60L drum medium temp Gamma build

[VincentG]

I have some of that, it needs to be completely flat and more rigid.

[Fool]

Castable ceramic foam? Tim Tinker paint on ceramic?

[VincentG]

Possibly, I'll look into it thanks.

[gwill67]

Mica if being used flat?

[VincentG]

Thanks for all the suggestions. I'm going to try polyurethane foam as it has similar performance to xps and can be had with adhesive backing. Its important to keep air completely away from the steel surfaces of the chamber wall.

Mica for instance has over 10x the conductivity of xps. That is still far better than raw steel but still far worse than xps or similar.

After testing yesterday I allowed the hot plate to sit in the open chamber. It stayed hot for a long time, but still no hot spots in the lower chanber area.

It should be an excellent test bed for measuring heat to work conversion.

[Tom Booth]

I have some of that, it needs to be completely flat and more rigid.

You can usually find rigid Styrofoam sheathing at just about any lumber yard or Lowe's/Home Depot. Usually foil faced on one side, but that can be peeled off if necessary. Comes folded up in a cube

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[VincentG]

My apologies, as usual I am not explaining myself well enough lol. By flat and smooth I mean no open cells, corrugations or rough surfaces. It has to wrap around the inside circumference of the chamber so stiff board like insulation is out.

[Fool]

What about foam in place polyurethane? Use a greased removable sheet metal form?

[Tom Booth]

My apologies, as usual I am not explaining myself well enough lol. By flat and smooth I mean no open cells, corrugations or rough surfaces. It has to wrap around the inside circumference of the chamber so stiff board like insulation is out.

This bends into a circle very easily without breaking.

[VincentG]

Ok Ill try that first. Its 14" in diameter.

[Fool]

A heat gun might help bend it?

[VincentG]

The area in square inches of the interior surfaces, as well as their temperature are as follows.

Cold plate- 132 35F
Chamber wall-450 85F
Hot plate-226 135F


With the chanber walls being by far the dominant heat exchanger surface, I'm surprised I had as much success as I did.

The response of the chanber seemed quite rapid, just driving the displacer by hand. Of course a real drive mechanism is needed to test actual response time.

[VincentG]

I have lined the inside of the chamber with reflectix foil faced bubble insulation, just because I had some.

Before insulation cold water in the lower exchanger and ambient above had no pressure swing or volume change. After insulation there was volume change and detectable pressure.

After insulation, cold water below with ambient above now had slightly better performance than hot water below and ambient above.

After insulation with hot water below and ice above, I am now seeing .5psi and much more volume change than before insulation.

There is a scaling issue but I don't believe it's exactly due to volume increasing more than the surface area.

The problem as I see it is the surface area of the displacer and chamber walls becomes too great compared to the heat exchanger area.

If it were a heat exchanger/volume issue, the pressure would gradually build after displacer movement as the air has a chance to heat/cool. But that does not happen. Instead, the gas changes temperature quickly, but never reaches supplied Tmax or Tmin.

The solution then is not to increase heat exchanger area but to decrease the area of anything but.

Better insulation will help alot, but I have a new Gamma chamber designed specifically to combat this issue.

I let the chamber sit for 30 minutes after testing. There was still enough heat to operate. Overall it functions well thermally.

[VincentG]

At this scale, even if chamber performance alone is less than desirable, the large area of the cold plate and chamber walls along with a valvular type displacer should be effective at maintaining isothermal compression.

This means the power piston can likely be sized quite a bit larger than one would think.

Another significant performance boost will come from increasing Tmax. I'd expect a non linear improvement as Tmax gets further away from ambient. Right now 135F is only 50F above ambient(85F)

[Fool]

Instead, the gas changes temperature quickly, but never reaches supplied Tmax or Tmin.

I'm wondering if the gas is flowing by, say Tmax, picking up some heat but not coming up to Tmax. Then being far enough away to not pick more heat up.

Another possible idea is that The gas is being heated and cooled by two temperatures?

How does it change with repeated strokes? I'm wondering now if the insulation takes repeated strokes to warm up???

Good work. Well done.