# Fluidyne (liquid-piston Stirling) manifold and tees for 1/2 in copper U-tubes

> How do I build a fluidyne liquid piston Stirling engine and what tube sizes does it use?

- Category: Stirling engines › Thermoacoustic & fluidyne
- Kind: Hardware
- Basis: West's fluidyne (Harwell, 1970s): a U-tube of water is the displacer, a second water column the power piston, gas spaces above; natural period of the U-tube sets the frequency: T = 2π√(L/2g) with L the water column length
- Page: https://xemstudio.com/objects/fluidyne-manifold/ · JSON: https://xemstudio.com/api/geometry?object=fluidyne-manifold · Plain text: https://xemstudio.com/objects/fluidyne-manifold/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- No solid moving parts: water in a U-tube rocks back and forth (the displacer), shuttling the gas above it between a heated and a cooled leg; a third column (the output) is pumped by the pressure swing. Frequency comes from the U-tube: T = 2π√(L/2g): a 400 mm water column oscillates at ~1.1 Hz. The hot leg is heated above the water line, the cold leg left bare or water-cooled.
- Tube: 1/2 in Type L copper (15.88 OD, 13.8 ID) for the U legs; glass (test tube or 16 mm borosilicate tube) for a see-through cold leg. Gas space above each leg 2–3 × the swept volume; tuning line (4 mm) between the gas spaces sets the phase (West's 'tuning column'). Output column: another 1/2 in leg with check valves for a pump.
- The printed manifold joins the two legs at the top with a gas passage and the tuning port; the tee adds the output column. Seal copper into the sockets with silicone; everything above the water stays under 60 °C so PETG is fine, the heated copper is below it.
- LES: the no-moving-parts demonstrator next to the thermoacoustic one: same spools, water instead of pistons.

## Sizes

| part | size | note |
|---|---|---|
| U legs | 1/2 in copper, 300–500 long | water column |
| manifold | 2 sockets @ 60 mm, 4 mm tuning port | printed |
| output tee | 1/2 in + 4 mm port | pump column |
| period | 2π√(L/2g) | L = water column length |

## Where the files are

- Wikipedia: [Fluidyne engine](https://en.wikipedia.org/wiki/Fluidyne) — web — CC BY-SA 4.0 (text)
- C. D. West: [Liquid Piston Stirling Engines (1983)](https://www.osti.gov/search/semantic:fluidyne) — book, reports (OSTI) — reference / US Gov reports public domain (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (2 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/fluidyne-manifold/index.json

- fluidyne-manifold-2x-1-2in-copper: [STEP](https://xemstudio.com/files/hardware/fluidyne-manifold/fluidyne-manifold-2x-1-2in-copper.step) · [STL](https://xemstudio.com/files/hardware/fluidyne-manifold/fluidyne-manifold-2x-1-2in-copper.stl) — fluidyne displacer manifold: two Ø15.88 (1/2 in copper) sockets 60 mm apart joined by a gas passage, 4 mm tuning port
- fluidyne-output-tee-1-2in: [STEP](https://xemstudio.com/files/hardware/fluidyne-manifold/fluidyne-output-tee-1-2in.step) · [STL](https://xemstudio.com/files/hardware/fluidyne-manifold/fluidyne-output-tee-1-2in.stl) — output-column tee for 1/2 in copper with a 4 mm side port

## Questions

**What is a fluidyne engine?**

A Stirling engine whose displacer and piston are columns of water in U-tubes; gas above the water is heated and cooled as the water rocks. No solid moving parts; it pumps water.

**What sets a fluidyne's frequency?**

The U-tube's natural period, T = 2π√(L/2g): a 400 mm water column gives about 1.1 Hz; longer columns are slower.

**What tube size for a fluidyne?**

1/2 in copper (15.88 mm) legs 300–500 mm long with the gas spaces above; a 4 mm tuning line between the gas spaces sets the phase.

## Fits with

- [Copper pipe (ASTM B88 Type L and M), 1/4 to 2 inch](https://xemstudio.com/objects/copper-pipe-type-l-m/)
- [Borosilicate test tubes, 10×75 to 30×200 mm](https://xemstudio.com/objects/borosilicate-test-tubes/)
- [Silicone and PU tube, hose barbs and push-fit fittings (the transfer port)](https://xemstudio.com/objects/silicone-tube-and-fittings/)
- [Stirling engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)
- [Thermoacoustic stacks (slicer-infill blanks, spiral), resonator lengths and a speaker driver cap for tri-clamp spools](https://xemstudio.com/objects/thermoacoustic-stacks-and-resonators/)