1. AEEM7063 AVIATION FIRE DYNAMICS CHALLENGES WITH HYDROGEN FIRES AND THEIR SUPPRESSION

2. INTRODUCTION

3. HYDROGEN: BACKGROUND INFORMATION
Hydrogen is the lightest element and its monatomic form (H1) is the most abundant chemical substance.
At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H2.
Hydrogen readily forms covalent compounds with most non-metallic elements and is present in the water molecule and in most organic compounds. So, naturally occurring atomic hydrogen is rare on earth.
Hydrogen gas produces water when burned, a property which later gave it its name (in Greek, hydrogen means "water-former“)

4. Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form.
In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquids. 
Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement leading to cracks and explosions.
Ideally, no oxygen should be present in the liquid hydrogen tanks in a fuel cell vehicle, but trace amounts of air may contaminate the hydrogen supply.

5. HYDROGEN COMBUSTION
Hydrogen is highly flammable and requires only a small amount of energy to ignite
Hydrogen gas forms explosive mixtures with air and with chlorine.
Hydrogen requires only one 10th as much energy to ignite as gasoline does.
A spark of static electricity from a person's finger is enough to set it off.

6. EXPOSIVE LIMITS OF COMBUSTIBLE GASES WHEN MIXED WITH AIR

7. With 4%-75% hydrogen, very little energy is required to ignite hydrogen. The speed of a hydrogen fire is ten feet per second to ft./sec
4% Barely burn and even then only upward
5% Fire starts burning sideways
18% Fire will now burn faster than the speed of sound
45% Burning at 6750 ft./sec -including shock wave
74% Barely burn
75% Will not burn-no air

8. Hydrogen gas leaking into external air may spontaneously ignite
Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns
If leaking from a pipe at a high enough pressure, hydrogen gas can self- ignite without the aid of an external energy source
Hydrogen has a much wider flammability range and detonability limits compared to most hydrocarbons
A hydrogen flame poses special dangers beyond those posed by hydrocarbon flames because human senses cannot easily detect it.

10. If you come upon a hydrogen fire, you will not see it – even up close
If you come upon a hydrogen fire, you will not see it – even up close. You might see sparkles, dust particles briefly burning
You won’t feel the heat of the hydrogen fire because very little infrared radiation (which gives us the sensation of heat) is present
The physical flame is stronger than a hydrocarbon flame.
The mixtures spontaneously explode by spark, heat or sunlight.
Hydrogen flames can burn in a strong wind and be stretched out away from its source a number of feet
HYDROGEN COMBUSTION

11. DISASTERS!!

12. THE HINDENBURG FIRE
Hindenburg disaster took place on Thursday, May 6, 1937
German passenger airship LZ 29 Hindenburg caught fire and was destroyed during its attempt to dock
Of the 97 people on board (36 passengers, 61 crew), there were 35 fatalities; there was also one death among the ground crew
The actual cause of the fire remains unknown, although a variety of hypotheses have been put forward for both the cause of ignition and the initial fuel for the ensuing fire

13. STATIC SPARK HYPOTHESIS
SABOTAGE HYPOTHESIS
STATIC SPARK HYPOTHESIS
LIGHTNING HYPOTHESIS
ENGINE FAILURE HYPOTHESIS
INCENDIARY PAINT HYPOTHESIS
PUNCTURE HYPOTHESIS
STRUCTURAL FAILURE
FUEL LEAK

14. HYDROGEN FIRE HYPOTHESIS
Initial thoughts:
Paint was both the point of ignition and the sole cause of the rapid spread of the fire.
“It did not look like a hydrogen fire”
The hydrogen within the airship is held blameless for both the start of the fire and its subsequent advance
The chemical composition of the Hindenburg paint is known, and it burns too slowly.
Offering support for the hypothesis that there was some sort of hydrogen leak prior to the fire is that the airship remained stern-heavy before landing.

