Brandenburg University of Technology
Faculty 4 Environmental Sciences and Process Engineering
Institute Mechanische Verfahrenstechnik
Bachelor Thesis Nr.42
Development and optimization of an electrostatic corona charger
BO YANG
3519809
Supervised by:
Prof. Dr. -Ing. Ulrich Riebel
Dipl. -Ing. Andreas Groll
09.05.2016
Contents
Declaration of Academic Integrity
Acknowledgement
List of symbols
1.
Introduction ...................................................................................................... 1
2.
Theory Background ......................................................................................... 3
3.
4.
2.1
Conception of the corona discharge and corona wind ................................... 3
2.2
Charging mechanism ................................................................................... 4
2.3
Onset voltage ............................................................................................... 6
2.4
The voltage limit and spark .......................................................................... 8
2.5
Positive and negative discharge .................................................................... 8
2.6
The relationship of current and voltage ...................................................... 10
2.7
Speed of ionic wind ................................................................................... 12
2.8
Particle concentration ................................................................................. 14
2.9
Coagulation ............................................................................................... 15
Experiment (1) Set-up .................................................................................... 17
3.1
Current and voltage characteristics ............................................................. 19
3.2
Voltage and velocity characteristic ............................................................. 22
3.3
The influence of polarity ............................................................................ 25
3.4
Influence of the diameter of needle ............................................................ 27
3.5
Influence of distance between electrodes .................................................... 30
3.6
Influence of mesh shape ............................................................................. 34
3.7
Influence of amount of needle .................................................................... 38
3.8
Conclusion ................................................................................................. 41
Experiment (2) Set-up .................................................................................... 43
4.1
Corona discharge texting............................................................................ 44
4.2
Different positions of setting corona discharge ........................................... 48
4.3
Conclusion ................................................................................................. 59
5.
Future Study ................................................................................................... 61
6.
Summary ........................................................................................................ 62
Reference ........................................................................................................ 64
Appendix………………………………………………………………………...66
List of Tables
Table 1 Different diameters of needle(Negative) .............................................. 28
Table 2 Different distance between electrodes(Negative) ................................. 31
Table 3 Different distance between electrodes(Positive) .................................. 31
Table 4 Different mesh shape........................................................................... 34
Table 5 Comparison of total number concentration in different conditions ....... 45
Table 6 Comparison of total number concentration in different positions ......... 51
Table 7 Comparison of total number concentration in faster velocity of aerosol 57
List of Figures
Figure 1 Positive corona discharge device .......................................................... 4
Figure 2 Particle charge limits ........................................................................... 6
Figure 3 The process of positive corona discharge before starting with onset
voltage ....................................................................................................... 9
Figure 4 The process of positive streamer .......................................................... 9
Figure 5 The process of negative corona discharge before starting with onset
voltage ..................................................................................................... 10
Figure 6 The process of negative streamer ....................................................... 10
Figure 7 positive corona discharge current-voltage characteristic curve ........... 11
Figure 8 Characteristic curve of velocity-voltage ............................................. 14
Figure 9 Experiment set-up design ................................................................... 19
Figure 10 The comparison between Theoretical and Experimental value in I-U 21
Figure 11 The comparison between Theoretical and Experimental value in V-U
curve ........................................................................................................ 23
Figure 12 The Current-Velocity curve ............................................................. 24
Figure 13 The comparison between different polarities in I-U curve ................ 25
Figure 14 The comparison between different polarities in V-U curve ............... 26
Figure 15 The comparison between different polarities in V-I curve ................ 26
Figure 16 The comparison between different diameter of wire in I-U ............... 29
Figure 17 The comparison between different diameter of wire in V-U ............. 29
Figure 18 The comparison between different distance between electrodes in VU(Negative) ............................................................................................. 32
Figure 19 The comparison between different distance between electrodes in VU(Negative) ............................................................................................. 32
Figure 20 The comparison between different distance between electrodes in VU(Positive)............................................................................................... 33
Figure 21 The comparison between different distance between electrodes in VU(Positive)............................................................................................... 33
Figure 22 Corona discharge with fan................................................................ 34
Figure 23 Different shape of mesh: Mesh 1(1), Mesh 2(2), Mesh 3(3), Mesh 4(4)
................................................................................................................ 35
Figure 24 The comparison between different shape of mesh in V-U(Negative) 37
Figure 25 The comparison between different shape of mesh in V-U with initial
velocity(Negative) ................................................................................... 37
Figure 26 The comparison between different shape of mesh in I-U(Negative) .. 38
Figure 27 The comparison between different amount of needle in I-U ............. 40
Figure 28 The comparison between different amount of needle in V-U ............ 40
Figure 29 The comparison between different amount of needle in V-I ............. 41
Figure 30 The corona discharge designing ....................................................... 42
Figure 31 Experiment set-up with FMPS ......................................................... 44
Figure 32 The comparison of different amount of needle in number concentration
with 25kv discharge ................................................................................. 45
Figure 33 Number concentration under different conditions ............................. 46
Figure 34 Mass concentration under different conditions ................................. 47
Figure 35 Position (a) (b) (c) ............................................................................ 48
Figure 36 Total number concentration in position(a) (b) (c) with 0kv,15kv,25kv
discharge.................................................................................................. 50
Figure 37 Number concentration with 15kv discharge under different positions.
................................................................................................................ 52
Figure 38 Mass concentration with 15kv discharge under different positions ... 53
Figure 39 Number concentration with 25kv discharge under different positions.
................................................................................................................ 53
Figure 40 Mass concentration with 25kv discharge under different positions ... 54
Figure 41 Increasing the velocity of aerosol in (d) and (e) ................................ 55
Figure 42 Total number concentration in position (d) (e) with 0kv,15kv,25kv
discharge.................................................................................................. 56
Figure 43 Number concentration with high-speed aerosol ................................ 58
Figure 44 Mass concentration with high-speed aerosol..................................... 59
Figure 45 Two stages corona discharge device ................................................. 61
Declaration of Academic Integrity
I promise that the thesis contains no material which has been
accepted for the award of any other degree or diploma in any
institutes of higher learning and that, to the best of my knowledge
and belief, the thesis contains no material previously published or
written by another person, except when due reference is made in the
text of the thesis.
I understand that to do so would mean that I had committed
plagiarism and that it is my responsibility to be aware of the
College’s regulations on plagiarism and their importance.
Signed:
Date:
Acknowledgement
I would like to express my appreciation to my home university (the University of
Shanghai for Science and Technology) for giving me the chance to study in Germany.
During this year, I have learned many things, not only the courses I studied here but
also the different cultures and the way to face difficulties.
I would like to express my acknowledgment to Prof. Dr. Riebel for supporting this
topic for me. I am an exchange student from China, and this is the first time I study in
Germany. Everything was difficult for me at beginning especially when I was searching
for the Bachelor thesis, but Prof. Dr. Riebel kindly gave me the chance to do my thesis
in his project. The topic is absorbing for me, and I can do the research as a real German
student, which is beyond my expectation before. From Prof. Dr. Riebel, I learned what
was the true “Spirit of Germany,” for his spirit of rigid, squareness, patience and
preciseness, which will have a huge influence in my future.
I would like to thank my supervisor Dipl.Groll, who helps me all the time, though
I cannot speak German. He tried his best to communicate with me in English, which
gives me a lot helps during my thesis. He is always humorous and cheerful, which gave
me much comforts when I first entered the laboratory. I feel very relax to communicate
him, and also, he is very patient for resolving all my questions even some of them are
very simple.
I would also like to thank Students in Lab: Zhe Lei, Yan Ma, Shouhui Li, Jiawei
Lu, who gave me many supports of my study and life in Germany.
Finally, I would like to thank my parents in China, who always support me and
whatever difficulties I may meet, they can always give me the strength to go through it.
List of symbols
𝑐𝑖 : Mean thermal speed of ions[m/s]
D: Diffusion coefficient of particles
d:Diameter of needle[m]
𝑑𝑝 : Diameter of particle[m]
e: Coefficient of restitution
E: Electrical field strength[V/m]
𝐸𝑤 : Onset corona field on surface of needle[V/m]
𝐸𝑝 : Corona field strength on the surface of needle[V/m]
𝐹𝑒 : field strength[𝑁]
G: Distance between needle and plate[m]
I: Current[A]
J: Current density[A/m²]
K: Geometric factor
𝐾0 : Coagulation coefficient[m³/s]
𝐾𝐸 : Electrostatic constant of proportionality
𝑁𝑖 : The concentration of ions.
