# Presentation: 607944

## Modeling of Single Bay Fuel Tank Inerting for FAA OBIGGS Research

- William CavageAAR-440 Fire Safety BranchWm. J. Hughes Technical CenterFederal Aviation Administration

- International Systems Fire Protection Working GroupPlace de Ville Tower C Ottawa, Canada February 14-15, 2005

## Outline

- Background
- Goals and Objectives
- Previous Work
- Analytical (calculation) Models
- Physical (scale) Models
- Results of Modeling Methods
- Analytical Model
- Scale A320 Tank in Altitude Chamber
- Summary

## 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

## 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

## 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

## Scale Model Inerting Data Comparison

## Previous Work – Average [O2] Predictions 

- Previous Work – Average [O2] Predictions

## Modeling Method – Altitude Analytical Model

- An analytical model of average ullage oxygen concentration based on inert gas added and altitude change was developed based on existing model
- Model changed from sea level model 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

## Altitude Inerting Calculation Model Data Comparison

## 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

## Block Diagram of Scale A320 CWT Experiment

## Results - Scale A320 CWT Model

- 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

## Full-Scale Data Compared with Scale System Performance

## Full-Scale Data Compared with Scale Model [O2] Results

## Local Full-Scale Measurement Compared with Scale Data

## Results – Comparison of Analytical & Scale Models

**Modeling Data for System Size Comparison**
**Ullage [O****2****] for Airbus v1972 Test Descent**

- Performed mock trade study of system sizing with both scale and analytical models
- Studied effect of decreasing the size of the system by 25% and 50%

- Used the previous discussed Airbus v1972 Test Descent
- Data trends compared well as did the peak and resulting values with exception of peak 75% system performance (off by 1%)

## Summary

- 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