22nd aiaa aerodynamic measurement technology and ground testing conference june 24th-26th, 2002...
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22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference
June 24th-26th, 2002 Adams Mark Hotel - St. Louis, MS
Modeling Inert Gas Distribution in Commercial
Transport Aircraft Fuel Tanks
William M CavageProject Manager - Fuel Tank Inerting
FAA AAR-440, Fire Safety R&D Branch
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Outline• Background• Equipment & Procedures
– B-747SP Ground Test Article
– 24% Scale Tank
• Data Analysis
• Modeling Methods
• Results
• Summary
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Background• Ongoing FAA Rulemaking Seeks to Improve on the
Existing and Future Fuel Tank Safety– Consistent Accident Trends are a Concern
– Focus of Concern is on Heated Center Wing Tanks (CWTs)
• Fuel Tank Inerting is a Well Established Method of Reducing/Eliminating Ullage Vapor Flammability– Has Been Meet with Resistance by Industry Leaders
• FAA Would Like to Develop Cost Effective Methods of Modeling Inert Gas Distribution in Commercial Transport Fuel Tanks
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Equipment• Boeing 747SP Full-Scale Inerting Test Article
– Decommissioned from Airline Service and Purchased by the FAA for Ground Testing Only
• All Major Systems Fully Operational
• Has Independent Power for Test Equipment and Instrumentation
– Full Complement of Ground Service Equipment
• Aircraft Modified to Study Inerting– Inert Gas Deposit System Installed on Aircraft
• Inerts CWT from Ground Source of Nitrogen Enriched Air (NEA)
– Instrumentation• Gas Sample Tubing at 8 Locations for Oxygen/THC Analysis
• 32 Thermocouples in Tank (Ullage, Fuel, Walls, Floor, and Ceiling)
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Boeing 747SP Aircraft
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Boeing 747-100/SP Center Wing Tank
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Boeing 747SP CWT Top Diagram
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
• Scale Tank Test Article– 24% Scale Model of Boeing 747 SP CWT was Built from 3/4
Inch Plywood By Scaling Drawings from Shepherd Report• Spars and Spanwise Beams Simulated with ¼ Inch Plywood
Installed in Slats with Scaled Penetration Holes
• Vent System Simulated with PVC Tubing Plumbed to an Aluminum Vent Channel Plumbed Similar to Aircraft
– Instrumentation• Oxygen Sensor in Each Bay and in One Vent Channel Plumbed in
Unique Sample “Drafting” Method Returned to Each Bay
• Thermocouple in Each Bay
– “Variable NEA Manifold” Allowed for NEA to be Deposited in Any and All Bays of the Tank at Different Flow Rates
Equipment
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Scale Plywood CWT Model
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Procedures• 24% Scale Tank Testing
– Series of Tests Done to Examine Different Deposit Schemes• Deposited Different Amounts of NEA in Different Bays to Determine the most
Efficient Method of Deposit in a Half Blocked Venting Configuration
• All Work Presented is for 95% NEA and 128 CFH Total Flow Rate
– Focus of Testing was to Find Best Method of Depositing NEA
• Boeing 747SP Full-Scale Inerting Testing– Series of Tests Done to Examine the Efficiency of Inerting
• Single Deposit (Optimal from Scale Testing)and Venting Case
• Tested for Different Day and Operational Conditions
• All Work Presented is for 95% NEA and 140 CFM Total Flow Rate with ACMs Running (Vertical Mixing Stimulated)
– Focus of Testing was on Operational Effects and Predictability
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Data Reduction Analysis• Volumetric Tank Exchange is the Ratio of the Volume of
Deposited Gas to the Volume of the Tank
– This Gives a Dimensionless Quantity of Inert Gas Given the Volume of the Tank
• Average Tank Oxygen Concentration
– This Gives a Representation of the Tank Oxygen Constituency Given Varying Oxygen Concentrations in Different Bays
VolumeTankFuel
RateFlowVolumeTimeExchangeTankVolumetric
]6[13.0]5[13.0]4[10.0
]3[10.0]2[23.0]1[31.][
222
2222
BayOBayOBayO
BayOBayOBayOOAverage
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Inert Gas Distribution Engineering Model
Bay
OBay V
tVtO
)()]([ 2
2
• Model Calculates Inert Gas Distribution in 6 Bay Tank, in terms of Oxygen Concentration Evolution, Given NEA Purity and Bay Deposit Flow Rates– Based on Original Single Bay Inerting Model, by FAA CSTA for Fuel
Systems, which Tracks Oxygen In and Out of Each Bay Assuming Perfect Mixing During the Time Step
– Assumes an “Outward” Flow Pattern and Splits Flow into a Bay to Adjacent Bays Using Out Flow Area Relationships
• Basic Formula for Volume of Oxygen in a Bay:
BayOsumBaynnBayOnBayNEAOO VtVtQVtVtQIGOFtQtVtV /)1(/)1()1()(2222
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Bay 1 Flow Out
Bay 2
Flow In Bay 3 Bay 4
Bay 5 Bay 6
Flow Out
Assumed Engineering Model Flow Pattern
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
CFD Model• A CFD Model was Developed with the Analysis
Package FLUENT – Used the Fluent CFD Solver Which Uses a Finite Volume Method
Where the General Conservation (transport) Equation (Mass, Momentum, Energy, etc.) is Solved for Each Finite Volume
– Has Ability to Track Fluid Species (O2 Concentration) at Given Locations
– Model was Solved Using a Laminar Flow Throughout (Oxygen Evolution is Based Entirely on Flow Diffusion)
– For Administrative Reasons, Model was Developed of the Scale Tank and not Full-Scale Test Article
– The Model Developed had Approximately 700K Cells and Ran on Several Platforms Over a Weekend.
