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Sound Waves At Work PUBLIC ACCESS

Researchers have Devised a new Technique to use Sound Waves, Opening the way for simple Acoustic Compressors, Speedy Chemical-Process Reactors, and Clean Electric-Power Generators.

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Associate Editor

Mechanical Engineering 120(03), 80-84 (Mar 01, 1998) (4 pages) doi:10.1115/1.1998-MAR-2

Researchers have devised a new technique to use sound waves, opening the way for simple acoustic compressors, speedy chemical-process reactors, and clean electric-power generators. MacroSonix Corp. in Richmond, Vermont, has developed a technique by which standing sound waves resonating in specially shaped closed cavities can be loaded with thousands of times more energy than was previously possible. Company’s wave-shaping technology is known as resonant macrosonic synthesis (RMS). With some clever engineering, he said, the elevated acoustic-energy levels produced using RMS can be tapped for a wide range of industrial applications, including simplified compressors, pumps, speedy chemical-process reactors, and clean electric-power generators. MacroSonix has already licensed the RMS technology to a large appliance manufacturer to develop acoustic compressors for home refrigerators and air conditioners. MacroSonix has demonstrated the ability to produce high-pressure amplitudes inside resonator cavities. The MacroSonix technology relates to pressure waves in gases, which tend to be nonlinear in behavior. MacroSonix is working on a new licensing deal for an RMS air compressor and another with an electronic-component supplier. The company would like to enter larger research consortia with private, university, or government research labs to explore the RMS electric-power-generation concept.

EVER SINCE ELECTRIC ITY became a familiar part of everyday life, people have grown accustomed to the idea of getting the power for various mechanical t~sks from unseen electromagn etic waves traveling through metal wires. Few, however, have witnessed the acoustic analogue of electromagnetism-sound waves, or pressure waves, propagating through gas-fill ed chambers- doing much in the way of useful work.

That situation may change, as a new way of packing large amounts of power into sound waves can now be used as a prime mover for a range of industrially significant processes. Over the past decade, Tim Lucas, an aco ustician who is president and chief executive officer of Mac roS onix C orp. in Richmo nd, Va., h as developed a technique by whi ch standing sound waves reso n at ing in sp eciall y sh ap ed cl osed caviti es ca n be loaded with thousands of times more energy than was previously possible. For example, the new devices can generate dynamic (oscillating) pressures exceeding 500 pounds per square inch in gases, more than what is needed fo r commercial applica tions. "You 've h eard about researchers using sound to levitate Ping-Pong balls inside tubes," he said. "Our technology should allow us to levitate bowling balls with sound waves."

Lucas and his research colleagues at MacroSonix are foc using on exploitin g w h a t h e ca ll ed "a new primary tec hn o logy" akin in signifi ca nce to lase rs or semicondu ctor transistors. "We have reach ed the p ower- den sity levels of machines we use every day-machines develop ed in the indu st r ial revoluti o n ," Lucas sa id. " N ow we can replace those m ac hine and their moving parts-with sound waves."

Lucas's wave- shaping technology is known as resonant macrosonic synthesis (RMS). With some clever engineering, he said, the elevated acoustic- energy levels produced using RMS can be tapped for a wide range of industri al applications, including simplified compressors, pumps, speedy chemical-process reacto rs, and clean electric- power generators. MacroSonix has already licensed the RMS technology to a large appliance manufacturer to develop acoustic compresso rs fo r home refrigerators and air conditioners.

The new technology should enabl e engineers to use sound to perform tasks commonly performed by mechanical devices, Lucas said. T his inI1erent simplicity could result in lower manufacturing costs, higher energy efficiency, lower operating costs, increased reliability and durability, and oilless operation, which is critical for the semiconductor and pharmaceutical industries.

RMS ranks among the most significant recent advances in aco ustics, along with thermoacoustics and sonoluminescence, according to other experts in the fi eld. Thermoacoustics is concerned with the thermal effects resultin g from ac ou sti cally driven gas compress ion and expansion processes (see "Cool Sounds" on page 83); sonoluminescence is the intense flashes of light that can be generated as an air bubble trapped in an aqueous sound fi eld collapses.

