THREE-DIMENSIONAL OPTICAL MEASUREMENTS IN AN ETHYLENE

FUELLED MODEL SCRAMJET ENGINE

A Thesis

Submitted to the Graduate School

of the University of Notre Dame

in Partial Fulfillment of the Requirements

for the Degree of

Master of Science

in

Aerospace Engineering

by

Bhargava K. Narayana

Hyungrok Do, Director

Graduate Program in Aerospace and Mechanical Engineering

Notre Dame, Indiana

December 2014

c© Copyright by

Bhargava Kumar Narayana

2014

All Rights Reserved

THREE-DIMENSIONAL OPTICAL MEASUREMENTS IN AN ETHYLENE

FUELLED MODEL SCRAMJET ENGINE

Abstract

by

Bhargava K. Narayana

This work documents the development of non-intrusive optical diagnostic methods

towards a qualitative study of ethylene flame dynamics in a laboratory scale model

scramjet engine. Planar laser Rayleigh scattering (PLRS) and OH based planar laser

induced fluorescence (PLIF) have been successfully developed and applied.

Prior to understanding the turbulent flame dynamics due to ethylene combustion

in the model scramjet, it is necessary to reveal the role played by turbulent struc-

tures in a combustion free environment. Also, shock/ turbulent boundary layers are

known to significantly impact unstart dynamics. Hence, PLRS has been chosen to

be employed considering its relevancy to the present experimental subject.

Visualizing flame structures in a transient combustion system is a key to estab-

lishing stable operational regimes. Imaging ground state OH is a proven, simple and

cost effective method amongst the LIF based techniques. In addition, these laser

based techniques are instantaneous in nature with temporal resolution as high as

10ns.

Flow physics in the scramjet model is complicated due to the interaction of tur-

bulence and flame structures. High intensities of turbulence are expected at such

high Reynolds number flows involving combustion. The high strain rates imposed

by turbulent structures might, in fact, contribute to flame extinguishment. In view

Bhargava K. Narayana

of turbulence being a 3-dimensional phenomena, there exists a need to visualize the

flow profile in a 3-dimensional domain. However, a truly 3-dimensional study is be-

yond the scope of current research methods. A closer and more accessible alternative

would be to apply 2-dimensional flow imaging techniques spanning over multiple

planes, provided that the flow exhibits a quasi-stable behavior. Although optical

investigations in the combustor regions have been reported, this study, to the best

of the author’s knowledge, is the first one to cater to the flow field investigation over

a significant region beyond the combustor/cavity in supersonic flows. Furthermore,

this study encompasses multiple planes to achieve a holistic reconstruction of the flow

physics.

A unique optical arrangement to aid such a visualization has been developed.

The results obtained provide supportive evidence underlining the applicability of

these laser based techniques to the present combustion system.

CONTENTS

FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

CHAPTER 1: INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.1.1 Laser-induced Fluorescence . . . . . . . . . . . . . . 41.2.1.2 Linear Regime . . . . . . . . . . . . . . . . . . . . . 41.2.1.3 Saturated Regime . . . . . . . . . . . . . . . . . . . 41.2.1.4 OH PLIF . . . . . . . . . . . . . . . . . . . . . . . . 61.2.1.5 Disadvantages . . . . . . . . . . . . . . . . . . . . . . 7

1.2.2 Chemiluminescence . . . . . . . . . . . . . . . . . . . . . . . . 71.2.3 Rayleigh Scattering . . . . . . . . . . . . . . . . . . . . . . . . 8

CHAPTER 2: EXPERIMENTAL SETUP . . . . . . . . . . . . . . . . . . . . 102.1 Hypersonic Wind Tunnel and Associated Instrumentation . . . . . . . 102.2 Optical Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Timing Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.4 Wavelength Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.4.1 Theoretical Spectral Database . . . . . . . . . . . . . . . . . . 182.5 Transient Combustion System . . . . . . . . . . . . . . . . . . . . . . 192.6 Condensed Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

CHAPTER 3: RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.1 PLRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2 Chemiluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.3 OH PLIF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

ii

CHAPTER 4: CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . 344.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.2 Recommendations for Future Work . . . . . . . . . . . . . . . . . . . 35

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

iii

FIGURES

1.1 Basic physical processes affecting the fluorescence rate depicted in atwo level system. Rate constants; b12 - stimulated absorption ; b21

-emission rate constants; A21 - spontaneous emission rate constant ;Q21 quenching rate constant; W2i photoionization rate constant; P -predissociation rate constant. Adopted from [16] . . . . . . . . . . . . 3

1.2 Depiction of LIF signal dependence on laser excitation energy. Signalresponse is linear for low pulse energies. Signal response is highest forsaturated regime and doesn’t increase with increasing laser energy [16] 5

1.3 OH (left) and CH2O (right) LIF signals from a co-axial burner fromLi[13]. Note the post flame existence of OH radicals denoting theregion of burned gases. Also notable is the prevalence of OH signatureover that of CH2O, denoting unburned gases. . . . . . . . . . . . . . . 6

1.4 Schlieren images of HyShotII combustor: (top) instantaneous; (mid-dle) averaged over test time by Laurence [12] and (bottom) Rayleighscattering images from the present scramjet model . . . . . . . . . . . 9