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
• Scale Tank Test Data Compares Well with Full-Scale Test Article Data– Bay 4 Does Not Compare Well for Any Modeling Method
• Engineering Model Compares Fair– Trend Data Very Good but Some Bays have Large Discrepancy in
Some Bay Oxygen Concentration Values when Compared with Full-Scale Data
• CFD Model Comparisons Initially Poor– Trend Data Marginal with Large Discrepancies in Some Bay
Oxygen Concentration Values when Compared with Full-Scale Data
– Subsequent Data Has Much Better Agreement
Results - Full Scale Comparison
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Scale Tank 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
B-747 SP CWT Inerting, Single Bay DepositComparison with Scale Tank Data
B-747 Data Scale Tank
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Engineering Model 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
B-747 SP CWT Inerting, Single Bay DepositComparison with Engineering Model
B-747 Data Engineering Model
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________CFD Model 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 DepositComparison with CFD Data
B-747 Data CFD Data
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Modeling Methods Compared
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Voumetric Tank Exchange
Ave
rag
e O
xyg
en C
on
cen
trat
ion
(%
vo
l)
Full-Scale Test Article
24% Scale Tank
Engineering Model
CFD Analysis
Weighted Average CWT Inerting DataSingle Bay Deposit, 95% NEA, 20-Minute Inerting
8% Line
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
• Comparing Scale Tank Test Article Data with Engineering Model Data for Different Deposit Scenarios Has Mixed Results– Bay to Bay Oxygen Concentration Comparisons Vary for
Different Deposit Scenarios
– The Average Oxygen Concentration Trend Data for the Different Deposit Scenarios is Consistently Biased High for the Engineering Model Except for the Single Deposit Case
– This Results in a Discrepancy Between Which Deposit Method is Optimal (Most Efficient) for Each Modeling Method
Results - Mock Trade Study
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Engineering Model Compared with Scale Tank
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
B-747 SP Scale CWT Inerting Compared with Engineering Model - Two Deposits
Engineering ModelScale Tank
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
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
B-747 SP Scale CWT Inerting Compared with Engineering Model - Four Deposits
Engineering ModelScale Tank
Engineering Model Compared with Scale Tank
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________Full-Scale Data Compared with Modeling Methods
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Overall VTE
Ave
rag
e O
xyg
en C
on
cen
trat
ion
(%
vo
l)
Balanced Deposit Balanced Deposit
Four Bay Deposit Four Bay Deposit
Two Bay Deposit Two Bay Deposit
Single Deposit Single Deposit
B-747 SP Scale CWT Inerting Compared with Engineering Model - Average Tank [O2]
Engineering ModelScale Tank
AAR-422 Fire Safety R&D
Inert Gas Distribution Modeling___________________________________
Summary• Scale Tank Testing Produced Good Results when
Compared with the “Good Mixing” Full-Scale Testing– Cost Effective Modeling Method
• Simple Engineering Modeling Methods Can Produce Fair Results in a Very Cost Effective Way – Additional Work Needed to Improve Model
– Additional Research Required to Resolve Discrepancies between Engineering Model and Scale Tank for for Multiple Deposits
• CFD Data Labor/Resource Intensive and Eventually Resulted in Good Comparison to Full-Scale Data
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