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Lecture 14: Applications of LIF, PLIF of Small Molecules
Planar Laser Induced Fluorescence
1. Intro to LIF & PLIF imaging applications
2. Flame applications of OH PLIF
3. PLIF with molecular vibrations in IR
4. Imaging hypersonic flows – challenges
5. Imaging hypersonic flows – PLIF of OH in reacting and non-reacting flows
6. Imaging hypersonic flows – T and dual species
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Planar Laser Induced Fluorescence (PLIF): A two-dimensional image can be acquired using LIF by Using a 2-D detector array (i.e. digital camera) Expanding the excitation laser beam into a sheet
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1. Introduction to LIF & PLIF Imaging
Laser
IR camera
Sheet opticsFlow facilityFilter
Fluorescence
Display
1-D LIF
PLIF
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Pulsed lasers are used to achieve single-shot imaging Shot excitation pulse effectively freezes the flow field Can suffer from lower SNR than 1-D LIF because the laser
energy is spread out over a much larger area May use averaged measurements for improved SNR with either
CW or pulsed laser excitation for steady systems
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1. Introduction to LIF & PLIF Imaging
Laser
IR camera
Sheet opticsFlow facilityFilter
Fluorescence
Display
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3-D PLIF scan laser sheet
Sheet must cross flow field during laser pulse ∆t Depth-of-field of lens must include outermost sheets Camera must frame quickly; #sheet/∆t
Display methods Orthogonal slices (cube display) Surface rendering Movie
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1. Introduction to LIF & PLIF: 3D-PLIF
Laser
Camera
Sheet opticsFlow facilityFilter
FluorescenceSpinning mirror
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Species density
Assume known: A21, B21;Iν, V & Ω are usually measured (i.e., by Rayleigh scattering)
Then SF ∝ C·ni, where C ∝ VΩfv,J/Q is from a calibration point For T-varying systems, need to select v and J to give fv,J/Q
independent of T5
1. Introduction to LIF and PLIF ImagingApplication of LIF to species density
Measure ni [molecules/cc] with LIF using linear excitation
Q
Tfn
QAABIVnS J
iF,v
2121
2112
01 4
photons/s
nALAP
nLPP
in
inout
scattered WWRayleigh:
intensity II
VIn
nVIP
Woutscattered
set in experiment
measure known
VICan infer
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Species mole fraction
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1. Introduction to LIF and PLIF ImagingApplication of LIF to species mole fraction
i
Ji
JiF
XCTTf
X
cnTf
nS
,v
,vphotons/s
Q = nσc, c = mean molecular speed C is obtained by calibration Requires selection of v and J to give Tf
Tf J
,v
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Temperature Strategy 1: Single-line method
Strategy 2
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1. Introduction to LIF and PLIF Imaging Application of LIF to temperature
TTf
XS JiF
,v
Use a tracer with fixed Xi
Select v and J for large T dependence of T
f J
,v
TFTfTf
QTf
n
QTf
n
SS
J
J
Ji
Ji
F
F
2,v
1,v
2
,v
1
,v
21
Select λ1 and λ2 to probe states with strong T-dependence for fv,J(T)1/ fv,J(T)2
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1. Introduction to LIF and PLIF Imaging30 Years: From Concept to Standard Tool
1982 – 1st PLIF in flames 1984 – 1st PLIF in turbulent flames 1987 – 3D digital flow field mapping with PLIF1992 – Acetone tracers to visualize flow mixing with PLIF1993 – PLIF investigation of scamjet fuel mixing1996 – PLIF imaging of temperature in supersonic jet2000-present – Transition of tracer-based PLIF imaging into industrial labsWhere is PLIF today:2012 – Use of kHz rate PLIF in practical aeropropulsion problems (e.g., swirl burner)
Now some applications of tracer-based PLIF
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2. Flame Applications for OH PLIF:A Tool for Turbulent Flame Research
Seitzman et al, 23rd International Symposium on Combustion (1990)
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2. Flame Applications for OH PLIF Flame Structure vs Reynolds Number via OH PLIF
OH PLIF of a hydrogen diffusion flame in air (2.2mm diameter jet)
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2. Flame Applications for OH PLIF Diffusion Flame Structure using OH PLIF
OH PLIF to visualize flame structure in a plane vs1. Axial distance from the nozzle exit2. Radial distance from the fuel jet axis
• H2/air diffusion flame• 2.2 mm diameter jet• Re~24,000
@41 D2-41D
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Forced CO2 jet
Infrared Planar Laser-Induced Fluorescence Imaging Potential to image species not easily accessible with Visible/UV PLIF
CO, CO2, H2O, hydrocarbons Advances in IR lasers sources and cameras have made IR PLIF possible First demonstrations of IR PLIF performed at Stanford (Kirby and Hanson)
2.0 μm excitation4.3 μm fluorescence
3. PLIF with Molecular Vibrations in IR
Laser
IR camera
Sheet opticsFlow facility
Filter
Fluorescence
Display
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Photophysics of Vibrational Fluorescence for CO and CO/CO2 PLIF
Emission rates are low (100 s-1 vs 106 s-1) Energy transfer process can be slow (μs vs. ns) V-V transfer is typically faster than V-T transfer
3. PLIF with Molecular Vibrations in IR
Energy transfer following 2.3 μm excitation of CO:
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Air
CO2 jet425 K
Kirby and Hanson, Appl. Opt., 2000.
