miet 2394 cfd lecture 11
TRANSCRIPT
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Computational FluidDynamics – Lecture 11
Prof. Jiyuan Tu
& Dr. Sherman C.P. Cheung
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Some Advanced Topics in CFD
Demands from real-world problems and industries
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Turbulent Reacting Flows
※ Combustion
Heavy modelling!
Turbulent flow
+Chemical reactions
Persistent Flame
Intermittent Flame
Buoyant Plume
A photographic image of a buoyant fire.
Complex Reacting Flow
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Concept of olumetric Heat ource
Heat
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tec"ler#s $xperiment
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isuali%ation of Computational model
X
Y
Z
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Results from olumetric Heat ource
X
Y
Z
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&ir Flow 'attern
X
Y
Z
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elocity 'rofile at Doorway
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Temperature 'rofile at Doorway
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Temperature Distribution
X
Y
Z
T
52#
5#$42"
4!6$42
45$263
473$64
462$"#5
45#$526
43$!47
427$36
4"5$7!
4#4$2""3!2$632
3"$#53
36!$474
357$!5
346$3"6
334$737
323$"5
3""$57!
3##
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&dvantages
(t is simple
)ive reasonable prediction both in velocities and
temperatures
ave computational time
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Disadvantages
Fire*flame height should be obtained prior from
experimental data or analytical approximation
+,rong prediction at the flame
.isinterpretation of flame structure
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,hy needs Combustion .odel
(mprove prediction at near fire field
ome of the Fire problems re/uire the shape of
flame structure
Flame spread along combustible materials
Fire suppression by sprin"lers or water mists
$ddy 0rea" 1p 2$013 or presumed 'DF
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Fire Triangle
Fuel
Heat Oxygen
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.ixed is 0urnt Concept
Fuel 4xidant
Combustion
Turbulent .ixing
'roduct
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Flame hape by Combustion .odel
X
Y
Z
T
"7##
"6##
"5##
"4##
"3##"2##
""##
"###
!##
##
7##
6##
5##
4##
3##
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Two Compartment Fire
3$6 m 3$6 m
2$4 m
%&en
'n(
Burn
Room
Adjacent Room
)P* Burner
2$# m
#$ m
+oor,ay
$xperimental setup by 5ielsen and Fleischmann 267773
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Temperatures at &d8oining Room
Temperature (K)
H e i g h t ( m )
3## 35# 4## 45# 5##
#
#$5
"
"$5
2
2$5
Meaurment
!rediction "y #om"ution Model
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Temperatures at Doorway
Temperature (K)
H e i g h t ( m )
3## 35# 4## 45# 5## 55# 6## 65##
#$5
"
"$5
2
Meaurment
!rediction "y #om"ution Model
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Temperatures above Fire ource
Temperature (K)
H e i g h t ( m )
4## 6## ## "### "2## "4## "6## "## 2### 22## 24#
#$5
"
"$5
2
$ncorrected experimental data
!rediction %ith #om"ution Model
#orrected experimental data (&' *)
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Disadvantages
$xtra e/uations involved in simulation
9nowledge of the fuel Thermal Decomposition is
re/uired 2Detail Chemistry3
Complicated to implement
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5eeds of Radiation and oot .odel
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oot Formation 'rocess
Describes fours chemical processes:
5ucleation
Coagulation
urface )rowth
4xidation
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5ucleation
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Coagulation
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urface )rowthinyl
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4xidation464H
C46
H64
C46
C46
H64
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Radiation and oot Contribution Revisit the two compartment fire experiment
(ncorporate the radiation and soot model
Compare the prediction with previous results
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Temperatures at &d8oining Room
Temperature (K)
H e i g h t ( m )
3## 35# 4## 45# 5###
#$5
"
"$5
2
2$5
Meaurment
!rediction %ith only gaeou radiation
!rediction %ith gaeou and oot radiation
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RMIT University 3"
Temperatures at Doorway
Temperature (K)
H e i g h t ( m )
3## 35# 4## 45# 5## 55# 6## 65##
#$5
"
"$5
2
Meaurment!rediction %ithout radiation
!rediction %ith gaeou and oot radiation
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Temperatures above Fire ource
Temperature (K)
H e i g h t ( m )
4## 6## ## "### "2## "4## "6## "## 2### 22## 24#
#$5
"
"$5
2
$ncorrected experimental data!rediction %ithout radiation
!rediction %ith gaeou and oot radiation
#orrected experimental data (&' *)
