Ultrasonic Bibliography Page 3 - Selected Articles.
CALL FOR CONTRIBUTIONS: I am writing a book on "High-Intensity Ultrasonic Technology and Applications", on the practical application of power (high intensity) ultrasonics, the use of ultrasonic energy to change materials. Contributions are welcome (see below).
THE CAVITATION BUBBLE
[image from University of Washington, Applied Physics Laboratory (Lawrence Crum, Ph.D.)
- bubble diameter approximately 1mm]
ULTRASONICS
Ultrasonic Whistles
(Nozzles, Atomizers, Nebulizers)
(23 Feb 04)
Ultrasonic whistles are perhaps best known as dog whistles;
whistles that sound at frequencies well above human hearing (~>18KHz).
However, such whistles can be used to perform useful work, primarily as
ultrasonic nozzles, also known as ultrasonic atomizers or
ultrasonic nebulizers.
An ultrasonic whistle is basically little more than an ordinary whistle with air (or other
gas, even steam) forced through under higher pressure such that the jet going over
the orifice or reed is moving fast enough to generate ultrasonic vibrations.
Englishman Francis Galton, author of the acclaimed seminal treatise on modern
scientific psychology: "Inquiries into Human Faculty and Its Development"
(1883), also invented many devices used in psychological investigation. One of
these was the Galton Whistle (1876) which was a clever combination of a
micrometer and a whistle, whereby he was able to precisely vary the length, and thus
the frequency, of the resonant chamber of a whistle:
(23 Feb 04 drawing by and © 2004 S. Berliner, III - all rights reserved)
This whistle was used to generate sounds to measure the upper limits of human
hearing.
Ultrasonic whistles have been used to levitate and also to stratify and sort them.
Further research developed whistles with resonant cavities into the orifices of which
liquids could be injected; the ultrasonically-induced cavitation in the liquid caused it to
atomize and the gas flowing out of the nozzle delivered the resultant fine droplets to
a volume or surface where they could be measured or used.
A typical ultrasonic nozzle might look like this:
(23 Feb 04 drawing by and © 2004 S. Berliner, III - all rights reserved)
{to follow}
One of the first uses for such nozzles was in humidification of Belgian lace works,
where fine airborne lints caused disastrous explosions. Another use is in spray
drying, whereby dissolved solids can be precipitated out and formed as microscopic
spheres in the nozzle effluent stream. Such nozzles have been used to form
ultrathin laydowns, such as in coating magnetic media.
Any dry gas can be use to drive ultrasonic nozzles. Steam can also be used but
has the drawback that condensate can sputter off the end of the nozzle.
Ultrasonic transducers and probes can also be used to generate a mist, as described
earlier in these pages under the "ultrasonic
fountain", but some propulsive means must be furnished. The popular
ultrasonic humidifiers sold publicly work in this manner.
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AM-9
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APPLICATIONS MONOGRAPH
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27 April 2004
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(27 Apr 2004)
THE USE OF ULTRASONIC PROBES
IN FUEL RESEARCH
[based on an undated paper written prior to 1976 by
Howard Alliger, President,
HEAT SYSTEMS-ULTRASONICS INCORPORATED
(now MISONIX INCORPORATED)
Revised and updated by
S. Berliner, III
27 April 2004
The potential of ultrasonic energy has often not been appreciated in such diverse
applications as emulsifying, extracting, and dispersing.
Instruments that perform these functions, like the SONICATOR®
Ultrasonic Processor from Misonix, have long been used in biological research and
have found wider application in physics, chemistry, and industry. The availability
of much larger power outputs and the use of continuous flow attachments have made
this transition possible. Better understanding of ultrasonic theory has helped,
as well.
When applied to fuel research, the SONICATOR probe has several interesting
applications: emulsifying fuel and water, breaking long chain polymers in fuel as
anti-mist agents, dispersing coal particles in oil, dissolving coal in solvents, extracting
oil from shale, and aiding the determination of chitin in contaminated fuels.
Some Theory
When an A.C. voltage is applied to a crystal, it changes shape in phase with the
electric field. This is the piezoelectric effect. These continuous changes
in shape are the pulsations or sound waves which travel through the probe into the
liquid. Alternate compressions and rarefactions of this wave produce a
phenomenon known as cavitation --- the creation and collapse of microscopic
bubbles. These bubbles or cavities take many cycles to grow to "resonant"
size. At this point, they collapse violently in one compression cycle, producing
on the order of 20,000 atmospheres of pressure. This mechanical shock is felt
at a distance of a few microns from the cavitation bubble collapse.
Although the SONICATOR is an interesting instrument and, also, relatively simple to
use, knowledge of ultrasonic theory as well as techniques developed since its
introduction are important for best possible results.
Almost any variable that tends to suppress cavitation --- such as higher ambient
pressure, higher surface tension or tensile strength, degassing, lower temperature ---
will increase intensity if there is enough power to produce cavitation.
