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Thermoacoustics   : Publications  

Recent thermoacoustics publications from Los Alamos.

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Mixture separation

Nonlinear pressure and flow

Fundamentals of engines and refrigerators

Vibration cancellation and mode locking

Toward practical engines--space power

Toward practical engines and refrigerators--natural gas liquefaction

Reviews and summaries

 

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Within each category, these recent publications are listed in reverse chronological order. Preprints are indicated by red italics. Clicking on a journal title takes you to that journal's website, where you might only have access if the journal recognizes you as a subscriber. If copyright rules allow it, the full article can be accessed here either by clicking on its title or on "View full article."


Mixture separation:

"High-purity thermoacoustic isotope enrichment," G. W. Swift, D. A. Geller, and S. N. Backhaus, submitted to the Journal of the Acoustical Society of America. View abstract or full article or external and internal hardware photos.

"Thermoacoustic mixture separation with an axial temperature gradient," D. A. Geller and G. W. Swift, Journal of the Acoustical Society of America 125, 2937-2945 (2009). View abstract or full article or typical DeltaE model or close-up hardware photo.

"Continuous thermoacoustic mixture separation," G. W. Swift and D. A. Geller, Journal of the Acoustical Society of America 120, 2648-2657 (2006); erratum 124, 2421 (2008). View abstract or full article (incl. erratum) or typical DeltaE model.

"Thermoacoustic enrichment of the isotopes of neon," D. A. Geller and G. W. Swift, Journal of the Acoustical Society of America 115, 2059-2070 (2004). View abstract or hardware (full view). or hardware (closeup) or full article.

"Thermodynamic efficiency of thermoacoustic mixture separation," D. A. Geller and G. W. Swift, Journal of the Acoustical Society of America 112, 504-510 (2002). View abstract or full article.

"Saturation of boundary-layer thermoacoustic mixture separation," D. A. Geller and G. W. Swift, Journal of the Acoustical Society of America 111, 1675-1684 (2002). View abstract or full article. Beware of a minor typographical error in Eq. (48).

"Thermoacoustic separation of a He-Ar mixture," P. S. Spoor and G. W. Swift, Physical Review Letters 85, 1646-1649 (2000). View abstract or full article.

"Thermal diffusion and mixture separation in the acoustic boundary layer," G. W. Swift and P. S. Spoor, Journal of the Acoustical Society of America 106, 1794-1800 (1999). Erratum 107, 2299 (2000); second erratum 109, 1261 (2001).


Nonlinear pressure and flow:

"Why High-Frequency Pulse Tubes Can Be Tipped," G. W. Swift and S. Backhaus, Proceedings of the 16th International Cryocoolers Conference (Atlanta, May 2010), edited by S. D. Miller and R. G. Ross, Jr., pages 183-192 (ICC Press, Boulder CO, 2011). View full article or typical DeltaEC model.

"The pulse tube and the pendulum," G. W. Swift and S. Backhaus, Journal of the Acoustical Society of America 126, 2273-2284 (2009). View abstract or full article or close-up hardware photo or partly-assembled hardware photo.

"Analytical solution for temperature profiles at the ends of thermal buffer tubes," Konstantin. I. Matveev, Gregory W. Swift, and Scott Backhaus, International Communications in Heat and Mass Transfer 50, 897-901 (2007). View abstract.

"An internal streaming instability in regenerators," J. H. So, G. W. Swift, and S. Backhaus, Journal of the Acoustical Society of America 120, 1898-1909 (2006). View abstract or full article.

"Gas Diodes for Thermoacoustic Self-Circulating Heat Exchangers," Greg Swift and Scott Backhaus, submitted to the Proceedings of the 17th International Symposium on Nonlinear Acoustics, The Pennsylvania State University, 18-22 July 2005.

"Temperatures near the interface between an ideal heat exchanger and a thermal buffer tube or pulse tube," Konstantin I. Matveev, G. W. Swift, and S. Backhaus, International Journal of Heat and Mass Transfer 49, 868-878 (2006). View preprint.

"Reduced-order modeling of vortex-driven excitation of acoustic modes," Konstantin I. Matveev, Acoustics Research Letters Online 6, 14-19 (2005). View abstract.

"A resonant, self-pumped,circulating thermoacoustic heat exchanger," G. W. Swift and S. Backhaus, Journal of the Acoustical Society of America 116, 2923-2938 (2004). View abstract or full article.

"Power dissipation and time-averaged pressure in oscillating flow through a sudden area change," B. L. Smith and G. W. Swift, Journal of the Acoustical Society of America 113, 2455-2463 (2003). View abstract or full article.

" A comparison between synthetic jets and continuous jets," B. L. Smith and G. W. Swift, Experiments in Fluids 34 467-472 (2003). The original publication is available at link.springer-ny-com or link.springer.de. Springer-Verlag is the copyright holder.