P: Pressure of cross section[N/m²]
q: Charge on single particle[C]
r: radius of curvature of needle[m]
S: Area of sectional surface[m²]
𝑆𝑊 : Area of wire[m²]
v: Flow velocity[v/m]
V: Power supply[V]
𝑉0 : Onset voltage[V]
𝑍𝑖 : Mobility of the ions[m²/sV]
μ: Electron mobility[m²/sV]
ρ: field density [C/m]
ρ𝑎𝑖𝑟 : Air density[kg/m³]
𝜀 : Relative permittivity
1. Introduction
The system of solid particles or liquid droplets suspended in a gas is called aerosol
[1], the sizes of particles are range from 0.001μm~100μm. The resources of an aerosol
can be classified into two types: natural resource and man-made resource. Natural
resources are usually generated by the eruption of a volcano, dust carried by the wind,
etc. [2]. The manmade resource is mainly relief of gasses from industries or burning
fossil fuels, etc. [2]. Aerosol can cause harm to human beings and be threaten to health.
Aerosol can be stably suspended in the air, sometimes could last for several years.
The elimination of aerosol in a natural way is mainly relied on coagulation [2]. However,
it is not far more enough compared to the much faster generation rate of an aerosol of
human activities. Thus, cleaning devices are necessary to be applied, which can be
achieved by particles charging, particles which are charged will move under electrical
forces if there is a collector, such as a tube or a plate, connecting to ground, charged
particles will move to the collector and eventually be collected.
The way to charge particles are flame charging, static electrification, diffusion
charging and field charging, but to have a highly charged particle, only the corona
discharge fulfilled it and is usually applied [1].
Corona discharge can be achieved by a corona discharge device connected to a
high voltage, this method can be widely used in air cleaning or cooling system. There
is a point-to-plane type of corona discharge device which is designed as a metal needle
connected to the high voltage and a plate with mesh used for collecting particles.
Several factors can have influences on the process of generating the corona wind.
This paper aims to set up the experiment to explore how these factors, such as
1
polarity of discharge, the diameter of the needle, the amount of needle, etc. For further
practice, the optimized device will be installed in a container, and aerosol will be
injected to create a range of particle sizes. By using the FMPS device, the data of
number concentration, mass and volume will be collected and analyzed, the result will
test how the device work.
2
2. Theory Background
2.1 Conception of the corona discharge and corona wind
There are several ways to have charging particles. However, only corona discharge
can satisfy the condition that high concentration unipolar ions will be produced, thus to
charge aerosols [2]. During the process of corona discharge, the wind will be generated
without any fans, this can be called corona wind.
Corona wind is a phenomenon which is conducted by the collisions between
ionized particles and neutral particles, and it can also be called ionic wind [3]. To give
the information about this process, an example of the point-and-plate type of positive
corona discharge is described in Fig 1. Corona electrode side is connected to high
voltage apply, this is usually can be achieved as a needle. Collector electrode will be
linked to ground, usually is a flat plate. Thus, an electrical field is established. As the
voltage rising gradually and reaching the onset voltage, the electrons around corona
electrode will be ionized [4], because electrons and positive ion will be separated from
air molecule due to the extremely uneven electrical field. The electron will move to
needle, and positive will move towards field as the effect of electrical force, theses
ionized molecules will eventually be collected by the collector electrode. During the
motions of positive ions, some natural air molecule will be collided and move with
ionized molecules along the field together. As a result, there will be a constant flow
between the electrodes [5].
3
Figure 1 Positive corona discharge device
Recent researches showed that corona wind can be mainly utilized in flow cleaning
process due to its higher efficiency [1], other researchers intended to design a cooling
device by corona wind, which is more silent and superior speed because of no machine
parts being existed [6]. Alternatively, some researchers were focused on dry technic [7].
2.2 Charging mechanism
Field charging is that in a strong electrical field, neutral particle in the field will
be collided by ions and be charged until it reaches the saturation charge and no ions can
collide it. The rate of charge can be given by:
3𝜀
𝐸𝑑 2
π𝐾 𝑒𝑍𝑖 𝑁𝑖 𝑡
n(t) = (𝜀+2)(4𝐾 𝑝𝑒)(1+𝜋𝐾𝐸
𝐸 𝑒𝑍𝑖 𝑁𝑖 𝑡
𝐸
4
)
(2.1)
Where 𝜀 is relative permittivity, E is electrical field strength, 𝑑𝑝 is particle
diameter, 𝐾𝐸 is electrostatic constant of proportionality, e is coefficient of restitution,
𝑍𝑖 is the mobility of the ions, usually is about 0.00015m²/V∙s [450cm²/stV∙ s], 𝑁𝑖 is
the concentration of ions.
Usually when 𝑁𝑖 > 1013 /𝑚3 , the sufficient time is about 3s to have the
saturation charge
3𝜀
𝐸𝑑 2
𝑛𝑠 = (𝜀+2)(4𝐾𝑝𝑒 )
(2.2)
𝐸
Another charging is diffusion charging, which does not need any external field,
because of Brownian motion of the ions and particles, collisions will happen and charge
the neutral particles. This can be described by:
𝑑 𝐾𝑇
𝜋𝐾𝐸 𝑑𝑝 ci̅𝑒 2 𝑁𝑖 𝑡
𝐸
2𝑘𝑇
n(t) = 2𝐾𝑝
ln(1 +
𝑒2
)
(2.3)
Where 𝑐𝑖 is the mean thermal speed of ions (𝑐𝑖 = 240𝑚/𝑠 at standard conditions) .
In a corona discharge, a field charging can be established by the needle and plate,
both of the corona and corona wind are generated by field charging. Particles which are
larger than 1.0μm are dominated by field charging while for particles less than 0.1μm
are charged by diffusion charging. Thus, corona discharge usually combines two types
of charging which is called combined charging. The limit of combined charging can be
shown in Fig 2.
5
Figure 2 Particle charge limits [1].
2.3 Onset voltage
As the voltage supply growing up, gas particles around corona electrode will be
ionized, and the corona discharge will be realized. The initial voltage that corona
discharge starts to work is called onset voltage. This field strength around the tip of
needle can be described by Peek’s law which is for wire-to-plate system [5]:
𝐸𝑤 = 𝐸0 (1 +
2.62∙10−2
√𝑟
)
(2.4)
Where 𝐸𝑤 [𝑉/𝑚] is the onset corona field on surface of needle, r[m] is the radius of
curvature of needle, 𝐸0 = 3.31 ∙ 106 [𝑉/𝑚].
The formula of onset voltage can be given as[8]:
𝐸𝑝 =
2𝑉
𝑟 ln(
6
4𝐺
)
𝑟
(2.5)
Where V[𝑉] is the voltage supply, 𝐸𝑝 [𝑉/𝑚] is the corona field strength on the
surface of needle,𝐺 [𝑚] is the distance between needle and plate.
Combine（2.4）with（2.5）when the field strength reaches to onset field strength，
𝐸𝑝 = 𝐸𝑤 , the onset voltage is given as：
𝑉0 =
𝐸𝑤∙ 𝑟 ln(
2
4𝐺
)
𝑟
（2.6）
Where 𝑉0 [𝑉] is the onset voltage.
The high voltage applied to the needle causes an intensive active electrical field
around the tip of the needle, the field is so strong that is stronger than the field of air
around it and causes air to be ionized. This onset field is the maximum field to have
corona discharge, of formula (2.6), this onset field largely depends on the size of needle,
the minimum value is 𝐸0 = 3.31 ∙ 106 [𝑉/𝑚] for a needle which has an infinitely large
diameter and is impossible to happen. Whether the size of wire is large or small, the
onset electrical field must exceed more than 𝐸0 ,but as the diameter increasing, the
field needed to start corona is getting smaller, so for formula (2.6), the onset voltage is
related to onset field 𝐸𝑤 , a small onset field can reduce the onset voltage, what is more,
the onset voltage is also linked to needle size and the distance between electrodes, a
smaller needle and closer distance can reduce the onset voltage. But latter in Chapter 3,
the actually results are completely opposite to formula (2.6), which shows that a sharper
needle have a quick discharge.
In conclusion, to start corona discharge quickly, 𝑉0 should be as small as
possible. The shape of needle and distance between electrodes will play the main role.
7
2.4 The Voltage limit and spark
Due to the mechanism of corona discharge, when positive particles or electrons
move to the needle, they can be soaked into the needles and generate a pulse current.
As power supply growing up higher enough, a current called streamer discharge current
is produced, as this current reaching the collector electrode, space will cause the
breakdown, and corona discharge will be covert to spark discharge. This dielectric will
shift to the conductor, causing a huge jump of current. If the power is not high enough,
a stable voltage was difficult to be sustained and causes the interruption of spark
discharge, so only a small voltage is generated between electrodes, this will be a
disadvantage to charge particles, which should be avoided. Normally the breakdown
field of air is 1.3 × 106 .
