Lecturer: Ronald K. HansonWoodard Professor, Dept. of Mechanical EngineeringStanford University
15 Lecture Short Course at Princeton University
Copyright ©2018 by Ronald K. HansonThis material is not to be sold, reproduced or distributed without prior
written permission of the owner, Ronald K. Hanson.
Underlying Science:Molecular Spectroscopy
Diagnostic Methods:Laser Absorption, LIF
Example Applications:Engines, Shock Tubes, Kinetics
Lecturer: Ronald K. HansonWoodard Professor, Dept. of Mechanical EngineeringStanford University
15 Lecture Short Course at Princeton University
Copyright ©2018 by Ronald K. HansonThis material is not to be sold, reproduced or distributed without prior
written permission of the owner, Ronald K. Hanson.
Today/Lecture 1:• Overview• Introductory Material
Course Objectives and Content• Introduction to fundamentals of molecular spectroscopy & photo-physics
• Introduction to laser absorption and laser-induced fluorescence
• Introduction to shock tubes as a primary tool for studying combustion chemistry, including recent advances and kinetics applications
• Example laser diagnostic applications including:• multi-parameter sensing in different types of propulsion flows and engines• species-specific sensing for shock tube kinetics studies• PLIF imaging in high-speed flows
Next: Spectroscopy and Roles of Lasers
Lecture 1: Course Overview
3
Course Overview:Spectroscopy and Lasers
What is Spectroscopy?• Interaction of Radiation (Light) with
Matter (in our case, Gases)• Examples: IR Absorption
Why Lasers?• Enables Important Diagnostic
Methods• LIF, Raman, LII, PIV, CARS, …• Our Emphasis: Absorption and LIF• Why: Sensitive and Quantitative!
Calculated IR absorption spectra of HBr
1000 1500 2000 2500 3000
0.01
0.1
1
10
100
1000CO2
CH3
C2H4
Min
imum
Det
eciti
vity
[ppm
]
Temperature [K]
1atm,15cm,1MHz
H2ONH2
1000 1500 2000 2500 3000
0.01
0.1
1
10
100
1000
OH
Min
imum
Det
eciti
vity
[ppm
]
Temperature [K]
1atm,15cm,1MHz
CH
CN
Minimum Detectivity using Laser Absorption
4
Spectrally resolved individual absorption line of NO at 600 K, 1 atm (in C2H4 combustion exhaust)
Course Overview:Role of Lasers in Energy Sciences
5
Example Applications:Remote sensing, combustion and gasdynamic diagnostics, process control, energy systems and environmental monitoring.
Common Measurements:Species concentrations, temperature (T), pressure (P), density (ρ), velocity (u), mass flux (ρu).
Coal-fired power plants
Coal gasifiers Swirl burners
IncineratorsOH PLIF in spray flame
Course Overview:Roles of Laser Sensing for Propulsion
TDL Sensing in Pratt & Whitney PDE
@ China Lake, CA
TDL Sensing in SCRAMJET @ WPAFB
Applicable to large-scale systems as well as laboratory science
248 nm beamSignal
PLIF in plume of Titan IV @ Aerojet
PLIF imaging of H2 jet in model SCRAMJET
@Stanford
TDL Sensing in IC-Engines @ Nissan & Sandia
Validate simulations and
models
Characterize test facilities
