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Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks

Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks. William M Cavage Lead Engineer - Fuel Tank Inerting FAA AAR-440, Fire Safety R&D Branch. North Atlantic Treaty Organization AVT Panel on Fire Safety and Survivability Aalborg, Denmark September 23-26, 2002. Outline.

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Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks

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  1. Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks William M CavageLead Engineer - Fuel Tank Inerting FAA AAR-440, Fire Safety R&D Branch North Atlantic Treaty OrganizationAVT Panel on Fire Safety and Survivability Aalborg, DenmarkSeptember 23-26, 2002

  2. Outline • Background • Previous Research • Preliminary Research • Experimental Equipment • Analysis and Modeling • Results • Summary AAR-440 Fire Safety R&D

  3. Background • Three Accidents in Recent History • All Accidents had Explosions in Center Wing-Tank • Explosions Occurred During/Just After Long Ground Operations on Hot Days with Empty (Residual Fuel) Center Wing Tanks • Exact Ignition Source Not Found During Any Investigation • FAA/Industry Seeking More Cost-Effective Inerting Methodologies • GBI is Inerting on Ground and then Continuing to Operate • ARAC Committee Stated Could be Cost-Effective if Focused on Center-Wing Tanks (CWTs) Only AAR-440 Fire Safety R&D

  4. Previous Research - Methods • Macdonald and Wyeth - Fire and Explosion Protection of Fuel Tank Ullage, Circa 1970s • Compares Different Methods of Reducing Flammability including Nitrogen Inerting • Klueg, McAdoo, and Neese - Performance of a DC-9 Aircraft Liquid Nitrogen Fuel Tank Inerting System, 1972 • FAA Developed and Tested an Inerting System Using Stored LN2 • Stewart and Starkman - Inerting Conditions for Aircraft Fuel Tanks, 1955 • Comprehensive Work on Flammability Limits and [O2] Requirements • Kuchta - Oxygen Dilution Requirements for Inerting Aircraft Fuel Tanks, 1970 AAR-440 Fire Safety R&D

  5. Preliminary Research • Ullage Washing Experiments (Inerting) • Quantitatively Determined Amount of Nitrogen Enriched Air (NEA) Required to Inert a Fuel Tank Test Article • Simple Rectangular Tank with Single Deposit Nozzle and Single Vent • Examined Different NEA % (Residual O2 Concentration) and Flow Rates as well as Examined Effects of Fuel Vapor and Temperature • Ullage Washing is Term Describing Inerting by Ventilation • Developed Nondimensional Relationships, Empirical Equation, and a Theoretical “Perfect Mixing” Solution • Data Illustrated that a VTE of 1.5 to 1.6 is Needed to Inert an Ullage to 8% Oxygen by Volume with 95% NEA (5% [O2]) • Published Report DOT/FAA/AR-01/6 AAR-440 Fire Safety R&D

  6. Rectangular Fuel Tank Inerting Data AAR-440 Fire Safety R&D

  7. Preliminary Research • Fuel Effects on an Inert Ullage • Quantitatively Determined the Effect an Unscrubbed Fuel Load Can Have on an Adjacent Inert Ullage • Simple Rectangular Tank with Single Deposit Nozzle and Single Vent • Examined Fuel Loads of 20, 40, 60, and 80 Percent Full at Sea Level and Two Altitudes; Inerted to 6, 8, and 10 Percent • Measured the Increase in Ullage [O2] due to Air Evolving from Fuel • Circulated the Ullage Through the Fuel to Bring Tank to Equilibrium Quickly - Represents Maximum Fuel Effect on Ullage • Study of Time Effects Showed Unless You Stimulate the Fuel, The Excess Air in Fuel had Small Effect on Ullage Compared to Air Entering Vent System (Fuel Burn) • Report Pending Publication AAR-440 Fire Safety R&D

  8. Inert Ullage Fuel Effects Data Max Increase in Ullage [O2] Due to Adjacent Fuel Load AAR-440 Fire Safety R&D

  9. Preliminary Research • Lower Oxygen Content Study • Performed Ignition Experiments with Model Fuel Tank in Pressure Chamber with both Propane and JP-8 at Reduced Oxygen Concentrations • Tank Instrumented with Thermocouples and Sample Lines for Hydrocarbon and Oxygen Concentration Measurement. • Heaters Placed Underneath the Tank Control the Liquid Fuel Temperature (Flammability). • Used Single High Power Spark to Ensure a Reaction if Probable • Study at Sea Level and Altitude to 38K Feet • Follow on Tests Validated Previous Testing that the Critical Oxygen Concentration is Approximately 12% at Sea Level • Report Pending Publication AAR-440 Fire Safety R&D

  10. Lower Oxygen Content Study Data AAR-440 Fire Safety R&D

  11. Experimental Equipment/Procedures • GBI Proof of Concept Ground/Flight Testing • Joint Project with Boeing that Evaluated the Concept of Ground-Based Inerting • Examined the Effects of Wind and Flight Conditions on Ullage Oxygen Concentration of a Model 737-700 • Tank Instrumented with Gas Sample Ports • NEA Distribution Manifold Installed in Tank by Boeing • Inerted Tank with Ground-Supplied NEA to Approximately 8% • Measured Oxygen Concentration at 8 Locations in CWT During “Normal” Ground and Flight Conditions • Examined Effects of Quiescent and Simulated Wind Conditions on Ground and Representative Flight Conditions in Air AAR-440 Fire Safety R&D

