Modeling of Fuel Tank Inerting for FAA OBIGGS Research William Cavage AAR-440 Fire Safety Branch Wm. J. Hughes Technical Center Federal Aviation Administration.

Modeling of Fuel Tank Inerting for FAA OBIGGS Research

William CavageAAR-440 Fire Safety Branch

Wm. J. Hughes Technical CenterFederal Aviation Administration

International Fire and Cabin Safety Research Conference

Parque das Nações Conference Center Lisbon, Portugal

November 15-18, 2004

AAR-440 Fire Safety R&D

Modeling of Fuel Tank Inerting____________________________________

Outline• Background

– 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



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 stepTankONEANEAOO VttVQtIGOFQtVtV /)1()1()(

222

Tank

O

V

tVtOTank

)()]([ 2

2

– 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

0

5

10

15

20

25

0 500 1000 1500 2000 2500

Time (seconds)

Oxy

gen

Co

nce

ntr

atio

n (

%)

94% NEA Model 94% NEA Data

96% NEA Model 96% NEA Data

6 CFM Flow Rate - Data vs Model



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

Bay 1Flow Out

Bay 2

Flow InBay 3 Bay 4

Bay 5 Bay 6FlowOut


Modeling of Fuel Tank Inerting____________________________________Engineering Model Inerting Data Comparison

0

5

10

15

20

25

5 10 15 20 25 30 35

Time (mins)

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

Bay 1 Bay 1

Bay 2 Bay 2

Bay 3 Bay 3

Bay 4 Bay 4

Bay 5 Bay 5

Bay 6 Bay 6

CWT Inerting, Single Bay Deposit in Bay 3Comparison with Engineering Model Data

B-747 Data Engineering Model

140 CFM Flow Rate95% NEA (5% O2)



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


Modeling of Fuel Tank Inerting____________________________________Scale Model Inerting Data Comparison

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Overall VTE

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

Bay 1 Bay 1

Bay 2 Bay 2

Bay 3 Bay 3

Bay 4 Bay 4

Bay 5 Bay 5

Bay 6 Bay 6

CWT Inerting, Single Bay Deposit in Bay 3Comparison with Scale Tank Data

B-747 Data Scale Tank

20 Minute Inerting Rate95% NEA (5% O2)

VolumeTankFuel

RateFlowVolumeTimeExchangeTankVolumetric



• Regardless of methodology, all modeling methods predict bulk ullage oxygen concentration well

Previous Work – Average [O2] Predictions

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Volumetric Tank Exchange

Ave

rag

e O

xyg

en C

on

cen

trat

ion

(%

vo

l) Full-Scale Test Article

24% Scale Tank

Engineering Model

747 CWT GBI Weighted Average Inerting DataSingle Bay Deposit, 95% NEA, 20-Minute Inerting

8% Line

VolumeTankFuel

RateFlowVolumeTimeExchangeTankVolumetric



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



Modeling Method – Altitude Calculation Model– Basic equation governing modeling method

21.*)*()1()*(

)1()1()(22

TankTank

OO

VtUGOFV

tUGOFmIGOFmtmtm

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 gasΔρ = Change in Ullage Density due to Altitude ChangeVTank = Volume of Tank UllagemTank = Mass of Gas in Tankmair = Mass of air entering tank

Ullage Gas Oxygen Fraction (UGOF) is given as:

)(2tmO

m

TankO mtmtUGOF /)1()1(2

– Simple effective model for predicting resulting ullage oxygen concentration given a system performance and mission profile


Modeling of Fuel Tank Inerting____________________________________Altitude Inerting Calculation Model Data Comparison

0

5

10

15

20

25

0 20 40 60 80 100 120

Time (mins)

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

Alt

itu

de

(ft)

)

Calculation Model

Flight Test Data

Altitude

A320 Flight Test - Single ASM Data



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


Modeling of Fuel Tank Inerting____________________________________Block Diagram of Scale A320 CWT Experiment

Altitude ChamberN

EA

Mix

er

OxygenAnalyzer

Flow Controller

DAS

Nitr

ogen

CompressedAir

NEA Generator

Com

pute

r

T

AltitudeOxygen

Analyzer

PressureTransducer

Sample Return

Scale Tank Model



• 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

Modeling Method – Scale A320 CWT


Modeling of Fuel Tank Inerting____________________________________Full-Scale Data Compared with Scale System Performance

0

5

10

15

20

25

65 70 75 80 85 90 95 100 105 110 115

Time (min)

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

0

20

40

60

80

100

120

140

160

Flo

w (

scfh

)

Scale Test NEA Purity

Flight Test NEA Purity

Scale Test NEA Flow

Flight Test NEA Flow (scaled)

Comparison of Model Data - Single ASM TestMeasured System Performance Data


Modeling of Fuel Tank Inerting____________________________________Full-Scale Data Compared with Scale Model [O2] Results

0

2

4

6

8

10

12

14

16

18

65 70 75 80 85 90 95 100 105 110 115

Time (mins)

Ox

yg

en

Co

nc

en

tra

tio

n (

% v

ol)

A320 Flight Test [O2]

Analytical Model [O2]

Scale Model [O2]

Comparison of Model Data - Average Ullage [O2]Airbus Test v1972 - Dual Flow System


Modeling of Fuel Tank Inerting____________________________________Full-Scale Data Compared with Modeling Methods

0

5

10

15

20

25

65 70 75 80 85 90 95 100 105 110 115

Time (mins)

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

0

5

10

15

20

25

30

35

40

45

Alt

itu

de

(kft

)

Scale Model [O2]

Flight Test [O2]

Flight Test Altitude

Scale Model Altitude

Comparison of Model Data - Single ASM TestUllage Sample Near Vent


Modeling of Fuel Tank Inerting____________________________________Comparison of Modeling Methods

0

2

4

6

8

10

12

14

16

18

65 70 75 80 85 90 95 100 105 110

Time (mins)

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

Single ASM Performance Single ASM Performance

75% System Performance 75% System Performance

50% System Performance 50% System Performance

Analytical Model Data for System Size ComparisonUllage [O2] Airbus v1972 Descent - Dual Flow System

Analytical Model Scale Tank Model

Modeling Data for System Size ComparisonUllage [O2] for Airbus v1972 Test Descent



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 0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60

Time (mins)

Oxy

gen

Co

nce

ntr

atio

n (

% V

ol)

Bay 1 Bay 2 Bay 3

Bay 4 Bay 5 Bay 6

Descent Inerting Data - 39K FeetStarting [O2] 2%, 10 Min Hold at 5K Feet



• 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

Summary

Modeling of Fuel Tank Inerting for FAA OBIGGS Research William Cavage AAR-440 Fire Safety Branch Wm. J. Hughes Technical Center Federal Aviation Administration.

Documents

inerting of tank

modeling of fuel tank

tank volume

volume of tank ullage

inerting systems

single bay tank

altitude change v tank

modeling methods