The Effect of Spray Refinement on Flame Extinguishment · 2017-10-09 · Acknowledgements...

The Effect of Spray Refinement

on Flame Extinguishment

Alexander Snegirev, A. Lipjainen, V. Talalov

St-Petersburg State Polytechnic University

Russia

[email protected]

Acknowledgements

• Microsoft Research (FireEx-MicroTEST project 2007–2009)

• Science and Higher Education Committee of the St.-Petersburg Administration (A. Snegirev, 2009, and A. Lipjainen, 2008)

• RFBR 10-08-08019-з

All models are wrong, but some are useful

(George E. P. Box)

1 University of Brighton, 19 April 2010

• Water mist fire suppression system (WMFSS) is a rapidly growing technology of fire protection

• Controversial debates in fire engineering community: completely opposite conclusions on water mist efficiency

• A likely reason is that WMFSS is applied in different regimes

• To classify the regimes of the flame-spray interaction, governing dimensionless criteria should be identified

• A validated CFD methodology is required to complement and, wherever possible, replace full-scale tests

• URANS: Yoon, Kim, DesJardin, Hewson, Tieszen and Blanchat, 2007, 2008

• LES studies of reacting multiphase flows with spray evaporation: Xia, Luo, Kumar, 2008; O’Grady and Novozhilov, 2009

Motivation


Objectives

• To develop and validate an LES methodology to predict flame suppression by water sprays

• To identify a dimensionless criterion suitable to distinguish fine and coarse sprays taking into account their structure and dynamics and governing criteria reflecting basics of flame-spray interaction regimes

• To assess the effect of spray refinement on flame suppression

University of Brighton, 19 April 20103

Water as an extinguishing agent

Fire suppression concepts

Total flooding Surface application

Halocarbons

Inert gases

Water mist

Foams

Powders

Water jetWater spray

Aerosols

Damage due to excessive water

Not suitable for combustible liquids

Motivation

to refine

water spray


• What happens to the spray when it is refined due to increased pressure?

• It may become not a ―surface application‖ agent, yet not a ―total flooding‖ one

Modelling methodology

• Fire3D model and code

• Turbulence

• Combustion

• Thermal radiation: Monte Carlo + WSGG (CO2, H2O, soot)

• Stochastic Lagrangian spray model

2

3 2SGS S

GC S C

S 2 ij jiS S S

1

2

ji

ij

j i

uuS

x x

min ,S Sl y C

2

2

O

,

O

min , 1fuel fuel ext vap

SGS

Yr C Y P

s

1

SGS S

Pr

t i

t i

gG

x

Subgrid turbulence production

due to buoyancy

Probability of local extinction due to

excessive vapour concentration


Buoyant flame modelling

Simulations of a 260-kW flame above a 1-m diameter burner (fuel is CH2O): a) — steady mean temperature field (URANS, mesh 64×64×80); b) —

instantaneous resolved temperature; c) — time averaged temperature; d) — flow vortical structure

as shown by the instantaneous iso-surface Q = S2–Ω2 = const (LES, mesh 88×88×128)

a) b) c) d)

Time averaged axial temperature (a) and

velocity (b). Case of a 260-kW flame above a 1-m

diameter burner (fuel is CH2O)

a)

b)


Spray model components

• Droplet movement

• Droplet dispersion by turbulence

• Droplet heating

• Droplet evaporation

• Inter-phase exchange

• Nozzle performance (initial droplet size and velocity distribution)