2.1 Schematic of the OH PLIF and PLRS measurements . . . . . . . . . 102.2 Sheet generation optics . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Sheet generation optics for OH PLIF and PLRS measurements . . . . 132.4 Schematic of conventional laser beam expansion optics . . . . . . . . 142.5 View of the stainless steel optical enclosure . . . . . . . . . . . . . . 152.6 View of the stainless steel optical enclosure: (left) without and (right)

with streamlined deflector hood . . . . . . . . . . . . . . . . . . . . . 162.7 Timing diagram of the simultaneous operation of OH PLIF, fuel in-

jection valve and the ICCD camera . . . . . . . . . . . . . . . . . . . 172.8 Sample wavelength scan in the range 282-284 nm using the sirah dye

laser by Jalbert [10] . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.9 Emission spectra generated by LIFBASE. Transition lines of interest

are marked. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.10 Schematic (not to scale) of the scramjet model used in the experiments

depicting the fuel injection port and cavity combustor. . . . . . . . . 20

3.1 Detailed flow features of Rayleigh scattering images in the scramjetcentral plane: (top) with and (bottom) without active fuel jet opera-tion. Free stream flow is at Mach = 4.5 and from left to right. . . . . 23

iv

3.2 Set of detailed Rayleigh scattering images arranged based on theirproximity to the central plane (x=0) of the model. Fuel jet injectionwith N2 is enabled. Free stream flow is at Mach = 4.5 and from leftto right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.3 Set of detailed flow features of Rayleigh scattering images in the scram-jet central plane: (top) with and (bottom) without active fuel jet op-eration (N2). Free stream flow is at Mach = 4.5 and from left to right. 25

3.4 Detailed chemiluminescence image of the combustion process. Brighter(blue) regions are indicative of intense heat release reactions. Freestream flow direction is from left to right. . . . . . . . . . . . . . . . . 26

3.5 Sequence of PLIF images taken ∆T =100 ms apart from each other(numbered), at one of the scramjet investigation planes. Free streamflow direction is from left to right. Fuel jet was active for 300 ms. . . 27

3.6 Detailed set of images comparing chemiluminescence (middle) andPLIF measurements planes at x = 0 mm (top) and x = 17 mm (bot-tom) in the model scramjet. Overall equivalence ratio (φ)= 0.83, M =4.5, P0 =100 kPa, T0 = 2600. Images were acquired during the quasi-stable state of the combustion process. Free stream flow direction isfrom left to right. The brighter (fluorescing) contours are indicative ofhigher OH concentrations. . . . . . . . . . . . . . . . . . . . . . . . . 28

3.7 A series of spatially varying OH distribution images obtained usingPLIF in the model scramjet. The bottommost image is at the scramjetcenter plane and top image is closest to the side wall. All the imageswere obtained at least 100 ms after fuel injection and can be consideredto be in stable mode of the quasi-steady combustion process. Freestream flow direction is from left to right. The brighter (fluorescing)contours are indicative of stronger OH concentrations. . . . . . . . . . 30

3.8 A depiction of flame residence (in quasi-stable mode) on the bottomwall of the scramjet model for conditions - φ)= 0.97, M = 4.5, P0 =100kPa, T0 = 2600 . Free stream flow direction is from left to right. . . . 31

3.9 Sequence of PLIF images with varying overall equivalence ratios ob-tained at a planar section 2 mm from the center of the model. All theimages were obtained 100 ms after fuel injection and can be consideredto be in quasi-stable mode. Free stream flow direction is from left toright. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

v

SYMBOLS

d Diameter of a lens

f Focal length of a lens

ICCD Intensified charge-coupled device

O2 Mass flow rate of O2

C2H4 Mass flow rate of C2H4

φ Equivalence Ratio

FWHM Full width at half-maximum

χOH OH mole fraction

T Temperature

T 0 Total temperature

P 0 Total pressure

λ Wavelength

Spp Fluorescence signal per pixel

fB(T) Temperature-dependent Boltzmann fraction of the absorbing state

vi

ACKNOWLEDGMENTS

I am deeply grateful to my advisor, Dr. Hyungrok Do, to have given me this

opportunity to partake in an enriching research experience. I would like to thank

my committee members, Dr. Flint Thomas and Dr. Scott Morris for their efforts in

going through the thesis and being part of the defense.

I would like to extend my thanks to my friends at Notre Dame without whom

this experience wouldn’t have been possible. Further, Qili Liu, Brian Neiswander and

Joanna have been extremely helpful with their advice and help. Stephen Hammack

and Constandinos have been very helpful in setting up the laser system and trans-

ferring operational skills to our team. The journey wouldn’t have been incredible

without the loving warmth of Eugene Heyse, Michael Sanders, Terry Jacobsen and

their amazing machining skills. I dedicate this work to my mother, father and sister

for supporting and motivating me with their annoying midnight phone calls from the

other side of the planet. This work would never have been possible for the love and

unrelenting support of my girlfriend, Jasmine.

vii

CHAPTER 1

INTRODUCTION

1.1 Objectives

With the development of laser based non-intrusive diagnostic techniques in re-

cent years, there has been renewed interest in combustion processes occurring in

hypersonic vehicles.

Some groups have exploited the more economical methods like schlieren and

chemiluminescene [12] for the study of flow inside scramjet models. However, sig-

nal quality of schlerien experiments is diminished in the absence of stark contrast in

the refractive index of the medium under observation. Furthermore, schlieren photg-

raphy is a line-of-sight technique. Rayleigh scattering technique offers a much better

alternative. Its attractiveness lies in the fact that it does not require doping with

particles or tracers [9].

Although non-intrusive laser based diagnostics were thought to be the best way

to retrieve flow parameters in hypersonic flows as far back as 1990[3], the techniques

weren’t economical enough until recently with the advancements in the development

of reliable and compact UV lasers. Previous studies in the facility by Do [6][7] and Liu

[14] have focused on inlet unstart in a model scramjet engine phenomenon utilizing

chemi-luminescence (for visualization part). Rayleigh scattering has been extensively

applied by Do [5] for study of inlet unstart phenomenon in supersonic flows. Work

presented in this thesis delves into the development and application of more advanced

flow visualization techniques (PLRS and PLIF) in the facility.

1

Planar laser-induced fluorescence of OH and CH radicals are commonly used for

experimental investigation of turbulent flame structures. Burned gas has a high

concentration of OH radicals and OH PLIF signals that can be used to separate

burned gas from unburned constituents. One must be careful in the interpretation of

the results of OH radical distribution though. In the case of low Reynolds number

turbulent flames, OH concentrations may correlate to the flame fronts. But for high

Reynolds number cases, such as the flow in a scramjet, this is less likely as the flame

front is heavily distorted and folded.

