toshio mogi, woo-kyung kim, ritsu dobashi the university of tokyo fundamental study on accidental...
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Toshio Mogi, Woo-Kyung Kim, Ritsu Dobashi
The University of Tokyo
Fundamental study on accidental explosion behavior of hydrogen/ air mixtures in open space
ICHS 2011International Conference on Hydrogen Safety
September 12-14, 2011San Francisco, California-USA
1
Background
Hydrogen Low ignition energy (0.019mJ)Extensive flammable region (4-75vol%)Easy leakage and high diffusivity
Clean energy carrier Renewable energy
Expected as an alternative fuel ( ex. fuel-cell vehicle)
Properties on safety
Hydrogen filling station
If hydrogen leaks from hydrogen handling system,electrostatic spark dischargeserious fire and/or explosion accidents.
2
Background
K. Wakabayashi, et al, 1st ICHS, 2005
M. Groethe, et al, 1st ICHS, 2005
To evaluate the strength of hydrogen/air mixture explosion, unconfined large scale experiments were recently carried out.
However, there has been little systematic research on the relation between flame propagation and blast wave in unconfined space.
Hazard analysis on an accidental explosion is very important.
Gas explosion causes indeed serious damages.
3
Objectives
To understand the relation between flame propagation and blast wave in open space
Hydrogen/air deflagration experiment using soap bubble method
The effect of hydrogen/air mixture concentration to behavior of flame propagation and blast wave
4
Experimental setup
Concave mirror
Concave mirror
Mercury lamp
High speed camera
Knife edge
Vacuum pump
Nozzle
Mixing chamber
Electrodes
Hydrogen cylinder
Air cylinder
Ignition coil
Battery
Microphone Amplifier
Oscilloscope
Control unit
Soap bubble
Gas supplying system
Ignition system
High speed Schlieren photography system
Sound pressure measuring system
5
Detail of Schlieren pictures
Before ignition After ignition
Bubble surfaceInsulator
Electrode
Nozzle
Bubble surface
Flame front
Boundary between mixture and surrounding air
6
Movie (f = 1.8 )
7
r =123mm, t =4msr =74mm, t =2.5ms r =89mm, t =3msr =42mm, t =1.5ms
r =39mm, t =3.5ms r =63mm, t =5.5ms r =83mm, t =7ms r =124mm, t =10ms
r =47mm, t =1.5ms r =84mm, t =2.5ms r =105mm, t =3ms r =125mm, t =3.5ms
Φ =0.7
Φ =1.0
Φ =1.8
100 mm
Flame propagation at equivalence ratios of 0.7, 1.0, 1.8.
Time
f
8
r =37mm, t =3ms r =66mm, t =5ms r =121mm, t =8.5ms r =149mm, t =10ms
Φ =4.0
r =40mm, t =2ms r =84mm, t =4ms r =128mm, t =6ms r =153mm, t =7ms
Φ =3.0
r =152mm, t =4.5msr =115mm, t =3.5msr =77mm, t =2.5msr =40mm, t =1.5ms
Φ =2.5
Flame propagation at equivalence ratios of 2.5, 3.0, 4.0.