A rise in temperature may allow more bubbles to nucleate and grow, enhancing the
scrubbing action of a tank type ultrasonic cleaner. But the increased vapor pressure
within a cavitation bubble reduces the violence of collapse; a violence which is
necessary for the probe SONICATOR's more difficult work. As fuels are reduced
in temperature, they can then more easily be emulsified with water. Similarly,
applying static pressure during treatment compresses and dissolves air bubbles in the
fluid. This increases intensity by removing the air which will cushion bubble
collapse. It is possible to completely eliminate cavitation in organic solvents by
saturating with certain gases. These gases fill the cavitation bubble and
prevent collapse.
Raising the frequency also lowers the intensity by reducing the size of the cavitation
bubble, which then collapses with less force. Cavitation is more difficult to
produce as the frequency is raised and cavitation can not be initiated at any power
level at frequencies over about 2.5 MHz.
Tensile strength is not a term normally applied to liquids. However, the forces
necessary to separate water molecules (probably van DerWaal's), like those of metals,
are quite high. In fact if water were absolutely pure, it would take about
15,000 psi to cause fracture or cavitation. Since water is never pure --- distilled
water has many thousands of impurities per cc, air and particles ---, far less pull, or
negative pressure, is necessary to produce cavitation. The greatest strength
achieved in a liquid is -277 atmospheres; this was produced in a highly purified water
which was "fractured" in a spinning tube by centrifugal force.
Solvents or fuel oil have lower tensile strengths than water and cavitation bubbles
produced in them collapse with less force. Ordinary degassed kerosene
requires about one atmosphere of negative pressure for cavitation, water 7
atmospheres. When comparing most organic fluids, higher viscosity and lower
vapor pressure may be used as a rough indicator of tensile strength --- or expected
cavitation intensity. Although other liquid properties such as density, surface
tension, and speed of sound are also involved, the former two are the most useful
or practical predictors. Adding a polymer to a particular fluid increases viscosity,
but this will not increase intensity of bubble collapse. The added viscosity is
due to friction, not greater molecular attraction, and the bubble would simply collapse
more sluggishly.
The addition of wetting agents also reduces tensile strength, or its close cousin,
surface tension. The use of solid combustible powders like lampblack as
emulsifying agents rather than the soap type, might be preferable since this will not
affect surface tension and intensity as drastically. One reason oil-in-water
emulsions are easier to form ultrasonically than water-in-oil, is possibly due to the
"stronger" disperse phase. Cavitation in a continuous water phase is
more intense for a given generator setting than for a continuous oil phase.
As a matter of interest, the height of Redwood trees is limited by the tensile strength
of the water rising to the top branches by capillary action. If the trees grew
higher, the liquid would "fracture" and the uppermost leaves would not be fed.
-
[As an aside here, the resultant steam explosion would blow the top off the tree!]
Making Emulsions
The use of water/oil emulsions is recognized as having several advantages in
combustion processes and engine fuel:
reduction of emissions --CO, NOx, smoke
utilization of heavier or less expensive fuel
small improvement in efficiency
The challenge facing us now:
find the optimum emulsion --- size, distribution, concentration
produce it without emulsifiers or wetting agents
produce it in fairly stable form
measure it
Here, ultrasonic techniques hold great promise. The SONICATOR will produce an oil
and water emulsion of 1 micron size in a few seconds; and in a minute, 1/10 micron.
The distribution curve is sharp compared to that of mechanical homogenizers.
As sonication proceeds, the particle size gets smaller and the size distribution
narrows. For any particular intensity there is a size and disperse phase
concentration limit. At some point in the sonication process, the particle size
will start growing rather than reducing, because flocculation or coagulation have
become more predominant than ultrasonic dispersing.
It takes less intensity to initiate an oil-in-water emulsion than its reverse and the
particle size is smaller. Oil droplets in and oil/water emulsion are electrically
charged which tends to stabilize the mixture.
As might be expected, the addition of stabilizers increases the concentration of the
emulsion as well as the rate of formation. Highly dispersed emulsions of 35%
concentration can be obtained without surface active agents, however.
Conceivably, if the disperse phase is broken down fine enough, say, approaching
1/100 micron, the dispersion might be stable without emulsifiers, like a colloid.
Certainly, there is less tendency to "cream out" due to sedimentation. A
water/oil ratio of up to 45% (water/water + fuel) can be burned. With just
natural surfactants in fuel, the ultrasonic emulsifying capacity may approach this
higher ratio. Oil/water/oil or multiple emulsions might be an interesting
addition to fuel research. These are frequently formed when inverting the
emulsion. Possibly the phases go on, ad infinitum, although this phenomenon
is not easily seen in a microscope or otherwise detected.
In the usual case, lower temperature produces better emulsions. Oil/water is
more stable under the effects of temperature than water/oil, and probably more
stable with time in general. If there is a change in the relative temperature of
the two phases, emulsification will be reduced. A little water, or a little oil, is
easier to emulsify.
The process becomes more difficult as the starting ratios of oil and water approach
50/50. It is also more difficult if one phase is or becomes more viscous than
the other.
