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Thermoacoustics  

Thermoacoustics is usually understood as a branch of acoustics studying sound generation by means of heat. Some pages of Rayleigh's book "The theory of sound" are dedicated to description of sound generation methods with the aid of heat (GTTI, 1944, v.2, p.221-228). The first edition of this book was issued in 1877. Rayleigh by himself refers to works of Faraday, Zeebeck, Tindal and others, which were carried out in the first half of the 19th century. These effects were only of theoretical sense at that time. They appeared to have been elegant and sufficiently complicated examples of instability.

Afterwards, as early as in the Fifties of our century, studying of thermoacoustical oscillations has got practical orientation because of instabilities in combustion chambers of jet propulsions and in other facilities having great temperature overfalls. Absolutely new and on the face of it unexpected direction appeared due to works of Rott in the 70s. Carrying out consecutive theoretical analysis of appearance of oscillations in non-uniformly heated pipe he noticed the reversibility of sound generation. Reversibility consists in the following. If one creates sufficiently great temperature overfall along the pipe axis, sound may be generated in the pipe. It turned out that the contrary phenomenon is possible also. Sound field in the pipe can create a monodirectional heat flux and a temperature overfall. This reversible phenomenon is not connected with heating due to conversion of mechanical (acoustical) energy to heat. Sound plays a part of thermocompressor pumping the heat or maintaining the temperature drop.

At the beginning of the 80s in the laboratory of Los-Alamos corresponding facility known as an acoustical thermocompressor was created and patented. Nowadays, scores of research groups (generally in America) are occupied with elaboration of acoustical refrigerator. The range of investigations is quite wide. Their applications spread to studying of heat exchange elementary processes in sound field in narrow gaps, as well as to designing of special facilities of common refrigerator. A feature of acoustical refrigerator that is the most attractive for practical use is a closure of cycle, as well as a disuse of low-boiling liquids, which are probably dangerous for ozone layer of the Earth's atmosphere. One of acoustical pump modifications is a so called impulse pipe. It is the most promising facility for cooling of electronic devices using high-temperature superconductivity.

At the present time, research on thermoacoustics, studying reciprocal transformations of heat and sound fluxes is actually combined into an independent branch of acoustics. Scientific articles being published only in the Journal of Acoustic Society of America are estimated at dozens per year. Also special symposia and trainings are conduced and special curriculums have been worked out. In Russia, works on thermoacoustics are carried out to a little extent only at Andreyev Acoustics Institute. A small thermocompressor was elaborated and manufactured. Theoretical investigations directed to creation of acoustical field with optimal structure are carried out. Relation of thermoacoustical processes with heat transport processes in another facility called Rank-Hilsh tube was found out.