SCHOOL OF SCIENCE AND ENGINEERING

Capstone Report CAPSTONE DESIGN

DESIGNING A SAND ABRASION AGING TEST FOR CSP SYSTEMS

By

Fatima Zahra EL MANSOURI

Fall 2016

Supervised by Dr. Asmae KHALDOUNE

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DESIGNING A SAND ABRASION AGING TEST FOR CSP SYSTEMS

Capstone Report

Approved by the Supervisor

_____________________________________________________

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Table of Contents List of Figures:  ............................................................................................................................................................  iii  

ABSTRACT  .................................................................................................................................................................  iv  

ACKNOWLEDGEMENTS  .....................................................................................................................................  v  STEEPLE ANALYSIS:  ...........................................................................................................................................  vi  

Social Implications:  ...............................................................................................................................................................  vi  Technical Implications:  ........................................................................................................................................................  vi  Environmental Implications:  ..............................................................................................................................................  vi  Ethical Implications:  ............................................................................................................................................................  vii  Political Implications:  ..........................................................................................................................................................  vii  Legal Implications:  ...............................................................................................................................................................  vii  Economic Implications:  ......................................................................................................................................................  vii  

1.   INTRODUCTION  ..............................................................................................................................................  1  1.1.   Sand Abrasion Accelerated Aging Test Technique:  .......................................................................................  1  1.2.   CSP Systems:  ...............................................................................................................................................................  2  

1.2.1.   CSP Systems, defined:  .....................................................................................................................................  2  1.2.2.   CSP mirror components:  .................................................................................................................................  2  

1.3.   Project Problematic and Steps Towards a Solution  ........................................................................................  4  2.   LITERATURE REVIEW  ................................................................................................................................  6  

2.1.   The Physics Behind Sandstorms:  ..........................................................................................................................  7  2.1.1.   Sand Properties:  .................................................................................................................................................  7  2.1.2.   Sand Transportation Modes:  ..........................................................................................................................  8  2.1.3.   Air Flow:  ..............................................................................................................................................................  9  2.1.4.   Wind Properties:  ..............................................................................................................................................  10  

2.2.   Sand Abrasion Tests:  ..............................................................................................................................................  11  2.2.1.   Air compressed Sand Blower:  .....................................................................................................................  11  2.2.2.   Another Type of Compressed Air Flow Sandblower :  .......................................................................  12  

3.   METHODOLOGY  ..........................................................................................................................................  14  3.1.   MIL-STD-810G:  .......................................................................................................................................................  15  3.2.   The sand blower design on SolidWorks:  ..........................................................................................................  16  3.3.   Air-Sand Simulation in SolidWorks:  .................................................................................................................  17  

4.   CALCULATIONS & INTERPRETATION  ..........................................................................................  20  4.1.   Two-Phase Flow Dynamic Analysis: Reynold’s Number :  .......................................................................  21  4.2.   Drag Force:  .................................................................................................................................................................  22  4.3.   Terminal Velocity of the particles:  .....................................................................................................................  25  4.4.   Effects of pressure on particle flow:  ..................................................................................................................  26  4.5.   Efficiency of the Sand Blower:  ...........................................................................................................................  27  

5.   LIMITATIONS:  ...............................................................................................................................................  28  5.1.   SolidWorks, as a first time user:  .........................................................................................................................  28  5.2.   Deciding on the criteria to be used for sand:  ...................................................................................................  29  5.3.   Average Efficiency:  .................................................................................................................................................  29  

6.   FUTURE WORK:  ...........................................................................................................................................  30  6.1.   Electrification of Sand and Electric Field:  .......................................................................................................  30  6.2.   Electrostatic Force, FES :  ........................................................................................................................................  32  

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References  ..................................................................................................................................................................  35  

List of Figures: Figure 1: Types of layers on different kinds of CSP mirrors ........................................................... 3 Figure 2: a) Without Reynold’s shear stress b) With Reynold’s shear stress ................................ 10 Figure 3: Sand Blower with pressure valve ................................................................................... 11 Figure 4: Sand blower with homogenization tube ......................................................................... 12 Figure 5: Sand blower apparatus composition ............................................................................... 13 Figure 6: Frontal dimensions of air duct ........................................................................................ 16 Figure 7: Complete Assembly of Sand Blower on SolidWorks ..................................................... 17 Figure 8: SolidWorks Design of Air and Sand Flow, in green are sand particles ......................... 18 Figure 9: Particles in yellow at Exit Face of Sand Blower ............................................................ 19 Figure 10: Drag Coefficient versus Reynold’s number curve ........................................................ 24 Figure 11: Separation points on particle producing form drag ...................................................... 26 Figure 12: Particles’ stopping point (in yellow) and their starting point at the injection whole (in

green) ...................................................................................................................................... 27 Figure 13: Charge-to-mass ratios of particles of different diameters in different wind velocities 31 Figure 14: Charge-to-mass ratio of particles given different velocities ......................................... 33