Nevertheless, considering the economical incentive, higher concentration and con-

sequently, easier detection over CH PLIF, it was decided to qualitatively analyze the

ethylene flame dynamics using OH PLIF.

1.2 Background

1.2.1 Fluorescence

When atoms or molecules spontaneously relax to a lower energy level, (typically)

due to vibrational and rotational energy transfer in the upper state and are accom-

panied by the emission of radiation, it is termed fluorescence. Fluorescence does not

possess directionality. A spectrally resolved fluorescence signal might contain more

than one wavelength even though excited at only one transition from a lower state

[8], [16].

A simplified energy structure with two energy levels is illustrated in figure 1.1.

The amount of fluorescence signal is affected by various collisional and optical pro-

cesses. A prerequisite for spontaneous emission to occur is that the molecule must be

in an excited state. This can be achieved through absorption of photons, following

which, the molecule might relax into a lower energy state through spontaneous emis-

sion/fluorescence. An alternate process is stimulated emission, wherein the excited

2

molecule is stimulated to emit a photon with the same energy, phase, polarization

and direction as the incoming one and settles into a lower state.

In addition, the molecule might leave the excited state without emitting fluores-

cence in the event of collisions with surrounding molecules. This process is colli-

sional quenching and its rate is higher for species at room temperature and pressure.

Photo-ionization and predissociation also contribute to increase in loss of sponta-

neous emission. Photo-ionization occurs when a molecule is ionized by a photon with

a large enough energy. Predissociation occurs when a molecule relaxes to an unbound

(dissociative) state from a bound state causing dissociation.

Figure 1.1. Basic physical processes affecting the fluorescence rate depictedin a two level system. Rate constants; b12 - stimulated absorption ; b21

-emission rate constants; A21 - spontaneous emission rate constant ; Q21

quenching rate constant; W2i photoionization rate constant; P -predissociation rate constant. Adopted from [16]

3

1.2.1.1 Laser-induced Fluorescence

Fluorescence can be conveniently achieved using lasers with the added features

of spatially, temporally and spectrally selective excitation. Owing to its simplicity

of operation, LIF has become one of the most widely used diagnostic techniques for

combustion studies in recent years. Also, it is well suited for pulsed flow facilities

when compared to probe based methods like hot wire anemometry. Additionally, the

fluorescence is usually at a longer wavelength than the laser radiation. This helps to

easily filter away the stray background radiation at the shorter wavelengths.

Application of LIF is limited to atoms or molecules which have bound states

accessible with laser radiation. Knowledge of emission spectrum of the atom or

molecule and rate of radiative decay of its excited state is a pre-requisite to LIF

measurements. For quantitative studies, losses in the form of non-radiative processes,

such as collisional quenching and predissociation, should also be accounted for.

1.2.1.2 Linear Regime

For low laser intensities, the fluorescence signal obtained is proportional to the

laser radiation. This is called the linear regime. Here, quenching rate (Q21) and spon-

taneous emission rate constants (A21) define fluorescence. Therefore, the quenching

rate must be estimated prior to quantitative concentation measurements. The fluo-

rescence signal is relatively weak in the linear regime compared to saturated regime.

1.2.1.3 Saturated Regime

It is the aim of any LIF experiment to achieve full spatial and temporal saturation.

At sufficiently high laser energies, the fluorescence signal becomes independent of

the laser intensity and of quenching. The energy transfers in the upper state are

dominated by the absorption and stimulated emission rates. This is described as the

saturated regime. Quenching can be disregarded in this regime. The fluorescence

4

signal is maximized, leading to maximized detection levels. Usually, the intensity in

the wings of the laser sheet are always low given the gaussian nature of the pumped

laser beam. Hence, full spatial resolution is never achieved. An optical setup to

achieve full spatial resolution is described later in this text. Also, because the laser

energy varies during the duration of a pulse, temporal saturation is very difficult to

achieve [8]. The dependence of fluorescence signal on laser energy is illustrated in the

schematic figure 1.4 with the linear and saturated regimes identified.

Figure 1.2. Depiction of LIF signal dependence on laser excitation energy.Signal response is linear for low pulse energies. Signal response is highestfor saturated regime and doesn’t increase with increasing laser energy [16]

5

1.2.1.4 OH PLIF

The fluorescence signal measured by the intensified CCD camera is proportional

to the OH mole fraction, found in the region of interest, and a temperature dependent

function. Experimental efficiencies for the current setup, such as the electronic gain of

the camera and transmission efficiency of the collection optics are assumed constant.

Figure 1.3. OH (left) and CH2O (right) LIF signals from a co-axial burnerfrom Li[13]. Note the post flame existence of OH radicals denoting the

region of burned gases. Also notable is the prevalence of OH signature overthat of CH2O, denoting unburned gases.

OH mole fraction depends on numerous factors including pressure, strain, local

equivalence ratio, exhaust gas recirculation and fuel. Considering its highly non-

specifc nature, caution must be exercised in the interpretation of results. Because

detection of OH is easier compared to other radicals it is usually chosen to characterize

6

the combustion activity.

Spp = const · χOH ·fB(T )

T

1.2.1.5 Disadvantages

One of the main disadvantages identified with PLIF is the quenching of fluores-

cence at higher pressures due to increased collisions of molecules. The key to avoid

quenching is to achieve short predissociation lifetimes, provided the fluorescence is

emitted only during predissociation lifetime. This is based on the fact that for suf-

ficiently short predissociation lifetimes, molecular collisions are eliminated. For the

combustion reactions in the scramjet model, which can be treated as a semi-enclosed

system, the temperature and pressure increases are strongly correlated. Therefore,

quenching ceases to be a problem during PLIF measurements.

