FLAME PROPAGATION DURING DUST EXPLOSIONS · Difference between Dust and Gas explosions Gas explosion Dust explosion Flame type Premixed Non-Premixed (homogeneous) (heterogeneous)

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.

Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, UK, 23-27 April 2007

16 www.see.ed.ac.uk/feh5

Flame Propagationduring Dust Explosions

Ritsu DOBASHIThe University of Tokyo, Tokyo, JAPAN

Background (1)�Appropriate understanding of

phenomena proceeded during dust explosion is necessaryto take appropriate measures against the accidental dust explosions.

Prevention techniquesMitigation techniquesRisk (consequence) analysis

(appropriate consequence prediction;computer model et.al.)

Background (2)�In most previous studies,

practical characteristics of dust explosions are focused;

such as,Explosion concentration limitsMinimum ignition energyExplosion pressureetc.

Also the fundamental studies areneeded for consistent measures.

Procedure of Dust Explosion

Flame Propagation

Combustible Dust

Ignition

Structural DestructionFragment Scattering Pressure Wave

Pressure Built-up

Damages

Dispersion

High Temperature Thermal DamagesFire

‘Flame Propagation’

�Understanding of flame propagation during dust explosion is essential

�Dust flame has some particular aspects compared with gas flame

�The fundamental studies on dust flame are needed

Characteristics of Dust flame Flame (Combustion zone) Unburned Burned Gas velocity

Su Sb

Particle velocity: not always equal to gas velocity

Thick combustion zone

Heterogeneous

Flame frontis not smooth

Radiative heat transfer



Difference between Dust and Gas explosions

Gas explosion Dust explosion

� Flame type Premixed Non-Premixed(homogeneous) (heterogeneous)

� Thickness Thin Thick

� Supply speed Almost same Often differentof O2 and Fuel (selective diffusion) (particle movement)

� Influence of Limited Sometime strongRadiation

� Flame front Usually smooth Not smoothoriginally

Propagating Flame Structure Dark zone Luminous zone

Blue spot flame Scale 10mm

Decane Spray Stearic Acid Dust Iron Dust

Larger Volatility Smaller

Premixed flame Diffusion flame? Surface combustion

Important points to be understood

�Flame propagation mechanism�Propagation limit Explosion limit�Propagation behavior

Risk (consequence) analysis

�Special aspects of Dust combustion�Effect of particle movement�Effect of turbulence�etc.

How the flame propagates?

Propagating

Relay Ignition Mechanism

Flame development

Ignition

Flame propagation

Heat transfer

Non-premixed flame(Diffusion flame)

Ignition can not propagate

sequential ignition

PremixedFlame

DiffusionFlame

Fuel Products + O2 Oxidizer + (O2) H2O

Flam

e

Fuel Oxidizer (O2) Products

Flam

e

PropagatePropagate



Questions on the Relay Ignition Hypothesis (stearic acid flame)� 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.

More detailed study is needed.

Experimental Approaches� Experiments for Organic Dust (resin)

– Limit of flame propagationChanging particle size distribution

– Temperature DistributionMeasuring thermal structure

– Ion Current MeasurementDetecting combustion reaction zone

– Observation by UV bandDetecting combustion reaction zone

� Experiments for Metal Dust (Iron)– Temperature Distribution

Measuring thermal structure

Experimentalsetting

Combustible dustStearic acid

(melting point 343 K)CH3(CH2)16COOH

1-Octadecanol(melting point 328 K)CH3(CH2)16CH2OH

Air tank

+ -

Heater

Copper mesh

Mov

able

cov

er

Nozzle

Elec

trode

s

Cyl

indr

ical

duc

t

T: Thermocouple

Pa: , Pl:

Pressure gauge

Neo

n tra

nsfo

rmer

T

Fuel

Air tank

EMV1 EMV2Pa Pl

d=60mm

d=96mm

d=84mm

425m

m

140m

m

40m

m

40m

m

40m

m

T

��94 mm94 mm

MeshMesh

Ignition Ignition ElectrodesElectrodes

Movable Movable CoverCover

AirAir AirAir

HeaterHeater SampleSample

StopperStopper

2. EXPERIMENTS

Flame propagates in open space

Particle size control

Fuel

Air

pa

pl

Two fluids Nozzle

Quick cooling of Liquid spray Solid particles

at liquid state (heated)

0 20 40 60 80 100 120 140 160 180 2000

1000

2000

3000

4000

5000

0 20 40 60 80 100 120 140 160 180 2000

1000

2000

3000

4000

5000

0 20 40 60 80 100 120 140 160 180 2000

1000

2000

3000

4000

5000

Case A:

pa=100kPa; pl=0.0kPa

Case B:

pa=60kPa; pl=60kPa

Case C:

pa=15kPa; pl=10kPa

Num

ber d

ensi

ty o

f par

ticle

s, n/

cm3

Num

ber d

ensi

ty o

f par

ticle

s, n

/cm

3

Diameter, �m

Diameter, �m

Diameter, �m

Num

ber d

ensi

ty o

f par

ticle

s, n/

cm3

Flammablelimit (Stearic acid)