2.5 Positive and negative discharge
The mechanism of charging particles under positive and negative discharge are
entirely different, also, will have influences on the process of charging particles. This
is mainly due to the different mobility, where the mobility of electron is much higher
than the mobility of positive ion [9].
For positive corona, electrons will move around needle while positive ions move
towards to the collector electrode, however, due to the slower mobility, many positive
ions are still around the needle. As a result of the interaction of electrical field between
positive and negative particles, the electrical field will be reduced around the needle,
so it will be harder to create corona discharge and thus need a higher onset voltage. This
shows in Fig 3.
8
Figure 3 The process of positive corona discharge before starting with onset voltage
While in the movement of positive streamer, because electrons move quickly to
the needle, the electrical field in front of the stream is stronger, which is easier to get to
the collector electrode. Thus, the stream is getting stronger and longer, which can reach
negative electrode faster. As a result, the breakdown between electrodes will happen,
which shows in Fig 4. So the breakdown voltage of positive discharge is lower.
Figure 4 The process of positive streamer
For negative discharge, positive ions are around needle while electrons are
moving towards the electrical field, due to the higher speed of electrons, the intensive
field around needle is much stronger than the field where the interaction is happening
in the cross section. The process is given in Fig 5. This causes corona discharges easily
to be established and lower onset voltage.
9
Figure 5 The process of negative corona discharge before starting with onset
voltage
In the process of negative streamer, when electrons move in front of positive ions,
new positive ions will be attracted, and new electrons will go to the positive electrode,
so a new negative streamer is generated. However, due to the weak electrical field in
the head of the streamer, the process of producing a new streamer is slower, so the
negative stream is harder to reach the positive side, which can be seen in Fig 6. Thus,
the breakdown voltage is higher than positive discharge.
Figure 6 The process of negative streamer
2.6 The relationship of current and voltage
Due to the theory of Townsend [10], the current during the process of corona
discharge can be described as:
10
I=KμVΔV=KμVΔV
（2.7）
Where I[A] is the current, K is the geometric factor, V[V] is the power supply,
μ[m²/Vs] is the ion mobility, and ΔV[V] can be described as:
ΔV=𝑉 − 𝑉0
(2.8)
Where 𝑉0 [𝑉] is the onset voltage.
Combine with (2.7) and (2.8), the current can be given by:
I = KμV(𝑉 − 𝑉0 )
(2.9)
Thus, the current-voltage characteristic curve can be described by Fig 7, after the
Current(μA)
onset voltage, the current has a relationship with voltage as I~𝑉 2 .
Onset voltage
Voltage(kv)
Figure 7 positive corona discharge current-voltage characteristic curve
11
2.7 Speed of ionic wind
From the introduction in Chapter 2.1, the ionic wind is generated by the collision
of charged particles and neutral air particles, the experiments in this essay show that
the velocity always increases with growing voltage, to find this relationship of speed
and voltage, some theoretical calculations can be given.
We assume that in the electric field, the field is even so field strength is always
the same, the force that particles get from the field can be described by the formula
below [11].
𝐹𝑒 = qE
(2.10)
Where 𝐹𝑒 [𝑁] is field strength, q[C] is the charge on single particle, E[V/m] is
field strength. If the amount of ionic particles in a volume unit is n, thus the total field
fore is
F = nqE
(2.11)
Also, no means the full amount of ionic particles in a volume unit, which can be
described as charge density ρ [8]. Thus, the force is
F=ρE
(2.12)
Where ρ [C/m] is field density. It should be noticed that Current can be described
as
I = J𝑆𝑊 = ρμ𝑆𝑊 E
12
(2.13)
Where J[A/m²] is current density, μ[m²/sV] is ionic mobility，𝑆𝑊 [m²] is area of
wire, so by dividing (2.12) and (2.13), the relation between force and current can be
described as
𝐼
F = 𝜇𝑆
𝑊
(2.14)
We assume that the wind speed is ultimately generated by the field force if the
field is in an insulating tube and there are no friction losses on the wall of tube, the
relationship between strength and flow speed is described by Bernoulli Equation:
𝐹
1
P = 𝑆 = 2 ρ𝑎𝑖𝑟 𝑣 2
(2.15)
Where P[N/m²] is the pressure of cross section，S[m²] is area of sectional
𝑘𝑔
surface, ρ𝑎𝑖𝑟 [𝑚3 ] is air density, v[m/s] is the flow velocity.
Thus by connecting (2.9), (2.14) and (2.15)，the velocity can be described as:
v = √(2 · ρ
1
𝑎𝑖𝑟 μ𝑆𝑊 S
·) V(𝑉 − 𝑉0 )
(2.16)
Thus, from the formula (2.16), the characteristic curve of velocity-voltage has a
linear relationship v~V, which is given by Fig 8.
13
Velocity(m/s)
Oneset voltage
Voltage(kv)
Figure 8 Characteristic curve of velocity-voltage
2.8 Particle concentration
The properties of corona discharge have been introduced before, the purpose of
corona discharge in this paper is aim to clean aerosols in the air. Therefore, to find out
how corona discharge can reduce aerosols in experiments, there are several properties
of the particle should be known.
Mass concentration is the most valuable goods and also commonly measured by
measurement, which is the mass of particulate matter in a unit volume of aerosol [1].
The unit is usually g/m³, mg/m³, orμg/m³.
Number concentration is another important property, which is the number of
particles in a unit volume of aerosol. The unit is usually number/cm³ or number/m³.
14
2.9 Coagulation
The result of the experiment shows that the number concentration of particles will
decrease as time going by when injecting aerosol into an airtight container having a
particular volume. This is due to coagulation of aerosols.
Coagulation is a process of collision where particles collide with each other and
coagulate into larger size particles. As a result, the number concentration of particles
will decrease while larger particles are getting more. There are two types of coagulation,
one is thermal coagulation caused by Brownian motion, and another is kinematic
coagulation which is caused by external forces such as gravity or electrical force [1].
Due to coagulation, the number of particles in a unit volume will decrease as time
passing, to describe this change, the formula is given as
𝑑𝑁
𝑑𝑡
Where
𝑑𝑁
𝑑𝑡
= −𝐾0 𝑁 2
(2.17）
is change rate of number, 𝐾0 [𝑚3 /𝑠] is the coagulation coefficient,
which is:
𝐾0 = 4𝜋𝑑𝑝 𝐷
(2.18)
Where D is the diffusion coefficient of particles, 𝑑𝑝 (m) is the diameter of particle.
In a standard condition, 𝐾0 can be simply quale to 𝐾0 = 3.0 × 10−16 𝐶𝑐 [𝑚3 /𝑠]. To
have the initial number of particles (𝑁0 ), a relationship of initial number and the number
at time t is given by:
𝑁
N(t) = 1+𝑁 0𝐾
0 0𝑡
15
(2.19)
Therefore, with an initial number, the calculation of collect efficiency is available.
Also, if the condition is not standard, a corrected coagulation coefficient K is
K = 𝐾0 𝛽
(2.20)
Whereβcan be found on references. Thus, as a consequence, the particles number
centration will reduce because of coagulation while the mass concentration remained
the same.
16
3. Experiment (1) Set-up
Firstly, to test the properties of corona discharge, an experimental stage has
established the outline of the operation can be shown in Fig 9.
A corona electrode is set inside a metal tube, where the total length of the tube is
44.50cm, out diameter is 11.30cm, inner diameter is 10.25cm. The distance between
the corona electrode and mesh of one side of the tube is 9.34 cm, the position of wire
in corona discharge can be adjusted, thus changing different diameter, amount of needle,
or removing the needle to change the gap distance between electrodes, these can be
conducted. It is important to cover the needle with pipe so that only a tip of needle of
one side can generate corona wind. The mesh is installed on the side of the tube where
is near the needle as the collector electrode, there are several types of mesh can be
changed, so comparing effects by changing different mesh is possible. Both of the tube
and mesh are linked to the ground as the collector electrode.
A power supply (PHYWE HV-POWER SUPPLY) is connected to corona
discharge, which has a range from 0KV to 25KV, the cables can be shifted to different
polarities so that the experiment can be conducted in the negative and positive side. A
voltammeter (PeakTeach 070 DMM) is linked to measuring the current, the
measurement range will be chosen as the range from 0~2000 μA.
