Funded by FCH JU (Grant agreement No. 256823)

busyicicleMechanics

Feb 22, 2014 (3 years and 3 months ago)

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Funded

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No
. 256823)

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Definitions


Deflagrations


Deflagration to Detonation Transition


Detonations


Prevention and Mitigation


Conclusions

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Deflagration


Transition from Deflagration to Detonation (DDT)


Detonation

Phenomenon

Flame Speed, m/s

Pressure

kPa

bar

Deflagration

< 340



800



8

DDT

> 700

Detonation




2000



1,600



16

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Effect of other species on the
combustion limits:


The flammable range
decreases with increasing
concentration of CO
2


The detonability range also
decreases with increasing
concentration
of
CO
2

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Temperature

LFL

UFL

Limiting Air
Concentration

Limiting Oxygen
Concentration

[
°
C]

[% v/v]

[% v/v]

[% v/v]

[% v/v]

20

4.1

75.6

20.4

4.3

100

3.4

77.6

19.1

4.0

200

2.9

81.3

15.0

3.2

300

2.0

83.9

10.9

2.3

400

1.4

87.6

6.2

1.3

The flammable range is extended significantly as the temperature is
elevated from a typical ambient temperature of 20
°
C

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LFL

UFL

Pressure

Limiting

Oxygen
Concentration

[bar]

[% v/v]

[% v/v]

[% v/v]

[% v/v]

1

4.3

78.5

21.5

4.5

10

3.9

72.4

27.6

5.8

20

5.8

74.1

25.9

5.4

It is not clear cut that an elevated pressure leads to either a narrowing or
extension of the flammable range. One factor that will influence the
measurement of the flammability limits is the way it is measured, that is to
say the actual measurement technique and the experimental set
-
up

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The burning velocity is
strongly dependent on the
hydrogen concentration


Notice significant scatter in the
experimental data


Experimental method greatly
influences the velocity
measurements


(Maximum) Laminar burning
velocity for H
2

is almost an
order of magnitude higher
than for many hydrocarbons,
for example CH
4

(


0.4 m s
-
1
)

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Ignition of hydrogen
-
air mixtures at or very near LFL


4.0 % v/v

ignition is not possible (?)


5.0 % v/v

a flame kernel is created, but the flame cannot be
sustained


5.5 % v/v

a

flame kernel is created and the flame can be
sustained, but it is highly buoyant


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We will assume


Stoichiometric hydrogen
-
air mixture at 20
°
C and 1 bar


Quiescent environment


No solid walls or obstacles in the immediate vicinity


The flame grows spherically and the flame surface has a smooth
appearance


At some late time, the flame surface becomes “wrinkled” due to


Thermal
-
diffusive effects


Hydrodynamic instabilities

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Why is this important?


The burning rate is proportional to
the flame surface area


Wrinkling increases the surface
area, which in turn leads to an
increase in the burning rate


Transition from laminar to
turbulent combustion may occur
when a threshold has been
exceeded, leading to


Increase in burning rate


Increase in pressure

where
S
l

and
S
T

are the laminar
and
turbulent burning velocity respectively,
and
A
l

and
A
T

are the flame surfaces of
the laminar and turbulent flame

𝑆
𝑇
=
𝑆
𝑙
𝐴
𝑇
𝐴
𝑙

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Fires can be


Non
-
premixed, for example jet fires


Premixed, for example hobs on a domestic cooker


Explosions are always premixed combustion events


The term
Deflagration

is commonly used to refer to explosions
(especially by lay persons) but does also apply to fires


We will refer to explosions as deflagration in this presentation

Fires is covered in another lecture and will
therefore not be considered further here

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To reiterate:


Lower Flammability Limit (LFL), below which no combustion can occur due to lack
of oxidiser; LFL is 4 % v/v for hydrogen in air


Upper Flammability Limit (UFL), above which no combustion can occur due to
lack of fuel; UFL is 75 % v/v for hydrogen in air


However, the flammable range and the energy required to ignite a
mixture are dependent on a number of factors:


Temperature of the mixture

o
Higher temperature extends the flammable range


Pressure of the mixture

o
Higher pressure narrows/extends the flammable range


Stoichiometry of the mixture

o
The lowest ignition energy required is for a mixture which nearly
stoichiometric

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Anatomy of a deflagration


Pressure wave travelling at the speed of sound (


340 m s
-
1

in air at 20
°
C and
101
kPa
)


Flame front, where the reaction takes place, travelling initially at the laminar
burning velocity


The expanding hot products push the flammable gas mixture ahead of it

Products

Unburnt
flammable
mixture

Reaction zone

Pressure wave

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Transition from Deflagration to Detonation (DDT) can occur in


Highly confined regions


Highly congested regions


Flame speed well in excess of the speed of sound in the unburnt
mixture at the onset of DDT


Different physical mechanism responsible for the high speed flame than
for a deflagration


Generates a large pressure spike at DDT, twice the Chapman
-
Jouguet
pressure (somewhere in the region of 3.0
MPa
)

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To reiterate:


Lower Detonation Limit (LDL), below which no detonation can occur due to lack of
oxidiser; LDL is 11 % v/v for hydrogen in air


Upper Detonation Limit (UDL), above which no detonation can occur due to lack
of fuel; UDL is 59 % v/v for hydrogen in air


Detonation cell size


Measure of the sensitivity of the mixture or chemical length scale


Depends on

o
The physical scale of the geometry


Detonability range increases with the size of the flammable cloud for the
same mixture stoichiometry

o
The stoichiometry of the mixture


Not all detonations are self
-
sustainable, that is to say may revert back to
a deflagration

