pda droplet measurement system national institute for aviation research non steady state...

PDA Droplet Measurement System

National Institute for Aviation Research

Non Steady State Flammability in Fuel Non Steady State Flammability in Fuel Tanks: Studies of Aircraft Fuel Tank Tanks: Studies of Aircraft Fuel Tank

Vapor Dynamics (FTVD)Vapor Dynamics (FTVD)

Fuel System Safety Group2003 CRC AVIATION

MEETINGSAlexandria, Virginia

April 29, 2003

David N. KoertWichita State UniversityWichita State University


Flammability in Fuel TanksFlammability in Fuel Tanks

2003 CRC AVIATION MEETINGS April 29, 2003

Motivation

Seek answers to key questions regarding:•Conditions at which fuel fog is formed •Flammability of the resulting air/fuel‑vapor/fuel‑aerosol mixture

Provide insight to supplement development of onboard inerting systems•Fuel fog formation occurs at conditions relevant to wing tanks

•Results may be pertinent in development of control systems for onboard inerting systems




Conduct experimental studies in Fuel Tank Test Cell (FTTC)

•Supersaturated fuel vapor will condense to form fuel fog

•Study fog formation by thermal/mass diffusion- Fuel (liquid) in bottom of tank is “hot” relative to the ullage above

- “Pre-fight scenario”- Primary mechanism of fog formation

•Study fog formation by adiabatic decompression- Fuel (vapor) in ullage is cooled during decompression

- “Takeoff scenario”- Secondary mechanism of fog formation

Identify conditions that promote droplet formation in fuel tanks

Understand vapor dynamics to minimize explosive conditions

Project Overview




Influence of Suspended Droplets on the Lower Flammability Limit

Fuel droplets in fuel-vapor air mixture may reduce the lower flammability limit •Dependent on fuel air ratio•Also dependent on droplet number density and droplet size distribution (Burgoyne and Cohen, 1954; Ott, 1970)

•Transition from vapor flammability (<10μ) to droplet flammability (>40 μ)

Burgoyne and Cohen•Tetralin aerosols formed by homogenous gas-phase nucleation in a 5.4 cm tube

•LFL of droplet-vapor-air ½ that of vapor-air

Variation with Droplet Size of (a) the Lower Limit ofFlammability, (b) Average Limit Flame Temperature[Burgoyne and Cohen, 1954]




Key Questions

How is condensed fuel replenished by additional fuel vapor?

Does convective mixing affect total mass of fuel vapor?

How does the Lower Flammability Limit (LFL) of evaporated fuel vapor and condensate compare to the published LFL for Jet A?

What is the impact of this on Fleet Flammability exposure?




Key Aspects of Experimental Observations

Fuel vapor transport is rate limited by convective fluid transport rather than molecular diffusion

Diffusional phenomena induced by temperature differences are primary “cause” of gas-phase condensation

Adiabatic decompression is only a secondary source of condensate

Surface condensation competes with gas-phase condensation

No simple relationship of fuel -vapor/-condensate mass to the equilibrium-based LFL estimate exists

These observations begin to answer Key Questions and lead to a Phenomenological Model of Fuel Tank Vapor Dynamics




Main Points of Remaining DiscussionDesign and measurement capabilities of the WSU FTTC experimental facility

Experimental and modeling results related to fuel vapor transport and gas-phase condensation

Experimental results related to competition between surface condensation and gas-phase condensation

Modeling results indicating the role of an “evaporation layer”

Phenomenological Model of Fuel Tank Vapor Dynamics

Discussion of additional needs for experimental work to answer questions about the non-equilibrium LFL and fleet flammability exposure




Test Facility Design•FTTC

•2 x 1 x 1 m (≈ 640 gal.)

•Design 1000 ft/min climb

•Optical access

•Eductor pump system•Water-Jet w/control valve

•Constant vacuum, Psat@Tw

•Surface heating-cooling•18 kW heater/chillers•Surface-mount heat exchangers

•Computer control system•Tank p-T control•Data logging

WindowHeater/Chiller

Heat Exchanger Eductor Pump

Fuel Tank Test Cell (FTTC) Indicating Related Systems




Measure Systems on FTTC Experimental Facility

Line Laser

Fuel Tank

Windows

VideoCamera

LaserSheet

PDA

PDAMeasurement

Point(20 cm inside)

