ULR, when I hear "thermo acoustic" I think V1 "buzz bomb" resonant pulse jet. If you want something to run efficiently find it's resonant frequency, but be careful the resonance doesn't concentrate the energy onto something that will fatigue fracture like metal. If you can keep the waves contained in gas or liquid you can do away with mechanical pumps.
High frequency resonant piezos may replace fans for computer cooling one day.
Thermoacoustic coolers powered by piezos to pump heat out of chips on PC boards IS one of the applications they're playing with.
Thermoacoustics is built on the fact that the high-pressure part of a sound wave is hotter than the low pressure part of it. At "normal" sound levels this effect is small. The main noticeable effect is that sound waves are gradually attenuated as the heat is conducted from the high- to the low-pressure part of the wave. This happens to higher frequencies in less propagation distance because the heat has a shorter distance to go. This is part of the reason distant thunder is a low rumble, though a nearby strike goes "crack".
But with extreme sound levels, which are easy to contain in plumbing, the compression is extreme and temperature differences are comparable to those in heat engines and heat pumps. You can design heat engines that input heat-across-temperature-difference and output mechanical power as sound, or consume sound and use the energy to pump heat across a temperature difference.
A common component is a "stack", which is a section of the plumbing where the sound wave flows along a surface and exchanges heat with it. A stack has a lot of surface area coupled to the working fluid but has limited friction with the moving fluid. (Imagine the middle, wavy part of a strip of corrugated cardboard. Coil it up into a disk with a lot of passages from one side to the other and stuff it in a pipe. Or take two plates and drill a bunch of holes in both, then move them apart, stuff tubes in the holes to connect them, and weld them into place.) A stack in use has a temperature gradient from one side to the other (perhaps due to radiator-like heat exchangers at either end.) Of course the material has to stand the temperatures involved and an conduction along its length represents a loss of energy, so a good stack may be made of (or coated with) an thermal insulating material.
There are two types of stack:
A tightly-coupled stack (heat transfers between the fluid and the walls of the stack very quickly) with a traveling wave through it (pressure and flow 90 degrees out-of-phase) acts as the equivalent of a sterling engine or heat pump. The stack is the regenerator. Cycle (as a heat engine):
- Fluid moves through the stack from colder location to warmer location, heating as it goes.
- Fluid sits at the hotter location during the pressure-rise and absorbs more heat from the walls or the heat-exchanger, raising its pressure further.
- Fluid moves through the stack from warmer location to cooler location, cooling as it goes.
- Fluid cools during the pressure drop, and also deposits heat on the walls or heat-exchanger, lowering its pressure further.
As you can see the heat flows from the hot end to the cold end and the pressure in the wave is boosted. For any given sound level there is a corresponding thermal gradient. If the actual thermal gradient is higher the stack runs in this sterling engine mode, pulling energy from the thermal gradient and amplifying the sound. (Similarly, if the actual thermal gradient is lower the stack runs as a heat pump, attenuating the sound and pumping heat up the thermal gradient. I'll let you figure out the details of the heat-pump version of the cycle.)
Note that the sound wave doesn't have to move the working fluid the whole length of the stack. Each cycle can bucket-brigade heat part way along the stack.
The other version is the loosely-coupled stack, interacting with a standing wave (pressure changes in phase with motion). In this case the cycle is:
- Fluid moves from a cooler to a warmer location, heating as the pressure rises.
- Fluid exchanges heat with the stack wall or heat exchanger.
- Fluid moves from a warmer to a cooler location, cooling as the pressure falls.
- Fluid exchanges heat with the stack wall or heat exchanger.
Again this will run as a heat-difference-to-sound-amplification motor if the thermal gradient is higher than that which corresponds to the sound level, and as a heat pump if the thermal gradient is lower. This version is less efficient than the traveling-wave version, because the 90-degree phase shift comes from heat flowing across a significant temperature difference, increasing entropy without doing useful work. But it's a lot easier to build the plumbing for this mode. (Also the stack can be located near a velocity node, where the pressure cycling is large and the flow is small - corresponding to a high impedance and minimizing losses from fluid friction in the stack.) The hot end is the end near the velocity node.
The rest of the plumbing is involved in routing and controlling the sound: Resonators, transmission lines, etc. Other devices that might be used are acoustic-electric transducers (think "loudspeakers" - in fact they may USE loudspeakers - or equivalents that can handle the temperatures and scales: Metal diaphragms with magnets moving near coils, for instance. Or piezoelectric transducers.) Another device is a shaped passage that uses some of the sound to encourage linear motion of the fluid, pumping it around. Slightly conical plumbing may be used to avoid a parasitic fluid circulation effect.
