Stirlingengineforum.com

Question for Tom:

Have you or do you know of anyone who has been able to directly measure the temperatures of the hot and cold faces of a displacer in motion? It just seems to me (intuitively, no math behind it) that a moving displacer with hot and cold air moving around it constantly should be at roughly the average temperature of the hot and cold ends of the cylinder on both faces. If true, this should reduce the temperature requirements for the displacer unless it's stopped and getting fried by the hot end radiation. I'm probably missing something..

Anyway, your testing and reports are good data for the community. Thanks for pushing ahead!

I'm sure that the displacer is not subject to as much heat as the hot side heat plate or cylinder, how much less would depend on a number of things.

I have an idea that by increasing "dwell", having the displacer in contact with the heat source longer it will pick up extra heat, even becoming nearly as hot as the hot plate. This should double the heat input.

If the displacer is non-heat conducting, then little heat would transfer through by conduction.

But in my mind, one side of the displacer heats up, then when it lifts away, cold air rushes in to fill the void and the AIR heats up and expands and drives the piston, but if the displacer moves up against the cold side, then both the cold side "sink" and the cold side of the displacer are isolated from the heat.

Ideally, by the time the displacer returns to the bottom for reheating, the heat in the air has already been converted to mechanical "work", so the heat, or hot air never touches the cold side of the displacer.

Now, of course, I'm not saying that this "ideal" is actually always the case in the various Stirling engines we have available. Some are better than others.

The presence and effectiveness of a regenerator has a bearing on the question also.

IMO the general notion, first introduced by Carnot, that a Stirling engine works by pushing hot air around to the cold side so the heat can be eliminated at the sink does not represent a true picture of how a working Stirling engine actually works. A Stirling engine works by converting heat energy to work output BEFORE it ever has a chance to reach the cold side.

Anyway, I opened up my browser and came here today because the cement displacer experiment has been sitting for about 24 hours since I last looked at it and I noticed that left alone, (without me trying to pick out the water beads with a toothpick, the beads have shrunken away from the sides of the cells quite well as the whole thing dries out more rapidly, having been removed from the plastic mold.


The cell structure is also becoming much stronger as it continues to dry and harden.

Actually cement will continue to harden and gain strength almost indefinitely. .

Yesterday, I happened across a good 3d printer for sale on Facebook marketplace at a reasonable price, so went and got it.

Now I'm researching high temperature 3d printing options.

Apparently most 3d printing plastic filament will get soft in a cup of warm water, but some are formulated to withstand higher temperatures, but not much higher.

I'm not sure why there doesn't appear to be any kind of filament with some kind of inner core that could act like a hardener, such as with an epoxy putty stick, but in the form of a 3d printing filament.

As the two part filament is heated and smeared around during the printing it would mix and go through some catalytic process of hardening.

Seems like a no-brainer, but as far as I've been able to find, there is no such filament available on the market

I found this:

Not exactly high temperature at all, but a presumably light weight 3d printed "foam".

It seems to only work in certain high end printers. Probably not mine, but I'm not certain at this point. I'm not sure why not.

Anyway, the... Well, just watch the video.


https://youtu.be/i5kJ366cxSg

For a lightweight displacer, just print it out, I guess, then soak the resulting displacer in water and it partly dissolves away leaving a sponge-like foam.

Now, if that could result in a high-heat resistant foam.

I know there are certainly high temperature plastics. I cooked my turkey in a plastic bag last thanksgiving. Or did I? Maybe not but I'm sure it's possible.

I guess the problem is, to print, it needs to be able to melt easily at some reasonable temperature. So plastic that doesn't melt can't be printed. A catch 22.

Not in a fiber type printer anyway. Maybe a resin printer?

Well, it looks like it is possible, sort of, I guess.


That image is from this video:

https://youtu.be/1p-XWqYHez4

Sorry, a bit hard to watch all the way through but it at least demonstrates the possiblity. I tried desperately to find a better video on the subject of this "porcelite" stuff, but there weren't many to choose from

There is this PDF porcelite Users guide.


https://tethon3d.com/wp-content/uploads ... -Guide.pdf

Kind of seems like there should be an easier way, but at least it's not outside the realm of possibility to 3d print ones own high temperature ceramic parts.

