L09 LectureSlides 2015-Spring CHY
description
Transcript of L09 LectureSlides 2015-Spring CHY
-
1AerodynamicsandFlow L9
Chungen Yin
[email protected]; 30622577
Spring 2015
Turbulent Reacting Multiphase Flows (5 lectures)
2
L9:TurbulentNonreactingMultiphaseFlows Turbulent particle dispersion Turbulence modulation Particle-wall interaction
2
-
3Turbulentparticledispersion(1)
3
Small particle A particle is referred to as small, if its diameter is smaller than the Kolmogorov
length scale;
as medium, if its diameter is between the Kolmogorov scale and the integral scale;
In dilute, particle-laden flows of interest, the majority of particles are small, based on this definition.
)(21 vuvuAC
dtvdm pfDp
vtdxd
For small, heavy particles/droplets in dilute two-phase flow,
The particle trajectory & velocity can be determined at each time, provided the knowledge of the flow field at that moment
4
Turbulentparticledispersion(2)
4
Number density
Position
The interaction between turbulent eddies and immersed small particles is referred to as turbulent particle dispersion.
Observation: dispersive effect on particles released from the same location; Key parameters: (1) particle size, with respect to eddy size; (2) fluid and
particle properties, e.g., fluid viscosity & density and particle density; (3) flow properties, e.g., distribution of turbulent kinetic energy.
inertia dominated viscosity dominated
-
5Stochastictracking(1). Randomvel.fluctuation
5
A Gaussian distributed random velocity fluctuation is used: also known as discrete random walk (DRW) model or eddy lifetime model.
)(21 vuvuAC
dtvdm pfDp
uUu
In DRW model, each eddy is characterized by
A Gaussian distributed random velocity fluctuation, u, v, w; A time scale
Random velocity fluctuations for k- and k-3/2''' kwvu : normally distributed random number
6
Stochastictracking(2).Interactiontime
6
),(minninteractio ce tt
A time scale, accounting for the Characteristic eddy lifetime and Crossing trajectory effect, is used as the integral time.
The time for particle to cross an eddy: estimated from eddy size/drift velocity
For small particle (moving with fluidzero drift velocity): the integral time becomes fluid Lagrangian integral time
kAfL
eL
fLe 2 )ln(rfLe orr : a uniform random number [0, 1]
18,with1ln
23/2pp
vev
evc
dkBLvu
Lt
-
7Stochastictracking(3)
7
In literature, a range of values have been suggested for A and B:
23ntconstraithewith41.0135.0
BAA
For assumptions of isotropic turbulence and that the characteristic size of the eddy is the Kolmogorov scale, the constants A and B can be approximated by
4/34/3 ; 23
CBCA )09.0( C
Over the interaction time, , the particle is assumed to interact with the fluid phase eddy with a given instantaneous velocity. When
this time is reached, a NEW value of will be applied to generate
new random velocity fluctuations ( new instantaneous velocity).
ninteractiot
DRW model constants (A & B)
(FLUENT: A=0.15 by default, 0.3 recommended for RSM)
8
Particlecloudtracking(1)
8
Number density
Position
Track the statistical evolution of a cloud of particles about a mean
trajectory: the concentration of particles about a mean trajectory is
represented by a Gaussian PDF whose variance is based on the degree
of particle dispersion due to turbulent fluctuations.
-
9Particlecloudtracking(2)
9
The PDF used is derived from Taylors analysis:
)(,)(),,,( ttxftzyxP ii
Its possible to define a normalized Lagranian autocorrelation function:
)(
)()(),(
22
2121
tu
tututtR
i
iipL
This can be assumed to exhibit an exponential decay.
10
Example
10
Instantaneous particle dispersion from simulation of plane wake
8
4
0
-4
-88
4
0
-4
-80 4 8 12 16 20 0 4 8 12 16 20
Particle Stokes number: St=0.01
St=1
St=10 St=100
-
11
Example(cont.)
11
1. Large-scale vortex structures are important controlling mechanisms for
the particle dispersion process.
2. Particle dispersion levels tend to maximize at intermediate values of
Stokes numbers, 0(1)0(10).
3. Particles of intermediate-St tend to concentrate preferentially near the
outer boundaries of large-scale vortex structures.
