the effect of fuel on an inert ullage in commercial airplane fuel tanks william cavage aar-440 fire...
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The Effect of Fuel on an Inert Ullage in Commercial Airplane Fuel Tanks
William CavageAAR-440 Fire Safety Branch
Wm. J. Hughes Technical CenterFederal Aviation Administration
International Systems Fire Protection Working Group
Tropicana Casino and Resort Atlantic City, NJ
November 1-2, 2005
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Outline
• Background
• Test Article
• Test Methods
• Calculations
• Results
• Summary
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Background• FAA developed a proof of concept inerting system to
illustrate the feasibility of fuel tank inerting– FAA intends to make a rule requiring flammability control of some or
all CWTS with an emphasis on inerting system technologies• The effect of adjacent fuel loads on an inert ullage has not
been studied thoroughly– Air in fuel can evolve and spoil the inert atmosphere in the ullage– Military work indicates “fuel scrubbing” is necessary to prevent large
increases in ullage oxygen concentrations with high fuel loads
• Need to know what considerations need be made to account for adjacent fuel loads (more NEA required?)– Commercial airlines have no intention of scrubbing fuel– Fuel tanks effected by rule tend to have low fuel loads
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Test Article• Used a 3x3x2 ft rectangular tank made for fuel tank
flammability and inerting research– Instrumentation panel installed to allow for gas samples,
thermocouples, and inerting agent to pass through
– Used lab oxygen analyer for sea level work and single channel altitude analyzer (similar to OBOAS) for altitude work
• Could deposit nitrogen, air, or NEA into the tank depending upon the needs of the experiment
• Had manifold installed in the bottom of tank to allow for gases to be passed through the fuel – Selector valve allowed for air, ullage gas, or NEA to be passed
through the manifold to scrub fuel, revive fuel, or equalize the ullage gases with the fuel gases
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Altitude Chamber
Air
Cle
aner
OxygenAnalyzer
DAS
Nitr
ogen
NEA Generator
Com
pute
r
T
Altitude O2
Analyzer
PressureTransducer
Sample Return
Fuel TankTest Article
ASM
Gas Sample
T
T
T
CompressedAir
SelectorValve
FuelPump
SelectorValve
Ull
age
Pum
p
Gas Sample/Pressure LineInstrumentation Wire
LegendNEA/Air DepositFuel Line
Block Diagram of Experiment Configuration
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Test Methods – 2 Primary Areas• Sea level testing focused on the change in [O2] due to fuel
load when tank is brought to equilibrium– Looked at how to bring fuel/ullage to stable state
– Quantified the change in oxygen concentration due to fuel load
– Examined the benefit of inerting the ullage through the fuel (rudimentary scrubbing)
• Altitude testing focused on quantifying the altitude effects for both equilibrium state and potentially what would be seen in a commercial transport fuel tank– Validated measured sea level changes and quantified altitude effects– Examined what stimulates oxygen evolution from fuel– Simulated two flight tests to determine modeling capability
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Calculations – Two Ways Ullage [O2] Increases• Tank air entry due to fuel consumption
– Tanks normally vented to atmospheric pressure
– Use inerting equation with fuel consumption is VTE and inerting gas is air (20.9% oxygen concentration)
)]1)(9.20[()( %222
FLStartStart eOOtO
• Change in [O2] due to air evolving from fuel– Solve a series of equations that equalize the partial pressure of
oxygen and nitrogen across the fuel given the Ostwald Coefficient
22 FU OO PP )1(
)(11
2
OUF
FU
OUF
FUFOUO
U
CTV
TVCTV
TVmm
Om
U
OUUO
U V
RTm
OP2
2
Mass of O2 in system is constantand partial pressure of O2 in
Ullage and fuel equal at state 2
calculate mass of oxygen at state 2 given conditions at state 1 calculate partial pressure O2
with equation of state
Static
UO
P
PO 2][ 2
1122 FUFU OOOO mmmm
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Results – Sea Level Testing• Stimulation Methods Studied
– Best method by far was ullage recirculation which highlights the fact that “oxygen evolution” is a misnomer, the process is an exchange of gases to bring partial pressures of fuel/ullage gases to equilibrium
• Resulting increase in oxygen concentration due to adjacent fuel (maximum increase) was measured/calculated– Calculations match measured numbers fairly well
• The benefits of inerting through fuel (rudimentary scrubbing)– Illustrated some benefit by depositing inert gas at the bottom of a fuel
tank, allowing the inert gas to displace some O2
