LTD Stirling Engine Development-Displacer Chamber Design

[VincentG]

This will be the second installment of LTD Stirling Engine Development after this thread viewtopic.php?f=1&t=5516 and likely the second most important design consideration after the displacer itself.

Design considerations in descending order of importance as I see them now;

-Ease of construction at any scale
-Thermal separation of hot and cold sides(that's right Tom) from each other
-Thermal separation of hot and cold exchanger from the chamber itself
-Rigid structure with good vibration dampening

Time to get back to reality and focus for a bit on a proven cycle(gamma displacer). The aim here is to produce the maximum pressure swing at the maximum cycle rate from the available Thigh and Tlow. What we do with that motive force is still up in the air for me, see this thread for more details viewtopic.php?f=1&t=5558 .

This is just my opinion on design. I might be wrong.

[VincentG]

Ease of construction and Rigidity/vibration dampening.


I fired up the modified LTD test bed engine for the first time in a while to test something I thought I observed this past winter. Even at meager pressure swings(maybe 1.25psi on the guage), I thought I was seeing the plates flexing.

This afternoon I put a dial indicator on the running engine. https://photos.app.goo.gl/Rhg6DTN3oRdfzR2Q7 Looks like around .004" under load. This might not sound like much but that's a lot of lost energy to volume change and harmonics. And it will only get worse with higher pressures and scaling up.

For now I'll just focus on the construction of the lower chamber. In a perfect world this would be cheap, simple, scalable, rigid and with good dampening.

For me the obvious answer is concrete. It's available all over the world and anyone can learn to build with it. Obviously concrete form techniques will vary at different scales, so this will just be a small scale example.

I started with a 3d printed concrete form that will be compatible with the LTD test bed I am using.
https://photos.app.goo.gl/Z9QK8TqqaQBrAany6

Then it's filled with concrete. At this scale I'm just using mortar with no aggregate. At large scale, standard 4000psi concrete with rebar and mesh should do. If possible the concrete should be fully wet cured for durability.
https://photos.app.goo.gl/CgjH6nFXvvwGdcs36

The form is poured in two stages to ensure flatness of the chamber floor. The first stage is as shown where the form is poured just up to what will be the bottom of inside the chamber. A plasticizer is helpful here to ensure the floor is as flat as possible. This will be important to create a good seal with the bottom of the displacer. The second stage will form the walls of the chamber. At large scale the earth itself can be used as the bottom of the form.

[Tom Booth]

(...)
-Thermal separation of hot and cold sides(that's right Tom) from each other
(...)

Not sure why you're dragging my name into this. Because I agree thermal separation is a good thing, or...

Because I don't think a cold side is required?

Or... Something else?

Anyway, very interesting about the deflection seen in the heat exchanger plates. It looks like you even have the gauge on a part that is double thickness. Presumably the bottom plate is doing the same thing also.

On larger LTD type engines I have soldered ribs or angle iron or whatever across the plates to try and mitigate the problem.

This engine was made from cookie tin but I never got it running. Sorry, it's a bit dusty.

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[VincentG]

Because I don't think a cold side is required?

This. And just a joke Tom. We could all use some levity from time to time here.

The bottom plate was doing the same, but because it's sitting on the rim of a bean can, it was not influencing the dial indicator much at all.

I found this engine, which actually used this flexing to power an inertia water pump. Clever idea, but still takes away from peak output if we can avoid it.

https://l.facebook.com/l.php?u=https%3A ... SH-bxZ0J3A

[Tom Booth]

Because I don't think a cold side is required?

This. And just a joke Tom. We could all use some levity from time to time here.

(...)

Emphasis on "required".

It still seems apparent though that a greater ∆T is helpful in some way, at least at times, to one degree or another, but probably to a limited degree. Not sure yet what that limit might be.

I think running a Stirling engine with ice acts much like insulation, allowing the engine to utilize the available heat more completely. But in the process of utilizing the available heat more completely, less heat remains, so you still don't have a lot of heat being transfered to the sink.

If the ice is kept very well insulated it will take a very long time to melt when used to help run a Stirling engine. The engine itself acts like insulation, or even perhaps a heat pump, slightly refrigerating the ice, but if the ice is still exposed to the surrounding ambient heat on all other sides, of course the ice will still melt rapidly.

I still need to try sandwiching ice in between two or more Stirling engines. Something suggested on the Science forum as a joke, but I took it as a serious suggestion. I thought it was a brilliant idea!

If there is anything to this Stirling engine as heat pump theory, that should demonstrate it or debunk it, one way or the other.

I think at minimum the ice will stay frozen a surprisingly long time.