 100
mJ of ignition energy!
Detonation of HC Fuels
[29]
Detonation of HC Fuels
• How to successfully detonate?
small-tube pre-detonator
• This also means that a pure detonation of HC
fuels is not likely to happen by accident, like a
deflagration explosion could. Good for our safety!
• But there is another means by which a detonation
could occur, purposefully or by accident…
DDT – Deflagration to Detonation
Conclusion?...direct detonations in HC fuels
NOT LIKELY.
Often times a detonation in HC fuels will be
produced by the transition of a subsonic
flame front from deflagration to detonation
(DDT).
DDT – Deflagration to Detonation
1D closed end tube explanation:
DDT – Deflagration to Detonation
Normal Shock Relations
We know from these relations that as the
F/A mixture crosses the shock, it’s
pressure is increased (compressed) and
the temperature is increased:
Ty
Py
My
Tx
Px
Mx
[32]
DDT – Deflagration to Detonation
Ty
Py
My
Tx
Px
Mx
[32]
As the mixture crosses the shock it
is compressed, heated and
autoignited, fueling the detonation.
DDT in Practice
PDE – Pulsed Detonation Engine
PDE – Pulsed Detonation Engines
• Pulsed detonation engines operate on the principle of
DDT.
• The ‘pulse’ comes from the detonation chamber having
to be cleared, and the F/A mixture renewed, after each
detonation.
• Theoretically can operate from subsonic fight to roughly
Mach 5.
• PDE thermal efficiency > turbofans/turbojets.
• Compressors/turbines are not necessary, weight 
PDE Schematic
Pulsed Detonation Research Facility in the Air
Force Research Laboratory at WPAFB[29]
Schelkin-type spirals
to accelerate flame
speed through
turbulence and flame
mixing [29] to achieve
DDT.
[29]
PDE - Flight
The AFRL
developed and
flew the Long E-Z
aircraft in a low
speed (120 mph)
low altitude flight
(60-100 feet)
producing 200 lbf
thrust.
[32]
Why not in practical use? - PDE
•
•
•
•
Engine pulses need to operate ≈ 1000 Hz.
Requires extremely fast and efficient mixing.
Integration of the inlet and nozzle.
Bulk of research done with gaseous fuels (C1C3) which are easier to detonate.
Focus of Study
•
•
•
•
Fire/Combustion
Hydrocarbon Deflagrations
Hydrocarbon Detonations
Short Intro to Detonations of High
Explosives
• How to prevent explosions
Additional Detonation Info
High Explosives
• Most true high explosives contain the oxygen they need
for burning.
• For instance, the chemical formula for nitroglycerin is:
C3H5(ONO2)3
• Dynamite = sawdust + nitroglycerin!
• Detonation of nitroglycerin can propel a shock at 30
times the speed of sound in air and produce
temperatures as high as 9030°F. [35]
Additional Detonation Info
TNT Equivalence
Common method for equating a known energy of a
combustible fuel to an equivalent mass of TNT:
[9]
mTNT = equivalent mass of TNT (mass)
η = empirical explosion efficiency (unitless)
m = mass of hydrocarbon (mass)
ΔHc = energy of explosion of flammable gas (energy/mass)
ETNT = energy of explosion of TNT (energy/mass)
Additional Detonation Info
Nuclear (Fission) Bombs [27]
• One particular type of nuclear weapon is a nuclear
fission bomb.
• Fission bombs are created by induced fission.
• These free neutrons are used to set off a massive chain
of fission reactions.
[26]
Additional Detonation Info
Nuclear (Fission) Bombs
‘Little Boy’, the bomb dropped on Hiroshima, was this type of nuclear
weapon. It produced an explosion equivalent to 14,500 tons of TNT, at
an efficiency of 1.5%...That means that only 1.5% of the material was
fissioned before being carried away by the explosion. [27]
[28]
Additional Explosion Info
Vapor Cloud Explosions (VCE)
Vapor cloud explosions usually occur through a
series of steps
1.
