Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
FLAME PROPAGATION DURING DUST EXPLOSIONS
Ritsu Dobashi
Dept. of Chemical System Engineering, Shool of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPAN
ABSTRACT
It is important to sufficiently understand the phenomena during dust explosions in order to take
appropriate measures preventing accidental dust explosions. However, at present basic knowledge on
flame propagation in combustible dust (dust flame propagation) is not enough. In this paper, the
particular characteristics of the dust flame are pointed out, which are different from those of gas flames.
Also the some recent experimental results on the dust flame are described.
The dust flame propagation has some special characteristics that the gas flame does not have. The
important points are as follows;
1. The flame propagates in heterogeneous medium
During dust flame propagation, the flame propagates in the region where combustible particles and
vaporized gas and/or liquefied particles coexist. The region is heterogeneous medium, where solid,
liquid and gas coexist. Therefore, complicated phenomena in such heterogeneous medium have to be
considered to understand the dust flame propagation.
2. The movement of dust particles is different from that of gas
The combustible dust particles are moving at the different velocity from gas flow, especially when the
gas flow is accelerated. Near the combustion field, the gas flow is usually accelerated and the
combustible particle can not follow the accelerating gas flow (velocity slip exists). It generates some
special characteristics of the dust flames.
Our recent experimental studies show some interesting results concerning above mentioned features.
1. Studies on the flame propagation mechanism
Flammable limit was examined in the experiments with Stearic acid particles changing the particle
size distribution. It was found that the lower flammable limit strongly depends on the mass density of
smaller particles (less than 60 μm on diameter). This result indicates that the smaller particles has
important role on the flame propagation. Flame structure was also examined using ion probe and UV
band observation system. It was found that the leading flame is maintained by the vaporization of the
smaller particles.
2. Studies on the particle movement
The effects on the velocity difference between the particle and gas was examined on the iron dust
flame. The iron particles can not follow the surrounding gas movement, and then the number density
of iron particle increases near the combustion zone. This behavior strongly affects the combustion
phenomena of iron dust.
The particular characteristics of dust flame propagation were explained with the results of recent
experimental studies.
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Background (1)
„ Appropriate
understanding of
phenomena proceeded during dust
explosion is necessary
to take appropriate measures against
the accidental dust explosions.
Prevention techniques
Mitigation techniques
Risk (consequence) analysis
Flame Propagation
during Dust Explosions
Ritsu DOBASHI
The University of Tokyo, Tokyo, JAPAN
(appropriate consequence prediction;
computer model et.al.)
Background (2)
Procedure of Dust Explosion
„ In
most previous studies,
practical characteristics of dust
explosions are focused;
such as,
Damages
Combustible Dust
Dispersion
Pressure Built-up
Flame Propagation
Explosion concentration limits
Minimum ignition energy
Explosion pressure
etc.
Structural Destruction
Fragment Scattering
Pressure Wave
Ignition
High Temperature
Thermal Damages
Fire
Also the fundamental studies are
needed for consistent measures.
Characteristics of Dust flame
‘Flame Propagation’
Flame (Combustion zone)
„ Understanding
of flame propagation
during dust explosion is essential
Unburned
Burned
Gas velocity
Su
„ Dust
flame has some particular
aspects compared with gas flame
Sb
Flame front
is not smooth
Particle velocity:
not always equal
to gas velocity
„ The
fundamental studies on dust
flame are needed
Radiative heat transfer
Thick combustion zone
17
Heterogeneous
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Propagating Flame Structure
Difference between Dust and Gas explosions
Decane Spray
Gas explosion
„
Flame type
Premixed
Non-Premixed
(homogeneous) (heterogeneous)
„
Thickness
Thin
„
Supply speed Almost same
Often different
of O2 and Fuel (selective diffusion) (particle movement)
„
Influence of
Radiation
Limited
Sometime strong
„
Flame front
Usually smooth
Not smooth
originally
Iron Dust
Dark zone
Luminous zone
Thick
Blue spot flame
Scale
10mm
Volatility
Larger
Premixed flame
Smaller
Diffusion flame?
