Thermoacoustics
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 General Description
Thermoacoustics is the study of the conversion of heat energy to acoustic energy or the transfer of heat using acoustics. Although naturally occurring thermoacoustic effects have been observed since the early 19th century, the mathematical understanding of the phenomena has only been achieved in the last 35 year. With this basis, practical refrigerators, gas liquefiers and prime movers have been built. Past research at NCPA has investigated the use of gas mixtures in optimizing thermoacoustic performance, the use of inert gas - condensing vapor mixtures to increase workflow and the measurement and evaluation of acoustic materials for use in thermoacoustic devices. Now we are working on small waste heat driven thermoacoustic prime movers for electrical power generation. This is cooperative research with the University of Utah.
Figure Caption: Schematic flow diagram of an acoustic heat engine. |
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 Importance of Research
High frequency thermoacoustic prime movers have high energy densities and are very simple. They have no moving parts. The acoustic field is used to drive piezoelectric transducers to generate power. These devices are being investigated to see if the cost and efficiency are competitive with thermoelectric power generation. The goal is development of a simple rugged power generator using waste heat.
Figure Caption: Miniature thermoacoustic prime mover supplied by Dr. Orest Symko, University of Utah. |
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 Major Accomplishments
A major accomplishment of past work is the development of theory for the inert gas - condensing vapor thermoacoustics. The figure at right indicates that the cooling power of the helium-condensing vapor mixture is higher than that of helium for low normalized temperature gradients. The pure helium system can achieve higher COP's relative to Carnot at higher critical gradients but for much lower cooling powers than the mixtures. This method of improving the efficiency and power density of thermoacoustic refrigeration has been patented and is a very promising technology.
Figure Caption: Coefficient of performance relative to the Carnot coefficient of performance versus normalized temperature gradients for pure helium (dark gray), helium and water vapor (purple), helium and ammonia (red), helium and butane (green), and helium and R134a (blue). |
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Present Research
 The working area of a thermoacoustic device is called the stack. In this region, heat is exchanged between the gas and solid matrix. The small engine utilizes random arrays of glass or metal fibers to minimize longitudinal heat conduction. We are presently working on  techniques to model, predict and measure performance of random stack materials.
Figure Captions: (Left) Examples of random stack materials -- RVC, aluminum foam, metal wools. (Right) 4-microphone impedance tube used to evaluate stack materials.
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Goal
The goal of this research is to develop predictive models to optimize power generation in small devices. These models will be used to investigate further miniaturization of thermoacoustic devices. |
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