1.2.2 Chemiluminescence

A brief description of chemiluminescence is provided as an overview of the pre-

existing flow visualization capabilities in the facility. Chemiluminescence is based on

the chemical excitation of species as opposed to excitation due to laser radiation. For

example, radiation emitted by chemically excited OH, denoted by OH∗, is captured

by the camera. The instrumentation required to perform chemiluminescence is eco-

nomical and therefore continues to still see regular application [11]. Band pass filters

may be used to observe defining spectral line of chemical species. For example, the

maximum spectral line for OH∗, CH∗ and C∗2 are known to occur at 308 nm, 431

nm nd 513 nm respectively. However, CO∗2 has emission over a broad spectral range.

Chemiluminescence, being a line-of-sight visualization, complicates the interpretation

of acquired images. Also, LIF is known to provide much more detailed information

due to its greater spatial and temporal resolution. Chemiluminescence, however, has

7

still been employed in this study because OH∗/CH∗ are known to unambiguously

characterize the overall equivalence ratio of laminar and turbulent flames.

1.2.3 Rayleigh Scattering

Rayleigh scattering imaging captures light emitted from particles illuminated by

the laser sheet, causing it to be more reliable than line-of-sight and path integrated

optical methods like schlieren photography. Therefore, it is reasonable alternative for

qualitative characterization of the shock and turbulence structures.

Filtered Rayleigh scattering and condensate enhanced Rayleigh scattering are

the variants considered for application in this study. Because condensate enhanced

Rayleigh scattering signals have the potential to be much stronger than molecular

Rayleigh scattering, it was chosen to be employed in this study. Condensate enhanced

Rayleigh scattering utilizing condensed CO2 particles was used for imaging the flow.

The size distribution of carbon dioxide clusters is shown to follow a very narrow trend

with a mean diameter of 6-10 nm [9]. As long as the molecular clusters satisfy the

Rayleigh criterion (diameter less than 1/10 the wavelength of incident light), Rayleigh

scattering is viable. Condensation of residual water vapour and CO2 are known to

provide a favorable medium in high speed test facilities, satisfying this criteria. Also,

the small size of clusters allows for faithful and rapid response to flow changes.

Rayleigh scattering is highly dependent on the thermal response of the particles

to the flow. The CO2 particles are prone to sublimation in regions with increased

local flow temperatures. Such regions are predominantly ones containing features like

shocks and boundary layers. Sublimation can lead to reduction or even elimination of

scattering signals. Boundary layers present a high temperature condition favorable for

sublimation and are accordingly marked by the mismatch in scattering signals from

the clusters present in cold core flow and their absence in the boundary layer. Also,

the presence of shocks in the flow causes clusters to sublimate due to strong changes

8

in flow temperature and consequently, eliminate the scattering signals downstream.

Figure 1.4 shows a set of schlieren images compared to Rayleigh scattering results

obtained in the facility. Rayleigh scattering is very effective in capturing the boundary

layers, but not as effective with shock propagation. However, because the excitation

and scattering occur at the same wavelength, imaging in near wall regions might be

an issue at higher wavelengths (∼ 532 nm).

Supercooling rates affect the equilibrium of condensation produced by nozzles.

Supercooling rates are lower in long, large, slow expanding nozzles compared to

their short, high expansion counterparts [9]. A process closer to equilibrium can be

expected for lower levels of supercooling.

Figure 1.4. Schlieren images of HyShotII combustor: (top) instantaneous;(middle) averaged over test time by Laurence [12] and (bottom) Rayleigh

scattering images from the present scramjet model

9

CHAPTER 2

EXPERIMENTAL SETUP

2.1 Hypersonic Wind Tunnel and Associated Instrumentation

Experiments described here were performed in the hypersonic wind tunnel facility

at University of Notre Dame. The wind tunnel is a pulsed-arc-heated facility. For

a more detailed description of the facility see [14]. A schematic of the experimental

setup is shown in figure 2.1.

Figure 2.1. Schematic of the OH PLIF and PLRS measurements

10

The OH PLIF laser system constitutes a Nd:YAG laser (Spectra-Physics, Quanta

Ray PRO, 532 nm, 450 mJ/pulse) and a dye laser (Sirah Precision Scan). Rhodamine

dye in ethanol solvent was used in the frequency doubled dye laser, which is pumped

by the Nd:YAG laser and emits 283.22 nm light corresponding to the Q1(7) line within

the OH A2Σ+ ← X2Π(1 − 0) transition band. The dye laser energy was varied in

the range 13.5 - 20 mJ/pulse and it was deduced that the laser intensities used in

the present study were in the saturated regime, as no marked increase was detected

in the fluorescence signal levels. Hence, it was decided to conduct experiments with

the dye laser power at 13.5 mJ/pulse.

Time-sequential chemiluminescence images were obtained using a high-speed movie

camera (Casio, Exilim Pro EX-F1) at 60 fps through a quartz optical access. A CCD

camera (LaVision Imager Intense) coupled with an intensifier (LaVision IRO) was

mounted near the other optical access window to capture the PLIF and PLRS sig-

nals. Fluorescence signals were focused onto the intensifier. The intensifier gain was

set at 7 for all experiments. A f/2.8 UV lens (Sodern Cerco) fitted with a band-

pass filter 306-320 nm (Asahi Spectra) was mounted on the intensifier. The acquired

fluorescence signals by the camera were digitized to 12 bits (equivalent to 4095 gray

levels). Windows on either side of the test section allowed optical access to the flow

conditions. DaV is 7.2 software was used for recording images acquired by the camera

and for controlling image acquisition. Image processing was done using Matlab.

Free stream flow of Mach 4.5 was generated with an axisymmetric converging

diverging nozzle 60 mm in diameter. The scramjet model has a flow channel cross-

section of 15mm × 40 mm (height × width). Hence, the scramjet can be safely

assumed to be in the core flow region of the nozzle. The maximum test time of the

facility was 1 s. Total pressure was fixed at 100 kPa for the tests. Fuel concentration,

controlled with fuel jet injection pressure, in the scramjet was varied through a wide

range (φ = 0.2 − 5.5). Free stream conditions of total pressure and temperature for

11

all the tests were kept constant.