Example of CO2 Imaging: Using 10.6 μm Excitation3. PLIF with Molecular Vibrations in IR
Transverse CO2 jet (425K) in air cross flow4.3 μm collection
Demonstrates ability to image hot CO2, mixing Potential for CO2 imaging in flames
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Benchmark lifted CH4 flame studied at Yale allows for quantitative comparisons
IR PLIF agrees with CFD and averaged Raman measurements within 20%
Demonstrates feasibility of single-shot imaging
LIF signal >> Raman
CO2(single-shot)
CO2(36-shot avg)
Raman(Yale)
~5,000 shots per row
CFD(Yale)
combustionproducts
(e.g., CO2)
Air
fuel65% CH4with N2
Air
imaged region(1.7 x 1.7 cm)
laser sheet(3 J @ 10.6)
Comparison of CO2 Images in a Lifted CH4 Flame3. PLIF with Molecular Vibrations in IR
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M > 6
Supersonic Combustor
HYPER-X Autoignition and Flame-Holding Mixing
Research Issues:
High-speed, harsh flow fields require non-intrusive diagnostics
Diagnostic Issues:
How do we do ground test beyond Mach 8?Impulse Facilities: reflected shock tunnels, expansion tubes
Diagnostic Challenges of SCRAMJETS: Opportunities for PLIF
4. Imaging Hypersonic Flows – Challenges
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Driver Section
Driven Section
Expansion Section
Dump Tank
Amplifiers &
Interval Counters
ICCD Camerafor OH-PLIF Imaging
578 x 384 Array
DoubleDiaphragm
8 Channel Data
Acquisition
System
KnifeEdge
YM 1200 Nd:YAG Laser
HD 500 Dye Laser
HT 1000Frequency
Doubler
Long durationXenon Arc
Light Source
DichroicMirror
IMACON 468Ultrafast Framing Camera
for Schlieren Imaging(inc. 8 ICCD modules,
each 576 x 384)
Image Acquisition
Computers
Driven/ExpansionDiaphragm
Mirror 2
Mirror 1
FocusingMirror
Single-shot PLIF imaging 8-frame schlieren imaging
Optical Diagnostics
Stanford Expansion Tube Facility4. Imaging Hypersonic Flows – Challenges
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N2He Fuel/O2/N2mixture
He
Driversection
Buffersection
Drivensection
Expansionsection
Viewing Section
A Tool to Study Ram Accelerator Phenomena
Virtue of expansion tube: accelerates combustible gas to high velocity, with reduced exposure to high temperatures
M > 1 Projectile Fix projectile, flow gases, employ PLIF
Measurement Strategy
Unsteady combustion/detonationKey issue
5. Imaging Hypersonic Flows – Reacting
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NO PLIF Imaging of Non-Reacting Flows
Mixture: 0.15% NO + 9.5% CH4 + 90.35% N2
P = 0.063 atm, V = 2230 m/s, M = 7, T = 280 K
19 mm Blunted Cylinder 40 degree Wedge
NO PLIF useful for shock wave visualization and quantitative flowfielddeterminations (T, P, χi)
5. Imaging Hypersonic Flows – Reacting
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V∞ = 2230 m/s , P∞ = 0.07 bar, M∞ = 6.3, T∞ = 300 K
25 mm Spherical-Nosed Cylinder
5% C2H4 + 15% O2 + 80% N2
Mixture CJ velocity, VCJ = 1685 m/stign ~ 1 µs
16.7% CH4 + 33.3% O2 + 50.0% N2
Mixture CJ velocity, VCJ = 1950 m/stign ~ 10 µs
25 mm Spherical-Nosed Cylinder
V∞/VCJ = 1.14 V∞/VCJ = 1.32
PLIF reveals steady combustion Peak OH in stagnation region
5. Imaging Hypersonic Flows – OH PLIF in Reacting FlowsSimultaneous OH PLIF and Schlieren Imaging of Smooth Flame Front Regime: V∞/VCJ >1
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Large Disturbance Regime: V∞/VCJ <1 Comparison with CFD Calculations
T∞ = 300 K, P∞ = 0.5 bar,V∞ = 1760 m/s
Stoichiometric H2/Air
Density contours from A. Matsuo & K. Fujii,AIAA 95-2565
T∞ = 420 K, P∞ = .23 bar,V∞ = 1730 m/s
20 mmDiameter
25 mmDiameter
10% C2H4 + 30% O2 + 60% N2
V∞/VCJ = 0.90V∞/VCJ = 0.90
Experimental results show good agreement with CFD calculations of large disturbance regime
5. Imaging Hypersonic Flows – OH PLIF in Reacting Flows
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Model problem: jet in supersonic crossflow
2-D imaging required to capture complex, unsteady flow structure
