Generating Uniform Sub-micron Solid Particles for Laser ...

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Generating Uniform Sub-micron Solid Particles for

Laser-based Flow Diagnostics

A Thesis

Presented in order to fulfill The Undergraduate Honors Program

At the Ohio State University

By

Keith Patrick Smelker

*******

The Ohio State University

2006

Dr. Mohammed Samimy

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ABSTRACT

New laser-based flow diagnostic techniques such as Particle Image Velocimetry

are an attractive method for characterizing the velocity field of high-temperature jet

flows. PIV required particle seeding and many current seeding techniques are not feasible

within a high temperature flow. This research focuses on developing a system to generate

uniform sub-micron solid particles and to seed them into a jet system with a high

temperature air flow using a pH stabilized colloidal dispersion. The pH stabilized

technique was first developed at NASA�s John Glenn Research Center by Dr. Mark

Wernet. This research covers the design and development of a system to use the pH

stabilized technique to produce sub micron particles for laser based flow diagnostics.

Seeding is the key to producing accurate laser based diagnostics results. The seed size

and material must be chosen to balance the ability to follow the flow, reflect measurable

amounts of light, and exist within the testing application. For this research, 0.6 µm

aluminum oxide was chosen as a balance of these three characteristics. The system to

introduce these particles into the flow was produced based on its storage ability, flow and

pressure control, and seeding capabilities. The system consists of an 8 gallon stainless

steel pressure vessel, a Tescom pressure regulator, stainless steel tubing, needle and ball

valves, and injector nozzles. The system produces seed for laser based flow diagnostics.

Experiments were run at Jet Mach 0.9, 0.83, and 0.73 and compared to a baseline case at

Mach 0.9 to see how well the seeder seeded the flow. The results show that the seeder

produces aluminum oxide seed particles, and each test showed steady improvement in

seeding the flow homogenously. However, this first generation seeder never seeded the

flow as well as the baseline case. Recommendations for running heated jet experiments,

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adding air atomizing nozzles, using a solenoid control valve, and running multiple

injection points are made to increase performance of the system.

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ACKNOWLEDGEMENTS

I want to thank Dr. Samimy for mentoring and teaching me throughout this

project. His constant mentoring and advising have helped me to learn a great deal about

research and working in a research environment. I would also like to thank Dr. Samimy

for sponsoring this research. Without his sponsorship or support of this research, none of

it would have been possible. I would also like to thank Dr. Wernet at NASA for allowing

me to use his notes and for answering any questions that I had throughout this whole

procedure. Finally, I would like to thank Dr. Jin-Hwa Kim, Dr. Marco Debiasi, Dr. Jacob

George, Jeff Kastner, Jesse Little, and Edgar Carabollo. Each of these people helped me a

great deal in the planning, designing, building, testing, and analyzing throughout this

project. Without them, this thesis would not have been finished. Thank you all for all of

your hard work and advise.

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TABLE OF CONTENTS

ABSTRACT...................................................................................................................ii ACKNOWLEDGEMENTS .........................................................................................iv List of Figures...............................................................................................................vi Introduction...................................................................................................................1 Background ...................................................................................................................4

2.1 Introduction ..........................................................................................................4 2.2 Particle Image Velocimetry...................................................................................4 2.2 Basic Particle Principles .......................................................................................8 2.3 pH stabilized Dispersion of Alumina in Ethanol ..................................................9 2.4 Particle Generation.............................................................................................11

Design of Experimental Apparatus and Experimental Setup....................................14 3.1 Major Seeder Design Parameters .......................................................................14 3.2 Seeder Design .....................................................................................................14 3.3 Seed Type and Generation ..................................................................................26 3.4 Equipment and Preparation of a pH Stabilized Dispersion of Ethanol ..............28 3.5 Particle Image Velocimetry System.....................................................................30 3.6 Experimental Methodology.................................................................................30

Discussion and Presentation of Results ......................................................................32 4.1 Introduction ........................................................................................................32 4.2 Mach 0.9 Case ....................................................................................................32 4.3 Mach 0.83 Case...................................................................................................35 4.4 Mach 0.73 Case...................................................................................................39

Conclusions and Recommendations for Future Work...............................................42 5.1 Conclusions ........................................................................................................42 5.2 Recommendation for Future Work .....................................................................43

References....................................................................................................................46

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List of Figures

Figure 2.1: GDTL Free Jet System with PIV Setup 7

Figure 2.2: Mie Scattering Cross Section 7

Figure 2.3: Stable Dispersion of Aluminum in Ethanol 10

Figure 2.4: Cyclone Entrainment 13

Figure 3.1: Operational Schematic 21

Figure 3.2: Top of Seeder Tank 22

Figure 3.3: Seeder Tank and Control Panel 22

Figure 3.4: Hollow Cone Spray Pattern 23

Figure 3.5: No. 3 Fine Spray Nozzle 23

Figure 3.6: CAD Drawing of Cart 24

Figure 3.7: Cart with Seeder 24

Figure 3.8: Swagelok Compression Fitting 25

Figure 3.9: First Injection Point with Nozzle 27

Figure 3.10: Sonicator with pH stabilized dispersion of aluminum oxide 29

Figure 4.1a: Mach 0.9 Baseline case with Ambient Seeding 34

Figure 4.1b: Mach 0.9 Experimental Case Tank Pressure 50 psig 34


Figure 4.2b: Mach 0.83 with Tank Pressure of 47 psig 37

Figure 4.3: Mach 0.83 with Tank Pressure of 80 psig

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Figure 4.4b: Mach 0.73 with Tank Pressure of 55 psi 40

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CHAPTER 1

Introduction

Recently, the Gas Dynamics and Turbine Laboratory (GDTL) at the Ohio State

University built a system to simulate free jet flows. The free jet nozzle is one inch

diameter and is located within an anechoic chamber that dampens the ambient noise

within the system. A heater has also been added to the system to experimentally create

heated flows that simulate jet exhaust. The goal of the research in this system is to

develop techniques to control the noise pollution radiated by different types of jet flows.

