# Thermoacoustic stack for tri-clamp spools: Ø34.8, Ø47.5 and Ø72.9, honeycomb and spiral

> What size stack does a tri-clamp thermoacoustic engine need?

- Category: Stirling engines › Thermoacoustic & fluidyne
- Kind: Hardware
- Basis: Stack OD = 16 ga spool bore (34.8 / 47.5 / 72.9), exact; channel width 2–4 thermal penetration depths (δκ ≈ 0.26 mm in air at 1 bar, 100 Hz; 0.15 mm at 300 Hz), Swift
- Page: https://xemstudio.com/objects/thermoacoustic-stack/ · JSON: https://xemstudio.com/api/geometry?object=thermoacoustic-stack · Plain text: https://xemstudio.com/objects/thermoacoustic-stack/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## Notes

- The stack is the core of a standing-wave thermoacoustic engine: many narrow parallel channels between a hot face and a cold face, sitting in the resonator tube. It is drawn at the spool bore, so it is held in the bore itself.
- Honeycomb stack (1.5 and 2 in, × 40): hexagonal cells 1.5 mm across flats with 0.4 mm walls, every cell whole inside a 0.4 mm outer ring. Spiral stack (all three sizes, × 30, 40 and 50): one 0.4 mm wall wound with a 1.0 mm gas gap from a Ø6 core to a 0.5 mm outer ring.
- Channel size: half-width 2–4 × δκ, δκ = √(κ / (π f ρ cp)), so about 1–1.5 mm channels at 100 Hz and about 1.0 mm at 300 Hz in air at 1 bar.
- Placement: centred about λ/8 from the closed (hot) end of a λ/4 resonator; a closed-open tube resonates at f = c / 4L (150 Hz → 572 mm, 300 Hz → 286 mm in air at 20 °C).

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| stack OD | spool bore (34.8 / 47.5 / 72.9) | tri-clamp spool bore | socket | stack-* |

## Sizes

| spool_in | stack_od_mm | forms | lengths_mm | quarter_wave_150hz_mm | half_wave_150hz_mm |
|---|---|---|---|---|---|
| 1.5 | 34.8 | honeycomb 1.5 mm cells / spiral 1.0 mm gap | 40 / 30, 40, 50 | 572 | 1143 |
| 2 | 47.5 | honeycomb 1.5 mm cells / spiral 1.0 mm gap | 40 / 30, 40, 50 | 572 | 1143 |
| 3 | 72.9 | spiral 1.0 mm gap | 30, 40, 50 | 572 | 1143 |

## Where the files are

- Wikipedia: [Thermoacoustic heat engine](https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine) — web — CC BY-SA 4.0 (text)
- G. W. Swift (LANL): [Thermoacoustics: A Unifying Perspective for Some Engines and Refrigerators (textbook; DeltaEC software free)](https://www.lanl.gov/org/ddste/aldps/materials-physics-applications/condensed-matter-magnet-science/thermoacoustics/) — web, software — LANL (verify at link)
- NASA (NTRS, public domain): [Thermoacoustic Stirling heat engine for space power](https://ntrs.nasa.gov/search?q=thermoacoustic%20Stirling) — PDF — US Government work, public domain
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (11 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/thermoacoustic-stack/index.json

- stack-honeycomb-1.5in-40-c1.5: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-honeycomb-1.5in-40-c1.5.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-honeycomb-1.5in-40-c1.5.stl) — honeycomb stack Ø34.8 × 40 (the 1.5 in spool bore, exact): 1.5 mm hexagonal cells, 0.4 mm walls
- stack-spiral-1.5in-30-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-30-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-30-g1.0.stl) — spiral stack Ø34.8 × 30 (the 1.5 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-1.5in-40-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-40-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-40-g1.0.stl) — spiral stack Ø34.8 × 40 (the 1.5 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-1.5in-50-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-50-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-50-g1.0.stl) — spiral stack Ø34.8 × 50 (the 1.5 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-honeycomb-2in-40-c1.5: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-honeycomb-2in-40-c1.5.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-honeycomb-2in-40-c1.5.stl) — honeycomb stack Ø47.5 × 40 (the 2 in spool bore, exact): 1.5 mm hexagonal cells, 0.4 mm walls
- stack-spiral-2in-30-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-30-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-30-g1.0.stl) — spiral stack Ø47.5 × 30 (the 2 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-2in-40-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-40-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-40-g1.0.stl) — spiral stack Ø47.5 × 40 (the 2 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-2in-50-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-50-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-50-g1.0.stl) — spiral stack Ø47.5 × 50 (the 2 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-3in-30-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-30-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-30-g1.0.stl) — spiral stack Ø72.9 × 30 (the 3 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-3in-40-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-40-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-40-g1.0.stl) — spiral stack Ø72.9 × 40 (the 3 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap
- stack-spiral-3in-50-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-50-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-50-g1.0.stl) — spiral stack Ø72.9 × 50 (the 3 in spool bore, exact): one 0.4 mm wall wound with a 1.0 mm gas gap

## How it fits

### Goes in or on
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/): stack OD = spool bore

### Connects to
- [Thermoacoustic heat exchangers, hot and cold, parallel plate, for 1.5, 2 and 3 in tri-clamp spools](https://xemstudio.com/objects/thermoacoustic-heat-exchanger/): hot and cold exchangers on the stack faces
- [Thermoacoustic driver cap: ISO 2852 cap with a Ø40 speaker socket, 1.5, 2 and 3 in](https://xemstudio.com/objects/thermoacoustic-driver-cap/): speaker cap closes the resonator

### Also needed
- [Tri-clamp end cap, 6 mm flat: 1.5 and 2 in](https://xemstudio.com/objects/tri-clamp-end-cap/): capped hot end
- [Tri-clamp clamp, two-bolt: every flange size ½ to 4 in](https://xemstudio.com/objects/tri-clamp-clamps/): clamp per spool joint

## Questions

**What channel size should a thermoacoustic stack have?**

2–4 thermal penetration depths: about 1–1.5 mm in air at 1 bar and 100–300 Hz.

**How long should a thermoacoustic tube be?**

Closed-open (λ/4): L = 343 / (4f): 286 mm for 300 Hz, 429 mm for 200 Hz, 858 mm for 100 Hz; closed-closed is twice that.

**Where does the stack go?**

Centred about λ/8 from the closed hot end: 107 mm in a 429 mm (200 Hz) λ/4 tube.

**What diameter is the stack for a 2 inch spool?**

Ø47.5, the 16 ga bore: a honeycomb stack × 40 or a spiral stack × 30, 40 or 50.

## Fits with

- [Thermoacoustic heat exchangers, hot and cold, parallel plate, for 1.5, 2 and 3 in tri-clamp spools](https://xemstudio.com/objects/thermoacoustic-heat-exchanger/)
- [Thermoacoustic driver cap: ISO 2852 cap with a Ø40 speaker socket, 1.5, 2 and 3 in](https://xemstudio.com/objects/thermoacoustic-driver-cap/)
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/)
- [Sanitary clamp spools: 3, 6 and 12 in, ½ to 4 in](https://xemstudio.com/objects/sanitary-clamp-spools/)
- [Stirling engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)