The Third International Workshop on Thermoacoustics will take place on Monday 26 and Tuesday 27 October 2015. The workshop encompasses two keynote lectures, which are introduced in the following.

Thermoacoustic engines: from what we know to what we do not understand

Guillaume Penelet, Associate professor, Laboratoire d'Acoustique de l'Université du Maine, LAUM UMR-CNRS 6613 Avenue Olivier Messiaen, 72085 Le mans cedex 9, France

The design and the development of thermoacoustic engines already has a three-decade history and there exists several examples of devices able to reach high performances. However, despite their simplicity in terms of geometry, thermoacoustic engines are not very well understood due to the complexity and the variety of the phenomena saturating heat transport and sound amplification.

This talk will be organized into two parts. The first part will be devoted to a general, academic presentation of thermoacoustic engines from the standpoint of acoustics. The second part will give an emphasis on some processes which are still poorly understood (notably entrance effects or acoustic streaming). As an illustrative example of the challenges that need to be taken up to get further insight into the operation of thermoacoustic engines, a focus will be given on recent research at LAUM, in which digital interferometric holography is used to characterize the highly nonlinear density fluctuations generated in the vicinity of the stack during the transient regime of wave amplitude growth in a standing wave thermoacoustic self-sustained oscillator.

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A way out of the acoustic to electric conversion limitations

Kees de Blok, Aster Thermoacoustics, Smeestraat 11, NL8194LG Veesen, The Netherlands

A common approach for converting acoustic power from thermoacoustic engines into electricity and vice versa is the use of so called resonant linear alternators or electro motors. The increase of moving mass when increasing power is found to set a practical limit to the power level which can be handled this way mainly by the extreme periodic forces in the construction and by the difficulty to maintain clearance seals of less than 50 m stable over large stroke amplitudes.

Linear alternators make use of the pressure variation of the acoustic wave. There is however no physical reason why not using the periodic velocity component of the acoustic wave. A way to convert such a bi-directional flow into rotation is known from shore and off-shore electricity production plants based on an oscillating water column (OWC). In a thermoacoustic system similar periodic flow conditions exist, so in principle, bi-directional turbines can be deployed for conversion of acoustic wave motion as well.

The idea of using such bi-directional turbines for converting high acoustic power flow was introduced by Aster in 2011, Since then, a lot of progress is made, by experimental and theoretical work, in understanding the principle of operation, modelling and design of such turbines. Results so far are encouraging and this type of turbine provides a cost effective device for converting acoustic wave energy into rotation and from there into electricity on arbitrary power levels.

The presentation will address the background and specific issues about the interaction of the turbine with the high power acoustic flow field as well as experimental results obtained in various prototypes.

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