2.2 Optical Setup

A collimated laser sheet of measured minimum thickness of 1 mm was gener-

ated using a unique setup. The sheet generation optics included a cylindrical plano-

concave lens of focal length f = −30mm, and cylindrical plano-convex lenses of focal

length f = 100mm and f = 700mm. An arrangement of the optics is shown in

figure 2.2. This setup was mounted on a traversable bread board operated on by

a computerized stepper motor drive. The design was such that the laser beam of

height around 20 mm was generated downstream of the scramjet model and directed

upstream towards the nozzle.

Figure 2.2. Sheet generation optics

12

Figure 2.3. Sheet generation optics for OH PLIF and PLRS measurements

The optics are enclosed in a stainless steel enclosure with a slit wide enough to

allow the ejection of the laser sheet. The enclosure is shown in the figures 2.3− 2.4.

The enclosure has a triangular protrusion for streamlining the flow around it. Owing

to space constraints in the enclosure, a Gallilean configuration was chosen for the

pair of f = 100mm plano-convex and f = −30mm plano-concave lenses. Care was

taken to acquire optics made from fused silica for the experiments. The enclosure also

features a slit to receive the laser beam from the dye laser. A circular UV fused silica

window was mounted behind the ejection slit of the enclosure to prevent any dust

accumulation on the optics within and also to prevent any burnt gases emanating

from the scramjet adversely affecting the sheet generation optics. This window was

replaced from time to time. This optical setup was used for both PLRS and PLIF

measurements in the facility.

13

Figure 2.4. Schematic of conventional laser beam expansion optics

The intensity of the laser beam emanating from the dye laser assumes a circular

Gaussian distribution. When this circular laser beam is molded into a sheet, the

Gaussian distribution is preserved and maintained, causing the wings of the sheet to

not possess the intensities required to excite fluorescence. Prior to the setup described

above, the laser sheet was generated using conventional means as shown in figure 2.5.

This setup required large divergence of the laser sheet, causing the sheet generation

optics to be spaced farther from the test subject, and therefore did not seem practical

for achieving full spatial saturation.

14

Figure 2.5. View of the stainless steel optical enclosure

With the current design, the sheet spanning across the scramjet is given access to

the intensities sufficient to excite OH radicals and elicit fluorescence and consequently,

achieve full spatial saturation. The only downside to the laser sheet generated in this

fashion is the depletion of laser intensity due to interaction with flow particles and

subsequent obstruction as the laser sheet traverses upstream. However, this could be

easily overlooked as the study was qualitative in nature.

15

Figure 2.6. View of the stainless steel optical enclosure: (left) without and(right) with streamlined deflector hood

2.3 Timing Circuit

Laser firing was synchronized with the ICCD camera exposure as illustrated in

figure 2.6. The laser Q-switches at a frequency of 10 Hz. A combination of relays

(built by Qili Liu as part of his dissertation) helped delay the trigger controlling the

tunnel injection valves to coincide with the Q-switching signal. The tunnel signal then

triggered a signal to the fuel jet injection valve, which could be altered temporally

as desired. For the current set of experiments, the fuel valve was opened at 100 ms

after the free stream flow was triggered. The fuel injection signal was also relayed

to the Programmable Timing Unit (PTU), triggering the ICCD camera which was

16

gated to 100 ns. The jet injection was controlled by a solenoid valve triggered by the

fuel injection signal.

Figure 2.7. Timing diagram of the simultaneous operation of OH PLIF,fuel injection valve and the ICCD camera

2.4 Wavelength Selection

Wavelength selection is key to PLIF experimentation. A wavelength pertaining to

Q1(7) (283.222 nm) transition of the OH spectra was selected for excitation because

it is strong and relatively temperature insensitive. Although some transitions might

appear to be much stronger in intensity during a peak finding scan, such as the peak

Q1(6) as seen in figure 2.7, the intensity is bound to vary relative to other peaks

due to pressure and temperature of the environment the OH radicals fluoresce in.

Therefore, peak finding scans must be run only to tune the dye laser wavelength to

17

a desired value.

Figure 2.8. Sample wavelength scan in the range 282-284 nm using thesirah dye laser by Jalbert [10]

2.4.1 Theoretical Spectral Database

Figure 2.8 shows the variation of emission spectra of OH excitation LIF at a

temperature of 2600 K in a thermalized system between 282.8 - 283.0 nm generated

in the software LIFBASE. Note the similar strengths of Q1(7) and Q1(6) transition

lines. For more information on how LIFBASE simulates LIF spectra see [15].

18

Figure 2.9. Emission spectra generated by LIFBASE. Transition lines ofinterest are marked.

2.5 Transient Combustion System

A cross-sectional view of the scramjet is provided in the figure 2.9. The scramjet

model was made of stainless steel and had constant internal flow channel dimensions

of 15 × 40 mm (height × width) stretching to a length of 600 mm. Sharp leading

edges are provided on the inlet lips of the model. The inner side of the upper inlet

lip had a 12 deg wedge to produce an incident shock into the scramjet for flow

deceleration. The fuel jet was injected obliquely 100 mm downstream of the inlet

lip at a 60 deg inclination from the centerline of the bottom wall of the model. A

solenoid valve, attached to a fuel reservoir, controlled the fuel jet injection. The

stagnation temperature of the flows was around 2600 K for the PLIF tests, which

is sufficient to auto ignite the partially premixed flames downstream of the fuel jet.

The bottom wall had a wall cavity located 100 mm downstream of the fuel jet nozzle

19

with dimensions of 3 mm in depth and 12 mm in length. The flame front position

in the model was assumed to be behaving in a quasi-stable mode of this transient

process.

Figure 2.10. Schematic (not to scale) of the scramjet model used in theexperiments depicting the fuel injection port and cavity combustor.