f
Time9
Flame radius versus time at various equivalence ratios
0 2 4 6 8 10 12 14 160
20
40
60
80
100
0.7 1.0 1.3 1.5 1.8 2.0 2.5 3.0 3.5 4.0
Fla
me
radi
us
r [
mm
]
Time t [ms]
3
u
bmean
r
r
dt
drS ru: initial soap bubble radius
rb: burned flame radius
Mean burning velocity calculation
10
Comparison between measured mean burning velocity and literature data
0 1 2 3 4 50
1
2
3
4
5 Aung et al. - bomb [4] Kwon et al. - bomb [5] Tse et al. - bomb [6] Liu et al. - burner [7] Gunther et al. - burner [8]
Mea
n bu
rnin
g ve
loci
ty
Sm
ean
[m/s
]
Equivalence ratioφ
Present work
11
Pressure wave histories with different equivalence ratio
0 2 4 6 8 10 120
50
100
150
200
0.7 1.0 1.3 1.5 2.0 1.8 2.5 3.0 3.5 4.0
Ove
rpre
ssur
e P
[P
a]
Time t [ms]
12
Comparison with existing simple model
The blast overpressure at the position d from the explosion point is equated by the theory of acoustics;
dt
dV
dt
d
dtp
4)(
d
p
t
Spherical flame
r
Pressure sensor (side-on)
p : pressuret : timedV/dt : volumetric rate of combustion
A.Thomas et al. (Proc. R. Soc. Lond. A 294: 449-466 ,1966)
Theory of acoustics
212)( rSd
tp
S : burning velocitye : volumetric expansion ratiorq : flame radius at quenching
ｒ
e S
Str 13
Comparison between measured and predicted peak overpressure
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
50
100
150
200
0 29.5 42.9 55.5 62.5 67.5
Exp Cal
P
eak
over
pre
ssur
e P
max
[P
a]
Equivalence ratio
Volumetric fraction of H2 [%]
φ
14
Discussion-Existing study on blast wave at acceleration of flame propagation
e S
ｒ
Str
Laminar flame propagates spherically
S : burning velocitye : volumetric expansion ratiorq : flame radius at quenching
212)( rSd
tp dt
dSr
d21
S=constant
dt
dV
dt
d
dtp
4)(
A.Thomas et al. (Proc. R. Soc. Lond. A 294: 449-466 ,1966)15
0 2 4 6 8 100
10
20
30
40
0
1
2
3
Measured burning velocity S
m
Exp. data Cal. (S = S
l )
Cal. (S = Sm)
Mea
sure
d bu
rnin
g ve
locity
Sm [m
/s]
Ove
rpre
ssur
e P
[Pa
]
Time t [ms]
φ =0.7
Time histories of flame radius, burning velocity, overpressure (f = 0.7)
3
u
b
r
r
dt
drS
≠constant
16
0 1 2 3 40
50
100
150
200
2.0
2.2
2.4
2.6
2.8
3.0
Mea
sure
d bu
rnin
g ve
loci
ty S
m [
m/s
]
Ove
rpre
ssur
e P
[P
a]
Time t [ms]
Exp. data Cal. (S = S
l )
Cal. (S = Sm)
Measured burning velocity S
m
φ =1.8
Time histories of flame radius, burning velocity, overpressure (f = 1.8)
17
Time histories of flame radius, burning velocity, overpressure (f = 3.0)
0 1 2 3 4 5 60
50
100
150
1.0
1.5
2.0
2.5
3.0O
verp
ress
ure
P
[Pa]
Time t [ms]
Exp. data Cal. (S = S
l )
Cal. (S = Sm)
Measured burning velocity Sm
Mea
sure
d bu
rnin
g ve
loci
ty S
m [
m/s
]
φ =3.0
18
DiscussionDiffusive-Thermal instability(Lewis number)
stable
unstableD
Le
Unburnedside
Burnedside
Mass diffusion DHeat diffusion
(Le>1,stable)
Unburnedside
Burnedside
(Le<1,unstable)
19
DiscussionDifferent type of wrinkled flame
f = 0.7 f = 4.0
Diffusive-thermal instability Wrinkled flame by rupture of a soap bubble
wrinkled flame by the rupture of a soap bubble is related with non-uniformity concentration distribution
20
Conclusions1) The measurements of the intensities of blast wave show that;
in lean hydrogen-air mixture the overpressure grew linearly with time in rich hydrogen-air mixture the overpressure grew linearly with time in the early stage and acceleratingly increase in later stage. The accelerating increase in the later stage resulted in a much larger peak overpressure than that in the stoichiometric mixture.
2) The overpressure of blast wave can be predicted by the acoustic theory if the real burning velocity could be known.
The theory indicates that the intensity of blast wave is affected by burning velocity, volumetric expansion ratio and flame acceleration.
In particular, the intensity of the blast wave is strongly affected by the acceleration of the burning velocity.
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