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ABSTRACT  In regions characterized with arid climatic conditions, sand soiling or abrasion of solar devices can occur. Sand abrasion is a sandstorm-specific phenomenon, which causes, sometimes-irreparable damage to solar components such as PV modules and CSP mirrors. In fact, sand carried by wind causes damage, which can lead to a drastic lowering of solar devices’ efficiency. This lowering is gauged in terms of the decrease in “transmittance and increase” of “the scattering of irradiation” (Klimm et al., 2015). In the literature, many sand-abrasion apparatuses have been developed to test the lifetime of solar devices. Sand blowers, which are the topic of this capstone design report, are the most commonly used. Aside from sand-blowers, wind tunnels, circulating chambers, as well as sand-trickling methods are also used to test the sand abrasion effects on solar components. Past work on sand blowers, indicates that most apparatuses are built horizontally, in a fashion that allows sand particles to enter a tube and exit the machine, with the aid of a fan, in a homogeneous way. In the previous semester, Spring 2016, Houssame Houmy, a student at Al Akhawayn University has developed a vertical sand blower to avoid the inertial effects on sand particles and the accumulation and adhesion of sand, an unnatural recreation of sandstorm conditions. My work, on the other hand, has recreated the apparatus’ same fundamental function but in a horizontal direction while generating a homogeneous distribution of sand. The latter design respects the MIL-STD-810G, or U.S military standards of sand abrasion testing while ensuring the solar component’s face is equally abraded by air-driven sand, so as to simulate desert-like windstorm environment. The first part of the report focuses on the different CSP mirrors that exist and the layers, which constitute them as well as the way they degrade with the continuous abrasion due sand particles. This phase’s purpose is to try and uncover the natural/real-life sand abrasion conditions in terms of wind velocity, particle concentration, and method of transport of sand and dust particles. In the literature, the physics behind sandstorms is subdivided into two main components: wind properties and sand properties characterizing sandstorm conditions. Moreover, the different sand blowers found in the readings are discussed and described in terms of shape, dimensioning, and efficiency. The second phase is the design of the horizontal sand blower in SolidWorks. The design includes the creation and assembly of different parts, the creation of quartz sand, SiO2, and the simulation of air flow and sand flow using the “Flow Simulation” toolbox in SolidWorks. The conditions to build the apparatus are those supplied by the US military standards of sand blowers, given a terminal velocity, concentration and mass flow rate of particles. Reducing the size of particles, of homogenization tube and lowering the velocity all contributed to the success of the project. The third phase of the capstone report showcases the results of the design, a free body diagram of the system, the forces applied to it, the terminal velocity of fall, the effects of particle to particle collisions, as well as the effect of pressure. Conclusions, limitations, as well as future work are parts that explore the constraints of the project in terms of knowledge of the topic and software. The future use of electrostatic or magnetic fields using coils are concepts that could help improve, drastically, the work done.

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ACKNOWLEDGEMENTS

First and foremost, I would like to thank my supervisor Dr. Khaldoune who provided help,

guidance and encouragement throughout these incredibly challenging past four months.

I would also like to acknowledge Mr. Rachid Lghoul who provided help and insight concerning

the design and simulation in SolidWorks as well as last minute ideas to conceptualize electric

field generation within the sand blower added in the future works section.

Dr. Alj was also a very responsive as capstone coordinator and provided the technicalities of

writing and developing the capstone design project in a scientific and conventional manner.

Also, I would also like to recognize Al Akhawayn University for providing the necessary

resources, as far as software is concerned, as well as spaces for studying and helpful staff.

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STEEPLE ANALYSIS:

SOCIAL IMPLICATIONS: The costs incurred on society: if the module’s irradiance is diminished/if it is damaged, it must be

replaced/recycled. Studying CSP systems impacts positively on society. In fact, any study related

to the renewable energies field has a detrimental effect on building a healthy environment for the

well being of individuals.

TECHNICAL IMPLICATIONS: The technical aspect of the project is about creating a way, different from those already existing,

to test the age of CSP systems in arid regions like deserts. In fact, sand in the surrounding areas is

the main cause of the spoiling of the CSP components. On a short-term basis, it hinders the

efficiency of the mirrors’ reflecting properties. In extreme cases, it might incur irreparable

damage to the cell itself. In fact, the technical aspect of the project is figuring out the extent of the

damage sand can cause. Thus, it is also a useful way to figure out not only the lifetime of

reflective mirrors but also the way sand abrades these components. It is also a recapitulative

research on the science behind sandstorms and Aeolian sand transport within a dry and controlled

medium.

ENVIRONMENTAL IMPLICATIONS: The module must be thrown away, or even recycled for the purpose of environmental protection.

Also, figuring out how and how fast solar mirrors age can help find ways to counteract their

aging process. It can help create a sustainable clean electricity model.

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ETHICAL IMPLICATIONS: Since the project studies the lifetime of solar CSP systems, which help save the environment

from harmful emissions and global warming, it is thus, an ethical project to undertake.

Furthermore, the project is based solely on the MIL-STD-810G, U.S. military standards which

predefines the procedures to take to ensure proper, safe and life-like results.

POLITICAL IMPLICATIONS: The political aspect of this project lies within the concept of green, sustainable energy

itself. Supporting environment-friendly measures is a factor in a country’s international political

recognition. For example, Morocco, with its project Noor has been quoted on an international

level for its efforts to create a sustainable environment-friendly and energy-producing model. In

fact, environment-friendly measures increase the popularity of the ruling parties themselves

within a country.

LEGAL IMPLICATIONS: The MIL-STD-810 G standards to be used have been created to ensure the safety of the

user of the machine to be modeled (in theory or later on as a real-life full on machine).

ECONOMIC IMPLICATIONS: Green sustainable energy is a factor in economic growth. It promotes the growth of job

offerings/opportunities. Job creation is one of the main goals in any country, since it is the

driving force behind a society of consumerism, of more revenue generation and a rise in national

GDP as well as various other sectors in the economy.

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1. INTRODUCTION

1.1. SAND ABRASION ACCELERATED AGING TEST TECHNIQUE: Once a solar product or component is created, aging tests can be performed on it to assess its

lifetime. The product can be subject to several environmental conditions that deteriorate its

performance such as temperature, humidity, sand abrasion, or even corrosion. Hence, this part

will focus on sand abrasion accelerated aging techniques. The latter are used to test solar

components present in regions of arid climates. Thus, they are dependent on the region in which

testing takes place. Since, there are over 90 deserts in the world, each with different sand particle

composition, a systematic way to test CSP mirrors would be by following the general guidelines

and recommendations provided by the U.S military standards for sand abrasion aging tests (MIL-

STD-810G, Method 510.5, 2008). These standards specify the concentration of sand that should

be used, the size of the sand grains, humidity and pressure, which should be kept constant, as well

as the duration of the test, for an optimal, yet generic testing procedure. However, the general

guidelines for this standard test still need to be regulated since they are deemed “too aggressive

for reflective materials and the testing conditions need to be modified.” They need to be “in

sophisticated sand storm simulating chambers able to control wind velocities and particle

concentrations” (Sutter et al., 2013). Hence, the design takes into consideration only some of the

guidelines that are fit to generate a reproducible, simple and effective sand blower.