"An acoustic streaming instability in thermoacoustic devices utilizing jet pumps," S. Backhaus and G. W. Swift, Journal of the Acoustical Society of America 113, 1317-1324 (2003). View abstract or full article.

"Measuring second-order time-average pressure," B. L. Smith and G. W. Swift, Journal of the Acoustical Society of America 110, 717-723 (2001). View abstract or full article.

"Synthetic jets at large Reynolds number and comparison to continuous jets," B. L. Smith and G. W. Swift, AIAA 2001-3030, Proceedings of the 31st AIAA Fluid Dynamics Conference, Anaheim CA, 11-14 June 2001.


Fundamentals of engines and refrigerators:

"Staging two coolers through a quarter-wave tube," G. W. Swift, D. L. Gardner, and S. Backhaus, Proceedings of the 17th International Cryocoolers Conference (Los Angeles, July 9-12, 2012), edited by S. D. Miller and R. G. Ross, Jr., pages 179-185 (ICC Press, Boulder CO, 2012). View full article.

"Quarter-wave pulse tube," G. W. Swift, D. L. Gardner, and S. Backhaus, Cryogenics 51, 575-583 (2011). View abstract or full article, movie of uninsulated setup with straight pulse tube, photo of uninsulated coiled pulse tube, or typical DeltaEC model (in two parts): part 1 and part 2 as described in the article.

"Thermoacoustic analysis of displacer gap loss in a low temperature Stirling cryocooler," Vincent Kotsubo and Greg Swift, CP823, Advances in Cryogenic Engineering: Transactions of the Cryogenic Engineering Conference - CEC 51, edited by J. G. Weisend II (2006, American Institute of Physics, Melville NY), pages 353-360. View abstract or full article.

"The effect of gravity on heat transfer by Rayleigh streaming in pulse tubes and thermal buffer tubes," Konstantin I. Matveev, Scott Backhaus, and Gregory W. Swift, to be published in the proceedings of IMECE 2004, International Mechanical Engineering Conference and Expo, November 13-19, 2004, Anaheim CA. View abstract.

"A cascade thermoacoustic engine," D. L. Gardner and G. W. Swift, Journal of the Acoustical Society of America 114, 1905-1919 (2003). View abstract or hardware or full article.

"Fabrication and use of parallel-plate regenerators in thermoacoustic engines," S. N. Backhaus and G. W. Swift, in the Proceedings of the 36th Intersociety Energy Conversion Engineering Conference, Savannah GA, 29 July-2 August 2001.

"A thermoacoustic-Stirling heat engine: Detailed study," S. N. Backhaus and G. W. Swift, Journal of the Acoustical Society of America 107, 3148-3166 (2000). View abstract or full article or typical DeltaE model.

"A thermoacoustic-Stirling heat engine," S. N. Backhaus and G. W. Swift, Nature 399, 335-338 (1999).

"Condensation in a steady-flow thermoacoustic refrigerator," R. A. Hiller and G. W. Swift, Journal of the Acoustical Society of America 108, 1521-1527 (2000). View abstract or full article or typical DeltaE model.

"Experiments with a flow-through thermoacoustic refrigerator," R. S. Reid and G. W. Swift, Journal of the Acoustical Society of America 108, 2835-2842 (2000). View full article.

"Acoustic recovery of lost power in pulse tube refrigerators," G. W. Swift, D. L. Gardner, and S. N. Backhaus, Journal of the Acoustical Society of America 105, 711-724 (1999). View full article.

"Superfluid orifice pulse tube refrigerator below 1 Kelvin," A. Watanabe, G. W. Swift, and J. G. Brisson, Advances in Cryogenic Engineering 41, 1519-1526 (1996). View full article.

"Measurements with a recuperative superfluid Stirling refrigerator," A. Watanabe, G. W. Swift, and J. G. Brisson, Advances in Cryogenic Engineering 41, 1527-1533 (1996). View full article.

"Uniform-temperature cooling power of the superfluid Stirling refrigerator," A. Watanabe, G. W. Swift, and J. G. Brisson, Journal of Low Temperature Physics 103, 273-293 (1996). View full article.


Vibration cancellation and mode locking:

"Mode locking of acoustic resonators and its application to vibration cancellation in acoustic heat engines," P. S. Spoor and G. W. Swift, Journal of the Acoustical Society of America 106, 1353-1362 (1999). View full article.

"The Huygens entrainment phenomenon and thermoacoustic engines," P. S. Spoor and G. W. Swift, Journal of the Acoustical Society of America 108, 588-599 (2000). View full article.


Toward practical engines--space power:

"High-temperature self-circulating thermoacoustic heat exchanger," S. Backhaus, G. W. Swift, and R. S. Reid, Applied Physics Letters 87, 014102 1-3 (2005). View abstract.