  12. Boeing 737-700 Test Article 1 5 7 2 3 4 6 8 AAR-440 Fire Safety R&D

  13. Experimental Equipment/Procedures • Boeing 747SP Test Article • Decommissioned from Airline Service and Purchased by the FAA for Ground Testing Only • All Major Systems Fully Operational • Has Independent Power for Test Equipment and Instrumentation • Center-Wing Tank Fully Instrumented • Gas Sample Tubing for Oxygen and Total Hydrocarbon Analysis • 32 Thermocouples in Tank (Ullage, Fuel, Walls, Floor, and Ceiling) • Additional Thermocouples and User Specified Oxygen Analysis Channels • Installed NEA Deposit Nozzle in Bay 3 • Inerted Tank with One Deposit Scheme Under a Variety of Conditions AAR-440 Fire Safety R&D

  14. 747SP Center Wing Tank AAR-440 Fire Safety R&D

  15. Experimental Equipment/Procedures • Scale Boeing 747SP Center Wing Tank • Quarter-Scale Model of Boeing 747SP CWT was Built from Three Quarter Inch Plywood By Scaling Drawings from Shepherd Report (24% Length Scale) • Variable Deposit Manifold Allowed for NEA to be Deposited at Any Rate in any Bay(s) Desired • Measured Oxygen Concentration in Each of the 6 Bays • Examined Various Distribution and Inert Gas Flow Scenarios • Varied Flow and Purity to Validate existing Nondimensional and Empirical Relationships • Examined Depositing at Different Locations with Different Venting Schemes AAR-440 Fire Safety R&D

  16. 747SP Scale Center Wing Tank Model AAR-440 Fire Safety R&D

  17. Analysis • Volumetric Tank Exchange is the Ratio of the Volume of Deposited Gas to the Volume of the Tank • Average Tank Oxygen Concentration for 747SP • Equation for Oxygen Concentration Given Inerting Efficiency k AAR-440 Fire Safety R&D

  18. Inert Gas Distribution Engineering Model • Model Calculates Inert Gas Distribution in 6 Bay Tank, in terms of Oxygen Concentration Evolution, Given NEA Purity and Bay Deposit Flow Rates • Based on Original Single Bay Inerting Model, by FAA CSTA for Fuel Systems, which Tracks Oxygen In and Out of Each Bay Assuming Perfect Mixing During the Time Step • Assumes an “Outward” Flow Pattern and Splits Flow into a Bay to Adjacent Bays Using Out Flow Area Relationships • Basic Formula for Volume of Oxygen in a Bay: AAR-440 Fire Safety R&D

  19. Results - 737 Proof of Concept Flight Test • CWT Was Rendered Inert from External Source with Few Problems • Slightly more NEA Required then Predicted • Oxygen Concentration of the CWT Rose Very Little over Time Provided Cross-Venting was Eliminated • CWT Stayed Inert During Flights with Small Fuel Loads and Provided Some Protection with Medium to High Fuel Loads AAR-440 Fire Safety R&D

  20. 737NG CWT Oxygen Concentration AAR-440 Fire Safety R&D

  21. 737NG CWT Oxygen Concentration Increase AAR-440 Fire Safety R&D

  22. Results - Full Scale GBI • Single Deposit of Inert Gas in the Compartmentalized Tank Worked Well • Provided Efficient Distribution of Inert Gas with Fair Bay-Bay Mixing • Quiescent Inerting of CWT Could Be Problematic • Vertical Mixing Problems Caused Inefficient Inerting in Some Cases • More Work Needed to Determine if Vertical Mixing is Easily Stimulated • Calculated and Compared Efficiency Factors AAR-440 Fire Safety R&D

  23. 747SP Inerting Data with Single Deposit AAR-440 Fire Safety R&D

  24. 747SP Inerting Data with Different Vertical Mixing AAR-440 Fire Safety R&D

  25. 747SP Calculated Inerting Curves with Different Vertical Mixing AAR-440 Fire Safety R&D

  26. Results - Inert Gas Distribution Modeling • Engineering Model Data Modeled Full-Scale Test Article Oxygen Concentration Trends Well but Magnitudes Off Somewhat • Scale Tank Data Modeled Full-Scale Test Article Very Well • Bay 4 Results Skewed in All Methods • CFD Data Modeled Fairly Well, with Some Changes to Model AAR-440 Fire Safety R&D

  27. Engineering Model Data Comparison AAR-440 Fire Safety R&D

  28. Scale Plywood CWT Model Data Comparison AAR-440 Fire Safety R&D

  29. CFD Model Data Comparison AAR-440 Fire Safety R&D

  30. Summary • Inerting a Fuel Tank from a Ground Source of NEA is Simple and Practical Method of Providing Protection to Heated CWTs When Needed Most • Limited Distribution of Inert Gas is Required to Inert a Compartmentalized Tank Provided Modeling is Performed and Vertical Mixing is Examined • Modeling of Inert Gas Distribution in a Compartmentalized Tank Can be Achieved with a Limited Amount of Resources AAR-440 Fire Safety R&D

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