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1.2. CSP SYSTEMS:

1.2.1. CSP Systems, defined: CSP systems are usually subdivided into three main systems: “a solar field, a thermal

storage unit and a thermal load.” The main application of CSP systems is to centralize incoming

solar irradiance and to transform it into heat. Moreover these systems are categorized under two

main classes: The first one is “’line focus.’” The incoming beams of solar energy are focused into

a “linear receiver” called “parabolic trough collectors.” The second system, “’point focus,’”

collects the solar energy into a “point or a disk” called “solar towers” or “parabolic dish systems”

or “solar furnaces.” As opposed to the first system, the point collection system can focus a higher

amount of solar energy and thus, it is more efficient in generating higher temperatures. It is used

in a variety of fields such as heating or cooling in AC systems as well as refrigerator cycles.

Additionally it has been developed to desalinize water as well as treat contaminated water or to

generate hydrogen. It is the most commonly used system as opposed to Linear Fresnel power

generators, but it has yet to surpass photovoltaic innovation, which is its fiercer rival (R. Martin,

2011).

1.2.2. CSP mirror components:   An important criterion for selecting materials for CSP mirrors is their capacity to absorb

direct normal irradiance (DNI), or the solar irradiance. Classical CSP mirrors are made up of 4

coats as shown in the first picture of the figure below (R. Martin, 2011) (Figure1):

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Figure 1: Types of layers on different kinds of CSP mirrors

Indeed, thick glass mirrors are constituted as follows (Fig. 2b) On the rear of an “iron

float glass,” by “wet chemistry” processing, silver coating is added. The glass’ characteristic

width is between 4 to 5 mm. Another layer, of “multilayer paint” this time is sprayed on the silver

to prevent it from quickly abrading. In classical systems, copper is usually the coating layer,

which prevents the oxidation of silver. The function of the “multilayer paint” has been to replace

the costly copper while preventing both degradation and oxidation of silver. Not only are these

types of mirrors more expensive, but they also reflect less irradiance ( >2 mm of “nominal

hemispherical reflectance”) than thin glass mirrors for instance.

The latter have a lower mass, thus cost less, but are more prone to breaking and damage.

“Aluminized reflectors,” like its glass-based counterpart mirrors is also constituted of 4 layers.

The reflective aluminum layer (Fig 2c) is set up using “electrochemical methods” is protected

with a layer of silicon dioxide, which prevents it from rusting and gives it overall protection

against abrasion. This type of mirrors is less costly than the traditional glass mirrors.

Additionally, their mass is lower, they are “flexible” and tough to shatter.

The last type of mirrors (Fig. 2d) is the “silvered polymers-”based (last picture on the

above illustration). It is also built of 4 coats. At the very top of the mirror a “base silver reflector”

coated with a “bonding layer” made of “poly-methyl-methacrylate (PMMA).” To the latter

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another coating preventing damaging can be added up. This type of system is less costly than the

previous ones, more effective, as well as flexible and difficult to fracture.

1.3. PROJECT PROBLEMATIC AND STEPS TOWARDS A SOLUTION   The aim of the project is to do a theoretical design and simulation on the 3D building

software, SolidWorks. Since sandblasting technology is not truly mature yet, the design described

in the later chapters fixes the latter problem. Thus, the goal is to recreate sandstorm conditions

within a simple apparatus that is low-cost, effective in abrading the tested specimen while

homogeneously distributing the sand particles within the designed apparatus while overcoming

inertial effects, which could drag down particles and create an uneven distribution at the exit of

the sand blower. Moreover, the velocity of the air blown out of a centrifugal pump should be

right under the velocity for which particles remain in suspension. The latter velocity, called the

terminal velocity is calculated in order to obtain a homogeneous suspension. The two-phase

model is to be taken as a continuum in order to facilitate the interpretation of the results. CSP

mirrors are important and their locations (deserts mainly) are inhospitable environments. It is

crucial to test a product once created to see how it reacts to its environment and how long it can

be used for.

A simulation is then made, using the “Flow Simulation” parameterization of SolidWorks.

Since quartz sand is not found amongst the materials list in the software, it is imperative to create

it. The data entered to describe silicon dioxide was found in various articles and websites

allowing a precise and optimal recreation of sandstorm conditions. Moreover, the sizing and

concentration of particles, through a series of trial and error is selected based off the success of

the simulation and the visual distribution of the particles. Since specular reflectance of solar

devices to be tested needs to decrease in an accelerated fashion, hence the title “accelerated aging

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test,” it is important to attempt compliance with U.S military standards but at the same time

personalize these standards according to the design’s implications and needed outcomes.

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2. LITERATURE REVIEW

The aim of this chapter is to introduce the science behind sandstorms. First, it explains

how particles travel in a sandstorm, the forces acting on them and their modes of transportation.

Then it focuses on the physics and equations modeling the wind during a sandstorm. The

literature review then summarizes the different types of sandblasting designs found in the various

articles read and how, in turn, they can be related to the future design.

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2.1. THE PHYSICS BEHIND SANDSTORMS: 2.1.1. Sand Properties:

  Sand varies from a geological region to another; it is divided into four categories: “gravel,

sand, silt and clay.” Clay sizes, the smallest of the four categories, range between 1 µm to 4 µm.

Clay is followed by silt and gravel. Silt diameters range from 4 µm to 62.5 µm, while gravel is

between 2 µm to 64 µm. Then, fine sand is between 62.5 µm and 200 µm. Last but not least,

coarse sand diameter sizes range from 0.5 mm to 2 mm.