and

• Flame extinction

• Spray-pool interaction


The spray model

,

, ,

31

4

p i Dp i i p i i i

p l l

du Cu u u u g

dt d

,

,

p i

p i

dxu

dt

2/324 11 Re ,Re 1000

Re 6

0.424 ,Re 1000

p p

pD

p

C

Droplet movement

Droplet dispersion

, min , 2 3p SGS p S SGSl k

*

*

*

ln 1 ,

,

SD S D p p

D p pp

S D p p

ll u u

u u

l u u

3/4 3/2

S S

S

l Cl

C k

2

2

1exp

22

ii

uf u

2 3SGSk i i iu u u

Subgrid eddy life time

model

SGS SGSk S

4

3

p l

D

D p

d

C u u

ρl >> ρg, spherical drag law

Spherical drag law


The spray model

Droplet heating

,

0 ,

p

p p vap p p boil

p l

p boil

dmdT q h T T T

m c dtdt

T T

, ,

boil

p

T

vap p vap boil p lT

h T h c T dT

, ,p p conv p radq q q

, ,NuPr

g

p conv p P g pq d c T T

1/2 1/3Nu 2 0.6Re Prp

2 4

,4

p rad p p p

Gq d T

4

0

G Id

Droplet evaporation

Shln 1 ,

Pr

,

p g m p boil

p

p

p boil

vap boil

d B T Tdm

qdtT T

h T

1/2 1/3Sh 2 0.6Re Scp

, ,

,1

vap sat p vap

m

vap sat p

Y T YB

Y T

The

classical

model with

Le ≈ 1,

cP, vap ≈ cP,g


Convective

and

radiative

incident

flux

The spray model

Inter-phase exchange

p p

M

n dmr

V dt

p

V p p

n dr m u

V dt

p

H p l p

n dr m c T

V dt

Source terms in gas

transport eqs

Spray atomization (cone angle φ)

00

2

,0 ,0

,0 2

1exp

22

p p

p

VV

V Vf V

50

50

exp ln 2

ln11 erf

2 2

v

v

d

d

dR d

d d

,0 02p lV P

Random choice of

velocity direction

and magnitudeRR or LN initial droplet

size distribution


The spray model

Spray-surface interaction• Droplet splash evaporation• Droplet escape

Two extreme

limits

Flame extinction

2

2

ln1 1exp

22

vap vap

vap

vap vapvap

Xf X

X

lnvap vapX 2 2ln 1 0.3vap vap vapX X

,

, , vap cr

ext vap vap vap crX

P P X X f X dX

2

,ln 21erfc

2 2

vap cr vap vap

vap

X X

Probability of

the super-

critical vapor

concentration

Presumed

PDF for

vapor mole

fraction to

allow for

turbulent

fluctuations

0.27


Nozzle spray characterization

00

2

l

PV

220

0 0 0 04

w

DQ V K P D P

1/32

1/350 0 0

1/3 1/3

0 0 0

1Wev l

l

d V DC C

D P D

2 2

0 050 1/3 2/3

0

v

w

D Dd

P Q

K-factorWater flow ratePressure drop Nozzle diameter

Initial droplet

velocity

Initial median

droplet diameter

Initial median

droplet diameter

depends on

pressure and

flow rate

[Yoon et al, 2004]

[SAND2007 3220]


Model validation

• Fire Laboratory for Accreditation of Models and Experiments (FLAME) – Sandia Labs, California, USA

• 2.5 MW pool fire (JP8 – C11H21)

• 30º cone angle, 5 m height

• Spray just covers the pool

90º nozzle

0P , kPa lQ , l/min 0V , m/s 50vd , mm We , 103 Re , 10

3

172.4 53.0 14.7 0.870 26.3 143.9

1310.0 147.6 41.0 0.370 204.1 400.9


Model validation

Extinguishing a 2500-kW flame above a 2-m diameter burner. Left to right: 0.05, 0.10, 0.15,

0.20, 0.25 s after nozzle activation (two right plots show the same time instant). Light colour

surface — vapour mole fraction 1%, dark colour surface — temperature 1000 K. Instantaneous

resolved temperature is shown in the axial plane

Low-pressure

spray:

no suppression

High-pressure

spray:

suppression


Fine

sprayCoarse

spray

When the spray is really fine?