TC1-TC4MeasuringUllage Gas

Temperature

TC1

TC2

TC3

TC4

13cm26cm

39cm52cm

13cm

TC6

TC5TC7

TC835cm

FTTC Measurement SystemsThermocouple Locations in FTTC




General Summary of FTTC Experiments

Date Name Gas P (atm) T (°F) Measurements Comments

1 10/20/01 Experiment 1 Ar 1.0 55, 90 video, T-P No PDA system, video taken with incandescent lighting (backlit)

2 12/15/01 Experiment 2 Ar 1.0 55, 90 Observation PDA Demo, no particle measurements



5 1/16/02 Experiment 5 Ar 1.0 55, 90 T-P, Droplet PDA Demo with particle measurements, no video

6 10/23/02 Experiment 6 Ar 1.0 55, 90 video, T-P, Droplet Video of horizontal laser sheet

7 11/7/02 Baseline 01 N2 1.0 55, 90 video, T-P, Droplet Video of horizontal laser sheet




11 11/21/02 Decompress01 N2 1.0 - 0.6 70 video, T-P, Droplet Video of horizontal laser sheet

12 11/22/02 Decompress02 N2 1.0 - 0.6 70 video, T-P, Droplet Video of horizontal laser sheet

13 2/28/03 Drydecomp01 N2 1.0 - 0.2 70 video, T-P, Droplet Fuel pan empty, video of vertical laser sheet

14 2/28/03 Baseline05 N2 1.0 55, 90 video, T-P, Droplet Video vertical laser sheet

15 3/1/03 Drydecomp02 N2 1.0 - 0.2 70 video, T-P, Droplet Fuel pan empty, video of vertical laser sheet




Results from Experiment 1

Pressure and Temperature HistoryDuring Droplet Formation Experiment #1

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P1 P2TC01 TC02TC03 TC04TC05 TC06TC07 TC08

Tank bottom heated, top cooled

Conditions:• Argon atmosphere• 10 gal Jet A (1% mass

loading)• Tank top 50 °F• Fuel 90 – 110 °F

Cloud 1st observed at 90 °F





Stills and video taken•Backlit with incandescent light

•Video quality is unconvincing

•Photo shows cloud but not the vigorous flow field





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ple

t D

ata

Tank Sides

Tank Top

Fuel Pan (bottom)

Droplet Diameter (um)

Droplet Concentration (in-3)/10 9̂




Results from Baseline01 (Experiment 7)

View the video named “clip05”

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Side Wall Temp.

Ceiling Temp.

Fuel Temp.

Vol. Ave. Droplet Dia.

Drop Number Density

Line Laser

Fuel Tank

Windows

VideoCamera

LaserSheet

PDA

MeasurementPoint

(20 cm inside)

Line Laser

Fuel Tank

Windows

VideoCamera

LaserSheet

PDA

MeasurementPoint

(20 cm inside)




Results from Baseline05 (Experiment 15)

View the video named “clip02”

Line Laser

Fuel Tank

Windows

VideoCamera

LaserSheet

Line Laser

Fuel Tank

Windows

VideoCamera

LaserSheet

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Time (min.)

Tem

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rop

et D

ia. (

m ),

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ts

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Vel

oci

ty (

m/s

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Ullage Temp.

Ceiling Temp.

Fuel Pan Temp.

Counts

Vol. Ave. Droplet Dia.

Dropet Velocity




Comments on the Videos from Experiments 7 &15

The presence and motion of droplets illuminated by the laser sheet indicates:•Cloud formation due to condensation of sub-cooled fuel vapor from evaporation of liquid

•Vigorous natural convection•Downward motion in vertical laser sheet is indicative of cellular structure in flow field

CFD Modeling also indicates cellular structure in flow field•2-D CFD calculations using FLUENT•Vertical temperature difference of 10°C, tank geometry, and standard gravitational conditions → Ra ≈ 109

•Standard k-ε turbulence model used




Fog Formation via Thermal Diffusion vs. Adiabatic Decompression

Experiment 13: “Dry” decompression•Uniform temperature, 70°F

•Fuel pan empty, residual vapor

•Nitrogen atmosphere

Cloud 1st observed at ~100 torr vac

View “clip_WSD01”

Experiment 7: Baseline 1 decompression•Tank bottom heated, top cooled- Tank top 50 °F- Fuel 90 – 110 °F

•Nitrogen atmosphere•10 gal Jet A (1% mass loading)

Cloud 1st observed at ~90°F

View “thermal_clip”




CFD Modeling Results for “End-View”




CFD Modeling Results for “Side-View”

Velocity(m/s)




Experimental, Modeling and Analytical Estimates of the Transport

of Fuel VaporVideo record indicates:

•Rapid convective transport and eddies indicating turbulence•Downward flow at vertical bisector of rectangular side

CFD velocity field indicates ≈0.2 m/s at outer edge of cells resulting in bottom to top surface a transit time ≈ 10 sec.

