22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference June 24th-26th, 2002...

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



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



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)


Inert Gas Distribution Modeling___________________________________Boeing 747SP Aircraft


Inert Gas Distribution Modeling___________________________________Boeing 747-100/SP Center Wing Tank


Inert Gas Distribution Modeling___________________________________Boeing 747SP CWT Top Diagram



• 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


Inert Gas Distribution Modeling___________________________________Scale Plywood CWT Model



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



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



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



Bay 1 Flow Out

Bay 2

Flow In Bay 3 Bay 4

Bay 5 Bay 6

Flow Out

Assumed Engineering Model Flow Pattern



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.



• 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


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


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


B-747 SP CWT Inerting, Single Bay DepositComparison with Engineering Model

B-747 Data Engineering Model


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


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



• 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


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


B-747 SP Scale CWT Inerting Compared with Engineering Model - Two Deposits

Engineering ModelScale 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


B-747 SP Scale CWT Inerting Compared with Engineering Model - Four Deposits


Engineering Model Compared with Scale Tank


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]




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

22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference June 24th-26th, 2002...

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Transcript of 22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference June 24th-26th, 2002...