"A self-circulating heat exchanger for use in Stirling and thermoacoustic-Stirling engines," S. Backhaus and R. S. Reid, Space Technology and Applications International Forum (STAIF-2005). Albuquerque, NM, February, 2005, AIP Conference Proceedings 746, p. 719-726. View abstract.

"Traveling-wave thermoacoustic electric generator," S. Backhaus, E. Tward, and M. Petach, Applied Physics Letters 85, 1085-1087 (2004). View abstract.

"Design of a high efficiency power source (HEPS) based on thermoacoustic technology," M. Petach, E. Tward, and S. Backhaus, Final report, NASA contract no. NAS3-01103, CDRL 3f (2004).

"Initial tests of a thermoacoustic space power engine," S. Backhaus, Space Technology and Applications International Forum (STAIF-2003). February, 2003. Albuquerque, New Mexico. AIP Conference Proceedings 2003 654, 641-647. Edited by M. S. El Genk. View abstract.

"Thermoacoustic space power converter," E. Tward, M. Petach, and S. Backhaus, Space Technology and Applications International Forum (STAIF-2003). February, 2003. Albuquerque, New Mexico. AIP Conference Proceedings 2003 654, 656-661. Edited by M. S. El Genk. View abstract.

"Thermoacoustic power systems for space applications," S. Backhaus, E. Tward, and M. Petach, Space Technology and Applications International Forum (STAIF-2002). February, 2002. Albuquerque, New Mexico. AIP Conference Proceedings 2002 608, 939-944. Edited by M. S. El Genk. View abstract.


Toward practical engines and refrigerators--natural gas liquefaction:

"Operation of thermoacoustic Stirling heat engine driven large multiple pulse tube refrigerators," Bayram Arman, John Wollan, Vince Kotsubo, Scott Backhaus, and Greg Swift, submitted to the proceedings of the 13th International Cryocooler Conference. Kluwer Academic/Plenum Publishers holds the copyright to this paper.

"Thermoacoustics for liquefaction of natural gas," G. W. Swift and J. J. Wollan, GasTIPS, Volume 8, Number 4, pages 21-26 (Fall 2002). (Erratum: page 23, column two, under the heading "How the refrigerator works": The first word of the 23rd line in that paragraph should be "falls" instead of "rises.")

"Development of a thermoacoustic natural gas liquefier," J. J. Wollan, G. W. Swift, S. N. Backhaus, and D. L. Gardner, AIChE Meeting, New Orleans LA, March 11-14, 2002.

"Thermoacoustic refrigeration --- A stirring concept for offshore associated gas liquefaction," W. C. van Wijngaarden, Monetizing Stranded Gas Reserves Conference, Houston TX, December 7-9, 1999.


Reviews and summaries:

"John Malone and Liquid Thermodynamics," G. W. Swift, issue 2010-3 of Stirling Machine World, edited by John Corey.

"Interactive analysis, design, and teaching for thermoacoustics using DeltaEC," W. C. Ward, G. W. Swift, and J. P. Clark, talk given at the spring 2008 meeting of the Acoustical Society of America (Paris). Also available as a pdf file. Abstract available at Journal of the Acoustical Society of America 123, 3546 (2008). The DeltaEC model described in the talk is available here.

"Thermoacoustic energy conversion," G. W. Swift, Chapter 7 of the Springer Handbook of Acoustics, edited by Thomas Rossing (Springer, 2007).

"New varieties of thermoacoustic engines," S. N. Backhaus and G. W. Swift, Paper number 502, Proceedings of the Ninth International Congress on Sound and Vibration, Orlando FL, July 8-11 2002.

"Separation of gas mixtures by thermoacoustic waves," D. A. Geller, P. S. Spoor, and G. W. Swift, Proceedings of the 17th International Congress on Acoustics, Rome, 2-7 September 2001; Volume 1, Part A, Session "Thermoacoustics," pages 16-17.

"Thermoacoustics," G. W. Swift, McGraw-Hill Encyclopedia of Science and Technology, 9th edition, Volume 18, 353-355 (2002)

"Thermoacoustics: A unifying perspective for some engines and refrigerators," G. W. Swift, graduate-level textbook, available at ASA Books. View table of contents.

"The power of sound," S. L. Garrett and S. Backhaus, American Scientist 88, 516-525, (2000).

"Streaming in thermoacoustic engines and refrigerators," G. W. Swift, Nonlinear Acoustics at the Turn of the Millennium: Proceedings of the 15th International Symposium on Nonlinear Acoustics, edited by W. Lauterborn and T. Kurz, 105-114 (American Institute of Physics, Melville NY, 2000).



