U.S. patent application number 12/599782 was filed with the patent office on 2011-03-03 for enhancing gas-phase reaction in a plasma using high intensity and high power ultrasonic acoustic waves.
This patent application is currently assigned to FORCE TECHNOLOGY. Invention is credited to Alexander Bardenshtein, Henrik Bindslev, Niels Krebs, Yukihiro Kusano.
Application Number | 20110048251 12/599782 |
Document ID | / |
Family ID | 39711085 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110048251 |
Kind Code |
A1 |
Bardenshtein; Alexander ; et
al. |
March 3, 2011 |
ENHANCING GAS-PHASE REACTION IN A PLASMA USING HIGH INTENSITY AND
HIGH POWER ULTRASONIC ACOUSTIC WAVES
Abstract
This invention relates to enhancing a gas-phase reaction in a
plasma comprising: creating plasma (104) by at least one plasma
source (106), and wherein that the method further comprises:
generating ultrasonic high intensity and high power acoustic waves
(102) having a predetermined amount of acoustic energy by at least
one ultrasonic high intensity and high power gas-jet acoustic wave
generator (101), where said ultrasonic high intensity and high
power acoustic waves are directed to propagate towards said plasma
(104) so that at least a part of said predetermined amount of
acoustic energy is absorbed into said plasma (104), and where a
sound pressure level of said generated ultrasonic high intensity
and high power acoustic waves (102) is at least substantially 140
dB and where an acoustic power of said generated ultrasonic high
intensity and high power acoustic waves (102) is at least
substantially 100 W. In this way, a high sound intensity and power
are obtained that efficiently enhances a gas-phase reaction in the
plasma, which enhances the plasma process, e.g. enabling more
efficient ozone or hydrogen generation using plasma in relation to
reaction speed and/or obtained concentration of the generated
compound. Other processes including plasma like exhaust gas
cleaning, pollution control, odor removal, fuel conversion,
sterilization, and oxidation can also be enhanced.
Inventors: |
Bardenshtein; Alexander;
(Hedehusene, DK) ; Bindslev; Henrik;
(Charlottenlund, DK) ; Krebs; Niels; (Hellerup,
DK) ; Kusano; Yukihiro; (Roskilde, DK) |
Assignee: |
FORCE TECHNOLOGY
Brondby
DK
TECHNICAL UNIVERSITY OF DENMARK
Kgs. Lyngby
DK
|
Family ID: |
39711085 |
Appl. No.: |
12/599782 |
Filed: |
May 13, 2008 |
PCT Filed: |
May 13, 2008 |
PCT NO: |
PCT/EP2008/055801 |
371 Date: |
November 12, 2010 |
Current U.S.
Class: |
99/451 ;
204/157.3; 315/111.21; 426/240 |
Current CPC
Class: |
H05H 1/2475 20130101;
C23C 16/4415 20130101; H01J 37/32321 20130101; H05H 2001/2493
20130101; Y10T 428/31 20150115 |
Class at
Publication: |
99/451 ;
315/111.21; 204/157.3; 426/240 |
International
Class: |
H01J 7/24 20060101
H01J007/24; B01J 12/00 20060101 B01J012/00; A23L 3/26 20060101
A23L003/26 |
Foreign Application Data
Date |
Code |
Application Number |
May 11, 2007 |
DK |
PA 2007 00717 |
Claims
1. A method of enhancing a gas-phase reaction in a plasma
comprising: creating plasma (104) by at least one plasma source
(106), and characterized in that the method further comprises:
generating ultrasonic high intensity and high power acoustic waves
(102) having a predetermined amount of acoustic energy by at least
one ultrasonic high intensity and high power gas-jet acoustic wave
generator (101), where said ultrasonic high intensity and high
power acoustic waves are directed to propagate towards said plasma
(104) so that at least a part of said predetermined amount of
acoustic energy is absorbed into said plasma (104), and where a
sound pressure level of said generated ultrasonic high intensity
and high power acoustic waves (102) is at least substantially 140
dB and where an acoustic power of said generated ultrasonic high
intensity and high power acoustic waves (102) is at least
substantially 100 W.
2. A method according to claim 1, wherein said sound pressure level
of said generated ultrasonic high intensity and high power acoustic
waves (102) is at least substantially 150 dB, at least
substantially 160 dB, at least substantially 170 dB, at least
substantially 180 dB, at least substantially 190 dB, or at least
substantially 200 dB.
3. A method according to claim 1, wherein said acoustic power of
said generated ultrasonic high intensity and high power acoustic
waves (102) is at least substantially 200 W, at least substantially
300 W, at least substantially 400 W, about 400 W, greater than
substantially 400 W, at least substantially 500 W, at least
substantially 1 kW, or selected from about 1-2 kW.
4. A method according to claim 1, wherein said plasma source (106)
comprises at least one source selected from a group of: a
dielectric barrier discharge (DBD) plasma source, a surface
discharge (SD) plasma source, a volume discharge (VD) plasma
source, a plasma torch source, an arc plasma torch, a gliding arc
plasma torch, a cold plasma torch, a pencil-like torch, a direct
current plasma source, a capacitively coupled plasma source, a
pulsed plasma source, a magnetron plasma source, an electron
cyclotron resonance plasma source, an inductively coupled plasma
source, a helicon plasma source, a helical resonator plasma source,
a microwave plasma source, an atmospheric pressure plasma jet
(APPJ) source, a barrier torch, an arc microwave torch, a corona
discharge plasma source, a micro-plasma source, a low pressure
plasma source, and a high pressure plasma source.
5. A method according to claim 1, wherein a working gas pressure at
an inlet of said at least one ultrasonic high intensity and high
power gas-jet acoustic wave generator (101) is between
approximately 1.9 and approximately 5 bar.
6. A method according to claim 1, wherein said plasma (104) is
created at atmospheric pressure.
7. A method according to claim 1, wherein said plasma source (106)
comprises at least one electrode (103; 103') and wherein one
electrode (103') of said at least one electrode (103; 103') is a
mesh type of electrode.
8. A method according to claim 1, wherein the generated ultrasonic
high intensity and high power acoustic waves are propagated towards
a membrane (401) so that any gases used by the at least one
ultrasonic high intensity and high power acoustic wave generator
(101) is not mixed with one or more gases (111) used by said plasma
source (106) to create said plasma (104).
9. A method according to claim 1, wherein the generated ultrasonic
high intensity and high power acoustic waves (102) are generated
using a gaseous medium (121) and where the acoustic waves (102) are
directed towards said plasma (104) and wherein said gaseous medium
(121) after exit of said at least one ultrasonic high intensity and
high power gas-jet acoustic wave generator (101) is directed away
from said plasma (104).
10. A method according to claim 1, wherein a gas mixture (111) used
for creating the plasma (104) is supplied to at least one electrode
(103; 103') of the plasma source (106) substantially in a direction
that said ultrasonic acoustic waves propagate towards said plasma
(104).
11. A method according to any claim 1, wherein said at least one
ultrasonic high intensity and high power gas-jet acoustic wave
generator (101) is selected from the group of: a Hartmann type
gas-jet generator, a Levavasseur type gas-jet generator, a
generator comprising an outer part (305) and an inner part (306)
defining a passage (303), an opening (302), and a cavity (304)
provided in the inner part (306), where said ultrasonic high
intensity and high power gas-jet acoustic wave generator (101) is
adapted to receive a pressurized gas and pass the pressurized gas
to said opening (302), from which the pressurized gas is discharged
in a jet towards the cavity (304), a generator of any of the above
mentioned types, which includes any type of concentrators or
reflectors of acoustic waves
12. A method according to claim 1, wherein a food item is subjected
to the plasma (104) where the creation of the plasma (104)
generates chemical radicals and sterilizes the food item.
13. A method according to claim 1, wherein said generating
ultrasonic high intensity and high power acoustic waves (102)
comprises: generating high intensity and high power acoustic waves
(102) by a first acoustic wave generator (101) using a gaseous
medium (121) where the gaseous medium (121), after exit from the
first acoustic wave generator (101), has a first principal
direction (A) that is different from a second principal direction
(B) of the high intensity and high power acoustic waves (102)
generated by the first acoustic wave generator (101), generating
high intensity and high power acoustic waves (102) by a second
acoustic wave generator (101'), where the first (101) and second
acoustic wave generators (101') are located in relation to each
other so that at least a part of the generated high intensity
acoustic waves (102), being generated by said second acoustic wave
generator (101'), is directed towards at least a part of the
gaseous medium (121) after exit from said first acoustic wave
generator (101).
14. A method according to claim 1, wherein said plasma (104) is
used in a process selected from the group of: ozone generation,
hydrogen production, exhaust gas cleaning, pollution control, odor
removal, fuel conversion, sterilization, and oxidation.
