Flammability Characteristics of JP-8 Fuel Vapors Existing Within a Typical Aircraft Fuel Tank

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Flammability Characteristics of JP-8 Fuel Vapors
Existing Within a Typical Aircraft Fuel Tank

• Steven M. Summer
• Department of Mechanical Aerospace Engg.
• Masters Thesis Defense
• December 21, 2000
• Faculty Advisor Prof. C. E. Polymeropoulos

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Overview of Problem

• Threat of ignition of fuel vapors within aircraft
  fuel tanks
• Has long been noted, but until recently, not much
  data
• Several protection systems have been researched
  and proposed, but none implemented in commercial
  aircraft

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Overview of Problem

• July 1996, TWA 800 crashes over East Moriches, NY
• NTSB cites an in-flight fuel tank explosion as
  cause
• Numerous research projects undertaken by CIT,
  UNR, ASU, SWRI and others
• Overall goal generate enough data on aviation
  fuel vapor generation/flammability to be able to
  develop a means of protecting against ignition

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Overview of Problem Aircraft Fuel Tanks

• Fuel is typically is stored in two wing tanks
• Larger aircraft also use a Center Wing Tank (CWT)
  located within fuselage

Definition Fuel Mass Loading - (Mass
of Liquid Fuel)/(Total Internal Tank Volume)
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Overview of Problem Aircraft Fuel Tanks

• In some cases, located directly underneath CWT is
  the Environmental Conditioning System (ECS)
• Hot bleed air from the ECS heats CWT fuel,
  resulting in an increase of the FAR
• ARACs FTHWG determined that these tanks are at
  risk 30 of the total flight time compared to 5
  for CWTs without ECS

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Overview of ProblemAviation Fuel

• Specifications for commercial grade fuel (Jet
  A/Jet A-1 Jet B) set forth by ASTM D1655
• Sets min/max values for things such as flash
  point, boiling point, freezing point, etc.
• Very vague criteria for actual composition of the
  fuel

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Overview of ProblemAviation Fuel

• These fuels shall consist of refined
  hydrocarbons derived from conventional sources
  including crude oil, natural gas liquids, heavy
  oil, and tar sands
• -ASTM D1655

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Summary of Problem

• CWTs with adjacent heat sources (ECS)
• Increases rate of fuel vapor generation
• Typically small amount of fuel in CWT
• Reduced impact on flammability because of
  increased evaporation of light ends
• Lack of a definitive composition of aviation
  fuels
• Leads to fuels consisting of hundreds of
  hydrocarbons, with varying properties

Result Fuel Tank Flammability Potential is
Increased Throughout Flight Profile
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Objectives

• Heated Fuel Vapor Testing
• Determine the effects of
• fuel mass loading,
• liquid fuel evaporative surface area and
• residual fuel on tank walls and
• on ullage vapor generation within an aircraft
  fuel tank environment

Definition Ullage - the unused internal portion
of the fuel tank
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Objectives

• Heated Fuel Vapor Testing With Tank Wall
  Cooling
• Determine the effects of cold tank wall
  temperatures on ullage vapor generation within an
  aircraft fuel tank environment

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Objectives

• Lower Oxygen Limit of Flammability Testing
• Determine the lowest oxygen level within the tank
  that will support ignition of the ullage fuel
  vapors (i.e. LOLF)

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Heated Fuel VaporTesting Objectives

• Determine the effects of
• fuel mass loading,
• liquid fuel evaporative surface area and
• residual fuel on tank walls and
• on ullage vapor generation within an aircraft
  fuel tank environment

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Heated Fuel VaporTesting Apparatus

• 88.21 ft3 vented, aluminum fuel tank
• 14 K-type thermocouples
• 1 Fuel
• 5 Surface (3 wall, 2 ceiling)
• 5 Ullage
• 2 hydrocarbon sample ports
• 150,000-Btu kerosene air heater
• Several sized fuel pans
• 1? x 1 ?, 2 ? x 2 ? and one covering tank bottom

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Heated Fuel VaporTesting Procedures

• Fuel measured and poured into fuel pan
• Fuel pan placed into tank
• Tank door sealed
• Kerosene air heater turned on
• Fuel heated to 10 above flash point (125 F)
• Hydrocarbon concentration monitored until
  equilibrium is reached

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Mass Loading Results
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Mass Loading Results
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Mass Loading Results
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Evaporative Surface Area Results
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Evaporative Surface Area Results
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Residual Fuel Results
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Residual Fuel Results
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Tank Wall CoolingObjectives

• Determine the effects of cold tank wall
  temperatures on ullage vapor generation within an
  aircraft fuel tank environment

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Tank Wall CoolingApparatus

• Same tank as Heated Fuel Vapor Testing with some
  modifications
• 3-in. shell surrounded the two side and rear
  walls for CO2 cooling
• Kerosene air heater replaced with a
  thermostatically controlled hot plate

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Tank Wall CoolingProcedures

• Fuel measured (1.5 gallons) and poured into fuel
  pan
• Fuel pan placed into tank tank door sealed
• Hot plate turned on
• Fuel heated to 10 above flash point (125 F) and
  maintained for 2 hours
• Walls were cooled to desired temperatures and
  maintained until significant decrease in HC
  concentration was observed

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Tank Wall Cooling Results
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LOLF TestingObjectives

• Determine the lowest oxygen level within the tank
  that would support ignition (i.e. the lower
  oxygen limit of flammability)

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LOLF Testing Apparatus

• 9 ft3 vented, aluminum fuel tank placed inside of
  10 m3 pressure vessel equipped with
• 12 K-type thermocouples
• 1 Fuel
• 7 Surface (3 floor, 1 on each side wall)
• 4 Ullage
• 9.5" x 9.5" fuel pan located in center of tank
• Thermostatically controlled hot plate
• 6" diameter mixing fan
• 2 hydrocarbon sample ports
• 1 oxygen sample port
• Spring loaded blow-out plate
• Two tungsten electrodes powered by a 20,000 VAc,
  20 mA transformer

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LOLF Testing Apparatus
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LOLF Testing Procedures

• Fuel measured (3/8-gallon) placed in pan
• Fuel pan placed in center of tank
• Nitrogen injected until desired O2 concentration
  reached
• Hot plates turned on
• Fuel heated to and maintained at 150F until HC
  concentration leveled off at 25000 ppm C3H8
• Spark initiated for 1, 2 3 second durations

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LOLF Testing Results (Preliminary Methane Tests)
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LOLF Testing Results
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Conclusions

• Heated Fuel Testing
• At mass loading of 0.08 0.15 kg/m3 significant
  reduction in HC concentration
• Evaporative surface area has no effect on HC
  concentration
• As evaporative surface area decreases, longer
  time necessary to obtain maximum HC concentration
• Residual fuel has no effects

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Conclusions

• Tank Wall Cooling Testing
• As tank wall temperatures decrease, the rate of
  decrease in HC concentration increases
• LOLF Testing
• Methane LFL of 5.3 5.35 determined
• LOLF determined to be 12 O2

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Recommendations

• Tank wall ullage temperatures need to be
  treated carefully
• Further LOLF experiments should include dynamic
  pressure instrumentation
• LOLF at altitude