Toshio Mogi, Woo-Kyung Kim, Ritsu DobashiThe 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 cameraKnife 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 =10msΦ =4.0

r =40mm, t =2ms r =84mm, t =4ms r =128mm, t =6ms r =153mm, t =7msΦ =3.0

r =152mm, t =4.5msr =115mm, t =3.5msr =77mm, t =2.5msr =40mm, t =1.5msΦ =2.5

Flame propagation at equivalence ratios of 2.5, 3.0, 4.0.

f

Time 9

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

Flam

e ra

dius

r [

mm

]

Time t [ms]

3

u

bmean

rr

dtdrS 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 S

mea

n [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;

dtdV

dtd

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 velocity : volumetric expansion ratiorq : flame radius at quenching

ｒ 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

pres

sure

Pm

ax [

Pa]

Equivalence ratio

Volumetric fraction of H2 [%]

φ

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Discussion-Existing study on blast wave at acceleration of flame propagation

S

ｒStr

Laminar flame propagates spherically

S : burning velocity : volumetric expansion ratiorq : flame radius at quenching

212)( rSd

tp dtdSr

d21

S=constant

dtdV

dtd

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 Sm

Exp. data Cal. (S = Sl ) Cal. (S = Sm)

Meas

ured b

urning

veloc

ity S

m [m

/s]

Overp

ressu

re P

[Pa]

Time t [ms]

φ =0.7

Time histories of flame radius, burning velocity, overpressure (f = 0.7)

3

u

b

rr

dtdrS

≠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 Sm

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

unstableDLe

Unburnedside

Burnedside

Mass diffusion DHeat diffusion

(Le>1,stable)

Unburnedside

Burnedside

(Le<1,unstable)

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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

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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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Thank you for your attention!

mogi.toshio@mail.u-tokyo.ac.jp

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