15. OTHER HYPOTHESIS
Hindenburg's skin was covered with the extremely flammable cellulose nitrate or cellulose acetate, added to help with rigidity and aerodynamics.
The skin was also coated with flecks of aluminum, a component of rocket fuel, to reflect sunlight and keep the hydrogen from heating and expanding.
These substances caught fire from an electric spark that caused the skin to burn.
At this point the hydrogen became the fuel to the already existing fire

16. HINDENBURG DISASTER

17. CHALLENGER DISASTER
The Challenger flew nine successful missions before that fateful day of the disaster in 1986
The Challenger was scheduled to carry some cargo, the Tracking Data Relay Satellite-2, as well as fly the Shuttle-Pointed Tool for Astronomy (SPARTAN- 203)
The Challenger had 7 crew members and it was the first flight of a new program called TISP (Teacher In Space Program).
Seventy three seconds into the mission, the Challenger exploded, killing the entire crew.

18. The commission's report cited the cause of the disaster as a the failure of an “O-ring” seal in the solid-fuel rocket.
The faulty design of the seal coupled with the unusually cold weather, let hot gases to leak through the joint.
Booster rocket flames were able to pass through the failed seal enlarging the small hole. These flames then burned through the Space Shuttle Challenger's external fuel tank and through one of the supports that attached the booster to the side of the tank.
That booster broke loose and collided with the tank, piercing the tank's side.
Liquid hydrogen and liquid oxygen fuels from the tank and booster mixed and ignited, causing the Space Shuttle Challenger to tear apart.

19. CHALLENGER DISASTER

20. MORE ON CHALLENGER DISASTER
MORE DISASTERS

21. WHY HYDROGEN SAFETY RESEARCH?

22. RESEARCH IN FIRE SAFETY FOR HYDROGEN-FUELED VEHICLES
Hydrogen is currently produced on an industrial scale of about 10 million tonnes annually
By 2040, it is anticipated that the use of hydrogen in fuel cell powered vehicles could replace the consumption of petroleum.
To be economically competitive with the present fossil fuel economy, the cost of fuel cells must be lowered by a factor of 10 or more and the cost of producing hydrogen must be lowered by a factor of 4.
Moreover, the performance and reliability of hydrogen technology for transportation and other uses must be improved dramatically. The role of hydrogen safety to achieve it can not be underestimated.

23. For automotive applications it is expected that the level of safety at the consumer interface with hydrogen is similar or exceeds that present with natural gas usage.
Hydrogen safety research was initiated decades ago as a result of accidents in the process industries
CFD tools are used to study the phenomena of hydrogen combustion, deflagration-to-detonation transition and hazard and risk assessment
In spite of recent progress there are still some unresolved issues of hydrogen

24. SLOW SUBSONIC LEAKS PERMEATION
Characteristic for connections and holes in delivery pipes.
The main area of concern is private and maintenance garages, car parks, and other automotive infrastructures
PERMEATION
Diffusion of high pressure hydrogen through storage vessel materials to surrounding space.
Permeation is a slow molecular flow regime through the surface material and must be modeled as diffusion through a solid rather than a flow.
The process of permeation involves the absorption and dissociation of molecular hydrogen to atomic hydrogen on the internal surface

25. HIGH-MOMENTUM LARGE-VOLUME RELEASE
Then atomic hydrogen diffuse through the metal to the opposite surface where atoms recombine and undergo desorption
HIGH-MOMENTUM LARGE-VOLUME RELEASE
Characteristic for accidental breaks of high-pressure tanks or activation of pressure relief device (PRD), e.g. by external fire
HYDROGEN-JET FLAMES
Hydrogen discharges through circular orifices larger than a critical diameter sustain stable lifted flames irrespective of the reservoir pressure driving the release.
At smaller diameters, however, stable burning will only be achieved at operating pressures higher than a particular value.

26. LIFT-OFF VELOCITY, LIFT-OFF HEIGHT, AND BLOW-OUT VELOCITY
Flame stability is usually characterized by lift-off velocity, lift-off height, and blow-out velocity.
LIFT-OFF VELOCITY
Defined as the mean jet velocity at which the flame becomes lifted above the jet exit rim.
If the jet velocity is further increased, the flame moves downstream to a position where it stabilizes.
LIFT-OFF HEIGHT
Distance between the lifted flame base and the jet exit.