Molecular diffusion time estimated using the Wilke-Lee modification of the Hirschfelder-Bird-Spot method indicates bottom to top surface transit time ≈ 105 sec.

Mixing process is the result of a combination of convective transport and turbulent and molecular diffusion dominated by large-scale convective transport




Effects of Surface Condensation in Experiments 1 - 5

Consider qualitative observations of droplet density vs. time

Gas-phase condensation diminished

•1, 2, and 4 fresh fuel•3 once used fuel•5 new fuel, scrubbed tank

Variations attributed to competition between gas-phase and surface condensation




Temperature of “Evaporation Layer” Above Fuel

Modeling with 10°C surface temperature difference indicates:•Thin, warm layer of ullage gas exists adjacent to the fuel in the bottom of the tank

•Transport of fuel vapor from this warm “evaporation layer” to the thick, cool layer of ullage gas above




Phenomenological Model of Aircraft FTVD

Mixing

Pool-SurfacePhase Changes

JET A

Droplet-SurfacePhase Changes

Evaporation

SurfaceCondensation

Jet A pool-surface temperature>

tank-wall/ullage-gas temperature

Interrelated phenomena in aircraft fuel tank vapor dynamics leading to fuel-cloud formation:a) pool-surface evaporation/condensation, b) “turbulent” mixing due to buoyancy driven

flow,c) homogeneous gas-phase nucleation, d) droplet-surface evaporation/condensation




Phenomenological Model of Aircraft FTVD

1. The ullage gases adjacent to the fuel surface (evaporation layer) approach saturated conditions at the fuel temperature, which is greater than that of the ullage-gases and bare tank walls above

2. A convective flow field is formed, which is characterized by Rayleigh numbers ranging from 108 - 1012 due to the geometry and surface temperature differences around the ullage space

3. The near-saturated fuel/air mixture adjacent to the fuel surface (evaporation layer) is mixed with the colder ullage-gases above, rate limited by convective fluid transport rather than molecular diffusion

4. The fuel vapor concentration in the colder ullage-gases above the evaporation layer becomes supersaturated, and homogeneous gas-phase nucleation occurs

5. Droplet growth and/or increasing fog density occurs at a rate limited by competing processes and can only persist while the rate of evaporation from the Jet A pool is sufficient to provide enough fuel to match




Discussion of Lower Flammability Limit: Results from Experiment 9

Tank bottom heated, top cooled

Conditions:- Nitrogen atmosphere

- 1 atmosphere- 10 gal Jet A (~1% mass loading)

- Tank top 50 °F- Fuel 90 – 110 °F

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Time (min.)

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per

atu

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Dro

ple

t D

iam

eter

(u

),

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nce

ntr

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n (

cm-3 x

10-4

)

Y[1] Y[2] Y[3] Y[4]

Y[5] Y[6] Y[7] Y[8]

D10 [µm] Conc.




Discussion of Lower Flammability Limit (Cont.)

Jet A LFL from Shepherd, et. al.

Estimates of eq. ratio for vapor and droplets for Experiment 9 data.

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Relative Time (min)

F/A

max. LFL

min. LFL

A/F forVapor + Droplets

A/F forVapor Only




Comments Regarding Key QuestionsExperimental results together and modeling/analysis leads to a model that provides answers to two key questions:

Q: How is condensed fuel replenished by additional fuel vapor?A: Results indicate formation and growth of droplets in a fuel

cloud which can only occur if the mass of vapor consumed in such

processes is replenished by continuous evaporation

Q: Does convective mixing affect total mass of fuel vapor?A: Yes. Convective transport dominates the mixing process

The temperature range at which this phenomena occurs indicates:•Fuel fog formation occurs at conditions relevant to wing tanks•Results may be pertinent to development of control systems for onboard inerting systems




Comments Regarding Further WorkFurther work is required to answer remaining questions regarding the LFL for non-equilibrium conditions and the risk, i.e.,•How does the Lower Flammability Limit (LFL) of evaporated fuel vapor and condensate compare to the published LFL for Jet A?

•What is the impact of this on Fleet Flammability exposure?

Additional research on cloud formation in the WSU FTTC is required •Map/Indentify cloud formation conditions/parameters•Identify characteristics of Jet A fuel-vapor-aerosol

Supporting research on flammability limits of Jet A fuel-vapor-aerosol mixtures would be helpful

pda droplet measurement system national institute for aviation research non steady state...

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