Fundamental study on accidental
explosion behavior of hydrogen/ air
mixtures in open space
Toshio Mogi, Woo-Kyung Kim, Ritsu Dobashi
The University of Tokyo
ICHS 2011
International Conference on Hydrogen Safety
September 12-14, 2011
San Francisco, California-USA
1
Background
Clean energy carrier
Renewable energy
Expected as an alternative
fuel ( ex. fuel-cell vehicle)
Hydrogen filling station
Hydrogen
Low ignition energy (0.019mJ)
Properties on safety Extensive flammable region (4-75vol%)
Easy leakage and high diffusivity
If hydrogen leaks from hydrogen handling system,
electrostatic spark discharge
serious fire and/or explosion accidents.
2
Background
Gas explosion causes indeed serious damages.
Hazard analysis on an accidental explosion is very important.
To evaluate the strength of hydrogen/air
mixture explosion, unconfined large scale
experiments were recently carried out.
K. Wakabayashi, et al, 1st ICHS, 2005
However, there has been little systematic
research on the relation between flame
propagation and blast wave in
unconfined space.
M. Groethe, et al, 1st ICHS, 2005
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
Soap bubble
High speed camera
Knife edgeIgnition system
Concave mirror
Ignition coil
Control unit
Concave mirror
Battery
Electrodes
Microphone
Oscilloscope
Amplifier
Nozzle
Sound pressure measuring system
Mercury lamp
High speed Schlieren
photography system
Mixing chamber
Vacuum pump
Air cylinder
Hydrogen cylinder
Gas supplying system
5
Detail of Schlieren pictures
Boundary between
mixture and surrounding air
Bubble surface
Bubble surface
Insulator
Electrode
Flame front
Nozzle
Before ignition
After ignition
6
Movie (f = 1.8 )
7
Flame propagation at equivalence ratios
of 0.7, 1.0, 1.8.
100 mm
r =39mm, t =3.5ms
r =63mm, t =5.5ms
r =83mm, t =7ms
r =124mm, t =10ms
Φ=0.7
r =42mm, t =1.5ms
r =74mm, t =2.5ms
r =89mm, t =3ms
r =123mm, t =4ms
Φ=1.0
f
r =47mm, t =1.5ms
r =84mm, t =2.5ms
r =105mm, t =3ms
r =125mm, t =3.5ms
Φ=1.8
Time
8
Flame propagation at equivalence ratios
of 2.5, 3.0, 4.0.
r =40mm, t =1.5ms
r =77mm, t =2.5ms
r =115mm, t =3.5ms
r =152mm, t =4.5ms
Φ=2.5
r =40mm, t =2ms
r =84mm, t =4ms
r =128mm, t =6ms
r =153mm, t =7ms
Φ=3.0
f
r =37mm, t =3ms
r =66mm, t =5ms
r =121mm, t =8.5ms
r =149mm, t =10ms
Φ=4.0
Time
9
Flame radius versus time at various
equivalence ratios
Flame radius r [mm]
100
80
0.7
1.0
1.3
1.5
1.8
2.0
2.5
3.0
3.5
4.0
60
40
20
0
0
2
4
6
8
10
Time t [ms]
12
14
16
Mean burning velocity calculation
dr  rb
Smean  
dt  ru



3
ru: initial soap bubble radius
rb: burned flame radius
10
Comparison between measured mean
burning velocity and literature data
Mean burning velocity Smean [m/s]
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]
Present work
4
3
2
1
0
0
1
2
3
Equivalence ratio φ
4
5
11
Pressure wave histories with different
equivalence ratio
Overpressure P [Pa]
200
0.7
1.0
1.3
1.5
2.0
1.8
2.5
3.0
3.5
4.0
150
100
50
0
0
2
4
6
8
Time t [ms]
10
12
12
Comparison with existing simple model
Theory of acoustics
p
The blast overpressure at the position d from the explosion point is equated by
the theory of acoustics;
Spherical flame
t
r
d
Pressure sensor
(side-on)
 d  dV 
p (t ) 


4d dt  dt 
p
: pressure
t
: time
dV/dt : volumetric rate of combustion
A.Thomas et al. (Proc. R. Soc. Lond. A 294: 449-466 ,1966)
S
ｒ
r   St
p(t )  2

d
   1rS 2
S : burning velocity
 : volumetric expansion ratio
rq : flame radius at quenching
13
Comparison between measured and
predicted peak overpressure
Peak overpressure Pmax [Pa]
200
Exp
Cal
150
100
50
0
0
0
0.5
1
29.5
1.5 2 2.5 3 3.5
Equivalence ratio φ
4
42.9
55.5
62.5
Volumetric fraction of H2 [%]
4.5
5
67.5
14
Discussion-Existing study on blast wave
at acceleration of flame propagation
 d  dV 
p (t ) 


4d dt  dt 
Laminar flame
propagates spherically
S
   1 2 dS
r
p(t )  2    1rS 
d 
dt
d
S=constant

2
ｒ
S : burning velocity
 : volumetric expansion ratio
rq : flame radius at quenching
A.Thomas et al. (Proc. R. Soc. Lond. A 294: 449-466 ,1966)
r   St
15
Time histories of flame radius, burning
velocity, overpressure (f = 0.7)
3
Cal. (S = Sm)
30
Measured burning
velocity Sm
2
20
1
10
0
0
2
4
6
Time t [ms]
8
Measured burning velocity Sm [m/s]
≠constant
φ=0.7
Exp. data
Cal. (S = Sl )
Overpressure P [Pa]
dr  rb 
S   
dt  ru 
40
3
0
10
16
Time histories of flame radius, burning
velocity, overpressure (f = 1.8)
φ=1.8
Exp. data
Cal. (S = Sl )
Overpressure P [Pa]
Cal. (S = Sm)
150
3.0
2.8
Measured burning
velocity Sm
2.6
100
2.4
50
2.2
0
0
1
2
3
Time t [ms]
4
Measured burning velocity Sm [m/s]
200
2.0
17
Time histories of flame radius, burning
velocity, overpressure (f = 3.0)
3.0
150
Overpressure P [Pa]
Cal. (S = Sm)
2.5
Measured burning velocity Sm
100
2.0
50
0
1.5
0
1
2
3
4
Time t [ms]
5
6
Measured burning velocity Sm [m/s]
φ=3.0
Exp. data
Cal. (S = Sl )
1.0
18
Discussion
Diffusive-Thermal instability(Lewis number)
Unburned
side
Unburned
side
Burned
side
(Le>1,stable)
Le 

D
Burned
side
stable
(Le<1,unstable)
Mass diffusion D
Heat diffusion 
unstable
19
Discussion
Different type of wrinkled flame
f = 0.7
Diffusive-thermal instability
f = 4.0
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
Conclusions
1) 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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Thank you for your attention!
[email protected]
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