Heat conductance losses in wire mesh stacks

Discussion on Stirling or "hot air" engines (all types)
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denisbudyak
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Heat conductance losses in wire mesh stacks

Post by denisbudyak » Fri May 15, 2015 4:05 am

Hi! Are there any estimates on that losses?

I only found that http://www.ncbi.nlm.nih.gov/pubmed/11543366
but I don't know how to get full text.

It is usually assumed that heat conductance losses are negligible, but I plan to do
regenerator from phosphor bronse and not from SS. Bronse have several times greater heat transfer capacity,
I worry if the loss will be acceptable?

Ian S C
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Re: Heat conductance losses in wire mesh stacks

Post by Ian S C » Sat May 16, 2015 2:37 am

It would be worth looking on a site like this one www.EngineeringToolBox.com This has all the propeties of metals, ie., Phos Bronze 28.9
Stainless steel 7 - 26 (Btu/(hr*F ft)) @ 68*F these are figures for heat conduction. I presume the two figures for SS depends on which grade of SS you use (there are dozens).
Apart from heat conduction in the regenerator material, there is also the problems of corrosion/oxidation, no problem in an inert gas.
Ian S C

denisbudyak
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Re: Heat conductance losses in wire mesh stacks

Post by denisbudyak » Sat May 16, 2015 3:43 am

The most complex part of the problem is the heat conductivity in the contact of subsequent layers in the stack. Data on individual metals won't help here :(
Also wire's surfaces are close enough to each other at some points, so gas heat conduction should be taken into account.

I plan to use hydrogen at temperature no more than 290C, so I hope no corrosion problems would occur.

Yesterday I undertook CAE modeling of a stack and obtained some results. In a hydrogen at 500C and 10 atm I get heat conductivity of 3,3W/(m*k) with steel meshes. I suppose that conductivity does not depend on scale (wire diameter), but it was 1mm.

I could make a mistake at any stage of process. Also the result must have been checked in a sequence of CAE meshes with different scales, but this was not done. If someone wants to re-check this, feel free to contact me for geometry file. Or you can redo the entire work. I attach a picture of my geomerty.
provolokiKusok.png
provolokiKusok.png (22.62 KiB) Viewed 6113 times

Ian S C
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Re: Heat conductance losses in wire mesh stacks

Post by Ian S C » Sun May 17, 2015 2:14 am

You may have, if not it would be worth down loading Andy Ross's book "Making Stirling Engines"(it's free), about 60 pages. Andy has done quite a bit of research into regenerators, and the conclusion of that is that the best material for a regenerator is stainless steel foil .0015"/ .04 mm thick with rows of dimples. If you use wire, or mesh, you want as fine as you can easily use, .25 mm would be a good start, I think you'll find that SS is cheaper than Phos Bronze.
If you want power from your engine, the hot cap should be at least a dull red heat. How are you going to keep the hydrogen in the motor? you will need very good seals on the crankshaft, or are you going to use an internal alternator/generator, there by eliminating the need to have any shaft coming out of the crankcase.
Ian S C

denisbudyak
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Re: Heat conductance losses in wire mesh stacks

Post by denisbudyak » Fri May 29, 2015 7:22 am

Hi!

Thanks, I've seen the book. Definitely, foil has much greater heat conductivity losses than mesh stack and I checked this. IMO 0.02mm thick foil is required. Even if the regenerator foil is cut as in Kirk V2 Rider, local losses would occur between cuts. Totally, mesh stack seem to have smaller losses (reheat loss + conduction loss), but suitability might depend on specific engine configuration. I think the most complex part of problem with foil is uniform spacing, I don't know how to solve it and I dislike what I have seen in Andy Ross's book. So for current model my desicion is to use meshes.

> If you want power from your engine, the hot cap should be at least a dull red heat.
I plan to use much lower heater temperature, about 280C, to keep oil from disintegration. Ordinary steel st20 is compatible to high pressure hydrogen at this temperature, according to Russian standards. Currently I'm looking for suitable oil. Heavy vapors are known to be stable, but they're too viscous. I consider using Dowtherm A, but its lubricity is questionable.The same goes to all other high-temperature heat transfer liquids I know. I still was unable to find any positive data on how to design crosshead and piston rings for any of those liquids. So this question is still open, some experiments are needed. Drop of power due to low heater temperature is compensated by higher pressure of good quality working fluid (hydrogen) - this is approved by modeling, by low leaks due to lubricated sealings and by low friction coefficient. I also plan internal alternator or magnetic coupling. I also plan to employ chemical hydrogen generator to compensate any leaks. Small opening would be available to wipe away any non-condensible gaseous contaminants, e.g. oil decomposition products, together with hydrogen excess. This is similar to ventilated crankcase concept of Philips MP1002CA.

Ian S C
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Re: Heat conductance losses in wire mesh stacks

Post by Ian S C » Sat May 30, 2015 4:38 am

Your best lubricant would be a Teflon based synthetic oil, but lubrication should be very minimal, the synthetic oil does not oxydize. If you are going to use Hydrogen, I imagine that to get an advantage over air, 300psi or more would be a good start. The greater the pressure, the larger amount of heat is required(more power/more heat)
Ian S C

denisbudyak
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Re: Heat conductance losses in wire mesh stacks

Post by denisbudyak » Sat May 30, 2015 5:35 am

Thank you, that's very interesting! Can you give some specific oil names and reference to data on their thermal stability?

I dislike teflon as its thermal decomposition in the flame gives off very toxic gases (e.g. smoking is prohibited in a room where PTFE is stored). But if it is the only way to go, I can accept its use in a sealed system.

I plan to use 15 atmospheres of average pressure (230 psi). But hydrogen always have an advantage over air do to its lower viscosity and better heat conductivity. E.g. it allows the same engine to run about 4 times faster with the same or better efficiency (it depends on the specific configuration). The only part of the engine that needs special attention is a displacer gap, where shuttle loss can increase with hydrogen. And, of course, safety needs special consideration.

Ian S C
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Re: Heat conductance losses in wire mesh stacks

Post by Ian S C » Sun May 31, 2015 2:22 am

At lower pressures it is not a great ecconomical advantage ie Philips MP1002 CA.
Can't name any particular lubricant. But you can get away fron liquid lubrication by using Xylan, this is a Teflon paint, that is baked on to a surface such as the skirt of an aluminium piston. Although quite warm, the power piston should not be any where near the temperature to effect the Xylan. Oil of any type is not really wanted in a Stirling Engine, specially a pressurised engine, oil under pressure/diesel engine,BANG, I believe this has happened.
Ian S C

denisbudyak
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Re: Heat conductance losses in wire mesh stacks

Post by denisbudyak » Mon Jun 01, 2015 6:49 am

Ok, thanks, I'll maybe take a look at Xylan.

masaruneema
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Re: Heat conductance losses in wire mesh stacks

Post by masaruneema » Sat Nov 21, 2015 12:14 am

there is also the problems of corrosion/oxidation, no problem in an inert gas.
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