Surface combustion
How the flame propagates?
Important points to be understood
„ Flame
Stearic Acid Dust
Dust explosion
propagation mechanism
z Propagation
limit
Explosion limit
z Propagation behavior
Risk (consequence) analysis
„ Special
aspects of Dust combustion
‡Effect
of particle movement
‡Effect of turbulence
‡etc.
Propagating
Relay Ignition Mechanism
Ignition
Ignition
Flame development
Premixed
Flame
can not
propagate
Diffusion
Flame
Heat transfer
sequential ignition
+
Oxidizer
(O2)
Fuel
O2
+
H2O
Flame
Non-premixed flame
(Diffusion flame)
Products
Propagate
Flame
Fuel
Oxidizer
(O2)
Products
Flame propagation
18
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Experimental Approaches
Questions on the Relay Ignition
Hypothesis (stearic acid flame)
„ Experiments
„ Why
is the propagating speed (about a
few tens cm/s) so fast?
„ No flame of asymmetrical shape is
observed.
„ It is uncertain that the leading visible
flame exactly corresponds to the
leading combustion reaction zone.
„ Experiments
Copper mesh
T: Thermocouple
Pressure gauge
CH3(CH2)16COOH
Electrodes
d=96mm
d=84mm
-
Fuel
EMV1
EMV2
Heater
Liquid spray
Solid particles
Two fluids
Nozzle
Number density of particles, n/cm3
Quick cooling of
4000
3000
Case A:
Flammable
limit (Stearic acid)
pa=100kPa; pl=0.0kPa
2000
1000
00 20 40 60 80 100 120 140 160 180 200
Diameter, Pm
5000
4000
Case B:
3000
pa=60kPa; pl=60kPa
Fuel
pl
Number density of particles, n/cm3
(heated)
2000
Air
1000
00 20 40 60 80 100 120 140 160 180 200
pa
5000
4000
Case C:
3000
pa=15kPa; pl=10kPa
Sample
140
5000
Diameter, Pm
at liquid state
Air
Heater
Air tank
Number density of particles, n/cm3
Particle size control
Air
Pl
120
Pressure of feeding air pa, kPa
Pa
Air tank
pa
Movable
Cover
Nozzle
T
Air
Stopper
T
1-Octadecanol
(melting point 328 K)
CH3(CH2)16CH2OH
+
㱂94 mm
Flame
propagates in
open space
Neon transformer
(melting point 343 K)
Movable cover
Stearic acid
Cylindrical duct
140mm 40mm 40mm 40mm
d=60mm
2. EXPERIMENTS
Mesh
Ignition
Electrodes
Pa: , Pl:
425mm
Combustible dust
for Metal Dust (Iron)
– Temperature Distribution
Measuring thermal structure
More detailed study is needed.