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A Buoyant FreeStanding Fire
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Advance in Computational!odels( ") ;arge $ddy imulation 2 LES 3
4nly resolve large eddies < sub-modeling of small eddies- ubgrid scale turbulent viscosity
-1nsteady simulation-Depending on grid si%e
-(ndustrial applications possible
RANS-LES Coupling
Reynolds &veraged-5~ 5ear-wall uses
~ (nternal domain uses ;$
Hybrid Coupling
SGS
T µ
V ∆
= - RANS κ ε
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Bac#ward Facing Step
+-. .olution /rom .tan/or( University
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Advance in Computational!odels( "")Direct 5umerical imulation 2 DNS 3
Resolve all scales of turbulent eddies
For fundamental understanding of turbulence
Challengesignificant grid numbers
mall time step
Higher order discreti%ation1n"nown initial < inlet 0C
>? Re
? t ∆ ↓↓
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!ultip$ase Flows( ")
※ )as olid particle flow
'hase@ 'hase6
.ining industry
'ollutants $nvironment
Coal fired power
DustA soilB
$lectricity
※ )as Droplet particle flow
&ir ;i/uid
5asal prayer
&erosol
(C $ngines pray
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!ultip$ase Flows( "")
※ 0ubble ,ater particle flow
0oilers
5uclear 'ower
Chemical reactor
&ir ;i/uid
※ 4il ,ater &ir olid particle flow
'hase@ 6 >
※ &ir ,ater free surface
5umerical methods
※$ulerian-$ulerian .ethod orTwo- fluid .ethod※$ulerian-;agrange .ethodor 'article trac"ing .ethod
※olume-of-fluid 2VOF 3Trac"ing free surface
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Typical Flow be$aviour inbubble columns
Bu00ly Flo, 1a&Bu00ly /lo, .lu Flo, 1urnTur0ulent Flo,
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Bu00le 1oaleseneBu00le 1oalesene 0ubbles may +merge together
forming larger bubbles
Coalescence reduces number of
bubbles but increases si%e of
bubbles
estern Miian University
-D p8
D p
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Bu00le Brea9aeBu00le Brea9ae
Revuelta et al$ 2##68 :$F$M$
0ubbles may brea"-up due toturbulence impact
0rea"age increases number of
bubbles but decrease si%e of
bubbles
-D p8
D p
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Po&ulation Balane ;&&roaesPo&ulation Balane ;&&roaes 'opulation 0alance $/uation 2'0$3
Three .ain &pproaches has been proposed: .oment of .ethod 2.4.3 ;east /uare .ethod 2;.3
Class .ethod 2C.3
( )( ) +−=⋅∇+
∂
∂3A23A23AA2
AAt f t bt r f v
t
t r f ξ ξ ξ
ξ
−∫ @
73A23A23AA2 dst s f t sbt sh ξ
+∫ @
73A23AA23A2 dst s f t sC t s f ξ
∫ −−ξ
ξ ξ 7
3A23A23AA26
@dst s f t s f t s sC
+eat rate (ue to 0rea9ae
Birt rate (ue to 0rea9ae
+eat rate (ue to 1oalesene
Birt rate (ue to 1oalesene
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1lass Meto(s /or Po&ulation Balane1lass Meto(s /or Po&ulation Balane
-D p8
D p
;verae -um0er
.im&le
)oss (istri0ution in/ormation
1om&le< an( e
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Average Bubble %umberDensity &AB%D'
&dopt a single averaged value to describe the
changes of local bubble number
4nly one additional e/uation is needed
&dvantages: Fast in terms of computational time
imple to implement
Disadvantages: 4nly one averaged value can be obtained
Fluid +elocity
n n n n n
n’
n’
n’ n’ n’ n’
( ) Breakagen
eCoalesen
n g n!t
nφ φ +=⋅∇+
∂
∂
n’
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!(ltiple S")e *roup &!(S"*'!odel &"' 1sed 5 si%e group 2scalar3 to describe bubble population
5 extra e/uations have to be solved 2depends on number of
group used3
&dvantages:
Higher resolution for bubble classes
Disadvantages: &dditional computational time needed
( ) BC BC " g " D D # # n!t
n−−+=⋅∇+
∂∂
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!(ltiple S")e *roup &!(S"*'!odel &""'
Fluid +elocity
n" n" n" n" n"
n"’ n’ n"’ n"’ n"’
# B D B
( ) $" $ N
" $
B nvv% # :@
∑+=
= "" B n% D =
# C DC
BC BC n D D # # S " −−+=
n" n"
n"n"
∑∑= =
="
k
"
l
$"kl &"C nn # @ @6
@ χ ∑=
= N
$
$""$C nn D@
χ
kl &"kl &" χ χ = if "l k vvv =+
else7=kl &" χ "l k vvv ≠+
-n"8
n"
-n"8
n"
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Bu00le MeanismBu00le MeanismBu00le 1oalesene
Ran(om 1ollision
Bu00le Brea9ae
a9e 'ntrainment
Tur0ulene Im&at
.earino// .ur/ae insta0ility
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Free Sur+ace Flow in BuildingDrainage system
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Fluid Structure "nteraction
Fluid flow tructural &nalysis
.ethod:
→ F # C
)eometry Ne'
32
32
CF(
Fl!ent 32 ANS)S
Fluid .odelling
2CFD3
!olid .odelling
2F$!.3
Coupling
Intera5tion
(5T$RF&C$
2middle-ware3
• Turbulence
• 'ressure
• Fluctuations
• !tress
• Deformation
• ibration.4are
In/ormation
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0iomedical &pplication
CT
.R(
)eometry
.odel
2C&D3
CFD .odel of &ir way
0lood vessel
'ressureA hear stressA
Drug depositionA Temperature etcBB
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Blood ,essel – -all S$earStress &-SS'
Carotid 0ifurcation
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Blood ,essel – FS"