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SCHOOL OF MECHANICAL ENGINEERING
Development of a High-Spectral-Resolution PLIF Technique for
Measurement of Pressure, Temperature, and Velocity in
Hypersonic Flows
Development of a High-Spectral-Resolution PLIF Technique for
Measurement of Pressure, Temperature, and Velocity in
Hypersonic Flows
Robert P. Lucht
School of Mechanical Engineering , Purdue University, W. Lafayette, IN
Presentation at the AFOSR MURI Review
College Station, TX
October 12, 2007
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Introduction and Motivation Introduction and Motivation
• Characterization of hypersonic turbulent flows in non-thermochemical equilibrium is critical for many DoD missions, including high-speed flight
• Optical measurements of instantaneous flow and thermodynamic properties is essential for the development of reliable predictive models
• We are pursuing high-spectral-resolution PLIF imaging of NO for P, T, V imaging in high-speed flows, combined with emerging pulse-burst laser technology offers the potential for instantaneous imaging of thes properties
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Optical Parametric Laser SystemsOptical Parametric Laser Systems
• At Purdue, we have developed tunable, pulsed, injection-seeded optical parametric systems capable of producing very narrow linewidth laser radiation
• These OP systems are similar to the more expensive ring dye lasers; all-solid state, rapidly tunable systems are ideal for high-resolution spectroscopy
• Underexpanded free jet is produced using a convergent nozzle supplied with 100 ppm NO in buffer N2 at stagnation pressure of about 6 atm
• High-spectral resolution PLIF, first demonstrated in the 1980’s with ring dye lasers by Hanson and Miles groups, performed using our OP systems
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Underexpanded Jet Flowfield Underexpanded Jet Flowfield Throat
Triple PointMach Disk
(Normal Shock)
Barrel Disk(Oblique Shock)
M >>1
M > 1
M > 1
M < 1
M > 1
M > 1
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Laser System Laser System
355 nmSeeded Nd:YAG
Pro 290
Doubling Crystal
DFB
OPO Stage
OPA Stage
/2
/2
-BBO Crystals
Pol.
Pol.
T
T
226 nm
452 nm
CM CM
Joule Meter
1630 nm
DFB can be current or temperature tuned
Spectral linewidth at 452 nm ~ 200 MHz = 0.007 cm-1
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Flow and Imaging System Flow and Imaging System
M > 1 M > 1
M > > 1
M = 1
Normal Shock (Mach Disk)
5 mm
Expansion Fan
NO/N2 Flow
Solenoid Valve
Pressure Gauge
Converging Nozzle
Solenoid Control Trigger
Andor MCDImaging Camera
Camera Trigger
UV Planar Laser Sheet
226 nm
Laser Q-Switch Trigger
~0.5 mJ/pulse
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Timing DiagramTiming DiagramLaser Q-Switch
Pulses
NO/N2 Flow Open(Solenoid Control)
T
t
AND Gate Trigger
Camera Trigger
0 < t < 100 ms
T = Duration of NO/N2 Flow before Image
100 ms
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Typical PLIF ImageTypical PLIF Image
Nozzle Exit (D)= 5 mm
Calibration Cuvette
Underexpanded Jet Flowfield
z
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Image Processing: Correction Factor Image Processing: Correction Factor NO, P = 1 atm, T = 300 K
Region of Interest (ROI)
Frequency (cm-1)
44096 44097 44098 44099
Th
eo
reti
ca
l A
bs
orp
tio
n (
arb
. u
nit
s)
0.00
0.01
0.02
0.03
0.04
Av
era
ge
La
se
r In
ten
sit
y (
arb
. u
nit
s)
0
50
100
150
200
250
300
CalculatedMeasured
Correction Factor
Theoretical Absorption=
ROI Average Laser Intensity
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Image Processing: Zero Degree Image Processing: Zero Degree
Raw Image Normalized ImageImages near NO Peak (44,097.53 cm-1)
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Image Processing: 45 DegreeImage Processing: 45 Degree
Normalized Image
Images near NO Peak (44,097.53 cm-1)
LaserSheet
Raw Image
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Spatially Resolved Spectra Extracted from Multiple Images
Spatially Resolved Spectra Extracted from Multiple Images
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Analysis of PLIF SpectraAnalysis of PLIF Spectra
• The PLIF spectrum is dependent on pressure, temperature, and velocity in the underexpanded jet
2
0
1~
21
, ,
, ,
, ,
LIF B NO
aa
a
B B NO NO
a a
AS f N
A Q
f f P T N N P T
Q Q P T P T
P T V
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Analysis of PLIF SpectraAnalysis of PLIF Spectra
• Spectral line width determined primarily by the pressure for this underexpanded jet
• Temperature profile can then be determined from the relative PLIF intensities at different spatial locations, complicated in this experiment by spatial profile of the laser sheet
• Flow velocity can be measured from spectral line shift for velocities in excess of ~ 100 m/s
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Determination of Pressure from PLIF Spectra
Determination of Pressure from PLIF Spectra
Frequency (cm-1)
44096 44097 44098 44099
LIF
Sig
nal
(ar
b. u
nit
s)
0
50
100
150
200
250MeasuredCalculated (X) z/D = 0.422
= 0.4
z/D = 0.422P = 1.28 atm
z/D = 0.567P = 0.86 atm
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Determination of Pressure from PLIF Spectra
Determination of Pressure from PLIF Spectra
z/D = 0.778P = 0.47 atm
z/D = 0.995P = 0.28 atm
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Determination of Pressure from PLIF Spectra
Determination of Pressure from PLIF Spectra
z/D = 1.35P = 0.12 atm
z/D = 1.50P = 1.27 atm
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Determination of Pressure from PLIF Spectra
Determination of Pressure from PLIF Spectra
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LIF Signals Before and After the Normal Shock
LIF Signals Before and After the Normal Shock
z/D = 1.35 (Before Normal Shock)
z/D = 1.50 (After Normal Shock)
Experiment Theory
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Spectral Line Shapes Just Before Normal Shock
Spectral Line Shapes Just Before Normal Shock
Fitting Parameters
T = 100 K P = 0.13 atm
= 0.05±0.01 cm-1
V = 500 ± 100 m/s
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Axial Velocity Profile in UE JetAxial Velocity Profile in UE Jet
z/D = 0M = 1
z/D = 1.45Normal Shock
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ConclusionsConclusions
• Injection-seeded optical parametric systems are used for high-spectral-resolution PLIF imaging in supersonic underexpanded free jet
• PLIF spectra were obtained from different laser pulses, measurements were not instantaneous
• Pressure and temperature values compare favorably with previous N2 CARS measurements, measurements in underexpanded jet complicated by large dynamic range of P and T
• Measured Doppler shift gives reasonable value of axial velocity profile in the supersonic region before the normal shock, measurement accuracy ~ 100 m/s