Abstracts of some of the publications above:


"A thermoacoustic-Stirling heat engine: Detailed study,"
S. N. Backhaus and G. W. Swift,
Journal of the Acoustical Society of America 107, 3148-3166 (2000).

Abstract: A new type of thermoacoustic engine based on traveling waves and ideally reversible heat transfer is described. Measurements and analysis of its performance are presented. This new engine outperforms previous thermoacoustic engines, which are based on standing waves and intrinsically irreversible heat transfer, by more than 50%. At its most efficient operating point, the engine delivers 710 W of acoustic power to its resonator with a thermal efficiency of 0.30, corresponding to 41% of the Carnot efficiency. At its most powerful operating point, it delivers 890 W to its resonator with a thermal efficiency of 0.22. The efficiency of this engine can be degraded by two types of acoustic streaming. These are suppressed by appropriate tapering of crucial surfaces in the engine and by using additional nonlinearity to induce an opposing time-averaged pressure difference. Data are presented which show the nearly complete elimination of the streaming convective heat loads. Analysis of these and other irreversibilities show which components of the engine require further research to attain higher efficiency. Additionally, these data show that the dynamics and acoustic power flows are well understood, but the details of the streaming suppression and associated heat convection are only qualitatively understood.


"Condensation in a steady-flow thermoacoustic refrigerator,"
R. A. Hiller and G. W. Swift,
Journal of the Acoustical Society of America 108, 1521-1527 (2000).

Abstract: Condensation may occur in an open-flow thermoacoustic cooler with stack temperatures below the saturation temperature of the flowing gas. In the experimental device described here the flowing gas, which is also the acoustic medium, is humid air, so the device acts as a flow-through dehumidifier. The humid air stream flows through an acoustic resonator. Sound energy generated by electrodynamic drivers produces a high-amplitude standing wave inside of the resonator, which causes cooling on a thermoacoustic stack. Condensation of water occurs as the humid air passes through the stack and is cooled below its dew point, with the condensate appearing on the walls of the stack. The dry, cool air passes out of the resonator, while the condensate is wicked away from the end of the stack. Thermoacoustic heat pumping is strongly affected by the form of the condensate inside of the stack, whether condensed mostly on the stack plates, or largely in the form of droplets in the gas stream. Two simple models of the effect of the condensate are matched to a measured stack temperature profile; the results suggest that the thermoacoustic effect of droplets inside the stack is small.


"Saturation of boundary-layer thermoacoustic mixture separation,"
D. A. Geller and G. W. Swift,
Journal of the Acoustical Society of America 111, 1675-1684 (2002).

Abstract: The theory for thermoacoustic boundary-layer mixture separation is extended to include the effects of a nonzero concentration gradient. New data are presented, which are in excellent agreement with this theory. The maximum concentration gradient which may be achieved in a binary mixture of gases through this separation process is intrinsically limited by the fractional pressure amplitude, by the tidal displacement, and by the size of the thermal diffusion ratio. Ordinary diffusion further detracts from the attainable final concentration gradient. Rayleigh streaming also works against thermoacoustic separation, and an estimate of the molar flux from streaming is given.


"Thermoacoustic separation of a He-Ar mixture,"
P. S. Spoor and G. W. Swift,
Physical Review Letters 85, 1646-1649 (2000).

Abstract: We report observation of a new mixture-separation process: an insonified mixture of helium and argon in a narrow duct spatially separates along the acoustic-propagation axis. We measure mole-fraction differences across the ends of the duct as large as 7%. We measure initial separation flux densities as high as 0.001 times M squared times c, where M is the acoustic Mach number and c is the sound speed. This initial separation flux, as a function of both the amplitudes and the relative phasing of the pressure and velocity oscillations in the duct, agrees well with a recent theory involving oscillating thermal diffusion in the acoustic boundary layer.


"Measuring second-order time-average pressure,"
B. L. Smith and G. W. Swift,
Journal of the Acoustical Society of America 110, 717-723 (2001).

Abstract: Measurements of the spatial distribution of the time-averaged second-order pressure in a plane standing wave in atmospheric air are reported. Several measurement pitfalls are identified, and solutions are described. These include accounting for slight nonlinearity of the piezoresistive transducer and careful mounting of the transducer. Streaming causes extra complication when a capillary-connected monometer is used. With the proper technique and instrumentation, results are in good agreement with theory.


"Thermodynamic efficiency of thermoacoustic mixture separation,"
D. A. Geller and G. W. Swift,
Journal of the Acoustical Society of America 112, 504-510 (2002).

Abstract: The acoustic power loss in the thermoacoustic mixture-separation process is derived, including the contributions due to a nonzero gradient in concentration. The significance of the gradient-dependent term is discussed. The limiting thermodynamic efficiency of the separation is calculated. Under reasonable circumstances, the efficiency approaches 0.01 times nH times nL times ( delta m / m avg ) squared, where nH and nL are the mole fractions of the two components of the mixture, and delta m / m avg is the fractional difference between the molar masses of the two components. This efficiency is of the same order of magnitude as that of some other, more conventional separation methods.


