HYDROGEN-AIR EXPLOSIONS IN A TUNNEL: LES ANALYSIS OF PRESSURE EFFECTS

HYDROGEN
-
AIR EXPLOSIONS IN A TUNNEL: LES ANALYSIS OF
PRESSURE EFFECTS

Verbecke

F
., Makarov D., Molkov V.

University of Ulster

Shore Road, Newtownabbey, Co.
Antrim, BT37 0QB, UK

f.verbecke
@ulster.ac.uk



SESSION
:

01.3


Abstract


Release of hydrogen from a v
ehicle and formation of
a
combustible mixture during
an
accident in a tunnel poses serious safety concerns. If ignited, the hydrogen
-
air cloud may
deflagrate or, in extreme case
s
,
a
transition to detonation
may occur
. To enable contemporary
hydrogen safety

engineering tools, i.e. CFD codes, reliable models should be developed and
codes validated against large
-
scale experiments.
L
arge
-
scale explosion experiments w
ere

performed in
a
1/5 real scale
horseshoe cross section

tunnel
1

of
78.5 m

length. Uniform 20%
and

30% hydrogen
-
air mixtures of 37 m
3

volume

were
ignited at floor level

in the centre of
the
tunnel
.

For stoichiometric hydrogen
-
air mixture an additional experiment was carried out
with obstructions.
The combustion model used is a further development of

the large eddy
simulation (LES) model applied for simulation of large
-
scale deflagrations at the University
of Ulster
2
. The combustion includes three known mechanisms of premixed flame
acceleration
;

the turbulence generated by flame front itself, the effe
ct of flow turbulence on
turbulent burning velocity
;
the fractal increase of
flame surface area

and

interplay between
them.
For spherically propagating near stoichiometric hydrogen
-
air flames

t
he growth of the
turbulence generated by the flame

front itself

to its maximum value, according to theoretical
predictions by Karlovitz
et al.
3
, is modelled based on observation
s

by Gotsintev
et al
.
4
.
The
Yakhot’s formula for turbulent premixed flame propagation velocity, based on the
renormalisation group theory, is
a core part of the mathematical model.
Once the

fully
developed turbulent combustion is established at
R
0

distance
R
0
=1.0
-
1.2 m from the ignition
source
the fractals term substitutes Kartovitz term in the Yakhot’s formula. The theoretical
fractal dimension

2.33 and characteristic value obtained during the model validation against
large
-
scale hydrogen
-
air hemispherical deflagrations were used in simulations.

Simulation
results on pressure dynamics along the tunnel are compared against experimental data. Good

agreement between simulations and experiments is obtained both for overpressures,

including positive and negative phases, and impulse at heights above the middle of the
tunnel. However, numerical simulations reveal
higher overpressure on the floor level i
n the
corners between floor and obstacles.

This contradicts the conclusion of experimentalists that
“the presence of vehicles had no effect on the deflagration”
1
. The phenomenon of
progressively growing overpressures in area between obstacle and floor is d
iscussed.


REFERENCES:

1.

M Groethe et al., Large
-
scale hydrogen deflagrations and detonations,
HYSAFE
ICHS International Conference on Hydrogen Safety, Pisa, Italy, 8
-
10 September 2005

2.

Molkov V., Makarov D., Schneider H.,
LES modelling of an unconfined large
-
scale
hydrogen
-
air deflagration
,
Journal of Physics D: Applied Physics
, 2006, Vol.39, Issue 20,
pp.4366
-
4376
.


3.

Karlovitz B, Denniston D W Jr and Wells F E 1951 Investigation of turbulent flames
The journal of Chemical Physics

19:5 541
-
547

4.

Gotsintsev Yu A,

Istratov A G, Shulenin Yu V 1988 Self
-
similar propagation of a
free turbulent flame in mixed gas mixtures Combustion, Explosion and Shock Waves
24:5 63
-
70