To gauge the velocity of corona wind, a little hole that can allow a LAMBRECHT
KLIMATOLOGISCHE MESSTECHNIK (flow measurement) to stretch in and be set
inside the middle of the tube to have the maximum speed. The precision range is
0.01m/s, there are options to by reading average values in 1s, 10s, the 30s, and 60s, to
have the accurate velocity and faster measure speed, we at this moment close to record
average velocity in 30s. In the field of corona discharge, there will be lots of charged
particles passing through the electrodes. Thus, the electrical field could have an
17
influence on recording results or even cause damages to measurement. Also, if the
measurement is set behind the mesh, the uneven surface of the mesh may cause the loss
of speed, so it is necessary to avoid them by establishing measurement on the back side
of corona discharge for 36.50 cm from the mesh.
18
Figure 9 Experiment set-up design
3.1 Current and voltage characteristics
From chapter 2.6, we assumed that the current follows the Townsend ’s theory by
having a quadratic relationship with voltage, to verify this prosumer, an experiment is
established. The diameter of the needle is chosen to be 0.50mm, the distance between
electrodes is adjusted for 5.12cm, the mesh has square units with the length of 2.25mm.
The needle is connected voltage within positive polarity, power supplies are improved
from 0kv to 25kv, increasing 1KV every time, the values of current and velocity are
recorded after every 30s when the voltage is changed. After finishing the recording, the
result will be compared with the theoretical calculations in which the formula is given
by (2.4). The value of the geometric factor k and electronic mobilityμis hard to obtain
because of the restraints of measurement equipment, but by reading another reference
who made their experiments at ambient conditions set the mobilityμto 0.0002[㎡
/Sv][12 ], and k could be 0.000112. The result can be shown in Fig 10. Also, the
theoretical onset voltage is also calculated to verify it which can be found in Chapter
2.3. According to formula (2.4), the onset electrical field can be calculated by:
𝐸𝑤 = 𝐸0 (1 +
2.62∙10−2
√0.00025
the onset voltage is:
19
) = 8.79 × 106 [V/m]
𝑉0 =
8.79×106 ∙0.00025 ln(
2
4×0.0512
)
0.00025
= 7.37[𝑘𝑉]
While as the records in this experiment, the onset voltage is about 7 kV, which is
slightly less than the theoretical value, there are no precise explanations to verify the
reasons, but the needle does not have an ideal shape and surface, this may cause the
electrical field intensity to be uneven and thus influence the onset voltage, in addition,
the power losses in the circuit may also play a role, the residences of current
measurement or the wire will separate the voltage supply. Thus, the actual voltage
provided on the needle will be reduced and make the onset voltage delay. However, the
tip of the needle is not a regular shape, this causes a sharper tip, and the real radius of
the needle may be less, which will reduce the onset voltage. Last but not least, the defect
of recording method may also cause the onset voltage too large because the voltage is
increased 1kv every time so the actual onset voltage may range from 6kv~7kv, which
is neglected by observation.
20
140
120
Current(uA)
100
80
60
40
20
0
0
5
10
15
20
25
Voltage(kv)
Experimental vlaue
Theoretical value
With theotrical onset voltage
Figure 10 The comparison between Theoretical and Experimental value in I-U
However, if using the same onset voltage as being recorded in the experiment, the
difference between theoretical calculation and experimental result is not very big.
However, it is still not acute because geometric factor k and electronic mobilityμare
still uncertain, and only a guess that both the theatrical calculations and experimental
result have a very similar increasing trend that the current shows a square relationship
with voltage. Despite the dramatic increasing on current when increasing voltage
supply from 19kv~23kv, the observed phenomenon including noise and slight lighting,
this may be caused by spark discharge. When the gas of the space between electrodes
has breakdown, the conductor will become good driver. Thus, residence primarily
decreases and causes current having a sharp growth.
In conclusion, both of the theoretical onset voltage and I-U curve are not far from
the actual values, which can verify the formulas can be suitable for the corona discharge.
21
3.2 Voltage and velocity characteristic
Follow by the result in Chapter 3.1, the speed of corona wind is also recorded as
the voltage supply increasing, and the mode of recording is selected by using 30s mode
to record the average values in 30s when the power increases by 1kv, the result shows
in Fig 11.
To verify the theoretical calculation in Chapter 2.7, the result of calculations is
also given by graph below. The density of air is valued as 1.2kg/m³, the diameter of
wire and channels are available to be measured as 0.0005m and 0.1025m, the onset
voltage is assumed the same as the experimental value, the result will be compared with
experimental records.
Although both lines show an upwards trend, the Theoretical value is much higher
than the value we measured. Even though they have the same onset voltage, the growing
rate of theoretical line is much faster than the experimental line and the maximum
velocity (23kv, 0.97m/s) in theoretical calculation almost double the value in laboratory
records (23kv, 0.46m/s).
22
Velocity-Voltage
1
0.9
Velocity(m/s)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
10 Voltage(KV) 15
Experimental value
20
25
Theoritical value
Figure 11 The comparison between Theoretical and Experimental value in V-U
curve
There are several factors can cause this result. First of all, the air going through
the tube is not an ideal flow and tube is not smooth which means there is always
frictions on the tube, and this will have dynamic losses when the air is going through.
Thus, the method of measuring has a significant influence on the little experimental
value because of the position where the speedometer is set on the back of corona
discharge for avoiding damages caused by charged particles. The long distance between
speedometer and corona discharge will get the rapid loss. In addition, the theatrical
calculation is based on the even field, which assumes that the whole field has an even
field strength so that the velocities of flow passing the field are remained same While
the real field strength is not even and the field strength around needle is much stronger
than other position, so the highest speed wind will be generated near the needle, as the
particles moving forward, the collisions of other particles may change their directions
and if the diffusion discharge appeared, the field will become more complicated and
23
uneven. So the actual speed will bias more. Also, because the tube is metal, some
charged particles will move to all sides and collected by the wall of the tube, which will
also cause the velocity of straight line decreasing.
Although there is a significant difference between theatrical and experimental
values, the growing trends are the same as the velocity showing a linear relationship
with voltage, which fits the conclusion in Chapter 2.7.
0.45
0.4
Velocity(m/s)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
20
40
60
80
100
120
140
Current(uA)
Experimental value
Figure 12 The Current-Velocity curve
By establishing the relationship between velocity and current, the velocity is also
increasing with current growing up. However, the trend shows that there is a decrease
in the growth rate, the velocity will firstly grow up sharply and then get more smooth.
Finally, the velocity will be stable of particular value where spark discharge occurs at
the same time, the spark limits the velocity so the corona wind cannot increase
unlimitedly as voltage growing up.
Thus, the experiment 3.1 and 3.2 show that both of current and velocity will
increase as well as the voltage increasing while due to spark discharge when the voltage
is high enough, there is always a limitation of corona wind speed.
24
3.3 The influence of polarity
The mechanisms of negative and positive discharge are quietly different, by the
established experiment, the same condition in Chapter 3.2 and Chapter 3.2, but
recording results under both positive and negative discharge. The result is given in Fig
13 and Fig 14.
Although the onset voltage under negative should be more advanced to happen,
the difference is quite small which is recorded in the experiment. The onset voltage
under negative discharge is 5kv while 7kv under positive condition, the negative
discharge will have less onset voltage due to the faster mobility of electron, the more
separation of electrons and positive particles around the tip of the needle will have more
intensive electrical field strength and easier to occur corona discharge. As a result,
negative discharge with lower onset voltage will have higher current and velocity which
can be seen in Fig 15.
160
140
Current(uA)
120
100
80
60
40
20
0
0
5
10
15
20
25
30
Voltage(kv)
Negative
Positive
Figure 13 The comparison between different polarities in I-U curve
25
0.6
Velocity(m/s)
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Voltage(kv)
Netagive
Positive
Figure 14 The comparison between different polarities in V-U curve
0.6
0.5
0.4
0.3
0.2
0.1
0
0
20
40
60
80
Negative
100
120
140
160
Positive
Figure 15 The comparison between different polarities in V-I curve
To summarize, the velocity within negative supply is slightly higher than positive
supply, and the current and ionic wind are more stable, generally the negative discharge
are more widely used in industries. However, it should be pointed out that spark is more
frequently and easily generated in positive supply, so it is harder for corona wind to
increase to a particular value in positive supply. On the other hand, during the process
of negative corona discharge, there is ozone being generated in the negative supply, this
is due to the negative electrons move from needle to everywhere where the electrons
26
will collide with an oxygen atom to become ozone, which is harmful to human health.
So the positive discharge is better to be used under indoor condition.
3.4 Influence of the diameter of needle
The Chapter 2.3 points out that the onset field and voltage are mainly depending
on the diameter of the needle. To compare the influence of different shapes of needles,
we tested three separate sizes of needles, which are separately d1=0.15mm, d2=0.50mm,
d3=1.35mm. The electrodes gap remains 5.12cm, the result can be given in Fig 16 and
Fig 17, which is conducted within negative voltage supply.
