SURFACE AREA OPTIMISATION

Discussion on Stirling or "hot air" engines (all types)
Ian S C
Posts: 1953
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: SURFACE AREA OPTIMISATION

Postby Ian S C » Thu Jun 28, 2012 7:32 am

Hi, first off, a displacer is only found in a BETTA, or GAMMA type motor, not in an ALPHA motor. In an ALPHA motor the hot piston usually has a thing that in a way resembles a displacer attached to it, this is called a Heylandt Crown, One of the main things this does is to isolate the piston its self from the heat, it also does a bit of regeneration (as does a displacer). It should be made as light as possible, proberbly from thin stainless steel (NOT aluminium). The hot cap that fits over this should also be of thin stainless.
It could be worth getting the free down load of Andy Ross's book "Making Stirling Engines". There are also a number of other books available, but they tend to be a bit pricy, but they do come up second hand, And they, along with this forum ,help you get your head around some of the more puzzling bits.
The displacer could well be roughed up as you describe, it also may be made of steel wool, or rolled up stainless shim. The traditional displacer is a thin walled cylinder, approximately 3 times longer than its diameter. It can be made of mild steel, but stainless is better. It's made of SS, or steel because this has a high resistance to heat conduction, this slows the conduction of heat from the hot end to the cold end(some will say use aluminium cans (I have), aluminium has high heat conductivity, and low strength when heated. Ian S C

Ian S C
Posts: 1953
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: SURFACE AREA OPTIMISATION

Postby Ian S C » Sat Jun 30, 2012 1:10 am

Vamoose, the simple term for the Heylandt crown is a hot cap (does'nt sound so high tech). One of the most important things is getting rid of waist heat, its easier to get heat in that out, similar occurrs with an internal combustion engine, easier to get the fuel/air in than get the exhaust out, and I think the same happens with steam engines. Ian S C

vamoose
Posts: 275
Joined: Tue Jul 03, 2012 12:16 am
Location: Australia

Re: SURFACE AREA OPTIMISATION

Postby vamoose » Wed Jul 04, 2012 12:23 am

Sorry if this topic now appears random. All my posts have been wiped from system and Boyd informs me cant be retrieved...

Ian S C
Posts: 1953
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: SURFACE AREA OPTIMISATION

Postby Ian S C » Wed Jul 04, 2012 4:03 am

Vamoose, stick them in again, or at least the first bit, it will be out of order, but that'll be OK.Had similar a year or two ago, happens occasionly, did'nt just loose a thread, I had to re enter the forum, and start all over. Good luck, Ian S C

vamoose
Posts: 275
Joined: Tue Jul 03, 2012 12:16 am
Location: Australia

Re: SURFACE AREA OPTIMISATION

Postby vamoose » Wed Jul 04, 2012 6:40 pm

Yep, same as you, this is vamoose no2... Didn't save a copy of my posts (didn't cross my mind i should). Ill do a quick Re-cap of this topic for other readers:

While looking at stirling engine designs in Alpha (and other) motors i started to observe that the internal cold side of the cycle in many designs seemed to have a lot less surface area for the removal/transfer of heat when compared to the hot side and its addition of heat, with its hot cap and extra sidewall surface area.
Why aren't there equivalent "cold caps" to at least equalize surface area on both hot and cold sides.. I wonder if this is due to denser air on the cold side (at least at the gas surface interface) requiring less surface area to achieve same amount of energy transfer (i think this possibly may be a wrong assumption).

I am also wondering that if the cap and adjacent wall surface areas are increased by some means (ie fins, grooves, pitting, knurling etc) would the increased surface area gains outweigh the turbulence losses (and increased dead space problems). Also could the turbulence (especially in the form of eddies and vortices) actually improve energy transfer with increased gas velocity/interchange on the metal surface/interface area - (what i mean is - replacing the gas that has reached temperature, for gas with a greater temp difference, reducing thermal stratification/insulation, and therefore increasing the overall rate of transfer of heat in and out).

Any thoughts...

Ian S C
Posts: 1953
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: SURFACE AREA OPTIMISATION

Postby Ian S C » Fri Jul 06, 2012 5:21 am

If you look at Andy Ross's "Making Stirling Engines" you'll see that he has used on some engines a finned interiour to the hot end . Ian S C

cbstirling2
Posts: 106
Joined: Sat Dec 17, 2016 9:35 pm

Re: SURFACE AREA OPTIMISATION

Postby cbstirling2 » Wed Mar 15, 2017 4:16 am

Vamoose and I had the same question: Does a Heylandt Crown belong on both hot and cold pistons in an alpha engine??
Graham Walker wrote "Heylandt Crown: The addition of an extension to a reciprocating piston to remove the hot or cold fluid from the region where the piston rings and seals operate."

But another feature is to dramatically increase surface area with little to no expansion of dead space.

The crown is a cone (usually truncated):
The formula for the top area of a cone is 3.14 times the radius times the hypotenuse side (πrl). http://www.web-formulas.com/Math_Formul ... f_Cone.asp

Flat top= πr2
Conic top=πrl
So if l is 3 while r is one, that's 3x more surface area to conduct in or out!

The cold side is just as important as the hot side for efficiency...

Thoughts?
CBStirling2

cbstirling2
Posts: 106
Joined: Sat Dec 17, 2016 9:35 pm

Re: SURFACE AREA OPTIMISATION

Postby cbstirling2 » Wed Mar 15, 2017 4:23 am

I am also thinking that for the cold side, an aluminum conic head is preferable to steel to get the heat out quickly.
CBStirling2

Ian S C
Posts: 1953
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: SURFACE AREA OPTIMISATION

Postby Ian S C » Thu Mar 16, 2017 3:01 am

My ALPHA motor has a cylindrical Heylandt Crown, it's about two diameters high, and made of stainless steel about .007" thick on a cast iron compression piston, there is a .040" gap between it and the cylinder wall. The cold piston is a flat topped cast iron one.
Ian S C


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