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1 1 Lecturer: Ronald K. Hanson Woodard Professor, Dept. of Mechanical Engineering Ph.D. Stanford, Aero/Astro; at Stanford since 1972 15 Lecture Short Course at Princeton University Copyright ©2013 by Ronald K. Hanson This material is not to be sold, reproduced or distributed without prior written permission of the owner, Ronald K. Hanson. Focus: Molecular Spectroscopy, Laser Absorption and LIF Today: Overview, Motivation, Examples Lecturer: Ronald K. Hanson Woodard Professor, Dept. of Mechanical Engineering Ph.D. Stanford, Aero/Astro; at Stanford since 1972 15 Lecture Short Course at Princeton University Copyright ©2013 by Ronald K. Hanson This material is not to be sold, reproduced or distributed without prior written permission of the owner, Ronald K. Hanson. Focus: Molecular Spectroscopy, Laser Absorption and LIF Today: Overview, Motivation, Examples 1 2 Objectives and Content • Introduction to fundamentals of molecular spectroscopy & photo-physics • Emphasis on laser absorption and laser-induced fluorescence in gases • 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 Lecture 1: Overview & Introductory Material 3 Course Overview: Spectroscopy and Lasers 4  What is Spectroscopy? • Interaction of Radiation (Light) with Matter (in our case, Gases). • Examples: IR Absorption, Emission  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 Typical emission spectra of high-temperature air between 560-610nm. 1000 1500 2000 2500 3000 0.01 0.1 1 10 100 1000 CO2 CH3 C2H4 M in im um D et ec iti vi ty [p pm ] Temperature [K] 1atm,15cm,1MHz H2O NH2 1000 1500 2000 2500 3000 0.01 0.1 1 10 100 1000 OH M in im um D et ec iti vi ty [p pm ] Temperature [K] 1atm,15cm,1MHz CH CN Minimum Detectivity using Laser Absorption 4 1 3 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 Ground Test 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 Understand complex reactive environments Optical Diagnostics 6 1 4 Course Overview: Role of Lasers in Combustion Kinetics: Shock Tubes 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) UV/Vis/IR Emission DetectorsIncident Beam Detector Transmitted Beam Detector Pressure PZT P5 T5 P2 T2 VRS Reflected Shock Wave 7 Advantages of Reflected Shock Wave Experiments • Near-Ideal Constant V or Constant P Platform • Well-Determined Initial T & P • Lack of Transport Effects  Negligible Non-uniformities • Clear Access for Sensitive, Quantitative Laser Diagnostics Course Overview: Lasers and Shock Tube: Time-Histories & Kinetics  Multi-wavelength laser absorption species time- histories provide quantitative kinetics targets form model refinement and validation  OH laser absorption provides high-accuracy measurements of elementary reaction rate constants 1494K, 2.15 atm 300ppm heptane, =1 JetSurF 2.0 H+O2 = OH+O 8 1 5 Useful Texts, Supplementary Reading 9  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. Lecture Schedule 10 1. Overview & Introduction Course Organization, Role of Quantum Mechanics, Planck's Law, Beer's Law, Boltzmann distribution 2. Diatomic Molecular Spectra Rotational Spectra (Microwaves) Vibration-Rotation (Rovibrational) Spectra (Infrared) 3. Diatomic Molecular Spectra Electronic (Rovibronic) Spectra (UV, Visible) 13. Laser-Induced Fluorescence (LIF) Two-Level Model More Complex Models 14. Laser-Induced Fluorescence: Applications 1 Diagnostic Applications (T, V, Species) PLIF for small molecules 15. Laser-Induced Fluorescence: Applications 2 Diagnostic Applications & PLIF for large molecules The Future 7. Electronic Spectra of Diatomics Term Symbols, Molecular Models: Rigid Rotor, Symmetric Top, Hund's Cases, Quantitative Absorption 8. Case Studies of Molecular Spectra Ultraviolet: OH 9. TDLAS, Lasers and Fibers Fundamentals and Applications in Aeropropulsion 4. Polyatomic Molecular Spectra Rotational Spectra (Microwaves) Vibrational Bands, Rovibrational Spectra 5. Quantitative Emission/ Absorption Spectral absorptivity, Eqn. of Radiative Transfer Einstein Coefficients/Theory, Line Strength 6. Spectral Lineshapes Doppler, Natural, Collisional and Stark broadening, Voigt profiles 10. TDLAS Applications in Energy Conversion Tunable Diode Laser Applications in IC Engines Coal-Fired Combustion 11. Shock Tube Techniques What is a Shock Tube? Recent Advances, ignition Delay Times 12. Shock Tube Applications Multi-Species Time Histories Elementary Reactions Monday Tuesday Wednesday Thursday Friday 1 6 Lecture 1: Introductory Material 1. Role of Quantum Mechanics - Planck’s Law 2. Absorption and Emission 3. Boltzmann Distribution 4. Working examples 11 ∆E Eelec Evib Erot  Quantum Mechanics:  Quantized Energy levels  “Allowed” transitions 12 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 7 1. Role of QM - Planck’s Law  Quantum Mechanics 13  Small species, (e.g., NO, CO, CO2, and H2O), have discrete rovibrational transitions  Large molecules (e.g., HCs) have blended features Quantized Energy States (discrete energy levels) Discrete spectra Planck’s Law: ∆E = Eupper (E’) – Elower (E”) = h = hc/λ = hc Energy in wavenumbers Energy state or level Absorption Emission “Allowed” transitions Energy ∆E c = λ  ~ 3 x 1010 cm/s Wavelength [cm] Frequency [s-1] Note interchangability of λ & ν 2. Absorption and Emission  Types of spectra:  Absorption; Emission; Fluorescence; Scattering (Rayleigh, Raman)  Absorption: Governed by Beer’s Law 14 Beer-Lambert Law      LSPLnT I I ij t       expexpexp 0 Number density of species j in absorbing state [molec./cm3] Cross section for absorption [cm2/molec.] Path length [cm] Absorbance I0, ν T, P, χi,v It L Gas Wavelength Tr an sm is si on 1 8 2. Absorption and Emission  Components of spectra: Lines, Bands, System. 