Pulse Tube Cryocooler - Part 2 (-75C)

Part 1:
   • Pulse Tube Cryocooler - Part 1  

This is the second part of my video series on attempting to build a Pulse Tube cryocooler. I managed to make significant progress by removing the linear motor and using a conventional rotary motor with a large gear reduction ratio and a flywheel to produce the larger forces needed for higher compression ratios.

For pistons, I used pneumatic actuators. I evaluated a 25mm bore and a 40mm bore piston, both with a 50mm stroke. Pneumatic actuators have more friction than conventional pistons due to their rubber lip seals, but theoretically have zero blowby, so they hold pressure, which makes them more effective for low frequency applications.

Here are some specifications for the cooler:

Pipe diameter: 18mm
Regenerator Length: 30mm
Regenerator Material: Fine steel wool
Pulse Tube Length: 100mm
Flow resistance source: 1/8 NPT needle valve
Inertance Tube Length: 10' (~3m)
Inertance Tube Diameter: 4.4mm
Buffer Tank Volume: 2L
Piston Swept volume (25mm): 23CC
Piston Swept volume (40mm): 57CC
Compression Ratio (25mm): 1.4
Compression Ratio (40mm): 2.0
Maximum Frequency: 15 Hz
Motor KV: 750
Motor Voltage: 16V
Motor reduction ratio: 5:1
Flywheel moment of Inertia: 0.012 kgm^2
Maximum recorded temperature drop below ambient: -91C
Lowest recorded temperature: -75C

I think with some more optimization, this system can probably reach -100C, although without helium or hydrogen as a working fluid, I think it's unlikely that I'll reach cold enough temperatures to liquefy oxygen/nitrogen.

In part 3 of this video, I'll do more investigation into hot-end heat exchanger design, regenerator design, and the effect of increasing the power density of the system by pressurizing it. I'll also be comparing the pulse tube performance to a similar spec alpha stirling cooler.

Links for parts:
Motor:
https://www.amazon.com/dp/B084QCLTM1?...

25: and 40mm pistons:
https://www.amazon.com/gp/product/B08...

https://www.amazon.com/dp/B08YYQZ5CQ?...

Music Used:
Kevin MacLeod - Lobby Time
Kevin MacLeod - Groove Groove