Thermoacoustic converters

Depending on the thermal levels of their heat sources and sinks, thermodynamic machines are divided into two categories: heat engines, which produce mechanical energy from thermal energy, and heat generators or coolers or heat pumps, which are receivers of mechanical energy and suppliers of thermal energy. In 1979, Ceperley discovered that Stirling machines were nothing other than thermoacoustic machines using the characteristics of progressive pressure waves. The two types of pressure wave, standing and travelling, give rise to the two corresponding classes of machine. In a standing wave, the gas oscillates with a pressure-velocity phase close to 90° and interacts with the stack wall, whose hydraulic diameter is close to the size of the thermal boundary layer, inducing a deliberately imperfect thermal contact. In a travelling-wave machine (to which the Stirling machine belongs), this phase is close to 0°; the fluid and the wall are in very good thermal contact (because the regenerator has a hydraulic diameter much smaller than the thickness of the thermal boundary layer), operation is close to thermodynamic reversibility and coefficients of performance approach the maximum values predicted by the Carnot coefficient.

This article follows on from the [BE 8 060] article on "thermoacoustic effects" and makes extensive use of the concepts and relationships developed there.

It is completed by the two articles [BE 8 062] and [BE 8 063] devoted respectively to the modeling and dimensioning of thermoacoustic systems and the study of thermoacoustic motor/generator combinations.