Currently, Particle Image Velocimetry (PIV) is used to map the velocity of the turbulent

flow structures. The study of these structures allows for new methods of noise reduction

and noise control to be developed.

Particle Image Velocimetry is a planar measurement technique used to map the

velocity field of a fluid flow (Melling 1998). A pulsed light sheet scatters light from the

particles. The particles are suspended in the flow, and two exposures are taken of the

particles. These images are compared in order to find the particle displacements between

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the two frames. This is done using a cross correlation technique where the images are

subdivided into regions and the regions are correlated using a digital Fourier Transform

to find out which particle regions correlate most to one another the (Wernet 1994). A

two-dimensional picture of the velocity field is then made using the cross correlation

data.

In order to seed a flow correctly, a good seed particle must be chosen for the

specific application. An acceptable seed particle will be both large enough to scatter light

and small enough to follow the flow accurately (Wernet 2004). The seeded particles also

must be distributed homogenously when introduced to the flow and not agglomerated

during introduction to the flow. There are many different types of seeding materials and

seeding techniques (Melling 1997). Currently, the GDTL utilizes a LaVision commercial

oil particle generator to seed the centerline flow and a fog generator to seed the co-flow.

For a heated flow the centerline seeder will not work because the oil particles will

evaporate at the elevated jet temperatures.

Research and design of a refractory metal seeding system for high temperature air

flows has previously been done (Wernet and Wernet, 1995). A pH stabilized colloidal

dispersion of alumina in ethanol is created using a technique such that the pH of the

solution is far away from the pH point of zero charge for the solution. The higher pH in

the solution allows the particles sufficient energy to create a double layer of repulsion

that overcomes the Van der Waals bonds that normally inhibit the use of a refractory in a

seeding solution. Tests showed that a pH 1 solution of alumina in 100% ethanol would

stay stable for 2.5 days (Wernet and Wernet, 1994). A commercial paint sprayer with an

atomizer can be used to create a high quality aerosol with this solution. In the previous

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research, a seeding system using a pH stabilized dispersion of alumina in ethanol was

designed and implemented for a high temperature compressor system and the tests were

successful (Wernet, et al. 1995).

This paper describes the design and testing of a portable system to generate sub

micron particles for laser based flow diagnostics. The system concentrates on generating

sub micron Aluminum oxide particles suspended in pH stabilized Ethanol and injected

with a commercial atomizer. Chapter 2 of this thesis details the theory behind generating

a well seeded flow. Chapter 3 details the design and testing procedure of the modular

apparatus for generating the seed material and the apparatus used for the making the pH

stabilized colloidal dispersion of Alumina. Chapter 4 shows and discusses the results of

using the pH stabilized colloidal dispersion of Alumina versus using a commercial

acetone seeder. Chapter 5 concludes on the work as well as makes recommendations for

further work.

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CHAPTER 2

Background

2.1 Introduction

Qualitative flow diagnostics within complex flow fields have been explored over

the last 100 years. With the technical progress within the last 20 years, qualitative

diagnostics have evolved to the point where detailed quantitative measurement of

complex flow fields can be obtained (Raffel, et al. 1998). Recently notable reviews of

planar velocimetry (Samimy 1998) and particle seeding techniques (Melling 1997) have

taken place. New techniques for seeding high temperature air flows have also been

developed (Wernet and Wernet 1994) and tested (Wernet, et al. 1995).

2.2 Particle Image Velocimetry

PIV is an indirect non-intrusive laser based flow measurement technique. The

entrained particle velocity is recorded by taking two successive images of the particles

illuminated by laser light. It is important to note that this is an indirect technique because

the velocity of the particles is measured not the actual velocity of the flow. Laser

illumination is generally carried out using a commercial laser and a series of mirrors and

splitters to shape the beam into a sheet of light. The sheet is directed over the flow in

either a cross-stream or stream-wise direction. Illumination of the particles happens

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according to Mie scattering theory which gives the cross sectional scattering property of

particles. Mie scattering dictates that a particle that is much larger than the wavelength of

light will have a scattering cross section with varying intensity based upon the

observation angle, refractive index, particle size, and wavelength of light (Raffel, et al.

1998). The cross sectional reflection of particles affects the seeding of the flow. As an

example, Figure 2.1 shows the Mie Scattering cross section for a particle. In this picture

the circles are logarithmic graphs of intensity based upon 100, and the x-y axis is the

scattering angle from 0-360º. Looking at this picture shows that the intensity of light

varies with the angle of observation. It also shows that light is reflected in a cross section

and not in a point. This means that each particle seen in PIV images is actually smaller

than the visualized particles.

Recording of the particle positions is accomplished with one of two techniques.

The first technique utilizes two lasers and a single CCD (charged coupled device)

camera. The laser sheets are placed as close together as possible. The lasers are fired

within microseconds of one another, and the camera takes two pictures of the flow based

on each laser pulse. The GDTL currently uses this technique. Figure 2.2 shows a

schematic of the GDTL jet system and the PIV setup. The second technique uses two

cameras and a single laser. The two cameras are each calibrated to pick up only one type

of polarized light, s or p light (Wernet 1995). When the laser illuminates the particle, the

two types of polarized light are given off and recorded.

