Big Beta Stirling 1kw, 100degC to 400degC

[zhivko]

GeoffV,

thanks for valuable info with the chart provided, but still its not clear if inox wool around displacer would be beneficial to heating cooling proces and how to properly mount it on displacer.

I will surely try disconnecting power piston from crank and test manually the movement of power piston. But, first I need to prepare gas heater - I plan to use copper drilled pipe that will be placed around inox cylinder in spiral in length of 300 mm.

I did more precise calculation as you suggested - results are here:

myCrankMechanism_latest.jpg (205.49 KiB) Viewed 9228 times

As you can see from calculation - I can only get 118Watt of heat inside through that 300mm length area. So inox steel wool would be needed; has anybody placed it inside beta stirling and how?

[Geoff V]

zhivko

By my calculation you have a heater of 26cm diam which needs to be heated by about the length of the displacer stroke say 15cm, so 26 x Pi x 15 / 10,000 = 0.122 sq mt, 3.5mm gap = 30W/M2/K therefore at 300K, heat transfer into the working gas will be about 1100W.

A regenerator will help if the heat exchangers are marginal and I know of one engine where the Regen matrix is part of the displacer, I am not keen on this arrangement as it increases the mass of the displacer which is never a good idea. The walls of the displacer case will act as a regenerator, but naturally the area will be quite small when compared to a proper matrix.

It appears that you are being misled by too many spreadsheet calculations, they are useful for analysis of a working engine but at this stage a more 'broad brush approach' will be less misleading.

Regarding your proposed heater, to stop it 'blowing back' the thickness of the burner tube wall must be more than the diameter of the jets, but you knew that anyway, I'm sure.

GeoffV

[zhivko]

zhivko

By my calculation you have a heater of 26cm diam which needs to be heated by about the length of the displacer stroke say 15cm, so 26 x Pi x 15 / 10,000 = 0.122 sq mt, 3.5mm gap = 30W/M2/K therefore at 300K, heat transfer into the working gas will be about 1100W.

Yes, you are right - I had an error in formula (forget to multiply it by heat transfer rate) - but actually I have additional question here? Why are you multiplying by 26? Heat transfer is happening around the area that is actually area of bore of cyleinder - shouldn't be 25cm used instead 26?

A regenerator will help if the heat exchangers are marginal and I know of one engine where the Regen matrix is part of the displacer, I am not keen on this arrangement as it increases the mass of the displacer which is never a good idea. The walls of the displacer case will act as a regenerator, but naturally the area will be quite small when compared to a proper matrix.

Actually now I have displacer with diameter 230 and with cylinder 250 it means 10mm gap - you will se that the heat rate from your chart is minimal - actually it is not even visible :) So I don't know if I cant count on 20W/m2/K in my case ...
If I go for 3.5mm gap - I think the displacer will rub cylinder if Stirling is placed in horizontal position - also how can I make such large displacer in such precission. Maybe Ante would suggest somebody in our area, who can do "metal sheet spining" - are there any other approach how to do precise thin wall cylinders? Currently for cylinder I use thin wall INOX pipe of inner diameter 250, wall thickness 2mm - and the tolerances of such pipe is almost 0.5mm in 250mm....

It appears that you are being misled by too many spreadsheet calculations, they are useful for analysis of a working engine but at this stage a more 'broad brush approach' will be less misleading.

Yes I know I should disconnect the crank and try manually moving displacer to see if current 10mm gap causes too much pressure drop along anular gap... I have this in plan...

Regarding your proposed heater, to stop it 'blowing back' the thickness of the burner tube wall must be more than the diameter of the jets, but you knew that anyway, I'm sure.

Actually not - and I must thank you again for a constructive comments. Can you be more precise regarding gas heater tube? Is it thickness of tube or thickness of tube wall that should be bigger than the diameter of the jets (you mean diameter of flame or gas nozzle in tube wall)?

