1. Modeling of Fuel Tank Inerting for FAA OBIGGS Research
William Cavage AAR-440 Fire Safety Branch Wm. J. Hughes Technical Center Federal Aviation Administration
International Fire and Cabin Safety Research Conference Parque das Nações Conference Center Lisbon, Portugal November 15-18, 2004

2. Outline Background Modeling Methods Summary Goals and Objectives
Previous Work
Analytical (calculation) Models
Physical (scale) Models
Modeling Methods
Scale A320 Tank in Altitude Chamber
Calculation Model
Scale 747 Tank in Altitude Chamber
Summary
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3. Background
FAA has been developing and testing an OBIGGS for fuel tank inerting to illustrate the feasibility of light weight, simplified inerting systems to reduce flammability in commercial transport airplanes
Modeling inert gas effects and distribution in commercial transport fuel tanks could assist in the development process and allow for cost effective systems analysis and trade studies
Capitalize on previous FAA modeling work done in support of ground based inerting research (sea level inerting only)
Models have to be simple to be useful in a cost analysis study or rulemaking exercise
FAA has a relatively large amount of flight test data to validate any developed models
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4. Previous Work – Ullage [O2] Calculation Model
Uses perfect mixing assumption and calculates the volume of oxygen in and out of tank at every time step
Uses a basic spreadsheet iterating calculation and runs immediately given the volume of the tank, the flow rate and purity of the NEA
Constant inerting only
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5. Previous Work – Multi-Bay Inerting Model
Engineering model that calculates inert gas distribution in 6-bay tank, in terms of oxygen concentration evolution, given NEA purity and bay deposit flow rates
Based on calculation method previously discussed and tracks oxygen in and out of each bay assuming perfect mixing
Designed for “localized deposit” methods and assumes an “outward” flow pattern, splitting flow to adjacent bays based on flow area relationships
Data agreed fairly well with full-scale data
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6. Engineering Model Inerting Data Comparison
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7. Previous Work – Scale 747SP CWT
FAA built and performed tests in 24% scale 747SP (classic type) center-wing fuel tank
Made from plywood using NTSB Shepherd report drawings
Scaled all penetrations (holes) between bays and vent system
Variable NEA deposit capability to allow for inerting of tank with scaled flows in each bay
Oxygen concentration measured in each bay
Model data duplicated full-scale results very well for localized deposit method studied
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8. Scale Model Inerting Data Comparison
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9. Previous Work – Average [O2] Predictions
Regardless of methodology, all modeling methods predict bulk ullage oxygen concentration well
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10. Modeling Method – Altitude Calculation Model
A calculation model of average ullage oxygen concentration based on inert gas added and altitude change was developed based on existing model
Model changed to calculate mass of oxygen added and removed at each time step, assuming perfect mixing, in a single bay tank given a tank volume and starting oxygen concentration
Must input system performance (NEA flow and purity) in terms of time and altitude
Calculates ullage gas removed from tank due to increase in altitude (decrease in pressure) to calculate mass of oxygen decrease in tank
Calculates air entering tank due to decrease in altitude (increase pressure) to calculate mass of oxygen increase
The model is a relatively simple time step spreadsheet that calculates instantaneously
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11. Modeling Method – Altitude Calculation Model
Basic equation governing modeling method
With: = Mass of oxygen in tank at time t
= Mass flow rate of inerting gas (in terms of t) IGOF = Fraction of oxygen in inerting gas
Δρ = Change in Ullage Density due to Altitude Change
VTank = Volume of Tank Ullage
mTank = Mass of Gas in Tank
mair = Mass of air entering tank
Ullage Gas Oxygen Fraction (UGOF) is given as:
Simple effective model for predicting resulting ullage oxygen concentration given a system performance and mission profile
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12. Altitude Inerting Calculation Model Data Comparison
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13. Modeling Method – Scale A320 CWT
FAA built and performed tests in 50% scale Airbus A320 center-wing fuel tank
Made from plywood using drawings given to us by Airbus in support of joint inerting flight test
Scaled all structural members of tank inside down to the smallest detail
Mass flow controller and NEA mixer used to inert the tank in altitude chamber
Altitude oxygen analyzer used to track ullage with additional basic instrumentation
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14. Block Diagram of Scale A320 CWT Experiment
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15. Modeling Method – Scale A320 CWT
Results indicate that duplication of the flight cycle with the system performance in the altitude chamber was accomplished with coordination of test personnel
Measured scale tank oxygen concentration data illustrated good agreement with flight test results considering the large differences in measurement systems sample lag
Localized oxygen concentration dynamics illustrated some fidelity when specific sample location data were compared with similar locations in the flight test aircraft
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16. Full-Scale Data Compared with Scale System Performance
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17. Full-Scale Data Compared with Scale Model [O2] Results
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18. Full-Scale Data Compared with Modeling Methods
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19. Comparison of Modeling Methods
Modeling Data for System Size Comparison
Ullage [O2] for Airbus v1972 Test Descent
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20. Modeling Method – Scale 747SP CWT
FAA performed tests in 24% scale Boeing 747 (classic type) CWT built for GBI inert gas distribution study
Used scale model in altitude chamber with NEA mixer and mass flow controller and simulated potential 747 missions with predicted system performance
OBOAS used to track bay [O2]
Single deposit in bay 6
Results appear to make sense but have no flight test data to make valid comparisons
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21. Summary
Simple calculation models of average ullage oxygen concentration have been developed and can duplicate flight test data in a fairly accurate manner
Requires no unique engineering skills
Very easy to develop and modify for a wide variety of flight cycles, OBIGGS capabilities and ullage conditions
Scale models can be tested with an altitude facility in a relatively cost effective manner to give good agreement with flight test data
Some specialized instrumentation and facilities needed
Provides some ability to analyze internal flow dynamics
More work is needed to validate this method for compartmentalized tanks
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