15. A system for enhancing a gas-phase reaction in a plasma
comprising: at least one plasma source (106) adapted to create
plasma (104), characterized in that the system further comprises:
at least one ultrasonic high intensity and high power gas-jet
acoustic wave generator (101) adapted to generate ultrasonic high
intensity and high power acoustic waves (102) having a
predetermined amount of acoustic energy and being directed to
propagate towards said plasma (104) so that at least a part of said
predetermined amount of acoustic energy is absorbed into said
plasma (104), and where a sound pressure level of said generated
ultrasonic high intensity and high power acoustic waves (102) is at
least substantially 140 dB and where an acoustic power of said
generated ultrasonic high intensity and high power acoustic waves
(102) is at least 100 W.
16. A system according to claim 15, wherein the sound pressure
level of said generated ultrasonic high intensity and high power
acoustic waves (102) is at least substantially 150 dB, at least
substantially 160 dB, at least substantially 170 dB, at least
substantially 180 dB, at least substantially 190 dB, or at least
substantially 200 dB.
17. A system according to claim 15, wherein said acoustic power of
said generated ultrasonic high intensity and high power acoustic
waves (102) is at least substantially 200 W, at least substantially
300 W, at least substantially 400 W, about 400 W, greater than
substantially 400 W, at least substantially 500 W, at least
substantially 1 kW, or selected from about 1-2 kW.
18. A system according to claim 15, wherein said plasma source
(106) comprises at least one source selected from a group of: a
dielectric barrier discharge (DBD) plasma source, a surface
discharge (SD) plasma source, a volume discharge (VD) plasma
source, a plasma torch source, an arc plasma torch, a gliding arc
plasma torch, a cold plasma torch, a pencil-like torch, a direct
current plasma source, a capacitively coupled plasma source, a
pulsed plasma source, a magnetron plasma source, an electron
cyclotron resonance plasma source, an inductively coupled plasma
source, a helicon plasma source, a helical resonator plasma source,
a microwave plasma source, an atmospheric pressure plasma jet
(APPJ) source, a barrier torch, an arc microwave torch, a corona
discharge plasma source, a micro-plasma source, a low pressure
plasma source, and a high pressure plasma source.
19. A system according to claim 15, wherein a working gas pressure
at an inlet of said at least one ultrasonic high intensity and high
power gas-jet acoustic wave generator (101) is between
approximately 1.9 and approximately 5 bar.
20. A system according to claim 15, wherein said plasma (104) is
created at atmospheric pressure.
21. A system according to claim 15, wherein said plasma source
(106) comprises at least one electrode (103; 103') and wherein one
electrode (103') of said at least one electrode (103; 103') is a
mesh type of electrode.
22. A system according to claim 15, wherein said system further
comprises a membrane (401) and where the system is adapted to
propagate the generated ultrasonic high intensity and high power
acoustic waves towards the membrane (401) so that any gases used by
the at least one ultrasonic high intensity and high power acoustic
wave generator (101) is not mixed with one or more gases (111) used
by said plasma source (106) to create said plasma (104).
23. A system according to claim 15, wherein the generated
ultrasonic high intensity and high power acoustic waves (102) are
generated using a gaseous medium (121) and where the acoustic waves
(102) are directed towards said plasma (104) and wherein said
gaseous medium (121) after exit of said at least one ultrasonic
high intensity and high power gas-jet acoustic wave generator (101)
is directed away from said plasma (104).
24. A system according to claim 15, said plasma source (106)
comprises at lease one electrode (103; 103') and wherein a gas
mixture (111) used for creating the plasma (104) is supplied to the
at least one electrode (103; 103') substantially in a direction
that said ultrasonic acoustic waves propagates towards said plasma
(104).
25. A system according to claim 15, wherein said at least one
ultrasonic high intensity and high power gas-jet acoustic wave
generator (101) is selected from the group of: a Hartmann type
gas-jet generator, a Levavasseur type gas-jet generator, a
generator comprising an outer part (305) and an inner part (306)
defining a passage (303), an opening (302), and a cavity (304)
provided in the inner part (306), where said ultrasonic high
intensity and high power gas-jet acoustic wave generator (101) is
adapted to receive a pressurized gas and pass the pressurized gas
to said opening (302), from which the pressurized gas is discharged
in a jet towards the cavity (304), a generator of any of the above
mentioned types, which includes any type of concentrators or
reflectors of acoustic waves.
26. A system according to claim 15, wherein a food item is
subjected to the plasma (104) where the creation of the plasma
(104) generates chemical radicals and sterilizes the food item.
27. A system according to claim 15, wherein said at least one
ultrasonic high intensity and high power gas-jet acoustic wave
generator (101) comprises a first acoustic wave generator (101) for
generating high intensity acoustic waves (102) using a gaseous
medium (101) where the gaseous medium (101) after exit from said
first acoustic wave generator (101) has a first principal direction
(A) that is different from a second principal direction (B) of
generated high intensity acoustic waves (102) being generated by
said first acoustic wave generator (101), and at least a second
acoustic wave generator (101') for generating high intensity
acoustic waves (102), where said first (101) and second acoustic
wave generators (101') are located in relation to each other so
that at least a part of the generated high intensity acoustic waves
(102), being generated by one of said first (101) and second (101')
acoustic wave generator, is directed towards at least a part of the
gaseous medium (101) after exit from the other of said first (101)
and said second (101') acoustic wave generator.
28. A system according to claim 14, wherein said plasma (104) is
used in a process selected from the group of: ozone generation,
hydrogen production, exhaust gas cleaning, pollution control, odor
removal, fuel conversion, sterilization, and oxidation.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method of enhancing gas-phase
reaction in a plasma. The invention further relates to a system for
enhanced gas-phase reaction in a plasma.
BACKGROUND OF THE INVENTION
[0002] A plasma is an ionized gas. Active species of ions,
electrons, high-energy neutrals, radicals as well as ultra-violet
emission in plasmas can be used for various things. Plasma
generation or a generated plasma may also advantageously be
utilized for various purposes or applications like ozone
generation, hydrogen production, exhaust gas cleaning, pollution
control, odor removal, fuel conversion, sterilization, oxidation,
etc.
[0003] Ozone can be formed by recombination in triplets of single
oxygen atoms which are split from the diatomic oxygen in a plasma.
Fuel conversion is the chemical or physical transformation of a
naturally occurring or already modified fuel to improve the quality
of the fuel. A fuel conversion process may result in one or more
upgraded fuel products which may be solid, liquid, or gaseous, and
may generate chemicals or raw materials for chemical manufacture.
For example, coal generally requires size reduction, washing, and
removal of inert species. Natural gas may need removal of H2S and
CO2 with separation of gas liquids and C2 compounds. However,
"gas-to-liquid" fuel conversion (liquefaction) is also of
significant importance. Here CH.sub.4, which is the major component
of natural gas, is converted to liquid fuels in one or more than
one step chemical reactions.
[0004] A variety of plasmas exists, including direct current
plasmas, capacitively coupled plasmas, pulsed plasmas, magnetron
plasmas, electron cyclotron resonance plasmas, inductively coupled
plasmas, helicon plasmas, helical resonator plasmas, microwave
plasmas, and plasma jets (see e.g. A Bogaerts et al. Spectrochimica
Pt.B 57 (2002) 609-658.). Many of them are operated at low
pressures, suffering from the drawbacks that they require expensive
vacuum systems. Furthermore, methods are only well-developed for
batch or semi-batch treatments. To overcome these drawbacks an
atmospheric pressure plasma surface modification system can be used
that not only avoids the need for vacuum equipment but also permits
both the surface modification of large objects and production line
continuous surface modification (see e.g. C Tendero et al.
Spectrochimica Pt.B 61 (2006) 2-30.).
[0005] A prior art plasma application system is shown in FIG. 1 and
is explained in more detail in the following. FIG. 1 illustrates an
example of capacitively coupled plasma of the well-known so-called
dielectric barrier discharge (DBD) type usable at atmospheric
pressure.
[0006] Other types or variations of plasma sources include
dielectric barrier discharges (DBDs) with a single dielectric
barrier located substantially in the middle between the two
electrodes or with a single dielectric barrier covering only one of
the electrodes. Such plasma sources are typically also referred to
as volume discharge (VD) sources where a micro-discharge can take
place in thin channels generally randomly distributed over the
electrode- and/or dielectric-surface. Other DBD plasma sources
include so-called surface discharge (SD) plasma sources typically
comprising a number of surface electrodes on a dielectric layer and
a counter-electrode on the reverse side of the dielectric layer.
Such SD plasma sources may include a so-called SPCP
(Surface-discharge-induced Plasma Chemical Processing) discharge
element or CDSD (Coplanar Diffuse Surface Discharge) element. In a
SPCP, electrodes are attached on the dielectric(s) and in a CDSD
the electrodes are embedded in the dielectric(s).