2.6 Condensed Manual

One of the purposes of this thesis is to also provide tips in areas of laser operation

and tuning to minimize the time spent on producing the efficient fluorescence signal.

Tips on laser operation and maintenance were adopted from literature review and

conversations with graduate students from UIUC and LaVision experts. A good part

of the combustion community uses the ND:YAG pump laser with the Sirah dye laser.

The author sincerely hopes the information provided here will be beneficial for any

beginner trying to use these systems.

Peak finding scans were run everyday prior to the experiments. As the tempera-

ture and humidity levels change during the day, multiple peak finding scans were ran

to keep track of the desired wavelength. Ideally, the peak finding scans need to be

performed using a Bunsen burner for credibility, as the fuel/oxidizer flow rate is con-

stant. However, reliable results could also be obtained using commercial-off-the-shelf

handheld butane burners. A notorious problem associated with such burners is the

20

loss of fuel flow rate/pressure within a few minutes of operation, rendering the results

of the scans inconclusive beyond the time frame of a few minutes. One such burner

was used for peak finding. Normally, the peak finding scans would take at least 15-20

minutes to cover the entire spectrum around the desired transition wavelength. One

way to overcome this problem would be to perform scans over small segments of the

spectrum and compare the peaks. It was found during our initial scans that the

peaks might have comparable strengths. As mentioned earlier, the pre-determined

peak must be adhered to. Once the peak finding scan has been performed, the energy

of the dye laser should be optimized near the desired peak.

DaV is 7.2 software and Sirah control software were used simultaneously for peak

finding scans. DaV is allows for real time integral counting of fluorescence signals and

hence was used in plotting the peaks. Usually a course step size was chosen and once

vicinity of peaks were identified, step sizes were tuned down to a finer scale.

The quartz windows on the scramjet model are prone to soot accumulation over

a few runs. This was found to cause considerable loss in the fluorescence signals. A

frequently used and well established way to remove the soot is to clean the quartz

surface with a cloth dipped in dilute hydrocloric acid. Glass windows are also known

to attenuate fluorescence signals. Installing glass windows either on the scramjet

model or wind tunnel test section should be avoided during LIF tests.

Overtime, there might be a decrease in the pump laser power output due to

buildup of condensation inside the flashlamp assemblies, rendering their reflective

surfaces cloudy. This affects their efficiency to reflect photons, and in turn, their

ability to generate a strong beam, ultimately affecting the fluorescence signal. Wiping

down each flashlamp assembly will help increase the dye laser output [10].

Because self-luminosity of the reacting flow in the scramjet can be overwhelming,

spatial filtering methods and electronic shuttering of the intensified detector might

be necessary for success of PLIF.

21

CHAPTER 3

RESULTS

3.1 PLRS

Rayleigh scattering was performed under two cases of flow conditions to ascertain

the extent of fast fuel jet mixing in the turbulent flow structures. The first case was

run without fuel jet to determine the flow structures. The second case was repeated

with an operational fuel jet under the same free stream conditions. Nitrogen was used

to simulate the effect of ethylene as both of the molecules possess similar molecular

weights. Longitudinal scans were run along the width of the scramjet cross-section

ranging from the central plane to the side walls in increments of 1 millimeter. Rayleigh

scattering at two wavelengths, 532 nm and 283 nm, were attempted. Typically,

532 nm is selected for Rayleigh scattering applications. However, higher signal-to-

noise ratios are achieved with UV light due to a larger Rayleigh cross-section of the

clusters, as well as, subdued reflectivity of metallic surfaces at UV wavelengths. It

was later realized that the lower pulse energies (∼ 20 mJ/pulse) at 283 nm were

not sufficient enough to elicit a strong Rayleigh scattering response. Hence, only the

results pertaining to experiments conducted at 532 nm light would be discussed in

this section.

Figure 3.1 is an illustration of the basic flow features generated in the model

scramjet. The cavity combustor and fuel jet injection port have been depicted in the

images for reference.

22

Figure 3.1. Detailed flow features of Rayleigh scattering images in thescramjet central plane: (top) with and (bottom) without active fuel jet

operation. Free stream flow is at Mach = 4.5 and from left to right.

The presence of a shock train was visualized near the inlet. The shock train

decelerates the flow to supersonic speeds, and the cavity combustor aids in holding

the flame. Additionally, a possible boundary layer transition due to shock/boundary

layer interaction was seen near the inlet lip due to the reflected shock. The successive

reflected shocks lost their strength, as perceived in the region prior to the fuel jet

injector. An expansion fan is created at the end of the cavity. Also, in the case of

an injected fuel jet, it can be seen that the turbulence characteristics downstream

cannot be finely defined. This is due to the loss of fluorescence signals from a strong

sublimation of CO2 clusters at the fuel jet, owing to the bow shock created by it.

23

Figure 3.2. Set of detailed Rayleigh scattering images arranged based ontheir proximity to the central plane (x=0) of the model. Fuel jet injection

with N2 is enabled. Free stream flow is at Mach = 4.5 and from left to right.

Figure 3.2 shows a collection of Rayleigh scattering images taken at different pla-

nar locations in the scramjet model. Prominent amongst the features is the tripping

of boundary layers (marked in red) by the fuel jet, even at locations as far as the

side wall of the model. The supersonic fuel jet injection was believed to cause an

increase in downstream pressure and temperature, which could have the potential to

trigger unstart. However, since the conditions selected for the experiments are not

conducive for unstart, no unstart was observed. As expected, the boundary layer at

the side wall is much thicker than at the central plane. Note the stark difference in

the flow structure between images at x=0 mm and x=18,19 m, as a result of the side

wall on the boundary layer development. The turbulent boundary layers developing

on the top and bottom walls seemed to merge beyond the cavity.

24

Figure 3.3. Set of detailed flow features of Rayleigh scattering images inthe scramjet central plane: (top) with and (bottom) without active fuel jetoperation (N2). Free stream flow is at Mach = 4.5 and from left to right.