In order to obtain the planar velocity map, the two images from either technique

are collected and analyzed via a computer. The analysis is performed using a cross

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correlation technique that divides the image into sub-regions. Each of these interrogation

windows is subjected to a cyclic FFT given by the following equation:

∑<<

==++=

nynx

yxydyxdxIyxIdydxC

,

0,021 ),(),(),(

2,

2ndydxn <<− (1)

where I1 and I2 are the image intensities of the 1st and 2nd interrogation windows, and the

C vector is the correlation strength of the displacement between the two interrogation

windows. The size of the interrogation window is given by n in pixels. More information

on this process can be found in the DaVis 7.0 Instruction manual. With these correlations,

a vector map can be drawn and the flow velocity can be studied.

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Figure 2.1: Mie Scattering Cross Section (Wernet 2004)

Figure 2.2: GDTL Free Jet System with PIV Setup

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2.2 Basic Particle Principles

When using laser based flow diagnostics, generating good seeding is key to

producing accurate results (Wernet 2004). In most seeding applications, the seed

material must be non-toxic, easily attainable, and can be readily introduced to the flow.

The particles� size must be chosen according to a couple of principles. Firstly since laser

based flow diagnostics do not measure the velocity of the flow directly but instead the

velocity of the particles within the flow, the particle must be small enough follow the

flow accurately. The time constant of a particle to achieve constant speed within a flow is

given by the following equation:

µρ

τ18

2 pps d= (2)

where dp is the diameter of the particles, τs is the time constant, ρp is the particle density

and µ is the dynamic viscosity of the seeded medium or flow (Raffel, et al. 1998). This

equation assumes that the fluid acceleration is constant. Most flows cannot be

characterized using this simplification, but this is still a convenient measure of particle

settling time. Looking at this equation, the diameter of the particle lends a large amount

of the time lag for the particle. With this, the smallest particles possible will follow the

flow most accurately. In general, a particle of 1 µm or smaller will acceptably follow

most turbulent flows or high speed gas flows (Melling 1997).

Even though the smallest particles will most accurately follow the flow, the

particles must also be large enough to scatter sufficient light for the camera. In general,

the amount of light scattered by a particle is governed by the particle�s size, shape,

refractive index, and the orientation of the particle (Raffel, et al., Willert, and

Kompenhans 1998). Thus, a balance exists between the particles being small enough to

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follow the flow accurately, and large enough to scatter light to be measured. The light

scattering characteristics of particles is very important in choosing an acceptable particle.

For particles of similar size, the one with the higher refractive index will be the most

suitable for a given application.

The flow application also dictates the material of the particle used. There are

many different materials that can be used to properly seed a jet flow. Oil droplets or some

type of Polystyrene Latex Spheres (PSL) are commonly used. However, for high

temperature flows neither of these seed materials will work. PSL has a melting point of

250 ºC causing agglomeration at elevated temperatures (Wernet 1995). Oil droplets

evaporate in flows where the temperature reaches levels far above ambient. Refractory

metals are used for flows in which the temperature is above ambient (Melling 1997).

Since the GDTL has started to use high temperature flows, this paper will focus on being

able to generate sub-micron aluminum oxide particles for flow measurements. Aluminum

oxide is a non-toxic, high refractivity, and easily attainable particle that is available in

submicron sizes. Normally, aluminum oxide particles agglomerate when introduced to a

flow. However, a new mixing technique has been made where the pH of the aluminum

oxide solution is stabilized to stop agglomeration (Wernet 1994).

2.3 pH Stabilized Dispersion of Alumina in Ethanol

Since aluminum oxide has a tendency to agglomerate when introduced to a flow,

a new technique was developed in order to make aluminum oxide a viable seeding

option. When a solid particle is suspended in a solution, the inter-particle forces dictate

how stable the suspension will be. A suspension is stable once the repulsive forces are

stronger than the attractive forces. When a particle is introduced into an acidic or basic

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solution, the polarity of the solution causes repulsive or attractive electrostatic forces

depending upon the sign and strength of the polar solution. In a solution, the point of zero

charge is the point where the polarity of the solution does not create an electrostatic layer

on the particle. When the pH is below this point, a repulsive electrostatic layer is

developed on the surface of the particles. To create a stable dispersion of alumina in

ethanol, the pH of the solution must be lowered far below the point of zero charge for

aluminum. Figure 2.3 illustrates how lowering the pH will create a stable dispersion

(Wernet 1994). With a stable dispersion of aluminum in ethanol, the dispersion can be

introduced to the flow without major agglomeration.

Figure 2.3: Stable Dispersion of Aluminum in Ethanol (Wernet 1994)

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2.4 Particle Generation

Particle generation for seeding a flow is done on either a global or a local level.

Local seeding is the process by which seed is introduced into only the section of the flow

field that is of interest for study. To accomplish local seeding, the seed is normally

introduced into the flow at the area of study. Global seeding introduces seed to the entire

flow field. Two parameters are important in global seeding. First of all, the injection

point must be far enough upstream that the particles will be evenly distributed throughout

the flow when the area of study is reached. Secondly, the injection point must not

severely interfere with the flow. For this study, global seeding will be used.

The density of particles entrained within a flow is critical to generating PIV

images that accurately approximate the movement of a complex gas flow. Firstly, there is

a limit to the density of particles the flow. This limit is based upon the scattering

characteristics of particles. Each particle has its own scattering cross section that the

camera records. As the particles become closer together the intensity cross sections begin

to overlap. This introduces noise into the image such that affects the accuracy of the PIV

recording (Samimy 1998). There is also a lower limit to the number of particles within a

region of flow. The flow field dictates that there enough particles to characterize the flow.

The general rule is that there should be at least 6-10 particles per interrogation window

(Wernet 2004).

Secondly, uniform particle number density is also important in accurately

characterizing a complex flow field. A homogenous particle distribution gives the same

number of particles in each interrogation window. A perfectly homogenously seeded flow

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will not show any flow structures but will have the same density of particles throughout

the flow. This ensures that each section of the flow is characterized accurately.

