Traveling wave engines:
This stirling cycle engines efficiency can be better one and a half times, twice than a standing wave ones! The machine has a denser core this is the regenerator. This has a higher output. This engine not work as a standing wave engine, which achieves the phase shift by the heat exchange delay (loose stack) but by other way. This has similar way as a stirling engine with displacer.
Versions of traveling wave engines:
There are two main types:
1. feedback or bypass engines (called TASHE type).
2. loop engines (called ASTER type).
And exotics:
3. cascad engines (two stages)
4. alpha-type engines and pulse tube type engines with one core
6. Active membrane and (membrane + piston displacement) engines.
7.Baypass and loop mixture
8.BITL.
Etc
TASHE (Thermoacoustic Stirling Heat Engine):
How does it work?
The gases in the tube has a flexibility, mass, and momentum. The gas springs in the bypass branch and with a phase delay it goes through the regenerator due to its momentum.
An empty membrane is the "compliance" part. This part helps the gas piston achieve greater momentum. If a thinner mechanical resonator piston is used, the momentum alone will be strong enough in the bypass.
The frequency of TASHE is much lower (although the frequency is adjustable) than the frequency of a loop (ASTER) type machine. This makes it easier to design a generator for a slower piston with a longer stroke length. The other is that in thermoacoustic engines , we can increase the power by widening the diameter of the engine (at least the regenerator), but it is difficult to inject heat into a thick tube and even harder to take it out, so if we slow down the frequency the gas will moveing larger to and fro. If the frequency is slower, the "air resistance" of the components, pipes and regenerator will also be lower. We can use a longer regenerator, the heat exchange in the engine will be easier and the engine will be better.
If I want to compare these thermoacoustic engines to traditional stirling, the TASHE is a free piston stirling engine and the loop engine is an alpha stirling motor!
In the TASHE engine, the gas in the bypass acts as a displacer. This engine has a stirling cycle but not any traveling wave in it. The TASHE engine equals to a tradicional striling engine with a displacer. In the loop engine is a traveling wave. It equals to an alpha stirling engine.
You can design a working TASHE, simply:
The loop thermoacoustic motor:
Structure :.
In this loop-shaped engine 1-4 regenerators can be placed at quarter-loop distances from each other. In this engine the length of the loop gives the frequency.
Its frequency is equal to If you divide length of the loop by four and with this amount You have to calculate a stand wave engine frequency.
How does it work?
If a regenerator is operated in a loop, a full-wave vibration is generated in it. The full-wave can also be imagined by placing four water pistons at equal distances in the loop. When the water pistons are set in motion, they will move 90 degrees relative to each other, so you actually have 4 alpha stirling machines you create in the loop.
If there is gas in the loop instead of the water pistons, it will work the same way.
The loop engine is simple, but it doesn't like the long regenerator. The regenerator strangles the gas pistons a lot because gas pistons have small weight and momentum. If the gas piston is too long, the resistance of the pipe is huge. Have you ever tried to blow through pipes of different lengths? You will be amazed at how hard it is! The regenerator is widened and made thinner how it not be strangled the gas pistons. This widening also creates a softer suspension space into which the gas piston can penetrate with greater momentum, and the bigger regenerator surface alsowill give the engine more power . A thin regenerator has a higher heat loss at high temperatures because heat passes through it, and in small size very cumbersome build because the heating or cooling fluid has to be introduced into the engine and expensive.
The ratio between width of the resonator and the width of the core must be 1/4. In radiator-like cold and hot heat exchangers, coolant and heating fluids circulate, including a thin regenerating layer. (In practice, car radiators are used.)
Another solution has been tried for heat exchange, namely to heat the wide flat units from the outside on both sides and to bridge the required hollow part between the heat exchangers and the top and bottom of the machine by means of heat radiation and heat conduction:
The third variation is a coaxial structure:
The most significant developer of the loop machine is the Aster.
http://www.aster-thermoacoustics.com/
If we want a longer regenerator and shorter tubes, then instead of a gas piston (resonator) we can use a water piston, but it will be too slow, the power of the engine will be less, or we can use membranes with mass, but they must be durable and tuned, or solid pistons which must be very accurate but these make construction difficult also. These are not as simple as they seem.