With the onset voltage provided by Chapter 2.3, the theoretical onset voltage is
U1=3.95kv, U2=7.37kv, U3=12.27kv respectively. While the onset voltage in the
experiment is Ue1=5kv, Ue2=7kv, Ue3=8kv, which is given in Table 1. Despite U2 and
Ue2 where there is a small difference between them, the needle with 0.15mm diameter
shows an onset voltage delay, about 1kv. This may be due to the loss in the circuit, also.
Due to the sheer size of needle, it is hard to keep straight and easy to have some curves
on the wire, this sharp curves may also sharp enough to have an electrical field which
will influence and reduce the field strength on the tip of the needle. However, the needle
with 1.25mm diameter has a significant advance in onset voltage, up to 4kv. Despite
the shape of the tip of the needle, the granulated surface may also play an important
part, it has a larger surface, and there some protuberances generated by the oxidation
over an extended period can be treated as another way to make the tip sharper, which
enhances the intensity of field around the tip. As a result, there is the quite small
difference in current when the voltage is given from 10kv~15kv, as voltage growing up,
the difference is becoming more apparent, on the other hand, velocity is keeping stable
because there is a linear relation between voltage and velocity.
27
Table 1 Different diameters of needle(Negative)
Diameter(mm) Theoretical
onset
voltage
(kv)
3.95
0.15
7.37
0.50
12.27
1.25
Onset
Voltage
(kv)
Maximum
Voltage
(kv)
Maximum
Current
(μA)
Maximum
velocity
(m/s)
5
7
8
25
25
25
134
114
100
0.59
0.53
0.48
Nevertheless, though the experimental records do not completely submit to the
formula, they have the same trend that smaller size of the needle can have an early onset
voltage. As a result, an advanced onset voltage will soon start to generate ionic wind,
and reach a higher speed. So the velocity shows a negative relation with a diameter of
wired, due to the smaller diameter, the field could be more inventive, a faster process
to have ionization. However, it should be pointed out that from formula 𝐸𝑤 = 𝐸0 (1 +
2.62∙10−2
√𝑟
), a larger diameter should reach the onset field early and easier to have corona
discharge, but from experiment result and calculation of onset voltage, the result is
completely opposite. There are no exactly explanations yet, but due to the mechanism
of corona discharge, the intensive field around tip will cause a part strong field strength
which can ionize the particles, a smaller diameter needle will have more intensive field
to cause corona discharge.
28
160
140
Current(uA)
120
100
80
60
40
20
0
0
5
10
15
20
25
30
Voltage(kv)
d1=0.15mm
d2=0.50mm
d3=1.25mm
Figure 16 The comparison between different diameter of wire in I-U
0.7
0.6
Velocity(m/s)
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
Voltage(kv)
d1=0.15mm
d2=0.50mm
d3=1.25mm
Figure 17 The comparison between different diameter of wire in V-U
29
30
3.5 Influence of distance between electrodes
According to Chapter 2.3, the onset voltage not only depends on the diameter of
the needle but also decided by the distance between needle and collector electrode. So
this experiment is based on the different distance G with the needle having 0.50mm
diameter and mesh with 0.225mm length of square units. The distance G can be adjusted
as moving the needle to change in corona discharge device. The experiment is
conducted under both positive and negative discharge. The results are demonstrated by
Fig 18 to Fig 19.
In the negative side, the records show a trend that the onset voltage is smaller as
distance reducing. When G=3.12cm, it is early to have corona discharge, and higher
current and velocity, due to the shorter distance, the field strength is stronger, and there
will be more particles being charged and enhance the intensity around the tip. So an
advanced onset is possible, however, a short distance is much easier to occur breakdown because steamer can reach another electrode quickly with a short distance. As
experiment demonstrating, even under negative condition, which shows in Table 2, the
voltage cannot reach 25kv, and the current assumption is much higher, while the
velocity is as same as it when distance is 4.12cm, but have lower current and more
smooth curve.
On the other hand, a longer length will cause the onset voltage delay and lower
speed because far from the collector electrode, the field strength between electrodes is
weak so it needs more field strength to have corona discharge. Also, a low electrical
field is not beneficial for ionized particles to move towards the field, which also causes
the velocity decreasing. However, the streamer travel in a longer distance will need
more field strength to be robust enough to reach the collector electrode so that the breakdown will delay.
30
It is more obvious in positive supply, which is given in Table 3, when G=3.12cm,
it only can reach 22kv and then break-down happened, the limitation of speed is much
lower than it when G=4.12cm. Nevertheless, the difference is still not so significant
when the distance ranges from 4.12cm~6.12cm. Especially when the voltage is lower
than 15kv, the lines demonstrated in graphs almost converge to one line. It may due to
the field is weak when the distance is longer than 3.12cm, which will cause the result
not to be very obvious compared with each other.
In collusion, as the distance decreasing, there is a trend that the onset voltage will
be lower and can get a higher velocity, while if the distance was too close, it is much
easier to have voltage break-down and reach to the limitation. In this experiment, the
best distance with high efficiency may range from 3.12cm~4.12cm.
Table 2 Different distance between electrodes(Negative)
Distance G
(cm)
3.12
4.12
5.12
6.12
Onset
Voltage
(kv)
4
5
6
6
Maximum
Voltage
(kv)
23
25
25
25
Maximum Maximum
Current
Velocity
(μA )
(m/s)
195
0.54
151
0.54
130
0.51
124
0.47
Table 3 Different distance between electrodes(Positive)
Distance G
(cm)
3.12
4.12
5.12
6.12
Onset
voltage
(kv)
5
6
7
7
Maximum
Voltage
(kv)
22
25
25
25
31
Maximum Maximum
Current
Velocity
(μA )
(m/s)
173
0.48
129
0.52
103
0.46
108
0.43
Negative
250
Current(uA)
200
150
100
50
0
0
5
10
15
20
25
30
Voltage(kv)
g=3.12cm
g=4.12cm
G=5.12cm
G=6.12cm
Figure 18 The comparison between different distance between electrodes in VU(Negative)
Negative
0.6
Velocity(m/s)
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Voltage(kv)
G=3.12cm
G=4.12cm
g=5.12cm
g=6.12cm
Figure 19 The comparison between different distance between electrodes in VU(Negative)
32
Positive
200
180
160
Current(uA)
140
120
100
80
60
40
20
0
0
5
10
15
20
25
30
Voltage(kv)
g=3.12cm
g=4.12cm
G=5.12cm
G=6.12cm
Figure 20 The comparison between different distance between electrodes in VU(Positive)
Positive
0.6
Current(uA)
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Voltage(kv)
G=3.12cm
g=4.12cm
g=5.12cm
G=6.12cm
Figure 21 The comparison between different distance between electrodes in VU(Positive)
33
3.6 Influence of mesh shape
The collector electrode is designed to a metal mesh, that can allow the air to pass
the mesh, and the ionic particles will be collected by the mesh, though there is no exact
theory about how the shape of the mesh may have influences on corona discharge which
is not mentioned before. The result of the experiment shows that mesh can play a major
role in the velocity of the wind. In this experiment, there is four mesh with different
shapes, which is given in Table 4 and Fig 23, the minimum unit size is measured as
L1=1.00cm, L2=0.23cm, L3=0.17cm, R4=0.55cm. The diameter of needle and distance
between electrodes remain 0.5mm and 5.12cm, the experiment is conducted under the
negative and positive condition, also, a fan has been used in front of mesh. So there will
be an initial velocity for discharge. The results are given by Fig 24 to Fig 25.
Figure 22 Corona discharge with fan
Table 4 Different mesh shape
Mesh 1
1.00
Length(Radius)
of unit(cm)
Mesh 2
0.23
34
Mesh 3
0.17
Mesh 4
0.55
Figure 23 Different shape of mesh: Mesh 1(1), Mesh 2(2), Mesh 3(3), Mesh 4(4)
The result shows that though there is a huge difference between the shapes of
mesh. Whether the negative or positive side has almost the same value of current, thus
the mesh has nearly non-influences on current because the current is mainly determined
by the needle and the distance between electrodes, also the media filled with an
electrical field, but the designing of collector electrode will not change these factors.