Experimental
setting
for Organic Dust (resin)
– Limit of flame propagation
Changing particle size distribution
– Temperature Distribution
Measuring thermal structure
– Ion Current Measurement
Detecting combustion reaction zone
– Observation by UV band
Detecting combustion reaction zone
100 A
80
Flammable
60
B
40
C
20
2000
Fuel
1000
00 20 40 60 80 100 120 140 160 180 200
pl
Non-flammable
00
20
40
60
80
Pressure of feeding fuel pl, kPa
Diameter, Pm
19
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
140
Effect
of particle size distribution
120
on flammable limit (Stearic acid)
a
Larger Particles
0.04
0.03
0.02
0.01
0
80
Flammable
60
Diameter, Pm
"Smaller particles govern flame propagation"
B
120
40
20
80
Unrealizable
condition
C
0
0
D
Flammable J
Non-flammable
20
40
60
80
40
Reference
diameter
= 60 Pm
0 20 40 60 80 100 120 140 160 180 200
A
100
160
Non-flammable
Mass density of particles, kg/m3
Smaller Particles
Mass density of
3 air p , kPa
Pressure of
feeding
smaller particles,
g/m
Reference Diameter
E
Non-flammable
0
240
280
320
360
Mass density of total particles, g/m 3
Measurement of temperature
distribution by thermocouple
Results (Limit of flame propagation)
„ Flame
propagation
near
lower
flammable limit is only supported by the
combustion of smaller particles
Direction of inserting into particle cloud
Thermocouple
Pt, (Pt+13%Rh) 20Pm
+ (Rh13%,Pt)
„ The
leading combustion zone might be
sustained by smaller particles, not by
larger particle combustion (blue spot
flame)
„ The
combustion character of dust can
not be indicated by averaged diameter
such as SMD
Sensor holder,
0.2 mm in diameter
-(Pt)
1.5mm
4 mm
Ceramics support tube
5 mm
Junction
Estimation of the heat release rate Q
Measured temperature distribution (1-Octadecanol)
Steady state assumption;
dT
U Cp U
dx
d 2T O 2 Q
dx
U represents the density
Cp the specific heat
U the flame propagation velocity
T the temperature
x the position
O the thermal conductivity
20
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Variation of the heat release rate (1-Octadecanol)
Results (Temperature Distribution)
„ Heat
release rate takes a maximum
value at the position about 2 mm ahead
of the visible leading flame edge
about 2 mm
ahead of visible flame
„ Combustion
reaction must already
starts at the position about 2-3 mm
ahead of the visible flame
Ion Current Measurement (Stearic acid)
(detecting combustion reaction)
Structure of the dust flame (Stearic acid)
Micro-electrostatic probe
Blue flame zone
Schlieren front
Burning particle with
luminous flame
t = 58 ms
15
th
10
5
Schlieren
front
Luminous zone
Dark zone
Sensor of
electrostatic probe
Blue flame zone
Low intensity combustion
at blue spots
Luminous zone
Dark zone
0
0
t = 80 ms
im / 2
20
im
t = 45 ms
im / 2
Ion current, 2u10-2 PA
25
50
100
Time after discharge t, ms
150
Scale
0
10mm
Scale
10 mm
UV band
observation System
Results (Ion Current Measurement)
High-speed video
+ Image intensifier
+ Band-pass filter
„ Combustion
reaction starts at the
position a few mm ahead of the visible
flame)
„ Thickness of the reaction zone (about
ten mm) is thicker than that of gaseous
hydrocarbon-air flame
Mesh
94mm
I
Ignition electrodes
Ordinary video
Pl
Pa
Pressurize
ⓨ᳇
Air
Fuel
Άᢱ
㵭
Air
ⓨ᳇
Pa
Air
ⓨ᳇
21
Hearter
Nozzle open
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Function of band-pass filter
Observation by UV band
Band-pass filter㧔308 nm㧕
Chemical emission
Blue flame
OH㧔281, 306 nm, UV㧕
CH㧔387, 432 nm, Blue㧕
CC㧔517 nm, Green㧕
Band spectrum
by UV band
Image intensifier + Band-pass filter
detecting emission from combustion reaction
Decreasing
Luminous
flame emission
Black body radiation
Luminous flame
„ Observation
To detect
combustion reaction
zone sensitively
Sensitive detection of combustion
reaction zone can be realized.
Blue spot flame
Scale
10mm
Observed propagating flames
(UV band, Ordinary system)
Observed flame propagation䋨at UV band䋩
Almost continuous propagation
Zone where
blue spot
flames exist
Luminous
flame
10mm
Observation Observation
at UV band by Ordinary system
White line indicates the leading edge of blue flame zone
5 mm
Flame propagation mechanism ?