Also additional question - you will see in my (like) last sinusoidal picture of displacer and piston movement - that actually phase angles are different - one is 90 degrees and the other is 50 degrees - is this OK? Or will this cause problems for stirling functioning? Actually it comes from nature of crank I use.

different phase angles - would that pose a problem?

Stirling Kinematics.jpeg (14.14 KiB) Viewed 9208 times

[Geoff V]

zhivko

26, I guessed with a displacer of 250mm diameter that the casing would be a little larger hence 26cm diameter.

The 10mm gap is far too large to get any real heat transfer to the working gas, which just flows down the middle of the gap outside the boundary layer, this is the problem with large scale plain heat exchangers. If one doubles the size of an engine, the area increases by 4 times yet the volume increases by 8 times (surface to volme ratio also know as the hydraulic radius). To pump this much more gas past the displacer, the gap has to increase and the heat transfer falls, hence the power falls which is why small SE's work and large plain heat exchanger engines don't. The only way to make large swept volume engines is to use slotted or tubular heaters and coolers. Slotted heat exchangers are the best because the are internal, hence smallest increase in volume, but are very difficult to machine (several hundred 0.5mm wide x 8-10mm deep slots I'd guess on your engine). Tubular exchangers are a little easier to make (if you furnace braze the tubes) but they add a lot of extra volume which reduces the pressure pulse. They are very good at adding surface area but if they a small enough to give good heat transfer then you need several times as many tubes as slots for Air. If you are prepared to run at 200bar on Hyrogen or Helium then fewer, larger tubes work because these gasses respond to heating and cooling much better than air and at 200bar there are many many times more molecules of gas per unit volume.

Regarding burners, I was refering to the thickness of the metal where the gas air mixture leaves the burner and combustion takes place, if too thin the flame will travel back into the mixing tube and ignite as soon as the burner gets hot.

FWIW I've been running some tests on an old plain heat exchanger beta engine with a scotch yolk crank, to compare air with helium, the bore is 100mm and the displacer stroke 31mm with a gap of 1.2mm at the cooler end and 1.6mm at the heater end. On Air the best speed I can achieve is 620rpm with peak power at 400rpm and just 16W at 1bar precharge, will publish the results on helium when complete. This engine has about 785 sqcm heater area, at 70W/M2/K that equates to about 1650 watts heat transfer at 300c, so the engine is less than 1% internal thermal efficiency.

Depressing isn't it?

Unless one is prepared to go to the trouble and expense of slotted heat exchangers

http://www.starspin.com/stirlings/jd6fs-4.html

then these engines really are only of novelty value.

GeoffV

[zhivko]

GeofV and others reading this topic,

thanks for an indepth explanation, I know I have quite a big volume of air to heat and cool and I am not keen to complicate with overpresure and different media than air.

The 10mm gap is far too large to get any real heat transfer to the working gas, which just flows down the middle of the gap outside the boundary layer, this is the problem with large scale plain heat exchangers.

I assume the chart you attached is for laminar flow?
If I would place inox wool in 10mm gap in full length of displacer (wool would move with displacer in that case) flow of air would be turbulent - I think I should get far better heating and cooling - what do you think?

I am also thinking in the direction to use a kind of car radiator for heating and also for cooling - isn't that far cheaper and more effective than using quite a lot of tubes that needs to be welded (I know it does not look nice).

I would take the air out from a centimeter above max top piston position, lead it through cooler car radiator, than regenerator, then through car radiator heater (kind a car radiator) and through the big pipe that is welded on forefront of hot cylinder.

used-radiators.jpg (47.89 KiB) Viewed 9214 times

What do you think about this idea - is it viable? Has it been done already somewhere - somehow?

Regards,
Zhivko

[Geoff V]

Zhivko

I've no experience with radiators, I have heard of flat plate heat exchangers being mentioned but again I've no experience with them, sorry.

GeoffV

[zhivko]

Hi everybody -

does sombedoy knows this guy? Peter Lynn
He states that displacer is 7mm les in diameter than cylinder - that means tha I need 3.5 mm gap. And regarding displacer length should be 2 times size of displacer diameter - when i look his "LSM 14 STIRLING CYCLE AIR ENGINE" - it doesn't looks so long!