27. BLOW-OUT VELOCITY
Jet velocity at which the reaction cannot be sustained and the flame is extinguished.
For positions between the jet exit and the flame base, a turbulent mixing region exists.
The stability limits of turbulent jet diffusion flames are important for operation of combustion systems and have safety implication for handling combustible fuels

28. EFFECTS OF THESE FACTORS ON HYDROGEN
Hydrogen has the much higher turbulent blow-off velocity in comparison to that of hydrocarbons.
This permits turbulent diffusion hydrogen flames to exist at velocities far in excess to those possible for turbulent diffusion hydrocarbon flames in air.
The flame lift-off height of the pure hydrogen jet diffusion flame was found to increase with the jet exit velocity.
The lift-off length and blow-off velocity have strong implications in safety considerations and hazard/risk assessment.

29. DEFLAGRATIONS AND DETONATIONS
There are two types of explosions:
DEFLAGRATIONS
Characterized by a flame propagation velocity below the speed of sound
Typically far below 100 m/s, and relatively modest overpressures, say below 0.5 bar.
 Subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it.
Most fire found in daily life, from flames to explosions, is deflagration

30. DETONATION
 Involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it.
flame propagation velocities, perhaps up to 2000 m/s, and substantial overpressures, up to 20 bars
Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases.
The velocity of detonations in solid and liquid explosives is much higher than that in gaseous ones, which allows the wave system to be observed with greater detail

31. The ability of a hydrogen-air mixture to detonate is greater than that of hydrocarbons.
For a stoichiometric hydrogen-air mixture, detonation once initiated will propagate as long as the mixture is within the detonability limits.
The direct initiation of hydrogen-air mixture detonation is possible by just 1.1 g of tetryl.
Only 1.86 g of TNT is needed to initiate detonation in a 34.7% hydrogen-air mixture in the open atmosphere.
For a 20% hydrogen-air mixture, however, the critical charge increases significantly to 190 g of TNT.

32. DEFLAGRATION-TO-DETONATION TRANSITION (DDT)
Geometrical conditions such as partial confinement and obstacles in the flame path may result in a subsonic flame to accelerate from subsonic speed to supersonic speed, transitioning from deflagration to detonation.
The exact mechanism is not fully understood
Existing theories are able to explain and model both deflagrations and detonations but there are no theories at present which can predict the transition phenomenon

33. DDT OF HYDROGEN
A specific feature of hydrogen-air deflagrations compared to of the majority of hydrocarbon-air deflagrations is the readiness to DDT
This occurs when mitigating confined deflagrations by the venting technique.
A needle-like structured flame front can be induced by the venting.
The onset of detonation is directly linked to the needle-like structured flame front formed by sudden venting.

34. HYDROGEN SAFETY ASSESSMENT IN AIRCRAFT AND GROUND VEHICLE

35. HYDROGEN FUELLED AIRCRAFT
An average airline passenger would opt for the aircraft fueled with Jet A.
Aircraft fueled with liquid hydrogen (LH2) would be declared unsafe.
Mainly because of Hindenburg fire disaster
Hazard due to hydrogen leaks : Liquid <-> vapor
Greater ignitability of hydrogen relative to jet fuel
“Flameless detonations“ : pressure pulses and fire balls
NASA has sponsored a number of studies to explore the potential of LH2, liquid methane (LCH4) and synthetic aviation-grade jet fuel (synjet) as alternatives to petroleum derived Jet A for use in future transport aircraft
It is highly available, less costly, produces high energy and less noise