There are multiple ways to introduce particles into a flow including oil particle

generators, fog generators, cyclone entrainment, fluidized beds, and atomizers (Wernet

2004). Oil particle and fog generators use a condensation technique to create sub-micron

oil particles. These particles are then injected into the flow. The GDTL employs both a

commercial atomizer from LaVision to seed the core jet flow, and a theatrical fog

generator for ambient air seeding. Cyclone entrainment and fluidized beds use a high

pressure air jet to create a cyclone that picks up the small particles on the bottom of the

tank and introduces them to the flow through the top of the tank. These techniques are

normally used for introducing dry solid particles to a flow. Figure 2.4 illustrates the

cyclone entrainment technique. Atomization is also a technique to introduce a solid

particle to a flow. Atomization requires solid particles to be suspended within a liquid,

and the liquid is pushed through a nozzle to create a quality aerosol. A liquid with a low

vapor pressure, and an injection point well upstream of the area of study is used to make

sure that the liquid is completely evaporated before the area of study.

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Figure 2.4: Cyclone Entrainment (Wernet 2004)

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Chapter 3

Design of Experimental Apparatus and Experimental Setup

3.1 Major Seeder Design Parameters

While the goal of the seeder is to produce uniform solid particles throughout the

flow, the design of the components was based on a few crucial parts: storage ability,

adaptability, pressure and flow control, and portability. Storage ability is broken down

into two components, corrosion resistance for handling acidic dispersions and capacity to

hold an hours worth of seeding solution. Adaptability is based on the seeder�s ability to

be used for multiple facilities in multiple arrangements. This means using parts and

pieces that can easily be removed and reattached without a significant overhaul on the

system. Pressure and flow control allow for multiple injection pressures and mass flow

rates be used with Mach numbers ranging from 0-2. Portability is important for moving

the seed generator to and from multiple facilities without a large amount of work.

3.2 Seeder Design

After reviewing the design of the seeder used from previous research and keeping

in mind the design parameters, a system schematic was designed to highlight the major

parts of the design. Figure 3.1 shows the schematic for the system. This schematic

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outlines the major components to introduce the pH stabilized dispersion into the free-jet

facility. Shop air is supplied to the tank to create a differential pressure between the tank

and the jet facility. The liquid will then flow from the tank and be injected into the jet

facility.

The tank is the central and largest part of the system. It is made of Schedule 80

10� 316 Stainless Steel pipe and the flanges are 300# 316 Stainless steel flanges.

Stainless steel was used because of its strength, durability, and corrosion resistance.

Flexitallic spiral wound gaskets were used to seal the tank. This type of gasket reduces

the effect of cathodic and anodic corrosion due to dissimilar metals coming into contact.

The height of the tank is 40� and is capable of holding 8 gallons of pH stabilized

dispersion. Two 2� holes were drilled in both the top and the bottom of the tank to drain

and put in the liquid. Both of these holes are tapped and plugged during operation. The

bottom of the tank was machined to make a sloping drain throughout most of the bottom.

Three 3/8� holes are also drilled and tapped into the top of the tank; one is the inlet for

the pressurized air, one is the outlet, and one is the pressure release. Two tubes reach into

the tank cavity. The first tube reaches into the mid-section of the tank and is the inlet for

the air. The second tube reaches to ¼� from the bottom of the tank and is the outlet for

the liquid. All of the tubes within the tank are 3/8� stainless steel connected with

Swagelok fittings. Pressure is released from the tank with a 3/8� Stainless steel ball valve.

Figure 3.2 shows the top of the tank.

A pressure regulator controls the pressure of the air coming into the tank. The

valve is a Tescom 44-1300s rated for 500 psi inlet and 0-150 psig outlet. It has a brass

body and has a single port for a pressure gauge. Brass was chosen as the material due to

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price. The maximum inlet pressure from the shop air is 120 psi and the maximum

pressure needed is 100 psi for a Mach 2 flow. Both the inlet and outlet pressure for the

regulator meet these specifications. Figure 3.3 shows how the Tescom Regulator is

situated with the controls to the seeding system. The Tescom regulator is on the left of

the picture with the inlet stream of air, and the flow rate controls are on the right with the

outlet streams of pH stabilized dispersion.

The outlet from the seeder is split into two streams and sent to the injectors. Two

outlets were designed in order to accommodate multiple injection layouts. An adaptable

injector layout allows the seeder to be used not only for the jet facility but also to seed

other facilities. It also gives flexibility in seeding the flow such that a co-flow seed can be

produced, as shown in Figure 3.1, or for seeding the core jet from multiple points. A co-

flow injection means that the ambient air around the jet is seeded such that the entrained

air is seeded as it is entrained within the jet. Both of the outlets from the seeder are made

of 3/8� 304 Stainless Steel tubing. Tubing connections are all stainless steel Swagelok

compression fittings. Stainless steel was used for its corrosion resistance and Swagelok

was used for its ease in attachment and removal. The fittings and tubing can be seen in

Figure 3.4.

Control for the seeder outputs is accomplished through a couple of different

mechanisms. As shown in Figure 3.4, the primary flow control is accomplished through a

ball valve followed in series by a needle valve. A pressure gauge in between the valve

gives the liquid output pressure and can be used to approximate the injection pressure.

The ball valve gives the system two features. Firstly, turning the flow on and off from

this point is accomplished without having to reduce the pressure within the tank. Thus,

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the system can be modified while it is still online. Sections locked down by ball valves

can be taken off and replaced without having to drain the tank. Secondly, this valve acts

as a quick disconnect point where the valve can be closed and all of the tubing after the

valve can be adapted without having to vent the tank or turn off the incoming air. The

needle valve gives the mass flow control for the system. Using a small needle valve, as

shown in Figure 3.4, allows for good resolution in control of the flow to the injector.