"An acoustic streaming instability in thermoacoustic devices utilizing jet pumps,"
S. Backhaus and G. W. Swift,
Journal of the Acoustical Society of America 113, 1317-1324 (2003).

Abstract: Thermoacoustic-Stirling hybrid engines and feedback pulse tube refrigerators can utilize jet pumps to suppress streaming that would otherwise cause large heat leaks and reduced efficiency. It is desirable to use jet pumps to suppress streaming because they do not introduce moving parts such as bellows or membranes. In most cases, this form of streaming suppression works reliably. However, in some cases, the streaming suppression has been found to be unstable. Using a simple model of the acoustics in the regenerators and jet pumps, a stability criterion is derived that predicts when jet pumps can reliably suppress streaming.


"A cascade thermoacoustic engine,"
D. L. Gardner and G. W. Swift,
accepted for publication in the Journal of the Acoustical Society of America in 2003.

Abstract: A cascade thermoacoustic engine is described, consisting of one standing-wave engine plus two traveling-wave engines in series. Most of the acoustic power is produced in the efficient traveling-wave stages. The straight-line series configuration is easy to build and allows no Gedeon streaming. The engine delivers up to 2 kW of acoustic power, with an efficiency (ratio of acoustic power to heater power) up to 20%. Understanding of the pressure and volume-velocity waves is very good. Agreement between measured and calculated powers and temperatures is reasonable. Some of the measured thermal power that cannot be accounted for by calculation can be attributed to Rayleigh streaming in the two thermal buffer tubes with the largest aspect ratios. Straightforward extension of this work should yield cascade thermoacoustic engines with efficiencies around 35-40% of the Carnot efficiency.


"Power dissipation and time-averaged pressure in oscillating flow through a sudden area change,"
B. L. Smith and G. W. Swift,
Journal of the Acoustical Society of America 113, 2455-2463 (2002).

Abstract: Experiments on oscillating flow at the abrupt transition between a 2-D channel and essentially infinite space are presented. It is shown that phenomena associated with the transition are functions of three independent dimensionless parameters including the dimensionless radius rounding the edge of the end of the channel. The effect of each of these three parameters on the time-averaged pressure difference across the transition and the acoustic power dissipation is explored by holding two parameters fixed while varying the third. Evidence is presented that the losses due to oscillatory flow in this geometry are smaller than would be expected from commonly accepted values for steady flow in similar geometry.


"Thermoacoustic enrichment of the isotopes of neon,"
D. A. Geller and G. W. Swift,
Journal of the Acoustical Society of America 115, 2059-2070 (2004).

Abstract: The enrichment of the neon isotopes in a thermoacoustic device is demonstrated. Because the thermal diffusion ratio of neon is small, an apparatus longer than a wavelength was necessary in order to easily observe the separation. The device was modular and extensible, so that arbitrarily large separations could in principle be obtained. The acoustic duct was a series of multiple, identical quarter-wavelength modules with side-branch drivers. In this way, waveforms close to that of a traveling wave were maintained in the duct, despite the high acoustic attenuation caused by the duct's small diameter and large length. The concentrations of the isotopes were measured at one end of the duct using a quadrupole mass spectrometer. For the operating frequency of 227 Hz, the maximum separation gradient obtained was 0.43%/m, and mole fluxes at zero gradient as high as 3 nmol/s were observed. Effects of turbulence, though not observed, are also discussed, and the scaling properties of this method are compared with those of traditional mixture-separation methods.


"Initial tests of a thermoacoustic space power engine,"
S. Backhaus,
Space Technology and Applications International Forum (STAIF-2003). February, 2003. Albuquerque, New Mexico. AIP Conference Proceedings 2003 654, 641-647. Edited by M. S. El Genk.

Abstract: Future NASA deep-space missions will require radioisotope-powered electric generators that are just as reliable as current-RTGs, but more efficient and of higher specific power (W/kg). Thermoacoustic engines at the similar to1kW scale have converted high-temperature heat into, acoustic, or PV, power without moving parts at 30% efficiency. Consisting of only tubes and a few heat exchangers, thernibacoustic engines are low mass and promise to be highly reliable. Coupling a thermoacoustic engine to a low mass, highly reliable and efficient linear alternator will create a heat-driven electric generator suitable for deep-space applications. Conversion efficiency data will be presented on a demonstration thermoacoustic engine designed for the 100-Watt power range.


"Thermoacoustic space power converter,"
E. Tward, M. Petach, and S. Backhaus,
Space Technology and Applications International Forum (STAIF-2003). February, 2003. Albuquerque, New Mexico. AIP Conference Proceedings 2003 654, 656-661. Edited by M. S. El Genk.

