Ongoing Fuel Flammability Work at the FAA Technical Center

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Ongoing Fuel Flammability Work at the FAA
Technical Center

• International Aircraft Systems Fire Protection
  Working Group
• London, UK
• June 13 14, 2002

Steve Summer Project Engineer Federal Aviation
Administration Fire Safety Branch, AAR-440
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Agenda

• Fuel vaporization computer model validation
  experiments
• Theoretical flammability limits as a function of
  MIE, FP, and O2 content
• Fuel vapor simulant for use in future ignition
  testing
• Reports to be published

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Fuel Vaporization Model Validation Experiments
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Acknowledgements

• Professor C. E. Polymeropolous of Rutgers
  University
• David Adkins of the Boeing Company

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Introduction

• The original model proved a good method of
  predicting the evolution of hydrocarbons.
• Results were presented by Prof. Polymeropolous
  (10/01 Fire Safety Conference)
• Could prove to be a key tool in performing fleet
  flammability studies.
• Fortran code has been converted to a
  user-friendly Excel spreadsheet by David Adkins
  of Boeing.

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Physical Considerations

• 3D natural convection heat and mass transfer
  within tank
• Fuel vaporization from the tank floor which is
  completely covered with liquid
• Vapor condensation/vaporization from the tank
  walls and ceiling
• Multi-component vaporization and condensation
• Initial conditions are for an equilibrium mixture
  at a given initial temperature

Gas, Tg
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Major Assumptions

• Well mixed gas and liquid phases within the tank
• Uniform temperature and species concentrations in
  the gas and within the evaporating and condensing
  liquid
• Rag 109, Ral 105-106
• Externally supplied uniform liquid and wall
  temperatures. Gas temperature is then computed
  from an energy balance
• Condensate layer is thin and its temperature
  equals the wall temperature.

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Major Assumptions (contd)

• Mass transport at the liquidgas interfaces was
  estimated using heat transfer correlations and
  the analogy between heat and mass transfer for
  estimating film mass transfer coefficients
• Low evaporating species concentrations
• Liquid Jet A composition was based on previous
  published data and and adjusted to reflect
  equilibrium vapor data (Polymeropoulos, 2000)

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Assumed Jet A Composition

• Based on data by Clewell, 1983, and adjusted to
  reflect for the presence of lower than C8
  components

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Assumed Jet A Composition
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20
MW 164
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by Volume
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5
0
5
6
7
8
9
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Number of Carbon Atoms
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User Inputs

• Equilibrium Temperature
• Final Wall and Liquid Temperatures
• Time Constants
• Mass Loading
• Tank Dimensions
• Note For comparison with experimental results,
  recorded wall and liquid temperature profiles
  were entered directly in lieu of the final
  temperatures and corresponding time constants

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Program Outputs

• Equilibrium gas liquid concentrations/species
  fractionation
• Species fractionation as a function of time
• Ullage, wall and liquid temperatures as a
  function of time
• Ullage gas concentrations as a function of time
• FAR, ppm, ppmC3H8

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Experimental Setup

• 17 ft3 vented tank placed inside environmental
  chamber.
• Thermocouples used to monitor ambient, ullage,
  surface and fuel temperatures.
• Blanket heater attached to bottom of tank used to
  heat fuel.
• Hydrocarbon analyzer used to monitor ullage fuel
  vapors.

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Future Testing

• Future tests to consist of
• Constant surface temperature tests.
• Various steady state pressure (cruise) tests.
• Varying pressure tests (flight profile).
• Varying wall to wall temperature tests.
• Varying fuel distribution tests.

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Future Model Improvements

• Capability of varying tank pressure.
• Capability of varying wall to wall temperature
  calculations.
• Capability of varying fuel distribution.

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Theoretical Flammability Limits as a Function of
MIE, FP O2 Content
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Background

• Present thinking in fuel tank inerting is that
  above x O2, the tank is at risk throughout the
  entire flammability envelope, below x O2 it is
  inert.

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Background

• Previous work has shown how flammability limits
  vary as a function of ignition energy.

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Background

• It follows intuitively that flammability limits
  will shift in a similar manner as inert gas is
  added to the fuel tank.
• Thus, if your fuel tank is only partially
  inerted, the flammability exposure time has still
  been reduced by a significant amount.
• How can this be quantified, validated and built
  into the flammability model?

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Computed Flammability Limits as a Function of O2

• Similar methodology as that in DOT/FAA/AR-98/26
  to compute flammability limits as a function of
  MIE.

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Computed Flammability Limits as a Function of O2

• Correlation of the variation of LOC with
  altitude.
• Previously determined with a large (20 J) spark
  source.

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Computed Flammability Limits as a Function of O2

• ,
  where
• Tmin is the minimum of the parabola given by Tmin
  Tfp 22 1.5Z.
• a is a constant, determined by matching the curve
  as best as possible to the calculated 21 O2
  curve for the given ignition energy.

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Resultant Curves for a 20 J Calculation
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Flammability Limits as a Function of MIE, O2 and
FP

• Combining this with the parabolic MIE
  calculations and LOC curves for various ignition
  energies, results in flammability limits which
  vary as a function of ignition energy, O2
  concentration and flashpoint.
• The sum of this work was put together into a
  working MS Excel model by Ivor Thomas, using the
  following LOC curves.

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Conclusions

• By a set of simple calculations, one can obtain
  varying flammability limits as a function of
  ignition energy, O2 concentration and flashpoint.
• Once validated, this data can be used in the
  flammability model to show reduction in fleetwide
  fuel tank flammability as a function of the
  amount of inert gas added to the tank.
• Future tests to validate these calculations are
  planned at the technical center.

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Fuel Vapor Simulant for use in Future Ignition
Testing
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Background

• Our current method for ignition testing of Jet-A
  fuel vapors is extremely time consuming (up to as
  long as 2 hours per test).
• If a gaseous mixture was available to simulate
  the flammability properties of Jet A, it would
  allow us to perform more tests quicker.

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Background

• Availability of said mixture would also have
  applicability to other issues (e.g. explosion
  proof testing of pumps, etc.)
• Subcommittee of SAE AE-5 currently being formed
  to look at this issue.

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Past Simulants - Hexane
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Past Simulants Caltech Mixture

• NTSB Docket No. SA-516, Exhibit No. 20O
• Volumetric Ratio of H2C3H8 of 51
• Examined the effect of fuel concentration, vessel
  size and ignition source on pressure history.

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Past Simulants Caltech Mixture
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Proposed Research Activity at Tech Center

• 20 L combustion vessel to be constructed within 4
  6 weeks.
• Variable energy (0.5 mJ 5 J) spark source to be
  obtained within 8 10 weeks.
• Tests to be conducted in a manner similar to the
  procedures layed out in ASTM flammability
  standards (e.g. E582, E2079, etc).

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Proposed Research Activity at Tech Center
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Reports to be Published
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Reports

• Summer, S. M., Fuel Flammability Characteristics
  of JP-8 Fuel Vapors Existing Within a Typical
  Aircraft Fuel Tank, DOT/FAA/AR-01/54
• Summer, S. M., JP-8 Ignition Testing at Reduced
  Oxygen Concentrations, DOT/FAA/AR-xx/xx