Maybe with biger diameter size I still have some chance to run my baby ;)

Thoughts - anybody?

Good luck (I will need it) ;)

[vamoose]

Zhivko,
Regarding your radiator question. Here's a post I've put in the links area and have recommended to others;

Philips stirling engine technology videos...

Really good explanation about stirling engine processes strait from the Guru's, Philips...
Starts slow, but is really worth the 15 minutes of your time in my opinion.
(there's even a 30hp stirling powered boat)

The Stirling Cycle part 1 (Stirling Cryogenics)
http://www.youtube.com/watch?v=GqIapDKtvzc

Stirling Cycle Part 2 (Stirling Cryogenics)
http://www.youtube.com/watch?v=GFfMruoR ... ure=relmfu

It kind of ties in with your question..

If you've seen them already then ignore my post. But if you haven't, I highly recommend you take the time to watch and maybe re-watch them a couple time. It's very informative about Beta engines, and was extremely cutting edge design then, and is even still so today...
Please suffer through the beginning as it gets way better as it progresses!! At about 5 minutes into the first video you will start to get more out of it (but still the begining is worth a watch)
vamoose

[zhivko]

Vamoose,

thanks for pointer - this video is very very good information and explains in detail how stirling function - Although I saw it already it was good for me to see it again...

Adding hot heat exchanger add complexity also hot seals in beta are necessary - this guy from new zealand is fascinating :) How he made fixation of displacer in 3.5 mm distance from cylinder for such big piston... Are there any plans for this?

The bigest problem for me is making a thin walled INOX displacer - obviously it should be light - I think lathe is out of question.. Has anybody some idea?

Anybody?


Klemen

[Geoff V]

Zhivko

The engine by Peter Lynn, if the power measurements are correct, is very good news for all builders of Hot Air engines. It appears to demonstrate that the heat transfer is also taking place from the displacer walls, presumably receiving heat by radiation from the outer casing. I've been testing a Beta recently trying to establish if this is the case as it would mean that the total area of the heater (outer case) can be factored by, say 1.5 or a little more to establish the actual heat transfer area. This would be very helpful in calculating the total heat input, during the design phase.

This engine, reported as 2.5 litres working volume, is probably around 150mm bore x 150mm stroke, whereas your engine at 250mm bore will be effected by the lower surface to volume ratio and the peak rpm would be lower with a 3.5mm gap, but worth a try.

Regarding construction of the displacer, this is probably the hardest part to make in a HAE, after the piston sealing rings. Supporting it such that it stays clear of the heater casing is difficult unless it is supported through the piston and at the opposite end of the crankcase. My prefered method of construction is a stainless steel tube with a semi hemispherical dome at the top made by spinning a disc to shape and TIG welding to the tube. Additional radiation baffles are also fitted to help keep the displacer round and are made by spinning a flange onto discs and spot welding in place. The base of the displacer I make by machining a deep aluminium disc with the displacer rod screwed into the middle and the SS part of the displacer is then attached by a length of SS studding which passes through the baffles and is welded to the domed top of the displacer. By using this method I can hold a clearance on my Alpha engine (60mm bore) of just 0.3mm so 3.5mm should be easily achieved on a 250mm bore.

Don't give up at the first hurdle as there are many many more down the road, if it was easy, after 200years nearly, these engine would be in mass production.

GeoffV

[Ian S C]

Peter Lynn, is one of the local Canterbury (NZ) experts on hot air engines, and I would follow his suggestions, he's fairly clued up as far as designing and building hot air engines. Ian S C

[zhivko]

Peter Lynn, is one of the local Canterbury (NZ) experts on hot air engines, and I would follow his suggestions, he's fairly clued up as far as designing and building hot air engines. Ian S C

Yes, I contacted him - but it looks he has too much work with kites... HOw old is this lad ?