12
Turbulentmodulation:Whatisit
12
Turbulence properties: characterized by, such as, turbulent kinetic energy, Reynolds stresses, spectra, or two-point correlations.
Turbulence modulation: one or more of the statistical properties of the carrier phase turbulence is changed by the presence of particles.
Most frequently, refers to changes in the carrier-phase turbulent kinetic energy (either enhanced or decayed).
However, TKE changes do not fully describe the changes to the turbulence.
-
13
Turbulentmodulation:Whyisitimportant
13
Attenuation of TKE as a function of mass loading ratio on the center-plane of fully developed channel flow for glass beads (150 m) and copper beads (70 m).
p 5.510-5; a mean inter-particle distance of 20dp. Dilute flowSuch a reduction (by a factor >7) in TKE may completely change the characters of the turbulence and the behavior of a reactor.
14
Turbulencemodulation:Keyparameters
14
Then even more parameters are important!
(A lot of turbulence length scales!)f
pld
f
pp
dvu
Re
f
vSt
Mass loading ratio, Particle volume fraction, Density ratio
If 2nd phase: one size, spheres. Additional dimensionless parameters
If 2nd phase: polydisperse, non-spherical
-
15
Turbulencemodulation:Mechanisms
15
The exact mechanisms: not very well understood; the available
theories often can NOT predict the level or even sign of the change in
TKE.
Some basic mechanisms: all them require the particles be large
enough that they can not follow the flow and there is a substantial
instantaneous relative velocity between two phases.
16
Turbulencemodulation:Mechanism1
16
Through the carrier-phase mean velocity
The effects of carrier mean flow distortions on the carrier turbulence can be
predicted by single-phase turbulence models.
ipj
i
jij
ij
i Fxu
xxp
xuu
tu
,)()(
If the mean carrier-phase velocity field is changed by the addition of particles,
the mean strain field and the turbulence production rate will also be changed.
-
17
Turbulencemodulation:Mechanism2
17
Unsteady particle wakes behind relatively large particles.Important when p is not small and Rep is in the vortex shedding regime.
(Experimental) Dye streaklines produced by dye introduced at cylinder surface (Rep=140)
18
Turbulencemodulation:Mechanism3
18
Extra dissipation of turbulence by particles.
-
19
Turbulencemodulation:Mechanism3(cont.)
19
Som
e
Fluctuating kinetic energy of the particles
Producing local flow distortions around each particle
A particle that cannot respond to fluctuations exerts a force on the fluid that opposes the relative motion. When the particles are heavy, the relative motion is produced mostly by the carrier-phase fluctuations.
The cloud of dispersed particles produces a non-uniform force field that instantaneously opposes the carrier-phase velocity fluctuations, extracting energy from the turbulence.
20
Turbulencemodulation:Mechanism4
20
Preferential concentration & sweeping of particles.
Preferential concentration occurs when particles (with particle time constants comparable to eddy time scales) are swept out of vortex cores and concentrated in convergence zones.
Particles apply an angular impulse opposing the vortex rotation as they are spun out of a simple vortex.
For St1, the ratio of the angular impulse to the initial angular momentum of the vortex is 1.3 MLR / St. This would act to suppress vortices (whose time scales are close to the particle time constant).
-
21
Turbulencemodulation:Mechanism5
21
Eddy distortion.
Particles are expected to produce significant local distortion of the small-scale motions.
The fact that the particle diameter is comparable to the Kolmogorov scale (dp/) indicates that the particles experience significant velocity gradients rather than the locally uniform flow assumed in most models. This means that the particles experience forces transverse to the relative velocity.
The fact that the average particle spacing (Lbetween-p) is so large means the force applied is not continuous. This is likely to produce a significant distortion of the energy-containing eddies and could lead to a higher turbulent dissipation rate.
22
Howtostudymodulation:Analytical(1)
22
A common approach to modeling the effects of particles on turbulence
is to treat the particles as applying a continuous force field onto the
fluid phase where the force is the reaction force (to the particle drag).
(1) Write down the modified N-S equations, by including fluid-particle inter-phase
forces
(2) Apply Reynolds decomposition; subtract the mean flow component; square
& average the fluctuating component to obtain the transport equation for k.
