Explosion of Hydrocarbon Fuels

1. Explosion of Hydrocarbon Fuels Aviation Fire Dynamics – Spring 2013 Final Presentation Derick Endicott

2. What is an ‘explosion’? • An explosion can be defined as: -“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] • -“the sudden conversion of potential (chemical in this case) into kinetic energy including the production and release of gases under pressure.”[1]

3. 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.

4. 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]

5. 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.

6. 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)

7. 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]

8. 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.

9. [6]

10. [9]

11. Detonation vs. Deflagration Applied – PDE http://www.youtube.com/watch?v=D-dLjHJuFWQ

12. 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

13. 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.

14. Focus of Study • Fire/Combustion • Hydrocarbon Deflagrations • Hydrocarbon Detonations • Short Intro to Detonations of high explosives • How to prevent explosions

15. Combustion Combustion is the exothermic oxidation reaction between a fuel (HC) and an oxidizer (AirOxygen). C and H easily break current bonds (when heated) and form new molecules with O + HEAT….this is how a fire thrives. Or more simply: [12]

16. 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.

17. 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.

18. Flame Buoyancy Effects [36]

19. Focus of Study • Fire/Combustion • Hydrocarbon Deflagrations • Hydrocarbon Detonations • Short Intro to Detonations of High Explosives • How to prevent explosions

20. Deflagrations [13]

21. Aircraft Deflagration Explosion http://www.youtube.com/watch?v=W-wlyDQ7xvI

22. 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

23. Molecular (Thermal) Diffusion • Thermal motion of particles at temperatures above absolute zero. Flame front (serves as concentration gradient) Mass flux across concentration gradient [Reactants] High [Reactants] Low Unburnt F/A Mixture (Cold) CombustionProducts (Hot) [Products] High [Products] Low [ ] denotes concentration

24. Deflagrations – Flame Speed • Flame speed, an important concept in the physics of flames can be understood using a basic 1-D tube assumption. [15]

25. δ – flame thickness SL – flame speed (laminar) δ [15] SL SL P T α Φ ~ 1 δ P T α Φ~ 1

26. Predicting Laminar Flame Speed – Theoretical Approaches[21] Maillard-LeChatelier theory: Fundamental Burning Velocity:

27. 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]

28. Predicting Laminar Flame Speed - Correlation Tref = 298 K Pref = 1 atm [7]

29. Predicting Laminar Flame SpeedExperimental – Case Western Reserve/Univ. of Connecticut [16]

30. Predicting Laminar Flame SpeedExperimental – Case Western Reserve/Univ. of Connecticut [16]

31. Reality: Turbulence Turbulent intensity • 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

32. [20]

33. 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. Important for confined explosions

34. Explosive Limits (U/L-FL/EL) [18]

35. 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

36. [23] • High turbulence • Fuel introduced at high rate • Intense F/A mixing • If damaged possible ignition • sources exist • Proper EL exists somewhere • Temperature/Pressure vary by • situation

37. [24]

38. Focus of Study • Fire/Combustion • Hydrocarbon Deflagrations • Hydrocarbon Detonations • Short Intro to Detonations of High Explosives • How to prevent explosions

39. Detonations500 ton TNT explosion [22]

40. Detonations http://www.youtube.com/watch?v=PgLzgdbfeJE

41. 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!

42. [11]

43. [1]

44. Detonation of HC Fuels • HC fuels hard to detonate! • Direct detonation heavy HC fuels oxygen-enrichment 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).

45. 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!

46. Detonation of HC Fuels [29]

47. 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…

48. 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).

49. DDT – Deflagration to Detonation Once flame front speed exceeds The sonic velocity in the Unburned F/A mixture, a normal shock will develop. 1D closed end tube explanation: Build up of temperature and pressure Accelerating the flame front T P (F/A) unburned Combustion products

50. 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: T P Tx Px Mx Ty Py My [32]