A series of Rayleigh scattering images captured in different planes of the scramjet

model is shown in Figure 3.3. Each set has an image taken with and without the

operation of the jet injection. Previously discussed rapid growth of the top wall

boundary layer triggered by the fuel jet injection, can be confirmed from the images.

A gradual decrease in the presence of the reflected shock near the inlet lip can be

noticed as one translates towards the side walls of the scramjet model. For flow

profiles near the wall, x = 18 mm and 19 mm, no shock train can be perceived in the

25

images. This indicates the strong effect of the side walls. Also, similar flow features

in each set of images can be observed until the fuel injection port. Fuel jet injection

does not seem to have any effect on the flow features upstream of the injection port.

3.2 Chemiluminescence

Images were acquired over a shutter period of 1/60 s. However, as no filters

were used, the chemiluminescence can be assumed due to the combined presence of

chemically excited species, mainly consisting of OH∗, CH∗, C∗2 and CO∗

2. Nevertheless,

the region with relatively higher intensities of chemiluminescence can be associated

with the strongest combustion reactions.

Shown in the figure 3.4 is a detailed, typical chemiluminescence image acquired

in the facility. All of the chemiluminescence images were focused onto the central

plane of the scramjet.

Figure 3.4. Detailed chemiluminescence image of the combustion process.Brighter (blue) regions are indicative of intense heat release reactions. Free

stream flow direction is from left to right.

3.3 OH PLIF

Prior to application of the discussed optical measurements in a reacting environ-

ment, it is quintessential to verify the quasi-stable assumption for the chosen test

26

conditions. Figure 3.5 substantiates the assumption that combustion reactions were

in fact in a quasi-stable mode in the model. The fuel jet was active for a period of 300

ms and the images seem to show similar characteristics, most conspicuous of which

is the strong OH signature around the cavity region; with the brighter (fluorescing)

contours indicative of stronger OH concentrations. OH distribution is an indicator

of intermediate reactions characteristic of ignition. It is well known that ignition

reactions are a precursor to the heat release reactions, and therefore, presence of

OH fluorescence may be suggestive of negligible heat release in respective regions of

interest.

Figure 3.5. Sequence of PLIF images taken ∆T =100 ms apart from eachother (numbered), at one of the scramjet investigation planes. Free stream

flow direction is from left to right. Fuel jet was active for 300 ms.

Figure 3.6 shows the differences in images acquired using simultaneous chemilu-

minescence and OH PLIF for runs under similar test conditions. The PLIF images

were taken at separate planes during different runs. The chemiluminescence and

topmost PLIF images complement each other. A striking feature is the absence of

signals in the central region of the longitudinal plane in both the images. This pecu-

27

liarity, apart from suggestive of a truly stable behavior of the flame, also indicates the

flame containment to regions sporting high mixing environments and consequently,

favoring flame residence. However, a contrasting perspective is obtained when the

chemiluminescence is compared to a PLIF measurements closer to the wall at x =17

mm (bottom image). A possible explanation could be that chemiluminescence, being

a line-of-sight technique, absorbs the signals in the planes encompassed in its depth

of field. Here, the planes in discussion are ones closest to the scramjet central plane.

If the depth of field were to also include the planes closer to the walls, then it can

be concluded that the reactions are stronger in the planes closer to the central plane,

and hence have more bearing on the chemiluminescence image.

Figure 3.6. Detailed set of images comparing chemiluminescence (middle)and PLIF measurements planes at x = 0 mm (top) and x = 17 mm

(bottom) in the model scramjet. Overall equivalence ratio (φ)= 0.83, M =4.5, P0 =100 kPa, T0 = 2600. Images were acquired during the quasi-stablestate of the combustion process. Free stream flow direction is from left to

right. The brighter (fluorescing) contours are indicative of higher OHconcentrations.

These can be seen predominantly in downstream regions of the cavity, which, as

expected, can be attributed to enhanced mixing.

28

PLIF measurements were acquired for flows at overall equivalence ratios varying

from lean to rich. PLIF images were helpful in ascertaining the interaction of tur-

bulence structures with flame fronts. Since the flame fronts have a 3D structure,

visualization of planar sections of the scramjet during fuel operation helped reveal

interactions unperceived in a single plane. Figure 3.4 shows a set of PLIF images

taken in various cross-sectional planes of the scramjet model.

Flame front fading towards the cavity was observed as one travels away from

the central plane of the model towards the side wall. However, there seems to be

combustion activity registered near the wall, (16-19 mm) downstream of the inlet

lip. This phenomenon could be attributed to development of dense boundary layers

at the side walls. A simple 2D scan at only one plane would not have provided

sufficient information to reach this conclusion. The OH distribution was seen to be

more concentrated in regions surrounding the lower boundary layers.

29

Figure 3.7. A series of spatially varying OH distribution images obtainedusing PLIF in the model scramjet. The bottommost image is at the

scramjet center plane and top image is closest to the side wall. All theimages were obtained at least 100 ms after fuel injection and can be

considered to be in stable mode of the quasi-steady combustion process.Free stream flow direction is from left to right. The brighter (fluorescing)

contours are indicative of stronger OH concentrations.

30

Figure 3.8. A depiction of flame residence (in quasi-stable mode) on thebottom wall of the scramjet model for conditions - φ)= 0.97, M = 4.5, P0

=100 kPa, T0 = 2600 . Free stream flow direction is from left to right.

A possible explanation of the flame residence in regions upstream of the fuel jet

could be due to a separation region induced by either mass loading or combustion

downstream of the fuel jet. In cases where the overall equivalence ratio is close to

stoichiometric ratio, as in the case just discussed, mass loading may be ruled out. As

the flow near the walls and upstream of the fuel jet is already separated, the presence

of flame might be ascribed to the pressure buildup due to combustion occurring

downstream and subsequently increased flame propagation speeds.