2.
3.
Sudden release of a large quantity of flammable vapor
Dispersion of mixture throughout building/plant/environment
while mixing with air.
Ignition of the vapor cloud
Additional Explosion Info
Flixborough, England Disaster (VCE)
[33]
Additional Explosion Info
Flixborough, England Disaster (VCE)[34]
• On June 1, 1974 a VCE destroyed a chemical plant in Flixborough,
England killing 28 people and injuring 36 more.
• The plant produced caprolactum, a chemical precursor to nylon, through
the process of oxidizing cyclohexane with air through a series of 6
reactors.
• 2 months prior to the explosion, a crack was discovered in reactor 5. The
company rerouted the the piping with a temporary bypass so that the plant
could still produce, until the crack was properly fixed.
• This bypass pipe ruptured, spewing 40 tons of cyclohexane into the plant
creating a vapor cloud, 100-200 meters in diameter.
• The VC was likely ignited by a nearby furnace and the explosion leveled
the plant, damaging over 1800 buildings within a 1 mile radius.
• The explosion was estimated to be of 15 ton TNT equivalence.
Focus of Study
•
•
•
•
Fire/Combustion
Hydrocarbon Deflagrations
Hydrocarbon Detonations
Short Intro to Detonations of High
Explosives
• How to prevent explosions
Methods for Preventing
Explosions – Inerting[9]
• Reducing the oxygen concentration is an
effective way to prevent explosions.
– Below the described oxygen concentration, the reaction cannot
generate enough energy to heat the unburned gases
adequately.
• Concentration of fuel doesn’t matter when [O2]
too low!
• Strongly dependent upon the inert gas species
(nitrogen in this case)
Limiting Oxygen Concentration (LOC)
[10]
Methods for Preventing
Explosions – Inerting[9]
• Inerting begins initial purge of the empty vessel
with inert gas to safe [O2] level:
– Generally ‘safe’ is considered 4% below the LOC (for
example 6% if the LOC is 10% for a particular fuel).
• Then the flammable material is added to the
vessel.
Methods for Preventing
Explosions – Inerting[9]
An inerting system is required to maintain
the safe oxygen concentration once the fuel
is under use.
Methods for Preventing
Explosions – Static Electricity[9]
• A static charge is the result of physically
separating a poor conductor from a good
conductor or another poor conductor.
• When different materials come into
contact, electrons move across the
interface from one surface to another.
Methods for Preventing
Explosions – Static Electricity[9]
• Physically separating two good
conductors?
• Physically separating good/poor or
poor/poor conductors?
Electrostatic build-up!
Methods for Preventing
Explosions – Static Electricity[9]
• Common producers of static electricity:
– Pumping nonconductive fluid through a pipe, mixing
immiscible liquids, pneumatically conveying solids,
and leaking steam that contacts an ungrounded
conductor, etc.
• A charge on the order of 0.1 mJ is considered
dangerous.
– Static build up of walking across a carpet averages
roughly 20 mJ.
Methods for Preventing
Explosions – Static Electricity[9]
• An electrostatic discharge occurs when
two materials at different potentials come
close enough to produce a charge
transfer.
• This transfer can be energetic enough to
be an ignition source.
How charge
[9]
accumulates
Contact and frictional charging: two materials,
one being an insulator, are brought into contact
and a charge separation occurs at the interface.
If the two objects are then separated, some of the
charges are separated, giving the two materials
equal but opposite charges.
How charge
[9]
accumulates
Double-layer charging: charge separation occurs
on a microscopic scale in a liquid at any interface
(solid-liquid, gas-liquid, or liquid-liquid)…
As the liquid flows, it carries a charge and it
leaves a charge of opposite sign on the other
surface, for example a pipe wall.
How charge
[9]
accumulates
3. Induction: electrons migrate towards the
opposite charge on opposing side of a
vessel, thus accumulating an equal quantity
of charge on the opposite side of the body.
This leaves the other part of the vessel
charged by induction.