Understandcomplex reactive
environments
Optical Diagnostics
6
Ring Dye Lasers(UV & Vis)
Diode Lasers(Near IR & Mid-IR)
CO2 Lasers(9.8-10.8 m)
Ti:Sapphire Laser(Deep UV)
He-Ne Laser(3.39 m)
DriverSection
Shock TubeDriven Section
emis
Incident Beam Detector
Transmitted Beam Detector
Diaphragm
7Course Overview:Role of Lasers in Combustion Kinetics: Shock Tubes
High-Speed Camera
EC-QCL Laser(11 m)
Temperature(4.3 m)
Ring Dye Lasers(UV & Vis)
CO2 Lasers(9.8-10.8 m)
Ti:Sapphire Laser(Deep UV)
emisIncident Beam
Detector
Transmitted Beam Detector
VS
P1T1
P2T2
IncidentShock Wave
8Course Overview:Role of Lasers in Combustion Kinetics: Shock Tubes
EC-QCL Laser(11 m)
He-Ne Laser(3.39 m)
Diode Lasers(Near IR & Mid-IR)
High-Speed Camera
Temperature(4.3 m)
Ring Dye Lasers(UV & Vis)
CO2 Lasers(9.8-10.8 m)
Ti:Sapphire Laser(Deep UV)
emis
Incident Beam Detector
Transmitted Beam Detector
P5T5
P2T2 VRS
ReflectedShock Wave
Applications of Shock Tubes• Ignition Delay Times• Elementary Reactions• Species Time-Histories
Species Accessible by Laser Absorption• Radicals: OH, CH3 …• Intermediates: CH4, C2H4, CH2O …• Products: CO, CO2, H2O …
9
Advantages of Shock Tubes• Near-Ideal Test Platform• Well-Determined Initial T & P• Clear Optical Access for Laser Diagnostics
Course Overview:Role of Lasers in Combustion Kinetics: Shock Tubes
High-Speed Camera
EC-QCL Laser(11 m)
Temperature(4.3 m)
Diode Lasers(Near IR & Mid-IR)
He-Ne Laser(3.39 m)
Course Overview:Lasers and Shock Tube: Time-Histories & Kinetics
Multi-wavelength laser absorption species time-histories provide quantitative targets for model refinement and validation
Laser absorption provides high-accuracy measurements of elementary reaction rate constants
1494K, 2.15 atm300ppm heptane, =1
JetSurF 2.0H+O2 = OH+O
10
Time-Histories Rate Constant
0.3 0.5 0.7 0.91010
1011
1012
1013
OH Laser Abs. Wang et al. (2017)
H2O Laser Abs. Hong et al. (2009)
H ARAS. Pirraglia et al. (1989)
2000K
k 1 [cm
3 mol
-1s-1
]
1000/T [1/K]
1428K 1111K
OH Laser Abs. Masten et al. (1990)
Useful Texts, Supplementary Reading
11
G. Herzberg, Atomic spectra and atomic structure, 1944. G. Herzberg, Spectra of diatomic molecules, 1950. G. Herzberg, Molecular spectra and molecular structure, volume II,
Infrared and Raman Spectra of Polyatomic Molecules, 1945. G. Herzberg, Molecular spectra and molecular structure, volume III,
Electronic spectra and electronic structure of polyatomic molecules, 1966. C.N. Banwell and E.M. McCash, Fundamentals of molecular spectroscopy, 1994. S.S. Penner, Quantitative molecular spectroscopy and gas emissivities, 1959. A.C.G. Mitchell and M.W.Zemansky, Resonance radiation and excited atoms, 1971. C.H. Townes and A.L. Schawlow, Microwave spectroscopy, 1975. M. Diem, Introduction to modern vibrational spectroscopy, 1993.
W.G. Vincenti and C.H. Kruger, Physical gas dynamics, 1965. A.G. Gaydon and I.R. Hurle, The shock tube in high-temperature chemical physics, 1963.
