LaserBased Imaging

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Laser-Based Imaging

• Nicole Dorner

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Background History

• Gained attention during the 1980s
• Used frequently with aerosol studies
• Can be used to measure different things
• Reaction kinetics
• Rate constants for molecular collisions
• Material velocities
• Stemmed from time-resolved streak camera imaging
  of laser interactions

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Why use laser imaging?

• Can give more detailed information than other
  types of measurements
• Other laser techniques can be used in tandem for
  even better results
• laser-induced fluorescence
• resonant multiphoton ionization
• Many types of lasers can be used
• NdYAG
• Ruby

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• Dual-pulse setup shown single-pulse can also be
  constructed
• Very customizable
• Uses stable and unstable resonator optics

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Laser setup example

• Wavelength/pulse duration can be chosen ?
  ?248nm, pulse17ns
• Planoconvex lens used
• Dimensions found by translation of a solid object
• Second laser orthogonal to high-energy pulse and
  motion of particles
• Digital delay unit synchronizes and varies
  high-energy and imaging pulses

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Example images with Al

• Shows both large atomic collision clusters (left)
  and individual atoms (right)
• Images taken at 45ns intervals

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Pros/Cons

• Advantages
• More detail than single-pulse method
• Delay of pulse can be accurately found using
  optical path length and speed of light
• Easy to change imaging pulse intervals
• Disadvantages
• Laser divergence and used
• optical path length
• Pulse width causes blurring

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Math to the Rescue!

• Arrhenius expression for a rate constant
• k(T) A exp(-Ea/RT) ?can be more detailed!

• State-to-state rate constants
• kif(Er) sif(Er)vr
• sif state-to-state cross section
• i initial state
• f final state
• vr relative velocity
• Er collision energy
• Uses initial and final states, as well as
  collision energy
• Can describe chemical lasing

• Energy-dependent states
• sif(Er) ? sif(Er,b,?)
• b unhindered dist. between 2 molecules
• ? angle of approach
• sif(Er) is an average!
• Gives a better connection between a measured
  cross section and the potential energy surface in
  the motions of atoms

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Pulse Detonation Engines (PDEs)

• Studied extensively at the U.S. Office of Naval
  Research
• Similar to pulse jet engine
• Oxygen Fuel Flammable Mixture!
• Pressure created from supersonic combustion
• Ignition becomes detonation
• Valves control direction of air

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Pulse Detonation Engines

• Operates at 50 (PJEs?30)
• Fast cycling lessens vibrations
• Requires a high energy to detonate
• Velocity of gas through chamber needed to
  determine chamber length
• Current chamber lengths too long for commercial
  use
• In process of becoming optimized

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PDE with a diode laser
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PDE with a diode laser

• Diode laser imaging can measure temperature
  (2000-4000K) and pressure (0.5-30atm)
  simultaneously
• Different laser configurations possible
• Time responses range in the µs
• Many fuel types can be examined
• Used to validate theoretical values

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Different setups uses

• Fuel concentration/velocity
• Fuel consumption

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Lasers and PDEs

• Current laser diagnostics measure
• Gas temperature
• Gas velocity
• Liquid fuel concentration
• H2O concentration
• Soot volume fraction
• Future laser diagnostics will measure
• Multi-phase engine capabilities
• Efficiency of nozzle geometry

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• Gabriel Roy, comp. Cumbustion Processes in
  Propulsion. Amsterdam Elsevier, 2005. 376-388.
• Paul L. Houston, New Laser-Based and Imaging
  Methods for Studying the Dynamics of Molecular
  Collisions. J. Phys. Chem. 100, 12757-12770
  (1996)
• S.A. Schaub, et. al. Measurement of hypersonic
  velocities resulting from the laser-induced
  breakdown of aerosols using as eximer laser
  imaging system. Rev. Sci. Instrum. 60, 3688-3691
  (1989)
• S. Sanders, D. Mattison, L. Ma, J. Jeffries, and
  R. Hanson, "Wavelength-agile diode-laser sensing
  strategies for monitoring gas properties in
  optically harsh flows application in
  cesium-seeded pulse detonation," Opt. Express 10,
  505-514 (2002)