Abstract: A thermoacoustic power converter for use in space in the conversion of radioisotope-generated heat to electricity is under development. The converter incorporates a thermoacoustic driver that converts heat to acoustic power without any moving parts. The acoustic power is used to drive a pair of flexure bearing supported pistons connected to voice coils in a vibrationally balanced pair of moving coil alternators. Initial tests of the small similar to100W thermoacoustic driver have demonstrated good efficiency. An alternator matched to the driver is now under construction. A description of the system and the results of development tests are presented.


"Thermoacoustic power systems for space applications,"
S. Backhaus, E. Tward, and M. Petach,
Space Technology and Applications International Forum (STAIF-2002). February, 2002. Albuquerque, New Mexico. AIP Conference Proceedings 2002 608, 939-944. Edited by M. S. El Genk.

Abstract: Future NASA deep-space missions will require radioisotope-powered electric generators that are just as reliable as current RTGs, but more efficient and of higher specific power (W/kg). Thermoacoustic engines can convert high-temperature heat into acoustic, or PV, power without moving parts at 30% efficiency. Consisting of only tubes and a few heat exchangers, these engines are low mass and promise to be highly reliable. Coupling a thermoacoustic engine to a low-mass, highly reliable and efficient linear alternator will create a heat-driven electric generator suitable for deep-space applications. Data will be presented on the first tests of a demonstration thermoacoustic engine designed for the 100-Watt power range.


"Traveling-wave thermoacoustic electric generator,"
S. Backhaus, E. Tward, and M. Petach,
Applied Physics Letters 85, 1085-1087 (2004).

Abstract: Traveling-wave thermoacoustic heat engines have been demonstrated to convert high-temperature heat to acoustic power with high efficiency without using moving parts. Electrodynamic linear alternators and compressors have demonstrated high acoustic-to-electric transduction efficiency as well as long maintenance-free lifetimes. By optimizing a small-scale traveling-wave thermoacoustic engine for use with an electrodynamic linear alternator, we have created a traveling-wave thermoacoustic electric generator; a power conversion system suitable for demanding applications such as electricity generation aboard spacecraft.


"The effect of gravity on heat transfer by Rayleigh streaming in pulse tubes and thermal buffer tubes,"
Konstantin I. Matveev, Scott Backhaus, and Gregory W. Swift,
to be published in the proceedings of IMECE 2004, the International Mechanical Engineering Conference and Expo, November 13-19, 2004, Anaheim CA.

Abstract: Thermoacoustic engines and refrigerators use the interaction between heat and sound to produce acoustic energy or to transport thermal energy. Heat leaks in thermal buffer tubes and pulse tubes, components in thermoacoustic devices that separate heat exchangers at different temperatures, reduce the efficiency of these systems. At high acoustic amplitudes, Rayleigh mass streaming can become the dominant means for undesirable heat leak. Gravity affects the streaming flow patterns and influences streaming-induced heat convection. A simplified analytical model is constructed that shows gravity can reduce the streaming heat leak dramatically.


"Reduced-order modeling of vortex-driven excitation of acoustic modes,"
Konstantin I. Matveev,
to be published in Acoustics Research Letters Online in late 2004 or early 2005.

Abstract: Vortex shedding that occurs in ducts with baffles in the presence of mean flow often leads to excitation of acoustic modes. Resulting flow oscillations may feed back to the process of vortex formation. A simple model is proposed for describing this complex interaction using the hypotheses for a quasi-steadiness of vortex shedding and for a short-period acoustic perturbation at the moment of vortex collision with a downstream baffle. The model is capable of predicting typical real-system phenomena, such as the lock-in of a dominant frequency of the vortex-acoustic instability in some ranges of the mean flow velocity.


"A resonant, self-pumped, circulating thermoacoustic heat exchanger,"
G.W. Swift and S. Backhaus,
Journal of the Acoustical Society of America 116, 2923-2938 (2004).

Abstract: An asymmetrical constriction in a pipe functions as an imperfect gas diode for acoustic oscillations in the pipe. One or more gas diodes in a loop of pipe create substantial mean flow, approximately proportional to the amplitude of the oscillations. Measurements of wave shape, time-averaged pressure distribution, mass flow, and acoustic power dissipation are presented for a two-diode loop. Analysis of the phenomena is complicated because both the mean flow and the acoustic flow are turbulent and each affects the other significantly. The quasi-steady approximation yields results in rough agreement with the measurements. Acoustically driven heat-transfer loops based on these phenomena may provide useful heat transfer external to thermoacoustic and Stirling engines and refrigerators.


"A self-circulating heat exchanger for use in Stirling and thermoacoustic-Stirling engines,"
S. Backhaus and R. S. Reid,
Space Technology and Applications International Forum (STAIF-2005). Albuquerque, NM, February, 2005, AIP Conference Proceedings 746, p. 719-726.