However, a pressure loss is introduced to the system when the valve is barely open such

that the gauges located at the control panel will not precisely give the injection pressure.

Both of these valves are rated for 3000 psi and are made of 304 stainless steel. The gauge

reads 0-300 psi and is rated for either liquid or gas.

Commercial Atomizers accomplish injection to the jet flow. Atomization is used

for seed generation because in order to generate a quality refractory metal aerosol the pH

stabilization technique had to be used. Since this technique introduces the seed to a

liquid, atomization fit the best for this process. A Spray Systems 1/4LND-316SS type

fine spray nozzle was selected. This is a 303 stainless steel wall mounted nozzle that

produces a spray by pushing a liquid through a small orifice. It produces the hollow cone

pattern as seen in Figure 3.5. A hollow cone pattern allows the seed to be distributed in a

wide angle throughout the flow such that mixing will occur rapidly within the flow. In

selecting this nozzle, four orifice sizes were selected to give control over the amount of

seed introduced to the flow at a given pressure. Table 3.1 shows the orifice sizes and

maximum flow for each of the nozzles. These four nozzles give a wide amount of

flexibility in choosing the right flow rate to produce a uniformly seeded flow. The flow

rates listed in the table are for a pressure of 300psi. Since the shop air is at 120 psi, this

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high of flow rate will never be seen and does not factor into the decision to use these four

nozzles. Figure 3.5 shows one of the nozzles used. The nozzle has a threaded head such

that it can be wall mounted directly into the injection point on the jet facility.

Orifice Number

Orifice Size (in.)

Maximum Flow rate at 300 psi (gph)

3 0.028 8.2 6 0.042 16.5

10 0.064 27 14 0.076 38

Table 3.1: Nozzle Orifice Sizes and Maximum Flow rates (Spray Systems Co. 2004)

The final major part to designing this system was making the system both

portable and adaptable to other facilitiess easily. Since the tank weighs in excess of 600

lbs while empty, making it portable presented a sizable challenge. Previously, an acetone

seeding system was designed to be moved using a dolly. However, when considering the

weight of this system compared to that of the acetone system, this proved to be unfeasible

and hazardous. The second option was to order a prefabricated cart that the tank could be

bolted to and moved using the cart. A stainless steel heavy duty cart was ordered to

accomplish this task. This seemed like a simple solution to moving the tank without a

large amount of design work being done. The wheelbase for this cart was in a diamond

pattern that was unstable when put under high point loads given by the support studs for

the tank.

Finally, a small cart with a rigid structure was designed using Solid Edge. Figure

3.6 shows the CAD picture of the cart. This design gives both horizontal and vertical

support for the tank while it is on the cart. The horizontal supports between the main

supports give rigidity to the cart structure, and the transverse supports give lateral support

to the structure as the cart moves. The base cart is a 24� x 36� commercially available

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cart rated for 1200 lbs equipped with 9� diameter wheels. Having large wheels ensures

that bumps and discontinuities on the floor will not compromise the stability of the

moving tank. The structure on the cart stands 44� high and is 24� square. This footprint is

6.5� greater than that of the tank such that the horizontal support can be taken off of the

structure and the tank can be moved from the cart. During construction of the cart, the

long transversal pieces were switched for shorter floor mounted brackets. These were

easier to make and smaller while giving adequate stability for the cart. Figure 3.7 shows

the completed cart with the seeder on it. The structure for the cart is made of U-Shaped

bar with ½� bolts slots located at every inch of the bar. The main supports are welded to

the cart. The side horizontal supports are bolted to main supports with ½� bolts. Overall,

this gives a stable and easily portable design for the seeder.

Adaptability for the system was accomplished in a couple of ways. Firstly, all of

the tubing and pipe fittings are done using Swagelok compression fittings. These fittings

do not use Teflon but are self sealing based upon compression of a metal ring that is

forced onto the tubing. The configuration of the components can be changed easily with

an adjustable wrench. Figure 3.8 shows the compression ring inside of the Swagelok

fitting. Secondly, all of the tubing and pipe fittings for the system were based upon 3/8�

stainless steel tube. With a single tube size throughout the system, all pieces can be

moved around within the system to find a suitable controls configuration for each

application. Finally, ball valves were used extensively within the design to increase the

modularity of the design. With these ball valves in place, each section of the tubing is its

own module that can be shut off from the rest of the system for changes or repair without

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having to drain the tank or stop the pressure. In these ways, the system is adaptable to

multiple configurations.

Looking at the overall design of the seeder system, it meets the major design

parameters in the following ways. As far as storage capabilities, the tank has storage

capabilities for 8 gallons of pH stabilized dispersion which is more than enough for

continuous operation for 2 hours, and all parts coming in contact with the acidic

suspension are stainless steel with good corrosion characteristics. The tank also has a

maximum pressure rating of 300 psi which is significantly higher than the 120 psi given

from the shop air. It is adaptable to multiple configurations using Swagelok components

and modularity achieved through ball valves. Pressure and flow control are achieved

through the main Tescom Regulator, needle valves, and multiple types of injector

nozzles. The cart makes the system portable to all points in the laboratory. Overall, the

design meets all of the major design parameters for making a seed generating system.