[0007] Other types of plasma sources are e.g. so-called plasma
torches such as arc plasma torches, cold plasma torches (see e.g. H
Mortensen et al. Jpn. J. Appl. Phys. 45(10B) (2006) 8506-8511.),
atmospheric pressure plasma jet (APPJ), pencil like torches,
barrier torches, and microwave torches (see e.g. C Tendero et al.
Spectrochimica Pt.B 61 (2006) 2-30.). Yet another type of plasma
source is the so-called gliding arc (see for example A Fridman et
al. J. Phys. D Appl. Phys. 38 (2005) R1-R24).
[0008] Additional types of plasma sources are low pressure plasmas,
corona discharge (see e.g. A Bogaerts et al. Spectrochimica Pt.B 57
(2002) 609-658) and microplasmas (see e.g. V Karanassios
Specrochimica Acta Pt.B 59 (2003) 909-928). See e.g. A Bogaerts et
al. Spectrochimica Pt.B 57 (2002) 609-658, U Kogelschatz Plasma
Chem. Plasma Proc. 23(1) (2003) 1-46, C Tendero et al.
Spectrochimica Pt.B 61 (2006) 2-30 and A Fridman et al. J. Phys. D
Appl. Phys. 38 (2005) R1-R24) for further details of plasmas and
atmospheric pressure plasmas.
[0009] The three articles `Ozone generation by hollow-needle to
plate electrical in an ultrasound field`, J. Phys. D: Appl. Phys.
37 (2004) 1214-1220,`Ultrasound and airflow induced thermal
instability suppression of DC corona discharge: an experimental
study`, Plasma Sources Sci. Technol. 15 (2006) 52-58, and
`Ultrasonic resonator with electrical discharge cell for ozone
generation`, Ultrasonics 46 (2007) 227-234, by Stanislav Pekarek,
Rudolf Balek et al. disclose suppression of DC corona discharge
where ultrasound or ultrasound combined with an airflow is used in
connection with a hollow needle-to-plate electrode system to
activate the corona discharge for ozone production. It was also
found that the application of ultrasound waves increases ozone
generation.
[0010] The article `Improvement of Charging Performance of Corona
Charger in Electrophotography by Irradiating Ultrasonic Wave to
Surrounding Region of Corona Electrode` (Kwang-Seok Choi, Satoshi
Nakamura and Yuji Murata Jpn. J. Appl. Phys. 44(5A) (2005)
3248-3252.) discloses improvement of the charging speed of a corona
charger in electrophotography using an ultrasonic wave where the
ultrasonic wave increases the charge density on an insulator layer
of a coated aluminum drum used instead of a photoreceptor drum used
for printing. At least some of the findings in the articles have
also been disclosed in patent application CZ 295687.
[0011] The ultrasonic generators disclosed in the above-mentioned
articles are based on piezoelectric transducers. No mention is
given of specific or preferred sound pressure levels of the emitted
acoustic waves or ultrasonic waves or the advantages thereof.
[0012] Furthermore, the articles mention that the acoustic
pressures developed by ultrasonic layouts are, respectively, of the
order of 2 and 10 kPa near the emitting surface of the transducer
at the frequency of generated ultrasound of 20.3 kHz. In the fourth
article, the ultrasonic generator is a 28-kHz 50-mm-diam and
80-mm-height bolt-clamped Langevin-type piezoelectric transducer.
The maximum input power is 50 W. These values give an estimation of
the emitted acoustic pressure value to be approximately 2 kPa. The
pressure values of 2 and 10 kPa correspond to very high sound
pressure levels of 160 and 174 dB above the reference pressure of
20 .mu.Pa. It can be estimated that the above-specified acoustic
pressures at the above frequency correspond to ultrasound
intensities of 4.4 and 20 kW/m.sup.2 or the sound intensity levels
of 156 and 163 dB above the reference intensity of 1 pW/m.sup.2.
This characterizes the ultrasonic acoustic waves being applied in
at least some of these articles as high-intensity. However, the
acoustic power provided according to the articles is in fact too
low and too localized to allow for efficient and high-volume
enhancement of the gas-phase in a plasma.
[0013] A similar situation can be outlined regarding the patent
specification U.S. Pat. No. 6,391,118. It discloses a method for
removing particles from the surface of an article in an apparatus
using corona discharge. The particles are supplied with an electric
charge and subsequently an ultrasonic wave or gas stream is applied
onto the surface of the article while an electric field is applied
for driving away the electrically charged solid particles from the
surface. The application of an ultrasonic wave and/or gas further
facilitate the removal of the electrically charged solid particles.
The variety of ultrasonic generators (oscillators) here includes a
piezoelectric oscillator, a polymer piezoelectric membrane, an
electrostrictive oscillator, a Langevin oscillator (that is as
mentioned above just a special type of piezoelectric transducers),
a magnetostrictive oscillator, an electrodynamic transformer, and a
capacitor transformer. Use of such oscillators provides acoustic
power that is low (no more than 50 W) and localized. It is too low
and localized to allow for efficient and high-volume enhancement of
the gas-phase in a plasma. Moreover, no disclosure is given of a
specific or preferred sound pressure level of the emitted
ultrasonic waves or the advantages thereof.
[0014] The article `Current Waveforms of Electric Discharge in Air
under High-Intensity Acoustic Standing Wave Field`, Japanese
Journal of Applied Physics, Vol. 43, No. 5B, 2004, pp. 2852-2856,
Nakane et al. also discloses an electric discharge phenomenon in a
high-intensity acoustic field. Here standing waves with a frequency
of 660 Hz are used, which limits a discharge volume for ozone
production. The enhanced ozone production according to this article
can be explained by Paschen's law signifying that the lower the
static pressure, the higher the growth rate of the discharge
streamers. Therefore, the streamers should grow more intensively in
the nodes of standing waves. Another part of the proposed mechanism
is that streamer channels oscillate in the acoustic field.
[0015] Patent application US 2003/0165636 discloses a process for
atmospheric pressure plasma surface modification of an object's
surface where excitation of the surface to be treated is done so
that it vibrates and undulates thereby activating the application
of plasma. The energy for excitation of the surface may come from
the process of creating the plasma, from an external source, or
from a combination thereof. The energy for excitation of the
surface may come from a vibration generator brought in contact with
the object to be treated or by indirect contact from a vibration
generator emitting acoustic waves, e.g. ultrasonic waves, to the
object to be treated so that it provokes turbulent plasma. No
disclosure is given of a specific or preferred sound pressure level
of the acoustic waves or the ultrasonic waves or the advantages
thereof. Therefore, exciting surface vibrations and undulations, or
in other words, generation of guided and surface acoustic waves on
the object is suggested in order to intensify a plasma treatment.
Correspondingly, it is disclosed that the vibration of the surface
to be treated can be the result of excitation at one or several
eigenfrequencies and their harmonics associated with the body of
the object to be treated. Thus, either the range of the
characteristic dimension of the modified object (primarily its
thickness) is strictly limited by the operating frequency of the
used source of acoustic energy, or the said frequency is strictly
determined by the dimension of the object. It is also disclosed
that the vibration of the surface can also result from forced
frequencies when an external generator of acoustic waves emits
frequencies that are not harmonics of the eigenfrequencies of the
object to be treated. This signifies generation of surface acoustic
waves (primarily the Rayleigh surface waves).
[0016] The following procedures of transfer of acoustic power into
ambient gas/plasma are mentioned: [0017] 1. External acoustic
generator.fwdarw.Treated object surface vibration.fwdarw.Gas
molecules (plasma particles) vibration. [0018] 2. Generation of the
treated surface object vibration directly, for instance through a
direct acoustic contact.fwdarw.Gas molecules (plasma particles)
vibration.
[0019] Both procedures require acoustic waves to overpass the
solid/gas interface at least once. However, due to more than
four-orders-of-magnitude difference in acoustic impedance for a
solid and a gas, most of generated acoustic power cannot be emitted
(and especially re-emitted) into the gas atmosphere and remains in
a solid being ultimately converted into thermal energy. Thus, it is
not possible in this way to generate sound or ultrasound in the air
with a power that would be enough to enable efficient and
high-volume enhancement of a gas-phase in a plasma. Therefore, it
is of prime importance not simply to "shake" the surface up and
provoke uncontrolled turbulent plasma with unknown efficiency and
spatial distribution in such a way, but rather to enable efficient
and high-volume enhancement of the gas-phase in a plasma.
[0020] The generated acoustic power is relatively low (at any rate,
below 100 W) because the power applied to the acoustic wave
transmitter is actually 100 W, and the efficiency of sound
generation in a gas atmosphere cannot exceed .about.30% even for
the most effective gas-jet ultrasonic transmitters, not to mention
other methods.