In order to study the physics of traveling sand, it is crucial to, first, list the forces acting

on a single sand particle. One particle free falling submits to two distinct forces. The first one is

gravity, which is directed downwards. The second one is the air resistance or drag force, which

acts upwards, in the opposite direction of the particle’s downward movement. Gravity of the

particle obeys the following formula, if we consider the particle to be a sphere of volume,

𝒱 =  43   .𝜋. 𝑟!  

!

So, 𝑤 = !!.𝜋.𝐷!. 𝜌! − 𝜌 .𝑔

Where ρD is the particle density, D is the particle’s diameter and (ρD – ρ) is the buoyancy force of

the sand particle in the airflow. The drag force on the other hand is contingent upon the shape of

the particle, the area exposed to airflow and its velocity in the wind stream. It is equal to:

𝐹! =  𝐶! .𝜋. 𝜈.𝐷. (𝑢!   − 𝑢)

Where 𝐶! =!"!"+ !

!!!" !.!+ 0.4 is the drag coefficient and 𝜈 is viscosity and u is velocity, and

Re is Reynold’s number.

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Once the value of gravitational force and air resistance reach the equilibrium state, the

particle’s speed reaches a constant value. The latter is called the “terminal velocity of fall”

(Bagnold, 1954). The latter velocity differs depending on the size of the particle. It is crucial for

the project’s end results to know this particular criterion. Indeed, its value represents the velocity

at which the particle will remain in suspension. As a matter of fact, if VY, the “vertical

component of the wind velocity” is lower than the final velocity of the grain, the particle will

continue to fall, and if the component is greater than the final velocity, then the particle will

remain in the air. It must be duly noted, “turbulences and vortices in the wind, collisions between

particles, and the electrostatic charge are all important factors regulating the motion of sand and

dust particles inside of an airflow.

2.1.2. Sand Transportation Modes:   There exist three types of transportation modes: “saltation, creeping and suspension.”

Saltation is generally less than one meter high above the soil. The particles describe a parabolic

direction. The movement starts from either grains impacting each other or winds creating enough

force to lift grains off the sand bed. There are two types of wind velocities in saltation: fluid

threshold and impact threshold. The latter is when the flight of the grains becomes self-sufficient.

Once past wind threshold, the particles on the surface begin to spin. While fluid threshold is a lot

greater, both velocities are contingent upon the size of the particles (R. Martin, 2011, p.13).

Unlike saltation, it is because of saltation particles having lost energy due to friction that

particles stay on the sand bed. Finally, the third way of particle travel is suspension. It concerns

particles of smaller sizes such as “clay, silt and also fine sand, but not medium or coarse sand.”

Once the “upward eddies currents” have a higher velocity than the “terminal velocity of fall” of

the grains, suspension occurs. These particles then adopt the same velocity magnitude as the air

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that suspends them (R. Martin, 2011, p.13). The design, however, will attempt to recreate

saltation and suspension modes of transport phenomena only.

2.1.3. Air Flow:  Basic Navier-Stokes equations can be used to model the wind field that is near the surface of

soils. However, since atmospheric winds are turbulent, to the Navier-Stokes equation will be

added the Reynold’s number, an equation called “Reynold- average Navier-Stokes” (Liu et al.,

2007,p.261):

𝜌.𝜕𝑢𝜕𝑡 =  −𝜌 𝑢.∇ .𝑢 −  ∇𝑝 + 𝜈. 2.𝑢 +  ∇. 𝜏 +  𝑓

In the previous equation, 𝑢  represents wind velocity, ρ the density, p for pressure, ν is the

viscosity of the air, while 𝑓   expresses external forces acting on the airflow and 𝜏 is the Reynold

shear stress which stands for the turbulent movements generated in the atmospheric layers. Its

expression is:

𝜏 = 𝜌.𝑑𝑢𝑑𝑦 .

𝑦!  𝐶!

In this equation, y is the distance of separation from the sand bed, while Ck is a constant

called the Von Karman constant where Ck =0.4.

Surrounding each sand grain is “vorticity confinement.” It is an external representative

force of sand interactions. The latter can be simulated using Reynold’s shear stress. Its presence

confers the model a more realistic sandstorm-like representation. The upcoming figure (Fig.2) is

a representation of the sand particle within the airflow with and without Reynold’s shear stress

(Liu et al., 2007,p.261):

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Figure 2: a) Without Reynold’s shear stress b) With Reynold’s shear stress

On a side note, if grains were not integrated into the sandstorm phenomenon, the wind

flow could be examined as an incompressible fluid, meaning:

𝛻 ·  𝑢 =  0

 

2.1.4. Wind Properties:  Winds are responsible for the detachment, transport and settlement of particles of sand during

sandstorm events. Although wind speeds are non-constant with regards to magnitude and

direction, using a logarithmic representation, it is feasible to report on the prevalence of the speed

of wind since it is a turbulent phenomenon. Velocity called V, as well as distance above the soil,

Z can be represented using the following formula (R.L. Martin, 2011, p.12):

𝑣 𝑧 =234 .𝑉 ∗. log

𝑧𝑘    

In the previous equation, V* represents the velocity potential or “drag velocity”, k is a constant

representing roughness. It is crucial to account for the fact that the roughness coefficient is

contingent upon the measurements of soil irregularities. If the soil has immobile sand particles,

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𝑘 = !!"

such that D is the diameter of the particles. If soil composition is altered, going from

extremely fine micro particles to larger ones, the above velocity will become lower.