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Figure 3.1: Operational Schematic

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Figure 3.2: Top of Seeder Tank

Figure 3.3: Seeder Tank and Control Panel

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Figure 3.4: Hollow Cone Spray Pattern (Spray Systems 2004)

Figure 3.5: No. 3 Fine Spray Nozzle

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Figure 3.6: CAD Drawing of Cart

Figure 3.7: Cart with Seeder

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Figure 3.8: Swagelok Compression Fitting

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3.3 Seed Type and Generation

From the beginning of this project, aluminum oxide was the material of choice,

but significant effort went into choosing a particle size and delivery system to the jet

core. In previous experiments, the seed used was Sumitomo AKP-15 particles with a

mean particle size of 0.7 ± 0.2 µm (Wernet 1995). However, at the time of this research

these particles were not available, so a particle size had to be chosen based upon the

systems present at the GDTL and current particle sizes offered by manufacturers. As

explained previously, the particle size is a balance between being small enough to follow

flow accurately and also large enough to refract a measurable amount of light. With this

in mind samples of two particles were obtained for experiments. Sumitomo contributed 2

kg each of AA-3 and AA-5 high purity aluminum oxide. The AA-3 and AA-5 particles

have a mean diameter of 0.3 µm and 0.6 µm, respectively, and are of 99.99% pure. Since

the previous research used a 0.7 µm particles with great success the 0.6 µm particles were

chosen as the primary seed keeping the 0.3 µm particles as an experimental. This size

particle has a lag time constant of 4.4 µs which does not affect its ability to accurately

follow the flow, and with a refractive index of 3.96 this particle will reflect enough light

to be measurable by the PIV system. Generation of the seeding particles is done through a

commercial atomizer as explained above. Two injection points are available on the jet

system. One is located at 9 ft upstream of the area of study and the second is located

about 12 feet from the area of study. Figure 3.9 shows the first injection point. This

picture shows that the nozzle injects the liquid directly into the flow. The second

injection point has a bend before the area of study which is thought to help in the mixing

of the particles within the flow.

27

Figure 3.9: First Injection Point with Nozzle

28

3.4 Equipment and Preparation of a pH Stabilized Dispersion of Ethanol

For complete instructions on how to prepare a pH stabilized dispersion of

Alumina in ethanol can be found in the literature (Wernet 1994). A 1% wt Aluminum

oxide suspension was prepared using the Mass Balance Equation:

ssOAl

OAl

VMM

wtOAl⋅+

=ρ

32

32%32 (3)

where M is the mass of aluminum oxide in grams, ρ is the density of ethanol, and V is the

volume of ethanol in milliliters. Using this equation 34 grams of aluminum oxide were

added per gallon of ethanol. The dispersion was prepared in segments of 200 ml where an

8.5% wt alumina dispersion was made and diluted to 1% with 800 ml of stabilized

ethanol solution. For a 200 ml dispersion of aluminum, the sonicator was used for 6-8

minutes with a 50% power level and a duty cycle of 1 second on and one second off.

Figure 3.10 shows a picture of the Sonicator transducer and the 8.5% wt. aluminum oxide

dispersion. Using this procedure, the pH stabilized dispersion of alumina in ethanol was

prepared and used.

29

Figure 3.10: Sonicator with pH stabilized dispersion of aluminum oxide in ethanol

30

3.5 Particle Image Velocimetry System

The Particle Image Velocimetry acquisition and processing system is a LaVision

PIV System. This system encompasses all of the acquisition and processing equipment

and controls. Images are acquired using a Red Lake 2000x2000 pixel CCD camera with a

90 mm Tamron lens. Control of the system is accomplished using a programmable timing

unit (PTU). A dedicated PC with Xeon Intel Processor houses all of the DaVis 7.0

software and essential hardware. The PTU controls the operation of a Dual Head Spectra

Physics 400 mJ Nd:Yag Laser. For this application, the laser was used in the second

harmonic (532 nm) and at 70% of its rated power. Image sets were acquired at a rate of

1Hz and the laser pulses were programmed for a 2 µs space between each image. The

laser sheet was produced using a set of cylindrical and spherical lenses. Great care was

taken in aligning each of the lasers and a dot card was used to calibrate the system and

focus the camera.

3.6 Experimental Methodology

Testing for this system was based on a couple of different goals. First of all, the

system needs to show that it can generate sub-micron Aluminum oxide particles.

Secondly, the new seeder should show that it is comparable in ability to the current setup

at the GDTL. In order to accomplish these goals a series of tests were run. The GDTL,

currently, uses an atomizer as its normal core flow seeder, and a fog generator for

ambient air. Mach 0.9 is the case for which this seed generating produces a well seeded

flow. To compare the aluminum oxide seeder with this case, an experiment at Mach 0.9

was done as well as two experiments at Mach 0.73 and 0.83. For the Mach 0.9 case, three

injector nozzles were tested numbers 14, 6, and 3. For the Mach numbers lower than 0.9,

31

the second injection point was used to see whether it improved uniformity in the seed.

Throughout each of these cases, the pressure within the tank was varied. At each of the

different Mach numbers and tank pressures, multiple PIV images were taken and

processed to compare with the baseline case. They were also studied to look at ways of

improving the new seeding technique. It is important to note that when implementing a

new seeding technique, an iterative scheme is needed to correctly seed a flow. Testing

using multiple Mach numbers, injection points, and tank pressures should show how each

of these variables improves the seeding technique.

32

Chapter 4

Discussion and Presentation of Results

4.1 Introduction

Using the aforementioned apparatus, as discussed in Chapter 3, and instructions to

prepare pH stabilized solutions of aluminum oxide in ethanol, experiments were run

according to Case 1, Case 2, and Case 3 by Mach number. Images were taken at each

Mach number and analysis was done by studying the images and qualitatively comparing

it to a well seeded baseline case of seeded with oil particles. Comparisons are based upon

the uniformity of the seed within the flow, a homogenous density, and also as the cases

progressed on how improvements changed the results. It is important to note that the

baseline images have the ambient air seeded and the test cases do not, so all of the

analysis and discussion focuses on the jet core.