36. PASSENGERS WILL BE EXPECTED TO SURVIVE THE IMPACT BECAUSE:
1) LH2 tanks are less apt to be ruptured, thus minimizing spillage potential :
It is important to minimize the surface area-to-volume ratios of these tanks
To minimize the weight of the tank and insulation system
To decrease the opportunity for heat to leak into the cold fuel and cause it to vaporize
Therefore it is preferable to locate the tanks for the cryogenic fuels in the fuselage for a hydrogen-fueled airplane.
This arrangement has been shown to provide lowest direct operating cost, a realistic measure of aircraft operating efficiency as well

37. LH2 tanks are less susceptible to damage:
In conventional aircrafts, fuel is tanked in the wing between the front and rear spars
LH2 tanks are less susceptible to damage:
a) They are mounted in the fuselage where they present a far smaller dimension to frontal impact.
b) They are protected by a significant amount of structure, both ahead of and beneath, which must be crushed or penetrated before the tanks are exposed.
c) They are designed for higher pressure than the rest of the fuselage and therefore are less apt to be the point of failure in an impact survivable crash.

38. In the event a tank is ruptured and fuel is spilled, LH2 evaporates almost immediately, and it dissipates into the atmosphere so rapidly. So the time and the area exposed to flammable mixtures are both quite small compared to the other candidate fuels
Two parts:
An assessment of the spreading and vaporization of the spilled liquid:
Results show that LH2 spreads the least and evaporates the quickest in all
The behavior of the fuel vapor cloud after vaporization has occurred:
Hydrogen rises rapidly in every case and therefore poses minimum risk to areas surrounding a spill. The other fuels are non-buoyant for extended periods and flammable clouds are blown along the ground

39. 3) If the spilled fuel is ignited, the duration of the hydrogen fire would be so brief that it would not heat the fuselage to the point of collapse, as would be the case with the other fuels, and the heat-affected area would be substantially smaller:
The fuselage should collapse after ~40 s in a methane flame, after ~ 50 s in hydrogen flame and after ~120 s in a JP-4 or Jet A flame.
When these fuselage collapse times are compared with the calculated burning times of the fuel spills for the worst survivable case it is seen that LH2 will pose no problem.
The pool fire would last only 15 s.
For LCH4 the pool fire would last 38 s, uncomfortably close to the 40 s calculated for fuselage collapse.
Although the pool burning times were not calculated for JP-4 and Jet A, it is obvious from accident experience that fuselage collapse would occur in event those fuels were used.

40. CONCLUSION
LH2 was found to be a safer fuel than any of the other candidates.
The passengers on board, and people and property in the immediate surroundings, would be exposed to less hazard if a crashed aircraft were fueled with LH2.

41. BLAST WAVES AND FIREBALLS GENERATED BY HYDROGEN FUEL TANK RUPTURE
This paper describes and analyzes the results of Type 4 hydrogen fuel tank fire exposure tests.
Project explored fire safety issues with an on-board hydrogen storage tank
Aimed at replacing compressed natural gas containers with hydrogen fuel tank.

42. TYPE-4 HYDROGEN CYLINDER RUPTURE
Comprised mainly of a high-density polyethylene inner liner, a carbon fiber structural layer, followed by a fiberglass protective layer
Composite material on the surface of the tank ignited approximately 45 seconds after the bonfire exposure.
After 6 minutes and 27 seconds, the cylinder failed catastrophically through the bottom, launching the 30.9 lb main portion 270 ft. from the test location.
Blast pressures were 300 KPa at 6.3ft. and 41 kPa at 21.3 ft.

43. CONCLUSIONS
A typical compressed hydrogen tank, when exposed to a bonfire, presents safety challenges.
The consequence of a rupture is catastrophic.
In the test, blast pressures of 6 psi were measured 21 ft. away from the tank, and debris was propelled more than 250 ft.
The test results suggest that bonfire protection and pressure relief sensing for Type 4 hydrogen tank will require some sophistication.

44. HYDROGEN SAFETY MEASURES

45. VENTILATION
Ventilation is useful for preventing the formation of explosive atmospheres when hydrogen is dealt with in confined space.
The basic principle of ventilation is to bring fresh outside air inside the room and remove the inside air. The main advantages of using ventilation in hydrogen systems are:
It prevents the accumulation of explosive gas inside the room.
It dilutes the explosive gas by bringing fresh air in.
It limits the resident time of explosive atmospheres.