-
23
Howtostudymodulation:Analytical(2)
23
1,1
1
11 1p
jjjj Fx
uxx
pxuu
tu
x1-momentum:
2,2
2
22 1p
jjjj Fx
uxx
pxuu
tu
x2-momentum:
3,3
3
33 1p
jjjj Fx
uxx
pxuu
tu
x3-momentum:
(1)
(2)
(3)
Eqns) RANS(')3(')2(')1(RANS of derivationin thoseas operationssimilar Use
321 uEquEquEq
23
22
21 ''' uuu The remaining is a transport equation for , i.e., k.
kpj
j Sxku
tk
,)('
T
Start with instantaneous NS equations
Operations
24
Howtostudymodulation:Analytical(3)
24
o The 1st main deficiency: The assumption that the force applied onto the fluid is linearly proportional to the fluid/particle velocity difference.
Most cases in which turbulence attenuation is significant involve Rep>1.
The instantaneous force on a particle is affected by the local straining field.
When dp Kolmogorov scale, the flow around particle is highly non-uniform. The force applied by the particle onto the fluid is almost never aligned with the
instantaneous velocity difference.
o The 2nd main deficiency: The analysis assumes that energy transferred to the particle is eliminated from consideration.
When a force is applied to a particle, that particle accelerates and creates localflow distortion. The assumption is that these fluid motions are at very smallscale and are quickly dissipated by viscosity. However, the particles are oftenlarger than the smallest scales of turbulence. In this case, it is moreappropriate to assume the particleturbulence interaction as energy transferbetween different scales of velocity fluctuation.
-
25
Howtostudymodulation:Experimental(1)
25
Heavily rely on laser-based instrumentation
LDA (Laser Doppler anemometry)
The challenge is to discriminate between signals from carrier-phase tracer particles and the generally stronger signals from the dispersed particles.
PDA (Phase Doppler anemometry)
Measure particle size and velocity simultaneously;
More robust but more difficult and expensive; Generally work well for phase separation if there is a significant difference between the tracer and the smallest dispersed particles.
PIV (Particle image velocimetry)
As with LDA, the challenge is to measure the fluid phase using tracer particles
Used extensively for multiphase flow research in recent years;
26
Howtostudymodulation:Experimental(2)
26
Compilation of experimental data on turbulence modulation
Data cover: Jets Pipe flows Gas-particle Gas-liquid
D : particle diameter
23
k : Turbulent length scale
-
27
Howtostudymodulation:Numerical
27
Difficult: due to an extremely large range of length scales
Not valid unless the particles are significantly smaller than both the turbulence Kolmogorov scale and the grid spacing.
Earlier efforts: Point-force coupling scheme
Adapt single-phase DNS codes for isotropic flows by using simple drag laws to compute the force applied onto each particle by the flow, and applying the reaction force back onto the fluid as a variable body force.
Recent efforts: Fully resolve the flow around a single particle in a small region of turbulent flow
28
Turbulencemodulation:Asummary
28
In gas flows, very small particles do not affect the flow. Particles with sizes of the order of Kolmogorov scale can cause significant attenuation. The attenuation reaches a maximum for Stk50 & Rep~the order of 10.
In liquid flows laden with solid particles, turbulence attenuation is rarely observed. Small particles very low Stokes numbers little effect on the flow. Large particles increase turbulence (wake effects).
Dispersed particles in the dilute regime can cause very large changes in the turbulence levels in typical shear flows.
As particle size increases, particle wake effect plays an increasing role, first decreasing turbulence attenuation and eventually causing large turbulence augmentation.
Turbulence modulation is much weaker in flows such as free shear flows.
Models: very poor, inconclusive; significant further research needed.
-
29
Particlewallinteraction
29
Lift force Lubrication
Mechanical interaction(in the absence of a fluid)
captured by wall van der Waals force (very small particles)
Collision (massive particle)rebounds but loses kinetic energy due to friction & inelasticity effects
y
v
Hydrodynamic interaction
30
Sphericalparticlesmoothwallcollision
30
ASSUMPTIONS:
Hard sphere; Particle deformation is neglected (i.e., throughout the collision process, the
distance between the particle center of mass and the contact point is constant and equal to the particle radius);
Coulombs friction law applies to particles sliding along a wall; Once a particle stops sliding, there is no further sliding.
-
31
Sphericalparticlesmoothwallcollision(cont.)