Fuel

Air

pa

pl 0 20 40 60 800

20

40

60

80

100

120

140

Pres

sure

of f

eedi

ng a

ir p a

, kPa

Flammable

Non-flammable

Pressure of feeding fuel pl, kPa

A

B

C



"Smaller particles govern flame propagation"

Diameter, �m

Mas

s den

sity

of p

artic

les,

kg/m

3

0 20 40 60 80 100 120 140 160 180 2000

0.01

0.02

0.03

0.04

Reference Diameter

Smaller Particles Larger Particles

Effect of particle size distributionon flammable limit (Stearic acid)

240 280 320 3600

40

80

120

160

Mas

s den

sity

of

smal

ler p

artic

les,

g/m

3

Mass density of total particles, g/m 3

�

�

�

Unrealizable condition

Non -flammable

Non

- flam

mab

le

Flammable

0 20 40 60 800

20

40

60

80

100

120

140

Pres

sure

of f

eedi

ng a

ir p a

, kPa

Flammable

Non-flammable

A

B

C

Reference diameter= 60 �m

Results (Limit of flame propagation)� Flame propagation near lower

flammable limit is only supported by the combustion of smaller particles

� 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

Measurement of temperature distribution by thermocouple

5 mm

1.5m

m

4 m

m

+ (Rh13%,Pt)

-(Pt)

Direction of inserting into particle cloud

Ceramics support tube

Junction

Thermocouple Pt, (Pt+13%Rh) 20�m

Sensor holder, 0.2 mm in diameter

Measured temperature distribution (1-Octadecanol) Estimation of the heat release rate

Qdx

TddxdTUC p

�� 2

2

�

Q�

Steady state assumption;

represents the densityCp the specific heatU the flame propagation velocityT the temperaturex the position� the thermal conductivity



Variation of the heat release rate (1-Octadecanol)

about 2 mmahead of visible flame

Results (Temperature Distribution)

� Heat release rate takes a maximum value at the position about 2 mm ahead of the visible leading flame edge

� 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)

Micro-electrostatic probe

10 mm

Scale

t = 45 ms

t = 80 ms

t = 58 ms

0 50 100 1500

5

10

15

20

25

Time after discharge t, ms

i m /

2 i m

i m

/ 2

Schlieren front

Dark zone

Blue flame zone

Luminous zone

Ion

curr

ent,

210

-2 �

A

th

Structure of the dust flame (Stearic acid)

Sensor of electrostatic probe

Schlieren front

Blue flame zone

Low intensity combustion at blue spots

Burning particle with luminous flame

Dark zone

Luminous zone

Scale 0 10mm

Results (Ion Current Measurement)

� 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

��

Pa PlPressurize

��

��

Pa Pl

Mesh��

��

��

Pa

��

Pa � ��

��

Pa Pl

��

Ignition electrodes

Fuel Air

Pa Pl

Hearter

94mm

Air

High-speed video+ Image intensifier+ Band-pass filter

��

Pa

Air

Pa

Nozzle open

Ordinary video

�

UV bandobservation System



Function of band-pass filterBand-pass filter�308 nm�

Chemical emission

OH�281, 306 nm, UV�

CH�387, 432 nm, Blue�

CC�517 nm, Green� Black body radiation Band spectrum

Blu

efla

me

Lum

inou

s fla

me

To detect combustion reactionzone sensitively


Decreasing Luminousflame emission

Observation by UV band

�Observation by UV bandImage intensifier + Band-pass filterdetecting emission from combustion reaction

Sensitive detection of combustionreaction zone can be realized.

Observed propagating flames(UV band, Ordinary system)

� � � � � Observation� � � � � � � Observation � � � � � at UV band� � � � � � by Ordinary system � � � � � � � � � � � � White line indicates the leading edge of blue flame zone

10mm Luminousflame

Zone where blue spot flames exist

5 mm

Observed flame propagation�at UV band

Almost continuous propagation

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

Development ofCombustion region

Ignition

Heat Transfer

Diffusion FlameIgnition

DiffusionFlame

Premixed Flame

Sequential Ignitions of Diffusion Flames (Relay Combustion)

Propagation of Premixed Flame

Fla

me

pro

pag

atio

n m

ech

anis

m ?