15 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, System. 16 Eint = Eelec + Evib + Erot Erot Line: Single transition λ Tλ r (distance between nuclei) E (pot.) 1 9 2. Absorption and Emission  Components of spectra: Lines, Bands, System. 17 Eint = Eelec + Evib + Erot Evib Line: Single transition Band: Group of lines with common upper + lower vibrational levels λ Tλ ∆v=vupper – vlower=1 is strongest for rovibrational IR spectra, but ∆v= 2,3, … allowed vupper vlower R P Two branches,e.g. P&R 2. Absorption and Emission  Components of spectra: Lines, Bands, System. 18 Eint = Eelec + Evib + Erot Evib Line: Single transition Band: Group of lines with common upper + lower vibrational levels λ Tλ ∆v=1∆v=2∆v=3 ∆v=1 But ∆v>1 possible vlower 1 10 2. Absorption and Emission  Components of spectra: Lines, Bands, System. 19 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 2. Absorption and Emission  Components of spectra: Lines, Bands, System. 20 Eelec System Example: High-temperature air emission spectra (560-610nm) C3Πu B3Πg A3Σ+uN2(1+) N2(2+) Nitrogen 12→8 11→7 10→6 9→5 8→4 7→3 6→2 vupper=v' vlower=v" v'-v"=4 Eint = Eelec + Evib + Erot 1 11 2. Absorption and Emission  Components of spectra: Lines, Bands, System. 21 System Example: Typical emission spectra of DC discharges UV Visible-NIR 2. Absorption and Emission 22 OH 2Σ−2Π (0,0) CH 2∆−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, System. 1 12  How is Tλ (fractional transmission) measured? 2. Absorption and Emission 23 Transmission (Tλ) Absorption λ Tunable Laser Test media; Flame Iλ; Detector 1.0 ∆λ = Full width at half maximum λ0 = Line center ∆λ = f(P,T) A resolved line has shape!  Do lines have finite width/shape? Yes!  3 key elements of spectra  Line position  Line strength  Line shapes 2. Absorption and Emission 24 Covered in course 1 13  How strong is a transition? 3. Boltzmann Distribution 25 Proportional to particle population in initial energy level n1 S12 Energy level 1 Energy level 2 ∆E=hν n1 Boltzmann fraction of absorber species i in level 1 Q kT g n nF i i i i     exp elecvibrot i i i QQQkT gQ    expPartition function - Equilibrium distribution of molecules of a single species over its allowed quantum states. defines T  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 Examples – 1 26 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  Future opportunities for use in flight 1 14  TDL Sensing to Characterize NASA Ames ArcJet Facilities High-Enthalpy Flow for Materials and Vehicle Testing 4. Working Examples – 2 27 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 Examples – 2 28 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 MW Difficult access (mechanical, optical, and electrical) Cooling water Anode Cathode Test cabin Inlet Air TDL Sensor Constrictor Tube Cooling Argon 1 15  Temperature from Atomic O Absorption Measurement 4. Working Examples – 2 29 Atomic oxygen energy diagram 777.2 nm 3P2 3P1 3P0 5P3 5P2 5P1844.6 nm 3P0,1,2 3S01 5S02 135.8 nm 130.5 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 0.0 777.28777.24777.20777.16777.12 Wavelength (nm) -0.05 0.00 0.05 Data Fitting Ab so rb an ce R es id ua ls Atomic oxygen absorption measured in the arc heater nO*= 6.64 x 1010 cm-3 Tpopulation= 7130±120 K 4. Working Examples – 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 performance 30 1 16 1392 nm 1469 nm 2678 nm Flow from Engine Nozzle Exit Fiber-Coupled Light to Engine Transmitted Light Caught onto Multi-Mode Fibers Detector for H2O Wavelengths Detector for CO2 Wavelength Pitch Optics Catch Optics H2O & T CO2 Nozzle Entrance 4855 nm COOR Port for Kistler Pressure Sensor 4. Working Examples – 3 Time-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 T gives velocity T, P, V & Xi yields Enthalpy Flux 31  Pulse-detonation combustor gives time-variable P/T  Time-resolved measurements monitor performance & test CFD Exhaust to ambient Pulsed detonations P chamber throat Assumption: Choked flow T gives velocity T, P, V & Xi Enthalpy Flux 1469 nm1392 nm Throat Sensors T & XH2O (CO@4.6m; CO2@2.7m) 32 4. Working Examples – 3 Time-Resolved High-P Sensing in PDC at NPS 1 17 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 33 4. Working Examples – 3 Time-Resolved High-P Sensing in PDC at NPS 4. Working Examples – 3 Time-Resolved TDL Yields Mass Flow ),,( sonicVPTfm   ),( mixsonic TfVV   T and P give V and mass flow in choked throat as f(t)  T, X, m and ideal gas can give enthalpy flow rate . 34 1 18 H   m hstag (T )  Time-resolved data provide key measures of engine performance  Power  Mass flow dynamics  H integrated over complete cycle for ηth 4 Consecutive Cycles Tref = 298 K 35 4. Working Examples – 3 Time-Resolved TDL Yields Enthalpy Flow Rate Next: Diatomic Molecular Spectra • Rotational and Vibrational Spectra 36