31
Pre-collision Compression period (1)
(from the contact with wall to the moment when Vy=0)
Recovery period (2)
(from the moment with an upward Vy to the moment no contact with wall)
Post-collision
Four periods
Pre-collision Post-collision
21tt dtFJ
y
32
Sphericalparticlesmoothwallcollision(cont.)
32
)0(
)2(
)0(
)2(
X
Xx
Y
Yy V
Ve
VV
e
)0(
)2(
Y
Y
V
Ve
)1(
)2(
Y
Y
J
Je
)0(
)2(
V
Ve
Coefficient of restitution (e): Four definitions
3
1
-
33
Sphericalparticlesmoothwallcollision(cont.)
33
y
sliding no:0, zx vv Three cases: Depending on the period in which the particle slides
along the wall.
Case1:Theparticlestopsslidinginthecompressionperiod.Case2:Theparticlestopsslidingintherecoveryperiod.Case3:Theparticlecontinuestoslidethroughoutthecompressionandrecoveryperiods.
34
Sphericalparticlesmoothwallcollision:Case1
34Impulsion equations (6*3=18 eqns)
In t
he t
able
: 9*
3=2
7 u
nkn
own
s
-
35
Sphericalparticlesmoothwallcollision:Case1
35
0)()( )()()()( kaviav sXsZsZsX (2 Equations)
0)()( )1()1()1()1()1( kavjviav XZYZX (3 Equations)
0)()( )2()2()2()2( kaviav XZZX (2 Equations)
Conditions at the end of different periods (2+3+2=7 eqns)
kavjviavrvU XZYZX )()(
Particle surface velocity:
(1) At the end of sliding period: tangential components of surface velocity are 0
(2) At the end of compression period: all components of surface velocity are 0
(3) At the end of recovery period: also no sliding.
36
Sphericalparticlesmoothwallcollision:Case1
36
)( )()()1()2( rYs
YYY JJeJeJ (1 Equation)
kJfiJfkJiJ sYZs
YXsZ
sX
)()()()( (2 Equations)
Coefficient of restitution, e (1 eqn)
Coulombs law: (friction F)= (normal F)*(friction coefficient)(2 eqns + 2 unknowns)
),( ZX : direction cosines of the approach velocity in X-Z plan
(1 eqn)122 ZX
Totally 18+7+1+2+1=29 eqns for 27+2=29 unknowns: closed!
-
37
Sphericalparticlesmoothwallcollision:Case2
37
Impulsion equations
38
Sphericalparticlesmoothwallcollision:Case3
38
Impulsion equations
-
39
Sphericalparticlesmoothwallcollision:Solution
3 9
Condition
Translationalvelocity
Angularvelocity
)1(72)0(
efvvY 0
)1(72 )0( v
vef
Y
)(
)(
)0(5
2)0(75)2(
)0()2(
)0(5
2)0(75)2(
Xa
ZZ
YY
Za
XX
vv
vev
vv
av
av
XZ
YY
ZX
)2()2(
)0()2(
)2()2(
)0()0()2(
)0()2(
)0()0()2(
)1(
)1(
YZZZ
YY
YXXX
vefvv
vev
vefvv
)0(25)0()2(
)0()2(
)0(25)0()2(
)1(
)1(
YXaZZ
YY
YZaXX
vef
vef
a:sphereradius;e:coefficientofrestitution;f:frictioncoefficient;X,Z:directioncosinesoftheapproachvelocityinXZplane;:translationalvelocitymagnitude.v
40
Exercises/Practices
40
(Optional) Repeat Tut. 19 Modeling evaporating liquid spray Follow the tutorial (Fluent15.0_Tutorial-19.pdf) step-by-step from
section 19.4.2, & reproduce the results as given in the tutorial.
The mesh (sector.msh) and the guide are available in the folder Files for lecture 9. Practice earlier for this non-reacting flow! More practices to come, for, e.g., reacting multiphase flows.
To install, license and use FLUENT on your PC, please visit\\Et-fs1.aau.dk\Fileshares\software_students\FLUENTand follow the guides there!
Calculation: A 100-micron nylon particle bounces off a flat, steel wall. The particle speed is 10 m/s and the angle of incidence is 30. The particle has no initial rotation and moves in the x-y plane. The coefficient of friction is 0.2 and the coefficient of restitution is 0.8. Find the rebound speed, the angle of rebound and spin rate (with correct sign). Here, X=1.0.