However, the shape of mesh can change velocity a lot, when there is no fan working,
the Fig 24 shows that the Mesh 4 has a large loss of speed. Mesh 4 is designed as a plate
with big holes, but also has a larger area of the metal plate, which will hinder the
particles going through the mesh, and the velocity will primarily reduce. For Mesh 2
35
and Mesh 3, they are very similarly designed, but the sizes of grid unit are not the same,
smaller group size will also cause the speed to drop, and there will be lots of pressure,
that is the reason why velocity with Mesh 3 can reach a higher speed. Nevertheless,
both Mesh 2 and Mesh 3 have much better than Mesh 4 for particles to go through.
While Mesh 1 is designed to have a much bigger size of the unit compared to Mesh 2
but has an even distribution of grids as Mesh 2, the result shows that Mesh 1 and Mesh
2 have similar velocity increasing trends, despite the velocity with Mesh 2 increases
more after 20kv under negative side.
If the experiment is conducted with fan, there will be an initial velocity of the
wind. So the difference between them will be more obvious to see. Mesh 1 has the
highest initial voltage because little pressure loss and also can accelerate to 7.6m/s.
Follow by Mesh 1 is Mesh2, but there is still a significant difference between them
while Mesh 3 Mesh 4 have a large loss in pressure because the grid restricts air to go
through it. However, Mesh 3 can have more velocity when the corona discharge is
working, may due to the more even in the distribution of grid units.
Considering that in pollution cleaning process, the corona discharge device should
have a good condition to have flow recycle, Mesh 1 has the highest initial velocity and
also the highest after particles being charged. So to have the most efficient grid, Mesh
1 is suitable for it.
36
Negative
0.6
0.5
Velocity(m/s)
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Voltage(kv)
Mesh 1
Mesh 2
Mesh 3
Mesh 4
Figure 24 The comparison between different shape of mesh in V-U(Negative)
Negative
0.8
0.7
Velocity(m/s)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Voltage(kv)
Mesh 1
Mesh 2
Mesh 3
Mesh 4
Figure 25 The comparison between different shape of mesh in V-U with initial
velocity(Negative)
37
Negative
140
120
Current(uA)
100
80
60
40
20
0
0
5
10
15
20
25
30
Voltage（kv)
Mesh 1
Mesh 2
Mesh 3
Mesh 4
Figure 26 The comparison between different shape of mesh in I-U(Negative)
3.7 Influence of amount of needle
Some articles say that a multi-needle can enhance the velocity of ionic wind
because more needles will create a high field density and more stable discharge [13].
We design three multi-needles by twisting 2,3 and four needles into one respectively.
The diameter of all the needle is 0.50mm, each needle in 2-needles design arranges into
a line for 180°angle, the angle is 120°in 3-needles designing and 90°in 4-needles
designing. All of the needles have the same distance(1.77cm) to the axes. The distance
between electrodes is 5.12cm. The result is given by Fig 27 and Fig 28, which is
conducted under negative discharge.
From the comparison, there is a vast improvement when the amount increases
from one to two, both of the velocity and current are much higher than them in 1-needle
design. That mat due to that two needles can have a wider area of discharge, and more
particles can be ionized, thus, the field density will be stronger and more ionized
particle can collide with air molecules so that the ironical wind will be faster. Also, it
38
should be noticed that the current and velocity do not have double values of them when
the amount increases from one to two, so the relationship cannot be simplified as 𝐼2 =
2𝐼1 or 𝑉2 = 2𝑉1 . There should be a more complicated relationship between them.
Although there is a significant difference between 1-needle and 2-needle design,
when the amount raises to 3 and 4, the improvement is not so obvious, especially when
it comes to 2-needles and 3-needles design, the lines of velocity nearly converge
together. While 4-needles has the highest line of speed, but the difference is still not so
remarkable. On the other hand, current-voltage characteristic curves in these designs
have more stable rising compared to each other. It should be mentioned that the
distances between each tip of the needle are not the same, but to the axes, a close range
will make the fields interactional, and that will weaken the field strength, so the
improvement will not as high as we expected. If we compare them with current velocity
curve which can be seen in Fig 29, the development on velocity is quiet little between
2,3,4-needles designs, but the current consumption will get much more. Also, the
change of some needles seems that has no influences on the onset voltage, theses four
designs have nearly the same onset voltage because the diameter of each needle and the
distance between electrodes remain no changes.
To sum up, a multi-needles design can enhance velocity compared to 1-neelde
design, but more needles will cause a mutual influence on each other, and the
improvement will be not as much as we wanted.
39
250
150
100
50
0
0
5
10
15
20
25
30
Voltage(kv)
1 Neeld
2 Needles
3 Needles
4 Needles
Figure 27 The comparison between different amount of needle in I-U
0.9
0.8
0.7
0.6
Velocity(m/s)
Current(uA)
200
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Voltage(kv)
1 Needle
2 Needles
3 Needles
4 Needles
Figure 28 The comparison between different amount of needle in V-U
40
0.9
0.8
Velocity(m/s)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
Current(uA)
1 Needle
2 Needles
3 Needles
4 Needles
Figure 29 The comparison between different amount of needle in V-I
3.8 Conclusion
From experiment 3.1 to 3.7, we obtained how several factors could affect the
corona discharge. As a result, we designed a corona discharge device as it can have
smallest onset voltage, bigger breakdown voltage, high wind speed. So due to the
comparisons from these experiments before, we at this moment use a needle with
0.15mm diameter as corona electrode, because a thin wire can have lower onset voltage
and higher speed, the needle is connected to high voltage cable and it can be changed
easily. Mesh 1 is used as collector electrode because the experiment shows it can have
not only more top speed but also the highest initial velocity so that the low-pressure
loss can allow the air cycling more smoothly. A plastic stick connects and fixes the mesh
and needle, both of their positions can be adjusted, so the distance G can be changed
flexibly, G=4.12cm because it will have a comparably lower onset voltage and higher
speed, but also can make the break-down delay. The wires connected to needle and
mesh will go through a plastic tube and link to the power supply and ground, the whole
41
length of the device is 20.50cm. This device will be tested in a container filled with
aerosol, because the device needs to go inside the container, the diameter of the mesh
has to be reduced to 10.25cm.
Figure 30 The corona discharge designing
42
4. Experiment (2) Set-up
A test stage will be set up to text the corona discharge device, There is a metal
barrel which has a diameter of 62cm, the height is 95cm, and the volume is 0.29m³.
Aerosol will be injected through a hole with a diameter of 8cm in the middle of the top.
Two holes on the both sides of the barrel, one has the diameter of 15cm and corona
discharge will be set in the barrel through it. Another is set 86cm from the top and has
the same diameter, where a fan will connect to and pump the air inside the barrel. The
whole barrel will be linked to ground.
Aerosol will be generated by a Laskin-nozzle aerosol generator, which container
the liquid named AEROSIL OX50, including 99.8% SiO2 and particle size is about
40nm. When putting air into the aerosol container, due to The Venturi Effect, aerosol
will be pulverized into tiny liquid particles and be injected from the top into the barrel
through a tube with a diameter of 2.54cm. The pressure of air pumped into the aerosol
container is keeping at 2bar.
A FAST MOBILITY PARTICLE SIZER™ SPECTROMETER MODEL 3091
(FMPS) is connected to the tube of existing, allows mixed air from container entering
it with a flux of 10L/min, or 0.6m³/h. This FMPS is used for measuring the distribution
of different particle sizes from 5.6~560nm, and the data can be demonstrated as total
number concentration, total mass concentration, and total volume concentration.
The mixed flow in the container will always be pumped out of it, and a fan
realizes this, a Testo 520D0E10 MODO2 digital manometer is linked to the tube of
existing and can measure the pressure range from 0~200mbar.
All of the experiments are conducted in the room with a temperature of 22℃ and
a standard pressure of 100Kpa. The whole operation device shows in Fig 31.
43
Figure 31 Experiment set-up with FMPS
4.1 Corona discharge texting
To test the practical function of corona discharge device, the device will be set on
the side of the container, the position can be shown in Fig 31. The volume flow rate of
the outlet remains 8m³/h. Air will be injected into aerosol generator under the pressure
of 2bar.
44
The fan will keep running while FMPS starts to record. As the total number
concentration stable, insert aerosol for 5 minutes, then stop to wait for 5 minutes. Again
pump aerosol for minutes but open corona discharge under negative discharge within
20kv. This operation will be conducted for three times while using a different amount
of needle, 1-needle, 2-needles, 3-needles design.
Figure 32 The comparison of different amount of needle in number concentration
with 25kv discharge
The data is collected, and we select data during the 5 minutes of injecting aerosol
and running corona discharge to have the average values, which is given in Table 5.
Then the result will be compared in Fig 33 and Fig 34.
