Modeling of Fuel Tank Inerting for FAA OBIGGS

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

Modeling of Fuel Tank Inerting __________________ Outline • Background – Goals and Objectives – Previous Work • Analytical (calculation) Models • Physical (scale) Models • Modeling Methods – Scale A 320 Tank in Altitude Chamber – Calculation Model – Scale 747 Tank in Altitude Chamber • Summary AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Previous Work – Ullage [O 2] 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Engineering Model Inerting Data Comparison AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Previous Work – Scale 747 SP CWT • FAA built and performed tests in 24% scale 747 SP (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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Scale Model Inerting Data Comparison AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Previous Work – Average [O 2] Predictions • Regardless of methodology, all modeling methods predict bulk ullage oxygen concentration well AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Modeling Method – Altitude Calculation Model – Basic equation governing modeling method With: IGOF Δρ VTank mair = = = = Mass of oxygen in tank at time t Mass flow rate of inerting gas (in terms of t) Fraction of oxygen in inerting gas Change in Ullage Density due to Altitude Change Volume of Tank Ullage Mass of Gas in Tank 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Altitude Inerting Calculation Model Data Comparison AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Modeling Method – Scale A 320 CWT • FAA built and performed tests in 50% scale Airbus A 320 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Block Diagram of Scale A 320 CWT Experiment AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Modeling Method – Scale A 320 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 AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Full-Scale Data Compared with Scale System Performance AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Full-Scale Data Compared with Scale Model [O 2] Results AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Full-Scale Data Compared with Modeling Methods AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Comparison of Modeling Methods Modeling Data for System Size Comparison Ullage [O 2] for Airbus v 1972 Test Descent AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ Modeling Method – Scale 747 SP 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 [O 2] – Single deposit in bay 6 – Results appear to make sense but have no flight test data to make valid comparisons AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting __________________ 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 AAR-440 Fire Safety R&D