Robert P. Lucht School of Mechanical Engineering , Purdue University, W. Lafayette, IN

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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