In most cases though, I think the old methods of simply casting ceramic in a plaster mold would be more straightforward. Usually.

Plaster casting something with sponge-like internal voids though, 3d printing might have an advantage.

Ouch!

At around $300 a bottle, aside from the cost of a resin printer to use it, I think I'll pass for now, though it looks as though there are some less expensive ceramic resins available now.

Porcelite was the result of a Kickstarter project about five years ago.

Another ceramic 3d print project from Kickstarter:

There are apparently also some experimental ceramic/resin fibers for 3d printing, but they are also quite expensive, like over $200 a roll that might work in my printer, but I think I would like this option a whole lot better.

The up front.cost for the printer is pretty high. Just under $1000, but it can be loaded with ordinary clay, which is relatively cheap, readily available and does not involve burning, washing or dissolving out any resin. Just fire in an ordinary kiln.


https://youtu.be/0r36G9LTAlk


There is, at least this one other REAL clay printer, "Potter bot" but is like $30,000 or something? Yikes! Another option anyway, if anyone can afford it. I don't think I'll be needing such a Full size clay printer just yet.


https://youtu.be/JgNK_zrH3jg

Maybe even better. Price wise, at least.

Prints up to 7 inches, at under $600


https://youtu.be/6QCkvh_2tOQ

Still not sure it has any real advantages over traditional methods of working clay. I also suspect that to print, the clay needs to be too soupy to "bridge" over internal voids.

It might just be more practical to layer clay over popsicle sticks. As I had proposed earlier.

I'll be trying various methods shortly.

I'm still waiting for some silica carbide grit and various other supplies, to make a microwave kiln, and also working on the workshop, where I'll be able to use the big kiln if necessary.

The cement honeycomb thing is developing interesting features as it dries and the Orbeez things fall out. There are channels where the water beads were touching together.
It is also becoming quite noticeably lighter as it continues to dry.

I'm still not sure what will happen to the water beads when trapped inside and fired at high temperature in a kiln, when the process is carried out with clay.

I've learned that water beads can be shrunken back down to a small size relatively quickly with salt/salt water. (Several videos on YouTube), so, soak the displacer in salt water and let the beads fall out of the matrix? Maybe. They tend to be sticky though. Maybe coat them with oil or cooking spray before adding cement.

Cement, is somewhat corroded by salt, but it would be good if the "Orbeez" could be removed, possibly to be reused, before firing the cement

However high alumina (refractory) cement is not sensitive to salt and high alumina concrete is sometimes used in areas where salt damage could be an issue. Oceanside roads, sidewalks etc.

I did try frying some dehydrated water beads with the propane torch. They are difficult to "burn" as they tend to pop like popcorn and scatter, but I managed to heat a few to glowing red/white hot.

They turn black, swell up and release an odor reminiscent of burnt plastic turning into small gray/black hard foam-like nodules. A kind of refractory-like material in itself, perhaps, but easily crushed to a powder.

The remnants are similar to ash from a wood or coal stove and seem to no longer have any capacity to absorb water.

The fumes generated while being reduced to ash would be my main concern. What exactly is being released and is it toxic?

Getting them out would be better. Possibly if dehydrated they would be small enough to get out through the porous structure before firing, or after fully drying.

Anyway, I think I have all of the supplies needed to build a microwave kiln.

I'm thinking that maybe, trapped inside the voids, the cement displacer itself might act as a kind of air filter trapping any noxious odors/substances inside the pores of the cement as the water beads are reduced to ash in the kiln.

Otherwise, if they can't be gotten out, I think I might rather stick with experimenting with foamed glass, though that may generate some fumes of its own? Don't know yet.

I was only able to really reduce a very few of the dehydrated water beads to ash with the propane torch and had to open the doors for some cross ventilation, due to the smell.

These water beads things do not seem to be entirely inert or non-toxic when actually burned at very high temperature. The dry powdered form of sodium Polyacrylate, seem to react to high heat/flame, in much the same way. (Pops like popcorn, turns black, produces a burned plastic odor, then finally reduces to ash)

The process, however, seems to be successful in terms of creating a fairly strong, lightweight cement fire-resistant honeycomb-like matrix.

The water beads may react differently when actually fired in a relatively oxygen free environment, inside the cement, but again, better to get them out if possible.