Table 5 Comparison of total number concentration in different amount of neeldes
Total number
concentration
(#/cm³)
Elimination
efficiency
Without
Corona
6.9 × 106
45
1-Needle
2-Needles
3-Needles
4.5 × 106
4.1 × 106
3.9 × 106
34.19%
40.55%
42.21%
From the experiment result, without corona discharge, the total number
concentration is about 6.9 × 106 , while after using 1-Needle corona discharge, this
value can be reduced to 4.6 × 106 , decreasing by 34.19% if compared with non-corona
discharge condition. 2-Needles design can reduce the total number concentration by
about 40.55%, which improves 6% while 3-Neeldes design only increases by 2%
compared 2-Needles design. This result can also match with Chapter 3.7 as the amount
of needle increasing, the efficiency does not show a significant growing trend.
Number concnetration
Number concentration(#/cm³)
800000
700000
600000
500000
400000
300000
200000
100000
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Without corona
1-Needle
2-Needles
3-Needles
Figure 33 Number concentration under different amount of needles
From the graph of the number concentration distribution, particle sizes range from
6.04~453.2nm, while particles range from 34~60.4nm take the largest par. Under
corona discharge, the separation of particles causes the bigger size particles this field
will shift to 25.5~45.3nm because the aerosol particles are ionized and collected by the
mesh. Thus from Chapter 2.9, where the rate of coagulation can be described as
𝑑𝑁
𝑑𝑡
=
−𝐾0 𝑁 2 . The decreasing of the number of particles N will drop which will reduce the
rate of coagulation. In addition, 𝐾0 can be described as 𝐾0 = 4𝜋𝑑𝑝 𝐷 ,so that when
46
the rate of coagulation is decreasing, there will be less big size particles to be generated,
which will also reduce the rate.
The result also shows that there is a drop of number concentration between three
types of corona discharge, but the difference between 2-needles and 3-needles is not
noticeable if compared to 1-Needle design. The number concentration of particles
ranges from 25.5~45.3nm has a slight reduce, but it is nearly the same compared with
2-Needles and 3-Needles design.
Mass concentration
Mass concnetration(μg/lm³）
250
200
150
100
50
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Without corona
1-Needle
2-Needles
3-Needles
Figure 34 Mass concentration under different amount of neeldes
For the mass concentration, thought particles range from 80.6~453.2nm have only
a small number concentration, they take a large part of mass concentration because
normally the large size particles have much heavier mass. While with corona discharge,
there is a significant decrease in mass concentration, which shows that the separation
impedes the coagulation. It is evident that there is almost non-difference between 2Needls and 3-Needles design, the improvement is very limited.
47
4.2 Different positions of setting corona discharge
This experiment aims to find how the different positions can influence the result
of corona discharge. For each test, firstly inject aerosol with the pressure of 2bar until
the number concentration keeping stable, then start the corona discharge under 15kv,
as the number concentration stable, adjust power to 25kv. The volume flow rate of
outlet remains 13.6m³/h, and the diameter of the tube which injects aerosol is 2.54cm.
The positions of discharge can be seen in Fig 35. Position(a) is the same in
Chapter 4.1, the corona discharge is set on the side of the container. While position(b)
is established under the nozzle, so aerosol particles go through discharge field vertically.
Position(c) is also placed under the nozzle, but it has the same direction as aerosol
particles
Figure 35 Position (a) (b) (c)
48
Position(a)
Position(b)
49
Position(c)
Figure 36 Total number concentration in position(a) (b) (c) with 0kv,15kv,25kv
discharge
The total concentration can be seen in Fig 36, the difference between them is very
obvious. The initial total number concentration is nearly the same, around3.0 × 106 .
When to start to have corona discharge under 15kv, concentration shows a reduced
trend. Position(a) under 15kv reduces less than position(b), and when power supply
rises to 25kv, only a slight drop in position(a) while decreasing in position(b) is obvious.
Nevertheless, position(c) has a significant reduce of total number concentration.
Concentration goes sharply to 6.5 × 105 , nearly one-third compared with position(b),
as power rising to 25kv, concentration drops again and it has the lowest values between
three different positions.
50
Table 6 Comparison of total number concentration in different positions
Position
Total number
concentration
under 0kv
(#/cm³)
a
3163255
b
3058151
c
2887848
Total number Total number
concentration concentration
under15kv
under 25kv
(#/cm³ )
(#/cm³)
(Elimination
(Elimination
efficiency)
efficiency)
2130997
1333182
(32.63%)
(57.85%)
1727934
711424
(43.50%)
(76.74%)
656291
299668
(77.27%)
(89.62%)
The values of total number concentration are selected during the time when the
concentration is stable, then they are calculated to have the average values, which are
given in Table 6. Total number concentrations in three positions are nearly the same
when the discharge is not working. As power rising up, concentration reduces by 32.36%
under 15kv and 57.85% under 25kv in position(a), nearly increases by 25%. It reduces
nearly the half by 43.50% under 15kv in position(b), when power is growing to 25kv,
it increases to 76.74%. The result in position(c) is the best of them, it can reduce by
77.27% under 15kv, the efficiency is much higher compared to others, and it can even
reach to 89.62% under 25kv.
The result in position(a) is not as well as others because discharge device is set
on the side of the barrel so only particles in a particular area can be ionized by discharge,
even when power turns to the maximum, the flux of corona wind is not sufficient to
ionize much more particles. While both position(b) and position(c) are very close to the
nozzle, where has the highest concentration of particles. Thus, it is much easier to ionize
more particles for them. However, in position(b), the direction of corona wind is
vertical to the direction of aerosol flow, so some particles may cross electric field
51
directly which will reduce the number of particles charged. As power growing up,
corona wind gets faster, and this will have a larger influence on the motions of particles
which can change the direction of particles. So that there are more chances to ionize
particles. In position(c), corona wind and aerosol flow have the same direction, more
particles can be charged, and corona wind can also get a higher speed, so it is much
easier to collect particles. Also, the distance between electrodes is the only 4.12cm, so
the space of electrical field is extremely limited. So position(c) has an advantage
because most of the particles can pass the needle and get ionized, while for the
position(b), not all of the particles can go exactly through the needle and be ionized.
Number concentration
Number concentration(#/cm³)
400000
350000
300000
250000
200000
150000
100000
50000
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Position(a):Without discharge
Position(b):Without discharge
Position(c): Without discharge
Position(a):With discharge under 15kv
Position(b):With discharge under 15kv
Position(c):With discharge under 15kv
Figure 37 Number concentration with 15kv discharge under different positions.
52
Mass concentration
90
Mass concentration(μg/m³)
80
70
60
50
40
30
20
10
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Position(a):Without discharge
Position(b):Without discharge
Position(c):Without discharge
Position(a):With discharge under 15kv
Position(b):With discharge under 15kv
Position(c):With discharge under 15kv
Figure 38 Mass concentration with 15kv discharge under different positions
Number concentration
Number concentration(#/cm³)
400000
350000
300000
250000
200000
150000
100000
50000
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Position(a):Without discharge
Position(b):Without discharge
Position(c): Without discharge
Position(a):With discharge under 25kv
Position(b):With discharge under 25kv
Position(c):With discharge under 25kv
Figure 39 Number concentration with 25kv discharge under different positions.
53
Mass concentration
Mass concentration(μg/m³)
90
80
70
60
50
40
30
20
10
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Position(a):Without discharge
Position(b):Without discharge
Position(c):Without discharge
Position(a):With discharge under 25kv
Position(b):With discharge under 25kv
Position(c):With discharge under 25kv
Figure 40 Mass concentration with 25kv discharge under different positions
Fig 37 to Fig 40 show how the number concentration and mass concentration
change during the experiment. It is evident that there is a huge drop in the number and
mass concentration when power turns to from 0kv to 25kv, especially for the position(c),
the number and mass concentration reduce nearly to 90%. Due to the separation, the
line of number concentration shifts to left side slightly. Position (c) has the best collect
efficiency even when it supplies with 15kv voltage, both of number and mass
concentration is much lower than it in position(a) and (b) with the 25kv voltage supply.
The number concentration in position(c) is lower than others, this mainly depends on
the hinder of mesh. When aerosol goes through discharge device, they will collide to
the mesh where can cause the pressure loss and give particles more time to have
coagulation. Which can also show in Fig 38, the line of mass concentration is slightly
higher compared to position(b). However, there are more big size particles in position(a)
when the discharge is not working, somehow coagulation occurs in advance, so there
are more big size particles though the total number concentration is nearly the same.
54
Nevertheless, it is still obvious that discharge set in position(c) can have the highest
collect efficiency.
Figure 41 Increasing the velocity of aerosol in (d) and (e)
Another idea aims to increase the velocity of aerosol by adding a muzzle with a
smaller diameter, reducing from 2.54cm to 1.32cm, which shows in Fig 4.10.
