Effectiveness of Water Mist Droplet Size on Fire suppression in Air Craft

Cabin and Cargo Compartment

Vahid Motevalli, Ph.D., P.E.Aviation Institute

The George Washington University, Washington D.C.

Fifth Aircraft Fire and Cabin Safety Research ConferenceOctober 29- November 1, 2007

Motivation• Water mist has been identified as a potential alternative to Halons• Smaller than 150 μm water mist droplets have been shown to be

effective in fire suppression• The physics of droplet size effects in terms of thermodynamic

cooling vs. oxygen displacement need to be better understood• Other applications of mist (other than water)

Advantages of water mist over sprinkler systems• Water damage reduction• Retrofit applications• Weight considerations, aircraft applications• Facilities where water vapor or run-off could cause higher damage

Applications include:• Aircraft Cargo Compartments and Cabins• Engine nacelle• Hidden spaces

Relevant Previous Research

• Study of the suppression Mechanism of Small Flames by Water Mist, Ndubizu et al. (1995, 1998, 2000)

• Minimum Performance Standard for Aircraft cargo compartment Halon Replacement Fire suppression Systems, Reinhardt et al. (2003)

• Spray as a fire suppression agent for aircraft cargo compartment fires, Marker et al. (2001)

• One zone model of water mist fire suppression systems, Li, Y. F. et al. (2004)

• Full-scale test effectiveness of water spray systems for improved aircraft fire safety, Sarkos et al. (1995)

• Cargo compartment fire protection in large commercial transport aircraft, Blake et al. (1998)

• Aircraft Cabin Fire Suppression by Means of an Interior Water Spray System, Whitfield et al. (1988)

• Small-scale and large-scale experimental research and testing at NRL

Relevant Previous Research

Competition between Oxygen Displacement and Thermodynamic Cooling

• Thermodynamic Cooling - Water mist droplet evaporation removes energy due to latent heat

• Oxygen Displacement – Water mist droplet evaporation creates a cloud of water vapor around each droplet that displaces the rest of gases including oxygen, thus causes a drop in oxygen concentration.

Water Mist Interaction in Flame Zone

Control Volume

Flame

Mist

Inflow

outflow

entrained flow

airairpmentenmoutpwwddvapmentinoutpentin

wcccfwwpvvapwwdd

TcmmTcVnYmmTcmm

hmTcLYVn

Δ−+Δ++−Δ+

=−+Δ+

,,

..

,,

.

,

....

,

.

,,

.

)(])[(])[(

)()(

ρ

ηηρ

Issues:• Mist droplet penetration into the flame zone, fn (droplet size, velocity, flame intensity)

• Evaporation rate and efficiency of oxygen displacement is a function of surface area (d2)

• Modeling the chemistry to account for oxygen displacement

Theoretical Overview

• Conservation equations with and without mist

xentrained

poutflow

pclow

p dTdAvcdTdAucQdTdAuc ∫∫∫∫∫∫ +=+ ρρρ.

inf

. . .

,inf

( ) ( )p p p x p wc m latentlow outflow entrained

uc dTdA Q Q uc dTdA vc dTdA mc Tρ ρ ρ+ − = + + Δ∫∫ ∫∫ ∫∫

wpmentpentpmoutpoutplatentmcc TcmTcmTcmTcmTcmQQQ )()()()()()(.

,

..

,

...

,

..Δ−Δ−Δ+Δ−Δ=+−

)()(()(,

.

,

..

.,2

.

,

..

mcccfmccfccfDisplOmcc hmhmhmQQQ ηηηη −Δ=Δ−Δ==−

A multi-zone model has been developed using conservation of mass and energy equations.

Four Distinct Zones

1. Upper layer2. Plume3. Flame4. Ambient zone

Uniformtemperature in each zone

1

2

3

4

Conservation of mass and energy with water mist interaction for the upper layer zone leads to the following

equation:

pevaporatedu

waterfireu

cmmQQ

dtdT

)( +

−=

&&

uuu ZTm ,,

wm&

( )( )buvapwbwpwwater TTcLYmTTcmQ e −++−= &&& )( 0,

Ye, Evaporation Rate

L, Latent heat of evaporation

uu AZm ρ= edtYmm wevaporated &=

pu

aupeu

cmTTcmQ

dtdT )( −−

=&&

Using the conservation of mass and energy equation for non-flaming zones

Upper Layerpuu cTm ,,

AmbientaTQ,&

uZ

ATcQ

Am

dtdZ

aapa

eu

ρρ

&&−−=

i

i

i uaap

i

a

eu Zt

ATcQ

Am

Z +Δ⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

+ ρρ

&&1

i

i

i

i upu

aupeiu Tt

cmTTcmQ

T +Δ⎟⎟⎠

⎞⎜⎜⎝

⎛ −−=

+

)(1

&&ii uau AZm ρ=

Applying forward difference discretization

Water Droplet Evaporation Rate

According to D2 Law for droplet evaporation:

