International Aircraft Fire and Cabin Safety Conference Fuel Tank Inerting Modeling Ivor Thomas...

International Aircraft Fire and Cabin Safety Conference

Fuel Tank Inerting Modeling

Ivor ThomasConsultant to FAA

1 425 455 [email protected]


Background

• Following TWA 800 the FAA undertook to examine fuel tank inerting to determine if a system could be made practical.– Two ARAC studies– FAA lab, ground and flight test programs– Computer simulations of system performance


Computer simulations of system performance

• Initial model to verify system performance in a single bay tank, using FAA Technical Center tests to confirm model.

• Flight simulation model to look at performance of an inerting system throughout a flight

• Air Separation Module (ASM) performance in flight (effects of available bleed air, system pressure drop etc.)

• Multi-bay simulations to examine in-tank performance (potential for one or more high oxygen bays when the tank average is satisfactory)


Initial Model

• Model Features:– Simple one bay tank,– Single source of Nitrogen Enriched Air, at a

fixed level of O2– Single vent overboard– Sea level conditions only


Initial Model

• Model Results:– Mixing was very rapid, and could be assumed

to be instantaneous– Starting from 21 % O2 model would predict

test results accurately


FAA Tech Center Test Data vs. Model4 %O2 NEA flow, 88 cu ft tank, 75 deg F

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40 45

Time Minutes

Ull

age

oxy

gen

co

nte

nt-

per

cen

t

Test; 2 cfm Flow

Test; 4 cfm Flow

Test; 5 cfm Flow

Model 2 cfm

Model 4 cfm

Model 5 cfm


Flight Model

• Model Features:– Simple one bay tank,– Single source of Nitrogen Enriched Air, at a variable

level of O2 and flow rate • Changeable for different flight conditions• Could add results of ASM performance for given

airplane/engine combination– Single vent overboard– Ground and Flight profiles can be simulated.– Fuel O2 Evolution and Fuel consumption included


Flight Model

• Model Results:– Very useful in determining effectiveness of

inerting options• Ground based inerting • Various sizes of ASM and flow modes• Substantiated concept of variable flow technique,

low flow in climb and cruise, high flow in descent


O2 and N2 Partial Pressure and %O2 in Ullage

0

5

10

15

20

25

30

0 100 200 300 400 500

Time-Minutes

Pa

rtia

l Pre

ssu

res-

ps

ia, %

O2

pTOT

%o2( vol)

Case Details

Init.Oxygen Level %

Fuel Oxygen level %

Tank type

Initial Fuel Load %

21

21

CWT

0

Ground Based Inerting


Total Pressure and %O2 in Ullage

0

5

10

15

20

25

30

0 100 200 300 400 500

Time-Minutes

Pa

rtia

l Pre

ssu

res-

ps

ia, %

O2

pTOT

%o2( vol)

Case Details

Init.Oxygen Level %

Fuel Oxygen level %

Tank type

Initial Fuel Load %

21

21

CWT

90

Ground Based Inerting



0

5

10

15

20

25

30

0 100 200 300 400 500

Time-Minutes

Pa

rtia

l Pre

ss

ure

s-p

sia

, %O

2

pTOT

%o2( vol)

Case Details

Init.Oxygen Level %

Fuel Oxygen level %

Tank type

Initial Fuel Load %

12

21

CWT

90

Dual-Flow On Board Inerting



0

5

10

15

20

25

30

0 100 200 300 400 500

Time-Minutes

Pa

rtia

l Pre

ss

ure

s-p

sia

, %O

2

pTOT

%o2( vol)

Case Details

Init.Oxygen Level %

Fuel Oxygen level %

Tank type

Initial Fuel Load %

12

21

CWT

0

Dual Flow On board Inerting


Air Separation Module (ASM)

• ASM performance strongly affected by available pressure and temperature

• Airplane bleed flow affected by airplane flight conditions.

• Model combined airplane/engine bleed data with ASM performance data to predict ASM performance in flight, which could then be used in the inerting flight model.


Screen Dump of ASM Model

Permeable Membrane Inerting System PerformanceUser Notes Heat ExchangerTo run a case, input values in Yellow Cells and click "Solve for exit pressure" button. Loss Coefficient 0.7

Don't change any other cells or program may not run correctly.Control OrificesFlow Mode 1(1= Descent, 2 = Cruise)

Input Required Waste/Vent outflow Input Orifice and tank dataAltitude 18,000 ft Pressure 7.34 psia Orifice coefficient 7Bleed Pressure 30.22 psia OEA flow 1.31 Lb/min Orifice Delta P 4.40 psigAir temperature 180 Deg F OEA O2 Level 28.3 % Tank Pressure 0.5 psig

ASM Inlet Data Outflow DataAmbient Pressure 7.34 psia NEA Flow 0.79 lb/minHeat Exchanger loss 2.74 psi NEA O2 Level 10.08 %

temperature 170 Deg FAir Flow 1.98 lb/min Outlet Pressure 12.2 psiaInlet Pressure 20.14 psig Outlet Pressure 4.90 psigInlet Pressure 27.48 psia Recovery 51.7 %Inlet Temperature 180 Deg F

ASM Delta P 10.59 psi

Solve for exit pressure


NEA Performance During Descent

0

2

4

6

8

10

12

14

16

0.00 5.00 10.00 15.00

Ambient Pressure psia

NE

A O

2 L

evel

, %

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

NE

A F

low

, L

b/M

in

NEA O2 Level

NEA Flow, lb/min

Typical Single ASM Performance


Multi-bay simulations

• Background:

• The performance of an inerting system can be estimated with the inerting flight model at a gross tank level.

• In-tank effects needed to be examined to understand the potential for bays of a tank to be left at a high O2 state even though the tank average O2 was acceptable.



• Model Features:– Specific airplane /tank set-up

• Bay sizes• Interconnect flow areas • vent geometry,

– Multiple NEA insertion points– Ground and Flight Operation– Leakage



• Model Approach:– Establish bay to bay flow paths, vent flow

paths and any leakage flows– Determine mass changes in each bay for

each time increment, based on NEA flow and altitude change and temperature change

– Solve for flows between bays and compute resultant O2 level in each bay

– Iterate along flight path


Typical Flight Profile, Oxygen Content by Bay

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100 120 140Time, Minutes

Oxy

gen

Con

ten

t %

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Alt

itu

de

ft

Bay 0

Bay 1

Bay 2

Bay 3

Bay 4

Bay 5

Bay 6

Inerting Limit

Altitude


Current Status

• FAA Multi-bay model developed for specific tanks

• NOT available to public as the model uses specific airplane data

• Specific model allows examination of different distribution techniques to minimize bay-to-bay O2 variation, particularly at Landing.


Conclusions

• FAA computer models, together with a large amount of testing to verify the assumptions in the models have allowed the FAA to understand the potential for On-board inerting systems and has allowed FAA to go forward towards an NPRM to address high flammability fuel tanks.

International Aircraft Fire and Cabin Safety Conference Fuel Tank Inerting Modeling Ivor Thomas...

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