– Requires more inert gas per volume of ullage to inert in this manner, but still less inert gas then required to inert empty tank
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Increase in [O2] Over Time with Different Stimulations
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 10 20 30 40 50 60 70 80 90 100
Time (Minutes)
Oxy
gen
Co
nce
ntr
atio
n I
ncr
ease
(%
Vo
l) Ullage Recirculation
Side-to-Side Slosh
Radiant Under Heat
40% Fuel Load, Inert to 8%: Different Stimulation Methods
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________Resulting Maximum Increase in Ullage [O2] due to Fuel
0%
2%
4%
6%
8%
10%
12%
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Fuel Load (% Full)
Oxy
gen
Co
nce
ntr
atio
n I
ncr
ease
(%
Vo
l)
6% Inert Inert to 6%
8% Inert Inert to 8%
10% Inert Inert to 10%
Comparison of Test Data with O2 Mass Balance Calculationsat Sea Level
Calculations Measured
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Benefit of Inerting Through Fuel
-1
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70 80 90
Fuel Load (% full)
Oxy
gen
Co
nce
ntr
atio
n I
ncr
ease
(%
vo
l) Inerting Through Manifold
Standard Ullage Washing
Total Increase in Ullage Oxygen Concentration from 8% at Sea Level
Note: Inerting through manifold requiredmore 5% NEA for the same ullage volume
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Results – Altitude Testing• Effect of altitude on ullage oxygen concentration quantified
– After ullage is at equilibrium with fuel at sea level, increase in altitude (decrease in ullage pressure) causes partial pressure imbalance
– Used ullage recirc at three altitudes and illustrated consistent results with poor agreement to calculations
• Altitude stimulation work increase examined qualitatively – Besides ullage recirc, examined fuel pumping and altitude change only
as potential methods of balancing the ullage/fuel gas partial pressures
• Simulation of Boeing GBI flight tests compared fair– Simulation was a performed with no fuel/ullage stimulation (altitude
only) and compared with GBI flight tests (not the descent portion)
– Results illustrated qualitatively that altitude stimulation was closest studied to flight test data but more work is needed to optimize results
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Change in [O2] Increase due to Altitude
0
2
4
6
8
10
12
14
0 50 100 150 200 250
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
Pre
ssu
re A
ltit
ud
e (f
t)
Oxygen Concentration
Pressure Altitude
Ullage Recirculation at 3 Altitudes
60% Fuel Load
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Resulting Max Increase in Ullage [O2] due to Altitude
0
2
4
6
8
10
12
14
16
0 5000 10000 15000 20000 25000 30000 35000
Altitude (ft)
Oxy
gen
Co
nce
ntr
atio
n I
ncr
ease
(%
vo
l)
20 % Fuel 20% Fuel
40 % Fuel 40% Fuel
60% Fuel 60% Fuel
80% Fuel 80% Fuel
Measured Calculated
Comparison of Test Data with O2 Mass Balance Calculations at Sea Level
Initial Oxygen Concentration 8%Stimulated with Ullage Recirc
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Comparison of Stimulation Methods at Altitude
0
2
4
6
8
10
12
0 50 100 150 200 250
Time (mins)
Ox
yg
en
Co
nc
en
tra
tio
n (
% v
ol)
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
Alt
itu
de
(ft
)
Ullage Recirc Fuel Pump Quiescent Pressure Altitude
40% Fuel Load
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
Comparison of Lab Simulation with Flight Test Data
0
5
10
15
20
25
0 20 40 60 80 100 120 140 160
Test Time (mins)
Oxy
gen
Co
nc
en
tra
tio
n (
%V
ol)
40% Fuel Lab Test 40% Fuel Boeing Test
80% Fuel Lab Test 80% Fuel Boeing Test
Flight and Lab Test Results Comparedwith Calculation Bands
Boeing 80% Fuel Test CalculatedBand with Air Evolution
Boeing 40% Fuel TestCalculated Band with Air Evolution
Note: Calculation Band Represents Maximum and Minimum Resulting Oxygen Concentration Given 0% and 100% Air Evolution From Fuel
AAR-440 Fire Safety R&D
Fuel Effects on an Inert Ullage____________________________________
• Oxygen evolving from fuel is a misnomer, changes in ullage oxygen concentration due to adjacent fuel are a result of the equalization of the partial pressures of gases at the fuel/ullage interface and is difficult to get without mixing fuel/ullage together
• Measured sea level increases in ullage oxygen concentration match well with calculations
• Some benefit can be garnered from remedial fuel scrubbing by inerting through fuel but more NEA / ullage volume is required
• Changes in ullage altitude cause additional increases in ullage oxygen concentration from fuel with calculations agreeing poorly
• Lab experiments can simulate flight test results with some accuracy with very little stimulation needed to match results
Summary
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