Leading combustion zone;locates ahead of visible flame,is supported by smaller particles,propagates continuously

Possible structure of the dust flame(1-Octadecanol and Stearic acid)

Schlieren front Blue spot flame zone Luminous flame Dark zone

Flame propagatingdirection

Leading reaction zone (a kind of premixed flame)

Diffusion flame

Real leading combustion zone must exist at the position ahead of the visible leading combustion zone

It mainly consists of the burning of smaller particles

"relay ignition mechanism" can not be adopted

Supposed flame structures for characteristic time scale

Thin combustion zone similar to pre-mixed flame Vaporization

p > g

Thick combustion zone Vaporization

p < g

1-Octadecanol, Stearic acid particle … Decane spray …

p : characteristic time of flame propagation

g : characteristic time of gasification

Experimental Approaches� Experiments for Organic Dust (resin)

– Limit of flame propagationChanging particle size distribution

– Temperature DistributionMeasuring thermal structure

– Ion Current MeasurementDetecting combustion reaction zone

– Observation by UV bandDetecting combustion reaction zone

� Experiments for Metal Dust (Iron)– Temperature Distribution

Measuring thermal structure

Flame propagation in Iron dust

Controller

Ignition system

Air

tank

Hi g

h pr

essu

re ta

nk

High speed video camera

Magnetic valve

Movable tube

Air nozzle

Dispersing cone

Argon ion laser

Combustion chamber

Structure of the flame propagating in Iron dust



Profile of the temperatureacross the combustion zone Iron particle diameter distribution

Velocity and max. temp.

Max. Temp. and flame Velocity

Max. Temp. and flame Velocity

� Maximum measured temperature and propagating velocity are in linear relation

� Effect of radiative heat transfer (~T 4) is not dominant in this case

Cassel, Liebman, and Mock, 1956

� � � �� inf

4inf

4000inf 2

TTCdTTFTT

Sadmm

ppadpadglad �

��

��

Result (Iron dust flame )� Combustion reaction occurs only on the

particle surface

� Thickness of combustion zone is about 5 mm

� Radiation is not the dominative heat transfer mechanism (propagating as a heat wave)








Leading combustion zone;locates ahead of visible flame,is supported by smaller particles,propagates continuously

Surface combustion reaction(no gas phase reaction)

Radiative heat transfer is not dominant(propagating as a heat wave)

�Flame propagation mechanism�Propagation limit Explosion limit�Propagation behavior

Risk (consequence) analysis

�Special aspects of Dust combustion�Effect of particle movement�Effect of turbulence

Important points to be understood

Characteristics of Dust flame Flame (Combustion zone) Unburned Burned Gas velocity

Vgu Vgb

Vpu

Vgu � Vpu

Particle vel.

Fuel Conc. Fuel Flux to reac. zoneOxygen Conc. Oxygen Flux to reac. zone�

similar as selective diffusion

Difficulty to define ‘Burning Velocity’in dust flame

Vgu or Vpu ?

buL UUS�1

��

��?difficult to determine

Laser scattering observation on Iron dust flame

Laser lightscattering

Iron particle trajectory

50 1001

2

3

Time from ignition, ms

Dis

tanc

e fr

om ig

nitio

n po

int,

cm

Leading edge of combustion zone

Particle



x,

mm

m/s

Gas velocity

Particle velocity

10

20

-10

-10

Combustion zone

10 20

Gas movement

Particle movement

Velocity slip must cause change of particle number density

Velocity slip between particle and gasChange of particle number density

A

�x

x

x+�x Control volume

x

Combustion zone

,, xxxx Nu ��

,, xx Nu

Dis

tanc

e fr

om le

adin

g ed

ge o

f com

bust

ion

zone

, x, m

m

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

5

10

15

Measured Calculated

Ratio of number density N/No

Flammability lean limit

Smaller

Fuel gas

Lean limit Equivalence

ratio

Dust

Lean limit Equivalence

ratio Methane 0.50 Adipic acid 0.28

Ethane 0.52 Lactose 0.22 Ethylene 0.40 Poly-ethylene 0.17

Benzene 0.49 Sulfur 0.10

Methanol 0.46 Aluminum 0.29

Ethanol 0.49 Iron 0.35

One of the reason might be concentration increase by velocity slip

Results (Behavior of particles)

� Difference exists between particle velocity and gas velocity (velocity slip)

� The velocity difference causes the increase of particle number density just ahead of the combustion zone.

in Stretched flow field

Particlevelocity

Gas flow Stream line

Combustion zone



Experimental settingto examine thestationary dust flame(work on progress)

Air

Dust

Combustion chamber

Summary

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.

Future work

� Theoretical study and Modeling� The effects to be examined

� Scale� Stretch� Turbulence

Special thanks to

Prof. T. HiranoDr. M. YashimaDr. J-L. ChenDr. W-J. JuDr. J-H. SunMr. K. SendaMr. M. MaruiMr. T. Anezaki

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).

References

� 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).



Documents

FLAME PROPAGATION DURING DUST EXPLOSIONS · Difference between Dust and Gas explosions Gas explosion Dust explosion Flame type Premixed Non-Premixed (homogeneous) (heterogeneous)