[0021] A principal impediment to the generation of a plasma or
other plasma processes are how to enable a very efficient gas-phase
reaction over large volumes in a plasma.
[0022] None of the mentioned prior art disclosures specify an
acoustic power level sufficient to efficiently allow for efficient
and high-volume enhancement of the gas-phase in a plasma.
[0023] Furthermore, the prior art involving piezoelectric
transducers or other transducers involving a solid to transfer the
energy, only provides the energy in a very localized fashion, e.g.
very close to the piezoelectric transducer (or other solid
transducer) and is therefore not suitable for high volume gas-phase
enhancement.
SUMMARY OF THE INVENTION
[0024] It is an object of the invention to provide a method of and
a system for enhanced gas-phase reaction in a plasma that
alleviates the above-mentioned shortcomings of prior art at least
to an extent.
[0025] It is another object to enable enhancement of a plasma
gas-phase reaction in a large volume.
[0026] An additional object is to speed a plasma process up.
[0027] A further object of some embodiments is to enhance a
generation process involving a plasma--e.g. hydrogen, ozone or
syngas generation using plasma, exhaust gas cleaning, pollution
control, odor removal, fuel conversion, liquefaction,
sterilization, oxidation, etc. using a plasma.
[0028] These objectives are obtained at least to an extent by a
method of enhancing a gas-phase reaction in a plasma comprising:
creating plasma by at least one plasma source, and wherein that the
method further comprises: generating ultrasonic high intensity and
high power acoustic waves having a predetermined amount of acoustic
energy by at least one ultrasonic high intensity and high power
gas-jet acoustic wave generator, where said ultrasonic high
intensity and high power acoustic waves are directed to propagate
towards said plasma so that at least a part of said predetermined
amount of acoustic energy is absorbed into said plasma, and where a
sound pressure level of said generated ultrasonic high intensity
and high power acoustic waves is at least substantially 140 dB and
where an acoustic power of said generated ultrasonic high intensity
and high power acoustic waves is at least substantially 100 W.
[0029] In this way, a high sound intensity and power are obtained
that efficiently enhances a gas-phase reaction in the plasma, which
enhances the plasma process, e.g. enabling more efficient ozone or
syngas generation using plasma, exhaust gas cleaning, pollution
control, odor removal, fuel conversion, liquefaction,
sterilization, oxidation, etc using a plasma in relation to
reaction speed and/or obtained concentration of the generated
compound.
[0030] Increasing the concentration of aimed products, reducing the
concentration of pollutants, odor etc. and/or reducing the reaction
time, reduces the cost of the process since the process is
expensive and requires a lot of energy.
[0031] As a comparison, the sound-emitting surface area of
transducers, e.g. like the ones described in the previous mentioned
prior art articles (`Ozone generation by hollow-needle to plate
electrical in an ultrasound field`; `Ultrasound and airflow induced
thermal instability suppression of DC corona discharge: an
experimental study`; and `Improvement of Charging Performance of
Corona Charger in Electrophotography by Irradiating Ultrasonic Wave
to Surrounding Region of Corona Electrode`) is of the order of
2.times.10.sup.-3 m.sup.2 and the emitted acoustic power is 10-40
W.
[0032] The acoustic power provided by a high-power gas-jet
generator is capable of much higher acoustic power outputs along
with the high sound pressure and intensity levels of 140 or 150 to
170 dB (see e.g. Y. Y. Borisov, Acoustic gas-jet generators of
Hartmann type, in L. D. Rozenberg (ed.) Sources of High-Intensity
Ultrasound (New York: Plenum: 1969) part I. and Levavasseur, R.
High power generators of sound and ultrasound. US patent, book
116-137, U.S. Pat. No. 2,755,767 (1956).).
[0033] It has been shown that an acoustic power of about 100 W or
more enhances the gas-phase reaction in a plasma.
[0034] A high-power gas-jet generator is normally capable of an
acoustic power of several hundreds watts (i.e. approximately one
order of magnitude higher than the piezoelectric transducer
acoustic power output) and typical SPL (sound pressure level) of
160 dB at 10 cm from the generator orifice at the frequency of
20-30 kHz. Even an acoustic power of 1-2 kW is attainable.
[0035] The main physical reason for such a dramatic difference in
acoustic power outputs of piezoelectric (or other solid-state
acoustic transducers) and gas-jet generators is that a
piezoelectric transducer works by vibrating (using sound) a solid
being in contact with a gas and thus transfers the vibrations to
the gas. Due to the tremendous difference in acoustic impedance for
a solid and a gas (a so-called acoustic impedance mismatch), most
of generated acoustic power cannot be emitted in the ambient gas
and remains in a solid. It is converted into thermal energy and
results in a transducer warming-up.
[0036] Consequently, it is not possible in this way to generate
sound or ultrasound in the gas with a power that would be enough to
enhance a gas-phase reaction in a plasma sufficiently. In fact, a
single piezoelectric transducer provides the high-intensity
ultrasound radiation only nearby its emitting surface and
irradiates a limited surface area that is comparable with the area
of its emitting surface. That is because of the acoustic wave
diffraction, which is significant when the transducer diameter is
comparable with the acoustic wavelength. Indeed, for ultrasound
frequency of 20-30 kHz in the air the wavelength is about 10-20 mm
that is of the order of the actual transducer diameter. In the case
of gas-jet ultrasonic transmitters, a vibrating media is not a
solid but a gas. It is clear that there is no any impedance
mismatch and high enough acoustic power can be emitted in the
ambient gas. Moreover, intensity and sound pressure levels of
ultrasound radiation remain very high at several tens of
centimeters from the gas-jet transmitter orifice while the acoustic
wave front is broad (it is sometimes just a spherical wave front).
In this way, it is possible to expose large volumes to
high-intensity ultrasound (sound intensity and sound pressure
levels of substantially 140 dB and above at approximately 10 cm
from the generator's orifice) and enable an efficient gas-phase
reaction in a plasma.
[0037] In one embodiment, the acoustic pressure level of said
generated ultrasonic high intensity and high power acoustic waves
is [0038] at least substantially 150 dB, [0039] at least
substantially 160 dB, [0040] at least substantially 170 dB, [0041]
at least substantially 180 dB, [0042] at least substantially 190
dB, or [0043] at least substantially 200 dB. where the sound
pressure level as they are at 10 cm from the generator orifice.
[0044] In one embodiment, the acoustic power of said generated
ultrasonic high intensity and high power acoustic waves is [0045]
at least substantially 200 W, [0046] at least substantially 300 W,
[0047] at least substantially 400 W, [0048] about 400 W, [0049]
greater than substantially 400 W, [0050] at least substantially 500
W, [0051] at least substantially 1 kW, or [0052] selected from
about 1-2 kW.
[0053] It is to be understood, that if several acoustic generators
are used even higher powers may be obtained.
[0054] In one embodiment, the plasma source comprises at least one
source selected from a group of: a dielectric barrier discharge
(DBD) plasma source, a surface discharge (SD) plasma source, a
volume discharge (VD) plasma source, a plasma torch source, an arc
plasma torch, a gliding arc plasma torch, a cold plasma torch, a
pencil-like torch, a direct current plasma source, a capacitively
coupled plasma source, a pulsed plasma source, a magnetron plasma
source, an electron cyclotron resonance plasma source, an
inductively coupled plasma source, a helicon plasma source, a
helical resonator plasma source, a microwave plasma source, an
atmospheric pressure plasma jet (APPJ) source, a barrier torch, an
arc microwave torch, a corona discharge plasma source, a
micro-plasma source, a low pressure plasma source, and a high
pressure plasma source.
[0055] In one embodiment, a working gas pressure at an inlet of the
at least one ultrasonic high intensity and high power gas-jet
acoustic wave generator is between approximately 1.9 and
approximately 5 bar.
[0056] In one embodiment, the plasma is created at atmospheric
pressure.
[0057] In one embodiment, the plasma source comprises at least one
electrode and wherein one electrode of said at least one electrode
is a mesh type of electrode.
[0058] This allows the gas/energy to pass through the `upper`
electrode in a very simple and efficient way.
[0059] In one embodiment, the generated ultrasonic high intensity
and high power acoustic waves are propagated towards a membrane so
that any gases used by the at least one ultrasonic high intensity
and high power acoustic wave generator is not mixed with one or
more gases used by said plasma source to create said plasma.
[0060] In one embodiment, the generated ultrasonic high intensity
and high power acoustic waves are generated using a gaseous medium
and where the acoustic waves are directed towards said plasma and
wherein said gaseous medium after exit of said at least one
ultrasonic high intensity and high power gas-jet acoustic wave
generator is directed away from said plasma.