Research on emulsions and their practical use in fuel oil has been hampered by a lack
of good means for measuring water droplet size, the disperse phase.
Indication of size distribution by an in-line detecting system would be especially
valuable.
Although conventional light microscopes are not too useful for size estimations much
below a micron, new "interference" methods might be applicable. There are
also several other possibilities. For example, since the viscosity of a
given phase ratio is influenced by mean droplet size as well as size distribution,
measurement of this parameter might provide a continuous flow measurement
system.
Electrical conductance has been used as a qualitative test for phase type, and high
voltage AC is known to break emulsions. Instinct would suggest that there is
an electrical approach to this problem. High and low voltages or currents, AC
superimposed on DC, or capacitance of a thin emulsion film, might be tried.
Still another possibility is the coherent light source of a laser. By monitoring
both the absorption and scattered radiation from a laser beam, the particle size and
distribution may be determined. The highest frequency available, 25MHz,
produces a wavelength in fuel oil of 70 microns. Only a small amount of sound
energy will therefore by scattered by one micron water droplets. However, if
the sound receiver is positioned to pick up scattered radiation only, and the source is
a multi-frequency transmitter, a suitable emulsion detecting device is possible.
[It is not the provenance if this monograph to examine droplet
measurement systems].
Making Anti-Mist Agents
Polymers are now being added to fuel oil as anti-mist agents. If the polymers
can be broken sufficiently when needed, for example, ... to start an engine, their use
becomes a practical solution to the fire hazard problem. Researchers have
used ultrasonics to break long-chain polymers since the early thirties. These
polymers are usually dissolved in organic solvents. Ultrasonic degradation or
depolymerizing of many molecules have been studied, including polystyrene, methyl
methacrylate, nitrocellulose, proteins, rubber, and starch.
The larger the molecule and more intense the ultrasonics, the faster the degradation.
An increase of intensity also reduces the ultimate particle size. The molecule is
usually broken in half at the C-C bond, and its fragments are in turn broken in half,
producing finally a narrow dispersion of molecular weights. As a contrast,
depolymerization due to heat or oxidation produces fragments of varying molecular
weights.
Polystyrene can be reduced to 1/6 its original molecular weight. The disruption
of these molecules is not due to frictional forces produced by the sound wave or by
heat. Neither is it due to a resonant effect in the long chain --- although the
largest polymers may reach a length of 1 micron, the sound wavelength in liquid even
at 1MHz is far longer. Cavitation produces this breaking effect; and
suppression of cavitation by degassing or high static pressures usually prevents it.
There is evidence that vibrating bubbles without collapse can cause breakage.
This phenomenon produces large local pressure changes also and is the probable
cause of cavitation erosion damage to ship propellers rather than actual bubble
collapse.
Dissolved gases within the organic solvent may have a significant effect. Highly
soluble gases such as sulfur dioxide, ammonia or carbon dioxide dissolved in benzene
can suppress cavitation activity by penetrating into the cavitation bubbles in great
amount, cushioning or preventing collapse.
Depolymerization may produce free radicals or ions concomitant with the splitting of
the C-C bond. These radicals can be detected by their ability to oxidize iodide
to iodine.
THE SONICATOR
As a research tool or production device the SONICATOR ultrasonic processor (as a
disruptor/emulsifier) is ideal in many respects. Periodic maintenance is
obviated since there are no "moving" parts and nothing to be cleaned.
Smoothly variable output intensities provide much needed flexibility for pilot
development. The continuous flow cell permits adjustment of static pressure
as well as fuel flow rate. This cell is made of polycarbonate resin (such as
LEXAN®) so that emulsification or other process can be viewed; and the
outlet may be placed directly in series with the combustion nozzle or measuring
device.
The standard 600 watt, 20KHz, SONICATOR will emulsify about ½ gal/min down
to 1 micron droplet size. Lower frequency 10KHz devices, which once were
available in the 8000 watt range, were more effective emulsifiers because of the
larger cavitation bubble, and were more efficient for a given wattage input.
10KHz SONICATORS were, however, quite noisy and physically much larger than the
equivalent 20KHz device and are no longer in production.
You may wish to visit the main ULTRASONICS page, et seq., with more on ultrasonics, as well as the Ultrasonics Cleaning page {in process} and the Ultrasonics Glossary page {also in process}.
Those persons interested in SONOCHEMISTRY might wish to look at the sonochemistry pages of:
Prof. Kenneth S. Suslick of the University of Illinois at Urbana-Champaign, and
Dr. Takahide Kimura at Shiga University in Japan.
THUMBS UP!
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S. Berliner, III
To contact S. Berliner, III, please click here.
To tour the Ultrasonics pages in sequence, the arrows take you from the
main Ultrasonics Page (with full index) to Pages A, 1, 1A, 2, 3, 4, and this page 5,
Glossary Page, Cleaning Page, and Bibliography Pages 1, 2, 3, and 4 (see Index,
above).
© Copyright S. Berliner, III - 1999, 2001, 2002,
2003,
2004
- All rights reserved.
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