2.2. SAND ABRASION TESTS:

2.2.1. Air compressed Sand Blower:  The first type of horizontal sand blowers encountered in the literature is one featuring an air

compressor, a stainless steel circular nozzle and a “vibrating hopper” through which sand

particles enter the apparatus. The stainless steel nozzle is “100 mm” long and has an 8 mm

“diameter.” The apparatus runs at room temperature and the pipe is exchanged with a new one

each time about “5000 g erosive particles had been blasted” (Gachon et al., 1999). The particles

are projected through a convergent then divergent cone. The space interval, L, as seen in the

figure below, between the nozzle and the sample “was kept constant at 30 mm whatever the

angle.” The latter distance permits the junction between “the convergent cone and a normal target

to be a circle of 10 mm in diameter” (Gachon et al., 1999). Figure 3 shows the design of the

apparatus.

Figure 3: Sand Blower with pressure valve

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2.2.2. Another Type of Compressed Air Flow Sandblower :

Figure 4: Sand blower with homogenization tube

In the next case (Karim et al, 2015) in Figure 4, the sand blower is described as an “open

circuit machine.” The sand crosses a nozzle having a diameter of 1.5 cm thanks to an air

compressor. Air is homogenized through a homogenization tube in order to ensure the particles

are well distributed once they attain the specimen. The three variables of the experiment are: the

exit velocity of the particles (once they hit the sample), “the mass as well as the nature of the

erosive particles.” Velocity of the air depends on its pressure and L, the distance between the

expulsion point of sand (the extremity of the nozzle) and the sample tested. Pressure in this case

is a constant, with P = 0.5 bars. The latter value has been obtained after several experiments

testing the optimal air speed versus pressure in order to appropriately homogenize the particles

within the tube. Making the pressure constant will render the velocity of air contingent upon

solely on the distance, L. Velocity’s different values, at different positions of the machine are

obtained using an anemometer. The sample holder is rotated horizontally, in order to give it an

angle ranging from 0° to 90°. The real-life image representing the described apparatus is

illustrated in the figure below. Unlike the first apparatus, the compressed air flow inlet comes

right after the particles insertion point. Figure 5 is a real-life picture of the said sand blower.

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Figure 5: Sand blower apparatus composition

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3. METHODOLOGY

The aim of this chapter is to provide an overview of the steps undertaken to obtain a

perfectly homogeneous distribution of sand particles within the sandblasting apparatus. First, the

chapter lists the U.S. military standards that are recommended to perform a sand abrasion test.

Then, the chapter moves to the steps taken in SolidWorks to assemble the parts and to create

quartz sand. Then, it discusses the simulation in SolidWorks and its successful outcome.

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3.1. MIL-STD-810G: The sand blower can be designed through a set of U.S. military standards, which have

been specifically established for the purpose of such endeavors. The sand blowing procedure

consists of determining the effects of sand on the reflective materials of CSP systems. In fact, this

accelerated aging test can help determine the lifetime of CSP light absorbing components, which

are consistently exposed to sandstorm conditions.

Since the purpose of the accelerated aging test of CSP light-absorbing components is to

simulate sandstorms conditions, it must simulate the wind speeds associated with storms as well.

As a base case, a wind velocity equating to 18 m/s can be responsible for lifting large sand

particles off the sand bed. In fact, wind velocities can reach up to 29m/s. Thus, the appropriate

range for the accelerated aging test is 18m/s-29m/s. Thus, the velocity at the exit of the sand

blower must be: V2 = 18 to 29 m/s. MIL-STD-810G furthermore specifies that between the sand

entrance and the sample locations, there should be a distance of 3 m. If the particles were to

achieve such velocities in under a shorter distance, then it can be used for the design of the

blower.

The sand particles used must follow a certain set of criteria. The type of sand used must

be silica sand. The sample must contain 95% SiO2 particles. The sand’s structure must be “sub-

angular,” with a “mean Krumbein number range of 0.5 to 0.7 for both roundness and sphericity.”

The sample particles should have “a hardness factor of 7 mhos.” Since there is a plethora of

deserts and possible sand samplings, the MIL-STD-810G standards facilitate the task by

recommending that 90% of sample to be 150µm to 850µm in diameter and the remaining 10% to

be at least greater than 600µm. The sand’s used concentration must be 0.18 g/m3 with a tolerance

of +/- 0.2 g/m3 (MIL-STD-810G, Method 510.5, 2008).

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3.2. THE SAND BLOWER DESIGN ON SOLIDWORKS:   The sand blower design consists of a centrifugal blower, which delivers air at high

pressure, generating a pressure gradient between the duct’s inlet, and the duct’s outlet. Thus, the

particles are incited to move within the empty duct. The latter’s inlet is connected to the

centrifugal blower, allowing air to pass though the duct. Parts design and assembly in

SolidWorks has been taught and covered throughout the course of Computer Aided Engineering.

The duct’s width is 0.13m, its length is 2 m and its height is 0.2 m. It is designed in silicon

material. At the very top of the tube, a whole is placed. Its diameter is: 64.92 mm and it is placed

632.95 m away from the entrance of the duct or the air blower (the part attached to the fan). The

picture below (Fig.6) illustrates the design of the rectangular duct and figure 7 illustrates the

whole assembly.

Figure 6: Frontal dimensions of air duct

Moreover, the duct is mated with a centrifugal blower operating at 4 m/s. The picture below is a

screenshot of the complete sand blower.

 17  

Figure 7: Complete Assembly of Sand Blower on SolidWorks

3.3. AIR-SAND SIMULATION IN SOLIDWORKS: The next step after designing the apparatus on SolidWorks is to simulate sandstorm

conditions inside the duct of the blower. Flow simulation in SolidWorks consists of first creating

lids to cover the areas through which the flow will be coming in and out, as well as the sand

injection whole. Moreover, since sand particles are not readily available on the software, the next

step was to design quartz sand particles of chemical formula SiO2. The characteristics of SiO2

entered in the material design toolbox are: the elastic modulus, Poisson’s ratio, shear modulus,

density, tensile strength, compressive strength, thermal expansion coefficient, thermal

conductivity and specific heat at 20ºC.