[0061] In one embodiment, the generated ultrasonic high intensity
and high power acoustic waves do not spatially overlap with the
working gas flow outgoing from the generator orifice. Moreover,
since the generated ultrasonic high intensity and high power
acoustic waves are directed toward the plasma and the gas outgoing
from the ultrasonic high intensity and high power acoustic wave
generator do not overlap in space, the said outgoing working gas is
not mixed with one or more gases used by said plasma source to
create said plasma.
[0062] In these ways, control of the gas environment for the plasma
generation process is enabled.
[0063] In one embodiment, a gas mixture, which is used for creating
the plasma, is supplied to at least one electrode of the plasma
source substantially in a direction that said ultrasonic acoustic
waves propagate towards said plasma.
[0064] In one embodiment, at least one of the ultrasonic high
intensity and high power gas-jet acoustic wave generators are
selected from the group of: [0065] a Hartmann type gas-jet
generator, [0066] a Levavasseur type gas-jet generator, [0067] a
generator comprising an outer part and an inner part defining a
passage, an opening, and a cavity provided in the inner part, where
said ultrasonic high intensity and high power gas-jet acoustic wave
generator is adapted to receive a pressurized gas and pass the
pressurized gas to said opening, from which the pressurized gas is
discharged in a jet towards the cavity, [0068] a generator of any
of the above mentioned types, which includes any type of
concentrators or reflectors of acoustic waves
[0069] In one embodiment, a food item is subjected to the plasma
where the creation of the plasma generates chemical radicals and
sterilizes the food item.
[0070] In one embodiment, the generating ultrasonic high intensity
and high power acoustic waves comprises: [0071] generating high
intensity and high power acoustic waves by a first acoustic wave
generator using a gaseous medium where the gaseous medium, after
exit from the first acoustic wave generator, has a first principal
direction that is different from a second principal direction of
the high intensity and high power acoustic waves generated by the
first acoustic wave generator, [0072] generating high intensity and
high power acoustic waves by a second acoustic wave generator,
[0073] where the first and second acoustic wave generators are
located in relation to each other so that at least a part of the
generated high intensity acoustic waves, being generated by said
second acoustic wave generator, is directed towards at least a part
of the gaseous medium after exit from said first acoustic wave
generator.
[0074] In one embodiment, the plasma is used in a process selected
from the group of: [0075] ozone generation, [0076] hydrogen
production, [0077] exhaust gas cleaning, [0078] pollution control,
[0079] odor removal, [0080] fuel conversion, [0081] sterilization,
and [0082] oxidation.
[0083] The present invention also relates to a system corresponding
to the method of the present invention. More specifically, the
invention relates to a system for enhancing a gas-phase reaction in
a plasma comprising: at least one plasma source adapted to create
plasma, wherein that the system further comprises: at least one
ultrasonic high intensity and high power gas-jet acoustic wave
generator adapted to generate ultrasonic high intensity and high
power acoustic waves having a predetermined amount of acoustic
energy and being directed to propagate towards said plasma so that
at least a part of said predetermined amount of acoustic energy is
absorbed into said plasma, and where a sound pressure level of said
generated ultrasonic high intensity and high power acoustic waves
is at least substantially 140 dB and where an acoustic power of
said generated ultrasonic high intensity and high power acoustic
waves is at least 100 W.
[0084] Advantageous embodiments of the system are defined in the
sub-claims and are described in detail in the following. The
embodiments of the system correspond to the embodiments of the
method and have the same advantages for the same reasons.
BRIEF DESCRIPTION OF THE DRAWINGS
[0085] These and other aspects of the invention will be apparent
from and elucidated with reference to the illustrative embodiments
shown in the drawings, in which:
[0086] FIG. 1 schematically illustrates a prior art plasma
apparatus;
[0087] FIG. 2 schematically illustrates a block diagram of a plasma
apparatus;
[0088] FIG. 3a schematically illustrates a (turbulent) flow without
application of ultrasonic high intensity and high power acoustic
waves;
[0089] FIG. 3b schematically illustrates a flow where the effect of
applying ultrasonic high intensity and high power acoustic waves
to/in air/gas is illustrated;
[0090] FIG. 4 schematically illustrates one embodiment of an
enhanced plasma apparatus;
[0091] FIG. 5 schematically illustrates an alternative embodiment
of an enhanced plasma apparatus;
[0092] FIG. 6 schematically illustrates an alternative embodiment
of an enhanced plasma apparatus;
[0093] FIG. 7 schematically illustrates an embodiment of an
enhanced plasma apparatus where the plasma source is a surface
discharge (SD) plasma source;
[0094] FIG. 8 schematically illustrates an embodiment of an
enhanced plasma apparatus where the plasma source is a torch plasma
source e.g. a gliding arc plasma source;
[0095] FIG. 9 schematically illustrates an embodiment of a high
intensity and high power gas-jet acoustic wave generator wherein a
converging supersonic gas jet outgoing a ring-shaped nozzle and
braking in a mushroom resonator has the form of a disk (i.e. the
so-called disk-jet Hartmann ultrasound generator);
[0096] FIG. 10 is a sectional view along the diameter of the high
intensity and high power acoustic wave generator (101) in FIG. 9
illustrating the shape of an opening (302), a gas passage (303) and
a cavity (304) more clearly;
[0097] FIG. 11 schematically illustrates another embodiment of a
high intensity and high power acoustic generator in form of an
elongated body; and
[0098] FIG. 12 schematically illustrates an embodiment of a high
intensity and high power acoustic generator comprising two
generators.
[0099] Throughout the figures, same reference numerals indicate
similar or corresponding features.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0100] FIG. 1 schematically illustrates a prior art plasma reaction
or generation apparatus. Shown is an example of a plasma generator
or plasma source, i.e. any device or method capable of creating a
plasma (forth only denoted plasma source) of the well-known
so-called dielectric barrier discharge (DBD) type usable even at
atmospheric pressure. Shown are a number of gases (111), such as
He, Ar, O.sub.2, CO.sub.2, and NH.sub.3 supplied to a gas mixing
unit (110) mixing the gases into a proper composition for the given
use or application. The selected gases for the plasma should be
selected based on the specific type of plasma reaction or compound
generation process being used and may be any gas, which is
ordinarily used for such processes. Specific and typical examples
include air, natural gas, CH.sub.4, He, Ar, Ne, Xe, air, N.sub.2,
O.sub.2, H.sub.2O, CO.sub.2, halogen compound gases such as Freon
gases (CF.sub.4, CHF.sub.3, C.sub.3F.sub.6, C.sub.4F.sub.8 etc.),
halon gases, NH.sub.3, NF.sub.3, SF.sub.6, organic compound gases
(CH.sub.4, C.sub.2H.sub.6, C.sub.2H.sub.4, C.sub.2H.sub.2,
C.sub.6H.sub.6, C.sub.2H.sub.5OH, etc.), NOx (nitrogen oxides),
SO.sub.2, silanes etc. and gas mixtures selected from them. In
order to stabilize the plasma, the gas(es) can be diluted with He,
Ar, see e.g. European patent EP 0508833 B1.
[0101] Further shown are two electrodes (103) placed apart with a
discharge gap between them, where at least one of the electrodes
(103) is adjoined or covered with an insulating or dielectric
material (105) on a side of the electrode facing the other
electrode in order to avoid arcing. In the figure, both electrodes
(103) are adjoined or covered with dielectric material (105). One
electrode (103) is connected to a suitable power supply (114),
being connected to ground and supplying AC high voltage, e.g. 0.1
kHz-500 kHz, between the electrodes (103).
[0102] Further, shown is a high voltage probe (113) connected to
the power supply (114) and an oscilloscope connected to the high
voltage probe (113). The high voltage probe is used for monitoring
the applied voltage, but is not relevant for and does not influence
plasma process.
[0103] The gas mix is supplied from the gas-mixing unit (110) to
the discharge area between the two electrodes (103) and, as a
result, plasma (104) is created when voltage is applied to the
electrodes (103). A specimen or object (not shown) may be located
in the plasma (104) e.g. for surface modification, treatment,
processing, etc.
[0104] The plasma process taking place may e.g. involve ozone
generation, hydrogen production, syngas production, exhaust gas
cleaning, pollution control, odor removal, fuel conversion,
sterilization, oxidation, etc.
[0105] Ozone can be formed by recombination in triplets of single
oxygen atoms which are split from the diatomic oxygen in a plasma.
Fuel conversion is the chemical or physical transformation of a
naturally occurring or already modified fuel to improve the quality
of the fuel. A fuel conversion process may result in one or more
upgraded fuel products which may be solid, liquid, or gaseous, and
may generate chemicals or raw materials for chemical manufacture.
