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

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

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

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)

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  ARAC’s FTHWG determined that these tanks are at risk 30% of the total flight time compared to 5% for CWT’s without ECS Aviation Rulemaking Advisory Committee Fuel Tank Harmonization Working Group

Overview of Problem: Aviation 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

Overview of Problem: Aviation 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

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

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

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

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)

Heated Fuel Vapor Testing: 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

Heated Fuel Vapor Testing: Apparatus  ft 3 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

Door T/C 5 T/C 2 Analyzer Port 2 T/C 1 T/C 0 T/C 3 Analyzer Port 1 T/C 4 Heat Inlet Heat Outlet

Heated Fuel Vapor Testing: 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

Mass Loading Results

Evaporative Surface Area Results

Residual Fuel Results

Tank Wall Cooling: Objectives  Determine the effects of cold tank wall temperatures on ullage vapor generation within an aircraft fuel tank environment

Tank Wall Cooling: Apparatus  Same tank as Heated Fuel Vapor Testing with some modifications: 3-in. shell surrounded the two side and rear walls for CO 2 cooling Kerosene air heater replaced with a thermostatically controlled hot plate

Tank Wall Cooling: Procedures  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

Tank Wall Cooling Results

LOLF Testing: Objectives  Determine the lowest oxygen level within the tank that would support ignition (i.e. the lower oxygen limit of flammability)

LOLF Testing: Apparatus  9 ft 3 vented, aluminum fuel tank placed inside of 10 m 3 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

LOLF Testing: Apparatus

LOLF Testing: Procedures  Fuel measured (3/8-gallon) & placed in pan  Fuel pan placed in center of tank  Nitrogen injected until desired O 2 concentration reached  Hot plates turned on  Fuel heated to and maintained at ~150°F until HC concentration leveled off at ~25000 ppm C 3 H 8  Spark initiated for 1, 2 & 3 second durations

LOLF Testing Results (Preliminary Methane Tests)

LOLF Testing Results

Conclusions  Heated Fuel Testing At mass loading of 0.08 – 0.15 kg/m 3 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

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% O 2

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