• Initial droplet size is a key parameter that switches extinguishment regimes

• Single value of characteristic droplet diameter is not enough to classify spray type (NFPA 750: dv99 < 1 mm)

15 University of Brighton, 19 April 2010 00:28

Fine spray is special

• Spray structure and dynamics changes dramatically due to spray refinement

• Focused jet instead of wide angle cone

• Faster evaporation, vapor cloud surrounds jet

Coarse, medium and fine water sprays in no flame conditions (flow rate 10 l/min, log-normal initial droplet size distribution, σ = 0.48): a) ― dv50 = 0.5 mm, b) ― dv50 = 0.2 mm, c) ― dv50 = 0.08 mm. Iso-surface shows vapour

mole fraction of 0.004 (a) and 0.01 (b and c)

Coarse spray:

maintains a

cone

shape until for a

long distance

away from the

nozzle; the

vapour cloud is

inside the spray

near its axis

Fine spray:

forms a thin,

focused jet

surrounded

by the

vapour cloud

S0=0.6 S0=0.15 S0=0.035


Fine spray is special

Photographs of atomizer sprays.

Parameters of water

upstream of the atomizer: 8 MPa,

20°C and 170°C


20°C

170°C

Time and length scales

• The deceleration (stopping) time is much smaller than the evaporation time

Evaporation timeDeceleration time

Deceleration length

≡ Stopping distance


Spray fineness criterion

• The deceleration length can be estimated neglecting evaporation:

• Deceleration length depends not only on d0 but also on V0

• Governing criterion:Spray fineness number

0 0 04

3

ld

d V dL f

0 00Re

V d

0

0

Re0

0 Re 00

ReReRe

Re Re 24D

df

C

0d

f

LS

H L

Stokes number


Regimes of flame-spray interaction

• Spray fineness number distinguish qualitatively different regimes of spray interaction with the flame

• Large Stokes number => Spray drag number, SD

• Small Stokes number => Spray momentum number, SM

Extinguishing a 260-kW flame above a 1-m diameter burner (0.2 s after nozzle activation):

a) — coarse spray, dv50 = 0.5 mm; b) — medium spray dv50 = 0.2 mm; c) — fine spray 0.08 mm.

Wide-angle nozzle (120° cone), nozzle height 3.0 m above the floor, water flow rate 10 l/min.

LES, mesh 88×88×128. Light colour surface — vapour mole fraction 0.01.

Coarse spray:

Drops penetrate

inside the flame

and evaporate

there =>Spray droplet drag

Plume momentumDS

Fine spray: vaporized jet suppresses the flame provided jet momentumis high enough

Spray momentum

Plume momentumMS

S0=0.6 S0=0.15 S0=0.035


Coarse spray

Fine spray

a) b) c)

The effect of spray refinement

260 kW, 1 m diam., wood volatiles CH2O, mesh

88×88×128, water flow rate 10 l/min

Coarse

spray,

dv50 =

0.50 mm

Medium

spray,

dv50 =

0.20 mm

Fine spray, dv50 = 0.08 mm Time step 0.05 s

No suppression

Suppression


Conclusions and future work

• The spray is fine when the Stokes number is low. The Stokes number is the spray fineness number

• Fine spray focuses in a narrow jet => the entire spray momentum is accumulated in the jet suppressing the flame

• This is neither a surface application, nor a total flooding regime

• Two opposite cases of coarse and fine sprays represent two physically different regimes of spray-flame interaction

• The structure and dynamics of the coarse spray is determined by the drag force due to droplet friction in the gas flow => Governing criterion is the dimensionless spray drug

• The structure and dynamics of the fine spray is governed by the total spray momentum => Governing criterion is the dimensionless spray momentum


Conclusions and future work

• Drop size reduction (due to higher pressure) may cause faster flame suppression at lower water supply rate

• Necessary conditions:

– sufficient spray momentum and

– proper spray orientation

• Unresolved theoretical problems: flame extinction, interaction with fuel surface, attenuation of radiant heat

Future work

• Further validation against full-scale measurement data

• More sophisticated spray atomization model

• A universal spray model?


Experience is what you get when you

didn’t get what you wanted(Randy Pausch)

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The Effect of Spray Refinement on Flame Extinguishment · 2017-10-09 · Acknowledgements...

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