In this video i expand on the resonant linear motor from my previous video and use it to drive a linear compressor for a pulse-tube refrigerator. A pulse tube refrigerator is used to reach cryogenic temperatures, with some going into the sub-kelvin temperature range. These are typically used in labs for scientific experiments that require extreme low temperatures, or for sensors on sattelites/spacecraft, like the optics on the James Webb Space Telescope.

From a thermodynamic standpoint, a pulse tube cryocooler is effectively the same as a Stirling or Gifford-McMahon (GM) cooling cycle, but it replaces the displacer (or expander piston in the case of an alpha-stirling) with a gas piston. With careful tuning of a needle valve, inertance tube (gas momentum tube), and buffer volume, the phase of the gas piston's motion is shifted from the pressure oscillations of the compressor (by ~60-90 degrees), allowing heat pumping to occur out of the cold end. The major advantage of this device is that it eliminates moving parts from the cold end, which would be the displacer piston in the case of a Stirling or GM cooling cycle.

While i did manage to create a working linear compressor with a tuned dynamic balancer to create pressure oscillations, the linear motor didn't seem to have enough force to create a reasonably large compression ratio, and i only managed to produce a minor temperature difference. However, actuating the piston by hand, i was able to produce a temperature drop of around 4C.

Pulse tubes are also extremely dependent on the tuning of the needle valve, inertance tube, and buffer volume, which act like an electrical RLC circuit to create a specific phase shift at a specific frequency. The pneumatic tuning in this video was far from optimal.

In the next part of this series, i'll be replacing the linear compressor with a rotary compressor that I plan to build with an air cylinder driven by a geared down brushless motor from an RC plane. I'll also be focusing heavily on tuning the inertance tube / buffer volume, and optimizing heat exchangers to remove energy from the system even at small temperature differentials.

Previous video on building a linear motor:
   • Electro-Mechanical Resonant Oscillator  

If you want to understand the basics of different cryocooler types, these articles are helpful:

https://www2.jpl.nasa.gov/adv_tech/co...

https://www2.jpl.nasa.gov/adv_tech/co...

This is a helpful overview of how different pulse tube configurations work:

http://large.stanford.edu/courses/200...

This paper explains more about the math in computing resistance/inertance/compliance and phase shift:

https://trc.nist.gov/cryogenics/Paper...

Another resource i found tremendously useful was https://cryocooler.org/. Under "Past Proceedings - Volumes 14 to 20" you can select a volume and under "Table of Contents" there's dozens of research papers on Stirling/GM/Pulse Tube cryocooler development.


STL Files (these are for the linear compressor with the TPU bellows piston):
https://www.thingiverse.com/thing:568...


Music Used:
Heatley Bros - Sunset Beach
Kevin MacLeod - George Street Shuffle
Kevin MacLeod - Lobby Time