How charge
[9]
accumulates
4.Charging by transport: when charged
liquid droplets or solid particles settle on an
isolated object, the object is charged.
The transferred charge is a function of the
objects capacitance and of the conductivities
of the droplet or particle and interface.
How electricity discharges[9]
• A charged object can discharge to a ground
or an oppositely charged object when the
field intensity between the two exceeds 3
MV/m (breakdown voltage of air), or when the
surface reaches a maximum charge density
of 2.7 x 10-5 C/m2.
How electricity discharges
Electricity may discharge by any one of these methods,
each of which produce enough energy to ignite common
HC fuels[9]:
(1) Spark
(2) Propagating brush
(3) Conical pile (Maurer discharge)
(4) Brush
(5) Lightning-like
(6) Corona discharges
Preventing discharges by type
Sparks – prevented by grounding and bonding.
Prevents two metallic objects from having different
potentials.
Conical Pile Discharges – prevented by keeping
nonconductive surfaces or coatings thin enough or
conductive enough such that it has a breakdown
voltage below 4 kV.
Lightning-like Discharges – prevented by keeping the
vessel volume less than 60 m3 or the vessel diameter
less than 3 m.
Preventing electrostatic ignitions
1. Prevent charges from accumulating to dangerous
levels
2. Include charge reductions by means of low energy
discharges
3. When dangerous discharges cannot be
eliminated, then an inertant must be used to
prevent explosions.
Preventing electrostatic ignitions
Relaxation: pumping fluid through a pipe into a
vessel, the separation process produces a
streaming current, which is the basis for charge
buildup in this situation.
Add in enlarged section of pipe before expansion
into vessel.
Preventing electrostatic ignitions
Bonding and Grounding: the voltage difference
between two conductive materials is reduced to
zero by bonding them together.
Eliminate all voltage difference between sets of
bonded materials by bonding all to ground!
Methods for Preventing
Explosions – Sprinklers
• Can help absorb heat, preventing
explosion from growing/gaining energy.
• Can help ‘knock down’ gas clouds.
• Many different types of systems
Summary
Deflagration
Detonation
 Subsonic combustion, driven
by thermal diffusion of heat
and mass.
 Speeds typically well below
100 m/s
 Main danger comes from
thermal damage.
 Standard combustion
reaction, travelling at
accelerated rate.
 Can transition to detonation
under certain conditions.
Often times this is how a HC
fuel will detonate.
 Supersonic combustion, led by a
shock wave.
 Speeds can be in excess of 2000
m/s (5-7 times speed of sound).
 Main danger from overpressure
 Shock compresses F/A mixture.
prior to reaction zone, causing it to
autoignite as it crosses the shock
wave, vigorously fueling the
reaction.
 Strength quickly dies down as
distance from the blast is
increased.
 Type of explosion produced from
high explosives and nuclear
weapons.
Resources
[1] Lilley, D., 2013. Some Fundamentals of Explosions. 51st AIAA Aerospace Sciences Meeting including the New
Horizons Forum and Aerospace Exposition. AIAA-2013-1193.
[2] Eidelman, S., Burcat, A., 1980. Evolution of a Detonation Wave in a Cloud of Fuel Droplets: Part I. Influence of Ignition
Explosion. AIAA Journal. Vol. 18 No. 9 September 1980.
[3] McIntosh, A., 1995. Influence of Pressure Waves on the Initial Development of An Explosion Kernel. AIAA Journal. Vol.
33 No. 9 September 1995.
[4] Kogarko, S., Adushkin, V., Lyamin, A., 1965. Investigation of Spherical Detonation of Gas Mixtures. Combustion,
Explosions, and Shock Waves. Vol. 1 No. 2 1965.
[5] Fletcher, R., 1968. Liquid-Propellant Explosions. AIAA Journal of Spacecraft and Rockets. October 1968, 1227-1229.
[6] Jeng, S.M. Gas Turbine Combustion Lecture Notes, Spring 2013.