J.B. Jeffries and K. Kohse-Hoinghaus, Applied combustion diagnostics, 2002. A.C. Eckbreth, Laser diagnostics for combustion temperature and species, 1988. W. Demtroder, Laser spectroscopy: basic concepts and instrumentation, 1996. R.W. Waynant and M.N. Ediger, Electro-optics handbook, 2000. J.T. Luxon and D.E.Parker, Industrial lasers and their applications, 1992. J.Hecht, Understanding lasers: An entry level guide, 1994. K.J.Kuhn, Laser engineering, 1998. R.K. Hanson et al., Spectroscopy and Laser Diagnostics for Gases, 2016
Lecture Schedule
12
1. Overview & IntroductionCourse Organization, Role of Quantum Mechanics,Planck's Law, Beer's Law, Boltzmann distribution
2. Diatomic Molecular SpectraRotational Spectra (Microwaves)Vibration-Rotation (Rovibrational) Spectra (Infrared)
3. Diatomic Molecular SpectraElectronic (Rovibronic) Spectra (UV, Visible)
13. Laser-Induced Fluorescence (LIF)Two-Level ModelMore Complex Models
14. Laser-Induced Fluorescence: Applications 1Diagnostic Applications (T, V, Species)PLIF for small molecules
15. Laser-Induced Fluorescence: Applications 2Diagnostic Applications & PLIF for large moleculesThe Future
7. Electronic Spectra of DiatomicsTerm Symbols, Molecular Models: Rigid Rotor, Symmetric Top, Hund's Cases, Quantitative Absorption
8. Case Studies of Molecular SpectraUltraviolet: OH
9. TDLAS, Lasers and FibersFundamentals and Applications in Aeropropulsion
4. Polyatomic Molecular SpectraRotational Spectra (Microwaves)Vibrational Bands, Rovibrational Spectra
5. Quantitative Emission/ AbsorptionSpectral absorptivity, Eqn. of Radiative TransferEinstein Coefficients/Theory, Line Strength
6. Spectral LineshapesDoppler, Natural, Collisional and Stark broadening,Voigt profiles
10. TDLAS Applications in Energy ConversionTunable Diode Laser Applications in IC EnginesCoal-Fired Combustion
11. Shock Tube TechniquesWhat is a Shock Tube?Recent Advances, ignition Delay Times
12. Shock Tube ApplicationsMulti-Species Time HistoriesElementary Reactions
Monday
Tuesday
Wednesday
Thursday
Friday
Lecture 1: Introductory Material
1. Role of Quantum Mechanics - Planck’s Law
2. Absorption and Emission3. Boltzmann Distribution4. Working Examples
13
ΔE
Eelec
Evib
Erot
Quantum Mechanics: Quantized Energy levels “Allowed” transitions
14
Eint = Eelec + Evib + Erot
1. Role of QM - Planck’s Law
How are energy levels specified?Quantum numbers for electronic, vibrational and rotational states.
We will simply accept these rules from QM}
1. Role of QM - Planck’s Law Quantum Mechanics
15
Small species, (e.g., NO, CO, CO2, and H2O), have discrete spectral features
Large molecules (e.g., HCs) have blended features
Quantized Energy States (discrete energy levels)
Discrete spectraPlanck’s Law:ΔE = Eupper (E’) – Elower (E”)
= h= hc/λ= hc Energy in wavenumbers
Energy state or level
Absorption
Emission“Allowed” transitions
Energy
ΔE
c = λ
~ 3 x 1010 cm/s Wavelength [cm]
Frequency [s-1]
Note interchangability of λ &
2. Absorption and Emission Types of spectra:
Absorption; Emission; Fluorescence; Scattering (Rayleigh, Raman)
Absorption: Governed by Beer’s Law
16
Beer-Lambert Law LSPLnTII
ijt
expexpexp
0
Number density of species j in absorbing state [molecule/cm3]
Cross section for absorption [cm2/molecule]
Path length [cm]
Absorbance
I0, ν T, P, χi,vIt
L
GasWavelength
Tran
smiss
ion
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
17
Eint = Eelec + Evib + Erot
r (distance between nuclei)
E(pot.)
Potential energy curve for 1 electronic state
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
18
Eint = Eelec + Evib + Erot
Erot
Line: Single transition
λTλ
r (distance between nuclei)
E(pot.)
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
19
Eint = Eelec + Evib + Erot
Evib
Line: Single transition
Band: Group of lines with common upper + lower vibrational levels
λTλ
Δv=vupper – vlower=1 is strongestfor rovibrational IR spectra,but Δv= 2,3, … allowed
vupper
vlowerR P Two branches,
Denoted byP&R
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
20
Eint = Eelec + Evib + Erot
Evib
Line: Single transition
Band: Group of lines with common upper + lower vibrational levels
λTλ
Δv=1Δv=2Δv=3
Δv=1
But Δv>1 possible
vlower
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
21
Eelec
System: Transitions between different
electronic states Comprised of multiple bands between
two electronic states Different combinations of vupper and
vlower such that “bands” with vupper-vlower=const. appear
C3Πu
B3Πg
A3Σ+uN2(1+)
Eint = Eelec + Evib + Erot
N2(2+)
Nitrogen
Example: N2 First positive SYSTEM:
B3Πg→A3Σ+u
The ground (lowest energy) state is X1Σ+
g
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
22
Eelec
System
Example: High-temperature air emissionspectra (560-610nm)
C3Πu
B3Πg
A3Σ+uN2(1+)
N2(2+)
Nitrogen
12→8
11→7 10→6
9→5
8→47→3
6→2
vupper=v'vlower=v" v'-v"=4
Eint = Eelec + Evib + Erot
2. Absorption and Emission Components of spectra: Lines, Bands, Systems
23
SystemExample: Typical emission spectra of DC discharges
UV Visible-NIR
2. Absorption and Emission
24
OH 2Σ−2Π (0,0)
CH 2Δ−2Π
CH 2Σ−2Π
CH 2Σ−2Π
NH 3Π−3Σ
In early days, spectra were recorded on film!But now we have lasers.