Result (Observation by UV)
„ The
leading flame front is not
discrete spot flames but continuous
reaction zone ahead of spot flames
„ The
leading flame was observed to
propagate continuously
22
Sequential Ignitions of Diffusion Flames
(Relay Combustion)
Diffusion Flame
Ignition
Ignition
Development of
Combustion region
Heat Transfer
Propagation of Premixed Flame
Diffusion
Flame
Premixed Flame
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Possible structure of the dust flame
(1-Octadecanol and Stearic acid)
Propagating Flame Structure
Decane Spray
Stearic Acid Dust
Iron Dust
Schlieren front Blue spot flame zone
Dark zone Luminous flame
Dark zone
Luminous zone
Blue spot flame
Scale
It mainly consists of
the burning of
smaller particles
10mm
Leading combustion zone;
Larger
Volatility
locates
ahead of visible flame,
is supported by smaller particles,
Premixed
Diffusion flame?
propagatesflame
continuously
Diffusion flame
Flame propagating
direction
Leading reaction zone
(a kind of premixed flame)
Smaller
Surface combustion
Wp : characteristic time of flame propagation
Wg : characteristic time of gasification
Thick combustion zone
Vaporization
„ Experiments
1-Octadecanol, Stearic acid particle …
for Organic Dust (resin)
– Limit of flame propagation
Changing particle size distribution
– Temperature Distribution
Measuring thermal structure
– Ion Current Measurement
Detecting combustion reaction zone
– Observation by UV band
Detecting combustion reaction zone
Thin combustion zone
similar to pre-mixed flame
Wp > Wg
Wp < Wg
"relay ignition
mechanism" can not
be adopted
Experimental Approaches
Supposed flame structures for characteristic time scale
Vaporization
Real leading
combustion zone
must exist at the
position ahead of the
visible leading
combustion zone
„ Experiments
for Metal Dust (Iron)
– Temperature Distribution
Measuring thermal structure
Decane spray …
Structure of the flame propagating in Iron dust
Flame propagation in Iron dust
Combustion
chamber
Argon ion laser
Ignition
system
High speed
video camera
Dispersing
cone
Movable
tube
Magnetic
valve
Controller
Air tank
High pressure tank
Air nozzle
23
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Profile of the temperature
across the combustion zone
Iron particle diameter distribution
Velocity
and
max.
temp.
Max. Temp. and flame Velocity
Max. Temp. and flame Velocity
„
„
Result (Iron dust flame )
„ Combustion
reaction occurs only on the
particle surface
Maximum measured temperature and
propagating velocity are in linear relation
Effect of radiative heat transfer (~T 4) is not
dominant in this case
S lad
„ Thickness
of combustion zone is about
5 mm
„ Radiation
is not the dominative heat
transfer mechanism (propagating as a
heat wave)
Og ˜ Tad Tinf K 0 2 ˜K 0 U p F V 0 Tad4 Tinf4 U p d p
U m Cm Tad Tinf Cassel, Liebman, and Mock, 1956
24
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Propagating Flame Structure
reaction
DecaneSurface
Spray combustion
Stearic
Acid Dust
(no gas phase reaction)Dark zone
Luminous zone
Radiative heat transfer is not dominant
(propagating as a heat wave)
Important points to be understood
Iron Dust
„ Flame
propagation mechanism
z Propagation
limit
Explosion limit
z Propagation behavior
Risk (consequence) analysis
„ Special
Blue spot flame
Scale
Leading combustion zone;
Larger
Volatility
locates
ahead of visible flame,
is supported by smaller particles,
Premixed
Diffusion flame?
propagatesflame
continuously
of particle movement
‡Effect of turbulence
Smaller
Surface combustion
Characteristics of Dust flame
Difficulty to define ‘Burning Velocity’
in dust flame
Flame (Combustion zone)
Unburned
Burned
Gas velocity
SL
Vgb
Vgu
Particle vel.
Vpu
Vgu
aspects of Dust combustion
‡Effect
10mm
Uu
Vgu or Vpu ?
z Vpu
similar as selective diffusion
Fuel Conc.