For example, coal generally requires size reduction, washing, and
removal of inert species. Natural gas may need removal of H2S and
CO2 with separation of gas liquids and C2 compounds. However,
"gas-to-liquid" fuel conversion (liquefaction) is also of
significant importance. Here CH.sub.4, which is the major component
of natural gas, is converted to liquid fuels in one or more than
one step chemical reactions. One well-known process is first
synthesize a syngas (CO+H.sub.2);
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2
CH.sub.4+C.sub.2O.fwdarw.2CO+2H.sub.2
or
CH.sub.4+O.sub.2.fwdarw.CO+2H.sub.2
and subsequently synthesize liquids; CO+H.sub.2.fwdarw.gasoline,
diesel, alcohol, etc.
[0106] Plasma can also synthesize liquids (methanol, ethanol,
benzene etc.) and other molecules (ethane, propane, ethylene,
acetylene, propylene benzene and heavier hydrocarbons) (K. V.
Kozlov, P. Michel, H. E. Wagner "Synthesis of organic compounds
from mixtures of methane with carbon dioxide in dielectric barrier
discharge at atmospheric pressure" Plasmas and Polymers, 5(3/4)
(2000) 129-150, S. Kado, Y. Sekine, T. Nozaki, K. Okazaki
"Diagnosis of atmospheric pressure low temperature plasma and
application to high efficient methane conversion" Catalysis Today
89 (2004) 47-55 etc.). Produced hydrogen gas can also be used for
other purposes such as an environmentally friendly fuel for fuel
cells. Petroleum refining and coal gasification are also examples.
Examples of quality upgrades are the manufacture of automotive
gasoline by cracking of petroleum components, and removal of
sulphur and nitrogen from liquid fuels by reacting the fuel with
hydrogen.
[0107] Exhaust gas cleaning can be performed by passing through a
plasma or injecting activated species in to the exhaust. Its
examples are NOx and SO.sub.2 reduction.
[0108] Plasmas are useful for decomposing toxic gas components such
as volatile organic compounds (VOCs) (K. Urashima, J. S. Chang
"Removal of volatile compounds from air streams and industrial flue
gases by non-thermal plasma technology" IEEE Transactions on
Dielectrics and Electrical Engineering 7(5) (2000) 602-614) and
odor such as NH3 (L. Xia, L. Huang, X. Shu, R. Zhang, W. Dong, H.
Hou "Removal of ammonia from gas streams with dielectric barrier
discharge plasmas" 152 (2008) 113-119). They are generally
decomposed by oxidation in a plasma.
[0109] A gas-phase reaction in the plasma (104) can be enhanced as
shown and explained in the following.
[0110] FIG. 2 schematically illustrates a block diagram of a plasma
process/apparatus. Illustrated are one or more plasma sources (106)
creating or supplying plasma (104).
[0111] Further illustrated are one or more ultrasonic high
intensity and high power acoustic wave generators (101) generating
high intensity and high power ultrasound (102). According to the
present invention, the generated ultrasound (102) is applied to at
least a part of the plasma (104) whereby at least a part of the
acoustic energy is absorbed by the plasma (104). The addition of
energy to the plasma (104) will enhance the gas-phase reaction and
will enhance the plasma process taking place. The application of
high intensity and high power ultrasonic acoustic waves with a
sound pressure level of at least substantially 140 dB and an energy
of at least substantially 100 W will enhance the gas-phase reaction
and thereby the plasma process significantly, as explained in the
following in connection with FIGS. 3a and 3b.
[0112] The working gas pressure at the inlet of the ultrasonic high
intensity and high power acoustic wave generators (101) may be
optimized so that high acoustic pressure can be generated. It is
preferably between 1.9 and 5 bar or between 2.5 to 4 bar and will
typically depend on the type generator used. The gas pressure at
the outlet of the high-power gas-jet generators is lower than that
at the inlet, and can be practically nearly equal to the gas
pressure for the plasma process.
[0113] The air-pressure required for operation of gas-jet
high-intensity and high-power ultrasonic generators is at least
over 1.9 bar for operation under normal conditions and the pressure
required for optimal operation providing stable generation of
ultrasound with a SPL over 140 dB at 10 cm from the generator
orifice is 2.5 to 4 bar depending on a generator type.
[0114] The one or more plasma sources (106) may be any plasma
source suitable for the given plasma process, e.g. such sources as
explained earlier and in the following and/or combinations thereof
or such sources e.g. using one or more gases, as explained earlier,
in creating the plasma. The plasma source(s) (106) can be chosen
among any existing ones (both low and high pressure plasmas), and
more specifically may be e.g. direct current plasmas, capacitively
coupled plasmas, pulsed plasmas, magnetron plasmas, electron
cyclotron resonance plasmas, inductively coupled plasmas, helicon
plasmas, helical resonator plasmas, microwave plasmas, DBDs, SDs,
plasma torches such as arc plasma torches, cold plasma torches,
APPJs, pencil like torches, barrier torches, arc plasma torches,
microwave torches, gliding arc, corona discharge, and
microplasmas.
[0115] The gas pressure for the plasma process is preferably higher
than 0.4 bar and may be around atmospheric pressure or more, so
that the acoustic energy can be delivered efficiently. One the
other hand, it is easier to generate plasmas at lower pressures.
Therefore, the gas pressure for the plasma process is preferably
more than 0.4 bar and less than the pressure at the inlet of the
high-power gas-jet generators. More preferable plasma source may be
DBDs, SDs, plasma torches such as arc plasma torches, cold plasma
torches, APPJs, pencil like torches, barrier torches, microwave
torches, gliding arc, corona discharge, and microplasmas, which can
be operated at the pressures mentioned above.
[0116] The one or more ultrasonic high intensity and high power
acoustic wave generator (101) is a gas-jet acoustic wave generator
and may e.g. be one or more Hartmann type gas-jet generators, one
or more Levavasseur type gas-jet generators, etc. or combinations
thereof and as explained in the following and as shown in FIGS.
9-12.
[0117] The use of a gas-jet acoustic wave generator has advantages
like described earlier in terms of acoustic power, high intensity,
acoustic impedance, etc.
[0118] If more than a single ultrasonic and high intensity and high
power acoustic wave generator is used they need not be of the same
type although they can be.
[0119] The plasma process may e.g. be ozone generation, hydrogen
production, exhaust gas cleaning, pollution control, fuel
conversion, sterilization, oxidation, etc.
[0120] In one embodiment, a food item is subjected to the plasma
process where the process will generate chemical radicals and
sterilize the food item in a very efficient way.
[0121] FIG. 3a schematically illustrates a (turbulent) flow without
application of ultrasonic high intensity and high power acoustic
waves.
[0122] Shown is a surface (314) of a solid object (100) where a gas
or a mixture of gases (500) surrounds or contacts the surface
(314).
[0123] Thermal energy can be transported through gas by conduction
as well as by the movement of the gas from one region to another.
This process of heat transfer associated with gas movement is
usually referred to as convection. When the gas motion is caused
only by buoyancy forces set up by temperature differences, the
process is normally referred to as natural or free convection; but
if the gas motion is caused by some other mechanism, it is usually
referred to as forced convection. With a condition of forced
convection, there will be a laminar boundary layer (311) near to
the surface (314). The thickness of this layer is a decreasing
function of the Reynolds number of the flow, so that at high flow
velocities, the thickness of the laminar boundary layer (311) will
decrease. When the flow becomes turbulent the layer are divided
into a turbulent boundary layer (312) and a laminar boundary layer
(313). For nearly all practically occurring gas flows, the flow
regime will be turbulent in the entirety of the streaming volume,
except for the laminar boundary layer (313) covering the surface
(314) wherein the flow regime is laminar. Considering a gas
molecule or a particle (315) in the laminar boundary layer (313),
the velocity (316) will be substantially parallel to the surface
(314) and equal to the velocity of the laminar boundary layer
(313). Heat transport across the laminar boundary layer will be by
conduction or radiation, due to the nature of laminar flow.
[0124] Furthermore, mass transport across the laminar boundary
layer will be solely by diffusion. The presence of the laminar
boundary layer (313) does not provide optimal or efficient
increased mass transport. Any mass transport across the boundary
layer will be solely by diffusion, and therefore often be the final
limiting factor in an overall mass transport.
[0125] One impediment to the transfer or transmission of energy
and/or mass from a gas to a solid surface is the boundary layer of
the gas, which adheres to the solid surface. Even when the motion
of the gas is fully turbulent, the laminar boundary layer exists
that obstructs mass transport and/or heat transfer. While various
methods and types of apparatus have been suggested for overcoming
the problem such as by means of driving the gas with sonic waves
and vibrating the solid object (100) with external vibration
generators, these methods while being effective to some extent, are
inherently limited in their ability to generate an effective
minimization of the laminar boundary layer and at the same time
covering an area large enough to make the method efficient.
[0126] FIG. 3b schematically illustrates a flow where the effect of
applying high intensity and high power ultrasonic acoustic waves
to/in air/gas (500) is illustrated.
