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.” -“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” 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.  • 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. • Fire Point: the lowest temperature at which a vapor above a liquid fuel will continue to burn once ignited; higher than the flash point. Important Definitions • Mechanical Explosion: an explosion resulting from the sudden failure of a vessel containing high-pressure nonreactive gas. • 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. • 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. • This energy may be dissipated by pressure waves, projectiles, thermal radiation, acoustic energy, etc.  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.   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 (AirOxygen). Or more simply:  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  Focus of Study • • • • Fire/Combustion Hydrocarbon Deflagrations Hydrocarbon Detonations Short Intro to Detonations of High Explosives • How to prevent explosions Deflagrations  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.  δ – flame thickness SL – flame speed (laminar)  Predicting Laminar Flame Speed – Theoretical Approaches  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  Predicting Laminar Flame Speed Correlation Tref = 298 K Pref = 1 atm  Predicting Laminar Flame Speed Experimental – Case Western Reserve/Univ. of Connecticut  Predicting Laminar Flame Speed Experimental – Case Western Reserve/Univ. of Connecticut  Reality: Turbulence • Reality Explosions highly turbulent! • High turbulence more surface area (of flame) increased flame speed.  Turbulence model developed by Klimov  for a turbulent intensity >> 1  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)  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   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  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!   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 . – The typical spark plug can provide only 100 mJ of ignition energy! Detonation of HC Fuels  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  DDT – Deflagration to Detonation Ty Py My Tx Px Mx  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 Schelkin-type spirals to accelerate flame speed through turbulence and flame mixing  to achieve DDT.  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.  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.  Additional Detonation Info TNT Equivalence Common method for equating a known energy of a combustible fuel to an equivalent mass of TNT:  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  • 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.  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.   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)  Additional Explosion Info Flixborough, England Disaster (VCE) • 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 • 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)  Methods for Preventing Explosions – Inerting • 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 An inerting system is required to maintain the safe oxygen concentration once the fuel is under use. Methods for Preventing Explosions – Static Electricity • 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 • Physically separating two good conductors? • Physically separating good/poor or poor/poor conductors? Electrostatic build-up! Methods for Preventing Explosions – Static Electricity • 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 • 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  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  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  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  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 • 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: (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  Lilley, D., 2013. Some Fundamentals of Explosions. 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. AIAA-2013-1193.  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.  McIntosh, A., 1995. Influence of Pressure Waves on the Initial Development of An Explosion Kernel. AIAA Journal. Vol. 33 No. 9 September 1995.  Kogarko, S., Adushkin, V., Lyamin, A., 1965. Investigation of Spherical Detonation of Gas Mixtures. Combustion, Explosions, and Shock Waves. Vol. 1 No. 2 1965.  Fletcher, R., 1968. Liquid-Propellant Explosions. AIAA Journal of Spacecraft and Rockets. October 1968, 1227-1229.  Jeng, S.M. Gas Turbine Combustion Lecture Notes, Spring 2013.  Turns, S. 2012. An Introduction to Combustion: Concepts and Applications. 3rd Edition. McGraw-Hill  http://www.youtube.com/watch?v=D-dLjHJuFWQ Resources Cont.  Crowl, D., Louvar, J., 2011. Chemical Process Safety Fundamentals and Applications. 3rd Edition. Prentice-Hall.  National Fire Protection Association, 1994. Guide for Venting of Deflagrations. Quincy, Mass. American National Standards Institute.  Clancey, V.J., 1972. Flammability Limits and Burning Velocities of Ammonia/Nitric Oxide Mixtures.  http://en.wikipedia.org/wiki/Combustion, 2/20/13.  http://en.wikipedia.org/wiki/Deflagration, 2/20/13.  http://io9.com/5846759/why-do-some-fires-go-supersonic. 2/20/13.  Cheetham, B. W. Speed of Vertical Pre-mixed Laminar Flame in Varying Propane-air Mixture.  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.  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.  Disimile, P., 2013. Aviation Fire Dynamics, Lecture Notes C3, Feb. 2013.  http://www.youtube.com/watch?v=W-wlyDQ7xvI  http://www.scidacreview.org/0703/html/cscads.html  Disimile, P., 2013. Aviation Fire Dynamics, Lecture Notes 4B, Feb. 2013.  http://en.wikipedia.org/wiki/File:TNT_detonation_on_Kahoolawe_Island_during_Operation_Sailoir_Hat,_sjot_Bravo,_1965.jpg  http://theaviationist.com/2010/02/25/exclusive-pictures-thomas-cook-leaking-fuel-after-take-off/#.US5LqqXlRUQ  http://www.militaryphotos.net/forums/showthread.php?73524-Post-your-favorite-Airliners-net-photo Resources Cont.  http://www.funenclave.com/reality-bites/china-airlines-plane-explosion-11027.html  http://www.atomicarchive.com/Fission/Fission1.shtml  http://science.howstuffworks.com/nuclear-bomb2.htm  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  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.  http://en.wikipedia.org/wiki/Pulse_detonation_engine  http://www.af.mil/news/story.asp?id=123099095  Shapiro, A.H., 1953. The Dynamics and Thermodynamics of Compressible Fluid Flow. Volume 1. Resources Cont.  http://i.ytimg.com/vi/ABRX34WlluU/0.jpg  http://en.wikipedia.org/wiki/Flixborough_disaster  http://en.wikipedia.org/wiki/Nitro-glycerine  https://ccse.lbl.gov/Research/Combustion/buoy.html  Questions?