Pulse Burst Laser system for high speed flow and combustion measurements Naibo Jiang, Matthew C. Webster, Kathryn N. Gabet, Randy L. Patton, Jeffrey A. Sutton and Walter R. Lempert Departments of Mechanical Engineering, The Ohio State University, Joseph D. Miller and Terrence R. Meyer Department of Mechanical Engineering, Iowa State University Jenifer A. Inman, Brett Bathel, Steve B. Jones, and Paul M. Danehy NASA Langley Research Center

Pulse Burst Laser Concept (Based on Lempert and Miles,et. al., AIAA , 1996) (a) CW laser is sliced into pulse-burst, repeated every 0.1s 0.1s 1  s (b) Nd:YAG gain curve (c) Result is high power "burst" of 1~99 pulses 0.1s 300  s-2 ms 0.1s

Why Operate In “Burst” Mode Two Basic Modes of Burst Mode Operation Temporally Resolve Unsteady Flow Behavior in “Long” Run Time Facilities (“Single Shot” Measurements at ~100 kHz - MHz Frame Rates) Acquisition of Images in Single Run of “Impulse” Facility (Shock Tunnels, etc).

Pulse Burst Laser System

OPO System-Frequency Tunable Output linewidth: ~300MHz; OPO Wavelength Tunable Range: 450nm-1.6µm With Frequency Doubling or Mixing, we can extend the range to deep UV. OPO is seeded at idler wavelength Nonlinear Crystals Double Pass 355 nm pump R s =R i  1 R s =R i  20% Diode Seed Laser Optical Isolators OPO Signal Output

Representative Burst Sequences 2 µs spacing - 20 pulses per burst 1064nm (~100mJ/pulse) 355nm (~35mJ/pulse) 622nm (~4mJ/pulse) 226nm (~0.2mJ/pulse)

Representative 1 MHz Burst (1 µs spacing) 355nm (~35mJ/Pulse) (a) (b) (c) (d) 1064nm (~100mJ/pulse) 622nm (~4mJ/pulse) 226nm (~0.2mJ/pulse)

8 NASA Langley 31” Mach 10 Facility Heater Bundle Laser Sheet Forming Optics Mach 10 Nozzle

9 Flat Plate Model With Cylindrical Trip Dotcard on model Sharp leading edge Laminar flow NO seeded from 11- mm wide spanwise slot Cylinder Trip (and shadow) Tripped, transitional flow Camera FOV Length Scale (1/4” increments) Sting

1MHz NO PLIF Imaging – Some Sequences 2 mm sphere trip mm above the surface

2 mm tall x 4 mm wide cyl trip - Reynolds Number Comparison ~ 1 mm above surface 2.4 MPa, Re = 1.5 million/m 500 kHz 9.2 MPa, Re = 5.6 million/m 500 kHz 4.9 MPa, Re = 3 million/m 1 MHz 3.4 MPa, Re =2.1 million/m 500 kHz

NO 2 Molecular Tagging Velocimetry in Mach 5 N 2 Flow NO 2 Injection Mach 5 Nozzle 226 nm laser sheet Cylinder Image 355 nm tagging line

500 KHz NO 2 MTV in Mach 5 Average flow speed is ~700m/s.

CH PLIF imaging in turbulent flames Pulse Burst Laser ECDL Air co-flow OPO 1064nm 838nm 615 nm 390 nm Fuel BBO Optical Isolators

CH PLIF (1)(2) (3)(4) (5) (6) (7) (8) (9) (10) 22.1% CH4 /33.2% H2 / 44.7% N2, 8 mm tube, 42.2 m/s for DLR A Flame Re = 15200, X/D=22

Rayleigh Scattering 10-kHz image sequence of instantaneous temperature fields in turbulent CH4/H2/N2 flame at Re = (DLR Flame B). Images are centered at x/d =25. Black = Low Temperature; White = High Temperature. Cross section is constant across the flame. Cold JetHot Jet

Summary and Conclusions Developed NO PLIF System with excitation at 226 nm, ~10 pulses / 1 MHz, kHz. Developed Quantitative NO2 Flow Tagging Velocimetry. 10 KHz Rayleigh Scattering in turbulent flames with ~200mJ/pulse, ~10 pulses / burst at 532 nm. 10 KHz CH PLIF imaging in turbulent combustions with excitation at 390 nm. Future Work Extend Temporal Envelope to ~10-20 msec. Develop Quantitative “Two Line” Measurement Capabilities (Temperature, Mixing, and Density Fields)

Acknowledgements Support NASA – Langley Research Center - Research Opportunities in Aeronautics (ROA) Grant NNX07AC34A. NASA – Langley Phase I SBIR Grant Air Force Research Laboratory-Propulsion Directorate Air Force Office of Scientific Research – Program in Unsteady Aerodynamics and Hypersonics Princeton Scientific Instruments, Inc. Greatful thanks to Dr. James Gord - Air Force Research Laboratory – Propulsion Directorate for many helpful discussions!