effectiveness of water mist droplet size on fire suppression in air … · 2007-11-06 ·...
Post on 02-Jun-2020
5 Views
Preview:
TRANSCRIPT
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
top related