MacroSonix has demonstrated the ability to produce high-pressure amplitudes inside resonator caviti es, acco rding to Greg Swift, a staff m ember of the Conde n se d Matter and Thermal Physics Group at Los Alamos National Laboratory in Los Alamos, N.M. " It's the equivalent of shaking a dishpan full of water and getting waves 16 inch es high. I've never seen anything quite this high before."

Anthony Atchley, head of the graduate program in acoustics at Pennsylvania State University in State College, was similarly impressed with how MacroSonix's technology manages to p rodu ce high-amplitude standing waves in reso n ators. At ch ley n oted th at, by suppressing the generation of harmonics, they sculpt the waveform in a way th at di storts it but doesn't shock it. "Of course, the big ques tion is: What else can yo u do with it?"

High- power standing sound waves can be generated inside an RMS bulb resonator (front) by driving it with a linear motor (left). A cutaway (rear) reveals that the specially shaped closed cavity is hollow.

Grahic Jump LocationHigh- power standing sound waves can be generated inside an RMS bulb resonator (front) by driving it with a linear motor (left). A cutaway (rear) reveals that the specially shaped closed cavity is hollow.

Finite-amplitude acoustic phen omena in resonant cavities have been of practi cal interes t since th e 1930s, when German researchers studied them in connection with the development of mufflers for tanks. Historically, scientists have believed that there is an intrinsic limit for sound waves in gases (in closed half-wavelength resonator cavities) that would never allow high-energy levels and acoustic pressures to exist. Previous experimental work had shown that sound waves in a resonator: would build up energy only to a ce rtain level and no more. T his acoustic saturation point occurs when shock waves start to form. Once a shock wave exists, any energy added to the wave is wasted as heat without any increase in the dynamic pressure of the wave.

The MacroSonix technology "relates to pressure waves in gases, which tend be nonlinear in behavior," Lucas said. A mechanical analogy would be a nonlinear spring: If you displace the spring onl y a small amount, the response will be mostly linear. A large displacement, on the other hand, will yield a nonlinear response. Similarly, if you drive small-pressure amplitudes into a cavity, the dynamic pressure will be small compared to the average gas pressure, and you can expect the resulting wave to propagate without distortion. Conversely, higher pressures typically produce distorted waves.

In general, a rigorous theoretical model for nonlinear standing waves, even in simple cavity geometries, is quite involved. In any real situ ation, thermal and viscous losses at the walls of the cavity must be considered in addition to the mainstream losses. This typically requires a three-dimensional solution to the nonlinear wave equation that includes a description of the motion of the compressible fluid within the boundary layer. To simplify its analysis, Lucas's research team uses a two- dimensional axisymmetric model, which is made possible by the axially symmetric shapes of"the resonator cavities they use.

To explain this complex condition, Lucas suggested considering a simplified example in which a sine-wave pressure oscillation is imparted to some average gas pressure in the cavity. These oscillating pressure values, he noted, can be added to the average pressure at each position along the length of the axisynunetric cavity in the same way an ac voltage riding on a de voltage is sununed at each location along the pathway of the current. The sine-wave oscillation superimposes itself on the average pressure, so the peak pressure is the difference between the average pressure and peak value of the sine wave. Similarly, a significant pressure deviation occurs at the low-pressure trough of the sine wave, he added. Company researchers work with relative pressure ratios normalized by dividing the peak pressure difference by the average pressure.

According to Lucas, the high-pressure regions are also areas of significant temperature deviation. "And since sound speed goes with temperature, a higher-temperature pressure peak tends to catch up to the trough [a low-pressure, lower-temperature region] ahead of it. The whole wave leans into a sawtooth shape." This pressure-steepening effect, in which the peaks catch up to the troughs, continues until the peaks reach the now near-vertical pressure drops and can go no farther. The resulting sawtooth- shaped waveforms are called shock waves. Finally, because of the high temperature gradient that exists across the shock front, "no matter how m.uch energy you try to put in, it's lost as heat," he said. "You can't store any more energy in the wave," which has reached what is known as acoustic saturation. Another way to consider this situation is to think of the added energy as going into higher harmonics instead of higher amplitudes.