46. Ventilation can be of two types:
Forced (mechanical)
Natural.
Selection of appropriate type depends on many factors.
Natural ventilation functions due to temperature differences and winds.
Hydrogen buoyancy and diffusion characteristics should be taken into account while selection the size of opening and their positions.
Although natural ventilation is a cheaper option than the mechanical ventilation, it is less controllable and is affected by weather conditions.

47. On the other hand mechanical ventilation can give constant and controlled flow.
The sizing of mechanical ventilation is not a big concern. It can be sized based on the previous experiences, based on calculations or based on hydrogen leak detection sensitivity.
The main disadvantage of this type of ventilation is that the gas cloud and air will be more turbulent and in this case if the gas cloud is ignited then the explosions will be more severe.
Using a combination of natural and mechanical ventilation can be a solution in some cases.

48. AUTOMATIC SHUTDOWN SYSTEM
INERTING :
It is defined as the replacement of a sufficient proportion of oxygen contained in a gaseous atmosphere by an inert gas, to avoid the ignition of atmosphere.
It is an important way to prevent the formation of explosive atmosphere.
However, it can be dangerous for the workers due to the asphyxiating property of inert gas.
The main inerting methods include:
Pressure swing method
Volume swing method
Flow-through method
Displacement method.

49. SCHEMATIC OF HYDROGEN-AIR-INERT MIXTURES:
The conditions of a hydrogenated atmosphere to be subjected to inerting can be derived from the triangular diagram
The three colored zones in the diagram represent:
Red zone: It corresponds to the explosion area
Orange zone: It corresponds to the mixtures which are not explosive but which may explode if they are mixed with air.
Green zone: It corresponds to mixtures which are not explosive and which will not explode if they are mixed with air.

50. Recombiners can be classified as:
A recombiner is a device that promotes the recombination of hydrogen and oxygen to form water.
It serves to avoid, remove or at least slow down the formation of flammable mixtures formed due to leakage of hydrogen.
Recombiners can be classified as:
Active devices:
Use heat to initiate conversion
Passive devices:
Use the property of hydrogen that, in presence of catalysts like platinum and palladium hydrogen reacts with oxygen at low temperatures in an exothermic reaction.
While using recombiners appropriate measures should be taken to keep the system temperatures below the self-ignition temperature.

51. CONTROL OF IGNITION SOURCES
Electrical equipments, hot surfaces, open flames, static electricity are the possible ways of ignition of hydrogen and propagation of the flame. By controlling these sources explosions can be avoided

52. DETECTION MEASURES DETECTION OF HYDROGEN LEAKS
The major hazards due to an unwanted release of hydrogen are connected with the possibility of building up explosive conditions.
Hydrogen is more dangerous due to its high flammability and low ignition energy. Thus appropriate detection measures should always be taken into consideration.
The main criteria for selecting a hydrogen sensor are listed below.
Response time
Detection range
Durability
Calibration / Maintenance
Cross sensitivity
Area coverage

54. DETECTION OF HYDROGEN FLAMES
Hydrogen burns with a pale blue flame and does not emit visible light or smoke. Hydrogen flames emit significantly less heat.
Thus it is difficult to detect the flames by human perception. Therefore hydrogen fire detectors are necessary.
Special imaging systems are required for determining the size and location of the flame. Many times it is difficult to differentiate hydrogen related signals and the parasitic signals. Thus to prevent wrong alarms human analysis may be advantageous.
The hydrogen fire detectors should posses following main characteristics.
Detect hydrogen flame at sufficient distance
Detect small flames

56. SAFETY MEASURES MITIGATION MEASURES
In case of fires the supply of hydrogen should be shut off as soon as possible since the hydrogen fires are not extinguished due to the danger of re-ignition or explosion.
The adjacent metal surfaces should be cooled by water or other means as they can be the possible source of re-ignition.
Water can be used as a fire extinguisher. Small fires can be extinguished by dry chemical extinguishers or with carbon dioxide, nitrogen, and steam.