[zhivko]

Zhivko
Regarding construction of the displacer, this is probably the hardest part to make in a HAE, after the piston sealing rings. Supporting it such that it stays clear of the heater casing is difficult unless it is supported through the piston and at the opposite end of the crankcase. My prefered method of construction is a stainless steel tube with a semi hemispherical dome at the top made by spinning a disc to shape and TIG welding to the tube. Additional radiation baffles are also fitted to help keep the displacer round and are made by spinning a flange onto discs and spot welding in place. The base of the displacer I make by machining a deep aluminium disc with the displacer rod screwed into the middle and the SS part of the displacer is then attached by a length of SS studding which passes through the baffles and is welded to the domed top of the displacer. By using this method I can hold a clearance on my Alpha engine (60mm bore) of just 0.3mm so 3.5mm should be easily achieved on a 250mm bore.

Ughh, since I am not native speaker I'm not sure if I understood correctly each word of the procedure - so if there is any picture of this (photo or sketch), it will be most welcome. Here the used to say, picture is worth thousand words. So you are using metal spining to form displacer? Are you using same process to shape a cylinder also?

Zhivko
Don't give up at the first hurdle as there are many many more down the road, if it was easy, after 200years nearly, these engine would be in mass production.

Yes I'm striving, but since I don't have lathe in my garage - it's a little more theoretical then practical aproach (from my side).

Here is video of latest geometry displacement ratio around 2. video

piston diameter= 250mm
Piston stroke= 105mm
displacer diameter= 243mm
displacer stroke= 198mm
displacer length= 290mm


I have put two chimneys for heater to ensure as much as possible symmetrical flow of hot air arround inox cylinder wall.

[Geoff V]

Zhivko

I have no pictures of the construction of my displacers, I am normally too involved to bother with the camera when I'm in the workshop, however there is a video of the internal parts of one of my engines showing a finished displacer, hope it helps.

http://youtu.be/jAQ5kr_yFwo

I have also been reading Peter Lynn's website and am reassured by the the caliber of the man that his power measurements are near enough to be trusted, which is encouraging for those interested in large scale hot air engines. The 200W per litre he is achieving will never compete with an ICE but there again the ICE will never run when fuelled by Logs or Solar heating. I also found his sceptic views of the Planet paralleling my own views, but we are both well into our 60's so it's not surprising!

Regarding your revised dimensions, they look about right for the engine to run OK, friction and piston ring sealing are the biggest enemies so care during construction is essential and how you manage without a lathe I don't know. Your engine will only ever run slowly but even though the pressure variations will be small, acting over the very large piston area you have, the engine should do some usefull work.

The weather here is raining and forecast to last all morning so I will do some calculations on displacer swept volume/gap pass area/rpm, there is enough verified data now for some meaningfull predictions.

Keep going.

GeoffV

P.S. the displacer cylinder is a tube, only the baffles and top cap are spun.

[zhivko]

Hi everybody,

I had some thoughts abot heat exchanger that are obviously crucial for such a big engine. Ante gave me great idea (on private message), how and where to put heat exchanger and I wanna share this thoughts with you, so you can brush it, refine, whatever...

heat exchanger

ExchangerAssPresentation.jpg (91.27 KiB) Viewed 9159 times

In this video I show heat exchanger that I could put in anular gap between cylinder and displacer Video of inox heat exchanger.


I will left 1mm on displacer side so if in vertical position displacer shouldn't rubb against exchanger.
Heat exchanger should be tighly inserted in cylinder for that reason I could longitudinaly cut it and with additional inox lamel inserted - it would stretch whole exchanger so it would become tight to cylinder wall.
There are 300 inox stainless steel lamels (thicknes 1.2mm) of dimension 150mm*9mm what gives heat exchange area of 0,81m2
Calculated by: 9mm(wide)*150mm(length)*2(sides of single lamel)*300(number of labels)

There was also idea to put same heat exchanger in cold side in anular gap, and between both exchanger regenerator in form of inox wool.

Lamels could be laser cutted, I still didn't get an price for that but will see what would be the cost...

Please comment, critisize, wahtever ;)