[0127] More specifically, FIG. 3b illustrates the conditions when
the surface (314) of a solid object (100) is applied with high
intensity and high power ultrasonic acoustic waves by a gas-jet
acoustic wave generator (not shown; see e.g. 101 in other Figures)
and where the high intensity and high power ultrasonic acoustic
waves is absorbed in the plasma so that the reaction is
enhanced.
[0128] Again consider a gas molecule/particle (315) in the laminar
layer; the velocity (316) will be substantially parallel to the
surface (314) and equal to the velocity of the laminar layer prior
applying ultrasound. In the direction of the emitted sound field to
the surface (314) in FIG. 3b, the oscillating velocity of the
molecule (315) has been increased significantly as indicated by
arrows (317). As an example, a maximum velocity of v=4.5 m/sec and
a displacement of +/-32 .mu.m can be achieved where the frequency
is f=22 kHz and the sound pressure level=160 dB. The corresponding
(vertical) displacement in FIG. 3a is substantially zero since the
molecule follows the laminar air stream along the surface. As a
result, the acoustic waves will establish a forced heat flow and/or
mass transport from the surface to surrounding gas/air (500) by
increasing the conduction by minimizing the laminar boundary layer.
The sound pressure level is in one embodiment substantially 140 dB
or larger. Furthermore, the sound pressure level may be selected
within the range of approximately 140-160 dB. The sound pressure
level may be above substantially 150 dB, above substantially 160
dB, above substantially 170 dB, above substantially 180 dB, above
substantially 190 dB or above substantially 200 dB.
[0129] The thinning or destruction of the laminar boundary layer
has the effect that heat transfer and mass transport from the
surface (314) to the surrounding or contacting gas (500) greatly is
increased, as the presence or the reduced size of the laminar
boundary layer no longer will hind heat transfer and/or mass
transport to the surface of the solid object(s) (100) being
subjected to plasma surface modification, i.e. the plasma will more
efficiently influence the surface of the object.
[0130] Furthermore, the high intensity and high power ultrasonic
acoustic waves is absorbed in the plasma so that the reaction is
enhanced.
[0131] Various embodiments are described in connection with the
following figures.
[0132] FIG. 4 schematically illustrates one embodiment of an
enhanced plasma apparatus.
[0133] Shown is at least one ultrasonic high intensity and high
power acoustic wave gas-jet generator (101) generating high
intensity and high power ultrasonic acoustic waves (102)
propagating towards and reaching a plasma (104), which will absorb
the substantial acoustic energy whereby a gas-phase reaction in the
plasma (104) will be enhanced due to the received energy.
[0134] A plasma (104) is created by a plasma source (106) using the
shown gas flow, the shown electrodes (103; 103'), and an insulator
or dielectric material (105) e.g. as explained in connection with
FIG. 1. The particular shown plasma source (106) is of the DBD type
but could be of another type.
[0135] The insulator or dielectric material (105) may e.g. comprise
Al.sub.2O.sub.3 or in general material having a dielectric property
or any kind of insulators such as ceramics, polymers and glasses.
Ceramics and glasses are more durable against plasma since they
have relatively high temperature resistance. They are often
preferred, since they typically have high dielectric constants and
thus plasma can be generated and sustained at lower AC
voltages.
[0136] Further shown is a horn or the like (402) or sound guiding
or directing means that ensures that the sound intensity and power
is contained and focused towards the plasma/object.
[0137] In one embodiment, a membrane (401) or similar is located
between the high intensity and high power ultrasonic acoustic wave
generator (102) and the plasma. This enables control of the gas
environment for the plasma generation process so that only the
received gas flow is used in creating the plasma. This may be
useful for gas driven generators (102) so that the gas from such
generators do not interfere with the gas mix used for plasma
creation. Other embodiments may exclude the membrane (401). The
membrane (401) is preferably relatively thin and relatively
transparent to ultrasound. The thickness, size, and/or shape of the
membrane (401) and tension applied to it may be optimized for
decreasing a loss of ultrasound.
[0138] In some embodiments, the membrane can be dispensed with even
though it is not preferred that a mix of the gaseous medium used
for generating the high intensity and high power ultrasonic
acoustic waves and the gas(es) used for creating the plasma occurs.
This can be achieved by having a high intensity and high power
acoustic wave generator where the generated acoustic waves
propagates generally in another direction than the general
direction of the gaseous medium after exiting the acoustic wave
generator.
[0139] In FIG. 12 is shown two such generators where the general
direction of the generated acoustic waves are at an angle to the
general direction of the gaseous medium after exit from the
generator.
[0140] Generators can be designed so that the two directions are
about opposite. For instance, stem-jet Hartmann generators
radiating ultrasound in the so-called high-frequency regime allow
such "natural" spatial separation of the ultrasound field and the
outgoing gas flow (see e.g. Y. Y. Borisov, Acoustic gas-jet
generators of Hartmann type, in L. D. Rozenberg (ed.) Sources of
High-Intensity Ultrasound (New York: Plenum: 1969) part I.). Such
generators can be very useful in avoiding the use of a membrane as
the gaseous medium may directed away from the plasma. In this way,
no gas(es) used for generating the acoustic waves will influence
the plasma gas(es). It is to be understood that even in such an
arrangement a membrane may still be useful (although it may be of a
different design) since it can contain the gas(es) used for
creating the plasma contained so they do not diffuse into the
surrounding environment, which may be useful since some have a
significant cost.
[0141] Any kinds of membranes can be used, as long as there is
neither significant loss of ultrasound nor significant gas leakage.
As long as they can form thin films, their materials can be chosen
from any thermoplastic and thermoset polymers such as polyesters,
polyethylene terephthalate, polyolefins (low density (LD)
polyethylene (PE), high density (HD) PE, ultrahigh density PE,
ultrahigh molecular weight PE, polypropylene, poly(vinyl chloride),
poly (vinylidene chloride), polystyrene, polyimide, polyamide, poly
(vinyl vinyl ether), polyisobutylene, polycarbonate, polystyrene,
polyurethane, poly (vinyl acetate), poly-acrylonitrile, natural and
synthetic rubbers, polymer alloys, copolymers, and their laminates.
They can be coated with organic and/or inorganic materials using
any existing techniques. Among them, lower density materials such
as PE can be used. Furthermore, a metal foil may be used as a
membrane. Other examples are metal coated (or material coated with
inorganic material) or laminated polymer membranes.
[0142] As an alternative, the membrane may comprise or consist of
Aerogel.
[0143] In one embodiment, the electrode located between the plasma
(104) and the generator (101) is a mesh type of electrode (103') or
another type of perforated electrode. This enables the generated
ultrasound to virtually pass unhindered to the object(s) (100)
without loosing a significant amount of energy whereby as much
energy as possible is present for influencing the laminar sub-layer
around the object(s) (100). Other embodiments may exclude the mesh
type/perforated electrode (103').
[0144] The direction of outgoing gas/gas mixture, used for creating
the plasma (104), and the ultrasound (102) is quite controllable
and the angle between their principal directions may vary. In the
shown embodiment, the angle is about 90.degree.. But the angle may
in principal be any angle. In FIG. 5 for example, the angle is
about 0.degree..
[0145] The gas or gas mixture used for creating the plasma (104)
may also be used for driving the gas-jet acoustic generator(s)
(101). The plasma (104) may be generated before, in or after the
acoustic generator (101).
[0146] FIG. 5 schematically illustrates an alternative embodiment
of an enhanced plasma apparatus. This embodiment corresponds to the
embodiment shown and explained in connection with FIG. 4 except
that the gas for the plasma is not fed from the side but rather
from the same direction as the high intensity and high power
ultrasonic acoustic waves (102).
[0147] FIG. 6 schematically illustrates an alternative embodiment
of an enhanced plasma apparatus. This embodiment corresponds to the
embodiment shown and explained in connection with FIG. 4 except
that it does not comprise a membrane. Such an embodiment is
suitable for ambient or normal air plasma. In such an embodiment
without a membrane, a high speed air flow used for the generation
of high intensity and high power ultrasound can also be used as a
process gas for the plasma.
[0148] FIG. 7 schematically illustrates an embodiment of an
enhanced plasma apparatus where the plasma source is a surface
discharge (SD) plasma source. The shown embodiment corresponds to
the embodiment shown and explained in connection with FIG. 4 except
that instead of a DBD plasma source it comprises a surface
discharge (SD) plasma source (106) comprising a single insulator or
dielectric material (105) and a number of electrodes (103) embedded
in the insulator or the dielectric material (105). The shown SD
plasma source is the so-called comprises a so-called CDSD discharge
element. Alternatively, it could comprise a SPCP discharge element
or by of another type of SD plasma source. As an alternative to the
gas flow being received from the side, it could be supplied in the
direction of the ultrasonic high intensity and high power acoustic
waves e.g. as shown in FIG. 5 or in another way.