57. ACTIVE INERTING, SUPPRESSION AND ISOLATION SYSTEMS
Inerting as described before is used for dilution of explosive atmosphere to prevent ignition.
In situations where no human activity is involved all air can be replaced by inert gas.
Typical inert gases used are nitrogen, carbon-di-oxide and some special mixtures, for example Inergen (mainly Ar and N2, some CO2), Argonite (Ar, N2).
A chemical suppression or isolation system may also be used in quenching the flame. But they pose more challenges. The flames may propagate faster due to the turbulence created by suppression systems.
In isolation systems, early detection of fire and fast isolation is necessary

58. WATER BASED PROTECTION SYSTEMS
Water can be used to extinguish the fire. Water spray system can be provided for hydrogen storage containers, grouped piping, and pumps where potential fire hazard exists.
The system should deliver a uniform spray pattern over 100 percent surface of the container, pump and adjacent piping.
EMERGENCY RESPONSE MEASURES
The emergency response plan should be based on systematic identification of hazards, followed by evaluation risk management.
The aim should be minimization of risk to people. Communication is a key element in any emergency response plan.

59. DETERMINING THE SAFETY DISTANCE
Effective communication will involve technical measures, organization, procedures and training adapted to each other and to the overall strategy.
If communication fails, effective emergency response is not possible. Effective emergency response will also require an organization intended and prepared for emergency response.
DETERMINING THE SAFETY DISTANCE
The location and distance of hydrogen storage from people and other facilities should be decided in such a way that the effects of fire, explosion and detonation, if occurs, can be reduced to tolerable levels.
While selecting a location the effects of possible combustible cloud drift and thermal radiations should be considered.
The hydrogen storage facilities should be located outdoors or in a building specially built for the purpose.

60. CONCLUSIONS

61. Hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas.
One cannot see hydrogen fire even up close and cannot feel the heat of the hydrogen fire but the physical flame is stronger than a hydrocarbon flame.
Hindenburg airship and challenger space shuttle disaster : Hydrogen fire disasters
LH2 was found to be a safer fuel than any of the other candidates.
Type 4 hydrogen tank will require some sophistication.
Prevent the hydrogen from escaping by following good process maintenance practices

62. In case of leak the area should be well ventilated to prevent hydrogen build up.
Gas detectors and flame detectors can quickly identify a gas leak or the resulting flame.
Combustible gas detection technologies:
Infrared
catalytic bead
Flame detection technologies:
Thermal detectors
UV flame detectors
Multi-spectrum IR flame detectors
Chemical suppression system and venting method should be handled with care.

63. REFERENCES

64. Blast waves and fireballs generated by hydrogen fuel tank rupture during fire exposure” : Robert Zalosh, Firexplo, Wellesley
“Large scale experiments to study fires following the rupture of high pressure pipelines conveying natural gas and natural gas/hydrogen mixtures”: B.J. Lowesmith, G. Hankinson
“An Assessment of the Safety of Hydrogen-Fueled Aircraft”: G. Daniel Brewer
“Research in Fire Safety for Hydrogen-Fueled Vehicles”: Kennerly H. Digges, R. Rhoads Stephenson
“Handling and safety of Hydrogen”: Itsuki Uehara
The Hindenburg Fire: Hydrogen or Incendiary Paint?”: A. J. Dessler, D. E. Overs, and W. H. Appleby
“Hydrogen safety research”: Molkov, V.
“Hydrogen Plant Explosion and Fire”
“Twenty Hydrogen Myths” : Amory Lovins

65. Analysis of Published Hydrogen Vehicle Safety Research” : U. S
Analysis of Published Hydrogen Vehicle Safety Research” : U.S. Department of Transportation “A numerical study of hydrogen or helium release and mixing in partially confined spaces” : K. Prasad, W. M. Pitts and J. C. Yang

66. QUESTIONS