Previously, the nonlinear nature of the gaseous medium through which sound waves propagate was considered the primary limiting factor in creating higheramplitude (more-powerful) sound waves. Lucas discovered that for resonant sound waves, the geometry of the resonator cavity through which the so und waves travel is the most important factor in determining the ideshape of the wave.

In fact, many researchers in the past had used cylindrical resonators, which is the one resonator configuration most likely to produce shock waves, he said. Most of the previous work was done in cylindrical cavities, which are pretty much guaranteed to give a shock wave in a standing wave; this contributed to the perception that the problem was intractable. Some researchers experimenting with other cavity shapes showed that the onset of shocking could be suppressed, but their estimates of the best dynamic gas-molecule velocity-the speed of a parcel of gas-was approximately Mach 0.1. Thus, the general perception remained that the research was a dead end.

As noted, the key realization leading to RMS is that the shape of the oscillating resonator can be used to control or tune the shape of the pressure waveform (the phase and amplitude of the wave's harmonics). While working at Los Alamos in 1990, Lucas found that he could create relatively large-amplitude, or macrosonic, sound waves up to 60 pounds per square inch by properly shaping the resonator.

He later demonstrated that the shape of the waves could be determined by the shape of the resonator even at extremely high pressure amplitudes. Thus, RMS allows the synthesis of nonshocked waveforms, which in turn lets large amounts of energy to be added to the wave so extremely high dynamic pressures can be achieved. "We get a specific series of harmonics that have a specific amplitude and phase to produce the desired waveform" via Fourier wave summing. MacroSonix's cavity shapes, which include various cones, cone-horn hybrids, and bulbs, are said to have high Q, the resonance-storage quality factor.

The next technical hurdle researchers addressed was to figure out how to transfer a lot of power into a cavity, Lucas said. "People typically use moving pistons or diaphragms, but at the vibration rates we wanted to work at-SOO or 600 cycles per second-you have to move the piston back and forth by 3 or 4 centimeters very rapidly," which is quite diffic ult to do. " I decided to shake the whole thing-the entire cavity-with a linear motor, a vibrator that is nothing more than a glorified electromagnet," he said. "That way, the whole inner surface of the resonator transfers energy to the enclosed gas. Now the cavity acts like one large piston."

Lucas has demonstrated his RMS technology using a resonator cavity shaped like an elongated pea r. When this cavity is vibrated with a linear motor so that its walls move back and forth a distance of about 100 rnicrons, it resonates with a smooth, shockless wave of high energy. Acoustic particle velocities in RMS cavities have reached Mach numbers greater than 0.5. He reported that there appears to be no theoretical reason why particle velocities cannot reach Mach 1.

This computer rendering shows the discharge and suction cycles of an ideshape alized acoustic compressor. The standing wave formed inside the resonator causes the gas pressure to oscillate, driving a discharge valve and a suction valve to convert the pressure oscillation into gas compression and flow.

Grahic Jump LocationThis computer rendering shows the discharge and suction cycles of an ideshape alized acoustic compressor. The standing wave formed inside the resonator causes the gas pressure to oscillate, driving a discharge valve and a suction valve to convert the pressure oscillation into gas compression and flow.

Since making his basic discovery eight years ago, Lucas has been working in secret on RMS, forming MacroSonix along the way. To date, the company has been awarded 10 US. patents. Now he is revealing the technology to the rest of the world in hopes of licensing the technology to companies that could exploit various RMS applications and forming research consortia to expand the company's advanced research on the technology.