[0149] FIG. 8 schematically illustrates an embodiment of an
enhanced plasma apparatus where the plasma source is a torch plasma
source e.g. a gliding arc plasma source. The shown embodiment
corresponds to the embodiment shown and explained in connection
with FIG. 4 except that instead of a DBD plasma source it comprises
a torch plasma source e.g. a gliding arc plasma source.
[0150] The torch plasma source could e.g. be a barrier torch design
or cold plasma torch design as well-known in the art.
[0151] FIG. 9 schematically illustrates an embodiment of a high
intensity and high power acoustic wave gas-jet generator in the
form of a disk-shaped disk jet (i.e. a disk-jet Hartmann ultrasound
generator). Shown is an embodiment of a high intensity ultrasound
generator (101), in this example a so-called disk-jet. The
generator (101) comprises a generally annular outer part (305) and
a generally cylindrical inner part (306), in which an annular
cavity (304) is recessed. Through an annular gas passage (303)
gases may be diffused to the annular opening (302) from which it
may be conveyed to the cavity (304). The outer part (305) may be
adjustable in relation to the inner part (306), e.g. by providing a
thread or another adjusting device (not shown) in the bottom of the
outer part (305), which further may comprise fastening means (not
shown) for locking the outer part (305) in relation to the inner
part (306), when the desired interval there between has been
obtained. Such an ultrasound device may generate a frequency of
about 22 kHz at a gas pressure of 4 atmospheres. The molecules of
the gas are thus able to migrate up to 33 .mu.m about 22,000 times
per second at a velocity of 4.5 m/s. These values are merely
included to give an idea of the size and proportions of the
ultrasound device and by no means limit of the shown
embodiment.
[0152] FIG. 10 is a sectional view along the diameter of the high
intensity and high power acoustic wave generator (101) in FIG. 9
illustrating the shape of an opening (302), a gas passage (303) and
a cavity (304) more clearly. As mentioned in connection with FIG. 9
the opening (302) is generally annular. The gas passage (303) and
the opening (302) are defined by the substantially annular outer
part (305) and the substantially cylindrical inner part (306)
arranged therein. The gas jet discharged from the opening (302)
hits the substantially circumferential cavity (304) formed in the
inner part (306), and then exits the high intensity ultrasound
generator (101). As previously mentioned the outer part (305)
defines the exterior of the gas passage (303) and is further
beveled at an angle of about 30.degree. along the outer surface of
its inner circumference forming the opening of the high intensity
ultrasound generator, wherefrom the gas jet may expand when
diffused. Jointly with a corresponding beveling of about 60.degree.
on the inner surface of the inner circumference, the above beveling
forms an acute-angled circumferential edge defining the opening
(302) externally. The inner part (306) has a beveling of about
45.degree. in its outer circumference facing the opening and
internally defining the opening (302). The outer part (305) may be
adjusted in relation to the inner part (306), whereby the pressure
of the gas jet hitting the cavity (304) may be adjusted. The top of
the inner part (306), in which the cavity (304) is recessed, is
also beveled at an angle of about 45.degree. to allow the
oscillating gas jet to expand at the opening of the high intensity
ultrasound generator.
[0153] FIG. 11 schematically illustrates another embodiment of a
high intensity and high power acoustic wave generator in form of an
elongated body. Shown is a high intensity and high power gas-jet
acoustic wave generator (101) comprising an elongated substantially
rail-shaped body, where the body is functionally equivalent with
the embodiments shown in FIGS. 9 and 10. In this embodiment the
outer part comprises one rail-shaped portion (305), which jointly
with a rail-shaped other part (306) forms an ultrasound device
(101). A gas passage (303) is provided between the rail-shaped
portion (305) and the rail-shaped other part (306). The gas passage
has an opening (302) conveying emitted gas from the gas passage
(303) to a cavity (304) provided in the rail-shaped other part
(306). One advantage of this embodiment is that a rail-shaped body
is able to coat a far larger surface area than a circular body.
Another advantage of this embodiment is that the high intensity and
high power acoustic wave generator may be made in an extruding
process, whereby the cost of materials is reduced.
[0154] FIG. 12 schematically illustrates an embodiment of a high
intensity and high power acoustic generator comprising two
generators. Shown is an example of two gas-jet high intensity and
high power acoustic wave generators (101; 101'), a first (101) and
a second (101'), where each generator (101; 101') generates high
intensity and high power acoustic waves (102) using a gaseous
medium (121). The gaseous medium (121) exits each generator (101)
in a principal direction schematically indicated by arrow (A; A')
in a cone-like shape, as represented by the hatched area, towards
the generated plasma.
[0155] The high intensity and high power acoustic waves (102)
generated by the first generator (101) propagate in a principal
direction as schematically indicated by arrows (B) that is
different than the general direction of the gaseous medium (A) from
the first generator (101) due to the design of the high power
acoustic wave generator (101).
[0156] The high intensity and high power acoustic waves (102)
generated by the second generator (101') propagate in a general
direction as schematically indicated by arrow (B').
[0157] One example of a high intensity and high power acoustic wave
generator operating in a way like this is shown and explained in
connection with FIG. 11. This design generates high intensity and
high power acoustic waves in a substantial line (seen from above),
whereas the design of FIGS. 9 and 10 generates waves in a
substantially circular way.
[0158] The first (101) and the second high power acoustic wave
generator (101) are located in relation to each other so that at
least a part of the generated high intensity and high power
acoustic waves (102) from the second acoustic wave generator (101')
has a general direction (B') that is directed towards at least a
part of the gaseous medium (121) from the first acoustic wave
generator (101) and that at least a part of the generated high
intensity and high power acoustic waves (102) from the first
acoustic wave generator (101) has a general direction (B) that is
directed towards at least a part of the gaseous medium (121) from
the second acoustic wave generator (101').
[0159] By directing high intensity and high power acoustic waves
generated by the second generator (101) directly towards the
gaseous medium (121) from the first generator (101), energy is
supplied in as a direct way as possible so that it directly
influences the gaseous medium (121) thereby increasing the
efficiency or turbulence of the gaseous medium.
[0160] This gives a very compact and efficient setup as the gaseous
medium of each generator is enhanced by the high intensity and high
power acoustic waves of another generator using a total of only two
generators.
[0161] If only a single generator (101) was used, the difference
between the general directions of the high intensity and high power
acoustic waves (B or B') and the general direction of the gaseous
medium (A or A') for a single generator (101) would cause a loss in
efficiency since the acoustic waves do not coincide with the
gaseous medium (121).
[0162] The location of the generators (101; 101') in relation to
each other may vary. One example is e.g. where the two generators
are facing each other displaced or shifted but where the high
intensity and high power acoustic waves still directly influences
the gaseous medium of the other generator.
[0163] In the figure, the shown sizes, directions, etc. of the
cones (121; 102) do not relate to any specific physical properties
like acoustic wave intensity, etc. but merely serve for
illustrational purposes. The intensities and/or power of the two
generators (101) may be equal or different (with either one being
greater than the other is). Furthermore, the shapes, sizes, and
directions may vary from application to application.
[0164] The specific location of one of the generator (101; 101')
may also vary in relation to the other generator and may e.g. be
placed above or higher than and/or e.g. facing, the other generator
(101); as long as the acoustic waves (102) of one generator (101)
directly influences the gaseous medium (121) of the other generator
(101) and vice versa.
[0165] Although this particular example shows two generators it is
to be understood that a given arrangement may comprises additional
generators.
[0166] The gaseous medium (102) may in general be any gaseous
medium. In one embodiment the gaseous medium (102) is steam. In an
alternative embodiment the gaseous medium (102) comprises one or
more gases used for creating the plasma.
[0167] It is to be noted, that one or more of the acoustic
generators shown in connection with FIG. 12 or any other Figure
could comprise one or more reflectors e.g. of generally parabolic
or elliptical shape for directing the acoustic energy to a
preferred region or spot.
[0168] It should be emphasized that the term "comprises/comprising"
when used in this specification is taken to specify the presence of
stated features, integers, steps or components but does not
preclude the presence or addition of one or more other features,
integers, steps, components and/or groups thereof.
[0169] In the claims, any reference signs placed between
parentheses shall not be constructed as limiting the claim. The
word "comprising" does not exclude the presence of elements or
steps other than those listed in a claim. The word "a" or "an"
preceding an element does not exclude the presence of a plurality
of such elements.
[0170] The invention can be implemented by means of hardware
comprising several distinct elements, and by means of a suitably
programmed computer or processor. In the system and device claims
enumerating several means, several of these means can be embodied
by one and the same item of hardware. The mere fact that certain
measures are recited in mutually different dependent claims does
not indicate that a combination of these measures cannot be used to
advantage.
* * * * *