Abstract: A major technical hurdle to the implementation of large Stirling engines or thermoacoustic engines is the reliability, performance, and manufacturability of the hot heat exchanger that brings high-temperature heat into the engine. Unlike power conversion devices that utilize steady flow, the oscillatory nature of the flow in Stirling and thermoacoustic engines restricts the length of a traditional hot heat exchanger to a peak-to-peak gas displacement, which is usually around 0.2 meters or less. To overcome this restriction, a new hot heat exchanger has been devised that uses a fluid diode in a looped pipe, which is resonantly driven by the oscillating gas pressure in the engine itself, to circulate the engine's working fluid around the loop. Instead of thousands of short, intricately interwoven passages that must be individually sealed, this new design consists of a few pipes that are typically 10 meters long. This revolutionary approach eliminates thousands of hermetic joints, pumps the engine's working fluid to and from a remote heat source without using moving parts, and does so without compromising on heat transfer surface area. Test data on a prototype loop integrated with a 1-kW thermoacoustic engine will be presented.


"High-temperature self-circulating heat exchanger,"
S. Backhaus, G. W. Swift, and R. S. Reid,
Applied Physics Letters 87, 014102 (2005).

Abstract: Thermoacoustic and Stirling engines and refrigerators use heat exchangers to transfer heat between the oscillating flow of their thermodynamic working fluids and external heat sources and sinks. An acoustically driven heat-exchange loop uses an engine's own pressure oscillations to steadily circulate its own thermodynamic working fluid through a physically remote high-temperature heat source without using moving parts, allowing for a significant reduction in the cost and complexity of thermoacoustic and Stirling heat exchangers. The simplicity and flexibility of such heat-exchanger loops will allow thermoacoustic and Stirling machines to access diverse heat sources and sinks. Measurements of the temperatures at the interface between such a heat-exchange loop and the hot end of a thermoacoustic-Stirling engine are presented. When the steady flow is too small to flush out the mixing chamber in one acoustic cycle, the heat transfer to the regenerator is excellent, with important implications for practical use.


"Continuous thermoacoustic mixture separation,"
G. W. Swift and D. A. Geller,
Journal of the Acoustical Society of America 120, 2648-2657 (2006).

Abstract: The superposition of nonzero time-averaged mole flux Ndot on a thermoacoustic wave in a binary gas mixture in a tube produces continuous mixture separation, in which one or more partially purified product streams are created from a feedstock stream. Significant product and feedstock flows occur through capillaries that are small enough to experience negligible thermoacoustic phenomena of their own. Experiments with a 50–50 helium-argon mixture show diverse consequences of nonzero flow, involving the addition of only one simple term n_H to the equation for the heavy component's time-averaged mole flux, where n_H is the mole fraction of the heavy component. A boundary condition for n_H must be imposed on the equation wherever products flow out of the separation tube, but not where feedstock flows in.


"An internal streaming instability in regenerators,"
J. H. So, G. W. Swift, and S. Backhaus,
Journal of the Acoustical Society of America 120, 1898-1909 (2006).

Abstract: Various oscillating-wave thermodynamic devices, including orifice and feedback pulse tube refrigerators, thermoacoustic-Stirling hybrid engines, cascaded thermoacoustic engines, and traditional Stirling engines and refrigerators, utilize regenerators to amplify acoustic power (engines) or to pump heat acoustically up a temperature gradient (refrigerators). As such a regenerator is scaled to higher power or operated at lower temperatures, the thermal and hydrodynamic communication transverse to the acoustic axis decreases, allowing for the possibility of an internal acoustic streaming instability with regions of counterflowing streaming that carry significant heat leak down the temperature gradient. The instability is driven by the nonlinear flow resistance of the regenerator, which results in different hydrodynamic flow resistances encountered by the oscillating flow and the streaming flow. The instability is inhibited by several other mechanisms, including acoustically transported enthalpy flux and axial and transverse thermal conduction in the regenerator solid matrix. A calculation of the stability limit caused by these effects reveals that engines are immune to a streaming instability while, under some conditions, refrigerators can exhibit an instability. The calculation is compared to experimental data obtained with a specially built orifice pulse tube refrigerator whose regenerator contains many thermocouples to detect a departure from transverse temperature uniformity.


"Analytical solution for temperature profiles at the ends of thermal buffer tubes,"
Konstantin I. Matveev, Gregory W. Swift, and Scott Backhaus,
International Communications in Heat and Mass Transfer 50, 897-901 (2007).

