Now let's make an analysis of another thermodynamic device:
There is another way to create a heat pump without a compressor and refrigerant, in which the working fluid itself (for example air or any gas) becomes a source of temperature differences and, under certain conditions, is capable of performing mechanical work itself.
Since all thermal phenomena are reversible, then by supplying thermal energy to the machine, mechanical work can be obtained at the output. Such devices are usually called thermoacoustic motors.
The simplest thermoacoustic machine consists of a resonator in the form of a tube and a regenerator made of a porous material, on the sides of which a temperature difference is created. In the simplest case, the regenerator can be a burner inserted inside the resonator (in the form of a vertically mounted glass tube) or a heated metal porous mesh, as shown in the figure below.
The optimal position of the flame is in the centre of the resonator, and the (heated) grid is approximately L / 4 distance from the lower end.
See the illustration 1 attached below.
The heat engine or thermoacoustic machine is of special interest for us, as it will help us to understand and draw analogy to various types of electrical self-feeding devices, which uses the principles of resonance.
The theory of the effect under consideration was created by Rayleigh in 1878, in which he discovered the principle that currently underlies all thermoacoustic.
And so, we will compare the principles of operation of the heat pump (generator) and the heat engine:
The principal of a heat pump:
1) if heat is transferred to the gas at the moment of maximum rarefaction and/or heat is taken away at the moment of maximum compression , then this stimulates the transfer of heat energy.
The thermodynamic phenomenon underlying thermoacoustic is reversible.
The principle of operation of a heat engine :
2) if heat is transferred to the gas at the moment of maximum compression and / or heat is taken away at the moment of maximum rarefaction, then this stimulates gas oscillations.
The thermoacoustic device can be based on two modes of resonant oscillations, on standing waves or on travelling waves.
Here standing wave device in heat-pump mode with the graphs for displacement, pressure and temperature(attachement2):
Below in attachement3 we can see a travelling wave device in mode of a heat engine.
And in this video you can see a travelling wave resonator, which is used to produce electric energy from the vibrations of an attached magnet in a practical implementation:
https://youtu.be/F2cTgt9WEBE