Heater & Regenerator Design

[Aviator168]

This is the doc for steel under high heat. You can find for other material on the net.
http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf
BTW. The way you made the cylinder, the wall can be very thin to take on 15 bars assuming the temperature is not too crazy.

[cbstirling2]

Rich,
Very impressive work. Why didn't you consider an alpha design? They generally produce the most power.

[richjbarnett]

Thanks for the info on the steels at high temp. I have for sure been conservative on the design. I have allowed minimum wall thickness of 2.5mm after the heat exchanger slots go in. The external fins i left so thick to allow me to get hold of it in the lathe chuck to open out the bore if needs be and to act as radial bands to stop any explosions!! Pressure make me paranoid, pressure and heat even more!! lol

The engine is for my Final Year Uni project so ive been fairly conservative i think.

I went for the gamma to avoid any hot cylinder / pistons issues, i thought id have more chance of getting the Gamma working for my first build. But ive hopefully minimised losses as the power cylinder sits under the displacer just off to one side so i can have a decent sized transfer port with very low dead volume.

[PetePeterson]

Rich,
Have you worked out the number of slots and their width and depth?
Their lengths are limited to your cylinder section lengths. To get the heat transfer you need enough surface area.
The slot size is very important: too narrow and you increase pumping losses, too wide and you increase dead space and
you loose the turbulence needed in the slots/tubes.

A good reference on tubes/slots: 'The Regenerator and the Stirling Engine' by Dr. Allan J. Organ, ISBN1860580106
On yahoo groups was a group SESUSA (no longer there), in the files area were different versions of an Excel spreed sheet CGS2002.xls
The group HotAirEngineSociety has a copy in the files section CGS2002-costing.xls

---
Pete

[Aviator168]

The slot size is very important: too narrow and you increase pumping losses, too wide and you increase dead space and
you loose the turbulence needed in the slots/tubes.

The slot size is just one the variables in heat transfer. The other variables are kinematic viscosity which has to do with density of the working fluid, velocity of the working fluid passing the heat exchanger and others. Of all, the kinematic viscosity has the largest effect in heat transfer and that's the reason most commercial stirling run on high pressure(increasing density and reducing the kinematic viscosity.) Given the slot size, velocity, density, and etc, you can calculate the effective heat transfer per unit area.
BTW. Very narrow slots are fine as long as you have lots of it.

[Ian S C]

And remember that the design of the cooling system is at least as important as the heating, it's no use getting more heat into an engine if you can't remove at least that amount, preferably more.
Ian S C

[Bumpkin]

"Looking to run at 15bar fairly quickly but then maybe more." - You probably already know, but remember not to mix air and oil at high pressures/temperatures.
From wikipedia - "At high temperatures and pressures, the oxygen in air-pressurized crankcases, or in the working gas of hot air engines, can combine with the engine's lubricating oil and explode. At least one person has died in such an explosion."
Bumpkin

[richjbarnett]

Will be using alternative gases above 10 bar.

[Ian S C]

I think that was the main reason that Philips restricted the MP 1002 CA/Bungalow motor to a restricting pressure of 200psi, if an inert gas had been use instead of air the pressure(after a redesign of the engine for the increase) could have been increased, boosting the power out put, but also increasing the price.
As for lubrication, modern synthetic oils overcome the problems of exploding engines, and build up of carbon from over heated mineral oil. It is also a better lubricant, and with the minute amount used in a Stirling Engine, cost is not a problem.
Ian S C

[PetePeterson]

Aviator168,
my reference was more a pointer to the spreed sheet, the slot width comment was in my notes from when the spreed sheet was discussed in the SESUSA group. With multi-tooth cutters (slitting saws, band saws ...) the width is from .015" to .035" for most cutters. When using a shaper to cut slots (as in the video) the slots can be very narrow. My notes show about .0075" as a cut-off point where cost and pumping losses are too high.
Since the SESUSA group is gone I can no longer search for the message my notes are from.

IanS,
when talking to Jim Symanski about his JD#6, his comment about the heat exchangers was he should have put more into the cold exchangers.
Jim was running 200 psi air at the time (fall 1999)
Others I have talked with agree, and a value of about 2.5X surface area of cold to hot is a number I can find reference to.

---
Pete

[Aviator168]

My notes show about .0075" as a cut-off point where cost and pumping losses are too high.

That's what I have come up with. About 200 micro

[richjbarnett]

Made a bit more progress this week.

Been working on the displacer. Finished the end cap that connects to the pushrod. As i mentioned i am venting the inside of the displacer to the crankcase so i dont get any crush as i start to increase the running pressure. This entailed drilling a 2.5mm diameter hole 130mm down the shaft, happily the drill didn't break.



So time to weld it all together this evening, was a bit nervouse about blowing a hole in the tube, but it all went ok.

[richjbarnett]

Just a little update and pictures.

Managed to put in a few hours the last couple of weeks.

Video of power piston, bounces, spins and takes 4-5 minutes to drop and that dry with no oil: https://www.facebook.com/richjbarnett/v ... 639446887/

https://www.facebook.com/richjbarnett/v ... 639446887/

Power cylinder assembly



Nice hearth decoration



Need to connect the displacer to crank this week.

[Bumpkin]

I like your crankshaft design. It always seems funny when people turn the crank back to the main center before swinging it back out 90 degrees, even without the purpose of providing another main bearing. That said; is there a key or something to register the clamping of the second throw for accurate assembly/reassembly? I look forward to seeing more.

Bumpkin

[Ian S C]

I solved the problem of hollow displacer rods by using stainless steel tube, it's not always easy to get the right size.
Ian S C