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Products

Unburnt
flammable
mixture

R
e
a
c
t
i
o
n

z
o
n
e

S
h
o
c
k

w
a
v
e


Anatomy of a detonation wave


Shock wave travelling at high
speed (


2,000 m s
-
1
) and
compresses and heats the unburnt
flammable mixture


The reaction zone is following
closely behind the shock wave


The pre
-
heating of the fuel
-
air
mixture leads to high velocity and
pressure

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The severity of an explosion depends on a number of factors:


Mixture

composition

concentration of hydrogen, additives (H
2
O(g), CO
2
, …)


Mixture uniformity


Degree of confinement

for example walls

and ceiling


Degree of congestion

obstacles such as pipes or vessels


Level of turbulence in the flammable cloud

Combustion in
a premixed gas
cloud

Increased
pressure

Expansion

Turbulence
enhances the
combustion

Flow interaction
with obstacles

Turbulence
generation

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Open atmosphere


Essentially unconfined and uncongested


Conventional wisdom suggests that the over
-
pressure would be low

o
P
open



10
kPa


Very high pressures were generated in the Buncefield incident (December 2005)


Closed vessel


Relatively slow flame


Can still generate high over
-
pressures


Uniform pressure in the vessel


Connected vessels


Pressure piling, pre
-
compression of the flammable mixture in the receiving vessel
can lead to much higher over
-
pressures

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Tunnels


Flame directed down the
tunnel


Increased turbulence due to
:

o
Surface roughness

o
Obstacles, such as cars, lorries and coaches

o
Ventilation

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H
2
release
Flame/
Jet fire
Strong blast
effects
Flash fire
Loss of
leak
tightness
Kinetic
effects
Equipment
failure
Escalation
Injury /
casualty
Unconfined
explosive
atmosphere
Confined
explosive
atmosphere
Blast effects
Passive

Flow restriction
Active

Detection and Isolation

Excess f low valve
Passive

Avoid unnecessary
conf inement

Natural ventilation
Active

Active ventilation

Detection and active
ventilation
Passive

Explosion
venting
Passive

Separation distance
Active

Emergency response
Passive

Separation distance
Active

Emergency response
Passive

No ignition sources
Active

Detection and power
shut
-
down
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Prevention


Passive

o
Flow restriction

o
Avoid unnecessary
confinement

o
Natural ventilation


Active

o
Detection and isolation

o
Excess flow valve

o
Mechanical ventilation



Mitigation


Passive

o
Explosion
venting

o
Separation distance

o
No ignition sources


Active

o
Emergency response

o
Detection

o
Power shut
-
down

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Flow restriction to reduce the amount of hydrogen released


Isolation of the system to minimise the amount of hydrogen released


Ventilation


More than one vent is needed and the locations of the vents are
important

remember that hydrogen is a very buoyant gas


Natural ventilation usually does not require any human activation


Mechanical ventilation might be more effective, but may require
activation either by a human or by a detection system


Detection can be difficult for low concentrations (at or below LFL) and
the reliability and drift over time of the sensors must be taken into
account

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The vents opens at a set
pressure


Combustion takes place in the
enclosure


Unburnt mixture is pushed out of
the enclosure


External deflagration


Combustion continues in the
enclosure


Ejected mixture combusts outside
the enclosure

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P1: opening pressure of the vent


P2: external pressure peak


P3: pressure at the peak
combustion rate in the enclosure


P4: oscillatory pressure peak

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Severity of a vented deflagration depends on


The ignition location

o
The closer the ignition location is to the vent the lower the
pressure

o
Very low or no external pressure generation if the ignition
location is right next to the vent


Size of enclosure

o
The most destructive external deflagration occurs when the
enclosure is quite small


The vent areas must be adequate


Simulations have indicated that even a non
-
combusting high
-
pressure release from a could compromise the structural integrity of
an enclosure


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Deflagrations generate overpressures less than about 800
kPa

and the
flame speed is generally less than the speed of sound in air


Transition from Deflagration to Detonation (DDT) is brought on by
turbulence and generates a pressure spike which can be in excess of
3.0
MPa


Detonation can be initiated by a DDT or by direct initiation, which
requires large ignition energy (typically by a high
-
explosive charge), and
generates pressure around 1.6
MPa

and travels at around 2,000 m s
-
1


Not all detonations are self
-
sustaining


Prevention

is better than mitigation

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Supplementary Information

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Fuel

Minimum

Ignition (Initiation) Energy

Deflagration,
mJ

Detonation,
mJ

Hydrogen

0.017

1.0∙10
7

Methane

0.25

2.3∙10
11

Propane

0.28

2.5∙10
9

Ethyne

0.007

1.29∙10
9

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Detonation cell size as a function
of the hydrogen concentration

Energy required for direct initiation
of a spherical detonation

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Damk
ö
hler

number
: Ratio of chemical reaction time scale to
characteristic flow (or integral) time scale


Karlovitz

number
: Ratio between chemical reaction time scale and the
Kolmogorov time scale


Lewis number
: Ratio between thermal conduction and molecular
diffusion


Peclet

number
: Ratio between heat convection and heat conduction


Prandtl

number
. Ratio between momentum diffusion and heat diffusion


Reynolds number
: Ratio between inertial force and viscous force


Schmidt number
: Ratio between momentum diffusion and molecular
diffusion


Zel’dovich

number:
Measure of the temperature sensitivity of the
reaction rate