Position(d)
55
Position(e)
Figure 42 Total number concentration in position (d) (e) with 0kv,15kv,25kv
discharge
The result is given in Fig 42, when the velocity of the aerosol is improved, there
is less number concentration in the barrel, especially in position(e). This result is mainly
caused by the good condition for coagulation. When the aerosol stream is coming inside
the barrel with high speed, the mixture is better than before and then, there will be more
chances for particles to collide with each other and coagulation will happen more
frequently. As a result, particles change to big size particles and the number will reduce.
For position(e), the aerosol stream will go through the mesh, which may have a hinder
function for particles. Thus, there is more resident time for particles to mix, and
coagulation occurs even more.
56
Table 7 Comparison of total number concentration in faster velocity of aerosol
Position
Total number
concentration
under 0kv
(#/cm³channel)
d
2447086
e
2127784
Total number
concentration
under15kv
(#/cm³channel)
(Elimination
efficiency)
1862913
(23.87%)
756734
(64.44%)
Total number
concentration
under 25kv
(#/cm³channel)
(Elimination
efficiency)
1483171
(39.40%)
412834
(80.60%)
By calculating the average values, the average total number concentration during
the time when total number concentration is stable can be seen in Table 7. Due to adding
a smaller diameter of the nozzle, there is a mechanical loss when aerosol goes through
it. Thus, the initial number concentration is lower than the result given in Table 6.
When the speed of aerosol streams increases, the collect efficiency is not getting
higher. By contraries, efficiency shows a trend of decrease, especially when the position
is set as position(d), almost drops a half compared to position(b). The reason may be
caused as when high-speed aerosol passes the electrical field, there is not sufficient time
for corona discharge to ionize the particles which travel in a vertical direction of corona
wind and across the electrical field directly. So a part of particles cannot be ionized,
and efficiency reduces.
Although there is also a drop of collect efficiency when the device is set as
position(e), the influence is not as much as position(d). The collect efficiency is still the
highest and also can reach to 80% when the voltage increases to 25kv. Because aerosol
particles and corona wind have the same direction, it is much easier for the needle to
catch and ionize particles. Thus, position(e) can still work well with high-speed aerosol.
57
From Fig 43 and 44, it shows that there are more big size particles in position(e).
Because aerosol stream must pass the complete discharge device, so there are more
speed loss and more resident time to have coagulation. The line of number
concentration shifts to the right side and higher mass concentration to a great size
particle.
Number concentration
300000
250000
200000
150000
100000
50000
0
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
Number concentration(#/cm³)
350000
Particle diameter(nm)
Position(d):Without discharge
Position(e):Without discharge
Position(d):With discharge under 15kv
Position(e):With discharge under 15kv
Position(d):With discharge under 25kv
Position(e):With discharge under 25kv
Figure 43 Number concentration with high-speed aerosol
58
Mass concentration
Mass concentration(μg/m³)
60
50
40
30
20
10
6.04
6.98
8.06
9.31
10.8
12.4
14.3
16.5
19.1
22.1
25.5
29.4
34
39.2
45.3
52.3
60.4
69.8
80.6
93.1
107.5
124.1
143.3
165.5
191.1
220.7
254.8
294.3
339.8
392.4
453.2
523.3
0
Particle diameter(nm)
Position(d):Without discharge
Position(e):Without discharge
Position(d):With discharge under 15kv
Position(e):With discharge under 15kv
Position(d):Wit discharge under 25kv
Position(e):With discharge under 25kv
Figure 44 Mass concentration with high-speed aerosol
4.3 Conclusion
Chapter 4 conducts several experiments to test the corona discharge designing in
a barrel with aerosol stream. From results, the corona discharge can have the effect on
collecting aerosol particles, which causes both of number and mass concentration
decreasing. Also due to the function of separation, more small particles are being
generated which hinder coagulation. As voltage growing up, this collect effect gets to
more obvious. Also, the collect efficiency relates to the velocity of corona wind. As we
see in Chapter 3.7, more needles can have a higher speed. Thus, greater collect
efficiency can be realized. However, the improvement of collect efficiency is limited
when needles get more.
Also, the experiment shows that the position of corona discharge has a huge
influence on collect ability. When corona discharge device has the same direction of
59
the aerosol stream, because particles will move towards the electrical field, it is much
easier for particles to get ionized and be collected. While if the corona discharge device
was set on the side of the barrel, the efficiency is not very high because the speed of
corona wind is very limited and cannot have sufficient time to ionize more particles in
the barrel. When the direction of corona wind is vertical to the direction of aerosol, the
result is not as well as that when corona wind and aerosol have the same direction but
also much higher than it when the device is set on the side of the barrel.
Thus, it is evident that when a corona discharge is set to close to the entrance of
aerosol, where gathers high concentration of particles, the collect efficiency can always
be high. However, when the velocity of aerosol gets higher, collect efficiency seems to
have a drop. Fast particles have the disadvantage to be ionized by the needle.
60
5. Future study
The previous research is focusing on corona discharge with only one high voltage
supply. Although there is a significant increasing of corona wind speed after several
improvements, this development is limited because voltage supply can only reach to a
certain value. Otherwise, there will be breakdown and spark.
So the idea is to set two or more voltage supplies, which can be seen in Fig 43.
The mechanism of this two-stages corona discharge is the same as the previous study,
but when natural molecules are collided by moving ionized particles and pass through
the mesh, another corona discharge needle will ionize them. Thus, these natural
molecules with initial dynamic energy will be accelerated again in the next stage, which
will have a higher speed and larger ionizing area.
While it is still cannot be sure whether this two-stages discharge device can
largely improve or not, and also how long the distance between two stages should be
set is remained unclear, which should be investigated in the further study.
Figure 45 Two stages corona discharge device
61
6. Summary
This project aims to optimize a corona discharge device. First of all, from the
theoretical knowledge we know that the corona discharge happens when there is an
extremely uneven electrical field, frequently this can be realized by a needle which is
connected to high voltage. Thus, this partial strong electrical field can ionize particles
around the needle. Also, as ionized particles move to the other electrode, some natural
air molecules will be collided and obtain the dynamic energy to travel towards the
electrical field, which can generate corona wind.
In a process of corona discharge, the on-set voltage is an imperative factor for a
corona discharge. From the experiment (1) set-up, we verify that the both of current and
velocity of corona wind can increase as voltage growing up, and an early onset voltage
discharge can reach to higher current and higher speed. While as the voltage is high
enough, the breakdown will happen, and corona wind cannot be faster anymore, corona
discharge has lost the effect.
Because the different mechanism of corona discharge in the positive and negative
side, negative discharge can always have a lower onset voltage and higher breakdown
voltage. A thinner needle with a sharp tip can have an absolute field strength, so it is
much easier to have corona discharge. Close distance between electrodes can also
reduce the onset voltage, but it will also make corona discharge to have break-down in
advance because the electrical stream can quickly reach to another electrode. The shape
of mesh does not have an influence on the onset voltage, so current is nearly the same.
However, it can significantly affect the speed of corona wind because of the pressure
drop when airflow goes through it. We also text a multi-discharge with two, three and
four needles. Some needles seem to have no influence on the onset voltage, but current
and velocity can largely increase when more needles are being set. This may due to a
larger ionize area so that more particles can be ionized. While as amount increasing
62
more, the improvement seems to be very limited because each needle can influence
other needles, so it is very complex to explain it.
Based on experiment (1) set-up, we designed a corona wind discharge which can
be set inside a barrel. Thus, experiment (2) set-up will test this corona discharge with
aerosol flows.
By using the discharge device, aerosol particles can be ionized and collected by
the electrode. The result shows that there is a noticeable drop in number and mass
concentration. Also, due to the separation which hinders coagulation, so there are less
big size particles existing in the barrel.
The position of setting the discharge device also has a significant influence. It is
vital to set discharge device nearby the entrance of aerosol so that a much higher collect
efficiency can be obtained, especially when the corona wind has the same direction of
aerosol flows, collect efficiency can even reach to 90%.
For further study, based on the previous conclusion, a two-stages corona
discharge can be tested with experiment set-up, which aims to have a higher collect
efficiency.
63
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versus approximate formulations, Ieee Transactions on Dielectrics
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65
of
High
Voltage
Appendix
Experiment(1)set-up
Corona wind speed measurement
66
Corona discharge device
Collect electrode
67
Experiment(2)set-up
Laskin-nozzle Aerosol generator
68
Fan in experiment(2)
FMPS Device
69
Corona discharge device
Position(a)
70
Position(b)
Position(c)
71
Aerosol with higher speed
72