Components of spectra: Lines, Bands, Systems
How is Tλ (fractional transmission) measured?
2. Absorption and Emission
25
Transmission (Tλ)
Absorption
λ
Tunable Laser Test media; Flame Iλ; Detector
1.0Δλ = Full width at half maximum
λ0 = Line center
Δλ = f(P,T)
A resolved line has shape!
Do lines have finite width/shape? Yes!
And shape is a f(T,P) an opportunity for diagnostics!
3 key elements of spectra Line positions Line strengths Line shapes
2. Absorption and Emission
26
Covered in course
3. Boltzmann Distribution How strong is a transition?
27
Proportional to particle population in initial energy level n1
S12
Energy level 1
Energy level 2
ΔE=h
n1
Boltzmann fraction of absorber species i in level 1
QkT
g
nnF
ii
ii
exp
elecvibroti
ii QQQ
kTgQ
expPartition function
- Equilibrium distribution of molecules of a single species over its allowed quantum states.
Statistical Mechanics: Defines T!
Hence measurements of two densities ni and nj → TSince ni/nj = gi/gj exp(‐(i‐j)/kT)
TDL sensing for aero-propulsion Diode laser absorption sensors offer prospects for time-resolved, multi-
parameter, multi-location sensing for performance testing, model validation, feedback control
4. Working Example – 1
28
Exhaust(T, species, UHC, velocity, thrust)
Inlet and Isolator(velocity, mass flux, species,
shocktrain location)
Combustor(T, species, stability)
l1 l2 l3 l4 l5
Diode Lasers
Fiber Optics
Acquisition and Feedback to Actuators
l6
Sensors developed for T, V, H2O, CO2, O2, & other species Prototypes tested and validated at Stanford Several applications successful in ground test facilities Now being utilized in flight
TDL Sensing to Characterize NASA Ames ArcJet Facilities High-Enthalpy Flow for Materials and Vehicle Testing
4. Working Example – 2
29
High pressure gas Arc heater Nozzle
High velocity low pressure
flow for hypersonic
vehicle testing
30ft
10ft
10ft
TDL Sensing to Characterize NASA Ames ArcJet Facilities High-Enthalpy Flow for Materials and Vehicle Testing
4. Working Example – 2
30
High pressure gas Arc heater Nozzle
High velocity low pressure
flow for hypersonic
vehicle testing
Goals: (1) Time-resolved temperature sensing in the arc heater: O to infer T(2) Investigate spatial uniformity within heater (multi-path absorption)
Challenges: Extreme Conditions T=6000-8000K, P= 2-9 bar, I~2000A, 20 & 60 MWDifficult access (mechanical, optical, and electrical)
Cooling water
Anode Cathode
Test cabinInlet Air
TDL Sensor
Constrictor Tube
Cooling Argon
Temperature from Atomic O Absorption Measurement
4. Working Example – 2
31
Atomic oxygen energy diagram
777.2 nm
3P23P13P0
5P35P25P1844.6 nm
3P0,1,2
3S01 5S0
2
135.8 nm130.5 nm
Fundamental absorption transitions from O are VUV but excited O in NIR
Equilibrium population of O-atom in 5S02 extremely temperature sensitive
0.6
0.4
0.2
0.0
777.28777.24777.20777.16777.12Wavelength (nm)
-0.050.000.05
Data Fitting
Abs
orba
nce
Res
idua
ls
Atomic oxygen absorption measured in the arc heater
nO*= 6.64 x 1010 cm-3
Tpopulation= 7130±120 K
4. Working Example – 2 Arc current at 2000A, power 20MW Last 200 seconds of run arc current decreased 100A Measured temperature captures change in arc conditions
Precise temperature measurements• 18K or 0.3% standard deviation• 200ms time resolution
