# Presentation: 280543

## Modeling In-flight Inert Gas Distribution in a 747 Center-Wing Fuel Tank

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

- 35th AIAA Fluid Dynamics ConferenceWestin Harbour Castle Toronto, Ontario Canada June 6-9, 2005

## Outline

- Background
- Previous Work
- Plywood Scale Test Article
- Modeling Methods
- Analytical Model
- CFD Model
- Results
- Cascading Inerting
- 747 SCA Test Simulations
- Summary

## Background

- FAA has developed and tested an OBIGGS for fuel tank inerting to illustrate the feasibility of simplified, light weight inerting systems to reduce flammability in some commercial transport airplane fuel tanks
- Modeling inert gas effects and distribution in commercial transport fuel tanks will assist in the development process and allow for cost effective systems analysis and trade studies
- Models have to be simple to be useful in a cost analysis study or rulemaking exercise
- FAA has a relatively large amount of commercial transport inerting flight test data to validate any developed models
- Need to capitalize on previous FAA modeling work done in support of ground-based (sea level) inerting research (see AIAA Paper 2002-3032)

## Previous Work – Multi-Bay GBI Analytical Model

- FAA developed analytical model that calculates inert gas distribution in 6-bay tank at sea level, in terms of oxygen concentration evolution, given NEA purity and flow rate
- Based on previous work and tracks oxygen in and out of each bay at each time step 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 illustrated marginal agreement with full scale

## Analytical Model Inerting Data Comparison

## Previous Work – Scale 747SP CWT GBI

- FAA built and performed tests in 24% scale 747SP (classic type) center-wing fuel tank
- Made from plywood using NTSB Shepherd report drawings and had scaled penetrations (low fidelity 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 – 747SP CWT GBI CFD Model

- Created wire mesh of a classic type 747 center-wing fuel tank with vent system and used Fluent CFD Solver which tracks fluid species (O2 & N2)

- Model assumed laminar flow so all gas distribution based entirely on flow diffusion
- Model had approximately 700K nodes and ran for several days on a multi-processor platform with some good results

## CFD Model Inerting Data Comparison

## Previous Work – Average [O2] Predictions 

- Previous Work – Average [O2] Predictions

## Previous Work – Altitude Analytical Model

- An analytical model of ullage oxygen concentration in a single bay tank based on inert gas added and altitude change was developed based on previous model
- Model uses system performance (NEA flow and purity) in terms of time and altitude
- Sea level model changed to calculate mass of oxygen added and removed from one bay at each time step, assuming perfect mixing, and given tank volume and starting oxygen concentration
- 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 calculation that runs nearly instantaneously

## Previous Work – Scale A320 CWT in Flight

- FAA built and performed tests in 50% scale Airbus A320 center-wing fuel tank using altitude chamber
- Made from plywood using drawings given to FAA 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 with scaled flow in altitude chamber
- Altitude oxygen analyzer used to track ullage oxygen concentration as well as additional instrumentation

## Flight Test Descent Data Compared with Model Results

## Test Article – Scale 747 SCA CWT In-flight

- FAA used existing 24% scale model in an altitude chamber to simulate 747 SCA flight test scenario
- Used model from previous GBI experiments
- Test personnel controlled the altitude/time flight cycle to simulate particular flight test
- Inerting system simulated by additional test personnel using mass flow controller and NEA mixer to inert the tank with scaled flow in altitude chamber
- 8-channel altitude oxygen analyzer used to track each bay ullage oxygen concentration as well as additional basic instrumentation to monitor experiment
- See Report DOT/FAA/AR-04/41 for details of 747 SCA flight tests

## Model Method - Multi Bay Analytical Model

- Developed analytical model of multi-bay inerting in-flight based on previous model to simulate 747 SCA flight test scenario

- First developed more simple “cascading” inerting model which has very few assumptions and is more easily validated
- Model has one inert gas deposit (bay 1) and one vent (bay n)

- Next, modified model to split flow to several bays and vent flow from several bays using flight test data for ratios

## Calculation of Mass of Oxygen in Bay i of a Tank with n Bays

## Results – Cascading Inerting

- Reconfigured the 747 scale tank to perform cascading inerting tests to compare results of scale tank with analytical model
- Changed deposit and venting configuration and made holes between bays small to promote mixing

- Results good but had significant discrepancies during descent
- Differences contributed to scale tank lid leaking air in due to worn seal
- Bulk average data matched identically further supporting conclusion

## Results – 747 SCA Simulation

- Used measured OBIGGS performance and flight cycle as input to scale tank and analytical model to simulate a test descent
- Results of both simple models illustrated good agreement with the data trends

- Some large discrepancies in values at individual times
- Peak and resulting values of bays 1, 2, & 3 critical to system design with deviations of about +/- 1% oxygen

## Results – 747 SCA Simulation (continued)

- Remaining bays of simulation illustrate same trends with even larger discrepancies
- Peak and resulting values very poor for bays 4 and 5 with bay 6 being marginal (bay 4 fight test data bad)

- Used data from a separate flight test to “tweak” both models
- Modified flow split ratios of analytical model and made small modifications to scale tank to better represent flight test trends

## Results – 747 SCA Simulation (continued)

- Could be an indication of problems with scale tank fidelity
- Simple ratio splits for analytical model probably too simplistic for actual vent system on descent

- Average ullage oxygen concentration data generated by both models very consistent with measured data for all three tests simulated

## Model Method - CFD Model

- Model allows for variable quantity and purity of inert gas deposited in a single bay of the tank at a single location
- Model allows for variable atmospheric pressure at the vent exit (changing altitude), and allows for ullage to exit the tank and air to enter the tank depending upon the difference between the descent rate and inerting conditions
- Model uses the Fluent CFD Solver, to track the amount of oxygen entering and leaving each bay and can determine bulk averages as well as local oxygen concentrations
- Fluent employs a finite volume method where the general conservation (transport) equation (Mass, Momentum, Energy, etc.) is solved for each finite volume
- Model assumes turbulent flow

## Results – 747 SCA Simulation (continued)

- Illustrates some of the same problems seen with the analytical model
- Room for improvement in CFD as analytical model relies highly on experimental data for improvement

- Results of CFD simulations encouraging but still considerable discrepancies between peak and resulting values with flight test
- Analytical model illustrated better agreement with trends and peak and resulting values

## Summary

- Peak and resulting values agree fair to good depending on case
- A scale model can be used with an altitude facility in a relatively cost effective way to give fair agreement with peak and resulting flight test [O2] values
- Some specialized instrumentation and facilities needed
- CFD data not any better than more simple modeling methods but additional work may yield significant improvements in simulation quality