Heating a LTD Stirling engine efficiently

[Stroller]

Andy Ross machined slots down the inside of the hot cap to increase surface area, in order to get more heat into the working fluid during the brief time the displacer was up at the cold end in his high RPM engines.

With an LTD engine, heat goes into the base rather than the sides of the displacer cylinder, so I've had a think about how we might make an equivalent improvement to the surface area of the flat bottom plate that is the hot-end.

Here's my proposal.

Make a milling cutter that machines parallel V shaped grooves with a 2mm pitch across the inside surface of the hot plate. Then rotate the plate 90 degrees and repeat the process. So now the plate is covered in little pyramids 2mm along each side and 2mm high at the point. If I did my math correctly, you'll now have a hot end with √5 more surface area than a plain flat plate. A factor of 2.236. In theory, you should be able to transfer 2.236 times more heat into the working fluid on each stroke, more than doubling performance, or reducing fuel requirements, if you do the same machining on the underside of cold top plate, to absorb 2.236 times more heat back out of the working fluid.

The downside is that this will create some dead space the displacer crown can't occupy, unless you machine the same profile on that and accurately align it so the inverted pyramids occupy the spaces between the pyramids on the hot plate surface. Seems like a big ask to me.

So the question would be: how much of the gain from the increased surface area will be offset by the dead space. Andy Ross was getting a good improvement in performance with his slots, so I think it might be worth trying this idea out.

You may well say, "It's a dumb idea Rog, and here's why". So have at it, and save me the effort of making that milling cutter and doing the machining work.

[Fool]

Good idea. As you've pointed out there are compromises to make. Deeper grooves. Square verses Vee. More and smaller. Wider valleys thinner fins. Fins verses pegs. Fitting tightly to bottom plate, thus increasing the Suction from lifting off the bottom, verses a bunch of vent gaps or space reducing that force. How much it costs to build. Maybe cheaper to mold it and cast it, and machine only the sealing surfaces. Maybe smaller on the inside larger on the outside. Strength of flat plate would also be factor. Different types of grooves might be stiffer.

[Stroller]

Thanks for the ideas.

Deeper grooves. More and smaller. - Makes no difference to surface area for any given profile. I'm limited to 2mm depth given my existing plates.

Square verses Vee. - Would give more surface area, gas flow needs thinking about.

Wider valleys thinner fins. - Concentric rings would be easier to machine on a lathe than milling grooves.

Fins verses pegs. - Less surface area, more problematic gas flow.

Fitting tightly to bottom plate, thus increasing the Suction from lifting off the bottom, verses a bunch of vent gaps or space reducing that force. - needs thinking about. Suction takes momentum from flywheel, but maybe makes a better job of gas velocity/flow/mixing?

How much it costs to build. Maybe cheaper to mold it and cast it, and machine only the sealing surfaces. - This is a one-off engine, but yes, if it went into production, hot stamping or rolling may be cheaper, plus the joint face machining. Casting would be pricier, but the roughness of sand casting would offer even more surface area.

Maybe smaller on the inside larger on the outside. - Depends on application. For my stove-top, flat is best for heat transfer at the hot end.

Strength of flat plate would also be factor. Different types of grooves might be stiffer. - Good point. Radii at the roots of the Vs or slots will help with stiffness/strength.

[MikeB]

If you do want to have the two faces have matching grooves, then think about using casting for one or both.

[VincentG]

Not sure if you will find this relevant.
https://youtube.com/shorts/AvZ_GvFu8xE? ... _zeX7PRnk7

I don't think chasing surface area with complicated design is needed until some other things are figured out. Imo the overall proximity of the heat exchanger to the gas is more important, and most importantly the "switching" on and off of the heat supply.

This is my epoxy engine operating on a small 20-30 degree Fahrenheit delta. I am spinning the displacer with a Dremel tool at 1500-2000rpm to test the response time of the, in this case, slave cylinder.

Shot in Samsung super slow motion, the total movement of the piston is darn near the same as spinning much slower by hand.

I have done the same test at much higher temperatures but the inertia of the piston begins to affect its movement.

[Stroller]

Thanks Vincent. Overspeeding power piston momentum is a problem I'd like to be having to deal with. At the moment, getting more heat in faster is the issue. My increased surface area idea isn't complicated, and can be machined onto the existing engine plates with a single point cutter on a CNC machine pretty quickly.

When I get more time to devote to the LTD engine I'll get the work done and report back on results.