Thermoacoustics is the interaction between temperature, density and pressure variations of acoustic waves. Thermoacoustic heat engines can readily be driven using solar energy or waste heat and they can be controlled using proportional control. They can use heat available at low temperatures which makes it ideal for heat recovery and low power applications. The components included in thermoacoustic engines are usually very simple compared to conventional engines. The device can easily be controlled and maintained.

Thermoacoustic effects can be observed when partly molten glass tubes are connected to glass vessels. Sometimes spontaneously a loud and monotone sound is produced. A similar effect is observed if one side of a stainless steel tube is at room temperature (293 K) and the other side is in contact with liquid helium at 4.2 K. In this case, spontaneous oscillations are observed which are named "Taconis oscillations". The mathematical foundation of thermoacoustics is by Nikolaus Rott. Later, the field was inspired by the work of John Wheatley and Swift and his co-workers. Technologically thermoacoustic devices have the advantage that they have no moving parts, which makes them attractive for applications where reliability is of key importance.

Historical review of thermoacoustics

Thermoacoustic-induced oscillations have been observed for centuries. Glass blowers produced heat-generated sound when blowing a hot bulb at the end of a cold narrow tube. This phenomenon also has been observed in cryogenic storage vessels, where oscillations are induced by the insertion of a hollow tube open at the bottom end in liquid helium, called Taconis oscillations, but the lack of heat removal system causes the temperature gradient to diminish and acoustic wave to weaken and then to stop completely. Byron Higgins made the first scientific observation of heat energy conversion into acoustical oscillations. He investigated the "singing flame" phenomena in a portion of a hydrogen flame in a tube with both ends open.

Physicist Pieter Rijke introduced this phenomenon into a greater scale by using a heated wire screen to induce strong oscillations in his Rijke tube. Feldman mentioned in his related review that a convective air current through the pipe is the main inducer of this phenomenon. The oscillations are strongest when the screen is at one fourth of the tube length. Research performed by Sondhauss in 1850 is known to be the first to approximate the modern concept of thermoacoustic oscillation. Sondhauss experimentally investigated the oscillations related to glass blowers. Sondhauss observed that sound frequency and intensity depends on the length and volume of the bulb. Lord Rayleigh gave a qualitative explanation of the Sondhauss thermoacoustic oscillations phenomena, where he stated that producing any type of thermoacoustic oscillations needs to meet a criterion: "If heat be given to the air at the moment of greatest condensation or taken from it at the moment of greatest rarefaction, the vibration is encouraged". This shows that he related thermoacoustics to the interplay of density variations and heat injection. The formal theoretical study of thermoacoustics started by Kramers in 1949 when he generalized the Kirchhoff theory of the attenuation of sound waves at constant temperature to the case of attenuation in the presence of a temperature gradient. Rott made a breakthrough in the study and modeling of thermodynamic phenomena by developing a successful linear theory.

350px|thumb| Fig. 2. a: schematic diagram of a thermoacoustic prime mover; b: schematic diagram of a thermoacoustic refrigerator.

If we put a thin horizontal plate in the sound field, the thermal interaction between the oscillating gas and the plate leads to thermoacoustic effects. If the thermal conductivity of the plate material would be zero, the temperature in the plate would exactly match the temperature profiles as in Fig. 1b. Consider the blue line in Fig. 1b as the temperature profile of a plate at that position. The temperature gradient in the plate would be equal to the so-called critical temperature gradient. If we would fix the temperature at the left side of the plate at ambient temperature T<sub>a</sub> (e.g. using a heat exchanger), then the temperature at the right would be below T<sub>a</sub>. In other words: we have produced a cooler. This is the basis of thermoacoustic cooling as shown in Fig. 2b which represents a thermoacoustic refrigerator. It has a loudspeaker at the left. The system corresponds with the left half of Fig. 1b with the stack in the position of the blue line. Cooling is produced at temperature T<sub>L</sub>.