( ) KtDtD −= 20

2

D (t): diameter of droplet as a function of timeD0 : initial diameter of the dropletK: Evaporation constant

t seconds

D0 D (t)

20D

Ktmm inwρ

••=

This relation would allow calculation of oxygen concentration diluted by water vapor

( )fg

boilpgq h

TTcB

−= ∞

Using formulation by Turns, S. R., Kg, the mean thermal conductivity of gas is calculated

( ) ( )TkTkk Fg ∞+= 6.04.0

2∞+

=TT

T boil

Tboil, the boiling temperature of the droplet (water droplet)

, thermal conductivity of gas∞k

( )1ln8

+= qpgl

g Bck

Kρ

Evaporation Rate Formulation

Evaporation constant for ( )290 2037T k≤ ≤

y = -3E-17x3 + 1E-13x2 - 2E-11x - 5E-09R2 = 0.9998

1.00E-09

5.10E-08

1.01E-07

1.51E-07

2.01E-07

2.51E-07

3.01E-07

290 590 890 1190 1490 1790 2090

T∞(k)

K(m

2 /s)

Evaporation Rate for Different Droplet Sizes

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.02 0.04 0.06 0.08 0.1 0.12

Δt (s)

Y vap

d=20 microns d=40 Microns d=60 micronsd=80 microns d=100 microns

Evaporation Rate for Different Droplet Sizes,Different Ambient Temperature

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500Diameter(μm)

Y eva

p

T=900K T=1000K T=1100K T=1200K

Results for Upper Layer TemperatureDimensions of Enclosure: Area : 12 m2 , Height: 3 m

Time Step (Δt=0.15 seconds)Slow Fire Growth

0

100

200

300

400

500

600

0 50 100 150t(s)

00.20.40.60.811.21.41.61.8

Upp

er L

ayer

Hei

ght (

m

Upper Layer Temperature FirmVB

Upper layer Height FirmVB

Δ Tu (

K)

Results for Upper Layer TemperatureDimensions of Enclosure: Area : 12 m2 , Height: 3 m

Time Step (Δt=0.15 seconds)Fast Fire Growth

0100200300400500600700800

0 20 40 60t(s)0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Upp

er L

ayer

Hei

ght

Upper Layer TemperatureFirmVBUpper layer HeightFirmVB

ΔTu

(K)

Fire Scenario: Fast Fire GrowthWater Flow rate: 0.5 kg/s, Droplet size: 150 μm, 100 μm, 50 mm

Total water used: 13.5 kgAverage rate of change for upper layer temperature, -1.102 º/s, -7.342 º/s, -21.08 º/s

0

100

200

300

400

500

600

700

0 5 10 15

t(s)

Upper Layer Temperature, Without Water MistUpper Layer Temperature, Droplet Size= 150 micronsUpper Layer Temperature, Droplet Size= 100 micronsUpper Layer Temperature, Droplet Size=50 microns

ΔT (K

)

300

350

400

450

42 42.5 43 43.5

t(s)

Plume ZoneTemperature, With Water MistPluem Zone Temperature, Without Water Mist

ΔTu

(K)

Fire Scenario: Fast Fire GrowthWater Flow rate: 0.35 kg/s, Droplet size: 100 μm, Total water used: 0.5775 kg

Average rate of change for Plume zone temperature, -8.115 º/s

Water Droplet Size Effect in The Flame ZoneThermodynamic Effect Only

1350

1400

1450

1500

1550

1600

1650

0 100 200 300 400 500

d(μm)

ΔT o

ut5 gr/s 6 gr/s

Proposed Experimental ApproachMEASUREMENTS

• Monodisperse mist

• Droplet size analyzer

• Temperature Measurement (TC)

• Thermal Mapping (IR Camera)

• Oxygen Concentration

• Mist evaporation Rate

Screen

Flame

Mist

Inflow

Co-flow

Premixed or Diffusion Flame

Mist Capture

TC Probe

Experiment Characteristics• Wolfhard-Parker burner with co-flow• 2-D flame, 10x1 cm, Up to 5 kW • Premixed and Diffusion Flames• Laminar flow in the burner

IR Camera

Conclusions• Simpler mathematical models, as opposed to CFD, can be

used to track mono-disperse droplets and examine flame-mist interaction.

• Smaller droplets are more effectiveness in thermodynamic cooling, because of the surface area per unit mass, but suffers with lower mass per droplet

• Oxygen displacement is a function of droplet surface area but is only important in the flame zone

• A carefully designed experiment proposed here can be used to resolve the question of oxygen displacement effect vs. thermodynamic cooling

• Quantity of water used can be reduced for several suppression applications, such as air cabin and cargo compartment