What if a shielding gas like carbon dioxide were used as the sodium Polyacrylate was heated?

I did actually hope the remaining ash or whatever would be unable to reabsorb water so that there would be no chance of the displacer becoming waterlogged in a high humidity environment

Anyway, for a form for the glass foam inside the microwave kiln, apparently stainless steel works, and does not damage the microwave?


https://youtu.be/qL5nJaiXZjI

Not much explanation needed here:


https://youtu.be/ED3LQfK75z0

You may be able to hear the sound of the remaining dehydrated water beads popping and frying.

The still not fully cured cement did not entirely survive the torture test, but that's not really unexpected.


The cement did not cool down rapidly as does the foamed glass.

After the heat blast, the honeycomb structure was also very fragile and mostly just brushed away at the touch of a finger.

The black globs in some of the cells are, of course, the charged remains of burnt "Orbeez".

BTW, I did not realize that silicon carbide (or carborundum) is actually a kind of ceramic.

Is it possible to make a silicon carbide foam?
I guess so:

https://www.samaterials.com/content/wha ... amics.html

Probably would require temperatures hotter than my microwave kiln could generate. Considering that it is what the kiln itself will be made of.

This stuff is used to filter molten metal. Pour molten metal through it to remove impurities from the metal.

Now that's some extreme heat resistance.

These silicon carbide foam ceramic filters, as well as other similar alumina etc. ceramic filters are available in flat sheets and various sizes, One I found on eBay:150mm diameter is just the right size, so I just sent for one. Why not? Something else new to play with.

They can also be used as a "remote" wireless heating elements, which gives me some additional ideas.

A microwave powered Stirling engine?

Basically, the heating element for a microwave kiln as the hot plate INSIDE a Stirling engine. Think of the surface area! And potentially, heated internally, from the outside by microwaves.

I'm not at all sure what the advantages, or ultimate applications for such a thing might be, but the idea is intriguing.

Water beads after soaking:

After removal from cured cement (dehydrated):


The used dehydrated water beads after being cooked with the propane torch, the remaining ash (including some adhering Portland cement residue) soaked for 24 hours+ to see if it could reconstitute.

Some of the "ceramic foam" I sent for already arrived.

Unfortunately, contrary to what I expected, (something very lightweight like Aerogel), these ceramic foam bricks are more like actual bricks as far as their weight.

The very thin pink Styrofoam packing material that it came with might actually be more useful as lightweight displacer material.

Except maybe, for very large (more than 4" diameter) displacers, I could have saved myself a whole lot of trouble.

While reading through the Myers Engine Works Stove fan material, it is stated, regarding the displacer, that the ideal material to use is a foam glass grill brick sold by many restaurant supply companies.

I found these advertised as "feather light":
https://www.konabbqstore.com/products/k ... 6bfd&_ss=r

And are also advertised as being a full 4"x4" in width, (I'll make a correction on that if it turns out not to be the case) which is perfect for fitting inside the 4 1/4" OD Coleman globe used for the stove fan. (With the corners rounded off the brick, of course)

There are many additional sources for similar grill bricks, but they all seem just a little too small for the fan, being only 3 1/2" x 4" but If the application does not require the full 4" diameter there are some better bargains

With all my searching for the ideal displacer material, I don't know how I managed to overlook this "grill brick" product before.

Myers Stove Works was way ahead of me on this, but came to the same conclusion: foam glass is a great high temperature displacer material.

I will still be needing to make some custom foam glass displacers for things like the large "pancake" type LTD or other larger engines, but I've already sent out for a pack of these bricks for use with the stove fan and any other small to medium size engines.

Awesome! can you tell if those bricks are open-cell such as they would flow for a regenerator?
Bumpkin

When they get here I can check to see how porous they are, but in general, I don't think foamed glass has very much heat capacity, but It might be worth a try.

The advertising details mention "Will not clog or absorb grease" so, this particular brand is probably mostly closed cell.

I:ve read some reviews of other brands where people complained about this problem and for that reason had to throw them away when clogged with grease, so those are probably more porous.

The glass foams up when heated, and the transition between closed and open cell is a matter of the temperature and heating time. A little more heat and the closed cell will pop and become open.

Every manufacturer has their own process.