It is also possible to fix the temperature of the right side of the plate at T<sub>a</sub> and heat up the left side so that the temperature gradient in the plate would be larger than the critical temperature gradient. In that case, we have made an engine (prime mover) which can e.g. produce sound as in Fig. 2a. This is a so-called thermoacoustic prime mover. Stacks can be made of stainless steel plates but the device works also very well with loosely packed stainless steel wool or screens. It is heated at the left, e.g., by a propane flame and heat is released to ambient temperature by a heat exchanger. If the temperature at the left side is high enough, the system starts to produces a loud sound.

Thermoacoustic engines still suffer from some limitations, including that:

  • The device usually has low power-to-volume ratio.
  • Very high densities of operating fluids are required to obtain high power densities
  • The commercially available linear alternators used to convert acoustic energy into electricity currently have low efficiencies compared to rotary electric generators
  • Only expensive specially-made alternators can give satisfactory performance.
  • TAE uses gases at high pressures to provide reasonable power densities which imposes sealing challenges particularly if the mixture has light gases like helium.
  • The heat exchanging process in TAE is critical to maintain the power conversion process. The hot heat exchanger has to transfer heat to the stack and the cold heat exchanger has to sustain the temperature gradient across the stack. Yet, the available space for it is constrained with the small size and the blockage it adds to the path of the wave. The heat exchange process in oscillating media is still under extensive research.
  • The acoustic waves inside thermoacoustic engines operated at large pressure ratios suffer many kinds of non-linearities, such as turbulence which dissipates energy due to viscous effects, harmonic generation of different frequencies that carries acoustic power in frequencies other than the fundamental frequency.

The performance of thermoacoustic engines usually is characterized through several indicators as follows:

  • The first and second law efficiencies.
  • The onset temperature difference, defined as the minimum temperature difference across the sides of the stack at which the dynamic pressure is generated.
  • The frequency of the resultant pressure wave, since this frequency should match the resonance frequency required by the load device, either a thermoacoustic refrigerator/heat pump or a linear alternator.
  • The degree of harmonic distortion, indicating the ratio of higher harmonics to the fundamental mode in the resulting dynamic pressure wave.
  • The variation of the resultant wave frequency with the TAE operating temperature

Travelling-wave systems

300px|thumb|Fig. 3. Schematic drawing of a travelling-wave thermoacoustic engine

Figure 3 is a schematic drawing of a travelling-wave thermoacoustic engine. It consists of a resonator tube and a loop which contains a regenerator, three heat exchangers, and a bypass loop. A regenerator is a porous medium with a high heat capacity. As the gas flows back and forth through the regenerator, it periodically stores and takes up heat from the regenerator material. In contrast to the stack, the pores in the regenerator are much smaller than the thermal penetration depth, so the thermal contact between gas and material is very good. Ideally, the energy flow in the regenerator is zero, so the main energy flow in the loop is from the hot heat exchanger via the pulse tube and the bypass loop to the heat exchanger at the other side of the regenerator (main heat exchanger). The energy in the loop is transported via a travelling wave as in Fig. 1c, hence the name travelling-wave systems. The ratio of the volume flows at the ends of the regenerator is T<sub>H</sub>/T<sub>a</sub>, so the regenerator acts as a volume-flow amplifier.

Just like in the case of the standing-wave system, the machine "spontaneously" produces sound if the temperature T<sub>H</sub> is high enough. The resulting pressure oscillations can be used in a variety of ways, such as in producing electricity, cooling, and heat pumping.

See also

  • Cryocooler
  • Photoacoustic effect
  • Rijke tube
  • Thermoelectric cooling
  • Pyrophone
  • Thermophone

References

  • Thermoacoustic research at Los Alamos National Laboratory
  • M. Emam, Experimental Investigations on a Standing-Wave Thermoacoustic Engine, M.Sc. Thesis, Cairo University, Egypt (2013)
  • M.E.H. Tijani, Loudspeaker-driven thermo-acoustic refrigeration, Ph.D. Thesis, Technische Universiteit Eindhoven, (2001)
  • Design Environment for Low-amplitude ThermoAcoustic Energy Conversion

pt:Refrigeração termoacústica

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