4.2 Mach 0.9 Case

Figure 4.1a shows the Mach 0.9 baseline case and Figure 4.1b shows an example

of an image taken with the Aluminum oxide seeder at Mach 0.9. In each of these

pictures, the color bar is a measure of intensity reflected from particles and the scales in

millimeters are the coordinates within the actual flow. This experiment was run with the

33

tank pressure at 55 psig, with an injector nozzle number of 3, and with injection at the

first inlet. Tests were run using both the number 6 and number 14 fine spray nozzles, but

the flow rate produced an extremely dense seed that did not characterize the flow well.

Looking at the baseline case, the seed is uniformly distributed throughout the flow. This

is seen by the uniform intensity particles throughout the picture. The density is also very

uniform and within the established bounds because single particles can be seen. Looking

at the picture for the test case, the seed is not uniform and it has too high of density. The

uniformity and density are both seen in the middle of the flow where the seed appears as

a single white streak. This streak is either a very dense section of seed or liquid ethanol

spread within the flow. Throughout this experimental case, ethanol was dripping out of

the jet nozzle due to the extensive amount of seed being introduced into the jet.

Comparing the baseline core flow with that of the experimental case, the experimental

case does not show the entire jet, but instead the seed is concentrated towards the top of

the jet. For this case, the seeder is producing a seed however it is not uniform and too

dense to acquire accurate PIV data.

34

Figure 4.1a: Mach 0.9 Baseline case with Ambient Seeding

Figure 4.1b: Mach 0.9 Experimental Case Tank Pressure 50 psig

35

4.3 Mach 0.83 Case

Figure 4.2a shows the Mach 0.9 baseline case and Figure 4.2b shows an

Experimental image at Mach 0.83 and a tank pressure of 47 psig. In analyzing these

pictures, it is important to note the changes that were made between this case and the

Mach 0.9 case. In this case, the injection point was placed at 11 ft upstream from the flow

and it is separated by a bend to enhance the mixing characteristics of the flow. Also,

multiple data sets where taken corresponding to different tank pressures to see how

changing the injection pressure changes the way seed is distributed throughout the flow.

Looking at the case with a tank pressure of 47 psig shows extensive improvement over

that of the Mach 0.9 case. The bright spots surrounded by sections of dark show that the

density and distribution of the seed is still not completely comparable to that of the

baseline case. In this case, the seed was mixed better than in the Mach 0.9 case, and the

amount of seed injected was closer to the baseline case. The 47 psig case still does not

completely characterize the flow.

Figure 4.3 shows an image of the Mach 0.83 case experimentally seeded with the

aluminum oxide seeder at a tank pressure of 80 psig. This picture shows a great

resemblance to that of the 47 psig case. This case again shows a fairly good cloud of seed

throughout the picture, but it also shows places where bright spots exist without any

cloud around them. Each of these bright spots is a place where the seed density is too

high and the camera cannot pick up individual particles. The higher tank pressure shows

less bright spots. This means that the increase in injection pressure helped to make the

particles mix more evenly with the flow. This case showed some progress towards the

baseline case.

36

Looking at the Mach 0.83 case as a whole, a couple of important things were

learned during this experiment. First of all, the second injection point helps with mixing

the particles throughout the flow a great deal. In comparing this case with that of the

Mach 0.9 case, the seeding was better distributed and the density was at a place where

PIV measurements could be made. However, there are still discontinuities in the

distribution and density of the particles. It was also noted in this case that the needle

valve did not give good control resolution. The valve only needs to be opened a very

small amount for there to be enough seed, so control resolution is a problem in

controlling the amount of seed entering the flow. It was seen that this case was a great

improvement over that of the Mach 0.9 case.

37


Figure 4.2b: Mach 0.83 with Tank Pressure of 47 psig

38


39

4.4 Mach 0.73 Case

Figures 4.4 and 4.5 show the comparison of the baseline case with that of the

experimental case at a Mach number of 0.73 and tank pressures of 50 psig and 80 psig.

Within in this experiment, the second injection point was used. Looking at the 50 psig set

of images, the experimental case shows a fairly homogenous distribution throughout the

flow. This is seen in the cloud that characterizes most of the jet. Some parts of this do

show places of bright spots and dark spots. This shows that the seed is still not

completely mixing with the flow. However improvement has been made between this

case and the Mach .83 cases. The 80 psig case shows too much seed within the flow. In

this picture there are bright spots littered throughout the seeded region. These spots are

places where the particle density is such that the scattering cross sections are overlapping

and discrete particles cannot be seen. The 80 psig case also shows a non homogenous

distribution of particles within the flow. The particles seem to be seeded towards the

bottom of the jet and not as many in the top part of the flow. Comparing the two

experimental cases shows that increasing the injection pressure does increase the amount

of seed within in the flow. It does not, however, increase the seeds ability to mix

throughout the flow. It is also interesting to note that throughout this experiment, ethanol

would begin to drip out of the jet nozzle when the seed generator was generating too

much seed. This is interesting because is shows that the ethanol is not completely

evaporating, and it also says that some of the particles viewed within the pictures could

be ethanol droplets and not alumina particles.

40


Figure 4.4b: Mach 0.73 with Tank Pressure of 55 psi

41


42

Chapter 5

Conclusions and Recommendations for Future Work

5.1 Conclusions

After analyzing all of the data and studying the results, the following conclusions

can be made. First of all, the new seeder does produce sub-micron particles for laser

based flow diagnostics. All of the cases show that seed is being generated however, the

seeder does not produce a uniform seed and it is not easily controlled. Non-uniformity is

seen in all of the experimental figures where bright spots exist at many places and a

consistent particle cloud does not exist as it does in the baseline case. Controller

resolution is seen in all three of the experimental cases where changing the needle valve

position a very small amount had a drastic effect on the amount of seed introduced to the

flow. Secondly, these experiments demonstrate the ability to produce a pH stabilized

dispersion of Aluminum oxide in ethanol. Being able to produce the seeding material

easily had great affects on making the experiments possible and on how the experiments

ran. Thirdly, the seed generator showed its portability, adaptability, and ease of use

throughout the experiments. The portability was demonstrated with the cart, adaptability

was demonstrated during experiments by changing injector nozzles without draining the

43

tank, and ease of use is demonstrated by the Mach 0.9 being the first test ever run with

this seeder and it working.