Defining parameter: jet momentum ratio: J = (ru2)jet/(ru2)∞22
INJECTANT(Hydrogen)
BOW
SHOCK
RECIRCULATION ZONE
M>1
RECIRCULATIONZONE
BOUNDARY LAYERMACH DISK
JET-SHEAR LAYERVORTICES
Autoignition Zones
Critical Scramjet Issue: Mixing of Fuel5. Imaging Hypersonic Flows – Reacting
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M= 3.4V= 2360 m/sP= 0.32 atmT= 1290 K
Instantaneous Average
Use of short gating times reveals unsteady flow structure
0.1 μs exposure8-frame movie with 1ms framing time shows flow structure
10 μs exposure
Nitrogen
Hydrogen
H2 Jet in Supersonic Crossflow Imaging with Schlieren5. Imaging Hypersonic Flows – Reacting
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PLIF Imaging of OH in Combusting Jet
Mach 13OxygenM∞= 4.7V∞= 3200 m/sP∞= 0.04 atmT∞= 1250 K
H2 , J=5x/d
Mach 10AirM∞= 3.4V∞= 2360 m/sP∞= 0.32 atmT∞= 1290 K
x/dH2 , J=1.4
Penetration increases with jet momentum ratio (J) Increased recirculation zone enhances flame holding Reaction zone width increases with x/d
Key Observations
Sf χOH·f(T)
Defining Eqn:
5. Imaging Hypersonic Flows – OH PLIF in Reacting Flows
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Flame Structure of Hydrogen Jet in Supersonic Crossflow(Combine Side-view, Plan-view and Transverse-plane Images)
Flat plate
Illumination sheet
Expansion tube exit
Fuel injection
Flow
xz
y
Schematic diagram of imaging experiments
M = 2.4P = 40 kPaT = 1500 KJ = 2.4
Planar laser-induced fluorescence of hydroxylradical marking the instantaneous reaction zoneof hydrogen jet in supersonic crossflow
5. Imaging Hypersonic Flows – OH PLIF in Reacting Flows
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Flame Structure of Hydrogen Jet in Supersonic Crossflow
Plan-views
y/d = 5 y/d = 3
y/d = 1 y/d = 0.5 y/d = 0.25
Images adapted from Heltsley (2009)
5. Imaging Hypersonic Flows – OH PLIF in Reacting Flows
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27Adapted from Heltsley (2009)
M = 2.4P = 40 kPaT = 1500 KJ = 2.4
Plan-view
Transverse Planeview
Side-view
Flame Structure of Hydrogen Jet in Supersonic Crossflow
5. Imaging Hypersonic Flows – OH PLIF in Reacting Flows
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5. Imaging Hypersonic Flows – OH PLIF in Reacting Flows
Hydrogen (M = 1)J = 2.4D = 2 mm
AirM = 2.4P = 40 kPzT = 1500 K
OH PLIF images trace the 3-D structure of the flame
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Extension of PLIF Imaging to Scramjet Flow Using 2 Lasers for T
Tk
CSS
Rf
f 1212
2
112 exp
Instantaneous temperature imaging Instantaneous multi-species imaging
H2 jet
Supersonic air
6. Imaging Hypersonic Flows – T w NO (2 camera)
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Instantaneous, Two-Line NO Temperature Images
Instantaneous T determined to ±50 K in 300 K-1400 K range Images reveal:
Jet mixing requires x/d ~ 12 Heated boundary layer provides ignition
With NO in freestream
Without NO in freestream
0 18x/d
0 18x/d
6. Imaging Hypersonic Flows – T w NO (2 camera)
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Simultaneous NO and OH Imaging
NO
OH
composite
Reveals mixing
Reveals combustion zones
6. Imaging Hypersonic Flows – Dual Species
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Comparison of Axial and Transverse Plane Images
NO
OH
NO image reveals efficient mixing by x/d = 15 OH image reveals that combustion occurs at jet periphery
and in boundary layer
11 mm
x= 22 mm
Freestream: M=1.4, P=0.4 atm, T=2200 K Jet: Po=3 atm, To=300 K, d=2 mm
6. Imaging Hypersonic Flows – Dual Species
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Next Lecture
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Applications of LIF and PLIF using flow tracers1. PLIF of ketone hydrocarbons
2. Acetone PLIF to image fuel mixing
3. 3-pentanone PLIF as a flow tracer
4. 3-pentanone PLIF in IC-engines
5. CW PLIF for high frame-rate imaging
6. Toluene PLIF for temperature
7. Two camera toluene PLIF for temperature