Ground Based Fuel Tank Inerting

1
An Update on FAA Fuel Tank Ullage Modeling
William Cavage, Steven Summer, and Robert Ochs
AAR-440 Fire Safety BranchWm. J. Hughes
Technical CenterFederal Aviation Administration
International Systems Fire Protection Working
Group CAA London, England UK April 19-20,
2006
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Outline

• Background
• Models Employed
• Flight Test Results Comparison
• Representative Flight Modeling

3
Background

• FAA developed a proof of concept inerting system
  to inert the CWT of classic style Boeing model
  747
• FAA intends to make a rule requiring flammability
  control of some or all CWTS with an emphasis on
  inerting system technologies
• Fire Safety research has been working to expand
  modeling capabilities to allow for systems
  analysis, trade studies, etc.
• Used lab research and flight test data to
  develop, expand, and validate a multiple bay fuel
  tank inerting model and an ullage flammability
  model
• Performed analysis of a single fictitious flight
  to examine the SOA of our modeling capability
• Developed a more typical 747 flight profile with
  a bigger OBIGGS than the NASA flight test system

4
Models Employed

• Multiple bay inerting model same as model
  discussed in AIAA paper (see web site) with some
  small improvements
• Tracks mass of oxygen in and out of each bay
  knowing NEA flow and given a set of generic flow
  splits between bays with no back pressure
• Tracks mass of ullage knowing the change in
  density (given altitude changes) with no mass
  storage in tank
• Model does these calculations in a time step
  process
• Ullage flammability model is the
  evaporation/condensation model by Polymeropoulos
  Ochs and discussed in Lisbon
• Uses a generic fuel species model based on flash
  point developed by Sagebiel
• Calculates the equilibrium state of the ullage
  given fuel temperature and then determines
  condensation effect from walls/ceiling in time

5
Results from Flight Test Comparison

• Inerting model illustrated excellent agreement
  with measured flight test data with some
  exception
• Good agreement is expected since a similar test
  was used to develop the bay to bay flow split
  ratios
• Bay 4 trends look good but value is considerably
  off (the problem bay)
• Resulting ullage average time values very
  consistent
• Flammability model data trend results very good
• Magnitude of data peaks and troughs illustrate
  significant deviation although measured data has
  some uncertainty so it is unclear how well the
  model duplicates flight test results
• Models uses no empirical relationships to obtain
  results

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Comparison of Flight Test Results with FAA Model
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Comparison of Flight Test Results with FAA Model
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Results Representative Flight

• Developed flight profile, OBIGGS performance, and
  temperature profile from existing flight test
  data taking into account a typical 747 flight
  profile
• Difficult to determine precise temperature peaks
  and troughs but did best to approximate
• OBIGGS was assumed to 33 bigger (1 more ASM)
• Data trends as expected but very little we can do
  to validate the magnitude of the results
• Regardless of peak oxygen concentration in single
  bay, tank remained nonflammable during entire
  flight
• Tank flammability dropped below LEL during peak
  oxygen concentration rise in bay 1

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Input Data for 747 Modeling Exercise
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Results of Modeling Exercise
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Summary

• Analysis of FAA Fire Safety model capabilities
  illustrates good fidelity for this single
  aircraft type
• Remains to be seen how this work could translate
  to other multiple bay CWT aircraft types
• Need additional work to be able to simulate
  different deposit and venting schemes
• No additional modeling work planned to date