Abstract: The distortion of temperature profiles at the ends of thermal buffer tubes is related to the time-dependent gas pressure and motion in both nearly adiabatic and nearly isothermal environments during one acoustic cycle. The analytical solution for the mean temperature distribution is derived assuming zero heat conduction between gas parcels and linear acoustics with the acoustic wavelength much longer than other system dimensions. Theoretical results are compared with some experimental data and with results of numerical simulations that assume high heat conductivity.


"Thermoacoustic mixture separation with an axial temperature gradient,"
D. A. Geller and G. W. Swift,
Journal of the Acoustical Society of America 125, 2937-2945 (2009).

Abstract: The theory of thermoacoustic mixture separation is extended to include the effect of a nonzero axial temperature gradient. The analysis yields a new term in the second-order mole flux that is proportional to the temperature gradient and to the square of the volumetric velocity and is independent of the phasing of the wave. Because of this new term, thermoacoustic separation stops at a critical temperature gradient and changes direction above that gradient. For a traveling wave, this gradient is somewhat higher than that predicted by a simple four-step model. An experiment tests the theory for temperature gradients from 0 to 416 K/m in 50-50 He-Ar mixtures.


"The pulse tube and the pendulum,"
G. W. Swift and S. Backhaus,
Journal of the Acoustical Society of America 126, 2273-2284 (2009).

Abstract: An inverted pulse tube in which gravity-driven convection is suppressed by acoustic oscillations is analogous to an inverted pendulum that is stabilized by high-frequency vibration of its pivot point. Gravity acts on the gas density gradient arising from the end-to-end temperature gradient in the pulse tube, exerting a force proportional to that density gradient, tending to cause convection when the pulse tube is inverted. Meanwhile, a nonlinear effect exerts an opposing force proportional to the square of any part of the density gradient that is not parallel to the oscillation direction. Experiments show that convection is suppressed when the pulse-tube convection number N_{ptc} = omega squared a squared sqrt( Delta T / T_{avg} ) / g / (D sin theta -L cos theta) is greater than 1 in slender tubes, where omega is the radian frequency of the oscillations, a is their amplitude, Delta T is the end-to-end temperature difference, T_{avg} is the average absolute temperature, g is the acceleration of gravity, L is the length of the pulse tube and D is its diameter, alpha is about 1.5, and the tip angle theta ranges from 90 for a horizontal tube to 180 for an inverted tube. Theory suggests that the temperature dependence should be Delta T / T_{avg} instead of sqrt( Delta T / T_{avg} ).


"Thermoacoustic analysis of displacer gap loss in a low temperature Stirling cryocooler,"
Vincent Kotsubo and Greg Swift,
CP823, Advances in Cryogenic Engineering: Transactions of the Cryogenic Engineering Conference - CEC 51, edited by J. G. Weisend II (2006, American Institute of Physics, Melville NY), pages 353-360.

Abstract: Thermoacoustic theory is applied to oscillating flow in a parallel-plate gap with finite and unequal heat capacities on the two bounding walls, and with relative movement of one wall with respect to the other. The motivation is to understand the behavior of displacer gap losses at low temperatures in a Stirling cooler. Equations for the oscillating temperature and enthalpy flux down the gap and down the moving solid as a function of pressure amplitude, flow, temperatures, wall velocity, and material properties are derived. General expressions, along with results illustrating the behavior of the solutions, are presented. The primary result is that losses may increase significantly below 25 K, due to vanishing wall heat capacities and reduced thermal penetration depth in the helium gas.


"Quarter-wave pulse tube,"
Greg Swift, David Gardner, and Scott Backhaus,
submitted to Cryogenics, 2011.

Abstract: In high-power pulse-tube refrigerators, the pulse tube itself can be very long without too much dissipation of acoustic power on its walls. The pressure amplitude, the volume flow rate amplitude, and the time phase between them evolve significantly along a pulse tube that is about a quarter wavelength long. Proper choice of length and area makes the oscillations at the ambient end of the long pulse tube optimal for driving a second, smaller pulse-tube refrigerator, thereby utilizing the acoustic power that would typically have been dissipated in the first pulse-tube refrigerator's orifice. Experiments show that little heat is carried from the ambient heat exchanger to the cold heat exchanger in such a long pulse tube, even though the oscillations are turbulent and even when the tube is compactly coiled.


"High-purity thermoacoustic isotope enrichment,"
G. W. Swift, D. A. Geller, and S. N. Backhaus,
submitted to the Journal of the Acoustical Society of America, 2014.

Abstract: In a tube many wavelengths long, thermoacoustic separation of a gas mixture can produce very high purities. A flexible wall allows a spatially continuous supply of acoustic power into such a long tube. Coiling the tube and immersing it in a fluid lets a single-wavelength, circulating, traveling pressure wave in the fluid drive all the wavelengths in the tube wall and gas. Preliminary measurements confirm many aspects of the concept with neon (20Ne and 22Ne) and highlight some challenges of practical implementation.

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