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Modeling of Single Bay Fuel Tank Inerting for FAA OBIGGS Research
William CavageAAR-440 Fire Safety Branch
Wm. J. Hughes Technical CenterFederal Aviation Administration
International Systems Fire Protection Working Group
Place de Ville Tower C Ottawa, Canada
February 14-15, 2005
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
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
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 [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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
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
AAR-440 Fire Safety R&D
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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
• 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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
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
AAR-440 Fire Safety R&D
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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
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
AAR-440 Fire Safety R&D
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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
• 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
Results - Scale A320 CWT Model
AAR-440 Fire Safety R&D
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
AAR-440 Fire Safety R&D
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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________Local Full-Scale Measurement Compared with Scale Data
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
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
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
• 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%)
Results – Comparison of Analytical & Scale Models
AAR-440 Fire Safety R&D
Modeling of Fuel Tank Inerting____________________________________
• Simple calculation models of average ullage oxygen concentration have been developed and can duplicate single bay 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
Summary