[7] Turns, S. 2012. An Introduction to Combustion: Concepts and Applications. 3rd Edition. McGraw-Hill
[8] http://www.youtube.com/watch?v=D-dLjHJuFWQ
Resources Cont.
[9] Crowl, D., Louvar, J., 2011. Chemical Process Safety Fundamentals and Applications. 3rd Edition. Prentice-Hall.
[10] National Fire Protection Association, 1994. Guide for Venting of Deflagrations. Quincy, Mass. American National
Standards Institute.
[11] Clancey, V.J., 1972. Flammability Limits and Burning Velocities of Ammonia/Nitric Oxide Mixtures.
[12] http://en.wikipedia.org/wiki/Combustion, 2/20/13.
[13] http://en.wikipedia.org/wiki/Deflagration, 2/20/13.
[14] http://io9.com/5846759/why-do-some-fires-go-supersonic. 2/20/13.
[15] Cheetham, B. W. Speed of Vertical Pre-mixed Laminar Flame in Varying Propane-air Mixture.
[16] Kumar, K., Sung, C.J, 2009. Combustion Data for Jet-A, its Constituent Components, and Surrogate Mixtures.
Multi-Agency Coordination Committee for Combustion Research – 2009 Fuels Summit. Sept. 17, 2009.
Resources Cont.
[17] Klimov, A. M., “Premixed Turbulent Flames – Interplay of Hydrodynamic and Chemical Phenomena”, in Flames, Lasers, and
Reactive Systems (J. R. Bowen, N. Manson, A. K. Oppenheim, and R. I. Soloukhin, eds.), Progress in Astronautics and
Aeronautics. Vol. 88, American Institute of Aeronautics and Astronautics, New York, pp. 133 – 146. 1983.
[18] Disimile, P., 2013. Aviation Fire Dynamics, Lecture Notes C3, Feb. 2013.
[19] http://www.youtube.com/watch?v=W-wlyDQ7xvI
[20] http://www.scidacreview.org/0703/html/cscads.html
[21] Disimile, P., 2013. Aviation Fire Dynamics, Lecture Notes 4B, Feb. 2013.
[22] http://en.wikipedia.org/wiki/File:TNT_detonation_on_Kahoolawe_Island_during_Operation_Sailoir_Hat,_sjot_Bravo,_1965.jpg
[23] http://theaviationist.com/2010/02/25/exclusive-pictures-thomas-cook-leaking-fuel-after-take-off/#.US5LqqXlRUQ
[24] http://www.militaryphotos.net/forums/showthread.php?73524-Post-your-favorite-Airliners-net-photo
Resources Cont.
[25] http://www.funenclave.com/reality-bites/china-airlines-plane-explosion-11027.html
[26] http://www.atomicarchive.com/Fission/Fission1.shtml
[27] http://science.howstuffworks.com/nuclear-bomb2.htm
[28] http://www.telegraph.co.uk/news/picturegalleries/worldnews/4162716/Photographs-of-Hiroshima-and-Nagasakitaken-by-a-British-serviceman-a-month-after-the-atom-bombs-were-dropped.html
[29] Schauer, F.R., Miser, C.L., Tucker, K.C., Bradley, R.P., Hoke, J.L., 2005. Detonation Initiation of Hydrocarbon-Air
Mixtures in a Pulse Detonated Engine. 43rd AIAA Aerospace Sciences Meeting, Jan. 10-13, 2005.
[30] http://en.wikipedia.org/wiki/Pulse_detonation_engine
[31] http://www.af.mil/news/story.asp?id=123099095
[32] Shapiro, A.H., 1953. The Dynamics and Thermodynamics of Compressible Fluid Flow. Volume 1.
Resources Cont.
[33] http://i.ytimg.com/vi/ABRX34WlluU/0.jpg
[34] http://en.wikipedia.org/wiki/Flixborough_disaster
[35] http://en.wikipedia.org/wiki/Nitro-glycerine
[36] https://ccse.lbl.gov/Research/Combustion/buoy.html
[25]
Questions?