Then, a “Wizard” window appears in which I select the meshing, the initial conditions of

the flow, temperature set at room temperature as well as environmental pressure. The inlet of the

airflow is given an initial velocity V= 4m/s and the diameter of the particles is set at 100 µm,

with a flow that is both turbulent and laminar as well as fully developed. The mass flow rate of

 18  

the particles is set at 1kg/s. The bottom layer of the duct is set as “absorbent”, such that it doesn’t

reflect the particles that get deposited on it, otherwise these particles would collide with each

other at a greater rate, creating a higher turbulence. The acquired result is captured in the

following picture (Fig.8).

Figure 8: SolidWorks Design of Air and Sand Flow, in green are sand particles

The exit face is practically covered homogeneously by the particles as can be seen below in

figure 9.

 19  

Figure 9: Particles in yellow at Exit Face of Sand Blower

 20  

4. CALCULATIONS & INTERPRETATION

This part lists all the calculations that help explain the phenomena that happen inside the designed sandblasting machine. This chapter comprises 4 distinct parts. First Reynold’s number

of the sand particles is computed only to find the sand flow to be laminar. Then, Reynold’s

number is computed for the airflow, which was found to be turbulent, a key characteristic of

sandstorms. Then the drag force is computed and equated to the gravitational force to find the

terminal velocity of fall of particles. A difference in pressure between the front and the back of a

particle help explain the motion of the particle within the designed apparatus.

 21  

4.1. TWO-PHASE FLOW DYNAMIC ANALYSIS: REYNOLD’S NUMBER : In order to better understand the type of flow involved in the upcoming analysis, one must first

compute Reynold’s number, a measure of the type of fluid flow. As seen in the Fluid Mechanics

and thermodynamics classes, when Reynold’s number is greater than 2300 then the flow is

turbulent, while a Reynold’s number less than 2300 is characteristic of a laminar flow. A single

grain of sand is taken as a sphere of diameter Dp, which is taken throughout all the calculations as

100 µm as convened by the U.S military standards for the building of sand blowers. It is flowing

within the confines of a gaseous flow (air in this case), with a fluid density of ρ f = 1.225 kg/m3, a

velocity, u∞ = 3.218 m/s representing the average flow of the particle upstream from the flow,

dynamic viscosity µ =1.983* 10(-5). Reynold’s number formula is:

𝑅𝑒 =   !!  .!!  .!!!

= 19.879 < 2300, thus the particle flow is laminar. However, since Re > 1,

viscous forces will dominate inertial forces. Reynold’s number of the airflow can be calculated

using the following formula, using the hydraulic diameter of a rectangular duct

𝐷!  =!.!!"#!!.!"#$!!!"#!!!!"#$!

= 0.1575 m

𝑅𝑒  !"#  =!!  ∗  !!"#$%  ∗  !!

!  = (1.225*4*0.1575)/(0.00001983) = 38,937.02.

Thus the flow of air can be considered as being turbulent. Moreover, a crucial force to be

taken into consideration is the drag force, or the force air has on particles. Force analysis such as

the drawing of free body diagrams as well as the computation of single forces using Newton’s

Second law of motion has been covered in the classes of statics and dynamics.

 22  

4.2. DRAG FORCE: To fully understand the impact the fluid has on a particle, it is important to

contemplate the effects of the drag force on the particle. In order to compute the latter force, it is

crucial to, first, start off by setting the equations of velocity and pressure of the air surrounding

the particles. As previously stated, the particle will be considered as a sphere, for simplicity

purposes. In an incompressible Newtonian fluid, which is air, the velocity and pressure are best

expressed in terms of the following continuity equation:

In this equation, ux, uy and uz are all components of the fluid’s velocity. The x-component of the

latter equation is given as:

In the latter equation gx is the x-component of gravity. In order to compute the force

applied by the fluid on a single grain of sand, one must understand that there are two distinct

components to the drag force: one is tangential while the other is normal to the surface of the

fluid. These components are as follows:

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In the latter equation, u∞ is the fluid velocity far from the particle. However, these

equations are representative of a creeping flow (Re 2.6

The latter value means that Stokes’ law doesn’t comply in computing the terminal velocity of

the particles. Additionally, since particles are no larger than 0.1 µm, then it further confirms

that Stokes’ law is not adequate for computing the terminal velocity of the particles.

Thus, the appropriate drag force for the suggested model is computed using the following

equation:

𝐹!  =   𝐶!  ∗   𝜋 ∗  𝐷!!

4 ∗12  ∗  (𝜌!  ∗  𝑉!  ))  

CD is the drag coefficient and Vf is the initial velocity delivered by the centrifugal pump.

 24  

Vf = 4 m/s. While CD is computed using:

𝐶!  =   (!"!"  +   !

!"!.!!) = 2.6986

CD can also be found using the CD versus Reynold’s number curve. The following is an

illustration of how CD can be founded directly by reading from the graph as well.

Figure 10: Drag Coefficient versus Reynold’s number curve

Thus, the drag force exerted by the air on the particle is:

FD = 2.15189*10-6 N, or FD = 2.1518 µN.

As previously mentioned, the drag force or body force, Fg due to inertial effect is computed as:

𝐹!  =!  ∗ !!!

!𝜌! − 𝜌! = 1.36054*10-8 N or Fg = 0.013605 µN

 25  

In this case, Fg < FD, thus the particles will not reach the bottom of the rectangular duct since air

forces outweigh gravity.

4.3. TERMINAL VELOCITY OF THE PARTICLES:

In order for the particles to levitate, by outweighing gravity, and come out homogeneously

while at the same time mixing with air, drag force must equal the force due to gravity. In other

terms, Fg = FD. By equating these two terms, Vt can be obtained as follows:

!  ∗ !!!

!𝜌! − 𝜌! = 𝐶!  ∗   𝜋 ∗  

!!!

!∗ !!  ∗  (𝜌!  ∗  𝑉!  ))

𝑉!  =  𝐷! ∗ 4 ∗ 𝜌! − 𝜌! ∗ 9.81

3 ∗  𝜌! ∗ 𝐶!  