In the image sequence shown in figure 3.8, high concentrations of OH can be

seen near and downstream of the cavity for leaner and stoichiometric fuel mixtures

(based on overall equivalence ratios). This is indicative of the role the cavity plays

in anchoring and stabilizing the flame for these mixture fraction regimes. Detectable

OH distributions are noticeable in the shear layers of the fuel jet, although these only

seem to become prominent during auto-ignition in fuel rich scenarios. In these cases,

the combustion activity is shifted to upstream locations of the cavity, indicative of

its subdued role in assisting combustion.

31

Figure 3.9. Sequence of PLIF images with varying overall equivalenceratios obtained at a planar section 2 mm from the center of the model. Allthe images were obtained 100 ms after fuel injection and can be consideredto be in quasi-stable mode. Free stream flow direction is from left to right.

32

The total temperature was such that the flame was auto-ignited in the windward

region of the fuel jet, and stretched downstream. A bow-shock was induced by the fuel

jet and the fuel is auto-ignited in the jet wake region. Strongest OH concentrations

were detected in the lower boundary layers in the periphery of the jet, supporting the

auto-ignition hypothesis. Another notable characteristic of the flames in the fuel rich

regime was the OH distribution, flanking, and what seems to be, a highly fuel rich

mixture convecting downstream of the fuel jet and residing in the central portion of

the longitudinal plane.

33

CHAPTER 4

CONCLUSIONS AND RECOMMENDATIONS

4.1 Conclusion

Planar Laser Rayleigh Scattering and OH Planar Laser Induced Fluorescence

techniques have been successfully developed and demonstrated to investigate flow

physics in a model scramjet engine in a free stream flow of Mach 4.5. These techniques

were extended to 3-dimensional flow domain for qualitative measurements with the

development of an unique optical system. Full spatial fluorescence (in the case of

PLIF) was achieved with the application of this optical arrangement. Particular

examples were discussed detailing the flow features.

Rayleigh scattering images were acquired using the second harmonic (532 nm)

of 1064 nm and at 283 nm (dye laser output). The incentive to utilize light at 283

nm was due to the fact that shorter wavelengths help achieve better signal-to-noise

ratio as a result of a larger Rayleigh cross-section, as well as, subdued reflectivity

of metallic surfaces; UV light is very strongly scattered and consequently, Rayleigh

light possesses highly exploitable contrast compared to surface scattered light. How-

ever, the laser intensity was not sufficient enough (∼ 13.5 mJ/pulse) to invoke strong

Rayleigh signals. Further work in this domain is possible. The results of the experi-

ments utilizing 532 nm light have been discussed. Rayleigh scattering has shown the

capability of highlighting detailed flow structures such as shock and expansion waves,

as well as, boundary layers in cold, non-reacting flows.

Chemiluminescence has been the preferred means of flow visualization in the fa-

cility. Differences between the chemiluminescence and PLIF have been delved into to

34

strengthen the case for PLIF. Chemiluminescence did help strengthen some deduc-

tions obtained through PLIF measurements as a result its long exposure time scales.

Compounded PLIF measurements from multiple planes and over various mixture

fractions have been examined to further establish the significance of this technique.

Results point to the effectiveness of cavities in the deceleration of the flow injection

and subsequent quasi-stable combustion in stoichiometric and fuel lean regimes based

on overall equivalence ratio. The examined results also attested to the fact that the

combustion reactions were in a quasi-stable mode, and that auto-ignition hypothesis

was valid for fuel rich scenarios. Fuel jet injection might be a defining operation in

generation of turbulence features downstream of the injection port. Hence, PLIF

and PLRS measurements can be combined to provide a holistic means for deduc-

ing combustion physics, discussed using examples. The boundary layer effect has

been shown to dominate upstream regions of the fuel jet (close to the walls), even in

reacting flows.

A small section composed of tips helpful for a trouble-free experimentation were

provided for beginners using the system. The measurement techniques discussed have

helped underline the reliability of instantaneous and very high spatially resolvable op-

tical methods for a complicated combustion system like a scramjet. These techniques

have to be extended to multiple planes for appropriate interpretation of results.

4.2 Recommendations for Future Work

Since the production of CH radicals occurs at the flame front, they can be re-

liably associated with the reaction zones [1], [17],[2]. Also, CH radical distribution

is narrower, and the lifetime much shorter than that of OH. Further, the heat re-

lease rate correlates betterl with the CH radical distribution than that of OH [19].

Nevertheless, CH PLIF doesn’t suffice to differentiate between unburned and burned

gaseous zones. Therefore, simultaneous OH and CH PLIF, or OH and CH2O PLIF

35

measurements could be applied to future experiments in the facility.

Bandpass filters corresponding to the maximum spectral lines of various chemical

species (OH∗, CH∗ and C∗2) could be used with the Casio camera to selectively and

cost effectively observe chemiluminescence in future experiments.

The best use of the current laser sheet generation scheme can only be exploited

if the camera is capable of imaging the entire length of the scramjet model. Un-

fortunately, due to the presence of struts in the optical access window this is not

possible with a single camera. A second imaging camera could be required to image

fluorescence in the part of the scramjet model obstructed by the strut.

Additionally, a pulse delay generator integrated into the timing circuit could po-

tentially help access time sequential Rayleigh signals and provide temporally resolved

(resolution as high as 0.5 ms) information on the development of turbulent and shock

structures [4].

For the same laser pulse intensities, Rayleigh scattering measurements at shorter

wavelengths help achieve better signal-to-noise ratio as a result of a larger Rayleigh

cross-section, as well as, subdued reflectivity of metallic surfaces. Therefore, the

third harmonic of 1064 nm light would be highly suited for application in the facil-

ity. Coupled with a multi-kHz, high output (∼ 120 - 200 mJ) pulsed laser system,

experiments with high temporal resolution could be attained. For convenience, these

experiments could be conducted post OH PLIF measurements by removing the dye

laser from the beam path.

36

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