While the seeder did generate seed, it did not generate a uniform particle density

number or homogenous distribution of seed throughout the flow. This was seen in all of

the experimental cases. The Mach 0.9 case had the worst distribution and seed density,

and as the experiments progressed the seed densities and distributions improved. The

pictures still did show that the improvements were not complete and that even in the

Mach 0.73 case, the seed exhibited many places with a high density and low density. Part

of the problem with the density and distribution was the lack of control resolution with

the system. The needle valve did give as much resolution in controlling the mass flow

rate of seed as was originally expected. This resulted in having very little control over

how much seed was introduced to a system. Ethanol condensation happened when the

seed mass flow rate was too high. This shows that evaporation was not occurring at a

high of rate as was needed to produce completely dry seed. This contributed to the high

density sections of seed within the images due to the agglomerations of liquid with

particles suspended in it. While the generator did not generate a perfectly seeded flow,

the experiments showed that the pH stabilized dispersion was a viable seeding solution

and the seeder worked for introducing the seed solution into the flow.

5.2 Recommendation for Future Work

After looking at the conclusions, a few of design and experiment parameter

improvements present themselves. First of all, experiments using the heated jet should be

run. This will better characterize the abilities of this system since its design was for a

heated application. Also, heating the jet will alleviate the problem of having ethanol

44

droplets within the flow. Secondly, the first addition to the system should be an air

atomizing nozzle. The currently used nozzle is a liquid atomizing nozzle which only

pushes a pressurized liquid through a small orifice. An air atomizing nozzle uses a second

stream of air within the nozzle to mix with the liquid before it goes through the orifice.

The main reason for using this type of nozzle is to enhance the mixing abilities of the

particles. By introducing air into the nozzle, the density of the injection stream is less, so

the air and dispersion mixture should mix with the flow more easily. This is also the same

injection system that Dr. Wernet used in the previous research (Wernet and Wernet

1994). This type of system will create a finer spray such that the system should create a

more uniformly seeded flow field. By using these recommendations, completely new sets

of results can be attained to see how well the seeder works within a heated jet and in

comparison to the previous research.

Adding a solenoid control valve presents a way to increase the control resolution

of the system. Controlling the system with good reliability was a problem that presented

itself throughout each of the cases. The needle valve that was used could only be opened

a very small amount to seed the flow with the right amount of seed. Opening the valve

only a small amount introduces a pressure loss into the system. The control of the

pressure is linked to that of the mass flow rate. Using a solenoid control valve would

allow separate control of both of these variables. A solenoid control valve works by

opening and closing a valve at a specified rate to produce a given mass flow rate without

affecting the pressure. The con to adding this is that it needs an electrical input with a

control system. It will, however, add much needed control to the system. Control should

allow the experiments to be

45

The final recommendation to improve the ability of the system to create a

uniformly distributed flow is to use multiple injection points. Currently, the seed is not

well distributed throughout the flow. Instead, the seed tends to be grouped within one

section of the flow and not in other sections. The main cause of this is that the injection

point is too close to the area of study. The heater does not allow the injection point to be

moved any further upstream. In order to increase the homogeneity of the seed, more

injection points should be added. With multiple injectors, mixing does not have to happen

throughout the entire flow but in each section of the injector. As a trial, a second injection

point could be added to have symmetry in the injection of the seed. Currently, the results

showed that the seed tended to be concentrated on the top or the bottom of the flow field

and a second injector would solve this problem. In all, running heated jet experiments,

adding air atomizing nozzles, using a solenoid control valve, and increasing the number

of injection points are recommendations to improve future work done with this system.

46

References

Melling, A. �Tracer Particles and Seeding for Particle Image Velocimetry,� Measurement Science and Technology, Volume 8, 1997. 1406-1416. Raffel, et al., M., Willert, C., and Kompenhans, J. �Particle Image Velocimetry: A

Practical Guide.� Springer-Verlag Berlin Heidelburg, New York, NY: 1998. LaVision GmbH, DaVis FlowMaster Software Manual for DaVis 7.0,

Gottingen, Germany, 2004. Samimy, M. �A Review of Planar Multiple-Component Velocimetry in High

Speed Flows,� 20th AIAA Advanced Measurement and Ground Testing Technology Conference, Albuquerque, NM, June 15-18, 1998.

Spray Systems, Inc. �Technical Specifications for Fine Spray

Nozzles.� www.spraysystems.com, 2004. Wernet, M., Wernet, J. and Skoch, G. �Demonstration of a Stabilized Alumina/

Ethanol Colloidal Dispersion Technique for Seeding High Temperature Air Flows,� 16th International Congress on Instrumentation for Aerospace Simulation Facilities, Wright-Patterson Air Force Base, 1995.

Wernet, M. �Fundamentals of Digital Particle Imaging Velocimetry,�

Course Notes, 2004. Wernet, M., �Fuzzy Inference Enhanced Information Recovery from Digital PIV

using Cross Correlation Combined with Particle Tracking,� SPIE Conference on Optical Diagnostics in Fluid and Thermal Flow, San Diego, CA, July 9-14, 1995.

Wernet, M. and Wernet, J., �Stabilized Alumina/Ethanol Colloidal Dispersion

for Seeding High Temperature Air Flows,� ASME Symposium on Laser Anemometry, Lake Tahoe, NV, June 19-23, 1994.

Wernet, M. �pH Stabilized Alumina Dispersions Instructions,� Notes, July 8, 1994.

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