ð Vt =1.023 m/s

Thus, in order for the particle to stay within the air streamlines and not accumulate in the

bottom of the tube, it should ideally have a velocity of Vt = 1.023 m/s. If the velocity of the two-

phase flow is less than Vt then it will float above the centerline of the duct. However, if the

velocity is greater than Vt, then the particle will succumb to inertial forces and accumulate on the

bottom of the tube.

The diameter of the particles also plays a great role in the adhesion to fluid streamlines of

the particles. A larger diameter would result in less particle levitation, while a very small

 26  

diameter would also result in particles deviating from fluid streamlines. A nanoparticle, for

instance would completely deviate from streamlines as opposed to a micro particle.

4.4. EFFECTS OF PRESSURE ON PARTICLE FLOW:

Since Reynold’s number is approximately 19.879 (as previously computed), the boundary

layer will segregate from the particle. At the shoulders of the spherical particle, as can be seen in

the picture below, there are 2 “separation points,” the pressure, p1 at both these segregated points

is roughly the same and throughout all of the surface connecting these two points. While at the

back of the sphere, there is a much higher pressure such that p2 > p1. This pressure differential is

one of the main drivers of the particles in the flow, conferring them the necessary force to move

through the flow directed from left to right. The latter is what is known as form drag.

Figure 11: Separation points on particle producing form drag

 27  

4.5. EFFICIENCY OF THE SAND BLOWER:

Solidworks can show all the particles that have made it out, labeled “opening” of the tube

and those that didn’t, labeled “absorbed” as they accumulated into the duct. The number of

particles that came out of the exit surface is 177, while the number of particles that didn’t come

out and fell into the tube is 184. The volume of the duct is Vduct =0.052 m3. In one cubic meter,

the volume out is: 177/0.052 = 3519.230 particles/m3 while the number of particles per m3 that

didn’t make it out of the duct is: 184/0.052 = 3403.846 particles/m3. The total particles/m3 is

6923.076. Figure 12 illustrates the starting point of the particles (at injection hole) in green and

where the particles stop (in yellow). In fact, not all particles come out of the tube, some fall inside

the duct before making it out.

Figure 12: Particles’ stopping point (in yellow) and their starting point at the injection whole (in green)

Thus, the efficiency of the sand blower designed can be computed as:

 28  

ηblower = 1- (Output particles/m^3)/(Total Input particles/m^3) *100

ηblower = 1- (3519.230/6923.076)*100 => ηblower = 50.83%

5. LIMITATIONS:

Chapter 5 lists the limitations encountered in the making of the project. Some of them

were overcome successfully while others couldn’t be outdone. Some work, including electrostatic

forces that can be used within the blower to obtain a more physically achievable outcome is

further explained in the next section.

5.1. SOLIDWORKS, AS A FIRST TIME USER:   Throughout my experimentation with SolidWorks, it took almost the whole semester to

learn how to build the apparatus and to simulate sand and air flowing through its confines.

However, my incessant attempts yielded positive results that could allow for future building of an

actual prototype and even an actual sandblasting machine. The errors committed in SolidWorks

were mainly related to the assembly and dimensioning of the parts. If a part were to be only a

millimeter bigger or smaller than it should, the simulation wouldn’t work. Thus, I had to learn the

hard way to give proper dimensions to the different parts. Once the gaps within the parts were

properly filled, the simulation worked perfectly.

I also struggled to actually draw the apparatus, I thought of making something out of the

ordinary but finally decided on a simple model that can be cost-effective as well as be built in the

future.

 29  

5.2. DECIDING ON THE CRITERIA TO BE USED FOR SAND:   A major part in the semester went into experimenting with different sizes of sand and

different air velocities in order to acquire a perfectly homogeneous flow. Once the velocity of

4m/s for air was found (by trial and error), and once the size of the particles was lowered way

below the U.S standards for sandblowers (set to 100 microns instead of 500 microns), the

machine simulated sand perfectly. Moreover, the mass flow rate calculated using the standards

was also unrealistic. Thus, once I set it to be 1 kg/s, the sand distributed along the blower’s duct

evenly.

5.3. AVERAGE EFFICIENCY:   The efficiency of the blower as computed in the above section was almost 51%, meaning

there is still room for improvement. The blower doesn’t eject all of the particles out of the

machine, as seen in previous sections, some of them fall down into the tube before reaching the

duct’s exit. Indeed, inertial effects are the main cause of such a phenomenon. To counteract this

effect, another proposed solution is discussed throughout the next section.

 30  

6. FUTURE WORK:

The aim of this chapter is the show that the sand blower could have been designed using

electrostatic forces. Since the particles’ friction within the duct generate negative charges, an

electric field or some sort or electrification by induction could be used to negatively charge the

duct so as to repel the particles from falling inside the conducting duct and to obtain a perfectly

homogeneous flow, and a higher efficiency for the sand blower.

6.1. ELECTRIFICATION OF SAND AND ELECTRIC FIELD:   Under average wind velocities, electric fields of more than 160kV/m are created when

saltation phenomenon occurs. These electric fields (“E-fields”) aid the raising of sand grains of

air from the sand bed. Particle impact produces negative charging for small grains, while it causes

larger ones to become positively charged. “Asymmetric rubbing” is the phenomenon, which, by

friction of small grains over large ones, induces the removal of electrons from the large grains

onto the smaller ones. Thus, since saltating clouds contain small particles, they will necessarily

be endowed with negative charging (Kok & Renno, 2008).

Based on previous experiments, formulas for the post-collision charges of large and small

grains has been deduced as (Kok & Renno, 2008):

qs’ = C1 (qs + qL) – C2 Δɸ and q’L = (1-C1 ) (qs + qL) +C2 Δɸ In the previous equation, qs’ and q’L represent post-collision charging of small particles

and large ones respectively and and qL, qs charges before impact. Also, Δɸ is the “’difference in

contact potential’” and C1 and C2 are dependent on “’mutual capacitances of the two particles’”

(Kok & Renno, 2008).