18 K Arc current decreased ~100A
TDL sensor provides new tool for routine monitoring of arcjet performance32
1392 nm1469 nm
2678 nm
Flow from Engine
NozzleExit
Fiber-Coupled Light to Engine
Transmitted Light Caught onto Multi-Mode Fibers
Detector for H2O Wavelengths
Detector for CO2Wavelength
Pitch Optics
Catch Optics
H2O & T
CO2
NozzleEntrance
4855 nmCOOR
Port for KistlerPressure Sensor
4. Working Example – 3Time-Resolved High-P Sensing in PDC at NPS
Pulse-detonation combustor gives time-variable P/T Time-resolved measurements monitor performance & test CFD
Assumption: Choked flow Tgives velocity
T, P, V & Xi
yields Enthalpy Flux
33
4. Working Example – 3Time-Resolved High-P Sensing in PDC at NPS
Optical sensors feasible in harsh, high pressure engine environment
34
Pulse Detonation CombustorAt Naval Post-graduate School in Monterey, CA
Pulse-detonation combustor gives time-variable P/T Time-resolved TDLAS measurements monitor performance & test CFD
Exhaust to ambient
Pulsed detonations
P
chamberthroat
Assumption: Choked flowT gives velocity
T, P, V & Xi Enthalpy Flux1469 nm1392 nm
Throat SensorsT & XH2O
(CO @ 4.6m; CO2 @ 2.7m)
35
4. Working Example – 3Time-Resolved High-P Sensing in PDC at NPS
T- Data Collected in Nozzle Throat vs CFD
T sensor performs well to >3500K, 30 atm! Data agrees well with CFD during primary blow down
36
4. Working Example – 3Time-Resolved High-P Sensing in PDC at NPS
4. Working Example – 3Time-Resolved TDLAS Yields Mass Flow
),,( sonicVPTfm
),( mixsonic TfVV
T and P give V and mass flow in choked throat as f(t) T, m, species (CO, CO2, H2O) and ideal gas law can
give enthalpy flow rate
.
37
H
m
hstag (T )
Time-resolved data provide key measures of engine performance Power (enthalpy flux) Mass flow dynamics H integrated over complete cycle for ηth
4 Consecutive Cycles
Tref = 298 K
38
4. Working Example – 3Time-Resolved TDL Yields Enthalpy Flow Rate
4. Working Example – 4Time-Resolved Sensing in HEG Shock Tunnel
39
Sensors developed for T, V, H2O, CO2, O2, & other species Prototypes tested and validated at Stanford Several successful demonstrations in ground test facilities Opportunities emerging for use in flight: sensing and control Measurements made in Mach 7.4 shock tunnel in Germany
– Diode laser absorption sensors offer prospects for time-resolved, multi-parameter, multi-location sensing for performance testing, model validation, feedback control
Exhaust(T, species, UHC, velocity, thrust)
Inlet and Isolator(velocity, mass flux, species,
shocktrain location)
Combustor(T, species, stability)
l1 l2 l3 l4 l5
Diode Lasers
Fiber Optics
Acquisition and Feedback to Actuators
l6
4. Working Example – 4Time-Resolved Sensing in HEG Shock Tunnel
40
41
4. Working Example – 4Time-Resolved Sensing in HEG Shock Tunnel
4. Working Example – 5First Multi-Species Sensing for Shock Tube Kinetics
Oxygen Balance:Methyl Formate Decomposition
Chemistry progress monitored by quantitative IR laser absorption
Multi-species time histories provide game-changing advantage for mechanism validation
Method accounts for nearly 100% of O-atoms
1420 K1.5 atm
Shock wave Test mixture
Detectors
Lasers
H-C-O-C-O
= __
Next: Diatomic Molecular Spectra
• Rotational and Vibrational Spectra
43