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

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


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


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


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


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


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


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


Typical PLIF ImageTypical PLIF Image

Nozzle Exit (D)= 5 mm

Calibration Cuvette

Underexpanded Jet Flowfield

z


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


Image Processing: Zero Degree Image Processing: Zero Degree

Raw Image Normalized ImageImages near NO Peak (44,097.53 cm-1)


Image Processing: 45 DegreeImage Processing: 45 Degree

Normalized Image

Images near NO Peak (44,097.53 cm-1)

LaserSheet

Raw Image


Spatially Resolved Spectra Extracted from Multiple Images

Spatially Resolved Spectra Extracted from Multiple Images


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


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


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




z/D = 0.778P = 0.47 atm

z/D = 0.995P = 0.28 atm




z/D = 1.35P = 0.12 atm

z/D = 1.50P = 1.27 atm





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


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


Axial Velocity Profile in UE JetAxial Velocity Profile in UE Jet

z/D = 0M = 1

z/D = 1.45Normal Shock


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

Documents

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