 31  

The sand surface is appraised as unlimited and positively charged. Thus, if particles are

negative, they will become attracted to the soil, which will stop their flight in the air. On the other

hand, if particles are positive, they will become repelled to the surface and keep saltating. The

electric field perpetrated by the surface is:

E is the electric field generated by the surface, ε is permittivity of air,“the space charge density”

is also included, while 𝛔 is the “surface charge density of soil.” Permittivity is a parameter, which

intervenes in the electric forces or in the middle of electrons. According to several previous wind

tunnel lab experiments, the following table of charge/mass ratios could be obtained (Kok &

Renno, 2008):

Figure 13: Charge-to-mass ratios of particles of different diameters in different wind velocities

The last experiment by Zhang et al. seems to fit the sand distribution and velocity range allowed

by U.S military standards.

Moreover, an in-depth investigation in the electrification process reveals that because of rainfall,

sand particles absorb water and are more apt to being electrified. In, fact the added moisture in

 32  

the air and the electrified atmosphere will provoke the ionization of the H2O molecule that gets

absorbed by the sand particles. The elevation of pH will produce silicic acid as a result of the

reaction of SiO2 particles with H20. The acid will then split into two categories of ions: H+ and

SiO4(4-). After particles collide, the level of friction rises, thus the temperature of sand grains

increases as well. However, the temperature will depend on the size of these particles: Small sand

grains will have a higher temperature than larger ones. Because H+ ions are of greater

proportions than the negative OH- ions, the positive ions will migrate towards the colder area of

the flow, thus, making the larger particles positively charged, while the smaller particles will

have a tendency to be negatively charged (Yu et al., 2006, p.542).

6.2. ELECTROSTATIC FORCE, FES :

As briefly mentioned in the literature review, sand particles of different sizes accelerated

by different wind velocities can become charged An increase in the electrostatic forces can cause

particles to adopt an adhesive/cohesive behavior. How the surface is charged as compared to the

particles will determine whether they will attract or repulse each other. Paints or sprays can help

charge the walls of the sand blower so as to confer them the same charge as that of the particles,

so the latter can flow out of the tube homogeneously (Sarver et al, 2012). The electrostatic force

of the particles, as previously learned in the physics 2 class can be computed as:

FES = !!  .!!

!!!!  !!  !!

In the previous equation, εr is the dielectric constant of the air and ε0 is the permittivity of free

space and S is the distance of separation between the centers of the two particles and q1 and q2 are

the charges of the particles. In an article published by Bo, Zheng, and Zhang, particles of uniform

 33  

shapes’ charge-to-mass ratio has been computed for different velocities of the flow and different

particle diameters. The last result will be used to find the charge to mass ratio by interpolation.

The following table illustrates all the results.

Figure 14: Charge-to-mass ratio of particles given different velocities

By effectuating an extrapolation, for a diameter of 100 microns (as used in the sand blower

experiment), the charge-to-mass ratio is;

CMR= -24.3 µC/Kg,

In order to obtain the charge of one particle of sand, CMR is multiplied by mass of a single sand

particle. The mass of a particle is:

mp = ρp*Vp ,with Vp =(4/3)*pi*(Rp^3)

The resulting number is the particle’s electric charge, CMR * mp = qp = -2.697*10^(-5) C. This

charge is considered to be the same for all particles, assuming they all have the same diameter,

thus, the same charge-to-mass ratio. Thus, the particles are negatively charged. By further

 34  

charging them, and exposing them to a negatively charged duct, they would be able to come out

from the duct without touching the walls and in a homogeneous way. This, however, could not be

tested since Solidworks doesn’t emulate electrostatic simulations. The negatively charged

particles, since they all have the same charge, would repel each other and migrate towards the

walls of the sand blower. In order to avoid such disturbance, it would be ideal to charge the tube

negatively as well (larger than that of the particles towards each other) in order for the particles to

travel in the center of the tube and come out in a homogeneous way.

 35  

References  Caron,  S.  (2011).  Accelerated  aging  of  thick  glass  second  surface  silvered  reflectors  under     sandstorm  conditions  (Master's  thesis,  Dalarna  University,  2011)  (pp.  1-‐99).     Tabernas:  CIEMAT-‐PSA.   Gachon, Y., Vannes, A., Farges, G., Catherine, M. S., Caron, I., & Inglebert, G. (1999). Study of sand particle erosion of magnetron sputtered multilayer coatings. Wear, 233-235, 263-274. doi:10.1016/s0043-1648(99)00224-0 Karim, M., Naamane, S., Delord, C., & Bennouna, A. (2015). Laboratory simulation of the surface erosion of solar glass mirrors. Solar Energy, 118, 520-532. doi:10.1016/j.solener.2015.05.044 Klimm, E., Ost, L., Spiegelhalter, B., & Weiss, K. (2015). Tests of functional coatings on glass adapted to extreme - arid and maritime - climatic conditions for solar energy systems. 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC). doi:10.1109/pvsc.2015.7355604 Kok, J. F., & Renno, N. O. (2008). Electrostatics in Wind-Blown Sand. Physical Review Letters, 100(1). doi:10.1103/physrevlett.100.014501 Sutter, F., Fernandez-García, A., Wette, J., & Heller, P. (2014). Comparison and Evaluation of Accelerated Aging Tests for Reflectors. Energy Procedia, 49, 1718-1727. doi:10.1016/j.egypro.2014.03.181 Yu, Z., Peng, Z., Liu, P., & Wu, X. (2006). The Influence of Charged Sand Particles on the External Insulation Performance of Composite Insulators in Sandstorm Condition. 2006 IEEE 8th International Conference on Properties and Applications of Dielectric Materials. doi:10.1109/icpadm.2006.284235