Oxygen Conc.
z
1
H
Ub
H?
difficult to determine
Fuel Flux to reac. zone
Oxygen Flux to reac. zone
Laser scattering observation on Iron dust flame
Distance from ignition point, cm
Iron particle trajectory
Laser light
scattering
3
Leading edge of
combustion zone
2
1
Particle
50
100
Time from ignition, ms
25
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Velocity slip between particle and gas
Change of particle number density
ux 'x , Nx'x , U
Control volume
Particle velocity
x+'x
Particle movement
20
x, mm
Velocity slip must cause change of
particle number density
'x
Gas movement
10
Gas velocity
20
m/s
x
x
A
-10
10
ux , Nx , U
-10
Combustion zone
Distance from leading edge of combustion zone, x, mm
Combustion zone
Flammability lean limit
15
10
5
0
„
Adipic acid
Lean limit
Equivalence
ratio
0.28
Ethane
0.52
Lactose
0.22
Ethylene
0.40
Poly-ethylene
0.17
Dust
Benzene
0.49
Sulfur
0.10
Methanol
0.46
Aluminum
0.29
Ethanol
0.49
Iron
0.35
Smaller
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
One of the reason might be concentration
increase by velocity slip
Ratio of number density N/No
Results (Behavior of particles)
„
Methane
Lean limit
Equivalence
ratio
0.50
Fuel gas
Measured
Calculated
in Stretched flow field
Difference exists between particle
velocity and gas velocity (velocity slip)
Combustion zone
The velocity difference causes the
increase of particle number density
just ahead of the combustion zone.
Gas flow Stream line
Particle
velocity
26
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Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007
Experimental setting
to examine the
stationary dust flame
(work on progress)
Summary
Combustion
chamber
The important aspects of dust explosion
were presented to well understand dust
flame propagation
„ Dust flame has some particular aspects
compared with gas flame (heterogeneous
combustion)
„ Particle can easily move at the different
velocity from gas flow. It causes some
specific behaviors.
Dust
Air
Future work
Special thanks to
Prof. T. Hirano
Dr. M. Yashima
Dr. J-L. Chen
Dr. W-J. Ju
Dr. J-H. Sun
Mr. K. Senda
Mr. M. Marui
Mr. T. Anezaki
Theoretical study and Modeling
„ The effects to be examined
„ Scale
„ Stretch
„ Turbulence
„
References
„
„
„
„
References
Jian-Lin CHEN, Ritsu DOBASHI, and Toshisuke HIRANO,
"Mechanisms of Flame Propagation through
Combustible Particle Clouds", J. Loss Prevention in the
Process Industries, 9-3, pp.225-229 (1996).
Wenjun JU, R. DOBASHI, and T. HIRANO, "Dependence
of flammability limits of a combustible particle cloud on
particle diameter distribution", J. Loss Prevention in the
Process Industries, 11-3, pp.177-185 (1998).
Wenjun JU, R. DOBASHI, and T. HIRANO, "Reaction
zone structures and propagation mechanisms of flames
in stearic acid particle clouds", J. Loss Prevention in the
Process Industries, 11-6, pp.423-430 (1998).
J-H.SUN, R.DOBASHI and T.HIRANO, "Combustion
Behavior of Iron Prticles Suspended in Air", Combustion
Sci. and Tech., 150, 99-114 (2000).
„
„
„
„
27
R.DOBASHI, J-H.SUN, W-J.JU and T.HIRANO, "Flame
Propagation through Combustible Particle Clouds", Fire
and Explosion Hazards - Proceedings of the Third
International Seminar, pp. 569-578 (2000).
J-F. SUN, R. DOBASHI and T. HIRANO, "Temperature
profile across the combustion zone propagating through
an iron particle cloud", J. Loss Prevention in the Process
Industries, 14(6), 463-467 (2001).
R. Dobashi, and K.SENDA, "Mechanism of flame
propagation through suspended combustible particles",
J. Phys. IV France, 12 Pr7-459 (2002).
J-H.SUN, R.DOBASHI and T.HIRANO, "Concentration
Profile of Particles across a Flame Propagating through
an Iron Particle Cloud", Combustion and Flame, 134,
381-387 (2003).
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