The first major application for RMS technology is likely to be a simple, low-cost acoustic compressor. Compresso rs, of course, are all but ubiquitous in the modern world. MacroSonix's licensing and development agreement with a Fortune 500 appliance maker was signed in 1994 for the manufacture of acoustic compressors for household refrigerators, air conditioners, and certain commercial refrigeration and cooling applications. "We are developing a lubricant-free acoustic compressor that is environmentally safe and promises to be more energyefficient than standard compressors," he said. In this compressor, the standing wave becomes the machine that provides the required gas compression, and it eliminates the need for oil and moving parts such as pistons connecting rods, crankshafts, and bearings.

A current version of the acoustic compressor has a cavity that oscillates back and forth by about 100 microns. "You can't see it move," Lucas said. "It's almost a solidstate compressor." Inside the cavity, dynamic gas-molecule displacement is about 5 to 6 centimeters, about one-third of the resonator length.

The compressor has a large-diameter end and a small-diameter end where a pair of reed valves are installed. "Since the losses are pretty much proportional to pressure, you don't want high pressure everywhere," he said, noting that the pressures at the small end can be seven to 10 times higher than those at the large end. MacroSonix engineers designed the cavity to concentrate most of its acoustic losses at the small high-pressure end. Peak-to-minimum-pressure ratios of 27: 1 were observed using tlus type of device. Practical compressors for air, refrigerants, or other gases require pressure ratios (discharge to suction) of3:1 or more.

In operation, the compressor enuts a nearly pure tone that is sufficiently loud to require a sound-deadening enclosure. "We are confident that we can meet what's become an appliance industry standard-about 35 decibels, which is lower than a whisper," he said.

Lucas reported that his team has also designed an acoustic compressor that doesn't need valves. It is based on creating a static pressure distribution in which a high-pressure area is located at a port on one end of the resonator and a low-pressure area is placed near another port at the other end. Although this design generates a lower-pressure head, it offers the benefit of noncontanunating operation. The valveless operation makes the compressor pump almost entirely solid-state.

For competitive reasons connected with MacroSonix's licensing deals, Lucas has not revealed the energy efficiencies attained by his acoustical compressors. "In these kinds of markets, cost and efficiency are king," he said. "If you can't compete on those issues, no one would be interested in the technology."

Cool Sounds

ACOUSTICIANS ARE WORKING on using the sound-wavedriven phenomenon known as the thermoacoustic effect in simple, reliable; low-cost, environmentally friendly refrigeration units. Although applications such as home refrigerators, electronics chillers, and natural-gas liquefiers have been explored, the efficiencies are not yet sufficiently high to generate the financial investment required for commercialization.

The thermoacoustic effect is surprisingly simple, according to Steven Garrett, United Technologies Corp. Professor of Acoustics at Pennsylvania State University in State College. "When you compress a gas, it gets warmer; expand a gas and it cools. A sound wave is really nothing more than a periodic compression and expansion of a gas." So a sound wave heats and cools small parcels of gas along the length of its propagation.

When a sound wave is sent down a half-wavelength tube with a vibrating diaphragm or a loudspeaker, the pressure pulsations make the gas inside slosh back and forth. This forms regions where compression and heating take place, plus other areas characterized by gas expansion and cooling.

A thermoacoustic refrigerator, Garrett said, is a resonator cavity that contains a stack of thermal storage elements (connected to hot and cold heat exchangers) positioned so the back-and-forth gas motion occurs within the stack. The oscillating gas parcels pick up heat from the stack and deposit it to the stack at a different location. Garrett said the device "acts like a bucket brigade" to remove heat from the cold heat exchanger and deposit it at the hot heat exchanger, thus forming the basis of a refrigeration unit.

Pioneering work on thermoacoustics began in 1982 at Los Alamos National Laboratory in Los Alamos, N.M., and continues under Greg Swift, who since 1994 has focused on developing a combustion-powered thermoacoustic natural- gas liquefier with no moving parts. Funded jointly by Denver-based Cryenco Inc. and the U.S. Department of Energy, the device burns natural gas to produce heat that is used to form a standing wave inside a helium-filled resonant cavity. The heat-driven cryogenic refrigerator, which has a cooling capacity of approximately 2,000 watts, reaches-150°C.

The Penn State group has developed several thermoacoustic refrigerators over the years. In 1992, the U.S. Air Force launched a thermoacoustic cooler the group built to chill optoelectronic devices (for eventual use on satellites) on the space shuttle Discovery. Two years later, the Shipboard Electric ThermoAcoustic Chiller (SETAC) cooled a radar azimuth converter on a Navy destroyer, the U.S.S. Deyo, for 18 hours. SETAC had the cooling power of a home refrigerator. The team is now fabricating a technically similar yet much larger chiller/air conditioner for the U.S. Navy. The 3- ton unit, called Triton, is being developed with $3 million in funding and is due for completion in April 1999. The 10-thermalkilowatt unit is expected to cool electronics at the rate of 36,000 Btus per hour.

Other thermoacoustics work is now being conducted at the Naval Postgraduate School in Monterey, Calif.; Purdue University in West Lafayette, lnd.; the University of Utah in Salt Lake City; and the University of Mississippi in University.

Thermoacoustic refrigerators use standing sound waves to generate low temperatures for cooling

Grahic Jump LocationThermoacoustic refrigerators use standing sound waves to generate low temperatures for cooling

This bulb resonator shape is only one of many cavity geometries that can be used for resonant macrosonic synthesis.

Grahic Jump LocationThis bulb resonator shape is only one of many cavity geometries that can be used for resonant macrosonic synthesis.

According to Lucas, the new RMS concepts will make previously unattainable physical effects possible. Such effects could be used in a range of new industrial devices and processes, with cavities shaped specifically for each application. For example, specialized noncontaminating acoustic compressors and pumps for commercial gases, ultrapure fluids, and hazardous fluids are a possibility, as are roughing vacuum pumps, which are important for the pharmaceutical and semiconductor industries.

Another application would be simplified process reactors for the chemical and pharmaceutical industries. RMS technology could be used to drive and control thermal and kinetic chemical reactions by producing localized heating with rapid pressure changes. "Theoretically," Lucas said, "you could change pressure and temperature rapidly, and turn the reaction on and off as needed. We've also demonstrated that we can move material in and out of cavities without disrupting the standing waves inside. This could allow us to turn batch processes into semi continuous processes," in which small an10unts of material are successively brought into the cavity, processed, and moved out.

Acoustic chambers could be used for separation, agglomeration, levitation, mixing, and pulverization of materials. For example, improved acoustic agglomeration of particulate contaminants- causing them to stick together-could lead to improved environmental gas scrubbers for power-plant flue gases by allowing smaller units to operate at higher flow rates. Rapidly solidified metal alloying operations might make good use of RMS-based levitation devices.

Electric-power generation using pulse combustion ofhydrocarbon fuels could also be realized through tlus technology. Using clean-burning, high-efficiency pulse-combustion engines similar to those used on Nazi Germany's V-I buzz-bomb weapon, engineers could create high-energydensity standing waves inside a football-shaped RMS cavity. These resonators would be attached to electromagnetic dynamos to generate electricity. In effect, this scheme would mean running the acoustic compressor cycle in reverse to produce electric power.

Lucas said MacroSonix is working on a new licensing deal for an RMS air compressor and another with an electronic- component supplier. · "We're also looking at letting a license for a special compressor for handling hazardous gases of interest to semiconductor firms." The company would like to enter into larger research consortia with private, university, or government research labs to explore the RMS electric-power-generation concept. Finally, Lucas is interested in setting up RMS research centers at universities to spread the tedllucal expertise needed to best exploit the innovation.

"Now that large amounts of energy can be transferred into resonant sound waves," Lucas said, "these sound waves can be used to perform industry's high-powered tasks in completely new and simpler ways. RMS quite literally unlocks the power of sound."

Copyright © 1998 by ASME
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